TGF-β1 mediated Smad1/Smad5 signalling modulates transdifferentiation of hepatic stellate cells

Abstract

Aims: The main cellular source of extracellular matrix (ECM) in the fibrotic liver are myofibroblast like cells (MFB). MFB originate from activated hepatic stellate cells (HSC) through a process called „transdifferentiation“. Both transdifferentiation and the fibrotic response of activated HSC are regulated by TGF-β1/ALK5/Smad3 signalling. In addition an involvement of TGF-β1/ALK1/Smad1 in these processes has been postulated [1]. Methods and Results: During transdifferentiation of primary HSC into MFB, Smad1/Smad5 proteins are highly phosphorylated at their C-termini. Inhibition of Smad1/Smad5-signalling using a specific compound (Compound C; [2]) causes complete block of transdifferentiation. Smad1/Smad5 are activated by TGF-β1-mediated phosphorylation in a time- and concentration-dependent manner in quiescent and activated HSC – and similar to Smad3– not in MFB. In contrast to other cell types this activation is independent of ALK5. In addition, we found a strong activation of the „classical“ BMP-target gene, i.e. Runx2, by TGF-β1 in HSC. In line with these findings, Runx2 has been shown previously to regulate TIMP-1 expression in HSC [3]. Conclusions: We show here for the first time that Smad1/Smad5-regulated signalling is critical for activation/transdifferentiation of HSC. This may be at least in part mediated by TGF-β1 in a fashion independent of ALK5. Moreover, we conclude that TGF-β1/Smad1/Smad5 regulate ECM homeostasis through induction of Runx2.