A non-autonomous role of MKL1 in the activation of hepatic stellate cells.

Although hepatic stellate cells (HSC) represent the major source of fibrogenesis in the liver under various pathological conditions, other cell types including hepatic parenchymal cells (hepatocytes) also contribute to HSC activation and hence liver fibrosis. The underlying mechanism, however, is poorly defined. Here we report that hepatocytes exposed to high concentrations of glucose (HG) emit a pro-fibrogenic cue as evidenced by the observation that primary HSCs cultured in conditioned media (CM) collected from hepatocytes exposed to HG up-regulated the production of extracellular matrix (ECM) proteins compared to CM collected from hepatocytes exposed to low glucose. We further identified the pro-fibrogenic cue from hepatocytes to be connective tissue growth factor (CTGF) because either depletion of endogenous CTGF in hepatocytes with siRNA or the addition of a CTGF-specific neutralizing antibody to the CM blunted the pro-fibrogenic effect elicit by HG treatment. Of interest, we discovered that genetic ablation or pharmaceutical inhibition of the transcriptional modulator MKL1 in hepatocytes also abrogated the HG-induced pro-fibrogenic effects. Mechanistically, MKL1 interacted with AP-1 and SMAD3 to trans-activate CTGF in hepatocytes in response to HG treatment. In conclusion, our data suggest that MKL1 contribute to HSC activation in a non-autonomous fashion by promoting CTGF transcription in hepatocytes.