Functional consequences of adiponectin oligomerization state: The role of adiponectin in maintaining systemic insulin sensitivity

Abstract

Adiponectin is an adipocyte-specific secretory protein that circulates in serum as a trimer-dimer of relatively low molecular weight (LMW) and a larger multimeric structure of high molecular weight (HMW). Serum levels of the protein generally correlate with systemic insulin sensitivity. The full-length protein affects hepatic gluconeogenesis through improved insulin sensitivity, and a proteolytic fragment of adiponectin stimulates beta-oxidation in muscle. We demonstrate that oligomer formation of Acrp30 critically depends on disulfide bond formation mediated by cys39. Mutation of cys39 results in trimers that are subject to proteolytic cleavage in the collagenous domain and have a decreased half-life in vivo, but surprisingly, are significantly more bioactive than higher order oligomeric forms of the protein with respect to reduction of serum glucose levels following intravenous injection. Furthermore, treatment of primary hepatocytes confirmed that increased bioactivity seen in vivo also translates into an augmented potency to reduce glucose output by Acrp30(cys39→ser) compared to higher-order oligomeric adiponectin complexes in the presence of gluconeogenic stimuli.;Based on our biochemical analysis and initial functional characterization, we hypothesized that each adiponectin complex may have differential abilities to sensitize the liver, muscle, adipose and even brain in order to maintain systemic glucose homeostasis. Through extensive correlative studies in various clinical settings, we find that the ratio, and not the absolute amounts, between the two oligomeric forms endogenously found in serum (HMW to LMW) is critical in determining insulin sensitivity. db/db mice, despite similar total adiponectin levels, display decreased HMW adiponectin compared to wildtype littermates, as do type II diabetic patients compared to insulin sensitive individuals. Furthermore, the HMW/total adiponectin ratio improves with PPARgamma agonist treatment (TZDs) in mice and humans, and we demonstrate that changes in this ratio in a number of type 2 diabetic cohorts serve as a quantitative indicator of improvements in insulin sensitivity obtained during TZD treatment, whereas changes in total serum adiponectin levels do not correlate well at the individual level. The predictive value of increased HMW adiponectin is strongly correlated with improvements in hepatic insulin sensitivity, and less relevant as a predictor of improved muscle insulin sensitivity, further underscoring the conclusions from previous clamp studies that suggested that the liver is the primary site of action for the full-length protein. These observations also suggest that the HMW complex is the bioactive adiponectin precursor relevant for the effects seen in hepatocytes, and may activate critical signal transduction pathways (AMPK, Akt) leading to increased insulin sensitivity in liver. (Abstract shortened by UMI.).