Lack of correlation between expression and function of P-glycoprotein in acute myeloid leukemia cell lines

In a panel of acute myeloblastic leukemia (AML) cell lines, representative of distinct differentiation stages, we investigated the possible correlation between drug-resistance and both expression and function of the multidrug resistance (MDR)-related P-glycoprotein (P-gp). The AML cell lines were KG1a, KG1, TF1, HEL, ML1, and two non drug-selected P-gp positive subclones originating from HL-60 (HL-60JD) and U937 (U937AQ). All these cells overexpressed the mdr1 gene (analyzed by RT-PCR) and displayed variable levels of P-gp expression. Flow cytometric semi-quantitative evaluation of P-gp with two P-gp specific monoclonal antibodies (MRK16 and UIC2) showed the following P-gp expression hierarchy: TF1 < KG1a < HEL < KG1 < HL-60JD < ML1 < U937AQ; the latter expressing 13 times more P-gp than TF1. When P-gp function was assessed by Rhodamine 123 (Rh123) efflux kinetics, we found that only KG1a and KG1 cells, which have an early (immature) CD34+ CD33- CD38- phenotype, and to a lesser extent TF1, with an intermediate (CD34+ CD33+ CD38+) phenotype, displayed significant P-gp activity which could be inhibited by both verapamil and SDZ PSC 833. In contrast, the other more mature CD33+ CD34- AML cell lines presented no Rh123 efflux capacity although they expressed higher P-gp levels. Daunorubicin (DNR) accumulation studies showed that inhibitors of P-gp increased DNR accumulation only in the immature AML cells whereas they had no impact on the mature AML cell lines. MTT drug cytotoxicity assay confirmed that the immature AML cells were 10-15-fold more resistant to DNR than the mature AML cells. Although P-gp inhibitors were able to increase the cytotoxicity of DNR in AML cells which displayed functional P-gp, they could not increase DNR cytotoxicity to levels comparable to that of the CD34- CD33+ cells, suggesting that DNR resistance of immature AML cells may not solely be related to P-gp. With drug-selection, AML subclones displayed higher levels of P-gp expression and higher extruding capacities, and therefore chemoresistance, and this independently of their initial differentiation phenotype. Finally, this study provides evidence for a lack of correlation between expression and function of P-gp in AML cells; this relationship being dependent upon leukemic cell differentiation in unselected myeloid leukemic cells.