Growth hormone and a partial mediator of its biological action, insulin-like growth factor I.

This review summarizes data on two hormones, growth hormone (GH) and insulin-like growth factor I (IGF-I). Genes encoding these hormones are actively expressed in various tissues which can synthesize and secrete the corresponding hormones into the intracellular space and blood. Apart from the pituitary, GH gene is also expressed in mammary gland, thymus, spleen, lymph nodes, and blood cells. GH activates expression of the gene of IGF-I and stimulates its secretion by the liver, adipose tissue, thymus, and other tissues. The growth-stimulating effect of GH is mediated (at least partially) by IGF-I, but direct (IGF-I-independent) influence on target tissues is also possible. Genes encoding GH and IGF-I receptors are expressed in all organs and tissues, including various cells of the immune system. GH and IGF-I regulate the function of the immune system via endocrine, paracrine, and autocrine mechanisms.