Fibroblast growth factor 21 (FGF-21) is a member of the FGF family. Using β-Klotho as a cofactor, FGF-21 signals through FGF receptor 1c and 4 to activate PI3K and MAPK pathways (Mattila and Härkönen; Kharitonenkov et al.). FGF-21 expression is regulated by tissue-specific peroxisome proliferator-activated receptors (PPARs). Upon PPAR-α stimulation FGF-21 is produced in the liver, and activation of PPAR-γ leads to FGF-21 production in adipose tissue. FGF-21 promotes insulin-independent glucose uptake and lipid accumulation in primary human adipocytes and in mouse 3T3-L1 cells. In pancreatic islets and INS-1 cells it inhibits glucose-mediated glucagon release and stimulates insulin production. FGF-21 does not induce proliferation in immortalized cell lines, unlike other FGFs (Kharitonenkov and Shanafelt). FGF-21 regulates thermogenesis in white and brown adipose tissue, and metabolic processes in cells of pancreatic origin (Kharitonenkov et al.).