Current Pharmaceutical Design - Volume 9, Issue 30, 2003

Heme oxygenase (HO) is the enzyme catalyzing the degradation of the heme group that produces carbon monoxide (CO), biliverdin and iron. Biliverdin is converted to bilirubin by biliverdin reductase, and iron is used in metabolism or sequestered by ferritin. HO-2 and HO-3 are constitutive isozymes, whereas HO-1 is induced by a variety of stimuli, many of them related to oxidative stress. This inducible isozyme is also a heat shock protein (HSP32). Despite the fact that bilirubin and CO are toxic at high concentrations and that free iron catalyzes the formation of free radicals and induces oxidative stresss, many lines of evidence indicate that HO-1 induction plays a protective role and can be considered part of a defensive response against stress. Expression of HO-1 has been reported in association with a wide range of conditions including fever and inflammation, ischemia / reperfusion damage, endotoxic shock, atherosclerosis, etc. In particular, Dr. Perrella and Dr. Yet [1] have presented data supporting the hypothesis that HO-1 plays an important role in regulating cardiovascular function, with protective effects on ischemia / reperfusion injury. Recent evidence also indicates the participation of HO in pathological situations such as atherosclerosis and restenosis. The mechanisms involved in vascular injury and the properties of HO-1 and bilirubin are discussed by Dr. Ishikawa [2]. Down-regulation of HO-1 is an important tool for establishing the role of this protein in physiological and pathological situations. This can be achieved using molecular biology techniques as most drugs inhibiting HO-1 activity, such as metalloporphyrins, present a limited selectivity. Up-regulation of HO-1 either by targeted gene transfer or by pharmacological means is a promising therapeutic strategy in a number of disorders due to the cytoprotective properties of this enzyme. Understanding the molecular mechanisms involved in HO-1 transcription and translation is the first step for effectively controlling HO-1 expression. In the current issue Dr. Alam and Dr. Cook review [3] the interesting topic of HO-1 gene transcription, which provides a number of targets to control HO-1 protein. Selective overexpression of HO-1 and HO-2 can be achieved by gene transfer. The potential of gene therapy for a number of human diseases such as hypertension, diabetes, cancer, neurological diseases, etc. is highlighted by the contribution of Prof. Abraham [4]. Since HO-1 effects are ascribed in part to the molecules produced by its activity, a possible therapeutic approach uses CO as a pharmacological agent. CO and NO are important mediators that share similar actions in a number of tissues. Due to lack of space, some physiopathological correlations between both molecules have not been discussed. An interesting pharmacological approach is presented by Dr. Motterlini and co-workers [5], who have investigated the potential of transition metal carbonyls as CO-releasing molecules with anti-inflammatory and cytoprotective effects. The presence of HO-1 may be part of an adaptive response against injury during inflammatory processes. Our group [6] has studied several in vitro and in vivo models to assess the contribution of mediators and establish the mechanisms involved in HO-1 protection in inflammatory disorders. This issue presents a general view of the role and interest of HO modulation in human diseases to explore therapeutic alternatives. Finally, I would like to thank all the experts who have agreed to share their experiences and views in this field for their valuable contribution. References [1] Perrella MA, Yet S-F. Role of Heme Oxygenase-1 in Cardiovascular Function. Curr Pharm Design 2003; 9(30): 2479- 2787. [2] Ishikawa K. Heme Oxygenase-1 Against Vascular Insufficiency: Roles of Atherosclerotic Disorders. Curr Pharm Design 2003; 9(30): 2489-2497. [3] Alam J, Cook JL. Transcriptional Regulation of the Heme Oxygenase-1 Gene Via the Stress Response Element Pathway. Curr Pharm Design 2003; 9(30): 2499-2511. [4] Abraham NG. Therapeutic Applications of Human Heme Oxygenase Gene Transfer and Gene Therapy. Curr Pharm Design 2003; 9(30): 2513-2524. [5] Motterlini R, Mann BE, Johnson TR, Clark JE, Foresti R, Green CJ. Bioactivity and Pharmacological Actions of Carbon Monoxide-Releasing Molecules. Curr Pharm Design 2003; 9(30): 2525-2539. [6] Alcaraz MJ, Fernandez P, Guillen MI. Anti-Inflammatory Actions of the Heme Oxygenase-1 Pathway. Curr Pharm Design 2003; 9(30): 2541-2551.

Heme oxygenase (HO) is a cytoprotective enzyme that degrades heme (a potent oxidant) to generate carbon monoxide (a vasodilatory gas that has anti-inflammatory properties), bilirubin (an antioxidant derived from biliverdin), and iron (sequestered by ferritin). Due to the properties of inducible HO (HO-1) and its products, we hypothesized that HO-1 would play an important role in the regulation of cardiovascular function. In this article we will review the role of HO-1 in cardiovascular function, and highlight our previous studies using gene deletion and gene overexpression transgenic approaches in mice. These studies will include the investigation of HO-1 in the setting of hypertension (renovascular), atherosclerosis and vascular injury (vein graft stenosis), hypotension (endotoxemia), and ischemia / reperfusion injury (heart). In a chronic renovascular hypertension model, blood pressure elevation, cardiac hypertrophy, acute renal failure, and acute mortality induced by one kidney-one clip surgery are more severe in HO-1 null mice. Moreover, absence of HO-1 leads to accelerated atherosclerotic lesion formation and vein graft disease. In addition, HO-1 null mice with endotoxemia have earlier resolution of hypotension, yet the mortality and the incidence of end organ damage are higher in the absence of HO-1. In contrast, mice with cardiac-specific overexpression of HO-1 have an improvement in cardiac function, smaller myocardial infarcts, and reduced inflammatory and oxidative damage after coronary artery ligation and reperfusion. Taken together, these studies suggest that an absence of HO-1 has detrimental consequences, while overexpression of HO-1 plays a protective role in ischemia / reperfusion injury.

