Dexamethasone-induced thymocyte apoptosis: Apoptotic signal involves the sequential activation of phosphoinositide-specific phospholipase C, acidic sphingomyelinase, and caspases

Glucocorticoid hormones (GCH) have been implicated as regulators of T-lymph ocyte growth and differentiation, In particular, it has been reported that GCH can induce thymocyte apoptosis. However, the molecular mechanisms respo nsible for this GCH-induced death have not been clarified. In this work, th e biochemical events associated with apoptosis induced by Dexamethasone (De x), a synthetic GCH, in normal mouse thymocytes, have been analyzed. Result s indicate that Dex-induced thymocyte apoptosis is attributable to an early ceramide generation caused by the activation of an acidic sphingomyelinase (aSMase). Caspase activity plays a crucial role in Dex-induced apoptosis a nd is downstream the aSMase activation in that inhibition of the early cera mide generation inhibits caspase activation and thymocyte death. Moreover, Dex treatment rapidly induces diacylglycerol (DAG) generation, through a pr otein kinase C (PKC) and G-protein-dependent phosphatidylinositol-specific phospholipase C (PI-PLC), an event which precedes and is required for aSMas e activation. Indeed, PI-PLC inhibition by U73122 totally prevents Dex-indu ced aSMase activity, ceramide generation, and consequently, caspase activat ion and apoptosis. All these effects require Dex interaction with GCH recep tor (GR), are countered by the GR antagonist RU486, and precede the GCH/GR- activated transcription and protein synthesis, These observations indicate that GCH activates thymocyte death through a complex signaling pathway that requires the sequential activation of different biochemical events. (C) 19 99 by The American Society of Hematology.