Mechanism of Renal Cell Injury in Diabetes

Major manifestations of diabetic nephropathy are increased enal hypertrophy involving glomeruli and tubules with expansion of matrix proteins, including fibronectin. These changes occur concomitant with increased expression of transforming growth factor-ø?(TGFø) that contributes to the pathogenesis of human and experimental diabetic nephropathy. The mechanisms by which hyperglycemia and/or TGFø ?result in hypertrophy and increased expression of fibronectin are poorly understood. We have shown recently that high glucose- as well as TGFø-induced downregulation of tumor suppressor protein PTEN (phosphatase and tensin homolog deleted on chromosome ten) contributes to renal cell hypetrophy and fibronectin expression. Our data show markedly reduced levels of PTEN in the kidney cortex and glomerulus of rats and mice with diabetes. Moreover, we demonstrate that high glucose and TGFø? increase two microRNAs, miR-21 and miR-214 in mesangial and proximal tubular epithelial cells. These microRNAs target PTEN for translational repression. In this proposal, using cultured mesangial and proximal tubular epithelial cells and renal tissues from streptozotocin-induced diabetic rats and diabetic OVE26 mice, we will test the hypothesis that transcriptional and post-transcriptional mechanisms suppress PTEN to induce hypertrophy and matrix expansion in diabetic nephropathy. Furthermore, our preliminary data demonstrate that high glucose and TGFø ?increase the levels of two transcription factors Brf1 and TBP, which cooperate with RNA polymerase III to induce 5S rRNA and tRNAs. We show an increase in the c-Myc protoonco-protein in mesangial and proximal tubular epithelial cells and in renal cortex of diabetic rats and mice. We hypothesize that the crosstalk between c- Myc and Brf1/TBP regulates diabetic renal hypertrophy and matrix protein levels. In the first specific aim, we plan to investigate the p53 tumor suppressor transcription factor as a candidate that regulates PTEN expression, hypertrophy and fibronectin expression. SIRT1 (silent information regulator 1), which deacetylates p53 to inactivate its transcriptional activity, is induced by high glucose and TGFø. We will elucidate the role of SIRT1 in cellular hypertrophy and fibronectin expression in mesangial and proximal tubular epithelial cells and in kidneys of rats and mice with diabetes. In the second aim, we will examine the role of two recently identified microRNAs, miR-21 and miR-214, in hypertrophy and fibronectin expression in response to high glucose and TGFø. In the specific aim 3, we will study the contribution of Brf1 and TBP in cooperation with c-Myc to high glucose- and TGFø-induced RNA polymerase III in regulating hypertrophy and fibronectin expression in mesangial and proximal tubular epithelial cells and in renal tissues of diabetic rats and mice. To address these specific aims, techniques including immunoprecipitation, immunoblotting, morphometry, immunohistochemistry, reporter transfection assays, chromatin immunoprecipitation assays, siRNA-mediated downregulation and conditional expression of proteins will be used. PUBLIC HEALTH RELEVANCE: Manifestation of diabetic nephropathy involves increased renal ypertrophy and matrix protein amassing, which result from hyperglycemia-induced expression of transforming growth factor-??(TGF?) that along with high glucose concentration regulates expression/activation of signaling molecules. The experiments proposed in this application will identify signaling molecules, which take part in mediating renal hypertrophy and matrix protein expression. The results obtained from these experiments will help designing drugs targeting fibrosis, which constitutes the pathology of diabetic nephropathy.