Roles of beta2- and beta3-adrenoceptor polymorphisms in hypertension and metabolic syndrome.

Hypertension, diabetes mellitus (especially type 2 diabetes mellitus), metabolic syndrome and obesity are rapidly growing public health problems. Sympathetic nerve activation is observed in obesity, hypertension and diabetes mellitus, which have strong genetic as well as environmental determinants. Reduced energy expenditure and resting metabolic rate are predictive of weight gain, and the sympathetic nervous system participates in regulating energy balance through thermogenesis. The thermogenic effects of catecholamines in obesity have been mainly mediated via the β2- and β3-adrenergic receptors in humans. Further, β2-adrenoceptors importantly influence vascular reactivity and may regulate blood pressure. Genetic polymorphistns of the β-adrenoceptor gene have been shown to alter the function of several adrenoceptor subtypes and thus to modify the response to catecholamine. β2-adrenoceptor polymorphisms (Arg16Gly, Gln27Glu, and Thr164Ile) have been studied in relation to hypertension. Genetic variations in the β3-adrenoceptor (i.e. Try64Arg variant) are also associated with both obesity and hypertension. However, the precise relationships of the polymorphisms of β2- and β3-adrenoceptor genes with sympathetic nervous system activity, hypertension, and metabolic syndrome have not been fully clarified. This paper will discuss the current topics involving the influence of the sympathetic nervous system and β2- and β3- adrenoceptor polymorphisms in hypertension and metabolic syndrome.


Introduction
Obesity, hypertension, and metabolic syndrome (type 2 diabetes mellitus) are major and growing health problems and are known as high-risk factors for subsequent cardiovascular and renal complications [1][2][3]. Obesity, hypertension, diabetes, and metabolic syndrome are intimately associated [4][5][6], and sympathetic nervous activation is frequently observed in those conditions. Thus, sympathetic nerve activation may play a major role in the onset and development of hypertension, obesity, and metabolic syndrome (diabetes mellitus) as well as cardiovascular complications in patients with hypertension, diabetes and obesity [2,7]. The sympathetic nervous system plays an important role in the regulation of energy expenditure. Reduced energy expenditure and resting metabolic rate are predictive of weight gain (obesity). The sympathetic nervous system participates in regulating energy balance through thermogenesis [8]. A large part of the sympathetic nervous systemmediated energy expenditure takes place in skeletal muscle, via the coupling of catecholamines with β2-adrenoceptors. Catecholamines are also powerful regulators of lipolysis and act via β1-, β2-, β3-(stimulatory), and α2-(inhibitory) adrenoceptor subtypes in adipose tissue, where their role becomes especially important during both exercise and energy restriction, when increased need for fat as a fuel exists. Thus, β-adrenoceptors play important roles in energy expenditure and control body weight [9][10][11][12][13].
Recently, there is evidence that human hypertension and obesity have strong genetic backgrounds [14][15][16]. Harrap et al. reported that about 46% of the phenotype of systolic blood pressure are determined genetically for hypertension [17,18]. Masuo et al. [18][19][20][21][22] have reported close relationships between β2and β3-adrenoceptor polymorphisms 2 International Journal of Hypertension accompanying elevated sympathetic nervous activity, blood pressure elevation (hypertension), weight gain (obesity), and insulin resistance in a series of longitudinal study. Many epidemiological studies on the relationships between β-adrenoceptor polymorphisms, hypertension, obesity, and diabetes (metabolic syndrome) have still been discordant. This paper will discuss the current topics involving the contribution of the sympathetic nervous system and β2and β3-adrenoceptor polymorphisms in the onset and the development of hypertension and metabolic syndrome (type 2 diabetes mellitus). (Table 1) The adrenoceptors (or adrenergic receptors) are a class of G protein-coupled receptors which specifically bind their endogenous ligands, the catecholamines (epinephrine and norepinephrine). Many tissues possess these adrenoceptors, and the binding of an agonist generally elicits a "typical" sympathetic response (i.e., the fight-or-flight response). Table 1 shows the effects of catecholamines bound to adrenoceptors (Table 1) and these effects on sympathetic nervous activity are through αand β-adrenergic receptors.

