Student Research: Robert Philip Baer
Mercury induces oxidative stress in rat kidney, characterized by increased production of mitochondrial H2O2 and oxidation of cell constituents. During prolonged exposure to mercury as methyl mercury hydroxide (MMH), this effect is accompanied by a two- to three-fold increase in reduced glutathione (GSH) levels in kidney cells.
Gamma-glutamylcysteine synthetase (GCS) is the first and rate limiting enzyme in the biosynthesis of GSH. Male fischer 344 rats were administered 1, 5, and 10 ppm (corresponding to 0.1, 0.6, and 12 mg/kg/day) MMH via drinking water in three chronic studies (9, 5, and 9 weeks). Controls received distilled water only. Kidney GCS large subunit mRNA levels were slot blot assayed vie the use of mucleic acid probes complimentary to a published rat kidney cDNA sequence.
Accompanying GSH increase, significant GCS mRNA upregulation was demonstrated in all three chronic studies during the third week after initiation of MMH exposure. Complicating the interpretation, separate Northern blot studies detected a high presumed mRNA band; detection of this band appeared to depend on treatment and labeling. GCS message length was found to be 3.2 kb. The toxicological dose-response strategy in rats contrasts the approach taken in recently published papers which confirm constitutively increased GCS mRNA expression in human cell lines, and augments recent toxicological studies in rat cell lines and mice which confirm upregulation of this message. The results suggest GCS mRNA upregulation is an important cellular adaptive response mechanism to minimize MMH-induced oxidative stress.