Role of Intracellular Free Calcium in the Obesity and Insulin Resistance Associated with Dominant Agouti Mutations

Several dominant mutations at the agouti locus in the mouse cause a syndrome of adult-onset obesity, hyperinsulinemia, and insulin resistance. Although ectopic overexpression of the agouti gene is directly responsible for the disease in these mutations, the precise mechanism is unclear. Intracellular Ca²⁺ ([Ca²⁺]i) appears to have a role in mediating insulin signal transduction, and altered handling of [Ca²⁺]i homeostasis and flux is observed in obese and insulin resistant animals and humans. Data reported here demonstrate that mice carrying the dominant agouti mutation, viable yellow (A^vy), exhibit an elevation of [Ca²⁺]i and Ca²⁺ influx rate in insulin-sensitive type I skeletal muscle. The degree of elevation in [Ca²⁺]i is highly correlated with the degree of expression of agouti gene and the elevation of body weight. Moreover, recombinant agouti protein directly induced a sustained increase in [Ca²⁺]i in cultured myocytes and adipocytes; this effect is substantially inhibited by Ca²⁺ channel blockade. Ca²⁺channel blockade was also effective in reducing fat pad mass and fatty acid synthase (FAS) mRNA levels and activity in adipocytes of transgenic mice expressing the agouti gene in a ubiquitous manner. These results are consistent with previous reports in which recombinant agouti protein directly stimulates FAS mRNA levels and activity and triglyceride content in cultured adipocytes in a Ca²⁺ dependent manner. Accordingly, altered [Ca²⁺]i metabolism appears to be involved in development of obesity syndrome in A^vy mice, and this defect in Ca²⁺ signaling may cause activation of FAS either directly or indirectly and subsequent de novo lipogenesis, contributing accumulation of fat depot in these mice.