In Vivo Evidence for Inverse Agonism of Agouti-Related Peptide in the Central Nervous System of Proopiomelanocortin-Deficient Mice

Abstract

OBJECTIVE-Melanocyte-stimulating hormone (MSH) peptides processed from proopiomelanocortin (POMC) regulate energy homeostasis by activating neuronal melanocortin receptor (MC-R) signaling. Agouti-related peptide (AgRP) is a naturally occurring MC-R antagonist but also displays inverse agonism at constitutively active melanocortin-4 receptor (MC4-R) expressed on transfected cells. We investigated whether AgRP functions similarly in vivo using mouse models that lack all neuronal MSH, thereby precluding competitive antagonism of MC-R by AgRP. RESEARCH DESIGN AND METHODS-Feeding and metabolic effects of the MC-R agonist melanotan II (MTII), AgRP, and ghrelin were investigated after intracerebroventricular injection in neural-specific POMC-deficient (Pomc Ϫ/Ϫ Tg/ϩ) and global POMC-deficient (Pomc Ϫ/Ϫ ) mice. Gene expression was quantified by RT-PCR. RESULTS-Hyperphagic POMC-deficient mice were more sensitive than wild-type mice to the anorectic effects of MTII. Hypothalamic melanocortin-3 (MC3)/4-R mRNAs in POMC-deficient mice were unchanged, suggesting increased receptor sensitivity as a possible mechanism for the heightened anorexia. AgRP reversed MTII-induced anorexia in both mutant strains, demonstrating its ability to antagonize MSH agonists at central MC3/4-R, but did not produce an acute orexigenic response by itself. The action of ghrelin was attenuated in Pomc Ϫ/Ϫ Tg/ϩ mice, suggesting decreased sensitivity to additional orexigenic signals. However, AgRP induced delayed and long-lasting modifications of energy balance in Pomc Ϫ/Ϫ Tg/ϩ, but not glucocorticoid-deficient Pomc Ϫ/Ϫ mice, by decreasing oxygen consumption, increasing the respiratory exchange ratio, and increasing food intake. CONCLUSIONS-These data demonstrate that AgRP can modulate energy balance via a mechanism independent of MSH and MC3/4-R competitive antagonism, consistent with either inverse agonist activity at MC-R or interaction with a distinct receptor. Diabetes 57:86-94, 2008 G enetic disruption of either mouse or human proopiomelanocortin (POMC) causes early-onset obesity (1-3), highlighting a major role of POMC in the regulation of energy homeostasis. POMC is processed posttranslationally into multiple peptides, including the opioid ␤-endorphin and the melanocortins ACTH, ␣-melanocyte-stimulating hormone (␣MSH), ␤MSH, and ␥MSH. POMC peptides in the central nervous system (CNS) are essential in the regulation of energy intake and expenditure as demonstrated in studies using compound mutant mice (Pomc Ϫ/Ϫ Tg/ϩ) expressing a Pomc transgene that selectively restored pituitary POMC in Pomc Ϫ/Ϫ mice to produce a neural-selective POMC deficiency (4). Lack of ␣MSH is likely the principal cause of obesity (3,5) due to the loss of agonist signaling at central melanocortin receptors (MC-R), melanocortin-3 receptor (MC3-R), and melanocortin-4 receptor (MC4-R), each of which plays a distinct role in the regulation of energy homeostasis (6 -8).The anorectic actions of centrally administered ␣MSH or the synthetic MC3/4-R agonist melanotan II (MTII) (9...

Obesity-programmed mice are rescued by early genetic intervention

“…The two notable parameters associated with weight loss after the induction of hypothalamic Pomc were reduction in food consumption and normalization of locomotor activity. The immediate phase of extreme hypophagia lasting for 2-3 weeks after induction of Pomc expression conceivably was due to MCR supersensitivity resulting from the previous lack of melanocortin signaling (30). Energy expenditure, as measured by oxygen consumption, was not altered after Pomc rescue in either sex and actually remained significantly decreased in female mice, despite their increased locomotor activity.…”

Pertussis Toxin-sensitive Signaling of Melanocortin-4 Receptors in Hypothalamic GT1-7 Cells Defines Agouti-related Protein as a Biased Agonist

“…In addition, it has been proposed that AGRP decreases basal as well as forskolinpromoted adenylyl cyclase activity, thus also acting as an inverse agonist on basal MCR activity (11). However, recent studies revealed that the effects of AGRP on appetite control are independent of melanocortin signaling (12,13). For example, in mice deficient of the melanocortin precursor proopiomelanocortin starvation after AGRP neuron ablation is independent of melanocortin signaling (13).…”

Hypothalamic and brainstem neuronal circuits controlling homeostatic energy balance

“…In general terms, POMC (anorexigenic) and AgRP (orexigenic) neurons have opposite physiological functions, which are largely the consequence of the contrasting actions of a-MSH and AGRP peptides on MCRs: while a-MSH is an endogenous MCR agonist, AGRP is an inverse agonist (Haskell-Luevano & Monck 2001, Nijenhuis et al 2001, Tolle & Low 2008. Indeed, substantial experimental evidence indicates that the agonism of MCRs attenuates appetite and enhances energy expenditure, whereas their antagonism has essentially the opposite effects (Fan et al 1997, Harrold et al 1999, Hwa et al 2001.…”