Adiponectin, also known as adipocyte complement-related protein of 30 kDa (Acrp30), is a hormone of adipocyte origin that is involved in the homeostatic control of circulating glucose and lipid levels.1,2 Adiponectin is a 147 amino acid protein that is similar in sequence and structure to the C1q complement factor. It possesses a short N-terminal variable region followed by several collagen repeats and finally a large C-terminal globular domain.1 Structurally, based on SDS-PAGE1 and crystallographic studies,3 Adiponectin appears to form a variety of higher order structures. Adiponectin monomers assemble into homotrimers with the three globular domains adjacent to one another and the three collagen-like regions forming a collagen triple helix. These trimers then assemble into hexamers and other high molecular weight (HMW) complexes.1,3
Mounting physiological and genetic evidence strongly implicates Adipo-nectin in the development of type 2 diabetes. Reduced Adiponectin levels are documented in obese, insulin resistant, and type 2 diabetes-afflicted patients.4,5 Insulin-sensitizing, anti-diabetic drugs given to insulin resistant individuals cause an increase in Adiponectin levels.6 Further, high Adiponectin levels are associated with reduced risk for type 2 diabetes.7 Adiponectin maps to a genome locus that is associated with susceptibility to type 2 diabetes8-10 and several adiponectin missense mutations associated with type 2 diabetes have been described.11-13
|Figure 1. The various Adiponectin species affect liver hepatocytes and skeletal muscle myocytes differently. Only the high molecular weight (HMW) multimer and the hexamer forms of Adiponectin act on hepatocytes via AMP-activated protein kinase (AMPK) to inhibit glucose production and reduce intracellular triglycerides (TGs) and insulin resistance. By contrast, the Cys39Ser mutant trimer and globular domain mutants, as well as all wild-type Adiponectin species, act on myocytes via AMPK to stimulate glucose uptake and reduce intracellular TGs and insulin resistance.
[Note: figure adapted from Waki, H. et al. (2003) J. Biol. Chem. 278:40352.]
Despite the growing evidence supporting a link between Adiponectin and type 2 diabetes, the details of the mechanisms involved were unknown. However, a recent study by Waki et al. implicates impaired Adiponectin multimerization in the development of type 2 diabetes.14 These investigators show that intermolecular disulfide bonds mediated by the N-terminal Cys39 residue are required for the formation of Adiponectin oligomers larger than the trimer species. Further, they describe the differential ability of the various Adiponectin species to activate AMP-activated protein kinase (AMPK) in hepatocytes and myocytes. While wild-type Adiponectin HMW complexes are capable of activating AMPK in hepatocytes, the Cys39Ser trimer and globular domain mutants are not. By contrast, both wild-type and mutant species are capable of activating AMPK in myocytes (Figure 1).14 Waki et al. then examine the oligomerization capabilities of several naturally-occurring mutant forms of Adiponectin associated with type 2 diabetes. Two mutants are incapable of forming the HMW species while three are incapable of forming a stable trimer and show impaired secretion as well.14 Because all of the documented Adiponectin mutations are heterozygous, Waki et al. then examine the effect of heterozygocity on the levels of the various Adiponectin species. As expected, there is an overall reduction in the level of the specific oligomers affected by the mutations in transfected cell lines. Further, serum Adiponectin levels from diabetic patients carrying these mutations are affected in a similar way.14