Binding of insulin-like growth factors to cultured rat calvarial cells with differing biologic responses

The insulin-like growth factors (IGFs) are considered important regulators of bone metabolism affecting a number of biologic responses in vitro. Primary fetal rat calvarial cells (PRC) and a cloned adult rat calvarial cell line (C3) both exhibit a concentration-dependent IGF stimulation of [3H]thymidine incorporation into DNA, but the C3 cells show a greater sensitivity and magnitude of response. IGF-I and IGF-II were nearly equipotent in PRC cultures, but IGF-I was more than twice as active as IGF-II in the C3 cultures. This effect of the IGFs on DNA synthesis in two bone cell cultures with different culture histories has been correlated with receptor and binding protein profiles. Specific high-affinity IGF binding sites were found in both cell types. In general, the sites present on PRC cells showed a preference for binding IGF-II over IGF-I, but C3 cells displayed two types of relatively specific binding sites. In both cell types [125I]IGF-I bound primarily to a protein with IGF type I receptor characteristics. However, in PRC cells, [125I]IGF-II cross-linked specifically with proteins that had IGF type II receptor characteristics plus several sites unique to these cells; in C3 cells, [125I]IGF-II bound to a 139 kD protein that could be displaced by either IGF-I or IGF-II. Finally, IGF-II-specific 85 and 67 kD proteins were common to both cell types. From these studies, it is apparent that the IGFs bind to a variety of high-affinity binding sites in bone cells and that these sites differ between a highly responsive and a less responsive bone cell population.