Peroxisome proliferator-activated receptor Gamma (PPAR Gamma) modulates hepatic stellate cells activation.

Hepatic stellate cells (HSC) have been identified as the precursor cell type mainly responsible for the development of liver fibrosis. During condition of chronic liver injury as well as following prolonged culture on uncoated plastic, HSC undergo a process of activation from the quiescent "fat storing" phenotype to a highly proliferative "myofibroblast-like" phenotype. This process is characterized by a gradual loss of lipid contets, increased expression of o-smooth-muscle actin, and collagen synthesis. PPARs are members of the nuclear receptor superfamily and regulate cell cycle, adipocyte differentiation and inflammation in response to various activators. AIM of our study was to evaluate the expression of PPARs in HSC and explore whether the transcriptional activation of these receptors are involved in HSC activation and phenotype transformation. An in situ hybridization procedure using a signal amplification system have showed a low PPARy and PPARa mRNA expression in activated HSC. Western blots of equal amount of nuclear protein from freshly isolated HSC and cultured HSC showed a progressive reduction of both PPAR isoforms during plastic induced HSC transdifferentiation. After seven days in colture PPARy protein expression was not detected and transfection experiments with a reporter plasmid consisting of three copies of PPAR response element (PPRE3-tk-luc) showed a progressive reduction of PPAR transcriptional activity. PPARy activators, 15-dPGJ2 (1-5J.LM) and ciglitizone( l-IOJ.LM), in a dose dependent manner, significantly reduced PDGFand serum- induced 3H-timidine incorporation and migration of HSC. Ligands of the heterodimerization partner RXR, 9-cis retinoic acid and LG268, strongly increased the degree of inhibition caused by PPARy activators. In addition, ciglitazone inhibited collagen synthesis and a-SMA expression during HSC activation. Cotransfection with human PPARy expression vector restored the PPRE3-tk-luc expression and the increased levels of the receptor inhibited cell proliferation in activated HSC. Our results indicate that PPARlRXR trancriptional control in HSC may contribute to mantain the "resting" phenotype of HSC and activation of this signal exerts a previously unrecognized inhibition of mitogenesis and extracellular matrix production in activated HSC