The vast majority of malignant gliomas relapse after surgery and standard radio-chemotherapy. Novel molecular and cellular therapies are thus being developed, targeting specific aspects of tumor growth. While histopathology remains the gold standard for tumor classification, neuroimaging has over the years taken a central role in the diagnosis and treatment follow up of brain tumors. It is used to detect and localize lesions, define the target area for biopsies, plan surgical and radiation interventions and assess tumor progression and treatment outcome. In recent years the application of novel drugs including anti-angiogenic agents that affect the tumor vasculature, has drastically modulated the outcome of brain tumor imaging. To properly evaluate the effects of emerging experimental therapies and successfully support treatment decisions, neuroimaging will have to evolve. Multi-modal imaging systems with existing and new contrast agents, molecular tracers, technological advances and advanced data analysis can all contribute to the establishment of disease relevant biomarkers that will improve disease management and patient care. In this review, we address the challenges of glioma imaging in the context of novel molecular and cellular therapies, and take a prospective look at emerging experimental and pre-clinical imaging techniques that bear the promise of meeting these challenges.
Anti-angiogenic therapy in glioblastoma (GBM) has unfortunately not led to the anticipated improvement in patient prognosis. We here describe how human GBM adapts to bevacizumab treatment at the metabolic level. By performing (13)C6-glucose metabolic flux analysis, we show for the first time that the tumors undergo metabolic re-programming toward anaerobic metabolism, thereby uncoupling glycolysis from oxidative phosphorylation. Following treatment, an increased influx of (13)C6-glucose was observed into the tumors, concomitant to increased lactate levels and a reduction of metabolites associated with the tricarboxylic acid cycle. This was confirmed by increased expression of glycolytic enzymes including pyruvate dehydrogenase kinase in the treated tumors. Interestingly, L-glutamine levels were also reduced. These results were further confirmed by the assessment of in vivo metabolic data obtained by magnetic resonance spectroscopy and positron emission tomography. Moreover, bevacizumab led to a depletion in glutathione levels indicating that the treatment caused oxidative stress in the tumors. Confirming the metabolic flux results, immunohistochemical analysis showed an up-regulation of lactate dehydrogenase in the bevacizumab-treated tumor core as well as in single tumor cells infiltrating the brain, which may explain the increased invasion observed after bevacizumab treatment. These observations were further validated in a panel of eight human GBM patients in which paired biopsy samples were obtained before and after bevacizumab treatment. Importantly, we show that the GBM adaptation to bevacizumab therapy is not mediated by clonal selection mechanisms, but represents an adaptive response to therapy.
Originally known as host defence peptides for their substantial bacteriotoxic effects, many cationic antimicrobial peptides also exhibit a potent cytotoxic activity against cancer cells. Their mode of action is characterized mostly by electrostatic interactions with the plasma membrane, leading to membrane disruption and rapid necrotic cell death. In this work, we have designed a novel cationic peptide of 27 amino acids (Cypep-1), which shows efficacy against a number of cancer cell types, both in vitro and in vivo, while normal human fibroblasts were significantly less affected. Surface plasmon resonance experiments as well as liposome leakage assays monitored by fluorescence spectroscopy revealed a substantial binding affinity of Cypep-1 to negatively charged liposomes and induced significant leakage of liposome content after exposure to the peptide. The observed membranolytic effect of Cypep-1 was confirmed by scanning electron microscopy (SEM) as well as by time-lapse confocal microscopy. Pharmacokinetic profiling of Cypep-1 in rats showed a short plasma half-life after i.v. injection, followed mainly by retention in the liver, spleen and kidneys. Extremely low concentrations within the organs of the central nervous system indicated that Cypep-1 did not pass the blood-brain-barrier. Local treatment of 4T1 murine mammary carcinoma allografts by means of a single local bolus injection of Cypep-1 led to a significant reduction of tumour growth in the following weeks and prolonged survival. Detailed histological analysis of the treated tumours revealed large areas of necrosis. In sum, our findings show that the novel cationic peptide Cypep-1 displays a strong cytolytic activity against cancer cells both in vitro and in vivo and thus holds a substantial therapeutic potential.
Recent results from 2 double-blind, placebo-controlled phase III trials (RTOG 0825) and (AVAglio) for first-line treatment of glioblastoma patients with the VEGF antibody bevacizumab, showed similar results, related to overall and progression-free survival. The RTOG 0825 trial indicated, opposed to the AVAglio trial, that patients treated with bevacizumab showed a decline in global neurocognitive function compared to untreated patients, -a decline that was most obvious after prolonged treatment. At present, there is a considerably controversy related to these observations. In the present work we point at the possibility that bevacizumab treatment of the normal brain can reduce synaptic plasticity in the hippocampus. We believe that such a phenomenon may partly explain the reduced cognitive function observed in patients in the RTOG 0825 trial. Since the same effects were not clearly defined in the AVAglio trial, further studies on putative neurocognitive effects after bevacizumab treatment are warranted.
Malignant melanoma is the most lethal form of skin cancer, with a high propensity to metastasize to the brain. More than 60% of melanomas have the BRAFV600E mutation, which activates the mitogen-activated protein kinase (MAPK) pathway . In addition, increased PI3K (phosphoinositide 3-kinase) pathway activity has been demonstrated, through the loss of activity of the tumor suppressor gene, PTEN . Here, we treated two melanoma brain metastasis cell lines, H1_DL2, harboring a BRAFV600E mutation and PTEN loss, and H3, harboring WT (wild-type) BRAF and PTEN loss, with the MAPK (BRAF) inhibitor vemurafenib and the PI3K pathway associated mTOR inhibitor temsirolimus. Combined use of the drugs inhibited tumor cell growth and proliferation in vitro in H1_DL2 cells, compared to single drug treatment. Treatment was less effective in the H3 cells. Furthermore, a strong inhibitory effect on the viability of H1_DL2 cells, when grown as 3D multicellular spheroids, was seen. The treatment inhibited the expression of pERK1/2 and reduced the expression of pAKT and p-mTOR in H1_DL2 cells, confirming that the MAPK and PI3K pathways were inhibited after drug treatment. Microarray experiments followed by principal component analysis (PCA) mapping showed distinct gene clustering after treatment, and cell cycle checkpoint regulators were affected. Global gene analysis indicated that functions related to cell survival and invasion were influenced by combined treatment. In conclusion, we demonstrate for the first time that combined therapy with vemurafenib and temsirolimus is effective on melanoma brain metastasis cells in vitro. The presented results highlight the potential of combined treatment to overcome treatment resistance that may develop after vemurafenib treatment of melanomas.
