Melanocortins and appetite

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Melanocortins and appetite

Overview of Pro-opiomelanocortin (POMC)

Pro-opiomelanocortin (POMC) is an important pro-hormone that gives rise to an array of bioactive peptide hormones that are implicated in energy balance, stress responses, pain, immune modulation, satiety, pigmentation and even exocrine gland secretions. It is expressed in the pituitary gland, skin, immune system and brain. In the brain it is expressed in the arcuate nucleus of the hypothalamus (ARH). POMC is post-translationally cleaved by a complex set of enzymes into a variety of hormones (see fig 1).

(PD) Image: Jessica Ivy
Fig1: Simplified diagram of the breakdown of POMC by prohormone convertase enzymes into the melanocortins and opioid hormone B endorphin.

What is also interesting is that both B endorphin and alpha MSH are cleaved from the same prohormone (POMC) alpha MSH is known to reduce feeding behaviour as we'll see later but B endorphine is shown to increase appetite. It is therefore important to explore how the prohormone may be processed in different tissues and what dictates this. In the same sense, how are the cleavage enzymes regulated to produce various concentrations of the different peptide hormones?

Experiments in which the propeptide convertase enzymes are knocked out are not particularly helpful as PC1 and PC2 are expressed in cells other than POMC cells and have important physiological functions other than POMC cleavage. For example PC1 is essential for the biosynthesis of insulin and PC2 the biosynthesis of glucagon. However, it has been seen that PC2 KO mice exhibit no alpha MSH expression at all and Prader Willi patients have reduced hypothalamic PC2 expression. Millington et al 2003. The PC2 knock out mice are so defective in other ways(Scamuffa et al 2006) it is hard to tell whether the lack of alpha MSH has an effect. Prader Willi sufferers do exhibit an obese phenotype but again this syndrome comes as a result of a mutation of several genes so it cannot be inferred that PC2 mutation is solely responsible for the obese and hyperphagic phenotype.

Some evidence that leptin modifies POMC processing as leptin deficient mice show an upregulation of POMC and PC2 expression (Pritchard et al 2002). You cannot however infer from this that the proteins themselves are increased nor does it mean that alpha MSH production is increased. Further proteomic studies need to be done to work out a technique to measure alpha MSH in vivo.

POMC was first noted for its importance in appetite and obesity in rodent studies. POMC-null mice were seen to exhibit hyperphagia and obesity without insulin resistance. Their adrenal gland was also atrophic and glucocorticoid levels were undetectable as a result of a lack of ACTH stimulating adrenal cell secretion and proliferation. This suggestion that POMC has a role in energy balance was supported by studies on rare individuals with POMC mutations. In humans a lack of POMC is fatal unless glucocorticoids are administered from birth as cortisol is essential for humans. A few rare individuals with the mutation have shown very similar phenotype to the POMC knockout mouse along with the characteristic red hair. Also saw

While these studies show that POMC-derived hormones may have a role in energy balance they don’t tell us which peptides are responsible for the effects and furthermore the lack of adrenal hormones as a secondary result of POMC lack may overshadow the primary POMC effects. Therefore we are required to look at the peptides in more detail.

The main system implicated in energy balance is the MELANOCORTIN SYSTEM.

Melanocyte-stimulating hormones and their Receptors

This is made up of several endogenous agonists such as alpha, beta, gamma MSH and interestingly has two endogenous inverse agonists as well namely the AgRP and agouti. These act on 5 different subtypes of the MCR (MCR1-5). See table…

The Melanocortin Pathway

The control and regulation of feeding involves a complex interplay between circulating hormones, neurotransmitters, neuropeptides and nutrients, and the melanocortin system is a central component in this.

The melanocortin system is the name collectively given for;

• Neurons arising in the arcuate nucleus of the hypothalamus that express AgRP or POMC.
• POMC neurons in the caudal brainstem
• Melanocortin receptors, predominately MC3R and MC4R that respond to POMC peptides and AgRP.

