# Metabolomics/Hormones/Leptin

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## Introduction to Leptin

Leptin, which comes from the Greek word leptos meaning “thin” was discovered in 1994 as the first factor in regulating feeding behavior and energy consumption. Composed of only 167 amino acids, it is a small protein that is produced in adipocytes (fat cells) and migrates through the blood to the brain and ultimately acts on neural receptors of the arcuate nucleus (of the hypothalamus) to inhibit appetite. Leptin basically conveys to the brain that the body’s fat reserves are met so food intake and fat synthesis should decrease while beta oxidation of fatty acids is allowed to increase for energy expenditure and heat.
Its identification was first found as a product of the obese (OB) gene in lab mice. Mice that carried two defective copies (one from each parent, ob/ob) had a defect in leptin production. Their behavior and physiology were seemingly in a constant state of starvation with increased serum cortisol levels, the inability to stay warm, abnormal growth, sterility, and an unrestrained appetite. As a consequence, they became very obese, weighing almost three times as much as a normal mouse. To control for the possibility of any other contributing factor, these mice were injected with leptin and the results confirmed that they lost weight and had increased locomotor activity as well as thermogenesis. Here is a picture and a follow-up article on humans.
Initially, leptin ignited human interest as a possibility to prevent obesity. But it has been found that obese individuals rarely have a defective leptin gene, so the system probably evolved to adjust an animal’s activity and metabolism during periods of severe nutritional deprivation, not to restrict weight.

## Brain Mechanisms: Role in Satiety

In the arcuate nucleus of the hypothalamus there are two types of neurons that regulate eating and metabolism, the orexigenic and anorexigenic neurons. Orexigenic neurons stimulate the appetite by producing and releasing neuropeptide Y (NPY). NPY also releases a second orexigen: agouti-related protein (AGRP). These (NPY secreting) neurons project to the paraventricular nucleus (PVN) which will ultimately stimulate the brain stem nuclei that controls the autonomic nervous system (ANS) to decrease: insulin secretion, breakdown of fatty acids, and body temperature. NPY neurons also project to the lateral hypothalamus, which will inhibit its secretion of peptide neurotransmitters, melanin-concentrating hormone (MCH) and orexin. These hormones generally stimulate appetite and reduce metabolic rate. During starvation, NPY in the blood rises as in the ob/ob mice, this elevated concentration sends signals to the brain to keep eating. NPY and AGRP are very potent; injections of these neuropeptides will cause animals to eat voraciously, even when they receive electrical shocks to the tongue. In addition, AGRP is long lasting, one injection into the third ventricle of rats resulted in six days of feasting. Release of leptin will inhibit NPY and AGRP. Anorexigenic neurons produce alpha-melanocyte-stimulating hormone (a-MSH) which acts as an agonist at the melanocortin-4-receptor (MC-4R) to inhibit eating. Moreover, this receptor is antagonized by AGRP, which stimulates eating. The amount of leptin produced and released by adipose tissue is proportional to the number and size of adipocytes. Here is a picture of the hormones that control eating.
The hormone is transduced by the JAK-STAT system which is also the mechanism used by interferon and growth factors. When leptin reaches the arcuate nucleus, it binds two leptin receptor monomers in the extracellular domain and dimerizes them. Both monomers are then phosphorylated on a tyrosine residue of the intracellular domain by a Janus kinase (JAK). The phosphorylated tyrosine resides become docking sitse for three proteins that are signal transducers and activators of transcription (STATS 3, 5, and 6, a.k.a. fat-STATS). These docked STATS are then phosphorylated on Tyr residues by the same JAK. Afterwards, the STATS dimerize and move to the nucleus where they will bind to specific DNA sequences to stimulate transcription of genes such as pro-opiomelanocortin (POMC), which is the precursor to a-MSH.
Leptin also stimulates the sympathetic nervous system to increase blood pressure, heart rate, and thermogenesis by uncoupling electron transfer from ATP synthesis in the mitochondria of adipocytes. The uncoupling protein (UCP or thermogenin) produces a pore in the inner mitochondrial membrane that enables protons to reenter the matrix- detouring them from the ATP synthase complex, so no ATP is generated but the continued oxidation of fatty acids from adipocytes releases energy as heat. Leptin stimulates the synthesis of this protein by releasing norepinephrine onto B3-adrenergic receptors on adipocytes. These are G-protein bound and will go on to activate adenylyl cyclase which will amplify a second messenger, cAMP to phosphorylate many downstream targets like Protein kinase A (which will migrate to the nucleus) to increase the expression of UCP gene.

