Inhibition of bile salt-induced apoptosis by cyclic AMP involves serine/threonine phosphorylation of CD95.
OBJECTIVE
Cyclic AMP (cAMP) inhibits bile salt-induced hepatocyte apoptosis; the underlying mechanisms are unclear.


METHODS
The effects of cAMP on taurolithocholate-3-sulfate-(TLCS)- or glycochenodesoxycholate (GCDC)-induced CD95 (Fas/APO-1) activation and apoptosis were studied in 24-hour cultured rat hepatocytes and in perfused rat liver.


RESULTS
TLCS induced a rapid oxidative stress response, c-Jun-N-terminal kinase (JNK) and epidermal growth factor (EGF) receptor (EGF-R) activation, subsequent EGF-R/CD95 association and CD95 tyrosine phosphorylation, CD95 membrane targeting, death-inducing signal complex (DISC) formation and hepatocyte apoptosis. None of these responses was triggered by cAMP; however, cAMP induced H89-sensitive serine/threonine phosphorylation of CD95. Similar data were obtained with GCDC, another proapoptotic bile acid. cAMP did not prevent the TLCS-induced oxidative stress response, JNK activation and EGF-R/CD95 association, however abolished EGF-R activation and subsequent CD95 tyrosine phosphorylation, CD95 membrane trafficking, and DISC formation in a H89-sensitive way. Also in presence of TLCS, cAMP induced rapid Ser/Thr phosphorylation of CD95 within 10 min. The effects of cAMP on the various steps of CD95 activation were also found in the intact perfused rat liver. Evidence is given that a cAMP-induced Ser/Thr phosphorylation favors internalization of CD95.


CONCLUSIONS
Inhibition of bile salt-induced apoptosis by cAMP involves both PKA-dependent Ser/Thr phosphorylation of the CD95 and inhibition of EGF-R activation, which results in an inhibition of CD95 tyrosine phosphorylation, CD95 membrane targeting, and DISC formation. CD95 regulation involves complex phosphorylations with CD95-tyrosine phosphorylation favoring CD95 membrane trafficking and DISC formation, whereas CD95 Ser/Thr phosphorylation inhibits these processes.
Reinehr
Roland
R
Clinic for Gastroenterology, Heinrich-Heine-University, Düsseldorf, Germany.
Häussinger
Dieter
D
D000241:Adenosine / Q000494:pharmacology; D000818:Animals; D017209:Apoptosis / Q000187:drug effects*; D002478:Cells, Cultured; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000494:pharmacology*; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D004789:Enzyme Activation; D066246:ErbB Receptors / Q000378:metabolism; D048031:JNK Mitogen-Activated Protein Kinases; D008297:Male; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism; D010766:Phosphorylation; D051381:Rats; D017208:Rats, Wistar; D012694:Serine / Q000378:metabolism*; D013658:Taurolithocholic Acid / Q000031:analogs & derivatives* / Q000494:pharmacology*; D013912:Threonine / Q000378:metabolism*; D019014:fas Receptor / Q000378:metabolism*
D019014:fas Receptor; C066776:taurolithocholic acid 3-sulfate; D005576:Colforsin; D013912:Threonine; D012694:Serine; D013658:Taurolithocholic Acid; D000242:Cyclic AMP; D066246:ErbB Receptors; D017868:Cyclic AMP-Dependent Protein Kinases; D048031:JNK Mitogen-Activated Protein Kinases; D020928:Mitogen-Activated Protein Kinases; D000241:Adenosine