Expression of smooth muscle cell markers in activated hepatic stellate cells is connected to MAP kinase regulated transcription levels of serum response factor (SRF)

Abstract

Activation of hepatic stellate cells (HSC) is a major event in liver fibrogenesis. A typical feature of activated HSC is the expression of α-smooth muscle actin (SMA) and other smooth muscle cell (SMC) marker genes. The transcription of these genes is mainly regulated by the transcription factor ‘serum response factor’ (SRF) [1]. Less is known about transcriptional control of the SRF gene itself. We could recently demonstrate that TGF-β upregulates SRF in activated HSC [2]. Primary rat HSC or CFSC were treated with specific inhibitors against the ALK5 receptor and members of the MAPK pathways (ERK, JNK, p38). The rate of SRF expression was determined on RNA (semi-quantitative PCR) and protein (Western blot) levels. Additionally, expression of SRF target genes SMA, SM22α, or c-fos was analysed in parallel and by use of respective luciferase reporter gene constructs. Expression of SRF was not only inducible by TGF-β but also and more effective by PDGF-BB, while treatment of cells with dexamethasone resulted in a strong decrease of SRF expression. We found no relevant effect on SRF protein levels by use of the ALK5 inhibitor. In contrast, inhibition of MAPK pathways influenced SRF and SMC marker expression. Administration of TAK1 or p38 inhibitors led to increased SRF contents in TGF-β-treated cells, while JNK inhibition strongly decreased SRF in these cells. Interestingly, we found no relevant change of SRF expression after PDGF induction by additional treatment with these inhibitors. In parallel, respective effects of MAPK inhibitors onto expression of SRF target genes were observed. A number of publications described stimulatory or inhibitory effects of members of the MAPK family onto SMA expression as a classical indicator of HSC activation [3,4]. Our results link the transcription factor SRF to these observations and give a mechanical explanation for TGF-β-regulated but Smad-independent regulation of genes during HSC transdifferentiation.