PI3K/Akt signaling pathway and bone destruction: problems and
mechanisms

doi:10.3969/j.issn.2095-4344.2728

Abstract

Abstract:

BACKGROUND: In recent years, the PI3K/AKT signaling pathway is closely related to bone destruction-related diseases. Inhibitors targeting this signaling pathway are also undergoing extensive research, which provides new ideas for clinical treatment of bone destruction-related diseases.

OBJECTIVE: To review the research of PI3K/AKT signaling pathway in bone destruction.

METHODS: WanFang and PubMed databases were searched for relevant articles published from January 1998 to August 2019. The retrieval key words were “PI3K/Akt signaling pathway; osteoclasts; osteoblasts; bone destruction” in Chinese and English, respectively. To exclude duplicate studies by reading the title and abstract, finally 67 articles were included in result analysis.

RESULTS AND CONCLUSION: An appropriate balance between osteoblast and osteoclast-mediated osteogenesis and bone resorption is necessary to maintain bone homeostasis, and an imbalance between these two biological processes will result in bone destruction. PI3K/Akt signaling pathway is a signal transduction pathway that regulates cell activities and plays an important role in cell proliferation, apoptosis and differentiation. The PI3K/Akt signaling pathway is involved in the inhibition of osteoporosis by promoting osteoblast proliferation, differentiation and bone formation. These results reveal that further exploration on the inhibitors of the PI3K/Akt signaling pathway related to bone destruction will provide new ideas for drug therapy in clinical practice.

Key words: PI3K/Akt signaling pathway, osteoclasts, osteoblasts, bone destruction

CLC Number:

Shi Dongmei, Dong Ming, Lu Ying, Niu Weidong. PI3K/Akt signaling pathway and bone destruction: problems and mechanisms[J]. Chinese Journal of Tissue Engineering Research, 2020, 24(23): 3716-3722.

