Transformation-dependent susceptibility of rat hepatic stellate cells to apoptosis induced by soluble fas ligand

Abstract

Cytokine-driven activation of hepatic stellate cells (HSC) in tissue injury and inflammation is a key pathogenetic event in liver fibrogenesis leading to an expanded pool of matrix producing myofibroblasts (MFB) which represent the transformed counterpart of HSC. We hypothesize that expansion of the pool of MFB might also be accomplished by modulation of apoptosis, which plays an opposite and complementary role to mitosis in the cellular homeostasis. We characterized the susceptibility of HSC in primary culture and of MFB in secondary culture to apoptosis induced by the soluble Fas ligand (sFasL) and related the effects to the expression levels of Fas (APO-1/CD95) and some major proapoptotic and contra-apoptotic protooncogenes. MFB showed a dose-dependent apoptotic reaction upon exposure to sFasL as evidenced by a strong increase of nucleosomal DNA fragments, loss of cellular DNA, positive TUNEL reaction, and annexin staining. The effect was found only if protein synthesis (cycloheximide) or RNA synthesis (actinomycin D) were arrested. HSC maintained for various times in primary culture were completely resistant to sFasL in combination with cycloheximide, but in late primary cultures (day 7 onward) an increasing susceptibility to sFasL-mediated apoptosis was developed. By semiquantitative reverse-transcriptase polymerase chain reaction (RT-PCR) analysis and alkaline phosphatase-anti-alkaline phosphatase staining Fas receptor was identified both in HSC and MFB at comparable expression levels. The expression of the contra-apoptotic protooncogenes bcl-2 and bcl-xl was found to be much stronger in early HSC than in late HSC and MFB as shown by ribonuclease protection assay. The expression of bcl-2 was additionally confirmed by semiquantitative RT-PCR and immunoblotting. Pro-apoptotic bax was found in comparable quantities at the RNA level in HSC and MFB but at the protein level MFB showed increased bax expression. It is concluded that transformation of HSC to MFB is paralleled by an increasing sensitivity to sFasL-mediated apoptosis, which might be related to a strong decrease of bcl-2 and bcl-xl expression, leading to a preponderance of proapoptotic gene expression in MFB. Modulation of apoptotic susceptibility of transforming HSC could be an important complementary pathway in the pathogenesis of fibrosis.