AMPK and the Endocrine Control of Metabolism.

Complex multi-cellular organisms require a coordinated response from multiple tissues to maintain whole-body homeostasis in the face of energetic stressors like fasting, cold and exercise. It is also essential that energy is stored efficiently with feeding and the chronic nutrient surplus that occurs with obesity. Mammals have adapted several endocrine signals that regulate metabolism in response to changes in nutrient availability and energy demand. These include hormones altered by fasting and refeeding including insulin, glucagon, GLP-1 (glucagon like peptide-1), catecholamines, ghrelin and FGF21 (fibroblast growth factor 21); adipokines such as leptin and adiponectin; cell stress-induced cytokines like TNFα (tumor necrosis factor alpha) and GDF15 (growth differentiating factor 15), and lastly exerkines such as IL-6 (interleukin-6) and irisin. Over the last two decades, it has become apparent that many of these endocrine factors control metabolism by regulating the activity of the AMPK (AMP-activated protein kinase). AMPK is a master regulator of nutrient homeostasis, phosphorylating over 100-distinct substrates that are critical for controlling autophagy, carbohydrate, fatty acid, cholesterol and protein metabolism. In this review we discuss how AMPK integrates endocrine signals to maintain energy balance in response to diverse homeostatic challenges. We also present some considerations with respect to experimental design which should enhance reproducibility and the fidelity of the conclusions.