Figures/Tables 3

2.2 PI3K/Akt信号通路对破骨细胞的影响 2.2.1 破骨细胞破骨细胞来源于骨髓造血干细胞，是一个高度分化的多核巨细胞，主要功能是直接参与骨吸收[23]。破骨细胞的形成和活化需要两个因素：RANKL和巨噬细胞集落刺激因子[24]。破骨细胞的数量与功能方面受到外界因素的影响，则会导致以骨破坏为主要特征的疾病[25-27]。RANKL与核因子κB 受体活化因子结合在破骨细胞存活、分化和活化方面是扮演着重要角色[28]。调控破骨细胞分化的信号通路主要集中在RANKL相关信号通路上。RANKL联合RANKL和激活肿瘤坏死因子受体相关因子。只有肿瘤坏死因子受体相关因子6在破骨过程中起着重要作用[29]。肿瘤坏死因子受体相关因子6激活多条下游信号通路，主要是核因子κB、丝裂原活化蛋白激酶和Akt，激活了抗酒石酸酸性磷酸酶和组织蛋白酶K等与破骨细胞有关基因的表达。 2.2.2 破骨细胞与PI3K/Akt信号通路有许多研究表明，破骨细胞分化、存活与多种细胞因子的之间相互作用以及PI3K/Akt信号通路有着重要关系[30-31]，但是具体作用机制仍不明确。有研究发现，PI3K参与多种细胞功能，包括有丝分裂、存活、运动和分化。这种脂质激酶产生D3-磷酸肌醇，吸收含有PH结构域的蛋白质，包括PDK-1和Akt，这种募集允许Akt的磷酸化和后续激活下游的相关靶蛋白[32]。在破骨细胞中，PI3K可刺激肌动蛋白丝的形成，调节细胞骨架功能，如趋化、附着和扩散[33-36]。此外，PI3K的抑制已被证明可以阻止成熟的破骨细胞的骨吸收[37-38]。LEE等[39]发现PI3K的特异性抑制剂LY294002能显著减少RANKL/M-CSF诱导的破骨细胞的产生。提示PI3K可能参与破骨细胞的功能及产生。肌醇-5-磷酸酶(SH2 domain containing inositol 5’-phosphatase，SHIP)被称为PI3K/Akt通路的负调节因子，能特异性水解磷PIP3中的5’-磷酸基团，PIP3是PI3K的主要产物，负调节PI3K活性[40]。尽管PI3K启动并调节促进破骨细胞前体存活和分化的信号，但SHIP -/-小鼠显示出破骨细胞前体数量增加和破骨细胞生成增强，导致严重的骨质疏松症[41]，表明SHIP可能不是PI3K的唯一负调节剂，另一种解释是可能与其他信号通路共同调节破骨细胞分化。RANKL和巨噬细胞集落刺激因子在一定程度上通过激活Akt通路来进破骨细胞的存活[42-43]。虽然以前的研究已显示PI3K /Akt途径对破骨细胞的生成、分化、存活很重要，但尚未揭示涉及破骨细胞分化的具体作用机制。MOON等[13]研究发现Akt通过调节GSK3β/ NFATc1信号级联诱导破骨细胞分化。KIM等[44]研究发现马齿苋通过在体外抑制Akt/GSK3-c-Fos-NFATc1信号传导来抑制破骨细胞分化和骨吸收活性，并在体内预防脂多糖诱导的骨丢失。表明PI3K/Akt/GSK3β/NFATc1信号轴在破骨细胞形成中起重要作用。 SUGATANI等[45]在破骨前体细胞中敲除Akt1和Akt2，发现由于IkB激酶α/β活性的下调，IkB激酶α的磷酸化，核因子κB p50的核转位和DNA的下调，导致Akt1和Akt2蛋白的缺失从而抑制破骨细胞分化，然而，Akt1和Akt2蛋白的缺失不刺激切割的胱天蛋白酶3 (Caspases-3)活性并且不能促进细胞凋亡，表明Akt1和 Akt2对于破骨细胞增殖和分化是必需的，但是对于破骨细胞的存活是不必要的。此外，Akt在骨形成的发展过程中扮演着重要角色。KAWAMURA等[46]发现缺乏Akt1的小鼠的骨质减少，并且破骨细胞中Akt1缺乏导致细胞自主功能障碍从而引起骨吸收受损。Akt1敲除小鼠表现为体型减少、骨长度变短及延迟继发性骨化[47]，而Akt1/Akt2双敲除的小鼠在出生后不久死亡，表现出侏儒症和骨骼骨化的发育迟缓[48]。许多研究表明，PI3K/Akt/mTOR信号传导在破骨细胞存活过程中发挥重要作用。mTOR是分离的破骨细胞前体细胞存活所必需的[45]。mTOR是造血细胞存活的关键下游介质[49]。mTOR对分离的破骨细胞前体的存活至关重要，使用雷帕霉素(mTOR抑制剂)完全阻断了破骨细胞的形成[50]。此外，巨噬细胞集落刺激因子，肿瘤坏死因子α和RANKL也通过mTOR信号传导促进破骨细胞存活[51]。 microRNA是内源的非编码小RNA，在调节各种生物学和病理过程中起着至关重要的作用。ZHAO等[52]研究表明，miR-214通过靶向Pten/PI3k/Akt途径促进破骨细胞生成。