Leptin acts on receptors in the hypothalamus of the brain where it inhibits appetite by (1) counteracting the effects of neuropeptide Y (a potent feeding stimulant secreted by cells in the gut and in the hypothalamus); (2) counteracting the effects of anandamide (another potent feeding stimulant that binds to the same receptors as THC, the active ingredient of marijuana); and (3) promoting the synthesis of α-MSH, an appetite suppressant. This inhibition is long-term, in contrast to the rapid inhibition of eating by cholecystokinin (CCK) and the slower suppression of hunger between meals mediated by PPY3-36. The absence of a leptin (or its receptor) leads to uncontrolled food intake and resulting obesity.
Several studies have shown that fasting or following a very low calorie diet (VLCD) lowers leptin levels.
It might be that on short term leptin is an indicator of energy balance. This system is more sensitive to starvation than to overfeeding. That is, leptin levels do not rise extensively after overfeeding. It might be that the dynamics of leptin due to an acute change in energy balance are related to appetite and eventually to food intake. Although this is a new hypothesis, there is already some data that supports it.
There is some controversy regarding the regulation of leptin by melatonin during the night. One research group suggested that increased levels of melatonin caused a downregulation of leptin. However, in 2004, Brazilian researchers found that in the presence of insulin, "melatonin interacts with insulin and upregulates insulin-stimulated leptin expression", therefore causing a decrease in appetite whilst sleeping.
To date, only leptin and insulin are known to act as an adiposity signal. In general,
- Leptin circulates at levels proportional to body fat.
- It enters the central nervous system (CNS) in proportion to its plasma concentration.
- Its receptors are found in brain neurons involved in regulating energy intake and expenditure.
- It controls food intake and energy expenditure by acting on receptors in the mediobasal hypothalamus
Interaction with amylin
Co-administration of two neurohormones known to have a role in body weight control, amylin (produced by beta cells in the pancreas) and leptin (produced by fat cells), results in sustained, fat-specific weight loss in a leptin-resistant animal model of obesity.
Satiety: appetite control
Leptin binds to neuropeptide Y (NPY) neurons in the arcuate nucleus, in such a way that decreases the activity of these neurons. Leptin signals to the brain that the body has had enough to eat, or satiety. A very small group of humans possess homozygous mutations for the leptin gene which leads to a constant desire for food, resulting in severe obesity. This condition can be treated somewhat successfully by the administration of recombinant human leptin. However, extensive clinical trials using recombinant human leptin as a therapeutic agent for treating obesity in humans have been inconclusive because only the most obese subjects who were given the highest doses of exogenous leptin produced statistically significant weight loss. It was concluded that large and frequent doses were needed to only provide modest benefit because of leptin’s low circulating half-life, low potency, and poor solubility. Furthermore, these injections caused some participants to drop out of the study due to inflammatory responses of the skin at the injection site. Some of these problems can be alleviated by a form of leptin called Fc-leptin, which takes the Fc fragment from the immunoglobulin gamma chain as the N-terminal fusion partner and follows it with leptin. This Fc-leptin fusion has been experimentally proven to be highly soluble, more biologically potent, and contain a much longer serum half-life. As a result, this Fc-leptin was successfully shown to treat obesity in both leptin-deficient and normal mice, although studies have not been undertaken on human subjects. This makes Fc-leptin a potential treatment for obesity in humans after more extensive testing.
Thus, circulating leptin levels give the brain input regarding energy storage so it can regulate appetite and metabolism. Leptin works by inhibiting the activity of neurons that contain neuropeptide Y (NPY) and agouti-related peptide (AgRP), and by increasing the activity of neurons expressing α-melanocyte-stimulating hormone (α-MSH). The NPY neurons are a key element in the regulation of appetite; small doses of NPY injected into the brains of experimental animals stimulates feeding, while selective destruction of the NPY neurons in mice causes them to become anorexic. Conversely, α-MSH is an important mediator of satiety, and differences in the gene for the receptor at which α-MSH acts in the brain are linked to obesity in humans.
The role of Leptin/Leptin receptors in modulation of T cell activity in immune system was shown in experimentation with mice. It modulates the immune response to atherosclerosis, which is a predisposing factor in patients with obesity.
Leptin promotes angiogenesis by increasing vascular endothelial growth factor (VEGF) levels.
Lung surfactant activity
In fetal lung leptin is induced in the alveolar interstitial fibroblasts ("lipofibroblasts") by the action of PTHrP secreted by formative alveolar epithelium (endoderm) under moderate stretch. The leptin from the mesenchyme in turn acts back on the epithelium at the leptin receptor carried in the alveolar type II pneumocytes and induces surfactant expression which is one of the main functions of these type II pneumocytes.
In mice, leptin is also required for male and female fertility. Leptin has a lesser effect in humans. In mammals such as humans, ovulatory cycles in females are linked to energy balance (positive or negative depending on whether a female is losing or gaining weight) and energy flux (how much energy is consumed and expended) much more than energy status (fat levels). When energy balance is highly negative (meaning a woman is starving) or energy flux is very high (meaning a woman is exercising at extreme levels, but still consuming enough calories), the ovarian cycle stops and females stop menstruating. Only if a female has an extremely low body fat percentage does energy status affect menstruation. Some studies have indicated that leptin levels outside an ideal range can have a negative effect on egg quality and outcome during IVF.
The body's fat cells, under normal conditions, are responsible for the constant production and release of leptin. This can also be produced by the placenta. Leptin levels rise during pregnancy and fall after parturition (childbirth). Leptin is also expressed in fetal membranes and the uterine tissue. Uterine contractions are inhibited by leptin.
There is also evidence that leptin plays a role in hyperemesis gravidarum (severe morning sickness), in polycystic ovary syndrome and a 2007 research suggests that hypothalamic leptin is implicated in bone growth.
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