PDGF receptor-β uses Akt/mTORC1 signaling node to promote high glucose-induced renal proximal tubular cell collagen I (α2) expression

Increased expression of PDGF receptor-beta (PDGFR beta) has been shown in renal proximal tubules in mice with diabetes. The core molecular network used by high glucose to induce proximal tubular epithelial cell collagen I (alpha 2) expression is poorly understood. We hypothesized that activation of PDGFR alpha by high glucose increases collagen I (alpha 2) production via the Akt/mTORC1 signaling pathway in proximal tubular epithelial cells. Using biochemical and molecular biological techniques, we investigated this hypothesis. We show that high glucose increases activating phosphorylation of the PDGFR alpha, resulting in phosphorylation of phosphatidylinositol 3-kinase. A specific inhibitor, JNJ-10198409, and small interfering RNAs targeting PDGFR beta blocked this phosphorylation without having any effect on MEK/Erk1/2 activation. We also found that PDGFR beta regulates high glucose-induced Akt activation, its targets tuberin and PRAS40 phosphorylation, and finally, mTORC1 activation. Furthermore, inhibition of PDGFR beta suppressed high glucose-induced expression of collagen I (alpha 2) in proximal tubular cells. Importantly, expression of constitutively active Akt or mTORC1 reversed these processes. As a mechanism, we found that JNJ and PDGFR beta knockdown inhibited high glucose-stimulated Hif1 alpha expression. Furthermore, overexpression of Hif1 alpha restored expression of collagen I (alpha 2) that was inhibited by PDGFR alpha knockdown in high glucose-stimulated cells. Finally, we show increased phosphorylation of PDGFR beta and its association with Akt/mTORC1 activation, Hif1 alpha expression, and elevated collagen I (alpha 2) levels in the renal cortex of mice with diabetes. Our results identify PDGFR beta as a driver in activating Akt/mTORC1 nexus for high glucose-mediated expression of collagen I (alpha 2) in proximal tubular epithelial cells, which contributes to tubulointerstitial fibrosis in diabetic nephropathy.