Insulin-like growth factor binding protein-5 interacts with the vitamin D receptor and modulates the vitamin D response in osteoblasts

The 1,25 dihydroxyvitamin D-3 [ 1,25(OH)(2) D-3]- induced differentiation of osteoblasts comprises the sequential induction of cell cycle arrest at G(0)/G(1) and the expression of bone matrix proteins. Reports differ on the effects of IGF binding protein ( IGFBP)- 5 on bone cell growth and osteoblastic function. IGFBP- 5 can be growth stimulatory or inhibitory and can enhance or impair osteoblast function. In previous studies, we have shown that IGFBP- 5 localizes to the nucleus and interacts with the retinoid receptors. We now show that IGFBP- 5 interacts with nuclear vitamin D receptor ( VDR) and blocks retinoid X receptor ( RXR): VDR heterodimerization. VDR and IGFBP- 5 were shown to colocalize to the nuclei of MG- 63 and U2- OS cells and coimmunoprecipitate in nuclear extracts from these cells. Induction of osteocalcin promoter activity and alkaline phosphatase activity by 1,25( OH) 2 D 3 were significantly enhanced when IGFBP- 5 was down- regulated in U2- OS cells. Moreover, we found IGFBP- 5 increased basal alkaline phosphatase activity and collagen alpha 1 type 1 expression, and that 1,25( OH) 2 D 3 was unable to further induce the expression of these bone differentiation markers in MG- 63 cells. Expression of IGFBP- 5 inhibited MG- 63 cell growth and caused cell cycle arrest at G(0)/G(1) and G(2)/ M. Furthermore, IGFBP- 5 reduced the effects of 1,25( OH) 2 D 3 in blocking cell cycle progression at G(0)/G(1) and decreased the expression of cyclin D1. These results demonstrate that IGFBP- 5 can interact with VDR to prevent RXR: VDR heterodimerization and suggest that IGFBP- 5 may attenuate the 1,25(OH)(2) D-3- induced expression of bone differentiation markers while having a modest effect on the 1,25(OH)(2) D-3 mediated inhibition of cell cycle progression in bone cells.