CD40 activation-induced, Fas-dependent apoptosis and NF-kappa B/AP-1 signaling in human intrahepatic biliary epithelial cells

Fas-mediated mechanisms of apoptosis are thought to be involved in the bile duct loss that characterizes diseases such as primary biliary cirrhosis (P BC). We have previously shown that activation of CD40 on hepatocytes can am plify Fas-mediated apoptosis; in the present study, we investigated interac tions between CD40 and Fas in biliary epithelial cells (BEC). We report tha t the bile ducts in PBC liver tissue frequently express increased levels of Fas, Fas ligand (FasL), and CD40 associated with apoptotic BEC. The portal mononuclear infiltrate contains CD40L+ve T cells and macrophages, thereby demonstrating a potential mechanism for CD40 engagement in vivo. Primary cu ltures of human BEC also expressed Fas, FasL, and CD40 but not CD40L protei n or mRNA. Activation of CD40 on BEC using recombinant CD40L increased tran scriptional expression of FasL and induced apoptosis, which was inhibited b y neutralizing antibodies to either Fas or FasL. Thus, CD40-induced apoptos is of BEC is mediated through Fas/FasL. We then investigated the intracellu lar signals and transcription factors activated in BEC and found that NF-ka ppaB and AP-1 were both activated after CD40 ligation. Increased functional NF-kappaB was seen early after CD40 ligation, but returned to baseline lev els after 4 h. In contrast, the rapid up-regulation of AP-1 was sustained o ver 24 h. This study provides further functional evidence of the ability of CD40 to induce Fas/FasL-dependent apoptosis of liver epithelial cells supp orting the importance of cross-talk between tumor necrosis factor (TNF) rec eptor family members as an amplification step in apoptosis induction. Susta ined activation of AP-1 in the absence of NF-kappaB signaling may be a crit ical factor in determining the outcome of CD40 engagement.