INCREASED TUBULIN ACETYLATION ACCOMPANIES REVERSION TO STABLE PLOIDY IN VINCRISTINE-RESISTANT CCRF-CEM CELLS

The T-cell leukemia line CCRF-CEM is unstable with respect to ploidy, whereas a vincristine-resistant subline, CEM/VCR R, maintains a stable pseudodiploid karyotype. Ploidy change in the parental cells requires the involvement of two cell cycle lesions. The first, in mitosis, pre vents cell division after S-phase. The second, in GZ, allows a cell wi th 4N DNA content to re-enter S-phase. We examined differences in expr ession of tubulin, a major component of the mitotic spindle and the ce llular target for vincristine, between the two cell lines. Levels of t he beta III isotype were decreased and levels of acetylated a-tubulin, a marker for microtubule stability, were increased in the CEM/VCR R c ells relative to the parental line, which suggests that the CEM/VCR R cells have a more stable mitotic spindle. Both cell lines exhibit some level of constitutive expression of p53 and c-myc. Constitutive expre ssion of and mutant p53 would contribute to the failure of these cells to recognise G1 checkpoints. Therefore, G1 checkpoint failure and the intrinsically less stable mitotic spindle in the CCRF-CEM cells may c ontribute to the observed ploidy instability. Conversely, the presence of markers of microtubule stability in the CEM/VCR R cells would pred ispose them to maintain their ploidy.