Q J Med 2000; 93:7–14
Review
QJM
The role of melanocortin signalling in the control of body
weight: evidence from human and murine genetic models
G.S.H. YEO, I.S. FAROOQI, B.G. CHALLIS, R.S. JACKSON1 and S. O’RAHILLY
From the Departments of Medicine and Clinical Biochemistry, Cambridge Institute for Medical
Research, Addenbrooke’s Hospital, University of Cambridge, and 1Department of Clinical
Biochemistry, Addenbrooke’s Hospital, Cambridge, UK
Summary
The peptide products of the pro-opiomelanocortin
(POMC) gene have established roles in the control
of physiological processes as diverse as adrenal steroidogenesis, skin pigmentation, analgesia and
inflammation. In the last 5 years, evidence accumulated from murine and human genetic models of
disrupted melanocortin signalling has firmly established a central role for a population of hypothalamic neurons expressing POMC in the control of
appetite and body weight. Of the five known melan-
ocortin receptors, the MC4R has been most closely
linked to body weight regulation. While a-MSH is
active at this receptor and suppresses appetite after
central injection, important roles for other POMCderived products have not been excluded. The development of pharmacological agonists acting on, or
mimicking, the hypothalamic melanocortinergic
pathway may provide exciting opportunities for the
therapy of human obesity.
Introduction
The POMC gene is expressed at physiologically
significant levels in a number of mammalian tissues
including anterior and intermediate pituitary, skin,
the immune system and hypothalamic neurons.1–3 In
these tissues the precursor peptide, POMC, is cleaved
into a range of smaller peptides including adrenocorticotropin (ACTH), b-endorphin and a-, b- and cmelanocyte-stimulating hormones (MSH) (Figure 1).
The repertoire of products derived from POMC by
any tissue is determined by the specificities of the
endoproteases (convertases) expressed in the secretory pathway.4,5 Thus, anterior pituitary corticotrophs
express prohormone convertase 1 (PC1) and
cleave POMC to ACTH, while melanotrophs in the
intermediate lobe of lower animals, express prohormone convertase 2 (PC2) and cleave ACTH further
to yield a-MSH. The physiological roles of the
various melanocortin peptides have been defined
with varying degrees of certainty. Thus, ACTH,
secreted from the anterior pituitary is the major
controller of adrenal steroidogenesis.6 In amphibians
and rodents, a-MSH from the intermediate lobe of
the pituitary is involved in the control of skin and
coat pigmentation.7 Mammals also express POMC in
skin, and locally secrete not only a-MSH but also band c-MSH, ACTH and b-endorphin, although their
physiological functions are not established.8 The
importance of at least some of these peptides to
human pigmentation is revealed by the association
of variants of the human melanocortin-1 receptor
with red hair and fair skin.9–11 The physiological
roles of central b- and c-MSH, CLIP and b-lipotropin
are also less than clear. Beta-endorphin, although a
product of the POMC gene, does not share the
Address correspondence to Professor S. O ’Rahilly, Departments of Medicine and Clinical Biochemistry, Level 5,
Addenbrooke’s Hospital, Hills Road, Cambridge CB2 2QQ, UK
© Association of Physicians 2000
8
G.S.H. Yeo et al.
Figure 1. Schematic diagram of POMC post-translational processing. The di-basic cleavage sites are indicated with black
arrows, and are denoted in single letter amino acid abbreviations: K, lysine and R, arginine.
receptor binding sequence common to the melanocortin peptides, and interacts with opioid receptors.
