Synergistic effects of c-Jun and SP1 in the promotion of TGFβ1-mediated diabetic nephropathy progression

Diabetic nephropathy (DN) is a major complication of diabetes mellitus. Transforming growth factor beta 1 (TGFβ1) is a well-distinguished mediator of progressive renal fibrosis in DN. However, the molecular mechanisms contributing to enhanced TGFβ1 expression in the progression of DN are not fully understood. Herein, we reported that c-Jun and specificity protein 1 (SP1) were critical upstream regulators of TGFβ1 expression in DN. The increase in c-Jun and SP1 expressions was positively correlated with TGFβ1 in both high glucose-treated human renal mesangial cells (HRMCs) and diabetic kidneys. Furthermore, c-Jun dose-dependently promoted SP1-mediated TGFβ1 transcription and vice versa. The synergistic effects of c-Jun and SP1 were attributed to their auto-regulation and cross-activation. Moreover, enhanced phosphorylation levels of c-Jun and SP1 were accompanied with increased TGFβ1 expression in diabetic kidneys. Accordingly, dephosphorylation of c-Jun and SP1 by the specific c-Jun N-terminal kinase (JNK) inhibitor SP600125 prevented the increase in TGFβ1 expression. These results suggested that c-Jun and SP1 synergistically activated profibrotic TGFβ1 expression in the development of DN by auto-regulation, cross-activation and phospho-modification.