Diffuse gliomas comprise a group of primary brain tumors that originate from glial (precursor) cells and present as a variety of malignancy grades which have in common that they grow by diffuse infiltration. This phenotype complicates treatment enormously as it precludes curative surgery and radiotherapy. Furthermore, diffusely infiltrating glioma cells often hide behind a functional blood-brain barrier, hampering delivery of systemically administered therapeutic and diagnostic compounds to the tumor cells. The present review addresses the biological mechanisms that underlie the diffuse infiltrative phenotype, knowledge of which may improve treatment strategies for this disastrous tumor type. The invasive phenotype is specific for glioma: most other brain tumor types, both primary and metastatic, grow as delineated lesions. Differences between the genetic make-up of glioma and that of other tumor types may therefore help to unravel molecular pathways, involved in diffuse infiltrative growth. One such difference concerns mutations in the NADP(+)-dependent isocitrate dehydrogenase (IDH1 and IDH2) genes, which occur in >80% of cases of low grade glioma and secondary glioblastoma. In this review we present a novel hypothesis which links IDH1 and IDH2 mutations to glutamate metabolism, possibly explaining the specific biological behavior of diffuse glioma.
It is well known that in vitro subculture represents a selection pressure on cell lines, and over time this may result in a genetic drift in the cancer cells. In addition, long-term cultures harbor the risk of cross-contamination with other cell lines. The consequences may have major impact on experimental results obtained in various laboratories, where the cell lines no longer reflect the original tumors that they are supposed to represent. Much neglected in the scientific community is a close monitoring of cell cultures by regular phenotypic and genetic characterization. In this report, we present a thorough characterization of the commonly used glioblastoma (GBM) model U-251, which in numerous publications has been wrongly identified as U-373, due to an earlier cross-contamination. In this work, the original U-251 and three subclones of U-251, commonly referred to as U-251 or U-373, were analyzed with regard to their DNA profile, morphology, phenotypic expression, and growth pattern. By array comparative genomic hybridization (aCGH), we show that only the original low-passaged U-251 cells, established in the 1960s, maintain a DNA copy number resembling a typical GBM profile, whereas all long-term subclones lost the typical GBM profile. Also the long-term passaged subclones displayed variations in phenotypic marker expression and showed an increased growth rate in vitro and a more aggressive growth in vivo. Taken together, the variations in genotype and phenotype as well as differences in growth characteristics may explain different results reported in various laboratories related to the U-251 cell line.
Patient-derived glioblastoma (GBM) stem-like cells (GSCs) represent a valuable model for basic and therapeutic research. GSCs are usually propagated in serum-free Neural Basal medium supplemented with bFGF and EGF. Yet, the exact influence of these growth factors on GSCs is still unclear. Recently it was suggested that GBM stem-like cells with amplified EGFR should be cultured in stem cell medium without EGF, as the presence of EGF induced rapid loss of EGFR amplification. However, patient biopsies are usually taken into culture before their genomic profiles are defined. Thus, an important question remains whether GBM cells without EGFR amplification also can be cultured in stem cell medium without EGF.
Glioblastoma (GBM) is known to be a heterogeneous disease; however, the genetic composition of the cells within a given tumour is only poorly explored. In the advent of personalised medicine the understanding of intra-tumoural heterogeneity at the cellular and the genetic level is mandatory to improve treatment and clinical outcome. By combining ploidy-based flow sorting with array-comparative genomic hybridization we show that primary GBMs present as either mono- or polygenomic tumours (64 versus 36 %, respectively). Monogenomic tumours were limited to a pseudodiploid tumour clone admixed with normal stromal cells, whereas polygenomic tumours contained multiple tumour clones, yet always including a pseudodiploid population. Interestingly, pseudodiploid and aneuploid fractions carried the same aberrations as defined by identical chromosomal breakpoints, suggesting that evolution towards aneuploidy is a late event in GBM development. Interestingly, while clonal heterogeneity could be recapitulated in spheroid-based xenografts, we find that genetically distinct clones displayed different tumourigenic potential. Moreover, we show that putative cancer stem cell markers including CD133, CD15, A2B5 and CD44 were present on genetically distinct tumour cell populations. These data reveal the clonal heterogeneity of GBMs at the level of DNA content, tumourigenic potential and stem cell marker expression, which is likely to impact glioma progression and treatment response. The combined knowledge of intra-tumour heterogeneity at the genetic, cellular and functional level is crucial to assess treatment responses and to design personalized treatment strategies for primary GBM.
Glioblastoma multiforme (GBM), the most aggressive brain malignancy, is characterized by extensive cellular proliferation, angiogenesis, and single-cell infiltration into the brain. We have previously shown that a xenograft model based on serial xenotransplantation of human biopsy spheroids in immunodeficient rodents maintains the genotype and phenotype of the original patient tumor. The present work further extends this model for optical assessment of tumor engraftment and growth using bioluminescence imaging (BLI). A method for successful lentiviral transduction of the firefly luciferase gene into multicellular spheroids was developed and implemented to generate optically active patient tumor cells. Luciferase-expressing spheroids were injected into the brains of immunodeficient mice. BLI photon counts and tumor volumes from magnetic resonance imaging (MRI) were correlated. Luciferase-expressing tumors recapitulated the histopathologic hallmarks of human GBMs and showed proliferation rates and microvessel density counts similar to those of wild-type xenografts. Moreover, we detected widespread invasion of luciferase-positive tumor cells in the mouse brains. Herein we describe a novel optically active model of GBM that closely mimics human pathology with respect to invasion, angiogenesis, and proliferation indices. The model may thus be routinely used for the assessment of novel anti-GBM therapeutic approaches implementing well-established and cost-effective optical imaging strategies.
The identification and significance of cancer stem-like cells in malignant gliomas remains controversial. It has been proposed that cancer stem-like cells display increased drug resistance, through the expression of ATP-binding cassette transporters that detoxify cells by effluxing exogenous compounds. Here, we investigated the side population phenotype based on efflux properties of ATP-binding cassette transporters in freshly isolated human glioblastoma samples and intracranial xenografts derived thereof. Using fluorescence in situ hybridization analysis on sorted cells obtained from glioblastoma biopsies, as well as human tumour xenografts developed in immunodeficient enhanced green fluorescence protein-expressing mice that allow an unequivocal tumour-stroma discrimination, we show that side population cells in human glioblastoma are non-neoplastic and exclusively stroma-derived. Tumour cells were consistently devoid of efflux properties regardless of their genetic background, tumour ploidy or stem cell associated marker expression. Using multi-parameter flow cytometry we identified the stromal side population in human glioblastoma to be brain-derived endothelial cells with a minor contribution of astrocytes. In contrast with their foetal counterpart, neural stem/progenitor cells in the adult brain did not display the side population phenotype. Of note, we show that CD133-positive cells often associated with cancer stem-like cells in glioblastoma biopsies, do not represent a homogenous cell population and include CD31-positive endothelial cells. Interestingly, treatment of brain tumours with the anti-angiogenic agent bevacizumab reduced total vessel density, but did not affect the efflux properties of endothelial cells. In conclusion our findings contribute to an unbiased identification of cancer stem-like cells and stromal cells in brain neoplasms, and provide novel insight into the complex issue of drug delivery to the brain. Since efflux properties of endothelial cells are likely to compromise drug availability, transiently targeting ATP-binding cassette transporters may be a valuable therapeutic strategy to improve treatment effects in brain tumours.
