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A Novel <em>in vivo</em> Gene Transfer Technique and <em>in vitro</em> Cell Based Assays for the Study of Bone Loss in Musculoskeletal Disorders

Differentiation and activation of osteoclasts play a key role in the development of musculoskeletal diseases as these cells are primarily involved in bone ...

University of California, Davis

In order to develop a highly efficient mammalian expression vector, we constructed a vector by the combination of the murine cytomegalovirus (MCMV) immediate early (IE) promoter and the human elongation factor one alpha (EF-1 alpha) first intron. The MCMV IE promoter was several fold stronger than the human cytomegalovirus (HCMV) immediate early (IE) promoter and the human elongation factor one alpha (EF-1 alpha) promoter in various mammalian cell lines such as NIH3T3, Neuro-2a, 293T or HT1080 and was only slightly weaker than the HCMV or the EF-1 alpha promoter in HeLa and CHO cell lines. We inserted the first intron of the human EF-1 alpha gene behind the MCMV and the HCMV promoter to enhance the gene expression through increasing RNA transcription and/or RNA stability. The insertion of the human EF-1 alpha first intron markedly enhanced the level of gene expression in many cell lines and the resultant MCMV promoter with the human EF-1 alpha first intron (MCMV/EF-I) was higher than the HCMV promoter from 4.3 to 65.5-fold in various cell lines. Also, the MCMV/EF-I promoter induced higher level of erythropoietin expression than the HCMV promoter in transiently transfected CHO cells.

The human elongation factor 1 alpha (EF-1 alpha) first intron highly enhances expression of foreign genes from the murine cytomegalovirus promoter.

Nm23-H1 encodes nucleoside diphosphate kinase A (NDPK-A) and is known to have a metastasis suppressive activity in many tumor cells. However, it has many other functions as well. Recent studies have shown that the interacting proteins with Nm23-H1 which mediate the cell proliferation, may act as modulators of the metastasis suppressor activity. The interacting proteins with Nm23-H1 can be classified into 3 groups. The first group of proteins can be classified as upstream kinases of Nm23-H1 such as CKI and Aurora-A/STK15. The second group of proteins acts as downstream effectors for the regulation of specific gene transcriptions, GTP-binding protein functions, and signal transduction in Erk signal cascade. The third group of proteins can be classified as bi-directionally influencing binding partners of Nm23-H1. As a result, the interactions with Nm23-H1 and binding partners have implications in the biochemical characterization involved in metastasis and tumorigenesis.

Regulators affecting the metastasis suppressor activity of Nm23-H1.

It has been shown previously that ribosomal protein S3 (rpS3) has an endonuclease activity, which is increased by protein kinase Cdelta (PKCdelta)-dependent phosphorylation. However, the reciprocal mechanism for rpS3 dephosphorylation is not known. In this study, we examined phosphatases involved in rpS3 dephosphorylation, and we determined that rpS3 is specifically dephosphorylated by protein phosphatase 2A (PP2A). By immunoprecipitation assay, rpS3 only interacted with PP2Ac but not with protein phosphatase 1. The interaction between rpS3 and PP2Ac occurred only in the nuclear fraction. Moreover, the PP2Ac association with rpS3 was identified in cells transfected with wild-type rpS3 but not with mutant rpS3 lacking PKCdelta phosphorylation sites. PP2A inhibition using okadaic acid induced rpS3 phosphorylation. The level of phosphorylated rpS3 in cells was decreased by the overexpression of PP2Ac and was increased by the down-regulation of PP2Ac. Taken together, these results suggest that oxidative stress regulates the phosphorylation status of nonribosomal rpS3 by both activating PKCdelta and blocking the PP2A interaction with rpS3.

Phosphorylation status of nuclear ribosomal protein S3 is reciprocally regulated by protein kinase C{delta} and protein phosphatase 2A.

The human ribosomal protein S3 (rpS3), a component of the 40S small subunit in the ribosome, is a known multi-functional protein with roles in DNA repair and apoptosis. We recently found that the arginine residue(s) of rpS3 are methylated by protein arginine methyltransferase 1 (PRMT1). In this paper, we confirmed the arginine methylation of rpS3 protein both in vitro and in vivo. The sites of arginine methylation are located at amino acids 64, 65 and 67. However, mutant rpS3 (3RA), which cannot be methylated at these sites, cannot be transported into the nucleolus and subsequently incorporated into the ribosome. Our results clearly show that arginine methylation of rpS3 plays a critical role in its import into the nucleolus, as well as in small subunit assembly of the ribosome.

Arginine methylation of ribosomal protein S3 affects ribosome assembly.

