Abstract
Heme oxygenase (HO) enzymes catalyze the breakdown of heme to iron, carbon monoxide (CO), and biliverdin, which is rapidly converted to bilirubin. HO-2 has been implicated in protection against oxidative stress, ischemia, and traumatic brain injury. The neuroprotective effects of HO-2 have been attributed to the generation of bilirubin, which is an important radical scavenger. However, the mechanism by which HO-2 provides protection is unclear. We utilized the olfactory system as a model to define the roles of HO-2 in glutathione depletion-induced oxidative injury, since olfactory receptor neurons (ORNs) express high levels of HO isoforms. We demonstrated that L-buthionine-[S, R]-sulfoximine (BSO), an inhibitor of glutathione biosynthesis, lowered glutathione levels and induced apoptosis of ORNs. Despite the presence of HO-1 in ORNs, HO-2 null animals displayed increased levels of neuronal death after BSO treatment compared to wild type mice. Levels of bilirubin and cGMP were also reduced in HO-2 null mice. Primary cultures of ORNs confirmed that the neuroprotective role of HO-2 was mediated by bilirubin and cGMP. Taken together, these results suggest that HO-2 plays a major role in neuroprotection from oxidative stress, an effect that is mediated by cGMP and bilirubin.
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