Gene therapy to control hypertension: current studies and future perspectives

Heme oxygenase in the regulation of vascular biology: from molecular mechanisms to therapeutic opportunities

Heme oxygenases (HOs) are the rate-limiting enzymes in the catabolism of heme into biliverdin, free iron, and carbon monoxide. Two genetically distinct isoforms of HO have been characterized: an inducible form, HO-1, and a constitutively expressed form, HO-2. HO-1 is a kind of stress protein, and thus regarded as a sensitive and reliable indicator of cellular oxidative stress. The HO system acts as potent antioxidants, protects endothelial cells from apoptosis, is involved in regulating vascular tone, attenuates inflammatory response in the vessel wall, and participates in angiogenesis and vasculogenesis. Endothelial integrity and activity are thought to occupy the central position in the pathogenesis of cardiovascular diseases. Cardiovascular disease risk conditions converge in the contribution to oxidative stress. The oxidative stress leads to endothelial and vascular smooth muscle cell dysfunction with increases in vessel tone, cell growth, and gene expression that create a pro-thrombotic/pro-inflammatory environment. Subsequent formation, progression, and obstruction of atherosclerotic plaque may result in myocardial infarction, stroke, and cardiovascular death. This background provides the rationale for exploring the potential therapeutic role for HO system in the amelioration of vascular inflammation and prevention of adverse cardiovascular outcomes.

Heme oxygenase in the regulation of vascular biology: from molecular mechanisms to therapeutic opportunities

Heme oxygenases (HOs) are the rate-limiting enzymes in the catabolism of heme into biliverdin, free iron, and carbon monoxide. Two genetically distinct isoforms of HO have been characterized: an inducible form, HO-1, and a constitutively expressed form, HO-2. HO-1 is a kind of stress protein, and thus regarded as a sensitive and reliable indicator of cellular oxidative stress. The HO system acts as potent antioxidants, protects endothelial cells from apoptosis, is involved in regulating vascular tone, attenuates inflammatory response in the vessel wall, and participates in angiogenesis and vasculogenesis. Endothelial integrity and activity are thought to occupy the central position in the pathogenesis of cardiovascular diseases. Cardiovascular disease risk conditions converge in the contribution to oxidative stress. The oxidative stress leads to endothelial and vascular smooth muscle cell dysfunction with increases in vessel tone, cell growth, and gene expression that create a pro-thrombotic/pro-inflammatory environment. Subsequent formation, progression, and obstruction of atherosclerotic plaque may result in myocardial infarction, stroke, and cardiovascular death. This background provides the rationale for exploring the potential therapeutic role for HO system in the amelioration of vascular inflammation and prevention of adverse cardiovascular outcomes.

Pharmacological and clinical aspects of heme oxygenase

This review is intended to stimulate interest in the effect of increased expression of heme oxygenase-1 (HO-1) protein and increased levels of HO activity on normal and pathological states. The HO system includes the heme catabolic pathway, comprising HO and biliverdin reductase, and the products of heme degradation, carbon monoxide (CO), iron, and biliverdin/bilirubin. The role of the HO system in diabetes, inflammation, heart disease, hypertension, neurological disorders, transplantation, endotoxemia and other pathologies is a burgeoning area of research. This review focuses on the clinical potential of increased levels of HO-1 protein and HO activity to ameliorate tissue injury. The use of pharmacological and genetic probes to manipulate HO, leading to new insights into the complex relationship of the HO system with biological and pathological phenomena under investigation, is reviewed. This information is critical in both drug development and the implementation of clinical approaches to moderate and to alleviate the numerous chronic disorders in humans affected by perturbations in the HO system.

ACE2: A novel therapeutic target for cardiovascular diseases

Hypertension afflicts over 65 million Americans and poses an increased risk for cardiovascular morbidity such as stroke, myocardial infarction and end-stage renal disease resulting in significant mortality. Overactivity of the renin-angiotensin system (RAS) has been identified as an important determinant that is implicated in the etiology of these diseases and therefore represents a major target for therapy. In spite of the successes of drugs inhibiting various elements of the RAS, the incidence of hypertension and cardiovascular diseases remain steadily on the rise. This has lead many investigators to seek novel and innovative approaches, taking advantage of new pathways and technologies, for the control and possibly the cure of hypertension and related pathologies. The main objective of this review is to forward the concept that gene therapy and the genetic targeting of the RAS is the future avenue for the successful control and treatment of hypertension and cardiovascular diseases. We will present argument that genetic targeting of angiotensin-converting enzyme 2 (ACE2), a newly discovered member of the RAS, is ideally poised for this purpose. This will be accomplished by discussion of the following: (i) summary of our current understanding of the RAS with a focus on the systemic versus tissue counterparts as they relate to hypertension and other cardiovascular pathologies; (ii) the newly discovered ACE2 enzyme with its physiological and pathophysiological implications; (iii) summary of the current antihypertensive pharmacotherapy and its limitations; (iv) the discovery and design of ACE inhibitors; (v) the emerging concepts for ACE2 drug design; (vi) the current status of genetic targeting of the RAS; (vii) the potential of ACE2 as a therapeutic target for hypertension and cardiovascular disease treatment; and (viii) future perspectives for the treatment of cardiovascular diseases.

Carbon monoxide and hypertension

The enzymatic action of heme oxygenase yields carbon monoxide, biliverdin and iron. Carbon monoxide is implicated in many physiological processes, including the regulation of vascular tissue contractility and apoptosis. By stimulating the soluble guanylyl cyclase (sGC)/cGMP pathway and activating K channels in vascular smooth muscle cells (SMCs), carbon monoxide relaxes vascular tissues under physiological conditions. Altered metabolism and functions of carbon monoxide have been linked to the pathogenesis and maintenance of hypertension. The expression and activity of heme oxygenase-1, sGC and cGMP in vascular SMCs are associated with different stages of development of hypertension in spontaneously hypertensive rats (SHRs). The importance of altered heme oxygenase-2 expression in vascular tissues in hypertension remains unclear. Increased vascular contractility, unbalanced cellular apoptosis and proliferation in the vascular wall, increased oxidative stress, and the altered interaction of carbon monoxide and nitric oxide are among the consequences of heme oxygenase/carbon monoxide system dysfunction in hypertension. Acute application of pharmacological inducers to upregulate the expression of heme oxygenase-1 or the use of gene delivery method to overexpress heme oxygenase-1 decreases blood pressure in young SHRs and other animal models of hypertension. These blood pressure-decreasing effects are annulled by metalloporphyrins. In adult SHRs, the heme oxygenase/carbon monoxide system appears to be normalized as a compensatory reaction. To date, acute manipulation of the expression of heme oxygenase-1 has not been successful in decreasing blood pressure in adult SHRs. In conclusion, abnormality of the heme oxygenase/carbon monoxide system has a critical role in the pathogenesis of hypertension, and novel therapeutic approaches should be pursued to achieve selective improvement in the function of this system in hypertension.

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