Ac-Trp-DPhe(p-I)-Arg-Trp-NH₂, a 250-Fold Selective Melanocortin-4 Receptor (MC4R) Antagonist over the Melanocortin-3 Receptor (MC3R), Affects Energy Homeostasis in Male and Female Mice Differently

Abstract

The melanocortin-4 receptor (MC4R) has been indicated as a therapeutic target for metabolic disorders such as anorexia, cachexia, and obesity. The current study investigates the in vivo effects on energy homeostasis of a 15 nM MC4R antagonist SKY2-23-7, Ac-Trp-DPhe(p-I)-Arg-Trp-NH2, that is a 3,700 nM melanocortin-3 receptor (MC3R) antagonist with minimal MC3R and MC4R agonist activity. When monitoring both male and female mice in TSE metabolic cages, sex-specific responses were observed in food intake, respiratory exchange ratio (RER), and energy expenditure. A 7.5 nmol dose of SKY2-23-7 increased food intake, increased RER, and trended towards decreasing energy expenditure in male mice. However, this compound had minimal effect on female mice’s food intake and RER at the 7.5 nmol dose. A 2.5 nmol dose of SKY2-23-7 significantly increased female food intake, RER, and energy expenditure while having a minimal effect on male mice at this dose. The observed sex differences of SKY2-23-7 administration result in the discovery of a novel chemical probe for elucidating the molecular mechanisms of the sexual dimorphism present within the melanocortin pathway. To further explore the melanocortin sexual dimorphism, hypothalamic gene expression was examined. The mRNA expression of the MC3R and proopiomelanocortin (POMC) were not significantly different between sexes. However, the expression of agouti-related peptide (AGRP) was significantly higher in female mice which may be a possible mechanism for the sex-specific effects observed with SKY2-23-7.

A Direct in Vivo Comparison of the Melanocortin Monovalent Agonist Ac-His-DPhe-Arg-Trp-NH2 versus the Bivalent Agonist Ac-His-DPhe-Arg-Trp-PEDG20-His-DPhe-Arg-Trp-NH2: A Bivalent Advantage

“…21, 56, 57 All mice were age matched to have surgeries at 8 weeks old. Briefly, mice were anesthetized with a mixture of xylazine (5 mg/kg) and ketamine (100 mg/kg) administered intraperitoneal (IP).…”

“…21, 56, 57, 60 The mice each received a saline treatment and a hPYY treatment on different days separated by a washout period of at least 3 days in a cross-over design nocturnal feeding paradigm. In the nocturnal feeding paradigm, mice have free access to food and water throughout the experiment.…”

“…21–36 Many of these studies focus on the development of bivalent ligands for their use as high affinity imaging tools or targeting agents for melanoma. 24–36 Given that chemical probes for the five melanocortin receptor subtypes (MC1-5R) have been utilized to study several diseases and disorders including Alzheimer’s disease, 44–46 sexual function disorders, 47, 48 social disorders, 49, 50 cachexia, 51–56 and obesity; 21, 57, 58 it is of interest to study the biological functions of bivalent ligands targeting these receptors. The limited reports of the in vitro functional effects of melanocortin homobivalent have all shown increased functional potency (up to 16-fold).…”

Bench-top to clinical therapies: A review of melanocortin ligands from 1954 to 2016

“…Lensing et al . reported a tetrapeptide, Ac-Trp-DPhe( p -I)-Arg-Trp-NH 2 , that displayed sex-specific metabolic responses in mice [182]. The Ac-Trp-DPhe( p -I)-Arg-Trp-NH 2 peptide possesses micromolar antagonist potency at the mMC3R (pA 2 = 5.4) and nanomolar antagonist potency at the mMC4R (pA 2 = 7.8) [132].…”

“…A 2.5 nmol dose significantly increased food intake, RER, and energy expenditure in female mice, but had minimal effects in males at this dose. These data suggest that Ac-Trp-DPhe( p -I)-Arg-Trp-NH 2 could be used as a sex-specific probe in vivo to study the underlying mechanisms of the melanocortin sexual dimorphism as it related to energy homeostasis [182]. From these results, it may be hypothesized that melanocortin ligands have different therapeutic windows in males and females, with clinically relevant implications.…”