Expression and characterization of basic helix-loop-helix proteins in hepatic stellate cells

The key player in liver fibrosis is the hepatic stellate cell (HSC) which e xists in normal liver as a quiescent vitamin A storing cell, however in res ponse to liver injury (viral, alcohol, iron etc) the cell undergoes a progr essive phenotypic transformation towards a proliferating, alpha smooth-musc le actin positive myofibroblast-like cell. In fibrotic liver these activate d HSCs are directly responsible for laying down excess extracellular matrix molecules. Transcriptional control of HSC activation is currently poorly u nderstood although there is growing evidence for a role for several transcr iption factors including members of the AP-I, NF-kappa B, SP-1 and bHLH fam ilies. Studies in other cell types have shown how basic helix-loop-helix (b HLH) proteins are important in the control of cell growth and differentiati on. Because of the dramatic nature of the phenotypic switch during the acti vation of the HSC, we have investigated the activity of bHLH proteins in HS Cs using the electromobility shift assay (EMSA) to study DNA binding activi ties on E-box DNA elements. Our results show that culture activation of rat HSC is associated with loss of a high mobility E-box DNA:protein interacti on and the induction of low mobility E-box DNA:protein complexes that may b e inclusive of the myogenic transcription factor MyoD.