Induced p21WAF1 expression acts to reverse myc myelomonocytic cell transformation

Two murine myelomonocytic cells lines were used to examine p21WAF1 expression in myc-induced cell transformation. tEMmyc4 and FDLV are two v-myc-transformed immortalised myeloid cell lines exhibiting different transformed phenotypes. 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 are growth factor-independent, constitutively express endogenous CSF-1, and are highly tumorigenic in syngeneic mice. Both cell lines were found to exhibit low p21WAF1 expression. When examined in tEMmyc4 cells, neither the p53-dependent pathway (mitomycin C or exogenous p53) nor p53-independent pathway (TPA or growth factor, CSF-1, stimulation) acted to increase p21WAF1 levels. Growth factor (IL-3) withdrawal, shown to reduce p21WAF1 levels in parental FDC-P1 cells, failed to do this in FDLV cells. The dependence of p21WAF1 expression on v-myc was further demonstrated by showing that a v-myc-targeted ribozyme, which acts to decrease v-myc RNA, increased p21WAF1 levels in tEMmyc4 cells. Enforced expression of exogenous p21WAF1 in tEMmyc4 cells with dysfunctional growth cycle (including growth arrest and increased susceptibility to apoptosis) was examined. p21WAF1 partially restored cell cycle regulation and apoptosis as well as inhibited the delayed cell cycle progression and apoptosis induced by mitomycin C or serum withdrawal. These results show p21WAF1 expression to be affected by v-myc and a restoration of p21WAF1 expression to partially reverse myc-mediated transformation.