Pro-opiomelanocortin and weight regulation: from mice to men.

Significant progress in our understanding of the mechanisms of weight homeostasis has been made by studying the many genetic mouse models of obesity. Positional cloning in the obese mouse led to the discovery of leptin as a feedback messenger indicating the adequacy of peripheral energy stores. This was the first in a series of important advances in this field. Shortly after this discovery, two research laboratories presented evidence for the role of hypothalamic pro-opiomelanocortinergic (POMC) neurons as important mediators in the regulation of feeding behavior, insulin levels and, ultimately, body weight. One of these mouse obesity models, the lethal yellow mouse, constitutively overexpresses the agouti protein, an endogenous antagonist of both the melanocortin 1 (MC1) and melanocortin 4 (MC4) receptors. A second mouse obesity model was created by knocking out the MC4 receptor. Investigations using both the autosomal dominant lethal yellow mouse and MC4 receptor knockout mouse have provided clear evidence for the role of hypothalamic POMC neurons and the MC4 receptor in the regulation of weight homeostasis in the rodent. Furthermore, the recent discovery of agouti-related protein (AGRP), an agouti-like peptide naturally found in the hypothalamus, provides further evidence for the importance of POMC neurons in the regulation of weight. Although the significance of central POMC and AGRP in the rodent is apparent, the role of POMC neurons in the regulation of weight and feeding behavior in humans is only now being appreciated.

The role of melanocortins in adipocyte function.

It has been demonstrated that adipocytes express high affinity ACTH and alpha-MSH binding sites, and that ACTH, alpha-MSH, and beta-LPH are potent lipolytic hormones. Considerable species variability exists in the lipolytic response to melanocortins, however. Recently, MC2 and MC5 receptor-mRNA was found in both murine adipocytes and in the 3T3-L1 murine embryonic fibroblast cell line, but only after the 3T3-L1 cells had differentiated into adipocytes. The 3T3-L1 cell line was used to characterize the pharmacological properties of both MC2 and MC5 receptors in situ. Both murine MC2 and MC5 receptors are functional in the adipocyte, although the MC5 receptor required high doses of alpha-MSH to activate cylase. ACTH potently stimulates cyclase with EC50 values that are consistent with the hypothesis that the murine MC2 receptor, not the MC5 receptor, mediates stress-induced lipolysis via release of ACTH from the pituitary.

Mahogany (mg) stimulates feeding and increases basal metabolic rate independent of its suppression of agouti.

The mahogany (mg) locus originally was identified as a recessive suppressor of agouti, a locus encoding a skin peptide that modifies coat color by antagonizing the melanocyte-stimulating hormone receptor or MC1-R. Certain dominant alleles of agouti cause an obesity syndrome when ectopic expression of the peptide aberrantly antagonizes the MC4-R, a related melanocyte-stimulating hormone receptor expressed in hypothalamic circuitry and involved in the regulation of feeding behavior and metabolism. Recent work has demonstrated that mg, when homozygous, blocks not only the ability of agouti to induce a yellow coat color when expressed in the skin of the lethal yellow mouse (AY), but also the obesity resulting from ectopic expression of agouti in the brain. Detailed analysis of mg/mg AY/a animals, presented here, demonstrates that mg/mg blocks the obesity, hyperinsulinemia, and increased linear growth induced by ectopic expression of the agouti peptide. Remarkably, however, mg/mg did not reduce hyperphagia in the AY/a mouse. Furthermore, mg/mg induced hyperphagia and an increase in basal metabolic rate in the C57BL/6J mouse in the absence of AY. Consequently, although mahogany is broadly required for agouti peptide action, it also appears to be involved in the control of metabolic rate and feeding behavior independent of its suppression of agouti.

Independent and additive effects of central POMC and leptin pathways on murine obesity.

The lethal yellow (AY/a) mouse has a defect in proopiomelanocortin (POMC) signaling in the brain that leads to obesity, and is resistant to the anorexigenic effects of the hormone leptin. It has been proposed that the weight-reducing effects of leptin are thus transmitted primarily by way of POMC neurons. However, the central effects of defective POMC signaling, and the absence of leptin, on weight gain in double-mutant lethal yellow (AY/a) leptin-deficient (lepob/lepob) mice were shown to be independent and additive. Furthermore, deletion of the leptin gene restored leptin sensitivity to AY/a mice. This result implies that in the AY/a mouse, obesity is independent of leptin action, and resistance to leptin results from desensitization of leptin signaling.

Targeted disruption of the melanocortin-4 receptor results in obesity in mice.

The melanocortin-4 receptor (MC4-R) is a G protein-coupled, seven-transmembrane receptor expressed in the brain. Inactivation of this receptor by gene targeting results in mice that develop a maturity onset obesity syndrome associated with hyperphagia, hyperinsulinemia, and hyperglycemia. This syndrome recapitulates several of the characteristic features of the agouti obesity syndrome, which results from ectopic expression of agouti protein, a pigmentation factor normally expressed in the skin. Our data identify a novel signaling pathway in the mouse for body weight regulation and support a model in which the primary mechanism by which agouti induces obesity is chronic antagonism of the MC4-R.

Role of melanocortinergic neurons in feeding and the agouti obesity syndrome.

