Independent Regulation of Transforming Growth Factor-β1 Transcription and Translation by Glucose and Platelet-Derived Growth Factor

Proximal tubular renal epithelial cells may contribute to the pathogenesis of renal interstitial fibrosis in diabetes by generation of cytokines such as transforming growth factor (TGF)-β1. We have previously demonstrated that proximal tubular renal epithelial cell TGF-β1 synthesis may be modulated by elevated glucose concentration and by cytokines such as platelet-derived growth factor (PDGF). The aim of the current study was to characterize the mechanism by which glucose and PDGF synergistically stimulate the generation of TGF-β1. Addition of either 25 mmol/L of d-glucose or low-dose PDGF increased TGF-β1 mRNA expression without stimulation of TGF-β1 protein synthesis. In contrast sequential stimulation with 25 mmol/L of d-glucose for 48 hours followed by low-dose (25 ng/ml) PDGF led to a significant increase in TGF-β1 synthesis. Elevated glucose concentration stimulated de novo gene transcription as assessed by stimulation of a TGF-β1 promoter-luciferase construct. This led to induction of a poorly translated TGF-β1 transcript determined by polysome analysis. PDGF at low dose did not influence TGF-β1 transcription, but led to alteration in TGF-β1 mRNA stability and translation. Without a previous glucose-induced increase in the amount of TGF-β1 transcript, PDGF did not stimulate significant TGF-β1 protein synthesis. At a high dose (100 ng/ml) PDGF stimulated TGF-β1 synthesis independent of glucose concentration. This was associated with increased TGF-β1 gene transcription and alteration in TGF-β1 mRNA translational efficiency. In conclusion the data suggests that in diabetic nephropathy, the role of glucose is to lower the threshold at which a stimulus such as PDGF stimulates TGF-β1 protein synthesis. The data also suggest that independent regulation of TGF-β1 transcription and translation by glucose and PDGF account for their synergistic effect on TGF-β1 protein synthesis. We hypothesize that the role of glucose in diabetic nephropathy is to prime the kidney for an injurious response to other stimuli.