Bcl-x_L is heterogeneously expressed by acute myeloblastic leukaemia cells and is associated with autonomous growth in vitro and with P-glycoprotein expression

Abstract

The cells from approximately 70% patients with acute myeloblastic leukaemia exhibit autonomous growth characteristics in vitro, which have been associated with a poor response to therapy. We have previously shown that leukaemic cells with autonomous growth characteristics express high levels of bcl-2 and are relatively resistant to apoptosis. As bcl-x_L is a bcl-2-related gene with anti-apoptotic activity which also confers resistance to cytotoxic drugs we have studied its expression in AML in relation to cellular growth characteristics and to the expression of P-glycoprotein. Cells from 15 patients were studied. Immunoblotting demonstrated bands at 31 kDa corresponding to bcl-x_L from the cells of all patients. Bcl-x_S was not detected in any sample. Using standardised, quantitative flow cytometry, bcl-x_L expression ranged from 0.25 × 10⁵ to 4.24 × 10⁵ bound FITC molecules, (median 1.35 × 10⁵). AML blasts with autonomous growth in vitro expressed more bcl-x_L (median 1.76 × 10⁵) than those which did not (median 0.86 × 10⁵, P = 0.01). Quantitative bcl-x_L expression strongly correlated with that of P-glycoprotein, also measured by quantitative flow cytometry using the MRK16 antibody (r = 0.95, P < 0.001), but not with mrpr1. these results provide a further explanation for the poor prognosis associated with autonomous in vitro growth of AML blasts and illustrate that these cells may co-express different modalities of resistance to cytotoxic drug therapy involving both anti-apoptotic pathways (bcl-x_L, bcl-2) and classic multidrug resistance (MDR1). The implication of these findings is that the use of agents to reverse MDR1 function in AML may be unsuccessful in the absence of strategies to reduce resistance to apoptosis.