GLUTATHIONE DEPLETION INDUCES HEME OXYGENASE-1 (HSP32) MESSENGER-RNA AND PROTEIN IN RAT-BRAIN

In mammalian systems, the heme oxygenase (HO) isozymes HO-1 (HSP32) and HO-2 oxidatively cleave the heme molecule to produce bile pigments and carbon monoxide. Although HO-1 is inducible by various chemicals in systemic organs and cell culture systems, this communication reports for the first time the induction of this stress protein and its transcript by a chemical in the brain. In addition, this study demonstrates expression of HO-1 in select populations of cells in the brain in response to GSH depletion. Specifically, treatment of adult rats with diethyl maleate (DEM; 4.7 mmol/kg) caused a pronounced decrease in brain GSH content within 1 h. GSH levels remained significantly depressed for at least 24 h postinjection. Northern blot analysis of brain poly(A)+ mRNA following DEM treatment revealed on the average a sixfold increase in the 1.8-kb HO-1 mRNA level compared with that of controls; concomitant with this change was a decrease in GSH levels. Total brain HO activity was not significantly altered along with the increase in HO-1 mRNA level. The increase in transcription of HO-1 was a direct response to GSH depletion, as judged by the observation that treatment of neonatal rats with L-buthionine-(SR)-sulfoximine (BSO) (3 mmol/kg, twice daily, for 2 days), a selective inhibitor of GSH synthesis, caused a marked depression in total brain GSH level and a concomitant increase in brain 1.8-kb HO-1 mRNA content. The magnitude of the increase was up to approximately 11.5-fold that of the control level, as evidenced by northern blot analysis. In contrast, the level of two homologous HO-2 transcripts (1.3 and 1.9 kb) did not increase in response to either DEM or BSO treatment. Analysis of brain HO-1-immunoreactive protein following DEM treatment for 9 h indicated induction of HO-1 protein in only select nonneuronal cell populations. In particular, the ependymal cells lining ventricles throughout brain, Bergmann glia of cerebellum, and leptomeninges lining brain and glia throughout brain responded to treatment by increasing the level of HO-1-like protein. We suggest that when GSH is depleted, an increase in HO-1 protein content, resulting in increased capacity to form bile pigments, may be of significance to cells with compromised antioxidant capability. Bile pigments are potent antioxidants in biological systems.