Progress in Dickkopf-1-mediated bone metabolism

doi:10.3969/j.issn.2095-4344.2013.02.027

Abstract

Abstract:

BACKGROUND: Dickkopf-1 is a soluble inhibitor of canonical Wnt/β-catenin signaling pathway. It blocks Wnt/β-catenin signaling by competitively binding with Wnt protein co-receptor LRP5/6 and then mediates downstream target gene transcription.
OBJECTIVE: To review the research progress of Dickkopf-1-mediated bone metabolism, and to provide theoretical foundation of anti-Dickkopf-1 therapy for bone disease.
METHODS: A computer-based online search was conducted in Medline and CBM databases from 1995-2012. Articles focusing on Dickkopf-1-mediated bone metabolism were collected using the keywords of “Dkk1; bone remodeling; bone diseases” in English and Chinese, respectively. High-quality studies were included, while repetitive studies were excluded.
RESULTS AND CONCLUSION: 118 articles were obtained initially, and after screening procedure 36 literatures were selectively included in this review finally. Bone tissue is always in dynamic remodeling process. The interaction between osteoblasts and osteoclasts determines the dynamic balance between bone formation and bone resorption. Dickkopf-1 not only inhibits bone formation by blocking Wnt/β-catenin pathway, but also promotes osteoclast differentiation and maturation through the crosstalk with osteoprotegerin/receptor activator of nuclear factor-kappa B ligand/receptor activator of nuclear factor-kappa B axis. Promoting bone formation and inhibiting bone resorption is the biological mechanism of anti- Dickkopf-1 for preventing bone loss which occurs in bone metabolic diseases, and the clinical results are favorable. Therefore, Dickkopf-1 should be a potential target in the treatment of bone disease.

Key words: tissue construction, academic discussion in tissue construction, Dickkopf-1, Wnt signaling pathway, bone remodeling, bone diseases, osteoclasts, bone metabolism, osteoblasts, bone induction, bone tissue engineering, National Natural Science Foundation of China

CLC Number:

R318

Wang Lu-fei, Bai Ding, Han Xiang-long. Progress in Dickkopf-1-mediated bone metabolism[J]. Chinese Journal of Tissue Engineering Research, 2013, 17(2): 337-341.

