The MRD1 (P-glycoprotein) and MRP (P-190) transporters do not play a major role in the intrinsic multiple drug resistance of Jurkat T lymphocytes.

The response of T cells in relation to the cell cycle has not been extensively studied. We have attempted to address this question using Jurkat T cells treated with cytostatic drugs known to arrest cells at various transition points of their cycle. We tested various concentrations of drugs that act at G1/S (hydroxyurea, lovastatin, thymidine), early S [aphidicolin, cyclosporin A (CsA), rapamycin] or G2 + M (colchicine, nocodazole) in 24 h cultures. Cytofluorimetric analyses showed that cycling Jurkat cells were equally distributed between the G1 (44.9 +/- 6.5%) and S (42.3 +/- 8.0%) phases. Cell distribution in G2 + M was 12.7 +/- 2.8%. Hydroxyurea but not lovastatin increased the percentage of cells in S phase to ca 60-70% and both drugs decreased it to ca 30% in G1. Thymidine had no effects. Aphidicolin increased the distribution in S phase to ca 70% with a decrease in G1 to ca 30%. CsA and rapamycin increased the percentage of the cells in G1 to ca 70% and decreased it to ca 25% in S phase. Nocodazole increased cell distribution in G2 + M to ca 60% and induced a decrease in G1 to ca 10%. The effects of the drugs were not related to their toxicity and their limited efficiency raised the possibility that Jurkat cells possessed an intrinsic resistance to these xenobiotics. Time-course analysis showed (scanning electron microscopy) that the early morphological changes induced by colchicine were reversible. Drug efflux experiments (vinblastine) suggested that an ATP-dependent process could be involved. However, Northern blot analyses showed a weak signal for MDR1 (MDR, multiple drug resistance). In contrast, a probe for multidrug resistance-associated protein (P-190; MRP) showed a strong signal in Jurkat and peripheral lymphocytes. The presence of drugs (CsA, nocodazole, thymidine) (24 h) did not up-regulate its message and cell treatment with BSO only moderately affected the efficiency of the glutathione S-conjugate MRP transporter. Our data suggest that the intrinsic multidrug resistance of leukemic Jurkat T cells does not appear to involve the MDR1 and MRP members of the ABC family of reverse drug transporters and these observations raise the possibility of the involvement of multi-faceted mechanisms.