title: Insulin-like growth factor binding protein-5 interacts with the vitamin D receptor and modulates the vitamin D response in osteoblasts.
abstract: The 1,25 dihydroxyvitamin D3 [1,25(OH)2D3]-induced differentiation of osteoblasts comprises the sequential induction of cell cycle arrest at G0/G1 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)2D3 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 alpha1 type 1 expression, and that 1,25(OH)2D3 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 G0/G1 and G2/M. Furthermore, IGFBP-5 reduced the effects of 1,25(OH)2D3 in blocking cell cycle progression at G0/G1 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)2D3-induced expression of bone differentiation markers while having a modest effect on the 1,25(OH)2D3-mediated inhibition of cell cycle progression in bone cells.
authors.affiliation: Kolling Institute of Medical Research, Royal North Shore Hospital, St. Leonards, New South Wales 2065, Australia. lyns@med.usyd.edu.au
authors.forename: Lynette J
authors.initials: LJ
authors.lastname: Schedlich
authors.forename: Anita
authors.initials: A
authors.lastname: Muthukaruppan
authors.forename: Michelle K
authors.initials: MK
authors.lastname: O'Han
authors.forename: Robert C
authors.initials: RC
authors.lastname: Baxter
mesh_terms: D000469:Alkaline Phosphatase / Q000037:antagonists & inhibitors / Q000378:metabolism; D015415:Biomarkers / Q000378:metabolism; D002117:Calcitriol / Q000494:pharmacology*; D002453:Cell Cycle; D002454:Cell Differentiation; D002467:Cell Nucleus / Q000737:chemistry / Q000378:metabolism; D002478:Cells, Cultured; D024042:Collagen Type I / Q000378:metabolism; D019281:Dimerization; D006801:Humans; D018975:Insulin-Like Growth Factor Binding Protein 5 / Q000378:metabolism*; D008024:Ligands; D010006:Osteoblasts / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D015675:Osteocalcin / Q000235:genetics; D011401:Promoter Regions, Genetic / Q000187:drug effects; D018167:Receptors, Calcitriol / Q000378:metabolism*; D047490:Retinoid X Receptor alpha / Q000037:antagonists & inhibitors / Q000378:metabolism; D014807:Vitamin D / Q000378:metabolism / Q000494:pharmacology; D030181:Vitamin D Response Element / Q000187:drug effects
chemical_list: D015415:Biomarkers; D024042:Collagen Type I; D018975:Insulin-Like Growth Factor Binding Protein 5; D008024:Ligands; D018167:Receptors, Calcitriol; D047490:Retinoid X Receptor alpha; D015675:Osteocalcin; D014807:Vitamin D; D000469:Alkaline Phosphatase; D002117:Calcitriol