Leptin in association with common variants of MC3R mediates hypertension

Leptin, Obesity, and Hypertension: A Review of Pathogenetic Mechanisms

The adipokine leptin is expressed at higher concentrations in obese subjects, who also incidentally have a higher prevalence of hypertension. The pathogenesis of this obesity-related hypertension is controversial and is believed to be related to many factors including increased sympathetic activity, abnormalities of the renin-angiotensin system, sodium retention, and an endotheliopathy acting independently or in concert with increased circulating leptin. This review discusses the potential mechanisms through which changes in leptin signal transduction pathways in tissues with the leptin receptor, especially the hypothalamus, mediate the pathogenetic relationships between obesity and hypertension. The hypothesis is explored that leptin effects on blood pressure (BP) are meditated by the downstream effects of hypothalamic leptin signaling and ultimately result in activation of specific melanocortin receptors located on sympathetic neurons in the spinal cord. The physiological consequences of this sympathetic activation of the heart and kidney are activation of the renin-angiotensin system, sodium retention and circulatory expansion and finally, elevated BP. This sequence of events has been elegantly demonstrated with leptin infusion and gene knockout studies in animal models but has not been convincingly reproducibly confirmed in humans. Further studies in human subjects on the specific roles of hypothalamic leptin in essential hypertension are indicated as elucidation of the signaling pathways should provide better understanding of the role of weight loss in BP control and afford an additional mechanism for pharmacologic control of BP in adults and children at risk of cardiovascular disease.

Mitochondrial DNA D-loop sequencing reveals obesity variants in an Arab population

Background: The association of mitochondrial DNA (mtDNA) variations with obesity has been investigated in diverse populations across the world. However, such obesity-associated mtDNA examinations are rarely conducted in Arab populations. Materials and methods: We re-sequenced mtDNA displacement loop (D-loop) region of 395 Arab individuals of Kuwait. We categorized the individuals based on their BMI scores as obese (n=232; BMI ≥30 kg/m²), overweight (n=110; BMI ≥25 kg/m² and <30 kg/m²), and lean (n=53; BMI <25 kg/m²). We performed all the statistical tests by combining obese and overweight individuals in one group. Association analyses were conducted applying Fisher's exact test and logistic regression model. Results: We identified that the mtDNA variations m.73A>G, and m.523delAC were positively correlated with obesity, while m.310T>C, and m.16318A>T were negatively associated. All these variants, except m.16318A>T, remain statistically significant after adjusting for age and gender. We found that the variant m.73A>G increases the likelihood of being obese by 6-fold, whereas haplogroup H decreases the probability of being obese in Arab individuals of Kuwait. Haplotype analysis revealed that a haplotype, A263G-C309CT-T310C, defining the H2a clade of H haplogroup, reduces the probability of being obese. Conclusion: Our study reports, for the first time, the obesity-related mtDNA variants in Arabs of Kuwait. Based on the mtDNA D-loop region variations, we detected particular variants and haplogroup that are related with increased and decreased probability of being obese in the Kuwait Arab population.

Associations of the melanocortin 3 receptor C17A + G241A haplotype with body composition and inflammation in African-American adults

BACKGROUND

The MC3R haplotype C17A + G241A, which encodes a partially inactivated receptor, has high prevalence in individuals of predominately African ancestry. In pediatric cohorts, homozygosity for this common variant has been associated with obesity, reduced lean mass, and greater fasting insulin. However, metabolic and body composition measures have not been well studied in adults with this haplotype.

METHODS

A convenience sample of 237 healthy African-American adult volunteers was studied. TaqMan assays were used to genotype MC3R variants. Labs were drawn in the morning in the fasted state. Body composition data was obtained via dual-energy X-ray absorptiometry. An analysis of covariance was used to examine the associations of genotype with metabolic and body composition measures controlling for age and sex.

RESULTS

Individuals homozygous for the MC3R C17A + G241A haplotype had significantly greater body mass index, fat mass, fat mass percentage, and C-reactive protein, with reduced lean mass percentage as compared to heterozygous and wild-type participants (all ps < 0.05); fasting insulin was marginally nonsignificant between groups (p = 0.053). After adjusting for fat mass, laboratory differences no longer remained significant.

CONCLUSIONS

Homozygosity for MC3R C17A + G241A is associated with increased adiposity in African-American adults. Further studies are needed to elucidate the mechanisms behind these associations.

