The transcription factor Nrf2, which normally exists in an inactive state as a consequence of binding to a cytoskeleton-associated protein Keap1, can be activated by redox-dependent stimuli. Alteration of the Nrf2–Keap1 interaction enables Nrf2 to translocate to the nucleus, bind to the antioxidant-responsive element (ARE) and initiate the transcription of genes coding for detoxifying enzymes and cytoprotective proteins. This response is also triggered by a class of electrophilic compounds including polyphenols and plant-derived constituents. Recently, the natural antioxidants curcumin and caffeic acid phenethyl ester (CAPE) have been identified as potent inducers of haem oxygenase-1 (HO-1), a redox-sensitive inducible protein that provides protection against various forms of stress. Here, we show that in renal epithelial cells both curcumin and CAPE stimulate the expression of Nrf2 in a concentration- and time-dependent manner. This effect was associated with a significant increase in HO-1 protein expression and haem oxygenase activity. From several lines of investigation we also report that curcumin (and, by inference, CAPE) stimulates ho-1 gene activity by promoting inactivation of the Nrf2–Keap1 complex, leading to increased Nrf2 binding to the resident ho-1 AREs. Moreover, using antibodies and specific inhibitors of the mitogen-activated protein kinase (MAPK) pathways, we provide data implicating p38 MAPK in curcumin-mediated ho-1 induction. Taken together, these results demonstrate that induction of HO-1 by curcumin and CAPE requires the activation of the Nrf2/ARE pathway.

Carbon monoxide (CO) is generated in living organisms during the degradation of heme by the enzyme heme oxygenase, which exists in constitutive (HO-2 and HO-3) and inducible (HO-1) isoforms. Carbon monoxide gas is known to dilate blood vessels in a manner similar to nitric oxide and has been recently shown to possess antiinflammatory and antiapoptotic properties. We report that a series of transition metal carbonyls, termed here carbon monoxide-releasing molecules (CO-RMs), liberate CO to elicit direct biological activities. Specifically, spectrophotometric and NMR analysis revealed that dimanganese decacarbonyl and tricarbonyldichlororuthenium (II) dimer release CO in a concentration-dependent manner. Moreover, CO-RMs caused sustained vasodilation in precontracted rat aortic rings, attenuated coronary vasoconstriction in hearts ex vivo, and significantly reduced acute hypertension in vivo. These vascular effects were mimicked by induction of HO-1 after treatment of animals with hemin, which increases endogenously generated CO. Thus, we have identified a novel class of compounds that are useful as prototypes for studying the bioactivity of CO. In the long term, transition metal carbonyls could be utilized for the therapeutic delivery of CO to alleviate vascular- and immuno-related dysfunctions. The full text of this article is available at http://www.circresaha.org.

As a by-product of heme catabolism by the heme oxygenase system, carbon monoxide (CO) has been neglected for many years, and only recently has its role as an essential signaling molecule been appreciated. In the past decade, the use of CO gas in pre-clinical experimental models of disease has produced some remarkable data indicating that its therapeutic delivery to mammals could alleviate inflammatory processes and cardiovascular disorders. However, the inherent toxic nature of CO cannot be ignored, knowing that inhalation of uncontrolled amounts of this gas can ultimately lead to serious systemic complications and neuronal derangements. From a clinical perspective, a key question is whether a safe and therapeutically effective threshold of CO can be reached locally in organs and tissues without delivering potentially toxic amounts through the lung. The advent of CO-releasing molecules (CO-RMs), a group of compounds capable of carrying and liberating controlled quantities of CO in cellular systems, appears a plausible alternative in the attempt to overcome the limitations of CO gas. Although in its infancy and far from being used for clinical applications, the CO-RMs technology is supported by very encouraging biological results and reflected by the chemical versatility of these compounds and their endless potential to be transformed into CO-based pharmaceuticals.

Carbon monoxide, which is generated in mammals during the degradation of heme by the enzyme heme oxygenase, is an important signaling mediator. Transition metal carbonyls have been recently shown to function as carbon monoxide–releasing molecules (CO-RMs) and to elicit distinct pharmacological activities in biological systems. In the present study, we report that a water-soluble form of CO-RM promotes cardioprotection in vitro and in vivo. Specifically, we found that tricarbonylchloro(glycinato)ruthenium(II) (CORM-3) is stable in water at acidic pH but in physiological buffers rapidly liberates CO in solution. Cardiac cells pretreated with CORM-3 (10 to 50 μmol/L) become more resistant to the damage caused by hypoxia-reoxygenation and oxidative stress. In addition, isolated hearts reperfused in the presence of CORM-3 (10 μmol/L) after an ischemic event displayed a significant recovery in myocardial performance and a marked and significant reduction in cardiac muscle damage and infarct size. The cardioprotective effects mediated by CORM-3 in cardiac cells and isolated hearts were totally abolished by 5-hydroxydecanoic acid, an inhibitor of mitochondrial ATP-dependent potassium channels. Predictably, cardioprotection is lost when CORM-3 is replaced by an inactive form (iCORM-3) that is incapable of liberating CO. Using a model of cardiac allograft rejection in mice, we also found that treatment of recipients with CORM-3 but not iCORM-3 considerably prolonged the survival rate of transplanted hearts. These data corroborate the notion that transition metal carbonyls could be used as carriers to deliver CO and highlight the bioactivity and potential therapeutic features of CO-RMs in the mitigation of cardiac dysfunction. The full text of this article is available online at http://www.circresaha.org.