Induced p21^WAF1 expression acts to reverse myc myelomonocytic cell transformation

Abstract

Two murine myelomonocytic cells lines were used to examine p21^WAF1 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 p21^WAF1 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 p21^WAF1 levels. 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 expression of exogenous p21^WAF1 in tEMmyc4 cells with dysfunctional growth cycle (including growth arrest and increased susceptibility to apoptosis) was examined. p21^WAF1 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 p21^WAF1 expression to be affected by v-myc and a restoration of p21^WAF1 expression to partially reverse myc-mediated transformation.Cancer Gene Therapy (2000) 7, 1491–1503