Identification of a P-glycoprotein-related protein (mini-P-glycoprotein) which is overexpressed in multidrug resistant cells.

Drug resistant variant cell lines were selected in vitro for resistance to adriamycin or vincristine, P388/ADR and P388/VCR-600, respectively, from P388 murine leukemia cells. These cells were demonstrated by immunoblot studies with P-glycoprotein specific monoclonal antibody (C219) to overexpress not only P-glycoprotein but also approximately 65 kDa protein, and the expression levels were found to correlate with the degree of resistance. Furthermore, in Northern blot analysis with an MDR1 cDNA as a probe, an overexpressed transcript with the length of about 2.4 kilobases which might encode this previously uncharacterized protein was also detected in the multidrug resistant cell line. Thus, it is suggested that this protein might be associated with multidrug resistant phenotype and be related to P-glycoprotein.