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

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• 作者：Jian Cui^a ; Chunling Ma^a ; Jia Qiu^a ; Xiaoli Ma^a ; Xin Wang^a ; Hong Chen ; ^a ; ^{hchen@public.bta.net.cn"" rel=""nofollow} ; Bingren Huang ; ^a ; ^{huangbr@public.bta.net.cn"" rel=""nofollow}
• 关键词：IGFBP-6 ; VDR ; Osteoblast differentiation
• 刊名：Molecular and Cellular Endocrinology
• 出版年：2011
• 出版时间：16 May 2011
• 年：2011
• 卷：338
• 期：1-2
• 页码：84-92
• 全文大小：938 K

文摘

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 D₃ (1,25(OH)₂D₃) 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)₂D₃. 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)₂D₃ and the VDR through interacting with the VDR and inhibiting its function. This is a novel mechanism for IGFBP-6.