Dominant alleles at the agouti locus (A) cause an obesity syndrome in the mouse, as a consequence of ectopic expression of the agouti peptide. This peptide, normally only found in the skin, is a high-affinity antagonist of the melanocyte-stimulating hormone receptor (MC1-R), thus explaining the inhibitory effect of agouti on eumelanin pigment synthesis. The agouti peptide is also an antagonist of the hypothalamic melanocortin-4 receptor (MC4-R). To test the hypothesis that agouti causes obesity by antagonism of hypothalamic melanocortin receptors, we identified cyclic melanocortin analogues that are potent agonists or antagonists of the neural MC3 (refs 11, 12) and MC4 receptors. Intracerebroventricular administration of the agonist, MTII, inhibited feeding in four models of hyperphagia: fasted C57BL/6J, ob/ob, and A(Y) mice, and mice injected with neuropeptide Y. Co-administration of the specific melanocortin antagonist and agouti-mimetic SHU9119 completely blocked this inhibition. Furthermore, administration of SHU9119 significantly enhanced nocturnal feeding, or feeding stimulated by a prior fast. Our data show that melanocortinergic neurons exert a tonic inhibition of feeding behaviour. Chronic disruption of this inhibitory signal is a likely explanation of the agouti obesity syndrome.

Characterization of melanocortin receptor subtype expression in murine adipose tissues and in the 3T3-L1 cell line.

It has been known for many years that adipocytes express high affinity ACTH and alpha-melanocyte stimulating hormone (MSH) binding sites, and that ACTH, alpha-MSH, and beta-lipotropin are potent lipolytic hormones. We show here that the adipocyte response to the melanocortin peptides results from the expression of both the MC2 (ACTH) receptor as well as the newly discovered MC5 receptor. Using RT-PCR and Northern blot hybridization, high levels of MC2 receptor messenger RNA (mRNA) were found in all adipose tissues examined in the mouse, whereas MC5 receptor mRNA was found in a subset of these. Both receptors mRNAs were also found in the 3T3-L1 cell line but only after the cells had been induced to differentiate into adipocytes. This cell line was then used to characterize the pharmacological properties of the MC2 and MC5 receptor sites in situ. The MC2 receptor exhibits properties similar to the ACTH receptor characterized in adrenocortical cells, coupling to activation of adenylyl cyclase with an EC50 of approximately 1 nM. An MSH binding site characterized in these cells is presumably the MC5 receptor, based on the observation that this is the only other melanocortin receptor mRNA detected in these cells. The MC5 receptor in the 3T3-L1 adipocyte activated adenylyl cyclase in response to alpha-MSH stimulation. Interestingly, Nle4, D-Phe7-alpha-MSH (NDP-MSH), a commonly used synthetic alpha-MSH agonist, was a potent antagonist of the MC5 receptor expressed in the 3T3-L1 cell line. Although the agouti signaling peptide is a potent antagonist of NDP-MSH binding to the MC1 and MC4 melanocortin receptors, agouti was unable to block NDP-MSH binding in the 3T3-L1 adipocyte.

Pilot prospective evaluation of counterpulsation with different intra-aortic balloon volumes on cardiac performance in humans.

UNLABELLED: Smaller intra-aortic balloons (IAB) may minimize peripheral vascular complications. To determine the influence of different IAB volumes on cardiac performance, we studied 20 hemodynamically stable patients on IAB counterpulsation. Variables were measured with either a 40cc or 32cc IAB displacement, at an assist ratio of 1:1 or 1:8: Heart rate, Ao and PA pressures, and Fick cardiac output. By echo-Doppler, the velocity time integral (VTI) across the LV outflow tract, a measure of stroke volume was also calculated. The mean age was 60 yr (range 18-77), height 5'6" (5'2"-5'11"), and body surface area 1.9M2 (1.5-2.3). Results presented as mean values were: [table: see text] *P < 0.005 1:1 vs 1:8 for both 40 and 32cc IAB. CONCLUSION: IAB pumping at 1:1 with either 40 cc or 32 cc volume displacement yields similar degrees of improvement in cardiac performance. A larger cohort is required to determine if smaller balloons may decrease complications without compromising efficacy.

Activating mutation in the stimulatory guanine nucleotide-binding protein in an infant with Cushing's syndrome and nodular adrenal hyperplasia.

Cushing's syndrome in infancy is uncommon. In this report, we describe an infant with ACTH-independent Cushing's syndrome in which an activating mutation in the stimulatory G-protein (Gs alpha) was detected. The patient presented at 3 months of age with Cushingoid features, poor linear growth, and elevated liver enzymes. Plasma ACTH and dexamethasone suppression test results were consistent with ACTH-independent Cushing's syndrome, and a subsequent adrenalectomy revealed bilateral adrenocorticonodular hyperplasia. Asymptomatic lesions consistent with fibrous dysplasia were later detected on bone scan. Genomic DNA was extracted from adrenal, liver, and blood and amplified by polymerase chain reaction with Gs alpha exon 8 primers. Using allele-specific oligonucleotide hybridization, the DNA was probed for known Gs alpha-activating mutations. A point mutation coding for an arginine to cysteine substitution at codon 201 of exon 8 was detected in genomic DNA from this infant's adrenal, liver, and leukocytes. The mutation was detected in nodular adrenal tissue, but was essentially absent in normal adrenal tissue. Activating mutations in the Gs alpha gene have previously been described in GH-secreting tumors, thyroid adenomas, and the McCune-Albright syndrome and are probably involved in the pathogenesis of adrenocorticonodular hyperplasia in this infant with Cushing's syndrome.

Reversible cardiomyopathy in an infant with unrecognized congenital adrenal hyperplasia.

We describe an infant who had a dilated cardiomyopathy and who was later found to have congenital adrenal hyperplasia. The cardiomyopathy resolved after replacement of glucocorticoid and mineralocorticoid. We believe that glucocorticoid deficiency may have played a direct role in the evolution of this cardiomyopathy.