Increased expression of cytosolic glutathione S-transferases in drug-resistant L5178Y murine lymphoblasts: chemical selectivity and molecular mechanisms.

The level of induction of three cytosolic glutathione-S-transferase (GST) classes has been compared in L5178Y murine lymphoblasts resistant to either the quinone-containing compound, hydrolyzed benzoquinone mustard (HBM), or the aromatic alkylating agent aniline mustard (AM). Three established cell lines, L5178Y/HBM2, L5178Y/HBM10, and the partial revertant, L5178Y/HBMR, were 2.5-, 6-, and 2.9-fold resistant to HBM and showed 3-, 11-, and 9-fold increases in GST activity, respectively, relative to the sensitive L5178Y cell line. Western blot analysis of cytosolic proteins showed overexpression of all three cytosolic GST classes pi, alpha, and mu, with predominance of the pi class. Northern blot analysis demonstrated corresponding elevations in the steady-state mRNA levels of each GST class. The level of GST-mu and -alpha isoforms correlated more closely with HBM resistance, whereas GST-pi, the predominant isoform in these cells, paralleled enzyme activity. These findings suggested that other factors such as quinone reductase may contribute to resistance. The AM-resistant cell line L5178Y/AM was 10-fold resistant to the alkylating agent AM, and GST activity was elevated 3.6-fold relative to the parental L5178Y cell line. Western blot analysis and Northern blot analysis provided evidence of overexpression of all three cytosolic GST classes but with marked predominance of the alpha class. These studies provide evidence that induction of GST isoforms in drug-resistant cells may have both a nonspecific as well as a selective component. The difference in isozyme profile between HBM- and AM-resistant cell lines emphasizes how structural differences, in particular, the nature of the electrophilic signal, may influence the pattern of induction of GST isozymes.