Figures/Tables 1

2.1 Wnt信号通路及Dkk1的生物学特征 Wnt信号通路有3条，即Wnt/β-catenin信号途径、Wnt/Ca2+信号途径和Wnt/平面细胞极性信号途径。在最为经典的Wnt/β-catenin途径中，胞外的Wnt蛋白与跨膜受体卷曲蛋白结合后与共同受体低密度脂蛋白受体相关蛋白5/6形成Wnt-Frizzled-低密度脂蛋白受体相关蛋白5/6复合体，传导信号到细胞质内，激活散乱蛋白(disheveled, Dsh)磷酸化，抑制下游的酪蛋白激酶-结直肠腺瘤性息肉蛋白-糖原合成酶激酶3β-轴蛋白(CK-APC- GSK3β-Axin)复合体对胞质内β-catenin降解的促进作用，使β-catenin在细胞内累积，进入细胞核与T细胞因子/淋巴样增强因子形成复合体，特异性结合靶基因转录启动子，共同调控转录过程进而影响细胞的行为[1-2]。 Dickkopf-1于1998年首次在两栖动物非洲蟾蜍胚胎细胞中被发现[3]，编码为Dkk1，是一种强大的Wnt信号通道的拮抗剂，随后的研究证实人类的Dkk1基因位于10号染色体10q11上。在脊椎动物中，Dkk家族共有4种：Dkk1、Dkk2、Dkk3和Dkk4。其中Dkk1的作用最强，对于胚胎头颈形成的过程发挥重要作用。哺乳动物的Dkk1含266个氨基酸残基，通过cDNA编码，含有两个富含半胱氨酸的区域称为保守区域，被50-55个氨基酸组成的连接区域分割[4-5]。在所有Dkks家族成员中，Dkk1呈现高度保守状态，它目前确认的受体有两类：①LRP-5与LRP-6。②含kringle结构域的Kremen蛋白受体。Mao等[6-7]发现Dkk1可通过与Wnt蛋白竞争结合LRP受体而直接抑制Wnt蛋白活性，或通过含kringle结构域的Kremen受体间接与LRP受体结合，形成三聚体，降低Wnt蛋白向细胞内传导信号，阻断经典Wnt/β-catenin-T细胞因子传导途径，以及c-Jun氨基末端激酶与Wnt/平面细胞极性传导途径。 2.2 Dkk1调节骨代谢的分子机制骨组织时刻处于重塑的动态平衡中，此平衡受全身激素及局部细胞因子的调节。在骨重塑过程中，成骨细胞和破骨细胞的活动及其相互作用决定骨形成和骨吸收之间的平衡，保持骨组织强度和结构的完整[8]。在Wnt 家族的相关信号分子中，任何分子的表达异常都可能破坏Wnt信号所维系的动态平衡，导致骨骼系统疾病[9]，Dkk1经Wnt/β-catenin信号通路调控骨重塑的具体机制叙述如下。 2.2.1 Dkk1通过阻断Wnt/β-catenin 信号通路抑制骨形成 Wnt信号途径促进骨形成的作用，主要表现为促进成骨细胞的增殖和分化。在成骨细胞分化的不同阶段，Wnt/β-catenin信号可以促使：①间充质干细胞向成骨细胞分化。②成骨细胞的增殖和成熟[10-11]。在Dkk家族中，Dkk1和Dkk4明显抑制Wnt通路，Dkk2可以抑制也可促进Wnt通路，而Dkk3对Wnt通路没有影响[12]。 Dkk1过表达导致β-catenin水平降低，抑制成骨细胞分化，并促进成骨细胞凋亡。Qiang等[13]通过在前成骨细胞中过表达Dkk1和沉默低密度脂蛋白受体相关蛋白5/6证实了这一点。以Col1A1 3.6 kb和Col1A1 2.3 kb为启动子的Dkk1过表达转基因小鼠出现了严重的骨量缺失，成骨细胞数目降低了49%，血清骨钙素浓度下降了45%；体外实验发现Dkk1重组蛋白对成骨细胞矿化结节形成的抑制作用呈剂量依赖性[14]。相反，Dkk1缺陷的杂合子小鼠Dkk+/-，成骨细胞数目和骨形成率均明显高于野生型[15]。可见，体内、外研究从正反两方面证实了Dkk1对成骨细胞的生物学活性具有强大的负性调控作用。下调Dkk1的表达有利于骨形成。在小鼠体内，骨量的多少与Dkk1的表达水平呈负相关，抑制Dkk1和低密度脂蛋白受体相关蛋白5/6的结合会刺激骨形成[16]。Glantschnig等[17]应用抗Dkk1抗体有效地治疗了雌激素缺失导致的骨质疏松，发现抗体治疗后的小鼠血清总骨Ⅰ型前胶原N端肽显著增加，新骨形成明显。