INVOLVEMENT OF MDR1 P-GLYCOPROTEIN IN MULTIFACTORIAL RESISTANCE TO METHOTREXATE

Cellular resistance to methotrexate (MTX) is believed to be unaffected by expression of MDRI 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 up take of MTX and DHFR gene amplification. These sublines show cross-res istance to drugs of the multi-drug resistance (MDR) family, which is c orrelated with relative resistance to MTX. The MTX-selected sublines s how increased expression and function of the MDRI gene, based on the m easurement of MDRI mRNA, Pgp and rhodamine 123 accumulation. Sequence analysis of the MDRI 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 (M Ab) UIC2 and a monovalent Fab fragment of UIC2. Our results indicate t hat Pgp can contribute to multifactorial resistance to MTX. (C) 1996 W iley-Liss, Inc.