Heme oxygenase (HO), an enzyme essential for heme degradation, shows anti-oxidative and anti-inflammatory properties via the production of bile pigments, carbon monoxide (CO) and ferritin induction under various pathophysiological conditions. A number of recent studies have shown biological effects of HO reaction in cardiovascular disorders. An inducible form of HO, HO-1, is induced by a variety of stresses such as oxidized lipoproteins, cytokines, hemodynamic changes, angiotensin II and nitric oxide (NO) in vascular wall. HO-1 induction seems to function as an adaptive response against these injurious stimuli. HO-1 induction in artery wall scavenges reactive oxygen species, which leads to the attenuation of monocyte adhesion and chemotaxis. HO-1 induction also reduces lipid peroxidation in plasma and artery wall. These properties of HO-1 suggest anti-atherogenic roles of this enzyme. In this review, roles of endothelial HO-1 expression and bilirubin in atherogenesis are also discussed. HO-1 also seems to play a significant role in restenosis after angioplasty, which is a major clinical problem associated with atherosclerosis. Recent progress in human HO-1 genetics supports these experimental results. This review aims to reaffirm current problems in the biological aspects of HO and suggest future research direction and clinical application.

The heme oxygenase-1 (HO-1) enzyme catalyzes the rate-limiting reaction in the catabolism of heme yielding products with pleiotropic, but ultimately, cytoprotective activities. High levels of HO-1 are frequently detected in various pathological states and generally in states of cellular oxidative stress. Induction of HO-1, regulated at the level of gene transcription, is essential for manifestation of the enzyme's cytoprotective function. Extensive analysis of the mouse gene, and to a lesser extent of the human gene, has identified a common mechanism - the stress response element (StRE) / Nrf2 transcription factor pathway - for gene regulation in response to a diverse array of HO-1 inducers including the substrate heme, various environmental and industrial toxins, and plant-derived polyphenolic compounds. In addition to Nrf2 complexes, numerous dimeric transcription factors bind to the StRE, permitting induction, repression and overall fine-tuning of gene activity. In principle, the multiplicity of StRE binding proteins also provides for a range of pharmaceutical targets for controlled production of the potentially therapeutic HO-1 protein.

The search for methods to control physiological levels of carbon monoxide (CO), a vasoactive molecule, and bilibrubin, an antioxidant, have improved our understanding of the protective role of heme oxygenase (HO) against oxidative injury. HO activity can assist other antioxidant systems in diminishing the overall production of reactive oxygen species, thus contributing to cellular resistance to such injury. Overexpression of HO gene by targeted gene transfer has become a powerful tool for studying the role of this human enzyme. Successful and functional HO gene transfer requires two essential elements. First, the HO gene must be delivered into a safe vector, such as the adenoviral, retroviral and leptosome based vectors that are currently being used in clinical trials. The use of non-viral vectors has also been described. Second, with the exception of HO gene delivery to ocular or cardiovascular tissue via catheter-based interventions, HO gene delivery must be site and organ specific. Site-specific delivery of HO-1 to renal structures in SHR, specifically mTAL, using Na+-K+ Cl - cotransporter (NKCC2 promoter), has been shown to normalize blood pressure and provide protection to mTAL against oxidative injury, respectively. Human HO-1 gene transfer into endothelial cells has been shown to attenuate Ang II- TNF- and hememediated DNA damage. Furthermore, delivery of human HO-1 into SHR has been shown to enhance somatic body growth and cell proliferation. The ability to transfect human HO gene and to demonstrate its expression may offer a new therapeutic strategy for treating pathological conditions, such as hypertension, trauma and hemorrhage.

Carbon monoxide (CO) is a resourceful gas as recent advances in the area of cell signaling are revealing an unexpected physiological role for CO in the cardiovascular, immune and nervous systems. Transition metal carbonyls have been lately discovered to function as COreleasing molecules (CO-RMs) and elicit distinct pharmacological activities in biological systems. Studies currently ongoing in our laboratories are investigating both the chemical and bioactive features of a series of water-soluble CO-RMs and their specific utilization as vasoactive mediators, anti-inflammatory agents and inhibitors of cellular and tissue damage. The data presented in this review corroborate the notion that transition metal carbonyls could be used as carriers to deliver CO in mammals and highlight the bioactivity and potential therapeutic features of CO-RMs in the mitigation of cellular and organ dysfunction.

Heme oxygenase 1 (HO-1) is induced by oxidative or nitrosative stress, cytokines and other mediators produced during inflammatory processes, likely as part of a defence system in cells exposed to stress to provide a negative feedback for cell activation and the production of mediators, which could modulate the inflammatory response. HO-1 activity results in the inhibition of oxidative damage and apoptosis, with significant reductions in inflammatory events including edema, leukocyte adhesion and migration, and production of inflammatory cytokines. HO-1 is induced by nitric oxide (NO) in different biological systems and can control the increased production of this mediator observed in many inflammatory situations. Regulatory interactions between HO-1 and cyclooxygenase (COX) pathways have also been reported. Modulation of signal transduction pathways by HO-1 or products derived from its activity, such as carbon monoxide (CO), may mediate the anti-inflammatory effects of this protein. Regulation of HO-1 activity may be a therapeutical strategy for a number of inflammatory conditions.