Two distinct modes of oncoprotein expression during apoptosis resistance in vincristine and daunorubicin multidrug-resistant HL60 cells

Apoptosis is a genetically regulated cell death process which results in a variety of morphological changes like chromatin condensation and DNA fragmentation. The decision between survival or death in response to an apoptotic stimulus is determined and regulated in part by oncoproteins which include proteins of the Bcl-2 family (bcl-2, bax, bcl-x_L) and bcr-abl. We investigated the effect of these proteins on the induction of this phenomenon in human promyelocytic leukemic HL60 cells and two multidrug resistant homologues selected respectively with vincristine (HL60/VCR) and daunorubicin (HL60R/DNR). We show that sensitive cells at 1 urn and HL60/VCR cells at DNR IC₅₀ were able to undergo apoptosis while HL60R/DNR did not even at much higher concentration of DNR. However, treatment with synthetic C₂-ceramide did not sensitize HL60/DNR cells to apoptosis. Cell death through apoptosis or necrosis was accompanied by acidification of the cytosol without mitochondrial membrane depolarization. Western blotting analysis shows that bax is expressed at slightly elevated level in HL60S/VCR in comparison with the other cells lines. Bcl-2 is overexpressed in HL60/VCR but not in HL60R/DNR. However, this cell line displayed a higher expression of bcl-x_L. Interestingly, bcr-abl, a dysregulated tyrosine kinase was detected only in HL60R/DNR cells. DNR at the IC₅₀, has no effect on expression of the oncoproteins. These data suggest that in addition of the multidrug resistance phenotype, bcr-abl translocation and bcl-x_L overexpression could also account for the development of resistance to cell death induced by anthracyclines in leukemic cells.