title: Calphostin C induces tyrosine dephosphorylation/inactivation of protein kinase FA/GSK-3 alpha in a pathway independent of tumor promoter phorbol ester-mediated down-regulation of protein kinase C.
abstract: The signal transduction mechanism of protein kinase FA/GSK-3 alpha by tyrosine phosphorylation in A431 cells was investigated using calphostin C as an inhibitor for protein kinase C (PKC). Kinase FA/GSK-3 alpha could be tyrosine-dephosphorylated and inactivated to approximately 10% of control in a concentration-dependent manner by 0.1-10 microM calphostin C (IC50, approximately 1 microM), as demonstrated by immunoprecipitation of kinase FA/GSK-3 alpha from cell extracts, followed by phosphoamino acid analysis and by immunodetection in an antikinase FA/GSK-3 alpha immunoprecipitate kinase assay. In sharp contrast, down-regulation of PKC by 0.05 microM calphostin C (IC50, approximately 0.05 microM for inhibiting PKC in cells) or by tumor promoter phorbol ester TPA was found to have stimulatory effect on the cellular activity of kinase FA/GSK-3 alpha, when processed under identical conditions. Furthermore, TPA-mediated down-regulation of PKC was found to have no effect on calphostin C-mediated tyrosine dephosphorylation/inactivation of kinase FA/GSK-3 alpha. Taken together, the results provide initial evidence that the PKC inhibitor calphostin C may induce tyrosine dephosphorylation/inactivation of kinase FA/GSK-3 alpha in a pathway independent of TPA-mediated down-regulation of PKC, representing a new mode of signal transduction for the regulation of this multisubstrate/multifunctional protein kinase by calphostin C in cells. Since kinase FA/GSK-3 alpha is a possible carcinoma dedifferentiation/progression-promoting factor, the results further suggest calphostin C as a potential anticancer drug involved in blocking carcinoma dedifferentiation/progression, possibly via inactivation of protein kinase FA/GSK-3 alpha in tumor cells.
authors.affiliation: Institute of Life and Biomedical Sciences, National Tsing Hua University, Hsinchu, Taiwan, R.O.C.
authors.forename: S C
authors.initials: SC
authors.lastname: Lee
authors.forename: S D
authors.initials: SD
authors.lastname: Yang
mesh_terms: D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000037:antagonists & inhibitors*; D002273:Carcinogens / Q000494:pharmacology*; D015536:Down-Regulation / Q000187:drug effects*; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D038362:Glycogen Synthase Kinase 3; D006801:Humans; D009281:Naphthalenes / Q000494:pharmacology*; D010766:Phosphorylation; D011493:Protein Kinase C / Q000037:antagonists & inhibitors*; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology*; D014407:Tumor Cells, Cultured; D014443:Tyrosine / Q000378:metabolism
chemical_list: D002273:Carcinogens; D004791:Enzyme Inhibitors; D009281:Naphthalenes; D014443:Tyrosine; D011493:Protein Kinase C; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D038362:Glycogen Synthase Kinase 3; C058819:calphostin C; D013755:Tetradecanoylphorbol Acetate