title: A novel interaction between insulin-like growth factor binding protein-6 and the vitamin D receptor inhibits the role of vitamin D3 in osteoblast differentiation.
abstract: Insulin-like growth factor binding protein-6 (IGFBP-6) is a secreted glycoprotein that reduces the bioavailability of IGFs. It has both IGF-dependent and -independent effects on cell growth, however the mechanisms responsible for its IGF-independent actions of IGFBP-6 are not fully understood. In previous studies, we have shown that recombinant IGFBP-6 can be internalized and translocated to the nucleus. The present study shows that IGFBP-6 interacts with the vitamin D receptor (VDR). Physical interactions between IGFBP-6 and the VDR were confirmed by GST pulldown and co-immunoprecipitation assays. We also determined that the interaction binding sites were on the C-terminal region of the VDR. This interaction can influence retinoid X receptor (RXR):VDR heterodimerization. Furthermore, immunofluorescence colocalization studies showed that IGFBP-6 colocalized with the VDR predominantly in the cell's nucleus. Inductions of osteocalcin and growth hormone promoter activities by 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) were significantly decreased when cells were co-transfected with IGFBP-6 and the VDR compared with cells transfected with the VDR only. Moreover, we found that alkaline phosphatase activity (ALP, a general marker of osteoblast differentiation) was significantly decreased in osteoblast-like cells when they were transfected with IGFBP-6 in the presence of 1,25(OH)(2)D(3). No obvious difference in ALP activity was observed when cells were transfected with IGFBP-6 and endogenous VDR was knocked down by siRNA. These results demonstrate that IGFBP-6 inhibits osteoblastic differentiation mediated by 1,25(OH)(2)D(3) and the VDR through interacting with the VDR and inhibiting its function. This is a novel mechanism for IGFBP-6.
authors: National Laboratory of Medical Molecular Biology, Institute of Basic Medical Sciences, Chinese Academy of Medical Sciences & Peking Union Medical College, Beijing, China.
authors: Jian
authors: J
authors: Cui
authors: Chunling
authors: C
authors: Ma
authors: Jia
authors: J
authors: Qiu
authors: Xiaoli
authors: X
authors: Ma
authors: Xin
authors: X
authors: Wang
authors: Hong
authors: H
authors: Chen
authors: Bingren
authors: B
authors: Huang
mesh_terms: D000943:Antigens, Differentiation / Q000378:metabolism; D002454:Cell Differentiation; D002460:Cell Line; D002762:Cholecalciferol / Q000378:metabolism*; D017930:Genes, Reporter; D006801:Humans; D047468:Immunoprecipitation; D018976:Insulin-Like Growth Factor Binding Protein 6 / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism; D007335:Insulin-Like Growth Factor II / Q000737:chemistry; D049409:Luciferases, Firefly / Q000096:biosynthesis / Q000235:genetics; D010006:Osteoblasts / Q000166:cytology* / Q000378:metabolism; D011485:Protein Binding; D054730:Protein Interaction Domains and Motifs; D055503:Protein Multimerization; D018167:Receptors, Calcitriol / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D047488:Retinoid X Receptors / Q000737:chemistry; D015533:Transcriptional Activation; D030181:Vitamin D Response Element
chemical_list: D000943:Antigens, Differentiation; D018976:Insulin-Like Growth Factor Binding Protein 6; D018167:Receptors, Calcitriol; D011993:Recombinant Fusion Proteins; D047488:Retinoid X Receptors; D002762:Cholecalciferol; D007335:Insulin-Like Growth Factor II; D049409:Luciferases, Firefly