INHIBITION OF C-MYC EXPRESSION INDUCES APOPTOSIS OF WEHI-231 MURINE B-CELLS

Treatment of WEHI 231 immature B-lymphoma cells with an antibody again st their surface immunoglobulin (anti-Ig) induces apoptosis and has be en studied extensively as a model of B-cell tolerance. Anti-Ig treatme nt of exponentially growing WEHI 231 cells results in an early transie nt increase in c-myc expression that is followed by a decline to below basal levels; this decrease in c-myc expression immediately precedes the induction of cell death. Here we have modulated NF-kappa B/Rel fac tor activity, which regulates the rate of c-myc gene transcription, to determine whether the increase or decrease in c-Myc levels mediates a poptosis in WEHI 231 cells. Addition of the serine/threonine protease inhibitor N-tosyl-L-phenylalanine chloromethyl ketone (TPCK), which bl ocks the normally rapid turnover of the specific inhibitor of NF-kappa B/Rel I kappa B alpha in these cells, caused a drop in Rel-related fa ctor binding. TPCK treatment resulted in decreased c-myc expression, p reventing the usual increase seen following anti-Ig treatment. Whereas inhibition of the induction of c-myc expression mediated by anti-Ig f ailed to block apoptosis, reduction of c-myc expression in exponential ly growing WEHI 231 cells induced apoptosis even in the absence of ant i-Ig treatment. In WEHI 231 clones ectopically expressing c-Myc, apopt osis induced by treatment with TPCK or anti-Ig was significantly dimin ished and cells continued to proliferate. Furthermore, apoptosis of WE HI 231 cells ensued following enhanced expression of Mad1, which has b een found to reduce functional c-Myc levels. These results indicate th at the decline in c-myc expression resulting from the drop in NF-kappa B/Rel binding leads to activation of apoptosis of WEHI 231 B cells.