Despite improved treatment options, glioblastoma multiforme (GBM) remains the most aggressive brain tumour with the shortest post-diagnostic survival. Arsenite (As2O3) is already being used in the treatment of acute promyelocytic leukaemia (APL), yet its effects on GBM have not been evaluated in detail. In U87MG cell monolayers, we have previously shown that arsenite cytotoxicity significantly increases upon transient inhibition of lysosomal protease Cathepsin L (CatL). As multicellular spheroids more closely represent in vivo tumours, we aimed to evaluate the impact of permanent CatL silencing on arsenite treatment in U87MG spheroids. CatL was stably silenced using shRNA expression plasmid packed lentiviruses. By using metabolic- and cell viability assays, we demonstrated that long-term CatL silencing significantly increased arsenite cytotoxicity in U87MG spheroids. Silenced CatL also increased arsenite-mediated apoptosis in spheroids via elevated p53 expression, Bax/Bcl2 ratio and caspase 3/7 activity, though with lower efficacy than in monolayers. Arsenite cytotoxicity was enhanced by lower CatL activity, since similar cytotoxicity increase was also observed using the novel CatL inhibitor AT094. The results have significant translational impact, since stable CatL silencing would enable the application of lower systemic doses of arsenite to achieve the desired cytotoxic effects on GBMs in vivo.
Biologic and therapeutic advances in melanoma brain metastasis are hampered by the paucity of reproducible and predictive animal models. In this work, we developed a robust model of brain metastasis that empowers quantitative tracking of cellular dissemination and tumor progression. Human melanoma cells labeled with superparamagnetic iron oxide nanoparticles (SPION) were injected into the left cardiac ventricle of mice and visualized by MRI. We showed that SPION exposure did not affect viability, growth, or migration in multiple cell lines across several in vitro assays. Moreover, labeling did not impose changes in cell-cycle distribution or apoptosis. In vivo, several SPION-positive cell lines displayed similar cerebral imaging and histologic features. MRI-based automated quantification of labeled cells in the brain showed a sigmoid association between metastasis frequency and doses of inoculated cells. Validation of this fully automated quantification showed a strong correlation with manual signal registration (r(2) = 0.921, P < 0.001) and incidence of brain metastases (r(2) = 0.708, P < 0.001). Metastasis formation resembled the pattern seen in humans and was unaffected by SPION labeling (histology; tumor count, P = 0.686; survival, P = 0.547). In summary, we present here a highly reproducible animal model that can improve the predictive value of mechanistic and therapeutic studies of melanoma brain metastasis.
Oligodendroglioma poses a biological conundrum for malignant adult human gliomas: it is a tumor type that is universally incurable for patients, and yet, only a few of the human tumors have been established as cell populations in vitro or as intracranial xenografts in vivo. Their survival, thus, may emerge only within a specific environmental context. To determine the fate of human oligodendroglioma in an experimental model, we studied the development of an anaplastic tumor after intracranial implantation into enhanced green fluorescent protein (eGFP) positive NOD/SCID mice. Remarkably after nearly nine months, the tumor not only engrafted, but it also retained classic histological and genetic features of human oligodendroglioma, in particular cells with a clear cytoplasm, showing an infiltrative growth pattern, and harboring mutations of IDH1 (R132H) and of the tumor suppressor genes, FUBP1 and CIC. The xenografts were highly invasive, exhibiting a distinct migration and growth pattern around neurons, especially in the hippocampus, and following white matter tracts of the corpus callosum with tumor cells accumulating around established vasculature. Although tumors exhibited a high growth fraction in vivo, neither cells from the original patient tumor nor the xenograft exhibited significant growth in vitro over a six-month period. This glioma xenograft is the first to display a pure oligodendroglioma histology and expression of R132H. The unexpected property, that the cells fail to grow in vitro even after passage through the mouse, allows us to uniquely investigate the relationship of this oligodendroglioma with the in vivo microenvironment.
Most tumors are typified by a chronic, unresolved inflammatory response that potentiates angiogenesis and therefore enables tumor progression. We have determined that dysfunctional tumor-associated adipocytes contribute to tumor-associated inflammation. In three tumor models, tumor-associated adipose tissue was characterized by thin and fragile adipocyte membranes, necrosis, robust expression of the pro-inflammatory factor HMGB1, and loss of the lipid storage mediator, perilipin-1. By transmission electron microscopy, macrophages in tumor-associated adipose tissue contained lipid droplets and resembled foam cells, which are commonly observed in inflamed tissues. In vitro co-culture studies showed that tumor-associated adipose tissue conditioned-medium stimulated monocyte-to-macrophage differentiation, adhesion, spreading, and lipid uptake. Compared with normal adipose tissue, tumor-associated adipose tissue secreted 3-fold higher levels of IL-6 and IL-6 was sufficient to stimulate macrophage differentiation and adhesion. These results suggest that, in tumors, loss of adipocyte specification, necrosis, and scavenging of adipocyte debris directly activates macrophages and contributes to tumor-associated inflammation. Thus, adipocyte dysfunction may facilitate tumor progression, especially in tumors closely aligned with adipose tissue, in particular, breast cancer.
Angiogenesis is regarded as a hallmark of cancer progression and it has been postulated that solid tumor growth depends on angiogenesis. At present, however, it is clear that tumor cell invasion can occur without angiogenesis, a phenomenon that is particularly evident by the infiltrative growth of malignant brain tumors, such as glioblastomas (GBMs). In these tumors, amplification or overexpression of wild-type (wt) or truncated and constitutively activated epidermal growth factor receptor (EGFR) are regarded as important events in GBM development, where the complex downstream signaling events have been implicated in tumor cell invasion, angiogenesis and proliferation. Here, we show that amplification and in particular activation of wild-type EGFR represents an underlying mechanism for non-angiogenic, invasive tumor growth. Using a clinically relevant human GBM xenograft model, we show that tumor cells with EGFR gene amplification and activation diffusely infiltrate normal brain tissue independent of angiogenesis and that transient inhibition of EGFR activity by cetuximab inhibits the invasive tumor growth. Moreover, stable, long-term expression of a dominant-negative EGFR leads to a mesenchymal to epithelial-like transition and induction of angiogenic tumor growth. Analysis of human GBM biopsies confirmed that EGFR activation correlated with invasive/non-angiogenic tumor growth. In conclusion, our results indicate that activation of wild-type EGFR promotes invasion and glioblastoma development independent of angiogenesis, whereas loss of its activity results in angiogenic tumor growth.