将Dkk1基因沉默可以限制糖皮质激素诱导的抑制成骨细胞分化的作用[18]。此外，系统性炎症是导致全身骨质缺失的重要因素，研究发现抗Dkk1抗体能够阻断并治疗肿瘤坏死因子转基因小鼠的骨质缺失[19]。 2.2.2 Dkk1能够诱导破骨细胞的分化与成熟 Wnt信号通路在胚胎发育、干细胞的增殖分化、肿瘤的发生发展、及骨形成过程中发挥关键性作用，可以通过成骨细胞实现对破骨细胞分化的影响。骨保护素/核因子ΚB受体活化因子配体/前体细胞表面受体骨调节系统是近年来骨生理研究领域中的重大突破[20]。破骨细胞的分化过程主要受源于基质/成骨细胞的巨噬细胞集落刺激因子、核因子κB受体活化因子配体调控。核因子ΚB受体活化因子配体能与破骨细胞前体细胞表面受体-核因子κB受体活化因子结合，通过一系列的级联反应和信号传导，使前体细胞分化、发育为成熟的破骨细胞；骨保护素是一种分泌型糖蛋白，骨保护素可与核因子ΚB受体活化因子配体结合，竞争性地阻断核因子ΚB受体活化因子配体与RANK之间的相互作用，从而抑制破骨细胞的分化与成熟。许多激素、细胞因子等均通过直接或间接地调节骨保护素/核因子ΚB受体活化因子配体/前体细胞表面受体轴来调控破骨细胞的分化和凋亡。近期研究表明，Dkk1与骨保护素/核因子ΚB受体活化因子配体/前体细胞表面受体轴之间存在“交互对话”效应，可通过调节成骨细胞分泌的核因子ΚB受体活化因子配体/骨保护素比值来影响破骨细胞的分化和成熟，对成骨细胞、破骨细胞具有双向调控作用：①Wnt信号通路可促进间充质干细胞向成骨细胞分化，上调其骨保护素的表达，从而抑制核因子ΚB受体活化因子配体发挥诱导破骨细胞分化的生物学效应。②Dkk1与成骨细胞表面的低密度脂蛋白受体相关蛋白5/6结合，阻断Wnt信号通路，抑制成骨细胞的增殖与分化，减少骨保护素的表达，从而增强核因子ΚB受体活化因子配体发挥诱导破骨细胞分化的生物学效应[21-22]。 2.3 Dkk1在治疗骨代谢相关疾病的应用前景通过阻断Dkk1来促进骨形成、抑制骨吸收，为治疗炎症、创伤、肿瘤等引起的骨量缺失提供了新的思路。尽管激活Wnt通路有可能导致肿瘤等其它疾病的发生，但由于Dkk1具有成骨细胞特异性，仍将成为首选的治疗靶点[23]。现将抗Dkk1治疗在骨代谢疾病中的转化医学研究叙述如下。 2.3.1 骨质疏松全球人类基因表达分析发现，在白种人的绝经后妇女中，Dkk1是与骨密度变化相关的基因[24]。在去卵巢鼠中通过反义寡核苷酸治疗下调Dkk1的表达会导致核因子ΚB受体活化因子配体表达下降，破骨细胞分化受抑制，组织形态学分析显示破骨细胞数量减少、成骨细胞数量增加，骨量增加[25]。通过小干扰RNA 和抗Dkk1抗体治疗均可有效减轻雌激素缺乏引起的骨质疏松症状[25-28]。Voskaridou等[29]的前期临床试验证实Dkk1中和抗体能够有效地改善地中海贫血造成的骨质疏松。可见，Dkk1是一个非常有潜力的骨质疏松治疗靶点，将会越来越多地被应用于临床治疗中[30]。 2.3.2 骨关节炎与类风湿性关节炎虽然Dkk1抑制Wnt/β-catenin信号通路的分子机制还未完全明了，但其阻止关节炎发生和进展的作用是可以肯定的。为炎症和机械应力导致的骨关节炎经注射抗Dkk1反义核苷酸治疗后，关节软骨和软骨下骨的破坏得到有效缓解[31]。类风湿关节患者血清Dkk1水平与疾病活动度成正比[32]，类风湿关节炎小鼠给予抗Dkk1抗体治疗后，发现骨吸收被抑制、炎症关节中有骨赘形成，说明抗Dkk1抗体引起了活跃的骨重塑[33]。 2.3.3 多发性骨髓瘤近年来，越来越多的研究发现伴严重骨损害的骨髓瘤患者都有血清Dkk1的高表达，且Dkk1是惟一一个与骨髓瘤骨病严重程度相关的指标[34]。Fulciniti等[35]评价了抗Dkk1中和抗体治疗多发性骨髓瘤的效果，发现它不仅能够促进骨髓基质细胞向成骨细胞分化，而且降低了白细胞介素6水平。抗Dkk1单克隆抗体(BHQ880)对不同动物模型的多发性骨髓瘤有显著疗效，主要表现为防止溶骨性病变的发展，并提高新骨形成率[35-36]。"

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