Regulation of inositol 1,4,5-trisphosphate receptors by transforming growth factor-beta: Implications for vascular dysfunction in diabetes

Diabetes in its early stages is associated with enhanced glomerular blood f low and systemic vasodilation. Possible consequences of enhanced glomerular blood flow are glomerular hypertrophy, increased shear stress, and subsequ ent glomerulosclerosis. The prosclerotic cytokine, transforming growth fact or-beta (TGF-beta), has been well established to play a key role in mesangi al matrix accumulation in diabetes; however, its role in regulating vascula r tone has not been studied in depth. Earlier studies have demonstrated tha t vascular smooth muscle cells and mesangial cells pretreated with TGF-beta have impaired calcium mobilization to inositol 1,4,5-trisphosphate (IPI) g enerating agonists, such as platelet-derived growth factor (PDGF) and Angio tensin II (Ang II). We postulated that this action of TGF-beta may be cause d by regulation of the key intracellular calcium channel, the inositol 1,4, 5-trisphosphate receptor (IP3R). Mesangial and smooth muscle cells primaril y contain the types I IP3R and III IP3R isoforms. Short-term exposure of me sangial cells to TGF-beta (15-60 min) leads to phosphorylation of the type I IP3R at specific serine residues. Long-term exposure of mesangial cells t o TGF-beta (24 hours) leads to downregulation of protein levels of both typ es I and III IP(3)Rs as assessed by Western blot and confocal analysis. Per meabilization of cells and exposure to IF, leads to impaired calcium mobili zation if cells are pretreated with TGF-beta. As an in vivo correlation, we found that streptozotocin-induced diabetic rats and mice have reduced rena l type I IP3R expression. By immunostaining, we found reduction of type I I P3R in glomerular cells and arteriolar smooth muscle cells of the diabetic rat kidney. Treatment of diabetic mice with a neutralizing anti-TGF-beta an tibody completely prevents diabetic glomerular hypertrophy. We conclude tha t: the vascular dysfunction of diabetes leading to glomerular hypertrophy i s mediated, in part, by TGF-beta-induced regulation of IP(3)Rs.