Subtypes of Adrenoceptors
There are several types of adrenergic receptors, but there are two main groups: α-adrenoceptors (α1-and α2adrenoceptors) and β-adrenoceptors (β1-, β2-, and β3adrenoceptors). Table 1 also summaries the distributions and functions of the α1-, α2-, β1-, β2-, and β3-adrenoceptors [24,25]. The α-receptors bind norepinephrine and epinephrine, though norepinephrine has higher affinity. Phenylephrine is a selective agonist of the α-adrenoceptors (both α1and α2-receptors), thus phenylephrine is usually used to investigate the α-adrenoceptors function. β-adrenoceptors are linked to G proteins, which are linked to adenyl cyclase. βadrenoceptor agonists cause the intracellular elevation of the second messenger cyclic AMP. Downstream effects of cyclic AMP include cyclic AMP dependent protein kinase, which mediates the intracellular events following hormone binding.

Sympathetic Nervous Activity and Insulin Resistance in Hypertension (Figure 1)
Insulin resistance in hypertension has been well documented in many epidemiological and clinical studies [8,26,27]. Several investigators have reported that chronic insulin administration elevates blood pressure in rats and in humans [28], although insulin also has effects on vasodilation. In addition, many clinical and epidemiological studies have demonstrated the close relationships between sympathetic nerve activity, insulin resistance and hypertension [19,[29][30][31][32]. Landsberg and other investigators examined the effect of feeding and starvation on sympathetic nerve activity in the cardiac tissue of animals, noting that feeding raised sympathetic nerve activity, and starvation had the opposite effect [33][34][35]. Energy intake stimulates hyperinsulinemia and sympathetic nerve activity resulting in blood pressure elevations in a cycle to inhibit thermogenesis. Insulinmediated sympathetic nerve stimulation in obese subjects is a compensatory mechanism aimed at restoring the energy balance by increasing the metabolic rate [33]. Therefore, hyperinsulinemia and insulin resistance in obese subjects are all part of a response to limit further weight gain via stimulating sympathetic nerve activity and thermogenesis [28].
On the other hand, Julius et al. [36] have hypothesized that increased sympathetic nerve activity in skeletal muscle causes neurogenic vasoconstriction, thereby reducing blood flow to muscle and consequently inducing a state of insulin resistance by lowering glucose delivery and uptake in hypertension and obesity. Both blood pressure elevation and weight gain may reflect a primary increase in sympathetic nervous tone. Masuo et al. [30,37] supported Julius's hypothesis. They described that high plasma norepinephrine might predict future blood pressure elevations and weight gain accompanying deterioration in insulin resistance observed in HOMA-IR (homeostasis model assessments of insulin resistance) [30,37]. Rocchini et al. [38] reported that clonidine prevented insulin resistance in obese dogs over a 6-week period. Their results suggest that sympathetic nerve activity might play a major role in the development of insulin resistance accompanying blood pressure elevations. Valentini et al. [39] reported attenuation of hemodynamic and energy expenditure responses to isoproterenol infusion in hypertensive patients, suggesting that sympathetic nerve activity-induced hypertension may subsequently lead to the development of obesity.
Many epidemiological studies showed close linkages of beta2-and beta3-adrenoceptor polymorphisms with obesity, hypertension, and the metabolic syndrome shown in Tables 2, 3, and 4. Sympathetic nervous activity is related to body weight or blood pressure through β-adrenoceptors. Thus, close linkages between sympathetic nerve activity and insulin resistance might depend on the β-adrenoceptor polymorphisms. Thus, one could speculate that the strong associations between β-adrenoceptor polymorphisms and insulin resistance might provide evidence that heightened sympathetic nerve activity followed by insulin resistance might play a major role in hypertension and obesity, because β-adrenoceptor polymorphisms might relate to insulin resistance through heightened sympathetic nerve activity ( Figure 1).

