Insulin-like growth factors; autocrine, paracrine or endocrine? New perspectives of the somatomedin hypothesis in the light of recent developments

Abstract

Introduction The list of growth factors which have generalized effects on the development and proliferation of many cells throughout the body has grown considerably; however, the insulin-like growth factors (IGFs) are still considered as a class in their own right. They are peptides with a high degree of structural homology to proinsulin with important effects on cell metabolism (insulin-like effects) and on cell growth, differentiation and mitosis. In man, there are two main forms which are known to exist; IGF-I, which is synonymous with somatomedin C, and IGF-II which in the human circulation is the most prevalent form. This review is not intended to cover the entire subject comprehensively but attempts to bring together some of the more recent developments in a cohesive manner to update our concepts on the mode of action of the IGFs. Although there have been many major advancements regarding IGF-II, particularly pertaining to its receptor,