The Effects of Glutathione (GSH) Depletion on Liver and Lung Heme Oxygenase(HO) in Neonatal Rats † 313

Heme oxygenase, the rate-limiting enzyme in heme catabolism, has been proposed to contribute to antioxidant defense. We previously reported that within 12 h of birth, hepatic HO-1 is inducible by CoCl₂ administration(Tom et al., 1996). Tomoro proposed that CoCl₂ induces hepatic HO-1 in adult rats via GSH depletion and oxidant stress mechanisms (Tomoro et al., 1991). In our studies in neonatal rats, hepatic GSH levels were decreased, but only by 10%, and lung GSH levels were unaffected by CoCl₂ administration. In the present study, we sought to determine whether comparable depletion of GSH achieved by diethyl maleate(DEM) administration would induce liver and lung HO-1 in neonatal rats. DEM depletes GSH by conjugation, but does not stimulate oxidation. Neonatal rats at 24 h of life were injected with DEM (0, 0.25, 0.5, 0.75, 1.0, or 1.5 mmol/kg BW) or an equal volume of corn oil in controls. At 0, 0.5, 1, 3, 6, 12, or 24 h later, liver and lung tissue were analyzed for GSH and HO-1 mRNA levels (n=3). At 0.5 h, increasing doses of DEM caused increasing GSH depletion in liver (33-78%) and lung (22-80%). In liver and lung at DEM doses of 0.25-1.5 mmol/kg, GSH levels decreased by 0.5 h, rebounded by 3-6 h, overshot at 12 h, and decreased to control levels by 24 h. In liver, increases in HO-1 levels were not observed until GSH depletion of > 44% (p < 0.05, 1-way ANOVA with Student Newman-Keuls test) were observed (DEM > 0.5 mmol/kg). HO-1 mRNA increased by 1 - 4-fold after a lag time of 3 - 6 h. With 0.25 mmol/kg DEM (33% GSH depletion), no increases in hepatic HO-1 mRNA levels were detected. In lung, despite GSH depletion, no increases in HO-1 mRNA were detected. The data suggest that CoCl₂ probably induces hepatic HO-1 in neonatal rats by mechanisms other than by GSH depletion. The increases in hepatic HO-1 mRNA associated with > 44% depletion of GSH suggest that similar levels of GSH depletion may initiate induction of hepatic HO-1. On the other hand, lung HO-1 mRNA was not induced by an even greater depletion of GSH. The difference observed in HO-1 mRNA between liver and lung tissue in response to GSH depletion suggests significant differences in regulation of the HO-1 gene in these two tissues.