TGFb-Stimulated MicroRNA-21 Utilizes PTEN to Orchestrate AKT/mTORC1 Signaling for Mesangial Cell Hypertrophy and Matrix Expansion

Transforming growth factor-b (TGFb) promotes glomerular hypertrophy and matrix expansion, leading to glomeruloscle-rosis. MicroRNAs are well suited to promote fibrosis because they can repress gene expression, which negatively regulate the fibrotic process. Recent cellular and animal studies have revealed enhanced expression of microRNA, miR-21, in renal cells in response to TGFb. Specific miR-21 targets downstream of TGFb receptor activation that control cell hypertrophy and matrix protein expression have not been studied. Using 39UTR-driven luciferase reporter, we identified the tumor suppressor protein PTEN as a target of TGFb-stimulated miR-21 in glomerular mesangial cells. Expression of miR-21 Sponge, which quenches endogenous miR-21 levels, reversed TGFb-induced suppression of PTEN. Additionally, miR-21 Sponge inhibited TGFb-stimulated phosphorylation of Akt kinase, resulting in attenuation of phosphorylation of its substrate GSK3b. Tuberin and PRAS40, two other Akt substrates, and endogenous inhibitors of mTORC1, regulate mesangial cell hypertrophy. Neutralization of endogenous miR-21 abrogated TGFb-stimulated phosphorylation of tuberin and PRAS40, leading to inhibition of phosphorylation of S6 kinase, mTOR and 4EBP-1. Moreover, downregulation of miR-21 significantly suppressed TGFb-induced protein synthesis and hypertrophy, which were reversed by siRNA-targeted inhibition of PTEN expression. Similarly, expression of constitutively active Akt kinase reversed the miR-21 Sponge-mediated inhibition of TGFb-induced protein synthesis and hypertrophy. Furthermore, expression of constitutively active mTORC1 prevented the miR-21 Sponge