PKC-β1 Mediates Glucose-Induced Akt Activation and TGF-β1 Upregulation in Mesangial Cells

• Published in: Journal of the American Society of Nephrology Vol. 20; no. 3; pp. 554 - 566
• Main Authors: Wu, Dongcheng, Peng, Fangfang, Zhang, Baifang, Ingram, Alistair J., Kelly, Darren J., Gilbert, Richard E., Gao, Bo, Krepinsky, Joan C.
• Format: Journal Article
• Language: English
• Published: American Society of Nephrology 01.03.2009
• Subjects: Basic Research
• ISSN: 1046-6673; 1533-3450
• DOI: 10.1681/ASN.2008040445

Accumulation of glomerular matrix is a hallmark of diabetic nephropathy. The serine/threonine kinase Akt mediates glucose-induced upregulation of collagen I in mesangial cells through transactivation of the EGF receptor (EGFR). In addition, in renal tubular cells, glucose-induced secretion of TGF-β requires phosphoinositide-3-OH kinase, suggesting a possible role for Akt in the modulation of TGF-β expression, but the mechanisms of Akt activation and its involvement in TGF-β regulation are unknown. Here, in primary mesangial cells, high glucose induced AktS473 phosphorylation, which correlates with its activation, in a protein kinase C β (PKC-β)-dependent manner. Glucose led to PKC-β1 membrane translocation and association with Akt, and PKC-β1 immunoprecipitated from glucose-treated cells phosphorylated recombinant Akt on S473. PKC is known to mediate glucose-induced TGF-β1 upregulation through the transcription factor AP-1; here, inhibitors of phosphoinositide-3-OH kinase, PKC-β and Akt, and dominant-negative Akt all prevented glucose-induced activation of AP-1 and upregulation of TGF-β1. Finally, pharmacologic and dominant negative inhibition of EGFR blocked glucose-induced activation of PKC-β1, phosphorylation of AktS473, activation of AP-1, and upregulation of TGF-β1. In vivo , the PKC-β inhibitor ruboxistaurin prevented Akt activation in the renal cortex of diabetic rats. In conclusion, PKC-β1 is an Akt S473 kinase in glucose-treated mesangial cells, and TGF-β1 transcriptional upregulation requires EGFR/PKC-β1/Akt signaling. New therapeutic approaches for diabetic nephropathy may result from targeting components of this pathway, particularly the initial EGFR transactivation.