Mechanical stiffness orchestrates distinct regulation of MRTFs and YAP/TAZ transcriptional cofactors in hepatocytes.

During liver fibrosis, hepatocytes respond to progressive increases in tissue stiffness by undergoing cytoskeletal reorganisation. This has important consequences such as immune cell infiltration, mainly triggered by the activation of YAP and TAZ proteins. However, other proteins that respond to mechanical modifications remain poorly characterised; in particular, the myocardin-related transcription factors A and B (MRTFs). Here, we referred to these four proteins as mechanical responsive transcriptional cofactors (MRTcoF). We analysed the regulation of MRTcoF proteins in fibrotic livers induced by carbon tetrachloride (CCl4) and observed that MRTcoF were differentially regulated during liver fibrosis, predominantly in hepatocytes. Transcriptome analysis suggested that fatty acid and glucose metabolisms, cell proliferation and signalling pathways were regulated by transcriptional enhanced associate domain (TEAD)- and serum response factor (SRF)-transcriptional programmes in CCl4-treated livers. Then, we evaluated the protein expression levels of MRTcoF in primary hepatocytes (PH) cultured on tissue culture petri dishes (TCPD). After culture, we observed upregulation of MRTcoF, and downregulation of TAZ after protein increase. Interestingly, despite the increase in MRTFB protein levels, no accumulation of MRTFB was detected in the nucleus. Culture on soft polyacrylamide hydrogels (PAA HGs) attenuated the activation of MRTcoF, having specific implications in the transcriptional regulation of target genes. In contrast, pharmacological inhibition of MRTcoF was not sufficient to halt transcriptional regulation. In summary, a progressive increase of stiffness activates MRTcoF transcriptional complexes in hepatocytes, suggesting a key role of mechano-transduction during liver fibrosis.