Caspase-9 (Casp-9) induces death signals by triggering other types of caspase activation, and its expression greatly influences the onset of apoptosis. During the isolation of apoptosis-related genes involved in glucocorticoid (GC)-induced cell death in murine thymic lymphomas, we found that the antisense gene of the transcription factor activator protein-4 (AP-4) inhibited dexamethasone-induced apoptosis. Western blot analysis revealed that the expression of Bcl-xL, Bax, and Apaf-1 was not affected in cells transfected with sense or antisense AP-4 genes. In contrast, both the expression and activation of Casp-9 were inhibited in the antisense AP-4 transfectants. We isolated the 2.4-kb 5'-flanking region of Casp-9, and the promoter activity was investigated. We found the AP-4-binding sites at -1.55 and -1.38 kb to be responsible for the promoter activity. Furthermore, a negative cis-element was expected to exist between bases -1140 and -944. When the cells were treated with dexamethasone, a rapid down-regulation of AP-4 and Casp-9 was observed whether the cells were GC-sensitive lymphomas or GC-insensitive L929 fibroblast cells. In addition, L929 cells pretreated with dexamethasone were found to be resistant to subsequent treatment with etoposide, an apoptosis-inducing reagent. GC has a two-sided effect on apoptosis, i.e. a pro-apoptotic effect on certain cell types and a prosurvival effect on other cell types. Our findings will explain, at least in part, this effect.