Ubiquitous expression of the mouse agouti gene mice results in a syndrome of obesity and hyperinsulinemia/insulin resistance. In humans, agouti is expressed in adipose tissue, and we have shown agouti protein to stimulate lipogenesis in adipocytes. However, transgenic mice expressing agouti in adipose tissue under control of the aP2 promoter do not become obese unless they also receive supplementary insulin. Accordingly, since we have previously shown the agouti protein to stimulate Ca2+ signaling in adipocytes and myocytes, we have examined the effects of agouti on intracellular Ca2+ ([Ca2+]i) and insulin release in the RIN-5F and HIT-115 pancreatic β cell lines. Agouti cDNA was expressed in insect cells infected with a baculoviral expression vector to produce secreted agouti protein which was then purified. Agouti (100 nM) stimulated sustained [Ca2+]i increases of 51±5 and 82±7 nM over baseline (i.e. 50% over baseline) in the RIN and HIT cells, respectively (p<0.001). This effect was abolished by glucose in the RIN cells, as glucose independently increases [Ca2+]i and insulin secretion. Agouti stimulated insulin release in a dose-responsive fashion, from 2.22±0.15 ng/mL to 3.18±0.56 and 7.68±0.28 ng/mL with 10 and 100 nM agouti, respectively (p<0.01). Thus, there is a potential role for agouti in the hyperinsulinemia of obesity; this increase in insulin may act synergistically with adipocyte agouti to stimulate lipogenesis and thereby contribute to obesity.
|State||Published - 1997|