Brain metastasis is associated with a particular poor prognosis. Novel insight into the brain metastatic process is therefore warranted. Several preclinical models of brain tumor metastasis have been developed during the last 60 years, and they have in part revealed some of the mechanisms underlying the metastatic process. This review discusses mechanisms of brain metastasis with a key focus of the development of animal model systems. This includes the use of rodent, syngeneic brain metastasis models (spontaneous, chemically induced and genetically engineered models) and human xenotransplantation models (ectopic inoculation and orthotopic models). Current information indicates that none of these fully reflect tumor development seen in patients with metastatic disease. The various model systems used, however, have provided important insight into specific mechanisms of the metastatic process related to the brain. By combining the knowledge obtained from animal models, new important information on the molecular mechanisms behind metastasis will be obtained, leading to the future development of new therapeutic strategies.
Two of the signature genetic events that occur in human gliomas, EGFR amplification and IDH mutation, are poorly represented in experimental models in vitro. EGFR amplification, for example, occurs in 40 to 50% of GBM, and yet, EGFR amplification is rarely preserved in cell cultures derived from human tumors. To analyze the fate of EGFR amplified and IDH mutated cells in culture, we followed the development over time of cultures derived from human xenografts in nude rats enriched for tumor cells with EGFR amplification and of cultures derived from patient samples with IDH mutations, in serum monolayer and spheroid suspension culture, under serum and serum free conditions. We observed under serum monolayer conditions, that nestin positive or nestin and SMA double positive rat stromal cells outgrew EGFR amplified tumor cells, while serum spheroid cultures preserved tumor cells with EGFR amplification. Serum free suspension culture exhibited a more variable cell composition in that the resultant cell populations were either predominantly nestin/SOX2 co-expressing rat stromal cells or human tumor cells, or a mixture of both. The selection for nestin/SMA positive stromal cells under serum monolayer conditions was also consistently observed in human oligodendrogliomas and oligoastrocytomas with IDH mutations. Our results highlight for the first time that serum monolayer conditions can select for stromal cells instead of tumor cells in certain brain tumor subtypes. This result has an important impact on the establishment of new tumor cell cultures from brain tumors and raises the question of the proper conditions for the growth of the tumor cell populations of interest.
Oligodendroglial tumors form a distinct subgroup of gliomas, characterized by a better response to treatment and prolonged overall survival. Most oligodendrogliomas and also some oligoastrocytomas are characterized by a unique and typical unbalanced translocation, der(1,19), resulting in a 1p/19q co-deletion. Candidate tumor suppressor genes targeted by these losses, CIC on 19q13.2 and FUBP1 on 1p31.1, were only recently discovered. We analyzed 17 oligodendrogliomas and oligoastrocytomas by applying a comprehensive approach consisting of RNA expression analysis, DNA sequencing of CIC, FUBP1, IDH1/2, and array CGH. We confirmed three different genetic subtypes in our samples: i) the "oligodendroglial" subtype with 1p/19q co-deletion in twelve out of 17 tumors; ii) the "astrocytic" subtype in three tumors; iii) the "other" subtype in two tumors. All twelve tumors with the 1p/19q co-deletion carried the most common IDH1 R132H mutation. In seven of these tumors, we found protein-disrupting point mutations in the remaining allele of CIC, four of which are novel. One of these tumors also had a deleterious mutation in FUBP1. Only by integrating RNA expression and array CGH data, were we able to discover an exon-spanning homozygous microdeletion within the remaining allele of CIC in an additional tumor with 1p/19q co-deletion. Therefore we propose that the mutation rate might be underestimated when looking at sequence variants alone. In conclusion, the high frequency and the spectrum of CIC mutations in our 1p/19q-codeleted tumor cohort support the hypothesis that CIC acts as a tumor suppressor in these tumors, whereas FUBP1 might play only a minor role.
A key challenge in the data analysis of biological high-throughput experiments is to handle the often low number of samples in the experiments compared to the number of biomolecules that are simultaneously measured. Combining experimental data using independent technologies to illuminate the same biological trends, as well as complementing each other in a larger perspective, is one natural way to overcome this challenge. In this work we investigated if integrating proteomics and transcriptomics data from a brain cancer animal model using gene set based analysis methodology, could enhance the biological interpretation of the data relative to more traditional analysis of the two datasets individually. The brain cancer model used is based on serial passaging of transplanted human brain tumor material (glioblastoma--GBM) through several generations in rats. These serial transplantations lead over time to genotypic and phenotypic changes in the tumors and represent a medically relevant model with a rare access to samples and where consequent analyses of individual datasets have revealed relatively few significant findings on their own. We found that the integrated analysis both performed better in terms of significance measure of its findings compared to individual analyses, as well as providing independent verification of the individual results. Thus a better context for overall biological interpretation of the data can be achieved.
There is at present a controversy in the literature whether MSCs are susceptible to spontaneous in vitro transformation or not. Several groups have reported spontaneous transformation of MSCs from various species. However, some of these reports were not true transformations and later proven to be due to cross-contaminating cancer cells. To date there is no solid evidence that MSCs can undergo spontaneous transformation in culture. Only two groups used DNA fingerprinting to authenticate their transformed cells, and both groups later showed cross-contamination of cancer cells in their cultures. In this commentary, we address the paper "Spontaneous transformation of adult mesenchymal stem cells from cynomolgus macaques in vitro" by Z. Ren et al. Exp. Cell Res. 317 (2011) 2950-2957. In this article the authors characterize the transformed mesenchymal cells (TMCs) and claim to have verified their origin. We question the authentication of the TMCs made by the authors and we also believe it is in the interest of the scientific community, that a highly controversial finding, such as spontaneous transformation of MSCs, should be properly verified by stringent methods, preferably DNA fingerprinting, in order to validate if an actual transformation event has occurred.
Glioblastoma (GBM) is a highly aggressive brain tumour, where patients respond poorly to radiotherapy and exhibit dismal survival outcomes. The mechanisms of radioresistance are not completely understood. However, cancer cells with an immature stem-like phenotype are hypothesised to play a role in radioresistance. Since the progenitor marker neuron-glial-2 (NG2) has been shown to regulate several aspects of GBM progression in experimental systems, we hypothesised that its expression would influence the survival of GBM patients. Quantification of NG2 expression in 74 GBM biopsies from newly diagnosed and untreated patients revealed that 50% express high NG2 levels on tumour cells and associated vessels, being associated with significantly shorter survival. This effect was independent of age at diagnosis, treatment received and hypermethylation of the O(6)-methylguanine methyltransferase (MGMT) DNA repair gene promoter. NG2 was frequently co-expressed with nestin and vimentin but rarely with CD133 and the NG2 positive tumour cells harboured genetic aberrations typical for GBM. 2D proteomics of 11 randomly selected biopsies revealed upregulation of an antioxidant, peroxiredoxin-1 (PRDX-1), in the shortest surviving patients. Expression of PRDX-1 was associated with significantly reduced products of oxidative stress. Furthermore, NG2 expressing GBM cells showed resistance to ionising radiation (IR), rapidly recognised DNA damage and effectuated cell cycle checkpoint signalling. PRDX-1 knockdown transiently slowed tumour growth rates and sensitised them to IR in vivo. Our data establish NG2 as an important prognostic factor for GBM patient survival, by mediating resistance to radiotherapy through induction of ROS scavenging enzymes and preferential DNA damage signalling.
