HIF-1 is a transcription factor that mediates the cellular responses to low oxygen environments, mainly as a result of having an oxygen-labile subunit, HIF-1?. HIF-1? has been carefully studied in the context of severe hypoxic stresses (<1% O2), but it is also known to be present at oxygen tensions commonly found in normal tissues in vivo (?1-13% O2), albeit at much lower levels. Its role under these physiological conditions is not fully understood. Here, we show that a transcriptionally active HIF-1? was up-regulated at 5% O2, both in normal and cancer cells, but only some of its target genes were elevated as a result. HIF-1? induction was in part dependent on the activation of the ERK1/2 MAPK signalling pathway, which we have previously shown is active at 5% O2. We also found that HIF-1? does not contribute to the protection against DNA damage that can be observed in low oxygen environments, and that there are certain DNA damaging agents, such as doxorubicin and actinomycin D, that prevent HIF-1? induction independently of p53. Moreover, absence of HIF-1? significantly reduced the growth advantage of cells cultured at 5% O2. In view of these data, we conclude that HIF-1? can be induced and activated at physiological oxygen tensions in a MAPK-dependent manner and that, although this does not lead to pro-survival responses to stress, it determines the increased cell proliferation rates that are common under these conditions.
There is now a plethora of new precision medicines for B-cell malignancy including classical kinase inhibitors, rationally designed inhibitors of anti-apoptotic proteins and antibody or antibody drug/toxin conjugates with functional properties. Some are showing spectacular single agent activity in early phase clinical studies and may reduce or, in combination, even obviate the need for chemotherapy. Nevertheless, significant problems remain if these medicines are to be introduced into routine clinical practice in a rational and affordable manner. Firstly, precision medicines must be carefully matched in a mechanistic fashion with specific subtypes of disease. Whilst sensitivity may be predicted by the detection of key mutations or by expression of target molecules, for therapies that depend on intact intracellular signalling pathways, functional assessment on viable primary malignant cells will be necessary using assays that faithfully mimic in vivo conditions. A second, but no less important challenge is to define mechanism-based synergistic combinations associated with minimal toxicities rather than simply adding new precision medicines to existing chemotherapeutic regimens. Finally, a closer, open, two-way interaction between academic medicine and the pharmaceutical industry will be necessary to achieve these aims. Implementing such changes would change radically how and where patients with B-cell malignancies are managed.
Oxygen availability has important effects on cell physiology. Although hyperoxic and hypoxic stresses have been well characterized, little is known about cellular functions in the oxygen levels commonly found in vivo. Here, we show that p53-dependent apoptosis in response to different DNA-damaging agents was reduced when normal and cancer cells were cultured at physiological oxygen tensions instead of the usual atmospheric levels. Different from what has been described in hypoxia, this was neither determined by decreases in p53 induction or its transactivation activity, nor by differences in the intracellular accumulation of reactive oxygen species. At these physiological oxygen levels, we found a constitutive activation of the ERK1/2 MAPK in all the models studied. Inhibition of this signaling pathway reversed the protective effect in some but not all cell lines. We conclude that a stress-independent constitutive activation of prosurvival pathways, including but probably not limited to MAPK, can protect cells in physiological oxygen tensions against genotoxic stress. Our results underscore the need of considering the impact of oxygen levels present in the tissue microenvironment when studying cell sensitivity to treatments such as chemotherapy and radiotherapy.
p21(Waf1/Cip1/Sdi1) is a cyclin-dependent kinase inhibitor that mediates cell cycle arrest. Prolonged p21 up-regulation induces a senescent phenotype in normal and cancer cells, accompanied by an increase in intracellular reactive oxygen species (ROS). However, it has been shown recently that p21 expression can also lead to cell death in certain models. The mechanisms involved in this process are not fully understood. Here, we describe an induction of apoptosis by p21 in sarcoma cell lines that is p53-independent and can be ameliorated with antioxidants. Similar levels of p21 and ROS caused senescence in the absence of significant death in other cancer cell lines, suggesting a cell-specific response. We also found that cells undergoing p21-dependent cell death had higher sensitivity to oxidants and a specific pattern of mitochondrial polarization changes. Consistent with this, apoptosis could be blocked with targeted expression of catalase in the mitochondria of these cells. We propose that the balance between cancer cell death and arrest after p21 up-regulation depends on the specific effects of p21-induced ROS on the mitochondria. This suggests that selective up-regulation of p21 in cancer cells could be a successful therapeutic intervention for sarcomas and tumors with lower resistance to mitochondrial oxidative damage, regardless of p53 status.
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