A Perception on Genome-Wide Genetic Analysis of Metabolic Traits in Arab Populations

Despite dedicated nation-wide efforts to raise awareness against the harmful effects of fast-food consumption and sedentary lifestyle, the Arab population continues to struggle with an increased risk for metabolic disorders. Unlike the European population, the Arab population lacks well-established genetic risk determinants for metabolic disorders, and the transferability of established risk loci to this population has not been satisfactorily demonstrated. The most recent findings have identified over 240 genetic risk loci (with ~400 independent association signals) for type 2 diabetes, but thus far only 25 risk loci (ADAMTS9, ALX4, BCL11A, CDKAL1, CDKN2A/B, COL8A1, DUSP9, FTO, GCK, GNPDA2, HMG20A, HNF1A, HNF1B, HNF4A, IGF2BP2, JAZF1, KCNJ11, KCNQ1, MC4R, PPARγ, SLC30A8, TCF7L2, TFAP2B, TP53INP1, and WFS1) have been replicated in Arab populations. To our knowledge, large-scale population- or family-based association studies are non-existent in this region. Recently, we conducted genome-wide association studies on Arab individuals from Kuwait to delineate the genetic determinants for quantitative traits associated with anthropometry, lipid profile, insulin resistance, and blood pressure levels. Although these studies led to the identification of novel recessive variants, they failed to reproduce the established loci. However, they provided insights into the genetic architecture of the population, the applicability of genetic models based on recessive mode of inheritance, the presence of genetic signatures of inbreeding due to the practice of consanguinity, and the pleiotropic effects of rare disorders on complex metabolic disorders. This perspective presents analysis strategies and study designs for identifying genetic risk variants associated with diabetes and related traits in Arab populations.

Cancer and comorbidity: The role of leptin in breast cancer and associated pathologies

Obesity is an important risk factor for postmenopausal breast cancer and also a poor prognostic factor among cancer patients. Moreover, obesity is associated with a number of health disorders such as insulin resistance/type-2 diabetes mellitus, hypertension, and other cardiovascular diseases. Frequently, these health disorders exhibit as components/complications of the metabolic syndrome. Nevertheless, obesity-related diseases may coexist with postmenopausal breast cancer; and these comorbid conditions could be substantial. Therefore, it may be assumed that different diseases including breast cancer could originate from a common pathological background in excessive adipose tissue. Adipocyte-released hormone-like cytokine (or adipokine) leptin behaves differently in a normal healthy state and obesity. A growing body of evidence suggests an important role of leptin in our major obesity-related health issues such as insulin resistance, hypertension, and neoplasia. In this context, this review describes the relationships of the abovementioned pathologies with leptin.

Polymorphisms and mutations in the melanocortin-3 receptor and their relation to human obesity

Inactivating mutations in the melanocortin 3 receptor (Mc3r) have been described as causing obesity in mice, but the physiologic effects of MC3R mutations in humans have been less clear. Here we review the MC3R polymorphisms and mutations identified in humans, and the in vitro, murine, and human cohort studies examining their putative effects. Some, but not all, studies suggest that the common human MC3R variant T6K+V81I, as well as several other rare, function-altering mutations, are associated with greater adiposity and hyperleptinemia with altered energy partitioning. In vitro, the T6K+V81I variant appears to decrease MC3R expression and therefore cAMP generation in response to ligand binding. Knockin mouse studies confirm that the T6K+V81I variant increases feeding efficiency and the avidity with which adipocytes derived from bone or adipose tissue stem cells store triglycerides. Other MC3R mutations occur too infrequently in the human population to make definitive conclusions regarding their clinical effects. This article is part of a Special Issue entitled: Melanocortin Receptors - edited by Ya-Xiong Tao.