Role of β-Adrenoceptor Polymorphisms in Hypertension, Obesity, and Diabetes
The sympathetic nervous system plays an important role in the regulation of energy expenditure and blood pressure regulation. A large part of the sympathetic nervous systemmediated energy expenditure takes place in skeletal muscle, via the coupling of catecholamines with β2-adrenoceptors.
Catecholamines are also powerful regulators of lipolysis and act via β1-, β2-, β3-(stimulatory), and α2-(inhibitory) adrenoceptor subtypes in adipose tissue, where their role becomes especially important during both exercise and    energy restriction, when increased need for fat as a fuel exists. Stimulation of β-adrenergic receptors by the sympathetic nervous system is a significant physiological modulator of pre-and postprandial energy expenditure [11][12][13] and total daily energy expenditure [9,10]. Recent studies show that β-adrenoceptors are polymorphic. Single nucleotide polymorphisms might have functional consequences in terms of receptor activity and regulation and hence may contribute to the pathophysiology of hypertension and obesity. On the other hand, there are few studies on the relationships between α-adrenoceptor polymorphisms, hypertension, obesity, and metabolic syndrome.

β1-Adrenoceptor
Polymorphisms. The β1-adrenoceptor is predominantly expressed in cardiac myocytes and adipose    [16,57,62,76,95] In lean subjects, β2-AR polymorphisms linked to obesity and obesity-related hypertension, but in obese subjects, β2and β3-AR polymorphisms relate to obesity and obesity-related hypertension. Morbid obesity is linked with β3-AR polymorphisms, but overweight or mild obesity is not associated with those.
Gender differences [71] Interaction between β1and β2-AR polymorphisms with changes in BMI was observed in men only, while in women an interaction between β1and β3-AR polymorphisms was observed in a longitudinal over a 24-year period large cohort study. Ethnic difference [103,104] Distributions of β-AR polymorphisms are different in 8 different ethnic populations. Haplotype [20,58,59,[105][106][107] Functions expressed of β-AR polymorphisms are different due to the other β-AR polymorphisms.
AR: adrenoceptor; BMI: body mass index. tissue, where its activation leads to increased heart rate and contractility and stimulation of lipolysis, respectively. The two most common β1-adrenoceptor polymorphisms are Ser49Gly and Arg389Gly, with relative allele frequencies of 0.85/0.15 and 0.70/0.30 in the Caucasian population, respectively. The β1-adrenoceptor is a candidate gene for obesity because of its role in catecholamine-mediated energy homeostasis [72,73]. For example, in obese individuals, the degree of weight loss during a very low calorie diet has been shown to correlate with changes in β1-adrenoceptor protein concentration in adipose tissue [72]. A population cohort of 761 women showed that women carrying the Gly49 genotype had greater increases in BMI over15 years compared to those with the Ser49 genotype [73]. Conversely, the distribution of the Arg389Gly polymorphism is similar in lean and obese subjects [74] and in a large cohort study including 3981 normotensive and 2518 hypertensive subjects [75]. The factors which might explain the discrepancy of published data are shown in the later section.

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International Journal of Hypertension

