Background and aims: Hydrophobic bile acids induce CD95 (Fas, APO-1)-dependent hepatocyte apoptosis, which involves epidermal growth factor receptor (EGFR)-catalyzed CD95 tyrosine phosphorylation. The mechanisms underlying bile salt-induced EGFR activation remain unclear.
Methods: Bile acid-induced EGFR activation was studied in 24-hour cultured rat hepatocytes and perfused rat liver.
Results: The proapoptotic bile salts taurolithocholate-3-sulfate (TLCS), glycochenodesoxycholate (GCDC) and taurochenodeoxycholate (TCDC), but not taurocholate (TC), activate within 1 minute the Src kinase family member Yes, followed by an association of Yes with EGFR and subsequent EGFR activation. EGFR phosphorylation by TLCS involves tyrosines 845 and 1173 but not 1045. Yes/EGFR association and EGFR activation were sensitive to inhibition by SU6656 but not by PP-2. cAMP had no effect on TLCS and GCDC-induced Yes activation but induced Ser/Thr phosphorylation of Yes and prevented Yes/EGFR association and subsequent EGFR activation. Both SU6656 and cAMP had no effect on bile salt-induced c-Jun N-terminal kinase activation, but blocked bile salt-induced CD95 tyrosine phosphorylation, membrane trafficking of CD95, formation of the death-inducing signaling complex, and apoptosis. In 4-day cultured hepatocytes, knockdown of either Yes or EGFR strongly attenuated bile salt-induced CD95 activation and apoptosis.
Conclusions: The data identify the Src kinase Yes as an upstream target of proapoptotic bile acids, which triggers EGFR activation, subsequent CD95 tyrosine phosphorylation, and apoptosis. The antiapoptotic effect of cAMP involves a protein kinase A-dependent inhibition of Yes/EGFR association, thereby preventing EGFR activation, which is required for CD95 activation.