THE AGOUTI GENE-PRODUCT INHIBITS LIPOLYSIS IN HUMAN ADIPOCYTES VIA A CA2-DEPENDENT MECHANISM()

Overexpression of the murine agouti gene results in obesity, The human homologue of agouti is expressed primarily in human adipocytes, and w e have shown recombinant agouti protein to increase adipocyte intracel lular Ca2+([Ca2+](i)) and thereby stimulate Lipogenesis. However, sinc e recent data demonstrate that increasing adipocyte [Ca2+](i) may also inhibit Lipolysis, we have investigated the role of agouti-induced [C a2+](i) increases in regulating lipolysis in human adipocytes, Short-t erm (1 h) exposure to recombinant agouti (100 nM) protein had no effec t on basal lipolysis, although longer term treatment (24 h) caused a 6 0% 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 deter mined whether agouti's antilipolytic effect is exerted through competi tive antagonism of the ACTH receptor (MCR-2), Forskolin (1 mu M), an a denylate cyclase activator, induced a 48% increase in Lipolysis in hum an adipocytes (P<0.05); this effect was reversed by 100 nM agouti (P<0 05), demonstrating that the antilipolytic effect of agouti is distal t o the ACTH receptor, To determine the role of [Ca2+]i in the antilipol ytic effect of agouti, human adipocytes were treated with KCI or argin ine vasopressin to stimulate voltage- and receptor-stimulated Ca2+ inf lux, respectively, Both agents caused inhibition of forskolin-induced lipolysis (P<0.005), Furthermore, agouti's antilipolytic effect was al so blocked by the Ca2+ channel blocker nitrendipine. These data demons trate that agouti exerts a potent antilipolytic effect in human adipoc ytes via a Ca2+-dependent mechanism. This effect, combined with agouti -induced lipogenesis, represents a coordinate control of adipocyte lip id metabolism that may contribute to an agouti-induced obesity syndrom e.