Theagoutigene product inhibits lipolysis in human adipocytes via a Ca²⁺‐dependent mechanism

Abstract

Overexpression of the murine agouti gene results in obesity. The human homologue of agouti is expressed primarily in human adipocytes, and we have shown recombinant agouti protein to increase adipocyte intracellular Ca2+([Ca2+]i) and thereby stimulate lipogenesis. However, since recent data demonstrate that increasing adipocyte [Ca2+]i may also inhibit lipolysis, we have investigated the role of agouti-induced [Ca2+]i increases in regulating lipolysis in human adipocytes. Short-term (1 h) exposure to recombinant agouti (100 nM) protein had no effect on basal lipolysis, although longer term treatment (24 h) caused a 60% decrease in basal lipolysis (P<0.0001). Short-term agouti treatment totally inhibited ACTH-induced lipolysis (P<0.05). Since melanocortin receptors (MCR) are involved in some actions of agouti, we next determined whether agouti's antilipolytic effect is exerted through competitive antagonism of the ACTH receptor (MCR-2). Forskolin (1 microM), an adenylate cyclase activator, induced a 48% increase in lipolysis in human adipocytes (P<0.05); this effect was reversed by 100 nM agouti (P<005), demonstrating that the antilipolytic effect of agouti is distal to the ACTH receptor. To determine the role of [Ca2+]i in the antilipolytic effect of agouti, human adipocytes were treated with KCl or arginine vasopressin to stimulate voltage- and receptor-stimulated Ca2+ influx, respectively. Both agents caused inhibition of forskolin-induced lipolysis (P<0.005). Furthermore, agouti's antilipolytic effect was also blocked by the Ca2+ channel blocker nitrendipine. These data demonstrate that agouti exerts a potent antilipolytic effect in human adipocytes via a Ca2+-dependent mechanism. This effect, combined with agouti-induced lipogenesis, represents a coordinate control of adipocyte lipid metabolism that may contribute to an agouti-induced obesity syndrome.

1α, 25-Dihydroxyvitamin D and corticosteroid regulate adipocyte nuclear vitamin D receptor

“…13,17 In addition, elevated [Ca 2 þ ]i also inhibits lipolysis, leading to an expansion of adipocyte triglyceride storage. 5,16 Conversely, the decrease in [Ca 2 þ ]i observed with increasing calcium appears attributable to suppression of circulating 1a, 25-dihydroxyvitamin D. 9,18,19 Consistent with this, data from our laboratory have demonstrated that 1a, 25-dihydroxyvitamin D induces rapid [Ca 2 þ ]i influx in adipocytes, whereas a specific membrane vitamin D receptor antagonist (1b, 25-dihydroxyvitamin D) blocked this effect. 4 This indicated a non-genomic action of 1a, 25-dihydroxy Role of 1a, 25-Dihydroxyvitamin D in adipocyte X Sun and MB Zemel vitamin D via a putative membrane vitamin receptor, which later was identified as the 1, 25D 3 -MARRS, 20 in modulating…”

“…15,16 Increasing [Ca 2 þ ]i in adipocytes via either receptor-or voltage-mediated Ca 2 þ channel activation stimulated fatty acid synthase expression and activity. 13,17 In addition, elevated [Ca 2 þ ]i also inhibits lipolysis, leading to an expansion of adipocyte triglyceride storage.…”

Agouti regulates adipocyte transcription factors

“…STAT5A is shown for each group to demonstrate even loading of the samples. olism in cultured adipose cells in vitro (19,47). Although the agouti gene is not normally expressed in adipose tissue in the mouse, we were able to induce the expression of high levels of agouti in white and brown adipose tissue by expressing the cloned mouse gene under the control of the aP2 promoter.…”

Dietary calcium regulates ROS production in aP2-agouti transgenic mice on high-fat/high-sucrose diets

“…8 We have reported that 1a, 25-(OH) 2 -D 3 plays a direct role in the modulation of adipocyte Ca 2 þ signaling, resulting in an increased lipogenesis and decreased lipolysis. 18,19 In addition, 1a, 25-(OH) 2 -D 3 also plays a role in regulating human adipocyte UCP2 expression, suggesting that the suppression of 1a, 25-(OH) 2 -D 3 and the resulting upregulation of UCP2 may contribute to increased rates of energy utilization. 10,11 Accordingly, the suppression of 1a, 25-(OH) 2 -D 3 by increasing dietary calcium attenuates adipocyte triglyceride accumulation and caused a net reduction in fat mass in both mice and humans in the absence of caloric restriction, 13 a marked augmentation of body weight and fat loss during energy restriction in both mice and humans, 13,20 and a reduction in the rate of weight and fat regain following energy restriction in mice.…”