Novel interactions between TGF-β1 actions and the 12/15-lipoxygenase pathway in mesangial cells

Diabetic nephropathy (DN) is characterized by mesangial cell (MC) hypertrophy and progressive accumulation of glomerular extracellular matrix (ECM). It was reported recently that 12/15-lipoxygenase (12/15-LO) expression is increased in high-glucose (HG)-stimulated MC and in experimental DN. 12-LO products could also directly induce MC hypertrophy and ECM expression and mediate growth factor effects, thus implicating the 12/15-LO pathway in DN. Because TGF-β is a major player in the pathogenesis of DN, whether there is an interplay between the TGF-β and 12/15-LO pathways in MC was evaluated. Treatment of rat MC (RMC) with TGF-β significantly increased levels of the 12/15-LO product 12(S)-hydroxyeicosatetraenoic acid [12(S)-HETE] and also 12/15-LO mRNA and protein expression. HG-induced TGF-β mRNA expression in RMC was inhibited by a specific ribozyme and siRNA targeted to knockdown rat 12/15-LO. It is interesting that direct treatment of RMC with 12(S)-HETE increased TGF-β mRNA and protein levels, as well as p-Smad2/3, which are TGF-β-specific target transcription factors. 12(S)-HETE also increased transcription from a minimal TGF-β promoter. Furthermore, TGF-β expression and p-Smad2/3 levels were lower in MC from 12/15-LO knockout mice relative to control mice. Reciprocally, mouse MC stably overexpressing 12/15-LO had greater TGF-β mRNA and also nuclear p-Smad2/3 relative to mock-transfected cells. 12/15-LO and TGF-β could functionally signal and increase ECM expression via the p38 mitogen-activated protein kinase signaling pathway. These results indicate for the first time that the 12/15-LO and TGF-β pathways can cross-talk and activate each other. These novel interactions may amplify the signal transduction cascades and molecular events that lead to DN.