The actions of the melanocortin peptides are
mediated by a family of five G-protein-coupled,
seven-transmembrane-domain receptors. The sites of
their expression and presumed physiological roles
are summarized in Table 1. Of note, the MC3R12,13
and the MC4R14,15 are expressed in areas of the
brain that have been implicated in the control of
feeding. The first evidence that central melanocortinergic pathways may be involved in feeding came
from studies of rodents in which the intracerebroventricular (ICV) administration of a-MSH led to suppression of food intake.16,17 More recently, ICV injection
of the cyclic hepta-peptide MTII, a melanocortin
agonist active at the MC3R and MC4R, reduced
food intake in a number of rodent models, this
effect being blocked by the specific antagonist
SHU9119.18,19 The central administration of
SHU9119 increases food intake, providing evidence
that endogenous melanocortins are indeed likely to
be involved in appetite control.19 The neuroanatomy
of hypothalamic neurons expressing POMC is consistent with a putative role for such cells in the
control of energy balance. Thus POMC-expressing
neurons within the arcuate nucleus (ARC) project
extensively to specific brain areas thought to regulate
feeding, including the paraventricular nucleus, lateral
hypothalamus, dorsomedial nucleus, ventromedial
hypothalamic nucleus and dorsal motor nucleus of
the brainstem (Figure 2). Immunocytochemistry and
in situ hybridization studies indicate that 40% of
POMC-containing neurons express mRNA for the
long form of the leptin receptor.20 POMC expression
in the arcuate nucleus is increased by leptin administration and reduced in hypoleptinaemic states such
as fasting or in the ob/ob mouse.21 Leptin administra-
tion to the ob/ob mouse restores POMC expression
in the rostral arcuate nucleus.22 Interestingly, a large
percentage of POMC-containing neurons also express
the recently described anorexic peptide CART
(cocaine and amphetamine regulated transcript).23
CART expression parallels POMC expression in
response to leptin treatment.24 Furthermore, ICV
injection of recombinant CART into both rats and
mice inhibits both normal and starvation-induced
feeding.24 Conversely, following central administration of anti-serum against CART in rats, the feeding
response was increased.24 Thus, this novel neuropeptide appears to co-operate with POMC in mediating
the appetite suppression effects of leptin.
Genetic defects involving
over-expression of melanocortin
antagonists
The first genetic evidence that melanocortin signalling may be important for the control of mammalian
body weight came from studies of the agouti (yellow)
mouse. The genetic basis for this dominantlyinherited murine phenotype was elucidated in
1994,25 and was the first of a number of such gene
discoveries in rodent obesity that have helped to
catapult the field of obesity into the forefront of
contemporary biomedical research. Yellow agouti
mice have a pleiotropic syndrome which is characterized by obesity, hyperphagia, hyperinsulinaemia, hyperglycaemia in males and a yellow coat
colour.26–28 The phenotype in these mice is caused
by inherited promoter rearrangements at the agouti
locus that result in constitutive ectopic expression of
the agouti peptide.27–31 Agouti is a 131-amino-acid
Control of hair and skin pigmentation33
ACTH receptor; controls adrenal steroidogenesis56
?
Appetite control,58 other?
Melanocytes
Adrenal cortex, adipocytes
Hypothalamus, thalamus, hippocampus, placenta, pancreas, stomach, duodenum
Hypothalamus, thalamus, limbic system, hindbrain, brainstem, cortex, spinal cord
Brain, skeletal muscle, adipocytes, spleen, thymus, testis, bone marrow, pituitary,
heart, lung, kidney, liver
MC1R
MC2R
MC3R
MC4R
MC5R
Control of sebaceous gland secretion57
Presumed physiological role
Sites of expression67
Receptor
Table 1 Summary of the sites of expression and presumed physiological role of the melanocortin receptors
Melanocortin and control of body weight
9
protein which is normally secreted only within the
hair follicle, where it acts in conjunction with MSH
to regulate hair pigmentation, producing the murine
wild-type coat coloration of black hairs with a
subapical yellow band.26,27 In the absence of the
agouti protein, MSH binds to the melanocortin-1
receptor (MC1R) on the surface of melanocytes,
leading to the production of brown-black (eumelanin)
pigment. Agouti functions as a paracrine molecule,
antagonizing the MCIR and inducing a switch from
eumelanin to yellow-red (phaeomelanin) pigment
production.25 The obesity phenotype seen in the
agouti syndrome is not dependent on the synthesis
of phaeomelanin or the disruption of MC1R signalling, since Avy mice carrying a gain-of-function MC1R
mutation are black yet still obese,32 while mice
lacking MC1R activity are yellow but not obese.33
A hypothesis for the mechanism of this obesity
syndrome was that ectopic agouti expression resulted
in aberrant antagonism of other melanocortin receptors in areas of the brain known to be involved in
the control of food intake and body weight.15,25
Agouti potently antagonizes the action of melanocortin peptides at the MC1R, the MC4R, and to a lesser
degree the MC2R, but not at the MC3R or MC5R.25,34
This proposed mechanism for the phenotype of the
agouti syndrome was supported by the identification
of the Agouti-related protein or AGRP, a hypothalamic neuropeptide found in both rodents and
humans, which shares high sequence homology with
Agouti.35,36 AGRP was found to be a melanocortin
receptor antagonist which primarily antagonizes
the MC3R and MC4R.36 Ubiquitous overexpression
of human AGRP results in an obese mouse sharing
the identical phenotype to the agouti mouse, but
possessing a normal coat colour.36 AGRP mRNA is
expressed almost exclusively in the arcuate nucleus
of the hypothalamus,35,36 and it appears that AGRP
is co-expressed in neurons containing the potent
orexigenic peptide neuropeptide Y (NPY).37 AGRP
mRNA levels are five to ten times higher in fasted
animals and in ob/ob mice, and suppress to normal
levels after leptin treatment.35,38
Genetic defects of POMC synthesis
The first direct evidence for the involvement of
melanocortin peptides in the control of human
appetite came from the description of two patients
with defects in the POMC gene.39 One proband was
compound heterozygous for two mutations which
prevented synthesis of ACTH and a-MSH, and the
second unrelated proband was homozygous for a
nucleotide transversion upstream of the start codon
which interferes with translational initiation by introducing an additional start site. The affected children
10
G.S.H. Yeo et al.
