Pleiotropic drug resistance and gene amplification in a SEWA mouse tumor cell line. Complex relations revealed by drug uptake data, and lipid and protein analysis.

SEWA mouse lines resistant to actinomycin D (AMD) or vincristine (VCR) exhibit the pleiotropic drug resistance (PDR) phenotype, and express a low-MW protein (p21) and numerous double minutes (DM). In drug uptake studies these lines were compared with the non-resistant parental line and with a methotrexate (MTX)-resistant line, not exhibiting PDR. On treatment with labelled AMD or VCR the two PDR lines displayed a highly reduced intracellular content of drug, whereas uptake of MTX was unchanged. Uptake of AMD was shown to be temperature-dependent. The MTX-resistant line did not exhibit any significant change in AMD or VCR uptake. Other workers have emphasized the role of a high-MW glycoprotein in the development of PDR. A search for a similar glycoprotein in our cells was unsuccessful. Since all indications point to membrane factors being important in the development of PDR, the lines were also subjected to lipid analysis. Compared with control cells distinct differences were detected in the lipid composition of all resistant lines (including the MTX-resistant line). In the course of our experiments, the DM in our most AMD-resistant line were replaced by two homogeneously staining regions (HSR). Simultaneously, the overproduction of p21 ceased, but the PDR phenotype persisted. This event tends to implicate a minor role for the p21 protein in PDR, but similar transitions from DM to HSR in other AMD-resistant SEWA lines were not accompanied by a decrease in p21 over-production. Our data point to a complex genetic control of multi-drug resistance.