Biphasic effect of protein kinase C activators on spontaneous and glucocorticoid-induced apoptosis in primary mouse thymocytes

Spontaneous and glucocorticoid (fluocinolone acetonide, FA)-induced apoptos is of primary mouse thymocytes was inhibited by protein kinase C (PKC) acti vators such as bryostatin-1 and phorbol ester 12-O-tetradecanoyl-phorbol-13 acetate (TPA) within the first 2-4 h of incubation but was enhanced upon p rolonged treatment. Only the anti-apoptotic but not the pro-apoptotic effec t of TPA was completely suppressed by the PKC inhibitor Goe 6983 and modera tely inhibited by Goe 6976. Immunoblot analysis revealed distinct PKC alpha , beta, delta, eta, theta, mu and zeta signals, a very faint PKC epsilon an d no PKC gamma signal. Upon prolonged TPA treatment all PKC isoenzymes beca me downregulated, albeit at different rates (PKC delta > alpha > mu >beta,t heta >>eta,zeta). No significant generation of caspase-derived catalytic PK C fragments, as found to be produced upon induction of apoptosis and to be pro-apoptotic in other systems, was observed in FA- or TPA-treated thymocyt es. It is concluded that the early anti-apoptotic effect of TPA depends on the activation of n-type PKC isoenzymes, whereas stimulation of spontaneous and FA-induced apoptosis by TPA ensues, at least partially, from a downreg ulation (or inactivation) of anti-apoptotic PKC species, i.e. in primary th ymocytes PKC activation is primarily involved in a negative regulation of a poptosis. (C) 2000 Elsevier Science B.V. All rights reserved.