Phorbol myristate acetate inhibits okadaic acid-induced apoptosis and downregulation of x-linked inhibitor of apoptosis in U937 cells

Okadaic acid is a specific inhibitor of serine/threonine protein phosphatas e 1 (PP-1) and 2A (PP2A). The phosphorylation and dephosphorylation at the serine/threonine residues on proteins play important roles in regulating ge ne expression, cell cycle progression, and apoptosis. In this study, phosph atase inhibitor okadaic acid induces apoptosis in U937 cells via a mechanis m that appears to involve caspase 3 activation, but not modulation of Bcl-2 , Bax, an Bcl-X-L expression levels. Treatment with 20 or 40 nM okadaic aci d for 24 h produced DNA fragmentation in U937 cells. This was associated wi th caspase 3 activation and PLC-gamma1 degradation. Okadaic acid-induced ca spase 3 activation and PLC-gamma1 degradation and apoptosis were dose-depen dent with a maximal effect at a concentration of 40 nM. Moreover, PMA (phor bol myristate acetate), PKC (protein kinase 3) activator, protected U937 ce lls from okadaic acid-induced apoptosis, abrogated okadaic acid-induced cas pase 3 activation, and specifically inhibited downregulation of XIAP (X-lin ked inhibitor of apoptosis) by okadaic acid. PMA cotreated U937 cells exhib ited 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 ap optosis by a mechanism that interferes with cytochrome c release and activi ty of caspase 3 that is involved in the execution of apoptosis. (C) 2001 Ac ademic Press.