The cell fate determinant NUMB orchestrates asymmetric cell division in flies and mammals and has lately been suggested to have a tumor suppressor function in breast and lung cancer. Here, we studied NUMB in the context of malignant gliomas. We used ectopic expression of NUMB in order to inhibit proliferation and induce differentiation in glioma cells by alteration of Notch, Hedgehog and p53 signaling. We found that NUMB is consistently expressed in glioma biopsies with predominance of NUMB2/4 isoforms as determined by isoform-specific real-time PCR and Western blotting. Upon lentiviral overexpression, in vitro proliferation rate and the grade of differentiation as assessed by morphology and expression of neural and glial markers remained unchanged. Orthotopic xenografts of NUMB-transduced human U87 glioma cells could be established in nude rats without impairing engraftment or causing significant changes in morphology based on magnetic resonance imaging (MRI). The previously reported alteration of Hedgehog and p53 signaling by NUMB could not be recapitulated in glioma cells. We thus show that in experimental gliomas, NUMB overexpression most likely does not exert a tumor suppressor function such as seen in epithelial cancers.
Bevacizumab, an antibody against vascular endothelial growth factor (VEGF), is a promising, yet controversial, drug in human glioblastoma treatment (GBM). Its effects on tumor burden, recurrence, and vascular physiology are unclear. We therefore determined the tumor response to bevacizumab at the phenotypic, physiological, and molecular level in a clinically relevant intracranial GBM xenograft model derived from patient tumor spheroids. Using anatomical and physiological magnetic resonance imaging (MRI), we show that bevacizumab causes a strong decrease in contrast enhancement while having only a marginal effect on tumor growth. Interestingly, dynamic contrast-enhanced MRI revealed a significant reduction of the vascular supply, as evidenced by a decrease in intratumoral blood flow and volume and, at the morphological level, by a strong reduction of large- and medium-sized blood vessels. Electron microscopy revealed fewer mitochondria in the treated tumor cells. Importantly, this was accompanied by a 68% increase in infiltrating tumor cells in the brain parenchyma. At the molecular level we observed an increase in lactate and alanine metabolites, together with an induction of hypoxia-inducible factor 1? and an activation of the phosphatidyl-inositol-3-kinase pathway. These data strongly suggest that vascular remodeling induced by anti-VEGF treatment leads to a more hypoxic tumor microenvironment. This favors a metabolic change in the tumor cells toward glycolysis, which leads to enhanced tumor cell invasion into the normal brain. The present work underlines the need to combine anti-angiogenic treatment in GBMs with drugs targeting specific signaling or metabolic pathways linked to the glycolytic phenotype.
Aberrant expression of the progenitor marker Neuron-glia 2 (NG2/CSPG4) or melanoma proteoglycan on cancer cells and angiogenic vasculature is associated with an aggressive disease course in several malignancies including glioblastoma multiforme (GBM) and melanoma. Thus, we investigated the mechanism of NG2 mediated malignant progression and its potential as a therapeutic target in clinically relevant GBM and melanoma animal models. Xenografting NG2 overexpressing GBM cell lines resulted in increased growth rate, angiogenesis and vascular permeability compared to control, NG2 negative tumours. The effect of abrogating NG2 function was investigated after intracerebral delivery of lentivirally encoded shRNAs targeting NG2 in patient GBM xenografts as well as in established subcutaneous A375 melanoma tumours. NG2 knockdown reduced melanoma proliferation and increased apoptosis and necrosis. Targeting NG2 in two heterogeneous GBM xenografts significantly reduced tumour growth and oedema levels, angiogenesis and normalised vascular function. Vascular normalisation resulted in increased tumour invasion and decreased apoptosis and necrosis. We conclude that NG2 promotes tumour progression by multiple mechanisms and represents an amenable target for cancer molecular therapy.
The somatic IDH1(R132) mutation in the isocitrate dehydrogenase 1 gene occurs in high frequency in glioma and in lower frequency in acute myeloid leukemia and thyroid cancer but not in other types of cancer. The mutation causes reduced NADPH production capacity in glioblastoma by 40% and is associated with prolonged patient survival. NADPH is a major reducing compound in cells that is essential for detoxification and may be involved in resistance of glioblastoma to treatment. IDH has never been considered important in NADPH production. Therefore, the authors investigated NADPH-producing dehydrogenases using in silico analysis of human cancer gene expression microarray data sets and metabolic mapping of human and rodent tissues to determine the role of IDH in total NADPH production. Expression of most NADPH-producing dehydrogenase genes was not elevated in 34 cancer data sets except for IDH1 in glioma and thyroid cancer, indicating an association with the IDH1 mutation. IDH activity was the main provider of NADPH in human normal brain and glioblastoma, but its role was modest in NADPH production in rodent brain and other tissues. It is concluded that rodents are a poor model to study consequences of the IDH1(R132) mutation in glioblastoma.
The "Side Population" (SP) discrimination assay is a flow cytometry method used to detect stem cells based on the dye efflux properties of ABC transporters. We discuss the SP assay and its applications in stem cell biology, with an emphasis on the technical challenges related to sample preparation, data acquisition, analysis, and interpretation. We highlight the value of multicolor phenotyping, the impact of DNA ploidy, and the importance of distinguishing graft versus host cells for an appropriate SP discrimination. To improve the consistency and reliability of data between laboratories, we propose a set of recommendations for SP assay data reporting.
We have previously established two distinct glioma phenotypes by serial xenotransplantation of human glioblastoma (GBM) biopsies in nude rats. These tumors undergo a gradual transition from a highly invasive nonangiogenic to a less-invasive angiogenic phenotype. In a protein screen to identify molecular markers associated with the infiltrative phenotype, we identified ?-basic-crystallin (?Bc), a small heat-shock protein with cytoprotective properties. Its increased expression in the infiltrative phenotype was validated by immunohistochemistry and Western blots, confirming its identity to be tumor-derived and not from the host. Stereotactic human GBM biopsies taken from MRI-defined areas verified stronger ?Bc expression in the infiltrative edge compared to the tumor core. Cell migration assays and immunofluorescence staining showed ?Bc to be expressed by migrating cells in vitro. To determine ?Bc function, we altered its expression levels. ?Bc siRNA depletion caused a loss of migrating tumor cells from biopsy spheroids and delayed monolayer wound closure. In contrast, glioma cell migration in a Boyden chamber assay was unaffected by either ?Bc knockdown or overexpression, indicating that ?Bc is not functionally linked to the cell migration machinery. However, after siRNA ?Bc depletion, a significant sensitization of cells to various apoptotic inducers was observed (actinomycin, tumor necrosis factor ?, and TNF-related apoptosis-inducing ligand [TRAIL]). In conclusion, ?Bc is overexpressed by highly migratory glioma cells where it plays a functional role in apoptosis resistance.
