Modulation of Blood Pressure by Central Melanocortinergic Pathways

Epidemiologic and physiological studies have consistently demonstrated that obesity is a major cause of hypertension.1,2 Studies of rodents with diet-induced obesity suggest that increased activation of the sympathetic nervous system is an important mediator of obesity-induced hypertension: alpha- and beta-adrenergic receptor antagonists and renal denervation significantly blunt the rise in arterial pressure associated with weight gain.3-5 However, the physiological mechanisms linking the development of obesity and hypertension are unclear.

The hypothalamic leptin–melanocortin pathway is critically involved in the control of energy balance, and genetic disruption of molecules in this pathway leads to severe obesity in rodents and humans.6 A key component of this pathway is the melanocortin system, which includes leptin-responsive neurons expressing neuropeptide Y and agouti-related-protein and those expressing proopiomelanocortin, which is cleaved to produce the melanocyte-stimulating hormones (MSH) (alpha-, beta-, and gamma-MSH).7 Downstream targets of these neurons express the melanocortin 3 receptor (MC3R) and the melanocortin 4 receptor (MC4R). Melanocortins are agonists of MC3R and MC4R, whereas agouti-related-protein is a high-affinity antagonist.8

Studies in rodents suggest that the melanocortin system is important in cardiovascular regulation.9 Acute central administration of alpha-MSH increases mean arterial pressure and heart rate.10 Pharmacologic studies indicate that this effect is attributable to activation of the sympathetic nervous system.11 Chronic pharmacologic blockade of MC3R and MC4R causes weight gain and a reduction in heart rate but no increase in arterial pressure.12 These pressor and depressor effects are mediated predominantly by signaling through MC4R-expressing neuronal pathways, because Mc4r-null mice maintain a normal blood pressure despite marked obesity and are unresponsive to the pressor effects of central alpha-MSH administration.10

MC4R deficiency is the most common form of inherited human obesity, with a mutation prevalence of approximately 6% in subjects with severe early-onset obesity,13 2.5% in unselected obese adults,14,15 and 0.1% in a population-based study of unselected subjects.16 We have previously described the clinical characteristics of human MC4R deficiency.13 Many of these phenotypes are closely replicated in the Mc4r-deficient mouse, suggesting that it is an excellent model for the human disease.17

To examine the effects of loss of function of the melanocortin pathway on blood pressure in humans, we studied adults who were heterozygous for complete loss-of-function mutations in MC4R (i.e., with haploinsufficiency) and equally overweight or obese control subjects who did not have MC4R mutations. To examine the effects of increased signaling through melanocortin receptors, we studied the short-term effects of a melanocortin receptor agonist in overweight or obese volunteers.
Methods
Physiologic Studies in MC4R-Deficient Subjects

We analyzed blood pressure and metabolic data for 46 MC4R-deficient adults from 32 nuclear families identified as part of the Genetics of Obesity Study.13 All subjects were heterozygous for complete loss-of-function mutations in MC4R (for details, see Table 1 in the Supplementary Appendix, available with the full text of this article at NEJM.org). We recruited 30 adult volunteers whom we determined to be overweight, which was defined as having a body-mass index (BMI, the weight in kilograms divided by the square of the height in meters) of more than 25, or obese (BMI, >30) and who were shown to have a normal MC4R genetic sequence. All subjects provided written informed consent.

Studies were conducted at the Wellcome Trust Clinical Research Facility at Addenbrooke's Hospital in accordance with the principles of the Declaration of Helsinki after approval by the ethics committee at Addenbrooke's Hospital. Eight MC4R-deficient adults of European descent and eight control subjects who were matched in age, ancestral background, and BMI agreed to undergo more detailed studies (Table 2 in the Supplementary Appendix). European descent was determined by the reported country of birth of at least three generations of family members. Details of measurement of blood pressure, autonomic nervous system activation, body composition, measurement of abdominal and liver fat by magnetic resonance imaging, hyperinsulinemic–euglycemic and hyperglycemic clamp studies, along with analytical methods, definitions, and calculations, are provided in the Methods section in the Supplementary Appendix.
Pharmacologic Study in Overweight or Obese Volunteers

We conducted a double-blind, dose-escalating, crossover trial of an MC4R agonist, LY2112688 (Eli Lilly), as compared with placebo, in 28 healthy men and women who were overweight or obese and whose weight had been stable during the previous 6 months (Table 3 in the Supplementary Appendix). The MC4R agonist was administered in two doses (0.45 mg and 1.0 mg) as a subcutaneous infusion over a 24-hour period. After each 24-hour infusion of either the MC4R agonist or placebo, there was a washout interval of 3 to 6 days before the next round of infusions was started in the same subjects. Seven-day infusions of the MC4R agonist at one dose or infusions of placebo were also administered with a washout interval of 7 days in a crossover design. Medications that were known to alter appetite, blood pressure, and autonomic function were prohibited.

Studies were conducted in the Clinical Research Unit at Biotrial in Rennes, France, under Good Clinical Practice guidelines as part of a program for the development of drugs targeting obesity. The study protocol was approved by the ethics review board (Comité de Protection des Personnes de Brest, France). Details regarding the study design, methods for measurement of blood pressure and biochemical values, statistical analysis, and a synopsis of the protocol are provided in the Methods section in the Supplementary Appendix.
Results
Blood Pressure and MC4R Deficiency

We studied the blood pressure and metabolic phenotype in 46 adults with MC4R haploinsufficiency and in 30 control subjects with a normal MC4R genotype (Table 1Table 1Characteristics of the Subjects.). The prevalence of hypertension was significantly lower in the MC4R-deficient group than in the control group (Figure 1AFigure 1Prevalence of Hypertension and Blood-Pressure Measures.). After the exclusion of subjects receiving antihypertensive medications, the mean systolic and diastolic blood-pressure levels were lower in MC4R-deficient adults than in control subjects (Figure 1B and 1C). The prevalence of diabetes was similar in the MC4R-deficient group and the control group (18% and 20%, respectively). After the exclusion of subjects with diabetes, fasting plasma glucose levels were similar in the two groups (Table 1). Consistent with our previous observation that the relative hyperinsulinemia associated with childhood MC4R deficiency becomes less marked with age,13 there was no significant difference in fasting plasma insulin levels between the two groups (Table 1).
Autonomic Nervous System Activity

To examine autonomic nervous system function, we measured variability in heart rate in MC4R-deficient subjects and in control subjects (Table 2 in the Supplementary Appendix). There was no difference in sleeping heart rate between the two study groups (Figure 2AFigure 2Heart Rate and Its High-Frequency Component and RMSSD.). On waking, the heart rate increased in the two groups; however, the magnitude of this increase was significantly attenuated in MC4R-deficient subjects (P=0.007) (Figure 2A). Infusion of insulin increased the heart rate in the two groups. Although the magnitude of the increase was similar, MC4R-deficient subjects maintained significantly lower heart rates throughout the clamp procedure, with a difference of 7 bpm between the two groups during the steady-state period (P<0.001) p="0.02)," p="0.002)">

DOWNLOAD PDF FULL

SHARECASH (U.S recommended)
ZIDDU (worldwide)

Mirror 1
Mirror 2