The enzyme 5‘-AMP-activated protein kinase (AMPK) assumes a significant role in the administration of cellular lipid and protein metabolism, and is a key negotiator of the metabolic effects of hormones including leptin, ghrelin, adiponectin, glucocorticoids and insulin.
Both leptin and insulin are understood to operate as adiposity signals. Insulin is a hormone which modulates the energy and glucose metabolism in the body, and induces cells in the liver, muscle, and fat tissue to extract glucose from the blood, depositing it as glycogen in the liver and muscle. Leptin, one of the principal adipose derived hormones, is a 16 kDa protein that drives the regulation of energy intake expenditure, including appetite and metabolism.
Generally, AMPK encourages catabolism (glycolysis, fatty acid oxidation and mitochondrial biogenesis) impedes anabolic pathways (gluconeogenesis, glycogen, fatty acid and protein synthesis), and directly influences the hypothalamus to control appetite. AMPK activation is thought to upregulate insulin receptor substrate-1 by inhibiting the mTOR pathway, which is associated with the pathogenesis of insulin resistance and multiple types of cancer.
Insulin has been proven to impede AMPK’s hypothalamic processes. Moreover, insulin can impede AMPK in fat by triggering the Akt complex, which leads to phosphorylation of α-AMPK at S485/491, causing a depletion of phosphorylation at T172 (necessitated by AMPK activation). In addition, AMPK and insulin are distinguished by their respective anabolic and catabolic activity.
One of leptin’s numerous metabolic duties is the upregulation of fatty acid oxidation in skeletal muscle, via the AMPK pathway. Cutting edge research has indicated that abnormalities in cellular lipid metabolism are implicated in the pathogenesis of the metabolic syndrome, perhaps as a result of dysregulation of AMPK and malonyl-CoA, an intimately related molecule. Leptin and insulin are also mutually regulative.
While there is some debate regarding mechanism, it seems that leptin impedes insulin secretion from pancreatic β-cells, either indirectly or directly. Additionally, further research suggests that leptin down-regulates insulin by negotiating the phosphorylation of IRS-1 through a JAK2, IRS-2 and PKCd dependent pathway. This serine site is understood to minimize the coupling of IRS-1 to the insulin receptor. Contrastingly, insulin is a powerful stimulator of leptin emission from white adipocytes.
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