Leptin is a hormone that is primarily secreted by adipose tissue, but is also produced and released by mast cells. In turn, mast cells also have leptin receptors. Leptin is primarily known for its action of part of the hypothalamus to inhibit the hunger response. Importantly, the body responds forcefully to leptin levels by engaging both biological and behavioral mechanisms to conserve energy. It is seen by researchers as less of a “hunger satiety” signal and more of a “starvation” signal.
Patients with obesity often have higher circulating levels of leptin than those without obesity. This occurs because leptin is secreted by adipose tissue, which obese patients have in higher amounts due to their higher percentage of body fat. These people seem to be resistant to the chemical action of leptin, possibly through a change in activity of leptin receptors in the hypothalamus. Some studies suggest that in obese patients, less leptin leaves the blood stream and crosses into the brain.
Leptin is now known to have a variety of other effects on the body, including modulating the immune system. It activates inflammatory cells, promotes T cell responses and mediates production of TNF, IL-2 and IL-6. In many inflammation models, cells express more leptin receptors than usual. In diet induced obese mice, mast cells have been observed to store and secrete TNF. In immune mediated diseases like autoimmune diseases, circulating levels of leptin are increased, and this in turn translates to higher levels of inflammatory cytokines.
Interestingly, leptin suppresses signals from the IgE receptor to make mediators. In leptin receptor deficiency models, magnified IgE anaphylaxis was observed. Leptin also seems to control the number of mast cells through some unclear mechanism. In leptin deficient mice, mast cell density is significantly higher in abdominal lymph nodes and fat deposits.
Leptin influences the release of many other molecules, including ghrelin. Ghrelin is the “hunger hormone,” released in the stomach and possibly elsewhere. It stimulates the hunger response in the body and also acts on the hypothalamus. The relationship between leptin and ghrelin is very complex and still being elucidated. However, it is thought that high levels of circulating leptin suppress secretion of ghrelin. This is especially of interest in inflammatory conditions as ghrelin suppresses production of a number of inflammatory markers, including TNF, IL-8, MCP-1, IL-1b, IL-6, CRP and others. This effect is so pronounced that it is being investigated as a treatment option for many conditions. Ghrelin has also been observed in one study in induce mast cell activation through a receptor independent pathway.
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