Cocaine- and Amphetamine-Related Transcript (CART) directly alters adipose tissue thermogenesis, insulin sensitivity, and leptin expression

Hypothalamic cocaine- and amphetamine-regulated transcript (CART) is stimulated by leptin, inhibits food intake, and increases thermogenesis in vivo via activation of the sympathetic nervous system. This neuropeptide is also expressed in peripheral tissues and nerve endings. A specific receptor has not yet been identified. We investigated direct effects of CART on signaling pathways as well as metabolic and endocrine functions of SV40T-immortalized brown adipocytes. Chronic stimulation of differentiated cells with 1µM CART did not affect the pattern or extent of fat accumulation as judged microscopically and using the fat–specific Oil Red O staining. However, acute exposure of cells to this neuropeptide (1µM, 10–40min) altered PI-3 kinase and JAK/STAT signaling pathways. Thus, Akt and STAT3 phosphorylation was decreased by 40 and 30%, respectively (p≤0.01). These effects were time-and dose-dependent with small decreases already detectable at a concentration of 10nM. By contrast, there was no effect on p44/42 MAP kinase phosphorylation. However, CART treatment (1µM) of differentiated adipocytes time-dependently induced protein expression of uncoupling protein-1 (UCP-1) to a maximum of 200% after 8h (p≤0.01). Furthermore, 1µM CART also time-dependently enhanced insulin-induced glucose uptake by a maximum of 25% after 4h pretreatment (p≤0.01). On the level of endocrine regulation, 8h of CART treatment (1µM) reduced leptin mRNA expression by 20%, thus potentially inducing a negative feedback signal to the central nervous system. Expression of adiponectin and monocyte chemoattractant protein-1 remained unaltered. In summary, our results provide evidence for direct effects of CART on adipocyte signaling, metabolism, and endocrine function. They suggest peripheral actions of this anorexigenic neuropeptide to enhance energy expenditure and provide a negative feedback via leptin.