## Further Web Resources

The Science Creative Quarterly: Leptin: A Piece of the Obesity Pie
Summary:
Obesity is an increasingly worldwide problem that leads to significant health risks. The traditional causes of obesity were overeating and a lack of exercise, this explanation may be true, but incomplete. The author of this page explores a biological factor found by researchers today that plays a role in weight regulation. He introduces the lipostatic theory within early studies and then goes into detail to explain the hormone leptin; particularly, what studies have shown, its feedback loop, and its other roles.
New Terms:
* VHM: Ventromedial hypothalamus
* NPY: Neuropeptide Y, a feeding stimulant
* a-MSH: Alpha-melanocyte-stimulating hormone, an appetite suppressant
* MCH: Melanin-concentrating hormone, a feeding stimulant
* CART: Cocaine-amphetamine-regulated transcript, an appetite suppressant
* CRF: Corticotropin-releasing factor
* Endocannaboids: Substances produced from within the body which activate cannabinoid receptors, leads to decreased appetite.
Connection to Metabolism:
* Fat metabolism
* Feedback regulation
Howard Hughes Medical Institute Bulletin: Leptin's Legacy
Summary:
This page addresses the study of leptin's role in metabolism with exploration of its therapeutic possibilities. Leptin has been found to treat successfully—possibly even cure—two rare diseases: a defect in the leptin gene and lipodystrophy, a disease caused by the absence of fat cells, which the body relies on to make leptin. Furthermore, this one fat-derived molecule affects metabolism, reproduction and the immune system, suggesting it may represent some evolutionary efficiency. An interesting hypothesis about leptin is that it is a protective mechanism against starvation. The leptin gene probably conferred a selective advantage on our ancestors by enabling them to survive lean times.
New Terms:
* Melanocortins: A group of pituitary peptide hormones that include adrenocorticotropin (ACTH) and the alpha, beta and gamma melanocyte-stimulating hormones (MSH) that derive from the prohormone proopiomelanocortin.
* SCD-1: Stearoyl-CoA desaturase-1, an enzyme that helps determine whether the body stores fat or burns it
* Type II Diabetes: Metabolic disorder that is primarily characterized by insulin resistance, relative insulin deficiency and hyperglycemia
* T cells: A group of cells that belong to a group of white blood cells known as lymphocytes, and play a central role in cell-mediated immunity
* Macrophages: Cells within the tissues that originate from specific white blood cells called monocytes; their role is to phagocytose (engulf and then digest) cellular debris and pathogens either as stationary or mobile cells, and to stimulate lymphocytes and other immune cells to respond to the pathogen
Connection to Metabolism:
* Disease states as a result of mutation in gene
* Efficiency: avoiding futile cycles