The promyelocytic leukemia (PML) protein participates in a number of cellular processes, including transcription regulation, apoptosis, differentiation, virus defense and genome maintenance. This protein is structurally organized into a tripartite motif (TRIM) at its N-terminus, a nuclear localization signal (NLS) at its central region and a C-terminus that varies between alternatively spliced isoforms. Most PML splice variants target the nucleus where they define sub-nuclear compartments termed PML nuclear bodies (PML NBs). However, PML variants that lack the NLS are also expressed, suggesting the existence of PML isoforms with cytoplasmic functions. In the present study we expressed PML isoforms with a mutated NLS in U2OS cells to identify potential cytoplasmic compartments targeted by this protein.
The development of novel therapeutic strategies for Alzheimers disease (AD) represents one of the biggest unmet medical needs today. Application of neurotrophic factors able to modulate neuronal survival and synaptic connectivity is a promising therapeutic approach for AD. We aimed to determine whether the loco-regional delivery of ciliary neurotrophic factor (CNTF) could prevent amyloid-beta (Abeta) oligomer-induced synaptic damages and associated cognitive impairments that typify AD. To ensure long-term administration of CNTF in the brain, we used recombinant cells secreting CNTF encapsulated in alginate polymers. The implantation of these bioreactors in the brain of Abeta oligomer-infused mice led to a continuous secretion of recombinant CNTF and was associated with the robust improvement of cognitive performances. Most importantly, CNTF led to full recovery of cognitive functions associated with the stabilization of synaptic protein levels in the Tg2576 AD mouse model. In vitro as well as in vivo, CNTF activated a Janus kinase/signal transducer and activator of transcription-mediated survival pathway that prevented synaptic and neuronal degeneration. These preclinical studies suggest that CNTF and/or CNTF receptor-associated pathways may have AD-modifying activity through protection against progressive Abeta-related memory deficits. Our data also encourage additional exploration of ex vivo gene transfer for the prevention and/or treatment of AD.
Although CD133 has been proposed as a marker for brain tumor-initiating cells, studies show that a tumorigenic potential exists among CD133(-) glioma cells as well. However, it is not established whether the ability of CD133(-) cells to form tumors is a property confined to a small subpopulation, rather than a common trait associated with most glioma cell types. Thus, we used lentiviral vectors expressing green fluorescent protein under lineage-specific promoters to identify CD133(-) glioma cells expressing Nestin, glial fibrillary acidic protein (GFAP), and neuron-specific enolase (NSE). Flow cytometry analysis showed the presence of CD133(-) subpopulations expressing these markers in glioma cell lines and in primary cultures from human glioblastoma (GBM) biopsies. Moreover, analysis of cell cycle distribution showed that subgroups expressing Nestin, GFAP, and NSE uniformly contained actively cycling cells, when cultured in serum-containing medium and stem cell medium. These subpopulations were fluorescence-activated cell sorted from CD133(-) U373 glioma cells and implanted intracerebrally in severe combined immunodeficient mice. Moreover, we implanted Nestin-, GFAP-, and NSE-positive glioma cells sorted from a human GBM biopsy, following removal of CD133-positive cells. All the CD133(-) subpopulations produced tumors, with no significant differences in survival or tumor take rates. However, there was a trend toward lower take rates for CD133(-) glioma subpopulations expressing GFAP and NSE. These findings suggest that the ability to form tumors may be a general trait associated with different glioma cell phenotypes, rather than a property limited to an exclusive subpopulation of glioma stem cells.
The interstitium, situated between the blood and lymph vessels and the cells, consists of a solid or matrix phase and a fluid phase, together constituting the tissue microenvironment. Here we focus on the interstitial fluid phase of tumors, i.e., the fluid bathing the tumor and stromal cells. Novel knowledge on this compartment may provide important insight into how tumors develop and how they respond to therapy.
To investigate the influence of serum-free medium (SFM) supplemented with epidermal growth factor and basic fibroblast growth factor compared with conventional serum-containing medium (SCM) on the phenotype of organotypic primary spheroids from seven gliomas.
Gliomas are highly invasive neuroepithelial tumors with a propensity of malignant transformation and very restricted treatment options. The neural cell adhesion molecule (NCAM) modulates cellular migration, proliferation, and synaptic plasticity by homophilic and heterophilic interactions. Hereby, we investigated its relevance as a glioma tissue marker for the biological aggressiveness of these tumors and compared these features with the carcinoma brain metastasis invasion zone.
The angiogenesis inhibitor histidine-rich glycoprotein (HRG) constitutes one of several examples of molecules regulating both angiogenesis and hemostasis. The antiangiogenic properties of HRG are mediated via its proteolytically released histidine- and proline-rich (His/Pro-rich) domain. Using a combination of immunohistochemistry and mass spectrometry, we here provide biochemical evidence for the presence of a proteolytic peptide, corresponding to the antiangiogenic domain of HRG, in vivo in human tissue. This finding supports a role for HRG as an endogenous regulator of angiogenesis. Interestingly, the His/Pro-rich peptide bound to the vessel wall in tissue from cancer patients but not to the vasculature in tissue from healthy persons. Moreover, the His/Pro-rich peptide was found in close association with platelets. Relesate from in vitro-activated platelets promoted binding of the His/Pro-rich domain of HRG to endothelial cells, an effect mediated by Zn(2+). Previous studies have shown that zinc-dependent binding of the His/Pro-rich domain of HRG to heparan sulfate on endothelial cells is required for inhibition of angiogenesis. We describe a novel mechanism to increase the local concentration and activity of an angiogenesis inhibitor, which may reflect a host response to counteract angiogenesis during pathologic conditions. Our finding that tumor angiogenesis is elevated in HRG-deficient mice supports this conclusion.
Most cancers contain tumor cells that display stem cell-like characteristics. How and when such cells appear in tumors are not clear, but may involve both stochastic as well as hierarchical events. Most likely, tumor cells that display stem cell-like characteristics can undergo asymmetric cell division giving rise to tumor cells that trigger angiogenic programs. As normal stem cells the cancer stem-like cells seem to adapt to hypoxic environments and will use metabolic pathways that involve increased conversion of glucose to pyruvate and lactate, and a concomitant decrease in mitochondrial metabolism and mitochondrial mass. The molecular pathways responsible for inducing glycolysis are now being explored. These pathways seem to mediate multiple metabolic functions in cancer stem-like cells, leading to a highly migratory and angiogenesis-independent phenotype. Future challenges will be to identify and validate molecular targets involved in anaerobic metabolic pathways active in cancer stem-like cells and to determine how these pathways differ from regulatory pathways involved in normal stem cell function.
