BCL-2 PROTEIN DOWN-REGULATION IS NOT REQUIRED FOR DIFFERENTIATION OF MULTIDRUG-RESISTANT HL-60 LEUKEMIA-CELLS

Parental and multidrug resistant HL60 leukemia cell lines were used to study coupling of expression of apoptotic/cytostatic (bcl-2, bax, bcl xL, p21/Waf1, and c-myc) genes during differentiation. The multidrug r esistant HL60 cell line, HL60/ADR, was less sensitive than parental ce lls to cytostatic activity of low (0.4-2 ng/ml) doses of PMA. However, during treatment with standard differentiating doses of PMA (10 ng/ml ), no difference between the two cell lines in cytostasis and differen tiation was found, Downregulation of c-myc and upregulation of p21/Waf 1 proteins showed the same time-course in both cell lines. The bcl-2 m RNA was rapidly downregulated while bax and bclxL gene expression was not altered in both differentiating HL60 and HL60/ADR cells. Significa nt downregulation of bcl-2 protein occurred only in parental HL60 cell s. In HL60/ADR, despite rapid cessation of bcl-2 protein synthesis, al most no change in steady-state bcl-2 protein level was found. The lack of bcl-2 protein downregulation was a result of the prolonged half-li fe of this protein in HL60/ADR cells. Thus, although downregulation of bcl-2 mRNA is coupled to differentiation, actual loss of bcl-2 protei n is not required for accomplishment of the differentiation program.