Figure 2. Schematic diagram of POMC and AGRP neuron projections. PVN, paraventricular nucleus; DMN, dorsomedial
nucleus; VMN, ventromedial nucleus; LH, lateral hypothalamus; ARC, arcuate nucleus; OC, optic chiasm; SCN,
suprachiasmatic nucleus. Peptides (−) inhibiting or (+) stimulating food intake.
presented with the metabolic consequences of hypocortisolaemia in the first few months of life, and
had undetectable plasma cortsiol and ACTH.
Glucocorticoid treatment reversed the severe metabolic derangement, but progressive obesity associated with hyperphagia developed in both children.
Intriguingly, both children had very pale skin and
red hair. The failure of adrenal steroidogenesis was
a predictable consequence of the lack of ACTH
action on the adrenal MC2R. Mutations in the MC1R
are common in red-haired humans and presumably
the lack of endogenous ligand for the MC1R was
responsible for the skin/hair phenotype.9,11 The
normal phenotype of the heterozygous parents indicates a recessive mode of inheritance. Presumably
the hyperphagia and weight gain is the result of lack
of melanocortinergic signalling within the appetite
control centres in the brain. Of interest, no other
behavioural or neurological dysfunction is apparent
in these children, implying that appetite control may
be the dominant function of POMC-expressing neurons within the central nervous system. These children provide the first compelling evidence for an
important role of POMC in the control of human
appetite.
Genetic defects of POMC processing
The fat/fat mouse displays a recessive syndrome of
adult-onset marked obesity associated with high
circulating concentrations of POMC and proinsulin.40
The defective prohormone processing is due to an
inactivating missense mutation in carboxypeptidase
E (CPE), an enzyme which co-operates with prohormone convertases in the proteolyic maturation of
many hormone and neuropeptide precursors.41
Interestingly, transgenic replacement of CPE activity
in the islets of fat/fat mice corrects the hyperproinsulinaemia, but does not alter their obesity, indicating
that islet dysfunction is unlikely to be its cause.40
While deficient melanocortin signalling within the
central nervous system, due to defective processing
of POMC, is an attractive candidate to explain the
obesity, the pathophysiology must be complex,
because CPE dysfunction will affect the processing
of numerous neuropeptides, some of which,
e.g. GLP1 and neurotensin, have putative roles in
the control of satiety whilst others, e.g. neuropeptide
Y and melanin concentrating hormone, are orexigenic.42–44 The cause of the uniquely late-onset obesity
of the fat/fat mouse remains a tantalising mystery.
Despite searching, mutations of the human CPE gene
have not been found.45
In 1995, we reported a woman with a novel
syndrome suggestive of defective prohormone processing.46 Her clinical features included extreme
childhood obesity, reactive hypoglycaemia and
hypogonadotrophic hypogonadism. In her plasma,
the ratios of proinsulin to insulin, and POMC
Melanocortin and control of body weight
to ACTH were enormously elevated. Subsequently,
she was found to be compound heterozygous for
PC1 mutations.47 As with the fat/fat mouse, while
this patient provides evidence which suggests that
the obesity and hyperphagia may have arisen from
failure of POMC processing, the broader role of PC1
implies that other candidate molecules may be
involved, for example GLP-1.48,49 Interestingly, unlike
the human with PC1 mutations, PC2 null mice are
not obese, despite reports that PC2 is necessary for
processing of POMC to MSH and the formation of
other anorexic peptides such as CRH and neurotensin5.50–52 A possible explanation may lie in the
particular role PC2 has in cleaving b-endorphin
from POMC and enkephalin from proenkephalin.
