1-ALPHA,25-DIHYDROXYVITAMIN D-3 INCREASES IGF BINDING PROTEIN-5 EXPRESSION IN CULTURED OSTEOBLASTS

Rat osteoblasts produce insulin-like growth factors (IGFs) and IGF bin ding proteins (IGFBPs), Expression of IGFBP-5, an IGFBP which stimulat es DNA synthesis of osteoblasts, was studied in vitro under the influe nce of 1 alpha,25-dihydroxyvitamin D-3 (1,25(OH)(2)D-3) and parathyroi d hormone (PTH), These two calcium-regulating hormones stimulated the expression of IGFBP-5 mRNA in cultured rat osteoblasts at low concentr ations (10 pM) and in a dose- and time-dependent manner, Intact IGFBP- 5 was not detected in the culture medium, but was found attached to th e cell layer, IGFBP-5 may thus direct IGFs to the sites of bone remode ling.