The combined effects of IL-3 and PSC 833 on daunorubicin- and mitoxantrone cytotoxicity in two growth factor-dependent leukemic cell lines

In the present study it was investigated whether and by which mechanisms the co-administration of interleukin-3 (IL-3) and the beta-glycoprotein blocker PSC 833 can augment mitoxantrone (MX) and daunorubicin (DAU) cytotoxicity in two human growth factor dependent P-glycoprotein (P-gp) positive myeloid leukemic cell lines, Mo-7 and GF-D8. Cytotoxicity was determined in MTT assay. Increased cytotoxicity occurred in Mo-7 cells preincubated with 24 h IL-3 followed by 1 h MX (cell survival: 85% +/- 6 vs 68% +/- 2, at 0.05 mu M MX, P < 0.005) or DAU (79% +/- 8 vs 62% +/- 9 at 0.8 mu g/ml DAU, P < 0.05). Similar results were obtained for the GF-D8 cell line. In this cell line, at 0.5 mu M MX the cell survival decreased from 84% +/- 13 to 61% +/- 19 (P < 0.05) and at 5.0 mu g/ml DAU from 102% +/- 8 to 69% +/- 5, P < 0.002). The IL-3 administration did not affect the P-gp and bcl-2 protein expression, cellular MX concentration or MX efflux but coincided with an increased percentage of cells in S-phase and topoisomerase II (topo II)-alpha mRNA and topo II activity especially in the Mo-7 cell line. PSC 833 enhanced DAU cytotoxicity in both cell lines. The administration of IL-3 plus PSC 833 in the Mo-7 cell line resulted in an additive effect on DAU cytotoxicity. At 0.8 mu g/ml DAU and 2,mu g/ml PSC 833, the percentage surviving cells decreased from 62% +/- 9 in the absence of IL-3 to 37% +/- 3 in the presence of IL-3 (P < 0.01). The additive effect of combined treatment was most pronounced in GF-D8 cells which also had the highest P-gl, expression. In contrast, PSC 833 did not modulate the MX effects, irrespective of the presence of IL-3. In summary, the results demonstrate that the combined administration of IL-3 and PSC 833 can enhance the cytotoxic effects of DAU but not MX in these P-gp positive cell lines whereas the effects of MX could be modulated by factors which influence topo II activity.