An early oxygen-dependent step is required for dexamethasone-induced apoptosis of immature mouse thymocytes

The roles of oxygen and reactive oxygen intermediates in apoptosis are uncl ear at present. Although oxygen and reactive oxygen intermediates are not r equired for the execution of apoptosis, oxygen may be involved in at least some forms of apoptosis, In this study we show that dexamethasone (Dex)-ind uced apoptosis of immature mouse thymocytes is completely inhibited by hypo xic culture. In contrast, anti-CD95 thymocyte apoptosis is unaffected by hy poxia, indicating the existence of two forms of thymocyte apoptosis: an oxy gen-dependent pathway (Des induced) and an oxygen-independent pathway (anti -CD95 induced). Furthermore, hypoxia inhibited mitochondrial permeability t ransition CPT in Des-treated, but not in anti-CD95-treated, thymocytes, sug gesting that the oxygen-sensitive step is upstream of mitochondria, Both De x- and anti-CD95-induced PT and apoptosis were dependent on activation of I i-converting enzyme-like protease, as PT and apoptosis were inhibited by pr eincubation with Cbz-Val-Ala-Asp-fluoromethyl ketone, an irreversible inhib itor of IL-converting enzyme-like proteases, In addition, hypoxia inhibited the activation by Dex of caspase-3 (CPP32)-like proteases, Our data show t hat the private signaling pathways of Dex (oxygen dependent) and anti-CD95 (oxygen independent) both converge upstream of mitochondrial changes. The o xygen-dependent step in Dex-induced apoptosis lies upstream of caspase-3-li ke protease activation, Our observations support a model of apoptosis signa ling in which independent pathways (oxygen dependent and oxygen independent ) particular to each stimuli converge at a central point in the apoptotic c ascade.