MKL1 mediates TGF-β1-induced α-smooth muscle actin expression in human renal epithelial cells

Transforming growth factor-beta 1 (TGF-beta 1) is known to induce epithelial-mesenchymal transition in the kidney, a process involved in tubulointerstitial fibrosis. We hypothesized that a coactivator of the serum response factor (SRF), megakaryoblastic leukemia factor-1 (MKL1), stimulates alpha-smooth muscle actin (alpha-SMA) transcription in primary cultures of renal tubular epithelial cells (RTC), which convert into myofibroblasts on treatment with TGF-beta 1. Herein, we study the effect of MKL1 expression on alpha-SMA in these cells. We demonstrate that TGF-beta 1 stimulation of alpha-SMA transcription is mediated through CC(A/T)(6)-rich GG elements known to bind to SRF. These elements also mediate the MKL1 effect that dramatically activates alpha-SMA transcription in serum-free media. MKL1 fused to green fluorescent protein localizes to the nucleus and induces alpha-SMA expression regardless of treatment with TGF-beta 1. Using proteasome inhibitors, we also demonstrate that the proteolytic ubiquitin pathway regulates MKL1 expression. These data indicate that MKL1 overexpression is sufficient to induce alpha-SMA expression. Inhibition of endogenous expression of MKL1 by small interfering RNA abolishes TGF-beta 1 stimulation of alpha-SMA expression. Therefore, MKL1 is also absolutely required for TGF-beta 1 stimulation of alpha-SMA expression. Western blot and immunofluorescence analysis show that overexpressed and endogenous MKL1 are located in the nucleus in non-stimulated RTC. Chromatin immunoprecipitation assay demonstrates that TGF-beta 1 induces binding of endogenous SRF and MKL1 to the alpha-SMA promoter in chromatin. Since MKL1 constitutes a potent factor regulating alpha-SMA expression, modulation of endogenous MKL1 expression or activity may have a profound effect on myofibroblast formation and function in the kidney.