is employed by AstraZeneca. FGF21 D-Melibiose mRNA were increased in the loaded mice compared with the control mice, we first evaluated the loading response in FGF21 over expressing mice with constant high D-Melibiose FGF21 levels. Leptin treatment, but not increased loading, decreased the BW in the FGF21-overexpressing mice, demonstrating that specifically the loading effect is usually attenuated in the presence of high activity in the FGF system. However, as FGF21 knockout mice displayed a normal loading response on BW, FGF21 is usually neither mediating nor essential for the loading response. In conclusion, the BW-reducing effect of increased loading but not of leptin treatment is usually blocked by high activity in the FGF system. We propose that both the gravitostat and the FGF system regulate BW independently of leptin and that pharmacologically enhanced activity in the FGF system reduces the sensitivity of the gravitostat. Obesity is usually a growing problem worldwide, and it is associated with increased mortality and morbidity (1). Today, there are few effective pharmacological treatment options to decrease obesity or to affect body weight (BW) homeostasis. In 1994, the fat mass regulating hormone leptin was identified (2), but unfortunately, leptin was not successful as a treatment of common obesity in humans (3). Only a D-Melibiose few people in the world suffering from leptin deficiency have a good effect from leptin treatment (4). Therefore, leptin has, so far, been of limited clinical use, although the marked obesity caused by leptin deficiency clearly proves the biological importance of leptin (5,6). The family of fibroblast growth factors (FGFs) regulates energy metabolism and provides a new approach to the treatment of obesity and other metabolic diseases (7). Some FGFs are released into the circulation and can then act as endocrine hormones. The binding of endocrine FGFs, such as FGF15, FGF21, and FGF23, to their FGF receptors (FGFRs) is usually promoted via D-Melibiose their interactions with coreceptors, such as-Klotho. The modulation of FGFR activity decreases BW independently of leptin (8,9). Additionally, we have recently published a homeostatic regulation of BW and fat mass, named the gravitostat, which affects the glucose metabolism, and like FGF, it is leptin impartial (10,11). The gravitostat can be activated by an artificial Rabbit polyclonal to AIM1L weight-loading treatment, which is effective in the decreasing of BW in mouse and rat. The effect of gravitostat activation seems to be larger in obese animals, contrary to leptin but the mechanism behind is not fully known (10,11). Both the family of FGFs (8,9) and the gravitostat (10) have been shown to regulate metabolism and BW independently of leptin, but there is a lack of knowledge of the interactions between these two systems. By studying the interactions between increased loading and other known systems affecting BW, such as the FGF system and leptin, we may identify novel targets for anti-obesity treatment. There are several FGFRs, and each receptor may bind several FGFs. The FGFR1c is usually abundant in the hypothalamus, and one of the FGFs binding to this receptor is usually FGF21. It has been shown that increased loading during exercise increases hepatic FGF21 production in mice, as well as serum FGF21 levels in both mice and humans (12,13). The aim of the current study was to determine the possible interactions between the effect of increased loading and the FGF system for the regulation of BW. == Materials and Methods == == Animals == All animal procedures were approved by the Ethics Committee on Animal Care and Use in Gothenburg, Sweden. C57BL/6 mice were purchased from Taconic (Ejby, Denmark). Transgenic FGF21 mice, FGF21 LoxP mice, and mice expressing Cre recombinase ubiquitously and from an early embryonic stage, under the control of the phosphoglycerate kinase-1 promoter (PGK-Cre), were from The Jackson Laboratory (Bar Harbor, ME). To generate the FGF21 knockout (KO) mice, PGK-Cre mice were crossed with FGF21 LoxP mice. Removal of the PGK-Cre allele and deletion of the FGF21 gene were confirmed by genotyping. Three different primer pairs were used to evaluate which mice had the FGF21 LoxP allele (forward 5-AAGCATTCCTGGTACCACGG-3 and reverse 5-AGCACTAAGGGAGGCAGAGGCAAGTGATT-3), the deleted FGF21 allele (forward 5-CCTCCAGATTTAGGAGTGCAG A-3 and reverse 5-AGGGAGGCAGAGGCAAGTGATT-3), and the PGK-Cre allele (forward 5-AACATGCTTCATCGTCGG-3 and reverse 5-TTCGGATCATCAGCTACACC-3). All genetically modified mice and their littermate controls were on a C57BL/6 background. == Loading == Mice were fed a high-fat diet (60% fat; D12492; Research Diets, New Brunswick, NJ) during 4 weeks, and then a capsule that weighed 15% of the BW (load) or 3% of the BW (control) was implanted intraperitoneally into the mice under isoflurane anesthesia. The BWs before capsule implantation.