The adipocyte: a model for integration of endocrine and metabolic signaling in energy metabolism regulation

The ability to ensure continous availability of energy despite highly varia ble supplies in the environment is a major determinant of the survival of a ll species. In higher organisms, including mammals, the capacity to efficie ntly store excess energy as triglycerides in adipocytes, from which stored energy could be rapidly released for use at other sites, was developed. To orchestrate the processes of energy storage and release, highly integrated systems operating on several physiological levels have evolved. The adipocy te is no longer considered a passive bystander, because fat cells actively secrete many members of the cytokine family, such as leptin, tumor necrosis factor-alpha, and interleukin-6, among other cytokine signals, which influ ence peripheral fuel storage, mobilization, and combustion, as well as ener gy homeostasis. The existence of a network of adipose tissue signaling path ways, arranged in a hierarchical fashion, constitutes a metabolic repertoir e that enables the organism to adapt to a wide range of different metabolic challenges, such as starvation, stress, infection, and short periods of gr oss energy excess.