Proline biosynthesis is a vent for TGFβ-induced mitochondrial redox stress.

The production and secretion of matrix proteins upon stimulation of fibroblasts by transforming growth factor-beta (TGF beta) play a critical role in wound healing. How TGF beta supports the bioenergetic cost of matrix protein synthesis is not fully understood. Here, we show that TGF beta promotes protein translation at least in part by increasing the mitochondrial oxidation of glucose and glutamine carbons to support the bioenergetic demand of translation. Surprisingly, we found that in addition to stimulating the entry of glucose and glutamine carbon into the TCA cycle, TGF beta induced the biosynthesis of proline from glutamine in a Smad4-dependent fashion. Metabolic manipulations that increased mitochondrial redox generation promoted proline biosynthesis, while reducing mitochondrial redox potential and/or ATP synthesis impaired proline biosynthesis. Thus, proline biosynthesis acts as a redox vent, preventing the TGF beta-induced increase in mitochondrial glucose and glutamine catabolism from generating damaging reactive oxygen species (ROS) when TCA cycle activity exceeds the ability of oxidative phosphorylation to convert mitochondrial redox potential into ATP. In turn, the enhanced synthesis of proline supports TGF beta-induced production of matrix proteins.