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Among the biologically active proteins secreted by fat cells, adiponectin is one of the most abundant protein products expressed by adipose tissue genes and is abundantly present in the blood circulation. It appears in circulating plasma in the human body at a concentration of 3-30 ug/ml. Adiponectin is also known as Acrp30, Apm1, AdipoQ, and GBP28. Initially, adiponectin was found in human subcutaneous adipose tissue, plasma, and murine fat cells. As an insulin hypersensitizing hormone (An Insulin-sensitizing Hormone), adiponectin can increase the fatty acid oxidation and sugar absorption of skeletal muscle cells, significantly enhance insulin's inhibition of gluconeogenesis and inhibit liver glycogen production. It is an important regulator of the body's lipid metabolism and regulation of blood glucose homeostasis. The nuclear transcription factor NF-κB is an important factor involved in the transcriptional regulation of VCAM-1, ICAM-1, and selectin-E by TNF-α. Ouchi and Kihara et al reported that adiponectin may regulate endothelial cell function by inhibiting the signaling pathway of NF-κB. NF-κB-inducible kinase (NIK) phosphorylates the IKB kinase (IKK) complex and activates NF-κB. TNF receptor binding factor 2 is a junction protein of NIK and TNF-α signaling pathway, which is a bifurcation point for TNF-α-induced NIK-NF-κB activation and activation of JNK or p38 pathway, and NIK is not involved in the activation of JNK and p38 kinase. Vascular smooth muscle proliferation induced by platelet-derived growth factor (PDGF) or heparin-binding epidermal growth factor (HB-EGF) plays an important role in vascular disease, while the physiological concentration of adiponectin is vascular smooth muscle induced by PDGF-BB with important inhibitory effects on proliferation and migration. In addition, adiponectin can inhibit the production and release of TNF with a certain anti-inflammatory effect, and an important cytoprotective effect on alcoholic liver injury.
Adiponectin is a novel protein secreted by adipocytes, containing 244 amino acids. Primary sequence analysis suggests that it contains four functional regions: the signal peptide of the first 18 amino acids, and the amino-terminal non-helical function of 23 amino acids, a segment of 22 collagen repeats (including 8 fully repeating Gly-X-Pro and 14 incomplete repeats of Gly-XY) and a carboxy-terminal carboxy-terminal end, which may be involved in the formation of a carboxy-terminal spherical functional domain. It has been found that the endogenous adiponectin produced by adipocytes is post-translationally modified into 8 isomers, 6 of which are glycosyl isomers, and the glycosylation sites are in 68, 71, 80 and 1044 Lys residues of the collagen-like functional regions. Wang et al. found that the glycosylation of the four Lys residues plays a key role in maintaining insulin sensitization. When Arg is used instead of glycosylated Lys, insulin sensitization is greatly attenuated. Since Yukio et al. from the University of Otsuka Medical Research Institute of Japan first isolated the fat-rich gene transcript 1/ablipin cDNA library from human plasma in 1996, a lot of research on apM1 has made us have a clearer understanding. Three clones containing apM1, two lambada clones, and one BAC clone have been discovered. apM1, a gene encoding adiponectin, is located on human chromosome 3q27 and has a length of 16 kb, including three exons ranging in size from 18 bp to 4277 bp, and two introns of 0.8 and 12 kb, respectively. The regulatory sequence of the apM1 gene contains a putative promoter element rather than a classical TATA box. The third exon of the gene has a longer 3' untranslated sequence, including three Alu repeats. This exon-intron of the apM1 gene is very similar to the ob gene encoding leptin.
Figure1. Adiponectin signaling pathway.
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