Ribosomal protein S3 (RpS3) is a well-known multi-functional protein mainly involved in protein biosynthesis as a member of the small ribosomal subunit. It also plays a role in repairing various DNA damage acting as a repair UV endonuclease. Most of the rpS3 pool is located in the ribosome while the minority exists in free form in the cytoplasm. We here report an additional function of rpS3 in which it represses its own translation by binding to its cognate mRNA. Through RT-PCR of the RNAs co-immunoprecipitated with ectopically expressed rpS3, rpS3 protein was found to interact with various RNAs-endogenous rpS3, 18S rRNA. The S3-C terminal domain was shown to be the major mRNA binding domain of rpS3, independent of the KH domain. This interaction was shown to occur in cytoplasmic fractions rather than ribosomal fractions, and then is involved in its own mRNA translational inhibition by in vitro translation. Furthermore, when Flag-tagged rpS3 was transiently transfected into 293T cells, the level of endogenous rpS3 gradually decreased regardless of transcription. These results suggest that free rpS3 regulates its own translation via a feedback mechanism.

RpS3 translation is repressed by interaction with its own mRNA.

Human ribosomal protein S3 (rpS3) acts as a DNA repair endonuclease. The multiple functions of this protein are regulated by post-translational modifications including phosphorylation and methylation. Using a yeast-two hybrid screen, we identified small ubiquitin-related modifier-1 (SUMO-1) as a new interacting partner of rpS3. rpS3 interacted with SUMO-1 via the N- and C-terminal regions. We also observed sumoylation of rpS3 in Escherichia coli and mammalian cell systems. Furthermore, we discovered that one of three lysine residues, Lys18, Lys214, or Lys230, was sumoylated in rpS3. Interestingly, sumoylated rpS3 was resistant to proteolytic activity, indicating that SUMO-1 increased the stability of the rpS3 protein. We concluded that rpS3 is covalently modified by SUMO-1 and this post-translational modification regulates rpS3 function by increasing rpS3 protein stability.

Ribosomal protein S3 is stabilized by sumoylation.

Bone destruction is a common feature of inflammatory arthritis and is mediated by osteoclasts, the only specialized cells to carry out bone resorption. Aberrant expression of receptor activator of nuclear factor kappa β ligand (RANKL), an inducer of osteoclast differentiation has been linked with bone pathology and the synovial fibroblast in rheumatoid arthritis (RA). In this manuscript, we challenge the current concept that an increase in RANKL expression governs osteoclastogenesis and bone destruction in autoimmune arthritis. We isolated human fibroblasts from RA, pyrophosphate arthropathy (PPA) and osteoarthritis (OA) patients and analyzed their RANKL/OPG expression profile and the capacity of their secreted factors to induce osteoclastogenesis. We determined a 10-fold increase of RANKL mRNA and protein in fibroblasts isolated from RA relative to PPA and OA patients. Peripheral blood mononuclear cells (PBMC) from healthy volunteers were cultured in the presence of RA, PPA and OA synovial fibroblast conditioned medium. Osteoclast differentiation was assessed by expression of tartrate-resistant acid phosphatase (TRAP), vitronectin receptor (VNR), F-actin ring formation and bone resorption assays. The formation of TRAP(+), VNR(+) multinucleated cells, capable of F-actin ring formation and lacunar resorption in synovial fibroblast conditioned medium cultures occured in the presence of osteoprotegerin (OPG) a RANKL antagonist. Osteoclasts did not form in these cultures in the absence of macrophage colony stimulating factor (M-CSF). Our data suggest that the conditioned medium of pure synovial fibroblast cultures contain inflammatory mediators that can induce osteoclast formation in human PBMC independently of RANKL. Moreover inhibition of the TNF or IL-6 pathway was not sufficient to abolish osteoclastogenic signals derived from arthritic synovial fibroblasts. Collectively, our data clearly show that alternate osteoclastogenic pathways exist in inflammatory arthritis and place the synovial fibroblast as a key regulatory cell in bone and joint destruction, which is a hallmark of autoimmune arthritis.

Article	Year
The human elongation factor 1 alpha (EF-1 alpha) first intron highly enhances expression of foreign genes from the murine cytomegalovirus promoter. Journal of biotechnology\| PubMed ID: 11738725	2002
Regulators affecting the metastasis suppressor activity of Nm23-H1. Molecular and cellular biochemistry\| PubMed ID: 19377884	2009
Phosphorylation status of nuclear ribosomal protein S3 is reciprocally regulated by protein kinase C{delta} and protein phosphatase 2A. The Journal of biological chemistry\| PubMed ID: 19458393	2009
Arginine methylation of ribosomal protein S3 affects ribosome assembly. Biochemical and biophysical research communications\| PubMed ID: 19460357	2009
RpS3 translation is repressed by interaction with its own mRNA. Journal of cellular biochemistry\| PubMed ID: 20217897	2010
Ribosomal protein S3 is stabilized by sumoylation. Biochemical and biophysical research communications\| PubMed ID: 21968017	2011
Rheumatoid and pyrophosphate arthritis synovial fibroblasts induce osteoclastogenesis independently of RANKL, TNF and IL-6. Journal of autoimmunity\| PubMed ID: 22867712	2012

Hyun Seock Shin

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