title: Phorbol myristate acetate inhibits okadaic acid-induced apoptosis and downregulation of X-linked inhibitor of apoptosis in U937 cells.
abstract: Okadaic acid is a specific inhibitor of serine/threonine protein phosphatase 1 (PP-1) and 2A (PP-2A). The phosphorylation and dephosphorylation at the serine/threonine residues on proteins play important roles in regulating gene expression, cell cycle progression, and apoptosis. In this study, phosphatase inhibitor okadaic acid induces apoptosis in U937 cells via a mechanism that appears to involve caspase 3 activation, but not modulation of Bcl-2, Bax, and Bcl-X(L) expression levels. Treatment with 20 or 40 nM okadaic acid for 24 h produced DNA fragmentation in U937 cells. This was associated with caspase 3 activation and PLC-gamma1 degradation. Okadaic acid-induced caspase 3 activation and PLC-gamma1 degradation and apoptosis were dose-dependent with a maximal effect at a concentration of 40 nM. Moreover, PMA (phorbol myristate acetate), PKC (protein kinase C) activator, protected U937 cells from okadaic acid-induced apoptosis, abrogated okadaic acid-induced caspase 3 activation, and specifically inhibited downregulation of XIAP (X-linked inhibitor of apoptosis) by okadaic acid. PMA cotreated U937 cells exhibited less cytochrome c release and sustained expression levels of the IAP (inhibitor of apoptosis) proteins during okadaic acid-induced apoptosis. In addition, these findings indicate that PMA inhibits okadaic acid-induced apoptosis by a mechanism that interferes with cytochrome c release and activity of caspase 3 that is involved in the execution of apoptosis.
authors.affiliation: Department of Immunology, School of Medicine, Keimyung University, 194 DongSan-Dong Jung-Gu, Taegu 700-712, South Korea. kwontk@dsmc.or.kr
authors.forename: T K
authors.initials: TK
authors.lastname: Kwon
mesh_terms: D017209:Apoptosis; D002273:Carcinogens / Q000037:antagonists & inhibitors* / Q000494:pharmacology; D053148:Caspase 3; D020169:Caspases / Q000096:biosynthesis; D015536:Down-Regulation / Q000187:drug effects; D004347:Drug Interactions; D004790:Enzyme Induction; D006801:Humans; D007527:Isoenzymes / Q000378:metabolism; D019319:Okadaic Acid / Q000037:antagonists & inhibitors* / Q000494:pharmacology; D051966:Phospholipase C gamma; D011506:Proteins / Q000235:genetics / Q000378:metabolism*; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology*; D014158:Transcription, Genetic / Q000187:drug effects; D010738:Type C Phospholipases / Q000378:metabolism; D020298:U937 Cells; D051636:X-Linked Inhibitor of Apoptosis Protein
chemical_list: D002273:Carcinogens; D007527:Isoenzymes; D011506:Proteins; D051636:X-Linked Inhibitor of Apoptosis Protein; C496866:XIAP protein, human; D019319:Okadaic Acid; D010738:Type C Phospholipases; D051966:Phospholipase C gamma; C505407:CASP3 protein, human; D053148:Caspase 3; D020169:Caspases; D013755:Tetradecanoylphorbol Acetate