Project Summary (1995-2000)

When humans and animals are exposed to toxic environmental chemicals, such as those present at Superfund sites, assessment of actual toxicant-induced injury often is hampered by uncertainty regarding the effects of long-term, low-dose exposures. Better information about the effects of such exposures can reduce these uncertainties and allow more accurate assessments. At low doses, many chemicals are known to cause changes in the expression of "stress response" genes, which can be measured as changes in the steady state level of messenger RNA transcribed from these genes. One common form of toxicant-mediated stress is oxidative stress, which results in mRNA induction for a number of enzymes involved in antioxidant defense. A very important enzyme involved in antioxidant defense is gamma-glutamylcysteine synthetase (GCS), the rate-limiting enzyme for the synthesis of the cellular antioxidant glutathione (GSH).

The primary goal of this project is to investigate the genetic and biochemical bases of GCS regulation in mice and humans. Researchers are testing three hypotheses relevant to this goal. The first hypothesis is that exposure of cells and/or animals to glutathione-depleting xenobiotics will result in an increase in the mRNA levels for one or both of the GCS subunits, which will be accompanied by an increase in GCS activity and GSH levels. The second hypothesis is that DNA-sequence regulatory elements in the 5' region of mouse GCS genes confer tissue-specific regulation of the basal and inducible levels of the heavy and light subunit GCS mRNAs. The third hypothesis is that individual variation among humans in GCS activity and inducibility may be partially attributable to genetic polymorphisms in the 5' regulatory regions of either the heavy or light subunit genes of GCS. Such information will be useful not only as a biomarker of exposure to toxicants, but potentially as a biomarker of susceptibility to such exposures. Ultimately this information will be useful for gauging the response of people and animals to low-dose contaminant exposures at Superfund sites.