In addition to its involvement in regulating energy homeostasis, the melanocortin system plays a role in mediating a number of physiological processes within the body including (3)

Although the melanocortin system is central to the regulatory mechanisms controlling appetite and satiety, the precise mechanism is not fully understood. Complexity arises from both the direct and indirect effects of a number of compounds including leptin, insulin, glucose, ghrelin, NPY, serotonin, peptide YY and endorphin, all of which act in isolation and in sync to mediate their effects on these POMC neurons.

Below is a table showing the opposing effects that POMC agonists and antagonists have on feeding behaviour.

Agonists Antagonists Reduce food intake Enhance food intake (hyperphagia) Increase energy expenditure Decrease energy expenditure Reduce body weight Increase body weight

This highlights the role of the melanocortin system in regulating energy homeostasis, and why disruption in the genes controlling this system, i.e. genetic mutations of the system, can result in individuals which are hyperphagic and consequentially obese.

A number of mutations of this system have been identified in mice, all of which show a dysregulation in energy homeostasis. Many of the mutations discovered involve excess production of POMC antagonists, so that melanocortin agonists can’t bind to melanocortin receptors to suppress appetite.

1. Excess production of AgRP induces its antagonistic effects through binding to both the MC1R and MC4R.
2. An increase in the expression of AgRP, which functions by antagonising receptors MC3R and MC4R. This prevents the potent appetite suppressor alpha MSH from binding.
3. A mutation which results in a deficiency in the number of MC4R receptors.
4. The insufficient production of POMC derived peptides to bind to these receptors.

Obese individuals have presented with POMC deleterious gene mutations, as well as with heterozygous mutations in the MC4R receptor.

POMC neurons and their peptides mediate satiety signals, while NPY neurons induce hunger signals and decrease energy expenditure. This integration involves both long term signals (leptin from adipose tissue and insulin), as well as acute hunger/satiety signals from the brainstem.

The melancortin alpha MSH is one of the most important regulators of energy homeostasis in the hypothalamus, where it induces a state of satiety within an individual through its actions on the MC4R. Additionally, administration of ACTH into certain regions of the hypothalamus has similar effects. Agouti is an antagonist at MC1R and MC4R receptors, while AgRP incurs antagonistic effects through its action on MC3R and MC4R receptors. Due to suppression of the alpha MSH anorectic signal, mutant mice with ectopic expression of these peptides are hyperphagic with an increase in adipose mass, lean mass, hyperinsulinemia and consequentially are obese.

Melanocortins are peptides derived form POMC, and are determined by tissue-specific post-translational cleavage. POMC gene expression has been identified in a number of tissues including the pituitary, skin, immune system and hypothalamus, which highlights the many physiological processes that POMC is involved in. POMC and CART are co-expression in a subpopulation of neurones in the arcuate nucleus, and the expression of both is stimulated by leptin. AgRP and alpha-MSH are involved in appetite regulation via their actions on MC4R. Melanocortin receptors MC3R and MC4R are unique in the sense that have both endogenous agonists and antagonists. Obesity can arise as a result of a POMC deficiency. Some melancortin receptors have been found on adipocytes (MC4R), suggesting that peripheral detection of these circulating melanocortins may also be involved in regulating energy homeostasis. Expression of these peptides in the anterior pitiutuary, results in the production of ACTH via the cleavage of POMC by the enzyme prohormone convertase1 (PC1). Stimulation of adrenal steroiogenesis then occurs via the action of ACTH on MC2R.

Leptin is produced primarily in white adipose tissue but also to some extent in brown adipose tissue, stomach, placenta, while recent studies suggest that it may also be produced in the brain. Numerous leptin receptors have been identified, all of which transmit their signal via the JAK-STAT pathway. The leptin receptor involved in regulating appetite is the receptor type Ob-Rb, which is present in many brain regions including ARC, PVN, DMH and LHA of the hypothalamus. When leptin binds to the Ob-Rb receptor, the subsequent signalling cascade results in a decrease in appetite through inhibition of the peptides involved in inducing orexigenic effects (NPY, MCY, AGRP and orexins). Additionally the anorexigenic peptides peptides (alpha-MSH, CART and CRH) are stimulated. Excess body weight results in excess leptin production from adipose tissue(released in a regular pulsatile manner) and hence leptin can act to restore normal energy homeostasis, by suppressing appetite and increasing energy expenditure through its effect on anorextic peptides.

