The melanocortin receptors (MCRs) belong to the G-protein coupled receptors (family A). So far, 5 different subtypes have been described (MC1R-MC5R) and of these MC2R and MC5R have been proposed to act directly in adipocytes and regulate lipolysis in rodents. Using ACTH and -melanocyte stimulating hormone (-MSH) generated from proopiomelanocortin (POMC), as well as synthetic MSH-analogues to stimulate lipolysis in murine 3T3-L1 adipocytes it is shown that MC2R and MC5R are lipolytic mediators in differentiated 3T3-L1 adipocytes. Involvement of cAMP, phosphorylated extracellular signal-regulated kinase (ERK) 1/2, protein kinase B (PKB), adenosine 5' monophosphateactivated protein kinase (AMPK) and Jun-amino-terminal kinase (JNK) in MCR mediated lipolysis were studied. Interestingly, results obtained in 3T3-L1 cells suggest that lipolysis stimulated by -MSH, NDP--MSH, MT-II, SHU9119and PG-901 is mediated through MC5R in a cAMP independent manner. Finally, we identify essential differences in MCR mediated lipolysis when using 3T3-L1 cells compared to primary adipocytes. INTRODUCTIONThe melanocortin system acts through multiple pathways relevant for the prevention of obesity and obesity-related complications (Cone 1999;Marks et al. 2002;Spiegelman & Flier 2001). The system is acknowledged to have an important function in regulation of satiety and energy expenditure (Yaswen et al. 1999;Spiegelman & Flier 2001;Cheung et al. 1997;Azzara et al. 2002). MC4R knockout mice exhibit a well-described phenotype defined by increased lean body mass and fat mass, hyperphagia and disturbances in the metabolic response to overnutrition (Mountjoy et al. 1994;Huszar et al. 1997;Tschop & Heiman 2001). Furthermore, loss of MC4R function is the most frequent monogenetic alteration in severely obese humans (Krude et al. 1998) and in severe, early onset childhood obesity the frequency of mutations in the MC4R locus is 4-6% (Farooqi et al. 2003). Besides the effect on satiety and energy expenditure, the melanocortin system is believed to influence insulin release and insulin sensitivity (Fan et al. 2000;Huo et al. 2009). The melanocortin peptides and their receptors are also suspected to have a direct lipolytic effect on adipose tissues in rodents (Cho et al. 2005;Boston 1999;Spirovski et al. 1975). However this effect is controversial in humans (Hoch et al. 2007). The effect of melanocortins on adipocytes have by some researchers been explained by neuronal regulation of lipolysis (Brito et al. 2007), since MC4R mRNA has been identified in sympathetic neurons connected to white adipose tissue (WAT), indicating that central MC4R might stimulate lipid mobilization in the periphery (Song et al. 2005). However, studies in differentiated murine 3T3-L1 adipocytes suggest a direct lipolytic effect stimulated by melanocortin peptides ACTH and -MSH (Bradley et al. 2005;Cho et al. 2005), Page 3 of 24 A c c e p t e d M a n u s c r i p t 3 which supports the earlier identification of MC2R and MC5R mRNA in this cell line (Boston & Cone 1996). MC1R...