p53-independent apoptosis and p53-dependent block of DNA rereplication following mitotic spindle inhibition in human cells

We have studied the response of human transformed cells to mitotic spindle inhibition. Two paired cell lines, K562 and its parvovirus-resistant KS der ivative clone, respectively nonexpressing and expressing p53, were continuo usly exposed to nocodazole. Apoptotic cells were observed in both lines, in dicating that mitotic spindle impairment induced p53-independent apoptosis, After a transient mitotic delay, both cell lines exited mitosis, as reveal ed by flow-cytometric determination of MPM2 antigen and cyclin B1 expressio n, coupled to cytogenetic analysis of sister centromere separation. Both ce ll lines exited mitosis without chromatid segregation. K562 p53-deficient c ells further resumed DNA synthesis, giving rise to cells with a DNA content above 4C, and reentered a polyploid cycle. In contrast, KS cells underwent a subsequent G1 arrest in the tetraploid state. Thus, G1 arrest in tetrapl oid cells requires p53 function in the rereplication check-point which prev ents the G1/S transition following aberrant mitosis; in contrast, p53 expre ssion is dispensable for triggering the apoptotic response in the absence o f mitotic spindle. (C) 1999 Academic Press.