Genetic risk variants for metabolic traits in Arab populations

Despite a high prevalence of metabolic trait related diseases in Arabian Peninsula, there is a lack of convincingly identified genetic determinants for metabolic traits in this population. Arab populations are underrepresented in global genome-wide association studies. We genotyped 1965 unrelated Arab individuals from Kuwait using Cardio-MetaboChip, and tested SNP associations with 13 metabolic traits. Models based on recessive mode of inheritance identified Chr15:40531386-rs12440118/ZNF106/W->R as a risk variant associated with glycated-hemoglobin at close to ‘genome-wide significant’ p-value and five other risk variants ‘nominally’ associated (p-value ≤ 5.45E-07) with fasting plasma glucose (rs7144734/[OTX2-AS1,RPL3P3]) and triglyceride (rs17501809/PLGRKT; rs11143005/LOC105376072; rs900543/[THSD4,NR2E3]; and Chr12:101494770/IGF1). Furthermore, we identified 33 associations (30 SNPs with 12 traits) with ‘suggestive’ evidence of association (p-value < 1.0E-05); 20 of these operate under recessive mode of inheritance. Two of these ‘suggestive’ associations (rs1800775-CETP/HDL; and rs9326246-BUD13/TGL) showed evidence at genome-wide significance in previous studies on Euro-centric populations. Involvement of many of the identified loci in mediating metabolic traits was supported by literature evidences. The identified loci participate in critical metabolic pathways (such as Ceramide signaling, and Mitogen-Activated Protein Kinase/Extracellular Signal Regulated Kinase signaling). Data from Genotype-Tissue Expression database affirmed that 7 of the identified variants differentially regulate the up/downstream genes that mediate metabolic traits.

Neurohumoral Integration of Cardiovascular Function by the Lamina Terminalis

The mechanisms involved in cardiovascular regulation, such as vascular tone, fluid volume and blood osmolarity, are quite often mediated by signals circulating in the periphery, such as angiotensin II and sodium concentration. Research has identified areas within the lamina terminalis (LT), specifically the sensory circumventricular organs (CVOs), the subfornical organ and the organum vasculosum of the lamina terminalis, as playing crucial roles detecting and integrating information derived from these circulating signals. The median preoptic nucleus (MnPO) is a third integrative structure within the LT that influences cardiovascular homeostasis, although to date, its role is not as clearly elucidated. More recent studies have demonstrated that the CVOs are not only essential in the detection of traditional cardiovascular signals but also signals primarily considered to be important in the regulation of metabolic, reproductive and inflammatory processes that have now also been implicated in cardiovascular regulation. In this review, we highlight the critical roles played by the LT in the detection and integration of circulating signals that provide critical feedback control information contributing to cardiovascular regulation.

BMI, body fat mass and plasma leptin level in relation to cardiovascular diseases risk factors among adolescents in Taitung

BACKGROUND

To investigate the risk factors associated with cardiovascular diseases and its relation to BMI, body fat mass and plasma leptin level among adolescents in Taitung, Taiwan.

METHODS

A cross-sectional Taitung Children Heart Study for 500 young adolescents between ages 13 and 15 was conducted. Gender-specific regression models were used to determine the associations between BMI, percentage of body fat mass, plasma leptin level and seven CVDs risk factors (systolic and diastolic blood pressure, mean arterial pressure, triglycerides, total cholesterol, HDL-cholesterol and LDL-cholesterol) before and after adjusting for weight status and age.

RESULTS

After adjusting for weight status and age, BMI was positively associated with systolic BP, triglycerides, LDL-cholesterol levels but negatively associated with HDL-cholesterol level in boys while positively associated with systolic and diastolic BP, mean arterial pressure, and LDL-cholesterol level in girls. The percentage of body fat mass was positively associated with triglycerides, total cholesterol, and LDL-cholesterol in boys while positively associated with systolic BP, total cholesterol, and LDL-cholesterol in girls. Plasma leptin was positively associated with triglycerides, total cholesterol and LDL-cholesterol in boys but no statistically significant associations with CVDs risk factors in girls. A strong relationship between the percentage of body fat mass and plasma leptin appeared among all participants (r=0.59, p<0.01).

CONCLUSIONS

BMI, body fat mass and plasma leptin level may be used to identify certain CVDs risk factors among Taitung adolescents. Future researches could consider measuring body fat mass in the relationship of CVDs risk factors instead of plasma leptin among young adolescents.

Central nervous system dysfunction in obesity-induced hypertension

The activation of the sympathetic nervous system is a major mechanism underlying both human and experimental models of obesity-related hypertension. While insulin and the adipokine leptin have long been thought to contribute to obesity-related neurogenic mechanisms, the evidence is now very strong that they play a major role, shown particularly in animal studies using selective receptor antagonists. There is not just maintenance of leptin's sympatho-excitatory actions as previously suggested but considerable amplification particularly in renal sympathetic nervous activity. Importantly, these changes are not dependent on short-term elevation or reduction in plasma leptin or insulin, but require some weeks to develop indicating a slow "neural adaptivity" within hypothalamic signalling. These effects can be carried across generations even when offspring are raised on a normal diet. A better understanding of the underlying mechanism should be a high research priority given the prevalence of obesity not just in the current population but also for future generations.