Malignant gliomas (glioblastoma multiforme) have a poor prognosis with an average patient survival under current treatment regimens ranging between 12 and 14 months. The tumors are characterized by rapid cell growth, extensive neovascularization, and diffuse cellular infiltration of normal brain structures. We have developed a human glioblastoma xenograft model in nude rats that is characterized by a highly infiltrative non-angiogenic phenotype. Upon serial transplantation this phenotype will develop into a highly angiogenic tumor. Thus, we have developed an animal model where we are able to establish two characteristic tumor phenotypes that define human glioblastoma (i.e. diffuse infiltration and high neovascularization). Here we aimed at identifying potential biomarkers expressed by the non-angiogenic and the angiogenic phenotypes and elucidating the molecular pathways involved in the switch from invasive to angiogenic growth. Focusing on membrane-associated proteins, we profiled protein expression during the progression from an invasive to an angiogenic phenotype by analyzing serially transplanted glioma xenografts in rats. Applying isobaric peptide tagging chemistry (iTRAQ) combined with two-dimensional LC and MALDI-TOF/TOF mass spectrometry, we were able to identify several thousand proteins in membrane-enriched fractions of which 1460 were extracted as quantifiable proteins (isoform- and species-specific and present in more than one sample). Known and novel candidate proteins were identified that characterize the switch from a non-angiogenic to a highly angiogenic phenotype. The robustness of the data was corroborated by extensive bioinformatics analysis and by validation of selected proteins on tissue microarrays from xenograft and clinical gliomas. The data point to enhanced intercellular cross-talk and metabolic activity adopted by tumor cells in the angiogenic compared with the non-angiogenic phenotype. In conclusion, we describe molecular profiles that reflect the change from an invasive to an angiogenic brain tumor phenotype. The identified proteins could be further exploited as biomarkers or therapeutic targets for malignant gliomas.
Establishing clinically relevant animal models of glioblastoma multiforme (GBM) remains a challenge, and many commonly used cell line-based models do not recapitulate the invasive growth patterns of patient GBMs. Previously, we have reported the formation of highly invasive tumour xenografts in nude rats from human GBMs. However, implementing tumour models based on primary tissue requires that these models can be sufficiently standardised with consistently high take rates.
Human mesenchymal stem cells (hMSC) aid in tissue maintenance and repair by differentiating into specialized cell types. Due to this ability, hMSC are currently being evaluated for cell-based therapies of tissue injury and degenerative diseases. However, extensive expansion ex vivo is a prerequisite to obtain the cell numbers required for human cell-based therapy protocols. Recent studies indicate that hMSC may contribute to cancer development and progression either by acting as cancer-initiating cells or through interactions with stromal elements. If spontaneous transformation ex vivo occurs, this may jeopardize the use of hMSC as therapeutic tools. Whereas murine MSC readily undergo spontaneous transformation, there are conflicting reports about spontaneous transformation of hMSC. We have addressed this controversy in a two-center study by growing bone marrow-derived hMSC in long-term cultures (5-106 weeks). We report for the first time spontaneous malignant transformation to occur in 45.8% (11 of 24) of these cultures. In comparison with hMSC, the transformed mesenchymal cells (TMC) showed a significantly increased proliferation rate and altered morphology and phenotype. In contrast to hMSC, TMC grew well in soft agar assays and were unable to undergo complete differentiation. Importantly, TMC were highly tumorigenic, causing multiple fast-growing lung deposits when injected into immunodeficient mice. We conclude that spontaneous malignant transformation may represent a biohazard in long-term ex vivo expansion of hMSC. On the other hand, this spontaneous transformation process may represent a unique model for studying molecular pathways initiating malignant transformation of hMSC.
Nucleoporins and the promyelocytic leukemia protein (PML) represent structural entities of nuclear pore complexes and PML nuclear bodies, respectively. In addition, these proteins might function in a common biological mechanism, because at least two different nucleoporins, Nup98 and Nup214, as well as PML, can become aberrantly expressed as oncogenic fusion proteins in acute myeloid leukemia (AML) cells. Here we show that PML and nucleoporins become directed to common cytoplasmic compartments during the mitosis-to-G1 transition of the cell cycle. These protein assemblies, which we have termed CyPNs (cytoplasmic assemblies of PML and nucleoporins), move on the microtubular network and become stably connected to the nuclear membrane once contact with the nucleus has been made. The ability of PML to target CyPNs depends on its nuclear localization signal, and loss of PML causes an increase in cytoplasmic-bound versus nuclear-membrane-bound nucleoporins. CyPNs are also targeted by the acute promyelocytic leukemia (APL) fusion protein PML-RARalpha and can be readily detected within the APL cell line NB4. These results provide insight into a dynamic pool of cytoplasmic nucleoporins that form a complex with the tumor suppressor protein PML during the G1 phase of the cell cycle.
Glioblastoma multiforme (GBM) has a very poor prognosis and novel treatment strategies are urgently needed. GBM appears to be an optimal target for anti-angiogenic therapy as the tumour shows a high degree of endothelial cell proliferation and pro-angiogenic growth factor expression.
Glioblastoma is the most frequent and most malignant primary brain tumor with a poor prognosis. The translation of therapeutic strategies for glioblastoma from the experimental phase into the clinic has been limited by insufficient animal models, which lack important features of human tumors. Lentiviral gene therapy is an attractive therapeutic option for human glioblastoma, which we validated in a clinically relevant animal model.
Glioblastoma multiforme (GBM; World Health Organization astrocytoma grade IV) is the most frequent and most malignant primary brain tumor in adults. Despite multimodal therapy, all such tumors practically recur during the course of therapy, causing a median survival of only 14.6 months in patients with newly diagnosed GBM. The present study was aimed at examining the expression of the DNA repair protein AlkB homolog 2 (ALKBH2) in human GBM and determining whether it could promote resistance to temozolomide chemotherapy.
Tumor-associated angiogenesis is one of the essential hallmarks underlying cancer development and metastasis. Anti-angiogenic agents accordingly aim to restrain cancer progression by blocking the formation of new vessels, improving the delivery of chemotherapeutic agents to the tumor site and reducing the shedding of metastatic cells into the circulation. This review article addresses some key issues in the use of angiogenesis inhibitors in cancer.
