The molecular sources of reactive oxygen species in hypertension.

In both animal models and humans, increased blood pressure has been associated with oxidative stress in the vasculature, i.e. an excessive endothelial production of reactive oxygen species (ROS), which may be both a cause and an effect of hypertension. In addition to NADPH oxidase, the best characterized source of ROS, several other enzymes may contribute to ROS generation, including nitric oxide synthase, lipoxygenases, cyclo-oxygenases, xanthine oxidase and cytochrome P450 enzymes. It has been suggested that also mitochondria could be considered a major source of ROS: in situations of metabolic perturbation, increased mitochondrial ROS generation might trigger endothelial dysfunction, possibly contributing to the development of hypertension. However, the use of antioxidants in the clinical setting induced only limited effects on human hypertension or cardiovascular endpoints. More clinical studies are needed to fully elucidate this so called "oxidative paradox" of hypertension.

The putative role of mitochondrial dysfunction in hypertension.

Hypertension is a condition associated with oxidative stress, endothelial dysfunction, and increased vascular resistance, representing probably both a cause and a consequence of elevated levels of reactive oxygen (ROS) and nitrogen (RNS) species. Mitochondria are important sites of ROS production, and a mitochondrial dysfunction, preceding endothelial dysfunction, might favor the development of hypertension. ROS production may also be induced by RNS, which inhibit the respiratory chain and may be generated through the action of a mitochondrial NO synthase. Mitochondrial uncoupling proteins are involved in both experimental and human hypertension. Finally, an excessive production of ROS may damage mitochondrial DNA, with resultant impairment in the synthesis of some components of the respiratory chain and further ROS production, a vicious cycle that may be implicated in hypertensive states.

The genetic basis of essential hypertension.

During the last few years the studies on the genetic basis of essential hypertension (EH) have been numerous, allowing however only a partial understanding of the underlying molecular mechanisms. The most used techniques were the candidate gene approach, genome-wide scanning, the intermediate phenotype approach and comparative-genomics in animal models. The renin-angiotensin-aldosterone system may play a prominent role in the genesis of hypertension, and polymorphisms of the genes coding for angiotensinogen, angiotensin-converting enzyme, angiotensin II type 1 and 2 receptors, and aldosterone synthase have been widely studied. Other mechanisms may involve the KLK 1 gene of tissue kallikrein, gene variants of endothelial nitric oxide synthase and polymorphisms of the endothelin-1 gene. Finally, a number of studies have highlighted the potential contribution of polymorphisms of genes coding for inflammatory cytokines, adrenergic receptors and intracellular G proteins, which can activate Na+/K+ exchangers. Further important information might derive from proteomic analysis and the study of mitochondrial genome. Overall, results have often been discordant, sometimes suggesting a different expression of the same gene variants in different populations. EH is a highly polygenic condition, caused by the combination of small changes in the expression of many genes, in conjunction with a variable collection of environmental factors.

Gene-based therapy for hypertension--do preclinical data suggest a promising future?

Many experimental studies have obtained a prolonged control of blood pressure through gene treatment. This consists in the administration of genes coding for vasodilator proteins (the 'sense' approach), or of nucleotide sequences that are complementary to the mRNA of vasoconstrictor proteins, which are consequently synthesized in smaller amounts (the 'antisense' approach). Examples of the sense approach include the genes encoding endothelial nitric oxide synthase and kallikrein. Examples of the second type of approach are the antisense oligodeoxynucleotides to angiotensin-converting enzyme and endothelin-1. Also, RNA molecules, such as ribozymes and small interfering RNAs, are capable to inhibit RNA function. Whole sense genes are usually administered through viral vectors, while antisense oligonucleotides may be administered with plasmids or liposomes. Both viral and non-viral vectors have advantages and disadvantages. Despite the still persisting limitations, the possibility exists that in the future some forms of genetic treatment will be extended to the clinical setting, allowing a prolonged control of essential hypertension and its end-organ sequelae.

Molecular aspects of atherogenesis: new insights and unsolved questions.

The development of atherosclerotic disease results from the interaction between environment and genetic make up. A key factor in atherogenesis is the oxidative modification of lipids, which is involved in the recruitment of mononuclear leukocytes to the arterial intima--a process regulated by several groups of adhesion molecules and cytokines. Activated leukocytes, as well as endothelial mitochondria, can produce reactive oxygen species (ROS) that are associated with endothelial dysfunction, a cause of reduced nitric oxide (NO) bioactivity and further ROS production. Peroxisome proliferator-activated receptors (PPAR) and liver X receptors (LXR) are nuclear receptors significantly involved in the control of lipid metabolism, inflammation and insulin sensitivity. Also, an emerging role has been suggested for G protein coupled receptors and for the small Ras and Rho GTPases in the regulation of the expression of endothelial NO synthase (eNOS) and of tissue factor, which are involved in thrombus formation and modulation of vascular tone. Further, the interactions among eNOS, cholesterol, oxidated LDL and caveola membranes are probably involved in some molecular changes observed in vascular diseases. Despite the relevance of oxidative processes in atherogenesis, anti-oxidants have failed to significantly improve atherosclerosis (ATS) prevention, while statins have proved to be the most successful drugs.

Different effects of antihypertensive drugs on endothelial dysfunction.

Since endothelial dysfunction may significantly contribute to the pathophysiology of hypertension and its complications, its modification seems to be a very attractive means to favourably affect the development of atherosclerosis and cardiovascular events in hypertensive patients. However, not all antihypertensive drugs consistently improve endothelial dysfunction. While first-generation beta-blockers showed contrasting or null effects on endothelial function, newer beta-blockers of the third generation, such as carvedilol and nebivolol, seem to be provided with specific endothelium-mediated vasodilating effects. Calcium channel blockers are generally able to increase endothelium-dependent vasodilation in several vascular beds, in patients with essential hypertension, probably through multiple mechanisms. Most studies have shown thatACE inhibitors favourably affect endothelial function mainly in the subcutaneous, epicardial and renal circulation, not only by inhibiting the effects of angiotensin II on the endothelium, but also by enhancing bradykinin-induced vasodilation, probably a hyperpolarization-related effect. On the other hand, discordant evidence is available about the effects of angiotensin II receptor type I blockers on endothelial function in patients with essential hypertension, atherosclerosis or diabetes.There are data suggesting that an increased activity of the endothelin- I system may play a role in the blunted endothelium-dependent vasorelaxation of hypertensive patients, an effect that could be contrasted by the use of endothelin-I receptor antagonists. However, to date no substantial clinical efficacy of endothelin-I receptor blockers has been shown in patients with essential hypertension. Finally, other possibly useful compounds in restoring impaired endothelial function in hypertension are some antioxidant agents such as vitamin C, folic acid, the cofactor tetrahydrobiopterin (BH4), L-arginine and the drugs of the statin class.