title: Cyclic AMP inhibition of proliferation of hepatocellular carcinoma cells is mediated by Akt.
abstract: Cyclic AMP (cAMP), one of the most important intracellular second messengers, has been reported to inhibit proliferation of human hepatocellular carcinoma (HCC) cells via negatively regulating p42/44 mitogen-activated protein kinase. Here, we reported that cAMP inhibited the proliferation of HCC BEL-7402 cells via a novel mechanism. Forskolin, an activator of adenylate cyclase, inhibited fetal bovine serum (FBS)-stimulated BEL-7402 cell proliferation in a dose- and time-dependent manner, along with the inhibition of FBS-stimulated serine/threoine protein kinase Akt (also known as PKB) phosphorylation which is required for Akt activation and this effect was mimicked by 8-Br cAMP. Forskolin also inhibited Akt phosphorylation stimulated by other growth factors such as IGF-1, epidermal growth factor, and insulin. These inhibitions were found not only in BEL-7402 cells, but also in another HCC cell line SMMC-7721 cells. Myr-Akt (myristolated-Akt), a constitutively active Akt which was relatively resistant to cAMP inhibition, conferred BEL-7402 cells resistance to cAMP treatment. However, overexpression of Myr-Akt alone was not sufficient to stimulate BEL-7402 cell proliferation. cAMP inhibited FBS-stimulated Akt phosphorylation in a cAMP-dependent protein kinase-dependent manner. Further studies demonstrated that cAMP inhibited FBS-induced membrane localization of 3-phosphoinositide-dependent kinase 1 (PDK-1) which is a required process for PDK-1 to phosphorylate Akt, but had no significant effect on phosphoinositide 3-kinase activity. These results indicate that cAMP inhibition of proliferation of HCC cells is mediated by Akt and cAMP inhibits Akt activation via blocking membrane localization of PDK-1.
authors.affiliation: Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, China.
authors.forename: Lunhua
authors.initials: L
authors.lastname: Liu
authors.forename: Yili
authors.initials: Y
authors.lastname: Xie
authors.forename: Liguang
authors.initials: L
authors.lastname: Lou
mesh_terms: D064413:3-Phosphoinositide-Dependent Protein Kinases; D006528:Carcinoma, Hepatocellular / Q000378:metabolism*; D045744:Cell Line, Tumor; D049109:Cell Proliferation / Q000187:drug effects*; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000378:metabolism* / Q000494:pharmacology; D017868:Cyclic AMP-Dependent Protein Kinases / Q000037:antagonists & inhibitors / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006801:Humans; D007700:Kinetics; D008113:Liver Neoplasms / Q000378:metabolism*; D010766:Phosphorylation / Q000187:drug effects; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D051057:Proto-Oncogene Proteins c-akt / Q000037:antagonists & inhibitors / Q000378:metabolism*
chemical_list: D004791:Enzyme Inhibitors; D005576:Colforsin; D000242:Cyclic AMP; D064413:3-Phosphoinositide-Dependent Protein Kinases; D017346:Protein Serine-Threonine Kinases; D051057:Proto-Oncogene Proteins c-akt; D017868:Cyclic AMP-Dependent Protein Kinases