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

Abstract

Mouse-to-rat cardiac transplants survive long term after transient complement depletion by cobra venom factor and T cell immunosuppression by cyclosporin A. Expression of heme oxygenase-1 (HO-1) by the graft vasculature is critical to achieve graft survival. In the present study, we asked whether this protective effect was attributable to the generation of one of the catabolic products of HO-1, carbon monoxide (CO). Our present data suggests that this is the case. Under the same immunosuppressive regimen that allows mouse-to-rat cardiac transplants to survive long term (i.e., cobra venom factor plus cyclosporin A), inhibition of HO-1 activity by tin protoporphyrin, caused graft rejection in 3–7 days. Rejection was associated with widespread platelet sequestration, thrombosis of coronary arterioles, myocardial infarction, and apoptosis of endothelial cells as well as cardiac myocytes. Under inhibition of HO-1 activity by tin protoporphyrin, exogenous CO suppressed graft rejection and restored long-term graft survival. This effect of CO was associated with inhibition of platelet aggregation, thrombosis, myocardial infarction, and apoptosis. We also found that expression of HO-1 by endothelial cells in vitro inhibits platelet aggregation and protects endothelial cells from apoptosis. Both these actions of HO-1 are mediated through the generation of CO. These data suggests that HO-1 suppresses the rejection of mouse-to-rat cardiac transplants through a mechanism that involves the generation of CO. Presumably CO suppresses graft rejection by inhibiting platelet aggregation that facilitates vascular thrombosis and myocardial infarction. Additional mechanisms by which CO overcomes graft rejection may involve its ability to suppress endothelial cell apoptosis.

Heme oxygenase‐1 in growth control and its clinical application to vascular disease

“…This inhibitory action of HO-1 is mediated by CO; however, it does not involve the cGMP signaling pathway since the administration of lipophilic analogues of cGMP or the inhibition of soluble guanylate cyclase has no effect on the expression of either of these mitogens. Similarly, studies in our laboratory and others found that HO-1-derived CO inhibits platelet aggregation, thereby preventing the release of growth factors from platelet alpha granules (Wagner et al, 1997;Sato et al, 2001). In this case, the anti-aggregatory effect of CO is mediated by cGMP since it is associated with a marked increase in platelet cGMP and is prevented by soluble guanylate cyclase inhibitors (Brune and Ullrich, 1987;Wagner et al, 1997).…”

“…Inhalation of low doses of CO (10-500 ppm) has been shown to protect tissues against hyperoxia, transplant rejection, and ischemia-reperfusion (Otterbein et al, 1999;Fujita et al, 2001;Sato et al, 2001). Presently, comparable concentrations of inhaled CO are used diagnostically to estimate lung diffusing capacity in patients, raising the possibility that inhaled CO may be used therapeutically .…”

Carbon Monoxide-Induced Early Thrombolysis Contributes to Heme Oxygenase-1-Mediated Inhibition of Neointimal Growth after Vascular Injury in Hypercholesterolemic Mice

“…It is a complex process influenced by coagulation and fibrinolytic pathways [9,10], inflammation [11], and hypercholesterolemic state [12]. The first line of evidence supporting the anti-thrombotic effect of HO-1 in vivo is the observation that CO suppressed vascular thrombosis occurring during the cardiac graft rejection likely through inhibiting platelet aggregation [13]. Subsequently, there was a study showing that CO protected ischemic lung injury through down regulating the expression of plasminogen activator inhibitor-1 (PAI-1), the principal regulator of the fibrinolytic system, in macrophages and derepressing fibrinolysis [14].…”

Non-heme Induction of Heme Oxygenase-1 Does Not Alter Cellular Iron Metabolism

“…Carbon monoxide, which is chemically very similar to nitrogen monoxide (NO) (11), can bind to the heme groups in various proteins to modulate their function. By this property, it has been shown that CO may initiate intracellular signaling cascades related to cell survival, mimicking the effects of HO-1 induction (2,5,12,13). Finally, when heme is provided to cells, the generation of Fe via the HO-1-catalyzed reaction induces ferritin (Ft; ubiquitous iron storage protein) synthesis (14), which Ferris et al (15) demonstrated to decrease reactive oxygen species within cells by sequestering catalytic iron.…”