Peripheral regulation of the growth hormone-insulin-like growth factor system in fish and other vertebrates

The growth hormone (GH)-insulin-like growth factor (IGF) system plays a major role in coordinating the growth of vertebrates including fish. Considerable research on the regulation of growth has focused on the production and secretion of GH from the pituitary. This review will synthesize recent work on regulating extrapituitary aspects of the GH-IGF system, which includes GH binding proteins (GHBP), GH receptors (GHR), IGF binding protein (IGFBP), and IGF receptors (IGFR). These components are widely distributed and they interact to coordinate growth as well as a host of other biological processes such as metabolism, osmoregulation, reproduction, behavior, and immunity. The GH-IGF system of fish is particularly interesting and complex because it consists of multiple subtypes of GHRs, IGFRs, and IGFBPs that arose through gene duplication events associated with the evolution of the teleost lineage. Peripheral regulation of the GH-IGF system results from adjusting peripheral sensitivity to GH and IGFs as well as from modulating the bioavailability and actions of GH and IGFs in target cells. Numerous chemicals, including hormones such as growth hormone, insulin, somatostatin, and sex steroids as well as a variety of transcription factors, proteases, and phosphatases, regulate the synthesis and activity of GHRs, GHBPs, IGFRs, and IGFBPs as well as the synthesis, secretion, and bioavailability of IGFs. In addition, numerous environmental factors such as nutritional state, photoperiod, stress, and temperature have dramatic effects on the expression and activity of peripheral components of the GH/IGF system. The complex regulation of these system components appears to be both organism- and tissue-specific.