The structure of P-glycoprotein and the secretion of lysosomal enzymes in multidrug-resistant cells.

We have previously demonstrated that multidrug-resistant cells have a lower content of lysosomal enzymes, a consequence of an increased rate of secretion. The question was therefore to know whether an intact functional P-glycoprotein was necessary for expression of this property. Control NIH3T3 and mdr1-gene-transfected cells (pHaMDR1) were used together with 2 variants either lacking 23 amino acids at the carboxyl terminal (pHaMDRC 23) or in which 4 extra amino acids are inserted (pHaMDRBL2). Transfected and variant cells exhibited reduced uptake of [3H]-vinblastine and [3H]-daunomycin, a finding consistent with their drug resistance. By contrast, only pHaMDR1 cells had a reduced level of N-acetyl glucosaminidase that paralleled an increased rate of secretion of the same enzyme. The mutant cells secreted lysosomal enzyme at the same rate and had the same intracellular lysosomal enzyme content as NIH3T3 cells. Abnormal behavior of lysosomal enzymes in multidrug-resistant cells therefore seemed to require an intact P-glycoprotein molecule. Although sequestration in lysosomes and then secretion of drugs may possibly contribute to protection, it would not be an essential component of multidrug resistance.