Peroxisome proliferator-activated receptors and hepatic stellate cell activation

The present study examined the roles of peroxisome proliferator-activated receptors (PPAR) in activation of hepatic stellate cells (HSC), a pivotal event in liver fibrogenesis. RNase protection assay detected mRNA for PPAR gamma1 but not that for the adipocyte-specific gamma (2) isoform in HSC isolated from sham-operated rats, whereas the transcripts for neither isoforms were detectable in HSC from cholestatic liver fibrosis induced by bile duct ligation (BDL), Semi-quantitative reverse transcriptase-polymerase chain reaction confirmed a 70% reduction in PPAR gamma mRNA level in HSC from BDL, Nuclear extracts from BDL cells showed an expected diminution of binding to PPAR-responsive element, whereas NF-kappaB and AP-I binding were increased. Treatment of cultured-activated HSC with ligands for PPAR gamma (10 muM 15-deoxy-Delta (12,14)-PGJ(2) (15dPGJ(2)); 0.1 similar to 10 muM BRL49653) inhibited DNA and collagen synthesis without affecting the cell viability. Suppression of HSC collagen by 15dPGJ(2), was abrogated 70% by the concomitant treatment with a PPAR gamma antagonist (GW9662). HSC DNA and collagen synthesis were inhibited by WY14643 at the concentrations known to activate both PPAR alpha and gamma (>100 muM) but not at those that only activate PPARa (<10 <mu>M) Or by a synthetic PPAR alpha -selective agonist (GW9578). 15dPGJ(2), reduced Phi1(I) procollagen, smooth muscle alpha -actin, and monocyte chemotactic protein-1 mRNA levels while inducing matrix metalloproteinase-3 and CD36, 15dPGJ(2), and BRL49653 inhibited alpha1(I) procollagen promoter activity. Tumor necrosis factor or (10 ng/ml) reduced PPAR gamma mRNA, and this effect was prevented by the treatment with 15dPGJ(2),, These results demonstrate that HSC activation is associated with the reductions in PPAR gamma expression and PPAR-responsive element binding in vivo and is reversed by the treatment with PPAR gamma ligands in vitro. These findings implicate diminished PPAR gamma signaling in molecular mechanisms underlying activation of HSC in liver fibrogenesis and the potential therapeutic value of PPAR gamma ligands for liver fibrosis.