High level resistance to glucocorticoids, associated with a dysfunctional glucocorticoid receptor, in childhood acute lymphoblastic leukemia cells selected for methotrexate resistance

The molecular basis for the clinical presentation of broad-range drug resistance in childhood ALL is poorly understood. In this study, high level cross-resistance to the glucocorticoid dexamethasone was encountered in a childhood ALL cell line selected for resistance to methotrexate (CEM MTX-R3). Compared with wild-type (WT) CEM cells, MTX-R3 cells had significantly fewer glucocorticoid binding sites, as well as reduced glucocorticoid receptor protein and mRNA levels. DNA sequencing and restriction fragment-length polymorphism (RFLP) analysis showed that WT cells expressed both a wild-type and a mutant (GR753F) glucocorticoid receptor allele, while MTX-R3 cells expressed only the GR753F allele. Therefore, the cross-resistance of MTX-R3 cells to dexamethasone appeared due to loss of expression of the wild-type glucocorticoid receptor allele. In an effort to gain insight into the underlying basis for the development of cross-resistance to methotrexate and glucocorticoids, glucocorticoid receptor nuclear translocation experiments were carried out. Exposure of WT cells to either dexamethasone or the cytotoxic agents cytarabine and methotrexate caused translocation of the glucocorticoid receptor from the cytoplasm into the nucleus. These data indicate that exposure of childhood ALL cells to cytotoxic agents may result in ligand-independent glucocorticoid receptor activation which, in the context of the outgrowth of drug-resistant cells, could lead to the co-selection of glucocorticoid resistance.

P-glycoprotein-mediated methotrexate resistance in CCRF-CEM sublines deficient in methotrexate accumulation due to a point mutation in the reduced folate carrier gene

We have previously described a series of methotrexate (MTX)-selected CCRF-CEM sublines (CEM/MTX R1-3) displaying increased resistance to drugs associated with the multidrug resistance phenotype and have provided evidence that MDR1 P-glycoprotein contributes to multifactorial MTX resistance in these cells. We have also suggested that P-glycoprotein-mediated MTX transport arises in these cells due to a deficiency in the normal MTX transport route, the reduced folate carrier (RFC). We have now determined the nucleotide sequence of the RFC gene in CEM/MTX R1-3 cells and confirm that the carrier is defective in these cells as a result of a premature stop mutation at codon 99, which severely truncates the encoded protein. CEM/MTX R3 cells were removed from MTX, and a series of sublines with increasing MDR1 expression were derived, following selection with vincristine. These cells show increasing cross-resistance to vincristine as well as other drugs associated with the multidrug resistance phenotype. More importantly, the increased P-glycoprotein expression correlates with increased resistance to MTX, supporting the hypothesis that in cells with a defective carrier protein, MTX can become a substrate for P-glycoprotein. Our data have implications for the P-glycoprotein-mediated transport of other hydrophilic drugs in situations where the relevant carrier protein has been functionally inhibited.

Involvement of MDR1 P-glycoprotein in multifactorial resistance to methotrexate

Cellular resistance to methotrexate (MTX) is believed to be unaffected by expression of MDR1 P-glycoprotein (Pgp), a pleiotropic efflux pump acting on different hydrophobic compounds that enter cells by passive diffusion. A series of human leukemic CCRF-CEM sublines, isolated by multi-step selection for very high resistance to MTX, exhibit multiple mechanisms of MTX resistance, including decreased carrier-mediated uptake of MTX and DHFR gene amplification. These sublines show cross-resistance to drugs of the multi-drug resistance (MDR) family, which is correlated with relative resistance to MTX. The MTX-selected sublines show increased expression and function of the MDR1 gene, based on the measurement of MDR1 mRNA, Pgp and rhodamine 123 accumulation. Sequence analysis of the MDR1 cDNA from MTX-selected CCRF-CEM cells revealed no mutations in the protein coding region. MTX resistance in these cell lines is partially reversible by a Pgp-specific monoclonal antibody (MAb) UIC2 and a monovalent FaB fragment of UIC2. Our results indicate that Pgp can contribute to multifactorial resistance to MTX.