Chandhoke Taranpreet K, Huang Yu-Feng, Liu Fei, Gronowicz Gloria A, Adams Douglas J, Harrison John R, Kream Barbara E
Department of Medicine, University of Connecticut Health Center, Farmington, CT 06030, USA.
Center for Molecular Medicine, University of Connecticut Health Center, Farmington, CT 06030, USA.
Bone. 2008 Jul;43(1):101-109. doi: 10.1016/j.bone.2008.03.012. Epub 2008 Mar 29.
ICER is a member of the CREM family of basic leucine zipper transcription factors that acts as a dominant negative regulator of gene transcription. Four different isoforms of ICER (I, Igamma, II and IIgamma) are transcribed from the P2 promoter of the Crem gene. We previously found that each of the ICER isoforms is induced by parathyroid hormone in osteoblasts. The goal of the present study was to assess the function of ICER in bone by overexpressing ICER in osteoblasts of transgenic mice. ICER I and ICER II cDNAs, each containing an N-terminal FLAG epitope tag, were cloned downstream of a fragment containing 3.6 kb of the rat Col1a1 promoter and most of the rat Col1a1 first intron to produce pOBCol3.6-ICER I and pOBCol3.6-ICER II transgenes, respectively. Multiple lines of mice were generated bearing the ICER I and ICER II transgenes. At 8 weeks of age, ICER I and ICER II transgenic mice had lower body weights and decreased bone mineral density of femurs and vertebrae. Further studies were done with ICER I transgenic mice, which had greatly reduced trabecular bone volume and a markedly decreased bone formation rate in femurs. Osteoblast differentiation and osteocalcin expression were reduced in ex vivo bone marrow cultures from ICER I transgenic mice. ICER I antagonized the activity of ATF4 at its consensus DNA binding site in the osteocalcin promoter in vitro. Thus, transgenic mice with osteoblast-targeted overexpression of ICER exhibited osteopenia caused primarily by reduced bone formation. We speculate that ICER regulates the activity and/or expression of ATF/CREB factors required for normal bone formation.
ICER是碱性亮氨酸拉链转录因子CREM家族的成员，作为基因转录的显性负调控因子发挥作用。ICER的四种不同异构体（I、Iγ、II和IIγ）由Crem基因的P2启动子转录而来。我们之前发现，每种ICER异构体在成骨细胞中都由甲状旁腺激素诱导产生。本研究的目的是通过在转基因小鼠的成骨细胞中过表达ICER来评估其在骨骼中的功能。分别将各自含有N端FLAG表位标签的ICER I和ICER II cDNA克隆到包含3.6 kb大鼠Col1a1启动子片段和大部分大鼠Col1a1第一内含子的片段下游，以分别产生pOBCol3.6-ICER I和pOBCol3.6-ICER II转基因。培育出了携带ICER I和ICER II转基因的多系小鼠。8周龄时，ICER I和ICER II转基因小鼠体重较低，股骨和椎骨的骨矿物质密度降低。对ICER I转基因小鼠进行了进一步研究，这些小鼠的股骨小梁骨体积大幅减少，骨形成率显著降低。ICER I转基因小鼠的离体骨髓培养物中，成骨细胞分化和骨钙素表达降低。在体外，ICER I在骨钙素启动子的共有DNA结合位点拮抗ATF4的活性。因此，成骨细胞靶向过表达ICER的转基因小鼠表现出主要由骨形成减少引起的骨质减少。我们推测，ICER调节正常骨形成所需的ATF/CREB因子的活性和/或表达。
