Increase in gamma-glutamylcysteine synthetase activity and steady-state messenger RNA levels in melphalan-resistant DU-145 human prostate carcinoma cells expressing elevated glutathione levels.

The biochemical and molecular basis for the elevation of glutathione (GSH) levels commonly detected in many drug-resistant cells has not been elucidated. In a series of L-phenylalanine mustard-resistant human prostate carcinoma cell lines (DU-145), resistance was associated with elevated GSH levels, increased activity of gamma-glutamylcysteine synthetase (GCS), the rate-limiting enzyme in GSH biosynthesis, and a marked increase in the steady-state levels of GCS-specific transcripts (4.0 and 3.2 kilobases). Loss of the resistant phenotype was accompanied by a reduction in GSH and a return of GCS activity and transcript levels to values comparable to those detected in the drug-sensitive parent cells. These data strongly implicate up-regulation of GCS activity as an important mechanism in the evolution of drug resistance associated with increased levels of intracellular GSH. The results further suggest that the ability to synthesize GSH may be more indicative of resistance than steady-state GSH levels per se.