Induced p21(WAF1) expression acts to reverse myc myelomonocytic cell transformation

Two murine myelomonocytic cells lines were used to examine p21(WAF1) expres sion in myc-induced cell transformation, tEMmyc4 and FDLV are two v-myc-tra nsformed immortalised myeloid cell lines exhibiting different transformed p henotypes. FDLV cells were derived from the transduction of v-myc into FDC- P1 cells and retain growth factor (IL-3) dependence, whereas tEMmyc4 cells were derived from the transduction of embryonal monocytes with v-myc and ar e growth factor-independent, constitutively express endogenous CSF-1, and a re highly tumorigenic in syngeneic mice. Both cell lines were found to exhi bit low p21WAF1 expression. When examined in tEMmyc4 cells, neither the p53 -dependent pathway (mitomycin C or exogenous p53) nor p53-independent pathw ay (TPA or growth factor, CSF-1, stimulation) acted to increase p21(WAF1) l evels. Growth factor (IL-3) withdrawal, shown to reduce p21(WAF1) levels in parental FDC-P1 cells, failed to do this in FDLV cells. The dependence of p21(WAF1) expression on v-myc was further demonstrated by showing that a v- myc-targeted ribozyme, which acts to decrease v-myc RNA, increased p21(WAF1 ) levels in tEMmyc4 cells. Enforced express ion of exogenous p21(WAF1), tEM myc4 eel Is with dysfunctional growth cycle (including growth arrest and in creased susceptibility to apoptosis) was examined. p21(WAF1) partially rest ored cell cycle regulation and apoptosis as well as inhibited the delayed c ell cycle progression and apoptosis induced by mitomycin C or serum withdra wal. These results show p21(WAF1) expression to be affected by v-myc and a restoration of p21(WAF1) expression to partially reverse myc-mediated trans formation.