Role of polypeptide growth factors in normal and abnormal growth

There are three general conditions in which cell growth occurs in animals. The first is during embryonic growth and subsequent development into an adult organism. Second, there is the cellular proliferation in response to an injury such as wound repair and, of special interest here, in compensatory growth. Finally, there is the rapid growth associated with the neoplastic transformation of cells and resulting tumor growth. In this short review we will present several examples of the best characterized factors which have been associated with one or more of these situations. As with most biologically active factors, it is impossible to make clear cut-cut dividing lines as to function and several factors can be implicated several places. For example, nerve growth factor may play a role both in normal neuronal development and wound repair. Also, growth factors active in neoplastic growth may be involved in embryonic development. Both these cases will be mentioned in detail later.Throughout this review, growth will be defined so as to include both an increase in the number of cells, hyperplasia, and the enlargement and extension of individual cells, hypertrophy. There are instances, however, where a distinction must be made. There has been an explosive proliferation in research findings concerning growth factors in the last 10 years, partially made possible by the commercial availability of several of the best characterized factors. Numerous literature makes it necessary to omit certain details here and, thus, no attempt will be made to detail the most recent theories on how growth factors transmit the mitogenic message once they interact with the target cells. For example, several recent reviews discussed in detail how this may occur as well as structural similarities of several factors [1–3].In brief, we will summarize the current knowledge of growth factors in relationship to the three common situations where they are known to act; normal growth, response to injury, and neoplastic growth. The participation of growth factors has been well established in these situations, although there are other factors involved such as the substratum supporting the cells [4, 5] and growth-inhibiting substances [6, 7], some of which will be mentioned where necessary