Zhou Renyi, Ma Yue, Tao Zhengbo, Qiu Shui, Gong Zunlei, Tao Lin, Zhu Yue
Department of Orthopedics, The First Hospital of China Medical University, Shenyang, China.
Department of Pulmonary and Critical Care Medicine, Shengjing Hospital of China Medical University, Shenyang, China.
Front Pharmacol. 2020 Dec 16;11:602307. doi: 10.3389/fphar.2020.602307. eCollection 2020.
Osteoporosis is a common disease resulting in deteriorated microarchitecture and decreased bone mass. In type 2 diabetes patients, the incidence of osteoporosis is significantly higher accompanied by increased apoptosis of osteoblasts. In this study, using the osteoblastic cell line MC3T3-E1, we show that high glucose reduces cell viability and induces apoptosis. Also, high glucose leads to endoplasmic reticulum (ER) stress (ERS) via an increase in calcium flux and upregulation of the ER chaperone binding immunoglobulin protein (BiP). Moreover, it induces post-translational activation of eukaryotic initiation factor 2 alpha (eIF2α) which functions downstream of PKR-like ER kinase (PERK). This subsequently leads to post-translational activation of the transcription factor 4 (ATF4) and upregulation of C/EBP-homologous protein (CHOP) which is an ER stress-induced regulator of apoptosis, as well as downstream effectors DNAJC3, HYOU1, and CALR. Interestingly, melatonin treatment significantly alleviates the high-glucose induced changes in cell growth, apoptosis, and calcium influx by inhibiting the PERK-eIF2α-ATF4-CHOP signaling pathway. Additionally, the MC3T3-E1 cells engineered to express a phosphodead eIF2α mutant did not show high glucose induced ER stress, confirming that melatonin protects osteoblasts against high-glucose induced changes by decreasing ER-stress induced apoptosis by impacting the PERK-eIF2α-ATF4-CHOP signaling pathway. The protective of melatonin against high glucose-induced ER stress and apoptosis was attenuated when the cells were pre-treated with a melatonin receptor antagonist, indicating that the effect of melatonin was mediated via the melatonin receptors in this context. These findings lay the provide mechanistic insights of melatonin's protective action on osteoblasts and will be potentially be useful in ongoing pre-clinical and clinical studies to evaluate melatonin as a therapeutic option for diabetic osteoporosis.
骨质疏松症是一种常见疾病，会导致骨微结构恶化和骨量减少。在2型糖尿病患者中，骨质疏松症的发病率显著更高，同时成骨细胞凋亡增加。在本研究中，我们使用成骨细胞系MC3T3-E1表明，高糖会降低细胞活力并诱导凋亡。此外，高糖通过增加钙通量和上调内质网伴侣结合免疫球蛋白蛋白（BiP）导致内质网（ER）应激（ERS）。而且，它诱导真核起始因子2α（eIF2α）的翻译后激活，eIF2α在蛋白激酶R样内质网激酶（PERK）的下游发挥作用。这随后导致转录因子4（ATF4）的翻译后激活以及C/EBP同源蛋白（CHOP）的上调，CHOP是一种内质网应激诱导的凋亡调节因子，以及下游效应分子DNAJC3、HYOU1和CALR。有趣的是，褪黑素处理通过抑制PERK-eIF2α-ATF4-CHOP信号通路，显著减轻了高糖诱导的细胞生长、凋亡和钙通量变化。此外，经基因工程改造表达磷酸化失活eIF2α突变体的MC3T3-E1细胞未表现出高糖诱导的内质网应激，证实褪黑素通过影响PERK-eIF2α-ATF4-CHOP信号通路减少内质网应激诱导的凋亡，从而保护成骨细胞免受高糖诱导的变化。当细胞用褪黑素受体拮抗剂预处理时，褪黑素对高糖诱导的内质网应激和凋亡的保护作用减弱，表明在这种情况下，褪黑素的作用是通过褪黑素受体介导的。这些发现为褪黑素对成骨细胞的保护作用提供了机制性见解，并可能在正在进行的临床前和临床研究中用于评估褪黑素作为糖尿病性骨质疏松症的治疗选择。