Leptin secreted form adipose tissue and insulin secreted from the pancreas, also regulate food intake. Excess adipose tissue (as occurs in obese individuals) results in an increase in leptin production (which normally induces a feeling of satiety), but excess production of NPY (as occurs in some mutations of the melancortin system), can suppress its effects. Similarly insulin levels show a marked increase in obese individuals, with hyperinsuliemia being one of the first metabolic disturbances identified in those obese subjects with mutations of the melanocortin system.

Insulin Following meal ingestion, there is a dramatic increase in insulin levels, some of which crosses the blood brain barrier in a concentration that is representative of circulating insulin levels. Central administration of insulin has been shown to act on both NPY and POMC systems, and increases POMC mRNA synthesis as well inhibiting food intake in fasting rats. Rats with untreated diabetes have shown to have a diminished amount of POMC mRNA which is indicative of its involvement in stimulating its synthesis.

Ghrelin Ghrelin stimulates arcuate nucleus production of NPY and AgRP, while simultaneously inhibiting the suppressive effects that leptin induces on appetite. Ghrelin administration in rats produces hyperphagia and increased body weight, proportional to the amount of ghrelin given. The role of ghrelin within the melanocortin system has been verified, whereby central administration of this peptide results in excessive eating, but if NPY antagonists are given in conjunction,a state of hyperphagia is not induced. During periods of meal deprivation, ghrelin levels increase. They induce their potent appetite stimulating properties by activating arcuate NPY and AgRP expression. In contrast, following food consumption, ghrelin levels show a marked decrease.

An example of the complexity that arises in fully understanding the precise mechanism governing this system arises from mice AgRP KO models. Interestingly, while the involvement of AgRP in the melanocortin system is undisputed, one would expect such KO to present with altered phenotypes and eating patterns, yet these AgRP were just like their wild type counterparts. NPY has been identified as one of the most abundant and potent orexigenic peptides of the hypothalamus. While ghrelin ahs short term effects, administration of NPY produces a prolonged and substantial increase in food consumption and a decrease in thermogenesis, resulting in an overall increase in body weight and leading to obesity. NPY exerts its action via G- protein coupled receptors, 5 of which have been identified, of which Y5 has been identified as being involved in meidatign its effect on feeding. Circulating levels of leptin and insulin can regulate NPY synthesis, while glucocorticoids serve to stimulate it. During times of fasting, low levels of insulin and leptin result in the up-regulation of NPY synthesis. Interestingly, an obvious increase in ARC NPY neuronal activity is not recorded in diet induced obese individuals, which suggests that it may be involved in ensuring a certain amount of energy stores, but when this store is in excess, NPY neurons exhibit diminished activity, potentially in an effort to restore normal energy homeostasis.

AgRP- inhibits the effects of alpha MSH on inducing its anorexigeneic effect. AGRP is potent in activating appetite through its anatagonistic activity on both MC3R and MC4R and is expressed only in the arcuate nucleus of the hypothalamus All neurons expressing AgRP also produce NPY, and project to various brain sites including PVN and DMH. Leptin inhibits the expression of AgRP, while periods of starvation result in an increase in its expession. Unlike NPY which induces strong, short lived effects on stimulating appetite, AgRP administration can induce a state of hyperphagia for up to one week.

Thus, when energy stores are low, there is reduced leptin released from adipose tissue, which in turn allows in an increase in the production of orexigenic peptides including NPY, AgRP and a decrease in alpha-MSH and CART. The opposite is true in times if positive energy imbalance.

Although Peptide YY is an appetite suppressor, it appears to mediate its effects through a distinct pathway that does not involve the melanocortin system as both POMC and MC4R KO mice continued to show a decrease in food intake following its administration, indicating that another system may be involved. This further highlights the complexity of the mechanisms controlling energy homeostasis.

Suggested future studies

Despite the unquestionable importance of melanocortins within appetite regulation much of the underlying molecular mechanisms that facilitate the melanocortin signalling pathways are still under debate and many aspects remain unidentified.

