title: cAMP-guanine exchange factor protection from bile acid-induced hepatocyte apoptosis involves glycogen synthase kinase regulation of c-Jun NH2-terminal kinase.
abstract: Cholestatic liver disorders are accompanied by the hepatic accumulation of cytotoxic bile acids that induce cell death. Increases in cAMP protect hepatocytes from bile acid-induced apoptosis by a cAMP-guanine exchange factor (cAMP-GEF)/phosphoinositide-3-kinase (PI3K)/Akt pathway. The aim of these studies was to identify the downstream substrate in this pathway and to determine at what level in the apoptotic cascade cytoprotection occurs. Since inhibitory phosphorylation of glycogen synthase kinase-3 (GSK) occurs downstream of PI3K/Akt and this phosphorylation has been implicated in cell survival, we conducted studies to determine whether GSK was downstream in cAMP-GEF/PI3K/Akt-mediated cytoprotection. Our results show that treatment of hepatocytes with the cAMP-GEF-specific analog, 4-(4-chlorophenylthio)-2'-O-methyladenosine-3',5'-cAMP, results in PI3K-dependent phosphorylation of GSK. Direct chemical inhibition of GSK in rat hepatocytes or human HUH7-NTCP cells with several structurally and functionally distinct inhibitors including bromoindirubin-3'-oxime (BIO), maleimides (SB216763, SB415286), thiadiazolidine derivatives, and LiCl attenuates apoptosis induced by glycochenodeoxycholate (GCDC). In addition, genetic silencing of the GSK β isoform with small interfering RNA attenuates GCDC apoptosis in HUH7-NTCP cells. Adenoviral inhibition of the Rap1 blocks both cAMP-GEF-mediated cytoprotection against GCDC-induced apoptosis and Akt/GSK3β phosphorylation. GCDC-induced phosphorylation of the proapoptotic kinase, c-Jun NH(2)-terminal kinase (JNK) is inhibited by GSK inhibition or cAMP-GEF activation. GCDC-induced apoptosis is accompanied by phosphorylation of the endoplasmic reticulum stress markers pIEF2α and IRE-1, and pretreatment with the cAMP-GEF analog or GSK inhibitors prevents this phosphorylation. Collectively, our results support the presence of a cAMP/cAMP-GEF/Rap1/PI3K/Akt/GSKβ survival pathway in hepatocytes that inhibits bile acid-induced JNK phosphorylation.
authors.affiliation: Department of Clinical Sciences, Tufts Cummings School of Veterinary Medicine, Grafton, Massachusetts 01536, USA.
authors.forename: A
authors.initials: A
authors.lastname: Johnston
authors.forename: K
authors.initials: K
authors.lastname: Ponzetti
authors.forename: M S
authors.initials: MS
authors.lastname: Anwer
authors.forename: C R L
authors.initials: CR
authors.lastname: Webster
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000502:physiology*; D001647:Bile Acids and Salts / Q000494:pharmacology; D006528:Carcinoma, Hepatocellular; D000242:Cyclic AMP / Q000031:analogs & derivatives / Q000494:pharmacology / Q000502:physiology; D019610:Cytoprotection; D004721:Endoplasmic Reticulum / Q000502:physiology; D005999:Glycochenodeoxycholic Acid / Q000502:physiology; D038362:Glycogen Synthase Kinase 3 / Q000502:physiology*; D020662:Guanine Nucleotide Exchange Factors / Q000502:physiology*; D022781:Hepatocytes / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D006801:Humans; D048031:JNK Mitogen-Activated Protein Kinases / Q000378:metabolism*; D008297:Male; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D010766:Phosphorylation; D051057:Proto-Oncogene Proteins c-akt; D051381:Rats; D017208:Rats, Wistar; D015398:Signal Transduction / Q000502:physiology*; D013312:Stress, Physiological / Q000502:physiology; D013873:Thionucleotides; D014407:Tumor Cells, Cultured; D020827:rap1 GTP-Binding Proteins / Q000502:physiology
chemical_list: C488718:2'-O-methyl-8-(4-chlorophenylthio)cAMP; D001647:Bile Acids and Salts; D020662:Guanine Nucleotide Exchange Factors; D013873:Thionucleotides; D005999:Glycochenodeoxycholic Acid; D000242:Cyclic AMP; D019869:Phosphatidylinositol 3-Kinases; C494920:Akt1 protein, rat; D051057:Proto-Oncogene Proteins c-akt; D048031:JNK Mitogen-Activated Protein Kinases; D038362:Glycogen Synthase Kinase 3; D020827:rap1 GTP-Binding Proteins