Adiponectin Resistance in Obesity: Adiponectin Leptin/Insulin Interaction.

The adiponectin (APN) levels in obesity are negatively correlated with chronic subclinical inflammation markers. The hypertrophic adipocytes cause obesity-linked insulin resistance and metabolic syndrome. Furthermore, macrophage polarization is a key determinant regulating adiponectin receptor (AdipoR1/R2) expression and differential adiponectin-mediated macrophage inflammatory responses in obese individuals. In addition to decrease in adiponectin concentrations, the decline in AdipoR1/R2 messenger ribonucleic acid (mRNA) expression leads to a decrement in adiponectin binding to cell membrane, and this turns into attenuation in the adiponectin effects. This is defined as APN resistance, and it is linked with insulin resistance in high-fat diet-fed subjects. The insulin-resistant group has a significantly higher leptin-to-APN ratio. The leptin-to-APN ratio is more than twofold higher in obese individuals. An increase in expression of AdipoRs restores insulin sensitivity and β-oxidation of fatty acids via triggering intracellular signal cascades. The ratio of high molecular weight to total APN is defined as the APN sensitivity index (ASI). This index is correlated to insulin sensitivity. Homeostasis model of assessment (HOMA)-APN and HOMA-estimated insulin resistance (HOMA-IR) are the most suitable methods to estimate the metabolic risk in metabolic syndrome. While morbidly obese patients display a significantly higher plasma leptin and soluble (s)E-selectin concentrations, leptin-to-APN ratio, there is a significant negative correlation between leptin-to-APN ratio and sP-selectin in obese patients. When comparing the metabolic dysregulated obese group with the metabolically healthy obese group, postprandial triglyceride clearance, insulin resistance, and leptin resistance are significantly delayed following the oral fat tolerance test in the first group. A neuropeptide, Spexin (SPX), is positively correlated with the quantitative insulin sensitivity check index (QUICKI) and APN. APN resistance together with insulin resistance forms a vicious cycle. Despite normal or high APN levels, an impaired post-receptor signaling due to adaptor protein-containing pleckstrin homology domain, phosphotyrosine-binding domain, and leucine zipper motif 1 (APPL1)/APPL2 may alter APN efficiency and activity. However, APPL2 blocks adiponectin signaling through AdipoR1 and AdipoR2 because of the competitive inhibition of APPL1. APPL1, the intracellular binding partner of AdipoRs, is also an important mediator of adiponectin-dependent insulin sensitization. The elevated adiponectin levels with adiponectin resistance are compensatory responses in the condition of an unusual discordance between insulin resistance and APN unresponsiveness. Hypothalamic recombinant adeno-associated virus (rAAV)-leptin (Lep) gene therapy reduces serum APN levels, and it is a more efficient strategy for long-term weight maintenance.