Abstract
Gamma‐glutamylcysteine synthetase (γ‐GCS) is a heterodimer consisting of heavy (γ‐GCSh) and light (γ‐GCSl) subunits. γ‐GCS catalyzes the rate‐limiting de novo biosynthesis of glutathione (GSH), an abundant physiological antioxidant that plays important roles for regulating oxidative stress. Expression of γ‐GCSh and γ‐GCSl are sensitive to oxidative stress. To investigate whether expression of γ‐GCS is correlated with tumor progression, we used immunohistochemical approaches to examine 16 human colorectal adenomas and resected 57 carcinomas from untreated patients. In adjacent normal colorectal epithelium, levels of γ‐GCSh expression were low. Strong cytoplasmic staining for γ‐GCSh was detected in 3 (18.8%) adenoma and 48 (84.2%) carcinomas. The frequency of γ‐GCSh expression in carcinoma was significantly higher than in adenoma (p<0.0001). We used RNase protation assay and Western blot to determine levels of γ‐GCSh mRNA and protein from 10 pairs of matched carcinomas with adjacent normal controls. Elevated expression of both γ‐GCSh mRNA and protein were found in 6 cases, suggesting that transcriptional and/or posttranscriptional regulation play an important role in the upregulation of γ‐GCS during colorectal carcinogenesis. We also examined the expression of another redox‐regulated gene, multidrug resistance protein 1 (MRP1). Strong staining for MRP1 was detected in 1 (6.3%) adenoma and 40 (70.2%) carcinomas. The frequency of MRP1 expression in carcinoma was significantly higher than in adenoma ( p<0.0001). Nuclear p53 expression was detected in 30 (52.6%) of carcinomas. There is a significant correlation between γ‐GCSh and MRP1 expression (p=0.013) but not between γ‐GCSh and p53. Since γ‐GCS is a sensor of oxidative stress, these results are consistent with the notion that oxidative stress is associated with colorectal tumor progression. © 2002 Wiley‐Liss, Inc.