Arsenic in the form of arsenic trioxide (ATO) is used as a therapeutic drug for treatment of acute promyelocytic leukemia (APL). The mechanism by which this agent cures this disease was previously shown to involve direct interactions between ATO and the promyelocytic leukemia protein (PML), as well as accelerated degradation of the APL-associated fusion oncoprotein PML/retinoic acid receptor ? (RARA). Here we investigated the fate of PML-generated nuclear structures called PML bodies in ATO-treated cells. We found that ATO inhibits formation of progeny PML bodies while it stabilizes cytoplasmic precursor compartments, referred to as cytoplasmic assemblies of PML and nucleoporins (CyPNs), after cell division. This block in PML body recycling is readily detected at pharmacologic relevant ATO concentrations (0.02-0.5?M) that do not cause detectable cell-cycle defects, and it does not require modification of PML by SUMOylation. In addition, PML and PML/RARA carrying mutations previously identified in ATO-resistant APL patients are impeded in their ability to become sequestered within CyPNs. Thus, ATO may inhibit nuclear activities of PML and PML/RARA in postmitotic cells through CyPN-dependent cytoplasmic sequestration.
Animal modeling for primary brain tumors has undergone constant development over the last 60 years, and significant improvements have been made recently with the establishment of highly invasive glioblastoma models. In this review we discuss the advantages and pitfalls of model development, focusing on chemically induced models, various xenogeneic grafts of human cell lines, including stem cell-like cell lines and biopsy spheroids. We then discuss the development of numerous genetically engineered models available to study mechanisms of tumor initiation and progression. At present it is clear that none of the current animal models fully reflects human gliomas. Yet, the various model systems have provided important insight into specific mechanisms of tumor development. In particular, it is anticipated that a combined comprehensive knowledge of the various models currently available will provide important new knowledge on target identification and the validation and development of new therapeutic strategies.
Tumor-associated stroma is typified by a persistent, non-resolving inflammatory response that enhances tumor angiogenesis, growth and metastasis. Inflammation in tumors is instigated by heterotypic interactions between malignant tumor cells, vascular endothelium, fibroblasts, immune and inflammatory cells. We found that tumor-associated adipocytes also contribute to inflammation. We have analyzed peritumoral adipose tissue in a syngeneic mouse melanoma model. Compared to control adipose tissue, adipose tissue juxtaposed to implanted tumors exhibited reduced adipocyte size, extensive fibrosis, increased angiogenesis and a dense macrophage infiltrate. A mouse cytokine protein array revealed up-regulation of inflammatory mediators including IL-6, CXCL1, MCP-1, MIP-2 and TIMP-1 in peritumoral versus counterpart adipose tissues. CD11b(+) macrophages contributed strongly to the inflammatory activity. These macrophages were isolated from peritumoral adipose tissue and found to over-express ARG1, NOS2, CD301, CD163, MCP-1 and VEGF, which are indicative of both M1 and M2 polarization. Tumors implanted at a site distant from subcutaneous, anterior adipose tissue were strongly growth-delayed, had fewer blood vessels and were less populated by CD11b(+) macrophages. In contrast to normal adipose tissue, micro-dissected peritumoral adipose tissue explants launched numerous vascular sprouts when cultured in an ex vivo model. Thus, inflamed tumor-associated adipose tissue fuels the growth of malignant cells by acting as a proximate source for vascular endothelium and activated pro-inflammatory cells, in particular macrophages.
Glioblastoma multiforme (GBM; WHO astrocytoma grade IV) is considered incurable owing to its inherently profound resistance towards current standards of therapy. Considerable effort is being devoted to identifying the molecular basis of temozolomide resistance in GBMs and exploring novel therapeutic regimens that may improve overall survival. Several independent DNA repair mechanisms that normally safeguard genome integrity can facilitate drug resistance and cancer cell survival by removing chemotherapy-induced DNA adducts. Furthermore, subpopulations of cancer stem-like cells have been implicated in the treatment resistance of several malignancies including GBMs. Thus, a growing number of molecular mechanisms contributing to temozolomide resistance are being uncovered in preclinical studies and, consequently, we are being presented with a broad range of potentially novel targets for therapy. A substantial future challenge is to successfully exploit the increasing molecular knowledge contributing to temozolomide resistance in robust clinical trials and to ultimately improve overall survival for GBM patients.
Glioblastoma multiforme (GBM) is the most aggressive type of malignant primary brain tumors in adults. Molecular and genetic analysis has advanced our understanding of glioma biology, however mapping the cellular composition of the tumor microenvironment is crucial for understanding the pathology of this dreaded brain cancer. In this study we identified major cell populations attracted by glioma using orthotopic rodent models of human glioma xenografts. Marker-specific, anatomical and morphological analyses revealed a robust influx of host cells into the main tumor bed and tumor satellites.
Mesenchymal (multipotent) stem/stromal cells (MSCs) may affect cancer progression through a number of secreted factors triggering activation of various cell signaling pathways. Depending on receptor status, phosphatase and tensin homolog (PTEN) status, or Wnt activation in the cancer cells, the signals may either result in increased growth and metastasis or lead to inhibition of growth with increased cell death. Thus, MSCs can play a dual role in cancer progression depending on the cellular context wherein they reside. The phosphatidylinositol-3-kinase (PI3K)/Akt signaling pathway has a central role in regulating tumor growth, and several MSC secreted factors stimulate activation of this pathway. A comprehensive understanding of the signals regulating MSC-tumor cross-talk is highly important for the development of MSCs as potential therapeutic vehicles. Thus, the presented review focuses on factors released by MSCs and on the dual role they may have on various stages of tumorigenesis.
Cysteine cathepsins play an important role in shaping the highly infiltrative growth pattern of human gliomas. We have previously demonstrated that the activity of cysteine cathepsins is elevated in invasive glioblastoma (GBM) cells in vitro, in part due to attenuation of their endogenous inhibitors, the cystatins. To investigate this relationship in vivo, we established U87-MG xenografts in non-obese diabetic (NOD)/severe combined immunodeficiency (SCID)-enhanced green fluorescent protein (eGFP) mice. Here, tumor growth correlated with an elevated enzymatic activity of CatB both in the tumor core and at the periphery, whereas CatS and CatL levels were higher at the xenograft edge compared to the core. Reversely, StefB expression was detected in the tumor core, but it was generally absent in the tumor periphery, suggesting that down-regulation of this inhibitor correlates with in vivo invasion. In human GBM samples, all cathepsins were elevated at the tumor periphery compared to brain parenchyma. CatB was also typically associated with angiogenic endothelia and necrotic areas. StefB was mainly detected in the tumor core, whereas CysC and StefA were evenly distributed, reflecting the observations in the xenografts. However, at the mRNA level, no differences in cathepsins and cystatins were observed between the tumor center and the periphery in both human biopsies and xenografts. Interestingly, in human tumors, cathepsin and stefin transcript levels correlated with CD68 and CXCR4 levels, but not with epidermal growth factor receptor (EGFR). Moreover, we reveal for the first time that an elevated StefA mRNA level is a highly significant prognostic factor for patient survival.
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