Coordinate regulation of feeding, metabolism, and growth: Perspectives from studies in fish

This paper develops a model for coordinate regulation of feeding, metabolism, and growth based on studies in fish. Many factors involved with the control of feeding [e.g., cholecystokinin (CCK) and ghrelin (GRLN)], energy metabolism [e.g., insulin (INS), glucagon (GLU), glucagon-like peptide (GLP), and somatostatins (SS), produced in the endocrine pancreas; and leptin (LEP) produced broadly], and growth [e.g., GRLN, growth hormone (GH), insulin-like growth factors (IGFs), GH receptors (GHR), IGF receptors (IGFR)] interact at various levels. Many such interactions serve to coordinate these systems to favor anabolic processes (i.e., lipid and protein synthesis, glycogenesis) and growth, including GH promotion of feeding and stimulation of INS production/secretion and the upregulation of GHR and IGFR by GRLN. As nutrient and stored energy status change, various feedbacks serve to curtail feeding and transition the animal from an anabolic/growth state to a catabolic state. Many factors, including LEP and IGF, promote satiety, whereas SS downregulates INS signaling as well as IGF production and GHR and IGFR abundance. As INS and IGF levels fall, GH becomes disconnected from growth as a result of altered linkage of GHR to cell signaling pathways. As a result, the catabolic actions of GH, GLU, GLP, LEP, and SS prevail, mobilizing stored energy reserves. Coordinate regulation involves relative abundances of blood-borne hormones as well as the ability to adjust responsiveness to hormones (via receptor and post-receptor events) in a cell-/tissue-specific manner that results from genetic and epigenetic programming and modulation by the local milieu of hormones, nutrients, and autocrine/paracrine interactions. The proposed model of coordinate regulation demonstrates how feeding, metabolism, and growth are integrated with each other and with other processes, such as reproduction, and how adaptive adjustments can be made to energy allocation during an animal's life history and/or in response to changes in environmental conditions.