β2-Adrenoceptor
Polymorphisms. The β2-adrenoceptor is the dominant lipolytic receptor in white human adipose tissue [13] and in skeletal muscle [12]. It also plays an important regulatory role in the peripheral vasculature. Genetic polymorphisms of the β2-adrenoceptor have been associated with hypertension, obesity, and metabolic syndrome (diabetes mellitus). The most common polymorphisms are Arg16Gly, with an allele frequency of 0.40/0.60, and Gln27Glu, with an allele frequency of 0.55/0.45, in the Caucasian population. The Thr164Ile polymorphism is rare, occurring in only 3 to 5% of the general Caucasians population.
Studies of agonist stimulation in cultured cells demonstrate that Gly16 receptors have a greater reduction in numbers or enhanced downregulation when compared with Arg16 whereas the Glu27 receptor is resistant to down regulation when compared with the Gln27 variant [108]. A number of clinical studies have investigated the impact of these polymorphisms on vascular responsiveness [40,109]. Gratze et al. [110] found that young normotensive white men homozygous for the Gly16 allele had higher blood pressure and lower peripheral vasodilation after infusion of the β2agonist salbutamol. Similar results were obtained by Hoit et al. [111] using the agonist terbutaline. On the other hand, three studies investigating isoprenaline induced increase in the limb blood flow Thus, volunteers homozygous for Gly16 exhibited larger vasodilatory responses than did volunteers homozygous for Arg16 [23]. Conflicting results have also been published with regard to the effects of genetic variants on the sympathetic nervous system modulation of energy expenditure. Bell et al. [112] reported that the response of resting energy expenditure to nonspecific β-adrenoceptor stimulation (with isoproterenol infusion) was not different between the 3 genotypes of Arg16Gly. Stob et al. [41] showed that individuals carrying the Arg16Arg variant of the β2adrenoceptor gene have a reduced thermogenic response to selective β2-adrenoceptor activation.
Associations of β2-adrenoceptor polymorphisms with hypertension and metabolic syndrome have been reported in many epidemiological studies but results are also discordant (summarised in Tables 2 and 3).

β3-Adrenoceptor
Polymorphisms. The β3-adrenoceptor, which is mainly expressed in adipose tissue, differs from the β2-adrenoceptor in two ways: it has a lower affinity for catecholamines, and it resists desensitisation (i.e., downregulation). These characteristic differences might lead to the different effects of catecholamine on β2-adrenoceptors and β3-adrenoceptors. β3-adrenoceptors stimulate the mobilization of lipids from the white fat cell and increase thermogenesis in brown fat cell. Decreased function of β3adrenoceptor in white adipose tissue could slow lipolysis and thereby cause the retention of lipids in fat cells. Slow lipolysis may contribute strongly to visceral obesity in human, and treatment of obese animal models with selective β3-adrenergic agonists reduces fat stores most effectively [94,113,114]. Many epidemiological studies have shown the strong relationships between β3-adrenoceptor polymorphisms (mainly Trp54Arg), hypertension, metabolic syndrome, and obesity [78,94,[113][114][115][116][117] (Table 4). (Table 5). Tables 2, 3, 4, and 5 show the discordant contributions of β-adrenoceptor polymorphisms to hypertension, metabolic syndrome (type 2 diabetes), and obesity. Table 5 summarizes factors which might explain the discrepancy of published data. Further, haplotypes of polymorphisms have strong influence on β-adrenoceptor function in each polymorphism [20,58,59,[105][106][107].

Conclusions
The role of the sympathetic nervous system β2and β3adrenoceptor polymorphisms in hypertension, metabolic syndrome (diabetes mellitus), and obesity is discussed through a literature review. Sympathetic nervous system activity and β-adrenoceptor polymorphisms (mainly β2and β3-adrenoceptor polymorphisms) might contribute to the onset and maintenance of hypertension, metabolic syndrome, and obesity; however, the findings have been discordant. Further, few studies have been performed to evaluate the relationship between β2and β3-adrenoceptor polymorphisms and sympathetic nervous system activity in the same study. A better understanding for the relationships of genetic background (polymorphisms) with sympathetic nervous system activity as the cause for hypertension (blood pressure elevation), metabolic syndrome (insulin resistance), and obesity (weight gain) might help for clinical treatment for obesity-related hypertension and metabolic syndrome. In fact, a number of studies have investigated genetic polymorphisms as determinants of cardiovascular response to antihypertensive drug therapy [103,104]. But further research on gene-drug interactions is necessary. In addition, to clarify the pathogenesis and mechanisms may lead to the prevention of hypertension and metabolic syndrome in obesity.