Macrophage-adipocyte cross-talk in the initiation of obesity-related insulin resistance and type 2 diabetes: role of adiponectin

Abstract

Background and Aims: Recent studies suggest that, in obesity, chronic inflammation of adipose tissues (AT) with macrophage infiltration and enhanced expression of pro-inflammatory cytokines plays a significant role in the development of obesity-related metabolic disorders, such as type 2 diabetes. We have previously demonstrated that adenovirus-mediated over-expression of adiponectin, an insulin-sensitizing adipokine with antiinflammatory action and reduced expression in obesity, results in a marked down-regulation of several inflammation-related genes, including F4/80 (a macrophage marker) and monocyte chemotactic protein-1 (MCP-1), in the white AT of db/db diabetic mice (Lam & Xu, Curr Diab Rep 2005;5:254-9). As MCP-1 has been suggested to contribute to macrophage infiltration of AT in db/db mice (Kanda et al, JCI 2006;116:1494-505), we hypothesize that the reduced expression of adiponectin in obesity may play an important role in initiating the macrophage-adipocyte cross-talk which leads to obesity-related insulin resistance and diabetes. This study aims at determining the time course and magnitude of macrophage infiltration and inflammatory activation in AT, in relation to changes in the AT levels of adiponectin, glucose tolerance and insulin sensitivity, in db/db diabetic mice. Materials and Methods: At the age of 5, 6, 7, 8, 9, 10, 11, 12 weeks, male db/ db mice and lean controls were tested for blood glucose (non-fasting, fasting, and in response to ip. insulin tolerance test and glucose tolerance test), insulin and lipid levels. The mRNA expression level of adiponectin, MCP-1 and F4/80 were determined by real-time PCR in epididymal fat pads obtained after sacrifice of mice at different ages. Immunohistochemistry was performed to assess the severity of macrophage infiltration in AT. Adiponectin and MCP-1 were also measured by ELISA in medium obtained after the fat pads were incubated for 24 hours. Results: Compared to lean controls, db/db mice had significant increases in the weight of AT in 3 depots (epididymal, perirenal and abdominal subcutaneous) at 5 weeks, accompanied by significant hyperinsulinaemia and hypercholesterolaemia, and a trend towards impaired responses during insulin tolerance test, compared to lean controls. A significant 80% reduction in adipose tissue adiponectin mRNA expression, accompanied by significant reductions in adiponectin in the explant mediium, was also seen at 5 weeks, when no significant increase was found in F4/80 mRNA expression or macrophage infiltration of epididymal AT. There was also no significant increase in the release of MCP-1 during AT explant incubation at 5 weeks. Greater weight than controls, accompanied by impaired glucose tolerance, was seen from 6 weeks. Non-fasting hyperglycaemia, compared to lean controls, became evident at 8 weeks. By 10 weeks, fasting hyperglycaemia was established. Conclusion: These data suggest that reduced adiponectin expression precedes the macrophage infiltration in AT of db/db mice and may contribute, at least in part, to the macrophage infiltration and inflammatory gene activation in white AT in obese animals. Hyperinsulinaemia precedes macrophage infiltration of AT which may, nevertheless, contribute to progressive insulin resistance and development of diabetes in the db/db mice. Supported by RGC grant.