## Peer Reviewed Articles

Hormonal Control of Food Intake
Summary:
Studies on obese mice (ob/ob) and diabetic mice (db/db) were instrumental in the studies of leptin. Ob/ob mice have a loss of function mutation in leptin which is produced in adipose tissue. Db/db phenotype has the ability to produce leptin but lacks the receptor for it. Studies suggest that leptin is used in the body during transitions between fed and starved states.
The hypothalamus of the brain has two cell populations of neurons that are responsive to leptin. The NPY/AgPR neurons can be upregulated through leptin. Loss of these neurons leads to hypophagia. POMC/CART neurons are stimulated by leptin. POMC deficiency leads to hyperphagia and obesity, having a role in appetite. Leptin receptors found outside the arcuate nucleus are critical in maintaining a normal body weight and possibly working to resist diet-induced obesity.
Leptin has an effect on neuronal growth and development by changing synaptic inputs to the arcuate neurons. It may be a growth factor during hypothalamic development. The ability to respond to leptin seems to be retained; therefore, leptin administration later in life works.
Leptin resistance has a few different mechanisms. In this case leptin administration does not result in weight loss even though the receptor is intact and there is a high level of the circulating hormone. It may not reach the receptor within the brain or there may be decreased levels of the receptor.
Leptin is regulated at several levels including transcription, translation, storage, turnover and secretion. It is upregulated through insulin and glucocorticoids and downregulated by anything that increases cAMP levels in the adipocytes.
New Terms:
* Hyperphagic: Increased appetite
* Hypophagia: Decreased appetite
* Arcuate nucleus: Collection of neurons in the hypothalamus of the brain
* Hypothalamus: Part of the brain that receives and integrates neural, metabolic and humoral signals from the periphery
* Parabiosis experiments: Surgical union of two animals allowing blood exchange
* Lipodystrophy: Metabolic disorder where there is a selective loss of body fat and low leptin levels
Connection to Metabolism:
* Many levels of regulation: transcription, translation, turnover and secretion; also regulated with other hormones
* Fed and starved states in the cell alter levels of leptin
Analysis of Circadian Pattern Reveals Tissue-Specific Alternative Transcription in Leptin Signaling Pathway
Summary:
Gene expression patterns change throughout the day along with the light/dark cycle, feeding behavior, sleeping patterns and other environmental factors. The daily light/dark cycle is in the hypothalamus. Peripheral tissues are controlled through sympathetic outputs. The leptin pathway was analyzed to look at oscillation in a biological system. Leptin regulates energy balance and makes an organism feel full after eating. It is expressed in adipose tissue and released into the bloodstream. There is an increase in the leptin receptor production before the leptin acrophase. The leptin signaling pathway is synchronized with the leptin receptor expression which follows an oscillating pattern. Alternative transcripts of long or short lengths are seen in the oscillation pattern. Additionally, if the balance of short and long lived transcripts are not maintained it may result in leptin resistance. These short and long lived transcripts compensate for the oscillation of a system to keep transcript levels constant. The oscillation pattern in leptin is tissue specific in brown and white adipose tissue.
New Terms:
* Circadian rhythms: daily rhythms that occur throughout nature
* Sympathetic: part of the nervous system that is always active at a low level, part of the fight-or-flight response
* Oscillation: repetitive variation between 2 or more states
* Acrophase: peak of a rhythm
* Brown adipose tissue: primary purpose is to generate body heat; contains high levels of mitochondria; present in many newborn or hibernating animals
* White adipose tissue: single fat droplets; account for large amount of body weight; have receptors for various hormones
Connection to Metabolism:
* Regulation in a biological system via transcription
* Light and dark cycles
* Energy balance in a cell

## Disease
### Hepatic Steatosis & Hyperinsulinemia

Infusion of leptin has been proven to devitalize hepatic steatosis and hyperinsulinemia. This had been discovered via decreasing the synthesis of hepatic triglycerides and improving insulin sensitivity, in diet-induced lipodystrophy model mice.
Conjugated linoleic acid (CLA)-containing lipodystrophy model diets were proven to cause substantial weight gain in the livers of mice. Despite this, there was no significant change between the initial and final body weights, or in food intake. Leptin infusion has been proven to insignificantly alleviate hepatomegaly, in diet-induced lipodystrophy model mice. There were significant decreases in waist subcutaneous and abdominal (perennial, epididymal, and omental) white adipose tissue (WAT), in lipodystrophy model mice, without a significant effect of leptin infusion on WAT weights.
The activities of fatty acid synthase (FAS) and malic enzyme (ME) were increased in the liver of mice that had been fed the lipodystrophy model diet. However, the infusion of leptin failed to change these lipogenic enzyme activities. The lipodystrophy model diet was discovered to raise activity of membrane-bound forms of Mg2+-dependent phosphatidate phosphohydrolase (PAP), the most significant enzyme for triglyceride (TG) de novo synthesis. The activity of this enzyme experienced substantial suppression when leptin treatment was administered.
Adiponectin and leptin are known to be secreted from adipose tissue, and have several physiological functions, which include insulin sensitivity regulation. As a result, adipocytokine secretion deficiency induced by scant levels of adipose tissue has been attributed to lipodystrophy. This lipodystrophy would be characterized by severe insulin resistance leading to hyperinsulinemia and hepatic steatosis. Hepatic steatosis and hyperinsulinemia are proven to be alleviated by leptin deficiency. Regardless, leptin infusion had no observable effect on adiponectin levels.
Mice that were fed the lipodystrophy model diet experienced an impaired insulin-mediated glucose lowering effect. However, leptin infusion in the mice resulted in an alleviation of insulin resistance. This indicated a weakening of hepatic steatosis and hyperinsulinemia via insulin resistance alleviation, and as a result of leptin treatment.
Link to Article:
http://www.lipidworld.com/content/7/1/8#IDAESI0J

## References

* Carlson, Neil R. Physiology of Behavior, 8th Ed. Boston: Allyn & Bacon, 2004
* Nelson, David L. and Cox, Micheal M. Lehninger Principles of Biochemistry, 4th Ed. New York: W. H. Freeman and Company, 2005.