Adiponectin

About Adiponectin:

The gene symbol is ADIPOQ [adiponectin, C1q and collagen domain containing].
Another designation is ACDC [adiponectin, C1q and collagen domain containing].

The protein is identical with apM1 (adipose Most abundant gene transcript-1), isolated by Maeda et al (1996) from an adipose tissue cDNA, and GBP-28 (gelatin-binding protein of 28 kDa) (Nakano et al, 1996), the gene of which was isolated by Saito et al (1999), who identified the protein as the one encoded by apM1.

Human adiponectin and murine adipocyte complement related protein of 30 kDa (Acrp30; also termed adipocyte-specific secretory protein) secreted by 3T3-L1 cells in the course of their differentiation into adipocytes are homologous proteins. The protein possesses significant homology to collagens VIII, X and complement factor C1q and has been described also as adipoQ. Wong et al (2004) have described a widely expressed and highly conserved family of seven adiponectin paralogs that can cause increased glycogen accumulation and fatty acid oxidation and/or lower blood glucose levels in ob/ob mice (see: CTRP1, CTRP2, CTRP9).

Adiponectin is a secreted protein of 244 amino acids that belongs to a protein family possessing a collagen-like domain through which they form homo-trimers, which further combine to make oligomeric complexes (Tsao et al, 2002). The human Adiponectin gene spans 17 kb on chromosome 3q27 (Saito et al, 1999), consisting of three exons and two introns. Adiponectin mRNA is found in adipose tissue but not in muscle, brain, heart, intestine, kidney, liver, lung, ovary, placenta, or uterus tissues.

Schaffler et al (1999) have shown that the Adiponectin gene is expressed by differentiated adipocytes. High levels of the protein are found in human plasma (approximately 2.0 to 17 micrograms/mL in plasma) (Hotta et al, 2000). Adiponectin is a hormone that plays a role in the regulation of glucose and lipid homeostasis (see also: adipokines) (Tsao et al, 2002; Berg et al, 2002).

Ouchi et al (1999) have determined that adiponectin suppresses TNF induced adhesion of monocytes to aortic endothelial cells and also suppresses the expression of vascular cell adhesion molecule 1 (VCAM-1), selectin E, and intercellular adhesion molecule 1 (ICAM-1 (CD54)) in these cells. Adiponectin may thus attenuate inflammation associated with atherogenesis.

Adiponectin specifically binds to collagen type 1, collagen type 3 and collagen type 5, which are present in vascular intima. As shown by Okamoto et al (2000), the protein is detected in the walls of the catheter-injured vessels but not in the intact vascular walls.

Levels of Adiponectin are decreased in obesity (Arita et al, 1999; Ouchi et al, 2003) and are reduced also in patients with coronary artery disease (Ouchi et al, 2003).

TNF-alpha reduces the expression and secretion of apM1 in differentiating primary human pre-adipocytes (Kappes and Loffler, 2000). Adiponectin inhibits the expression of endothelial adhesion molecules induced by TNF-alpha through a mechanism involving the suppression of TNF-alpha induced I-kappa-B-alpha phosphorylation and subsequent NF-kappa-B activation that does not affect other signals mediated by TNF-alpha (Ouchi et al, 2000. Adiponectin has an inhibitory effect on proliferation of vascular smooth muscle cells. Arita et al (2002) have reported that Adiponectin suppresses proliferation and migration of human aortic smooth muscle cells stimulated with PDGF-BB (but not HB-EGF or PDGF-AA), by acting as a plasma binding protein for PDGF-BB. Adiponectin also strongly and dose-dependently suppresses phosphorylation of p42/44 extracellular signal-related kinase (ERK) phosphorylation induced by PDGF-BB, PDGF-AA, and HB-EGF.

As shown by Yokota et al (2000), adiponectin also suppresses functions of mature macrophages, inhibiting their phagocytic activity and their production of TNF-alpha induced by bacterial lipopolysaccharides. Ouchi et al (2001) have shown that adiponectin suppresses lipid accumulation and class A scavenger receptor expression in human macrophages derived from monocytes.

Some studies by Yokota et al (2000) reveal Adiponectin to be an important negative regulator in hematopoiesis and the immune system. Adiponectin predominantly inhibits proliferation of myelomonocytic lineage cells (see also: hematopoiesis). Some of the effects may be due to apoptosis. Kobayashi et al (2004) have reported that a high molecular weight form of adiponectin suppresses apoptosis and caspase-3 activity in human umbelical vein endothelial cells.

Adiponectin suppresses colony formation from the hematopoietic progenitor cells CFU-GM, CFU-G, and CFU-M but does not affect BFU-E or mixed erythroid-myeloid CFU (Yokota et al, 2000). Adiponectin also inhibits proliferation of several myeloid cell lines (4/9 studied) but has no effects on erythroid or lymphoid cell lines except for one cell line.

DiMascio et al (2007) have investigated the role of adiponectin in hematopoietic stem cell function and shown that adiponectin is expressed by adipocytes in the bone marrow and that adiponectin receptors are expressed by hematopoietic stem cells. Adiponectin increases the proliferation of hematopoietic stem cells and retains them in a functionally immature state. Adiponectin signaling is required for optimal proliferation of hematopoietic stem cells in vitro and in long term hemopoietic reconstitution in vivo (see also: LTRC [long-term repopulating cells].

Yamauchi et al (2004) have cloned the cDNAs for two adiponectin receptors, both of which bind full-length adiponectin and globular adiponectin and mediate antidiabetic metabolic effects. T-cadherin has been identified as a receptor for hexameric and high molecular weight forms of adiponectin and may act as a co-receptor (Hug et al, 2004).

Maeda et al (2002) have studied transgenic knock-out mice lacking expression of adiponectin. Clearance of free fatty acid in plasma in these mice is delayed. The animals also low levels of fatty acid transport protein 1 (FATP-1) mRNA in muscle, high levels of TNF-alpha mRNA in adipose tissue, high plasma TNF-alpha concentrations, and impaired insulin signaling, all of which contribute to the severe diet-induced insulin resistance observed