1,25-DIHYDROXYVITAMIN-D3 [1,25-(OH)2D3] INCREASES INSULIN-LIKE GROWTH FACTOR-I (IGF-I) RECEPTORS IN CLONAL OSTEOBLASTIC CELLS - STUDY ON INTERACTION OF IGF-I AND 1,25-(OH)2D3

We previously reported a cooperative effect between insulin-like growth factor I (IGF-I) and 1,25-dihydroxy-vitamin D3[l,25-(OH)2D3] in murine clonal osteoblastic cells, MCT3T3-E1. In the present study, the possible mechanism of interaction between these hormones was investigated. The effect of IGF-I on l,25-(OH)2D3receptors in MC3T3-E1 cells was examined. The affinity and hormone binding capacity of l,25-(OH)2D3receptors were not altered by IGF-I. Immunoblot analysis showed about 54 kilodaltons (kDa) l,25-(OH)2D3receptors, similar to that observed for mouse fibroblasts. The synthesis of IGF-I by the cells under a serum-free condition was determined by RIA. The assay revealed immunore-active IGF-I secreted by MC3T3-E1 cells (1.79 ± 0.04 x 10-9M, mean ± SE, n = 5). Rat GH significantly increased the concentration of IGF-I, but l,25-(OH)2D3did not. IGF-I radioligand-receptor assay revealed specific binding of IGF-I to MC3T3-E1 cells. The relative potency of IGF-I-related peptides to bind with the cells was in the order of IGF-I >> multiplication-stimulating activity (the rat homologue of IGF-II) > insulin, and the receptor protein migrated as a 130-kDa band in autoradiography. Scatchard analysis showed a significant increase in IGF-I binding sites by 50% after 3-day treatment with 5 X 10-11M l,25-(OH)2D3, without any change in affinity. These results indicate that the interaction of IGF-I and 1,25-(OH)2D3in the culture of MC3T3-E1 cells may be mediated by the effect of 1,25-(0H)2D3on IGF-I receptors. © 1990 by The Endocrine Society.