INVOLVEMENT OF cPLA2 INHIBITION IN DEXAMETHASONE-INDUCED THYMOCYTE APOPTOSIS

Various molecular mechanisms have been suggested to be involved in dexamethasone induced thymocyte apoptosis. In this study we show that pharmacological inhibition of cytoplasmic PLA(2) in mouse thymocytes for 18 h with arachidonyl trifluoromethyl ketone (AACOCF3) (10 mu M) and palmitoyl trifluoromethyl ketone (PACOCF3) (10 mu M) induced a drastic increase of thymocyte apoptosis comparable to that observed following Dex (10(-7) M) treatment, while inhibition of secretory PLA(2) with p-bromophenacyl bromide (pBPB) (20 mu M) did not. AACOCF3-induced thymocyte apoptosis, similarly to Dex-induced thymocyte apoptosis, was eliminated by cell pre-treatment with the PI-PLC beta inhibitor, U73122, but not by the PC-PLC inhibitor D609. These observations were corroborated by the ability of AACOCF3, like Dex, to induce a rapid and transient increase in DAG generation. In addition, AACOCF3-induced apoptosis involved the activation of the acidic sphingomyelinase (aSMase) but not of the neutral sphingomyelinase (nSMase), as evaluated by measurements of enzyme activity in cell extracts following thymocyte exposure to AACOCF3 and by the ability of monensin to inhibit AACOCF3-induced thymocyte apoptosis. In addition, the AACOCF3 apoptotic effect resulted in an early increase of ceramide levels. AACOCF3-induced thymocyte apoptosis involved the activation of caspase 3, and cell pre-treatment with a caspase 3 inhibitor prevented AACOCF3-induced apoptosis. These observations suggest that cPLA(2) inhibition may have a role in Dex-induced thymocyte apoptosis and highlight the importance of cPLA(2) activity in thymocyte survival.