Induction of Heme Oxygenase-1 Expression in Vascular Smooth Muscle Cells

Abstract

Endotoxic shock is a life-threatening consequence of severe Gram-negative infection characterized by vascular smooth muscle cell relaxation and severe hypotension. The production of nitric oxide (NO), through the inducible NO synthase pathway, has been implicated as a major contributor in this process. We now demonstrate that heme oxygenase (HO), an enzyme that generates carbon monoxide (CO) in the course of heme metabolism, may also be involved in the hemodynamic compromise of endotoxic shock. Inducible HO (HO-1) mRNA levels are dramatically increased in aortic tissue from rats receiving endotoxin, and this increase in vascular HO-1 message is associated with an 8.9-fold increase in HO enzyme activity in vivo. Immunocytochemical staining localizes an increase in HO-1 protein within smooth muscle cells of both large (aorta) and small (arterioles) blood vessels. Furthermore, zinc protoporphyrin IX, an inhibitor of HO activity, abrogates endotoxin-induced hypotension in rats. Studies performed in rat vascular smooth muscle cells in vitro show that the induction of HO-1 mRNA is regulated at the level of gene transcription, and this induction is independent of NO production. Taken together, these studies suggest that the up-regulation of HO-1, and the subsequent production of CO, contributes to the reduction in vascular tone during endotoxic shock.Endotoxemia leading to shock is a detrimental consequence of severe Gram-negative bacterial infection. Endotoxic shock is initiated by the release of bacterial cell wall-derived lipopolysaccharide (LPS) 1 and the subsequent production of cytokines and vasoactive mediators that result in vascular smooth muscle cell relaxation and hypotension (1, 2). One of the most important cytokines in the cascade of events leading to LPSinduced hypotension is interleukin (IL)-1␤ (1, 3). We have demonstrated previously that IL-1␤ stimulates the inducible isoform of nitric oxide synthase (NOS) and increases the production of NO in vascular smooth muscle cells (4). NO is a labile, free radical gas that acts as a potent vasodilator (5, 6). The importance of NO in the pathogenesis of endotoxic shock has been emphasized by recent studies demonstrating that mice carrying a disrupted inducible NOS gene have an attenuated hypotensive-response to LPS (7) and are resistant to LPS-induced death (7,8). However, the study by MacMicking and colleagues (7) also suggested that an inducible NOS-independent pathway contributes to LPS-induced hypotension and death, and we hypothesize that one potential pathway involves heme oxygenase (HO).HO is the enzyme that generates carbon monoxide (CO) and biliverdin (subsequently reduced to bilirubin) in the course of heme metabolism (9). CO is a gas molecule that shares some of the properties of NO, inasmuch as CO binds to the heme moiety of cytosolic guanylyl cyclase to produce cGMP (10). Two distinct forms of heme oxygenase have been identified (9): HO-1 (an inducible isozyme) and HO-2 (a non-inducible isozyme). Morita and colleagues (11) have demonstrated that HO-...

Carbon Monoxide Generated by Heme Oxygenase-1 Suppresses the Rejection of Mouse-to-Rat Cardiac Transplants

“…This include the ability of CO to promote vasodilatation (36) by the induction of smooth muscle cell relaxation (26,37). Vasodilatation may contribute to prevent vascular thrombosis, which would be important in suppressing graft rejection.…”

The reciprocal relationship between heme oxygenase and nitric oxide synthase in the organs of lipopolysaccharide-treated rodents

“…The similarities and diversities of the NO/NOS and CO/ HO systems were well established in numerous studies (11,14), but the interactions between these systems have not yet been elucidated. The interactive behavior of the two systems has been examined in monocytic cells (7), vascular endothelial cells (4), and vascular smooth muscle cells (22), and accumulating evidence from these studies has shown that the NO/NOS system induces the CO/HO system, while the CO/HO system reciprocally regulates the NO/NOS system. The relationship between the two systems in whole organs, on the other hand, has not been well studied, and it is unknown whether the two systems interact in the same manner in all organs or whether the two systems interact in an organ-specific manner.…”

Induction of Heme Oxygenase 1 by Nitrosative Stress

“…The common conception that these enzymes are merely components of a catabolic pathway that facilitates the elimination of toxic products from the organism has been disputed by strong evidence demonstrating that endogenously generated CO and bilirubin act as crucial effector molecules in the mitigation of vascular and cellular dysfunction (4 -12). Disparate conditions and a number of pathological states including hypoxia, endotoxic shock, atherosclerosis, and inflammation have been found to promote overexpression of the HO-1 gene and increased heme oxygenase activity (13)(14)(15)(16)(17)(18). Although the molecular mechanism(s) leading to HO-1 induction by these and other conditions remains to be fully elucidated, a common denominator that characterizes the prompt stimulation of HO-1 under most circumstances is the transient decrease in cellular glutathione levels and a drastic change in the redox status of the intracellular milieu (15, 19 -21).…”