Agouti regulation of leptin expression in adipocytes

Overexpression of the mouse agouti gene results in obesity. Humans express agouti in adipose tissue, and we have demonstrated that agouti protein stimulates lipogenesis in isolated human adipocytes. Since leptin (ob) is also expressed in human adipocytes, and other agents which stimulate lipogenesis, such as insulin, appear to increase leptin expression, we investigated the role of agouti in regulating leptin expression. Adipose tissue isolated from transgenic mice ubiquitously expressing agouti exhibited an approximately two-fold increase in leptin mRNA, and these mice exhibited a five-fold increase in circulating leptin (9.0±3 vs. 48.8±13.5 ng/mL, p<0.0005). To determine if this effect was specifically attributable to agouti or if it was secondary to the obesity exhibited by these animals, agouti cDNA was expressed in insect cells via a baculoviral expression vector to produce secreted agouti protein, which was then purified. Human adipocytes were then exposed to this agouti protein (50 nM) for 48 hours, and mRNA was isolated and probed for ob. Agouti exposure resulted in a 356% increase in ob mRNA levels compared to untreated cells (p<0.005). Thus, agouti specifically regulates ob expression; this stimulation may provide a mechanism to limit agouti-induced lipogenesis and obesity.