POSSIBLE COEXISTENCE OF 2 INDEPENDENT MECHANISMS CONTRIBUTING TO ANTHRACYCLINE RESISTANCE IN LEUKEMIA-P388 CELLS

Murine leukaemia P388 and L1210 cell sublines with varying degrees of resistance to the anthracycline daunomycin (DNM) have been used to mon itor (i) intracellular accumulation of DNM, (ii) expression of the dru g efflux pump P-glycoprotein (pgp) and (iii) cytoplasmic pH changes. D rug-resistant L1210/65 cells (65-fold resistance), overexpress pgp, an d display decreased intracellular accumulation of DNM and identical in tracellular pH as compared to the parental drug-sensitive L1210 cell l ine. On the other hand, moderately drug-resistant P388/20 cells (20-fo ld resistance), which also exhibit a decreased intracellular drug accu mulation with respect to drug-sensitive P388/S cells, display only mod erate pgp-encoding mdr1 gene transcription without detectable levels o f pgp protein, and undergo cytoplasmic alkalinisation (up to approxima tely 0.2 pH units). A further increase in the level of drug resistance (P388/100 cells, 100-fold resistance), results in a more pronounced d ecrease in drug accumulation, significant pgp expression and slightly higher intracellular alkalinisation. Alterations in the degree of prot onation of DNM have been shown previously to influence processes such as the rate of uptake and the intracellular accumulation of the drug. On this basis, we propose that the changes in intracellular pH, observ ed at low levels of drug resistance (P388/20 cells), could constitute an early cellular response aimed at decreasing the intracellular accum ulation of ionisable anti-neoplastics. As the level of resistance incr eases (P388/100), the cells seem to require more efficient mechanisms of defense against the drug, such as that represented by the expressio n of pgp. Since there is no apparent correlation between the extent of the changes in intracellular pH and the level of pgp expression in DN M-resistant P388 cell sublines, it is suggested that these two cellula r responses contributing to drug resistance could operate independentl y.