Down-regulation of bcl-2 in AML blasts by all-trans retinoic acid and its relationship to CD34 antigen expression

High levels of expression of the bcl-2 oncoprotein in acute myeloblastic leukaemia (AML) cells have been associated with low complete remission rates and poor survival. The sensitivity of AML blasts to drugs such as Ara-C can be increased by the down-regulation of bcl-2 expression by antisense oligonucleotides. All-trans retinoic acid (ATRA) has been reported to increase the sensitivity of AML cell lines to Ara-C and to induce differentiation in the HL60 promyelocytic cell line, with both effects being accompanied by a decrease in bcl-2 expression. Using flow cytometry and a monoclonal antibody to bcl-2, we have investigated the effects of ATRA (1 mu M) on bcl-2 expression in the blast cells of 25 AML patients and the K562 cell line after incubation for 72 or 24 h, respectively. Using kolmogorov-Smirnov statistical analysis where a D value of > 0 . 12 was statistically significant, we found that in 8/25 AML samples and the K562 cells there was a significant decrease in bcl-2 protein expression after incubation with ATRA (D value range 0 . 14-0 . 44). The mean peak fluorescence (MPF) values for the bcl-2 levels of the ATRA responders (n = 8) was reduced to 35 . 5 +/- 6 . 9 following incubation with ATRA compared to 47 . 6 +/- 8 . 2 (mean +/- SEM) for control samples incubated in the absence of ATRA (P = 0 . 014). There was no significant difference between the baseline bcl-2 molecules of equivalent soluble fluorochrome (MESF) levels in the ATRA responders (48 . +/- 5 . 7, n = 8) and the non-responders (41 . 3 +/- 3 . 9, n = 17) (mean +/- SEM) (P = 0 . 28). The down-regulation of bcl-2 expression by ATRA was particularly associated with CD34-negative AML and of the eight AML patients' cells that responded to ATRA by down-regulating bcl-2, seven were CD34 negative (P < 0 . 05). Our data suggest that the addition of ATRA to combination chemotherapy would increase the chemosensitivity of some patients with AML, particularly CD34-negative AML, due to downregulation of bcl-2 expression.