Altered expression of DNA-topoisomerase IIalpha is associated with increased rate of spontaneous polyploidization in etoposide resistant K562 cells.

Molecular mechanisms of drug resistance and alterations in chromosome number were studied in two independently etoposide selected clonal cell lines, K562-VP16-1 and K562-W16-2, and in bulk cell line K562-VPI6. In all cell lines was observed down-regulation of topo IIalpha gene expression, while topo IIbeta mRNA content was unchanged compared to parental cell line. Antiapoptotic bcl-2 mRNA content was decreased in all drug resistant cell variants, the level of bcl-X(L) mRNA was greatly diminished only in one of cell lines, K562-VP16-1. Proapoptotic bax mRNA was down regulated to an undetectable level in all resistant cell lines analysed. These data indicate that abrogation of bax-mediated apoptosis can be implicated in development of etoposide resisance. Cytogenetic analysis revealed increased rate of spontaneous polyploidization in K562-VPI6 bulk and K562-VP-16-2 cells, while in K562-VP-16-1 cells was observed only moderate accumulation of polyploid cells. The degree of changes in topo IIa but not bcl-2 family members expression correlated positively with dynamics of accumulation of polyploid cells. Our findings suggest that down regulation of topo IIalpha in association with p53 deficiency can confer chromosomal instability in etoposide-resistant K562 cells.