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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. [ABSTRACT FROM AUTHOR]