1α,25-Dihydroxyvitamin D3 increases IGF binding protein-5 expression in cultured osteoblasts

Rat osteoblasts produce insulin-like growth factors (IGFs) and IGF binding proteins (IGFBPs). Expression of IGFBP-5, an IGFBP which stimulates DNA synthesis of osteoblasts, was studied in vitro under the influence of 1α,25-dihydroxyvitamin D3 (1,25(OH)2D3) and parathyroid hormone (PTH). These two calcium-regulating hormones stimulated the expression of IGFBP-5 mRNA in cultured rat osteoblasts at low concentrations (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 the cell layer. IGFBP-5 may thus direct IGFs to the sites of bone remodeling.