Adiponectin (also referred to as GBP-28, apM1, AdipoQ and Acrp30) is a protein which in humans is encoded by the ADIPOQgene. It is involved in regulating glucose levels as well as fatty acid breakdown.
Adiponectin is a 244-amino-acid-long polypeptide. There are four distinct regions of adiponectin. The first is a short signal sequence that targets the hormone for secretion outside the cell; next is a short region that varies between species; the third is a 65-amino acid region with similarity to collagenous proteins; the last is a globular domain. Overall this gene shows similarity to the complement 1Q factors (C1Q). However, when the 3-dimensional structure of the globular region was determined, a striking similarity to TNFα was observed, despite unrelated protein sequences.
Adiponectin is a protein hormone that modulates a number of metabolic processes, including glucose regulation and fatty acidoxidation. Adiponectin is exclusively secreted from adipose tissue (and also from the placenta in pregnancy) into the bloodstream and is very abundant in plasma relative to many hormones. Levels of the hormone are inversely correlated with body fat percentage in adults, while the association in infants and young children is less clear. Similarly, circulating adiponectin concentrations increase during caloric restriction in animals and humans, such as in patients with anorexia nervosa. This observation is surprising, given that adiponectin is produced by adipose tissue; however, a recent study suggests that adipose tissue within bone marrow, which increases during caloric restriction, contributes to elevated circulating adiponectin in this context.
Adiponectin is secreted into the bloodstream where it accounts for approximately 0.01% of all plasma protein at around 5-10 μg/mL. Plasma concentrations reveal a sexual dimorphism, with females having higher levels than males. Levels of adiponectin are reduced in diabetics compared to non-diabetics. Weight reduction significantly increases circulating levels.
Adiponectin automatically self-associates into larger structures. Initially, three adiponectin molecules bind together to form a homotrimer. The trimers continue to self-associate and form hexamers or dodecamers. Like the plasma concentration, the relative levels of the higher-order structures are sexually dimorphic, where females have increased proportions of the high-molecular weight forms. Recent studies showed that the high-molecular weight form may be the most biologically active form regarding glucose homeostasis. High-molecular-weight adiponectin was further found to be associated with a lower risk of diabetes with similar magnitude of association as total adiponectin. However, coronary artery disease has been found to be positively associated with high molecular weight adiponectin, but not with low molecular weight adiponectin.
Adiponectin exerts some of its weight reduction effects via the brain. This is similar to the action of leptin, but the two hormones perform complementary actions, and can have synergistic effects.
These have distinct tissue specificities within the body and have different affinities to the various forms of adiponectin. The receptors affect the downstream target AMP kinase, an important cellular metabolic rate control point. Expression of the receptors is correlated with insulin levels, as well as reduced in mouse models of diabetes, particularly in skeletal muscle and adipose tissue.
Adiponectin was first characterised in 1995 in differentiating 3T3-L1 adipocytes (Scherer PE et al). In 1996 it was characterised in mice as the mRNA transcript most highly expressed in adipocytes(Maeda, 1996 ((citation #1, below))). In 2007, adiponectin was identified as an transcript highly expressed in preadipocytes (precursors of fat cells) differentiating into adipocytes.
The human homologue was identified as the most abundant transcript in adipose tissue. Contrary to expectations, despite being produced in adipose tissue, adiponectin was found to be decreased in obesity. This downregulation has not been fully explained. The gene was localised to chromosome 3q27, a region highlighted as affecting genetic susceptibility to type 2 diabetes and obesity. Supplementation by differing forms of adiponectin were able to improve insulin control, blood glucose and triglyceride levels in mouse models.
Adiponectin prevented leptin(obesity)-induced fibrosis in mice.
Because this substance is a protein, oral administration of adiponectin is not effective, as enzymes in the digestive system break it down into its constituent amino-acid parts. One way to introduce the hormone into the blood stream is intravenous administration, which itself poses significant logistical challenges due to the difficulty in producing in vitro adiponectin in its most potent, "high molecular weight" form using mammalian cells.
Extracts of sweet potatoes have been reported to increase levels of adiponectin and thereby improve glycaemic control in humans. However a systematic review concluded there is insufficient evidence to support the consumption of sweet potatoes to treat type 2 diabetes mellitus.
This section requires expansion. (May 2013)
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