Combined effects of insulin treatment and adipose tissue-specific agouti expression on the development of obesity

Abstract

The agouti gene product is a secreted protein that acts in a paracrine manner to regulate coat color in mammals. Several dominant mutations at the agouti locus in mice cause the ectopic, ubiquitous expression of agouti, resulting in a condition similar to adult-onset obesity and non-insulin-dependent diabetes mellitus. The human agouti protein is 85% homologous to mouse agouti; however, unlike the mouse agouti gene, human agouti is normally expressed in adipose tissue. To address whether expression of agouti in human adipose tissue is physiologically relevant, transgenic mice were generated that express agouti in adipose tissue. Similar to most humans, these mice do not become obese or diabetic. However, we found that daily insulin injections significantly increased weight gain in the transgenic lines expressing agouti in adipose tissue, but not in nontransgenic mice. These results suggest that insulin triggers the onset of obesity and that agouti expression in adipose tissue potentiates this effect. Accordingly, the investigation of agouti's role in obesity and non-insulin-dependent diabetes mellitus in mice holds significant promise for understanding the pathophysiology of human obesity.

Agouti regulates adipocyte transcription factors

“…The Ϫ5.4-kb aP2 promoter exhibits low levels of ectopic expression in many tissues (30,34), and the changes that we observed in adipose tissue may have been indirectly caused by ectopic agouti expression. To examine the direct effects of agouti on adipocytes, mature 3T3-L1 adipocytes were treated with recombinant murine agouti.…”

“…In our original study (30) performed at ORNL, we did not observe any significant differences in body weight between transgenic mice that expressed agouti in adipose tissue (Tg/ϩ) and their littermates (ϩ/ϩ), but the transgenic mice gained more weight if they were given insulin injections. The ORNL mice harbored a number of pathogens and parasites.…”

“…Transgenic mice were generated at Oak Ridge National Laboratory (ORNL) as previously described (30). The aP212 and aP273 lines were rederived at Charles River Laboratory and maintained on the FVB/N background at Pennington Biomedical Research Center (PBRC; Baton Rouge, LA).…”

“…Mice were euthanized by cervical dislocation, and adipose tissue was quickly removed, weighed, and frozen in liquid nitrogen for future analysis. PCR genotyping and Northern blot hybridization analyses were performed as previously described (30).…”

“…We (30) reported that transgenic mice that have the aP2-promoter driving agouti expression are sensitized to insulin. In this paper, we further describe the phenotype of the aP2-agouti transgenic mice and identified a potential mechanism of increased insulin sensitivity and obesity.…”