表明microRNA对于破骨细胞的分化、功能和存活是必需的。此外，先前已证明microRNA通过调节成骨细胞和破骨细胞分化来调节骨代谢。因此，LI等[53]研究表明，MYB诱导的miR-363-3p上调通过PTEN/PI3K/AKT信号通路调节骨质疏松症的发病机制。SHAO等[54]研究表明microRNA-497的下调通过骨肉瘤中的PI3K/Akt途径促进细胞生长和顺铂耐药性。这提示可以通过靶向microRNA来研究相关疾病的治疗药物，从而减少患者的痛苦，提高患者的生存质量。 2.3 PI3K/Akt信号通路对成骨细胞的影响 2.3.1 成骨细胞成骨细胞的主要功能是参与骨形成。成骨细胞起源于间充质干细胞，间充质干细胞可以分化成许多谱系，包括骨、脂肪、肌肉和软骨。这种分化依赖于细胞外信号传导和影响常见细胞内信号传导网络的转录因子的激活或抑制[55]。骨形成是由成骨细胞、间充质干细胞参与调节的，其寿命和活性通过生长因子信号网络调节进行的。几种生长因子家族，如骨形态发生蛋白和转化生长因子β已被证明通过调节其他生长因子的作用直接或间接调节成骨细胞分化[56]。此外，Wnt、骨形态发生蛋白、Notch等信号通路均在间充质干细胞向成骨细胞分化过程成扮演重要角色[56]。 2.3.2 成骨细胞与PI3K/Akt信号通路 PI3K/Akt途径可被生长因子和其他细胞外信号激活，以调节包括细胞生长、增殖、分化及存活、凋亡在内的许多基本细胞过程。MUKHERJEE等[57]研究发现IGF激活的PI3K/Akt信号通路调节骨形态发生蛋白2介导的成骨细胞分化。ZHANG等[58]研究表明转化生长因子β1通过PI3K/AKT/ mTOR/S6K1(S6激酶1)信号传导途径促进成骨细胞的存活、成骨分化和迁移。SUZUKI等[59]研究发现重复的转化生长因子β1处理降低了MC3T3-E1细胞中的碱性磷酸酶活性。然而，组成型活性(CA)-Akt的强制表达在重复转化生长因子β1给药的条件下改善了这种碱性磷酸酶活性的抑制。虽然转化生长因子β1处理后碱性磷酸酶活性增加，但骨钙素的mRNA水平降低。因此结果表明，Akt活化是转化生长因子β1诱导的小鼠前成骨细胞成骨细胞分化中的关键步骤，但其功能作用根据成骨阶段而不同。然而，MUKHERJEE等[57]研究发现，在成骨细胞分化和功能的所有阶段都需要Akt活性。这有可能是在成骨分化细胞的过程中存在几个不同的Akt靶标，这些靶标控制着成骨细胞发育和功能的不同方面。另外，GHOSH-CHOUDHURY等[60]研究发现用LY294002和显性失活的PI3K均阻止了骨形态发生蛋白2诱导的碱性磷酸酶，证实了该脂质激酶参与了成骨分化的过程。骨形态发生蛋白2在成骨细胞前体细胞中以PI3K依赖性方式刺激Akt的活性。Akt的显性失活突变体对Akt活性的抑制阻断了骨形态发生蛋白2诱导的成骨细胞碱性磷酸酶活性。以上结果表明PI3K/ Akt信号通路参与成骨细胞的分化。细胞增殖和分化之间的关系是相辅相成的，增殖为了使行使功能的细胞提供足够的数量，而分化则为了使增殖的细胞能够行使功能。PI3K/Akt信号通路除了参与成骨细胞的分化外，还能够增加成骨细胞增殖活性。GU等[12]在3种不同Ti表面(分别为SLActive、SLA、PT)上，培养MC3T3-E1细胞，分别加入LY294002后，与Ti表面相关的成骨细胞增殖活性的减少(SLActive>SLA>PT)，与成骨分化有关的碱性磷酸酶活性的增加。表明PI3K/ Akt信号通路在促进成骨细胞分化的同时抑制其增殖，这可能与细胞在分化增加的同时减少了细胞增殖有关系。 BURGERS等[61]研究发现PTEN基因(在人类10号染色体上缺失的磷酸酶和张力蛋白同源物)编码的PIP磷酸酶使PIP3脱磷酸并负调控PI3K诱导的激活。PTEN阻断Akt的激活，影响细胞周期、翻译和凋亡。当Pten功能被阻断时，PIP3会累积并且Akt不断被激活，导致细胞增殖、存活和迁移增加。携带Cre介导的成骨细胞特异性Pten基因缺失的小鼠具有正常的体型，但在整个生命过程中骨量和密度显着增加。来自这些突变小鼠的成骨细胞对细胞凋亡的敏感性较低，并且加速了分化能力。这些发现提供了体内证据，即通过PI3K/Pten途径的信号传导促进成骨细胞存活和功能。 DUFOUR等[62]研究表明，转化生长因子β2通过PI3K/Akt靶向Bcl-2以防止在卸载骨骼中成骨细胞凋亡。XI等[63]在大鼠骨质疏松模型中，阻断PI3K/Akt信号通路促进大鼠成骨细胞的凋亡。DENG等[64]研究发现，地塞米松通过ROS-PI3K/AKT/GSK3β信号通路诱导成骨细胞凋亡。这些发现表明PI3K/Akt信号通路促进了成骨细胞的凋亡。 