Melanocortin Receptors G-proteins: A question of cross-talk? POMC expression is up-regulated in αMSH neurones as result of increased leptin and insulin signalling. This has been proposed to sequentially increase αMSH release which acts predominantly through activation of the G-protein coupled receptors MC3R and MC4R which are Gs linked receptors. Gs stimulate the up-regulation of adenylase cyclase and thus increase cAMP levels, and a multidude of sequential kinase signalling resulting in a complex cascade of second messengers that facilitate its anorexigenic effects and prevent orexigenic pathways.

Recent studies have unveiled the possibility for the involvement of alternative G-proteins or the coupling of G-proteins to create a specific effect. However, there are contradictory studies between the involvement of Gq’s and the activation of PLC and calcium signal which has been seen in some studies but not in others however, the cell lines used were different and furthermore the results are yet to be confirmed in vivo. Additionally, another study found that the inhibition of Gi/o specific cells expressing MC4R treated with agonist created a suppression of ERK1/2 phosphorylation indicating a possible role for Gi’s and downstream PKC/calcium activation of ERK 1/2 as opposed to conventional Gs PKA signalling initially described in certain cell types. Clearly, these areas of research is open to clarification in terms of the G-proteins involved in melanocortin signalling and activation of important downstream activity of molecules such as ERK 1/2 and thus enable us to fine tune the understanding of appetite regulation.

αMSH: Please release me let me go Although, as we mention above it is thought that leptin and insulin are actively responsible for increasing POMC expression and consequently αMSH release very little is understood about how and where αMSH is actually released.

Melanocortin 3 Receptor Although there are implications for the MC3R in appetite regulation it does not have such in-depth association with obesity as the MC4R has. Needless to say its involvement should not be overlooked especially since studies have shown that mutations, heterozygous genotype and knock out experiments can create the pathophysiological state or at least predispose humans and murine species to obesity either in childhood or later in life which is especially apparent when coupled to an obesigenic environment. Moreover, there is confusion as to the roles that this receptor has in an anorexic effect since it is also thought that it is part of the autoregulation of POMC providing negative feedback. Especially since it seems that MC3R that from studies by A. A Butler et al. 2000 that MC3R knock out shows primarily alterations to in metabolic syndromes whereas MC4R alters feeding behaviour and intake as well as energy expenditure. The distinction between MC3Rs actions in terms of obesity need further elucidation such that in future it may be required in combined therapeutic techniques to target the melanocortin receptors.

Agouti-related protein signalling: antagonist or inverse agonist? AgRP is thought to have a competitive antagonistic role on MCR to prevent αMSH signalling however, research has unveiled that AgRP can produce autonomous orexigenic effects by reducing the cAMP increase caused by binding of αMSH potentially through Gi signalling suggesting that it has inverse agonist qualities. This is primarily observed in terms of its ability to reduce cAMP levels as shown by researchers such as W A. J. Nijenhuis et al. (2001.) The exact composition of MCRs and the complexes that are formed at the membrane is an avenue that requires exploration especially since the picture is far more complex than you might think as MCR could potentially interact with a variety of G-proteins, accessory proteins such as syndecans, mahoganoid, MRAPs especially with MC4 in order to manipulate various aspects of αMSHs signalling capacity.

Lipid rafts The cholesterol composition of the lipid membrane affects cAMP signalling, with higher levels of cholesterol dampening the cAMP response because the lipid rafts segregate the G-protein signalling components accessibility to each other. Accordingly, this reveals that the lipid micro-domain surrounding the MCR’s location within the membrane may alter the result of αMSH signalling that possibly produces cell-specific effects. However, this is also a vague area in terms of solid evidence and requires studies across different cell lines and into cell membrane composition specific to the MCR location which could potentially be quite challenging.

A role for BNDF/Trk signalling The neurotrophic brain derived neuronal growth factor (BNDF) acts through the tropomysin receptor kinase B (Trk) and is thought to be activated downstream of melanocortin signalling through MC4R. This is proposed due to the ability of synthetic mineralocorticoid peptides to promote BDNF expression within hypothalamic nuclei, the ventro medial hypothalamus (VMH), in addition to the ability of BDNF to blunt the feeding in MC4R knockout mice. However, the manner which they interact is not clear and further investigation at the molecular level will shine light on the downstream signalling involved.

Discussion

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