Investigation into the assembly of adiponectin as a target for countering obesity related diseases

Abstract

Adiponectin, a collagenous hormone secreted abundantly from adipocytes, possesses potent anti-diabetic and anti-inflammatory properties. Mediated by the conserved Cys39 located in the variable region of the N-terminus, the trimeric (low molecular weight (LMW)) adiponectin subunit assembles into different higher order complexes, e.g. hexamers (middle molecular weight (MMW)) and 12-18-mers (high molecular weight (HMW)), the latter being mostly responsible for the insulin-sensitizing activity of adiponectin. The endoplasmic reticulum (ER) chaperone ERp44 retains adiponectin in the early secretory compartment and tightly controls the oxidative state of Cys39 and the oligomerization of adiponectin. Biologically active, recombinant and pure adiponectin oligomers are difficult to produce hampering the analyses of the oligomerisation process. To mitigate this production problem, we engineered and synthesized a model peptide of the N-terminal domain of adiponectin. We demonstrated that the peptide could be used for probing the influence of the variable domain on the multimerization of this important circulating hormone as well as the interaction between the adiponectin and ERp44. Using cellular and in vitro assays, we showed that ERp44 specifically recognizes the LMW and MMW forms but not the HMW form. Binding assays with short peptide mimetics and the aforementioned peptide model of the N-terminal domain of adiponectin suggest that ERp44 intercepts and converts the pool of fully oxidized LMW and MMW adiponectin, but not the HMW form, into reduced trimeric precursors. In vivo, these ERp44-bound precursors in the cis-Golgi may be transported back to the ER and released to enhance the population of adiponectin intermediates with appropriate oxidative state for HMW assembly, thereby underpinning the process of ERp44 quality control. Obesity-induced ER stress causes dysregulation of ER chaperone activity in vivo including ERp44 action, resulting in a decreased level of secreted HMW, which is associated with insulin resistance and type 2 diabetes. Using adipocyte cells and genetically obese mice we demonstrated that designed peptide mimetics derived from ERp44 clients can restore dysregulated ERp44 activity and in turn facilitated adiponectin assembly into HMW form and promote adiponectin release from the ER. Therefore, these peptides can act as reagents to counteract impaired adiponectin multimerization caused by dysregulation in ER chaperone activity.