PI3K/Akt信号通路除了参与在成骨细胞的增殖、分化及存活、凋亡的基本细胞过程外，也参与了骨质疏松、骨关节炎等以骨破坏为主要特征的相关疾病中。XI等[63]在大鼠骨质疏松模型中，体内和体外研究表明，PI3K/Akt细胞信号通路通过促进成骨细胞增殖、分化和骨形成而参与骨质疏松的抑制。LIN等[10]研究发现抑制PI3K/AKT/NF-κB轴能够预防骨关节炎小鼠的异常骨形成并减轻软骨变性。LIAN等[65]首先证实了PI3K/Akt/ mTOR信号通路相关基因在骨肉瘤患者中的高表达。这提示在临床治疗中可以通过靶向信号通路中关键因子，从而研制出新的药物，造福更多的患者。 2.4 抑制剂与PI3K/Akt信号通路 PI3K抑制剂有2种类型，即pan-PI3K和同种型特异性抑制剂。泛抑制剂也称为PI3K的第一代抑制剂，包括可以结合所有Ⅰ类PI3K的化合物Wortmannin(一种真菌代谢产物)和LY294002。为了改善药代动力学性质，已经出现了能够选择性抑制PI3Kp110α，β，δ或γ催化亚基的同工型特异性抑制剂。这种更大的特异性提供了减少副作用的可能性，从而允许更高的耐受剂量。此外，双重PI3K/ mTOR抑制剂也被研究中，其可靶向PI3K的所有p110亚型以及两个mTOR复合体的ATP位点，从而更完全地抑制了整个PI3K/AKT/mTOR信号轴。最近研究的抑制剂除了在浆细胞恶性多发性骨髓瘤、人B细胞恶性肿瘤、骨肉瘤等肿瘤方面取得进展外，对破骨细胞、成骨细胞的分化、增殖也有一定的影响，这为以后研制治疗骨破坏相关疾病的抑制剂提供了新思路。 MARTIN等[66]发现PI3K/Akt途径介导浆细胞恶性多发性骨髓瘤(MM)浆细胞中的增殖、存活和药物抗性，并且还参与调节骨形成成骨细胞和骨再吸收破骨细胞的形成和活性。NVP-BKM120(Buparlisib，Novartis)是一种PI3K抑制剂，目前正在几种肿瘤环境中进行临床评估。在这项研究中，检查了BKM120在免疫活性小鼠多发性骨髓瘤模型中的抗肿瘤发生作用及其对成骨细胞和破骨细胞形成和功能的影响。BKM120治疗(40 mg/kg)导致血清副蛋白和肿瘤负荷显著降低，并且胫骨近端的μCT分析显示BKM120治疗的动物中溶骨性骨病变的数量显著减少。BKM120还介导血清水平的成骨细胞标记物P1NP显著增加，并且血清水平的破骨细胞标记物TRAcP5显著降低。在体外，BKM120降低了多发性骨髓瘤浆细胞增殖、破骨细胞的形成和功能，促进了成骨细胞的形成和功能。这些研究结果表明，除了其抗肿瘤特性外，BKM120还可用于治疗多发性骨髓瘤患者的溶骨性骨病。 YEON等[67]在此研究了破坏细胞分化如何受到idelalisib(也称为CAL-101)的影响，idelalisib是p110δ选择性抑制剂，被批准用于治疗特定的人B细胞恶性肿瘤。研究发现RANKL的受体激活剂诱导PI3Kδ蛋白表达，而idelalisib抑制RANKL诱导的破骨细胞分化。接下来，通过蛋白质印迹分析和实时PCR证实了idelalisib对RANKL诱导的Akt-c-Fos/NFATc1信号级联激活的抑制作用。最后，idelalisib通过下调Akt-c-Fos/NFATc1信号级联反应在破骨细胞分化的最后阶段抑制破骨细胞前迁移，有可能扩展idelalisib的临床用途以控制破骨细胞分化。总之，目前的结果有助于理解PI3Kδ作为可药物靶标的临床价值以及相关治疗剂(包括破骨细胞生成)的功效。 PI3K/Akt/mTOR信号通路是一种有吸引力的抗癌靶标，因为它在细胞生长、分裂和分化的调节中起关键作用。LIAN等[65]首先证实了PI3K/Akt/mTOR信号通路相关基因在骨肉瘤患者中的高表达。因此，进一步研究了A005(一种新合成的双PI3K Akt/mTOR抑制剂)对骨肉瘤细胞和小鼠异种移植肿瘤模型的影响。结果证实A005抑制人骨肉瘤细胞的增殖、迁移和侵袭。此外，A005还抑制RANKL诱导的破骨细胞受体激活剂体外分化和骨吸收。因此，A005进一步应用于SaOS-2骨肉瘤诱导的小鼠骨溶解模型。A005通过调节PI3K/Akt/ mTOR途径抑制肿瘤生长并预防骨肉瘤相关的骨溶解。总之，结果显示A005抑制破骨细胞生成并通过抑制PI3K/Akt/mTOR信号传导来预防骨肉瘤诱导的骨溶解。这些发现表明A005可能是治疗人骨肉瘤的有希望的候选药物。这提示可以研究PI3K/Akt信号通路在骨破坏相关疾病中的抑制剂，从而为临床药物治疗提供新思路。 "

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