Insulin-like growth factors and bone: the osteoporosis connection.

In the last five years significant progress has been made defining the role of insulin-like growth factors (IGFs) in the process of bone remodeling. In this paper, we present critical evidence that IGF-I and IGF-II are produced by bone cells and regulate specific osteoblastic and osteoclastic functions. In addition, we review work from several laboratories establishing the role of the skeletal IGF binding proteins as an integral component of a unique IGF regulatory system. Data presented suggest that the calciotropic hormones active in the bone remodeling process may exert their effects through the IGF regulatory system. In contrast to the well-defined local action of IGF-I and IGF-II on the skeleton, the relationship between circulating IGF-I and bone remodeling is less certain. Newer data are presented which suggest the potential utility of serum growth factor measurements in certain clinical states. Finally, this paper presents an overview of the most current efforts to stimulate bone formation using recombinant IGFs. However, work on the beneficial aspects of IGFs for the skeleton remains preliminary at best with the eventual therapeutic role of IGF-I in osteoporosis yet to be defined.