Heme Oxygenase-1: Function, Regulation, and Implication of a Novel Stress-Inducible Protein in Oxidant-Induced Lung Injury

Accumulating evidence suggests that oxidative stress plays a central role in the pathogenesis of many pulmonary diseases including adult respiratory distress syndrome, emphysema, asthma, bronchopulmonary dysplasia, and interstitial pulmonary fibrosis. The morbidity and mortality of these diseases remain high even with optimal medical management. In our attempts to devise new therapies for these disorders, it is crucial to improve our understanding of the basic mechanism(s) of oxidant-induced lung injury. A major line of investigation seeks to characterize the cellular and molecular responses of the lung to oxidant insults. Much progress has been made in our understanding of the role of the "classic" antioxidant enzymes (e.g., superoxide dismutase, catalase, glutathione peroxidase) in mediating the lung's resistance against oxidant lung injury. However, it is becoming clear that other oxidant-induced gene products may also play vital roles in the lung's adaptive and/or protective response to oxidative stress. One such stress-response protein is heme oxygenase-1, HO-1. Since the identification of HO-1 in 1968, many of the studies involving this enzyme were understandably focused on the regulation and function of HO-1 in heme metabolism. This emphasis is self-evident as HO-1 catalyzes the first and rate-limiting step in heme degradation. Interestingly, however, evidence accumulated over the past 25 years demonstrates that HO-1 is induced not only by the substrate heme but also by a variety of non-heme inducers such as heavy metals, endotoxin, heat shock, inflammatory cytokines, and prostaglandins. The chemical diversity of HO-1 inducers led to the speculation that HO-1, besides its role in heme degradation, may also play a vital function in maintaining cellular homeostasis. Further support for this hypothesis was provided by Tyrrell and colleagues who showed in 1989 that HO-1 is also highly induced by a variety of agents causing oxidative stress. Subsequently, many investigators have focused their attention on the function and regulation of HO-1 in various in vitro and in vivo models of oxidant-mediated cellular and tissue injury. The magnitude of HO-1 induction after oxidative stress and the wide distribution of this enzyme in systemic tissues coupled with the intriguing biological activities of the catalytic byproducts, carbon monoxide, iron, and bilirubin, makes HO-1 a highly attractive and interesting candidate stress-response protein which may play key role(s) in mediating protection against oxidant-mediated lung injury. This review will focus on the current understanding of the physiological significance of HO-1 induction and the molecular regulation of HO-1 gene expression in response to oxidative stress. We hope that this discussion will stimulate interest and investigations into a field which is still largely uncharted in the pulmonary research community.