Hydrophobic bile salts trigger ceramide formation through endosomal acidification

Hydrophobic bile salts activate NADPH oxidase through a ceramide- and PKC zeta-dependent pathway as an important upstream event of bile salt-induced hepatocyte apoptosis. The mechanisms underlying bile salt-induced ceramide formation have remained unclear to date and thus were studied in rat hepatocytes. Proapoptotic bile salts, such as taurolithocholylsulfate (FLCS), lowered the apparent pH(ves) within seconds from 6.0 to 5.6 in an FITC-dextran-accessible endosomal compartment that also contains acidic sphingomyelinase. Simultaneously, a rapid decrease in N-(ethoxycarbonylmethyl)-6-methoxyquinolinium bromide (MQAE) fluorescence was observed, suggestive of an increase in cytosolic [Cl-], which is known to activate vacuolar-type H+-ATPase. No vesicular acidification or increase in cytosolic [CF] was found in response to the non-apoptotic bile salt taurocholate or the anti-apoptotic bile salt tauroursoclesoxycholate. Inhibition of TLCS-induced endosomal acidification by bafilomycin or 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid largely abolished the TLCS-induced ceramide-formation and downstream ceramide-dependent processes, such as p47(phox)-serine phosphorylation, NADPH oxidase activation, CD95 activation and apoptosis. These responses were also abolished after knockdown of acidic sphingomyelinase in rat hepatocytes. In conclusion, hydrophobic, proapoptotic bile salts stimulate ceramide formation through chloride-dependent acidification of endosomes, with subsequent activation of acidic sphingomyelinase. Our data suggest that changes in ion homeostasis underlie the stimulation of ceramide formation in response to hydrophobic bile acids as an important upstream event of bile salt-induced apoptosis.