RESISTANCE TO APOPTOSIS IN CTLL-2 CELLS CONSTITUTIVELY EXPRESSING C-MYB IS ASSOCIATED WITH INDUCTION OF BCL-2 EXPRESSION AND MYB-DEPENDENT REGULATION OF BCL-2 PROMOTER ACTIVITY

c-Myb, the cellular homologue of the transforming gene of the avian my eloblastosis virus, is preferentially expressed in all hematopoietic l ineages, including T and B lymphocyte lineages, In T lymphocytes, c-My b expression appears to be required for cell cycle progression and pro liferation, To further investigate the role of c-Myb in T cell prolife ration and survival, interleukin (IL) 2-dependent CTLL-2 cells were tr ansfected with a constitutively active c-myb or with a c-myb antisense construct able to down-regulate endogenous Myb levels, and the transf ectants were assessed for proliferation and survival in low concentrat ions of IL-2 and for susceptibility to dexamethasone-induced apoptosis , Compared with control cells, CTLL-2 cells constitutively expressing c-Myb proliferate in low concentrations of IL-2 and are less susceptib le to apoptosis induced bq IL-2 deprivation or treatment with dexameth asone. In contrast, cells transfected with an antisense c-myb construc t do not proliferate in Low concentrations of IL-2 and undergo apoptos is upon IL-2 deprivation or dexamethasone treatment more rapidly than parental cells, Overexpression of c-Myb was accompanied by up-regulati on of BCL-2 expression, In transient transfection assays, the murine b cl-2 promoter was efficiently transactivated by c-Myb, but such effect was observed also in cells transfected with a DNA binding-deficient c -myb construct, Moreover, in gel retardation assays, a 38-bp oligomer in the shortest bcl-2 promoter segment regulated by c-Myb formed a spe cific complex with nuclear extracts from c-Myb transfected CTLL-2 cell s, Thus, these results strongly suggest that c-Myb? in addition to reg ulating T cell proliferation, protects T lymphocytes from apoptosis by induction of BCL-2 expression, which involves a c-Myb-dependent mecha nism of promoter regulation.