DIMINISHED RETINOIC ACID SIGNALING IN HEPATIC STELLATE CELLS IN CHOLESTATIC LIVER FIBROSIS

Hepatic stellate cells (HSC) participate in liver fibrogenesis via myo fibroblastic activation, the extent of which appears to correlate with the loss of cellular vitamin h. The present study has tested a hypoth esis that HSC activation is associated with diminished retinoic acid ( RA) signaling. Pure HSC were isolated from rats with cholestatic liver fibrosis induced by bile duct ligation (BDL) and sham-operated animal s (Sham). Northern blot analysis of HSC RNA from BDL confirmed fibroge nic activation of the cells with enhanced mRNA levels for procollagen- alpha 1(I) and transforming growth factor-beta 1 (TGF-beta 1). Competi tive polymerase chain reaction analysis showed selective reductions in the mRNA levels of RA receptor (RAR)-beta and retinoid X receptor (RX R)-alpha to 20 and 17% of the Sham HSC. The mRNA level for cellular re tinol binding protein I, a gene with RA responsive element (RARE), was also suppressed by 78% in BDL. The concentrations of all-trans-RA and 9-cis-RA were decreased in HSC from BDL. Nuclear extracts of these ce lls showed diminished binding activity to the RARE, whereas activity o f AP-1, a transcription factor known to be antagonized by RAR and RXR, was enhanced. These results demonstrate diminished RA signaling in HS C from cholestatic liver fibrosis, which appeared to have resulted fro m RA deficiency and suppressed expression of RAR-beta and RXR-alpha. F urthermore, the reciprocal enhancement of AP-1 activity and coordinate ly increased expression of an AP-1 responsive gene, TGF-beta 1, sugges t a permissive role of the diminished RA signaling in promoting AP-1 a ctivity and TGF-beta 1 expression.