Mature enkephalin levels in the brains of PC2 null
mice are markedly reduced and endogenous opioid
pathways may have a role in signalling within
orexigenic networks.44,53
We recently reported the development of obesity
in a child first described by Nussey and co-workers,
who has undetectable plasma ACTH, markedly elevated plasma POMC and normal proinsulin processing.54,55 The genomic sequences of POMC
and PC1 are normal, and the phenotype is distinct
from that of PC2 or CPE deficiency. Though the
precise nature of the defect in this child remains
obscure, she provides further suggestive evidence
that defective processing of POMC is associated with
obesity in man.
Genetic defects of melanocortin
receptor signalling
The phenotypic effects of disruption of
the MCIR, MC2R and MC5R do not involve alterations in energy balance and have their main effects
on pigmentation, adrenal function and sebaceous
gland secretion, respectively.33,56,57 In contrast, targeted disruption of the MC4R in mice results in an
obesity syndrome characterized by hyperphagia, hyperinsulinaemia, hyperglycaemia, and increased linear
growth, with no abnormality of the reproductive or
adrenal axes.58 MC4R knockout mice do not respond
to the anorectic effects of the agonist MTII.59
Heterozygotes for the null MC4R allele exhibit a
phenotype intermediate between that of wild-type
and homozygous littermates.58 These data provide
strong evidence that the MC4R receptor is involved
in the mediation of the effects of melanocortins on
appetite. Results from the knock out of the MC3R
are eagerly awaited, and it remains possible that this
receptor, which is widely expressed in the CNS, may
also play a role in some aspects of energy balance.
Subsequently, we and others have identified a
number of different families in which mutations in
11
the human MC4R are associated with severe early
onset obesity.60–63 It is notable that most of the
affected patients reported thus far have been heterozygous for their respective mutations, and that these
obese subjects show no evidence of impaired adrenal
or reproductive function, all of which are consistent
with the rodent model data. The finding of a phenotype associated with heterozygous mutations in
the MC4R is consistent with the murine data, and
suggests that this receptor may represent a tightly
regulated control point in the homeostatic control of
body weight. The fact that heterozygous null mutations result in an obese phenotype in both humans
and mice is highly suggestive of haplo-insufficiency
rather than dominant negativity, and indicates that
the regulatory system is sensitive to quantitative
variation in MC4R expression.
Summary and conclusions
Accumulating evidence from humans and mice with
genetic defects in POMC synthesis, processing and
melanocortin receptor signalling clearly points to an
important role for the hypothalamic melanocortinergic neuronal population in the control of appetite
and energy balance. Is there any evidence that
dysfunction of this pathway might contribute to more
common forms of human obesity? Intriguingly, a
number of independent linkage studies attempting to
identify quantitative trait loci (QTLs) involved in the
determination of obesity and obesity-associated traits
have come up with positive LOD scores for a locus
at chromosome 2p21 which encompasses the POMC
gene. Thus Comuzzie et al. (1997), studying a
Mexican-American population, reported that this
locus accounted for 47% of the variation in serum
leptin levels.64 Subsequent studies on French families65 as well as a population of African Americans66
have reported the same association. Of course, this
region of chromosome 2 encompasses many more
genes than POMC and there is, as yet, no proof that
sequence variations in or around POMC are responsible for these findings.
The potential therapeutic implications of this body
of work are readily apparent and have not been lost
on the pharmaceutical industry. Previous pharmacological approaches to the development of appetite
suppressants involving manipulation of serotoninergic and or noradrenergic signalling have run
aground as a result of the toxicity associated with
the interference with the broader functions of these
transmitters in other tissues and other CNS pathways.
Given the fact that the MC4R has a much more
anatomically restricted pattern of expression and
given the relatively ‘pure’ obesity phenotype resulting
12
G.S.H. Yeo et al.
from its dysfunction, this receptor clearly represents
an exciting potential therapeutic target.
17.
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