Dexamethasone and 1,25-Dihydroxyvitamin D₃Modulation of Insulin-Like Growth Factor-Binding Proteins in Rat Osteoblast-Like Cell Cultures

Using the method of Western ligand blot, we have found that the major form of insulin-like growth factor-binding protein (IGF-BP) secreted by rat osteoblastic-like cells in culture is a 31-kDa protein that is immunologically identical to BP-2, the binding protein originally identified in conditioned medium of Buffalo rat liver cells (BRL-3A). Two minor forms of IGF-BPs with apparent mol wt of 24 kDa (BP-24) and 42 kDa (BP-42) have also been identified. The amount of IGF-BPs in serum-free conditioned medium increased 3-fold on day 3 compared to the day 1 level. We also studied the modulation of IGF-BPs by dexamethasone (DEX), 1,25-dihydroxyvitamin D [1,25-(OH)2D3], and insulin-like growth factor-I (IGF-I). DEX coordinately reduced the level of IGF-BPs in a dose- and time-dependent manner, which resulted in less than 10% of the BP-2 remaining at 100 nM. In contrast, 1,25-(OH)2D3 at 100 nM enhanced the amount of BP-2 by 1-fold. In combined treatments, 1,25-(OH)2D3 at 10 nM was unable to antagonize the inhibitory effect of DEX in the dose range of 1-10 nM. IGF-I, at 1 and 10 nM, proved to be a potent stimulator of all IGF-BPs, and at 10 nM, it completely reversed the inhibition by 100 nM DEX. Although the roles of IGF-BPs have not been clearly defined in bone cells, they are capable of modulating the biological actions of IGFs in other cell culture systems. Modulation of the IGF-BP level by DEX, 1,25-(OH)2D3, and IGF-I suggests important roles for these binding proteins in altering IGF-I action in rat osteoblast-like cell cultures.