Diabetic kidney disease is associated with podocyte (glomerular epithelial cell) dysfunction, which results in proteinuria and eventually glomerulosclerosis. The TRBs (TRB1,2,3) are a newly recognized family of kinase-like proteins that modulate signaling pathways, transcriptional and proteasomal events. We provide evidence that TRB3 is expressed in podocytes. Moreover, our studies demonstrate that TRB3 is upregulated in the kidneys of diabetic mice, and in this proposal our aim is to investigate the relevance and mechanisms underlying this finding. Hyperglycemia and elevated free fatty acids (FFA) induce the generation of reactive oxygen species (ROS), and ROS in turn induce podocyte injury, and amplify fibrotic and inflammatory pathways in diabetes. Our studies demonstrate that ROS upregulate TRB3 expression in podocytes. ROS also trigger the Unfolded Protein Response (UPR), an Endoplasmic Reticulum stress response that activates a series of signaling cascades designed to globally reduce transcription and translation. C/EBP Homologous Protein (CHOP/GADD153) expression is a hallmark of the UPR/ER stress response, and we have shown that CHOP expression is upregulated in diabetic kidneys. In this grant we propose that TRB3 is activated in podocytes during diabetes as part of the ER stress response, and that it functions as a brake to reduce the transcription of AP1-driven inflammatory genes. Our preliminary studies also demonstrate that free fatty acids potently upregulate podocyte TRB3 expression and we will investigate the timing and mechanisms underlying this novel observation. The overall hypothesis of this proposal is that in podocytes, TRB3 is upregulated in diabetes during the Endoplasmic Reticulum stress response, and that TRB3 functions to limit inflammation induced by ROS and to preserve podocyte function. In the following studies we will breed Akita diabetic mice with TRB3 knockout mice to generate diabetic mice that are deficient in TRB3. We propose that TRB3 is protective in diabetic nephropathy, therefore we hypothesize that diabetic mice deficient in TRB3 will have more enhanced manifestations of diabetes. We will also study whether the expression of TRB3 increases in the kidneys and podocytes of diabetic mice. In this Merit Review we also propose to study the markers and the time course of the ER stress response in diabetes. Next, we will study whether podocyte TRB3 inhibits the expression of inflammatory molecules including MCP- 1, TGF-2 and MMP-9. Additionally, we will evaluate the molecular mechanisms underlying the ability of TRB3 to alter inflammatory gene expression. In the final aim of this proposal we will investigate our novel observation that FFA induce TRB3 expression. We will evaluate whether this is related to the ability of FFA to induce the expression of ROS. This application proposes to investigate the ER stress response, which is a novel stress pathway in diabetic kidney disease. We also will elucidate the function of TRB3, a unique protein that interferes with key signaling pathways. We will investigate the relevance of TRB3 expression in diabetic kidney disease and investigate whether manipulation of TRB3 expression can alter outcomes in diabetic nephropathy. These findings are likely to lead to the development of novel therapeutic strategies in diabetic kidney disease.
