Objective: To explore the mechanism by which inositol-requiring enzyme-1α (IRE1α) regulates autophagy function of chondrocytes through calcium homeostasis endoplasmic reticulum protein (CHERP).
Methods: Cultured human chondrocytes (C28/I2 cells) were treated with tunicamycin, 4μ8c, rapamycin, or both 4μ8c and rapamycin, and the expressions of endoplasmic reticulum (ER) stress- and autophagy-related proteins were detected with Western blotting. Primary chondrocytes from ERN1 knockout (ERN1 CKO) mice and wild-type mice were examined for ATG5 and ATG7 mRNA expressions, IRE1α and p-IRE1α protein expressions, and intracellular calcium ion content using qPCR, Western blotting and flow cytometry. The effect of bafilomycin A1 treatment on LC3 Ⅱ/LC3 Ⅰ ratio in the isolated chondrocytes was assessed with Western blotting. Changes in autophagic flux of the chondrocytes in response to rapamycin treatment were detected using autophagy dual fluorescent virus. The changes in autophagy level in C28/I2 cells overexpressing CHERP and IRE1α were detected using immunofluorescence assay.
Results: Tunicamycin treatment significantly up-regulated ER stress-related proteins and LC3 Ⅱ/LC3 Ⅰ ratio and down-regulated the expression of p62 in C28/I2 cells (P < 0.05). Rapamycin obviously up-regulated LC3 Ⅱ/LC3 Ⅰ ratio (P < 0.001) in C28/I2 cells, but this effect was significantly attenuated by co-treatment with 4μ8c (P < 0.05). Compared with the cells from the wild-type mice, the primary chondrocytes from ERN1 knockout mice showed significantly down-regulated mRNA levels of ERN1 (P < 0.01), ATG5 (P < 0.001) and ATG7 (P < 0.001), lowered or even lost expressions of IRE1α and p-IRE1α proteins (PP < 0.01), and increased expression of CHERP (P < 0.05) and intracellular calcium ion content (P < 0.001). Bafilomycin A1 treatment obviously increased LC3 Ⅱ/ LC3 Ⅰ ratio in the chondrocytes from both wild-type and ERN1 knockout mice (P < 0.01 or 0.05), but the increment was more obvious in the wild-type chondrocytes (P < 0.05). Treatment with autophagy dual-fluorescence virus resulted in a significantly greater fluorescence intensity of LC3-GFP in rapamycin-treated ERN1 CKO chondrocytes than in wild-type chondrocytes (P < 0.05). In C28/I2 cells, overexpression of CHERP obviously decreased the fluorescence intensity of LC3, and overexpression of IRE1α enhanced the fluorescence intensity and partially rescued the fluorescence reduction of LC3 caused by CHERP.
Conclusion: IRE1α deficiency impairs autophagy in chondrocytes by upregulating CHERP and increasing intracellular calcium ion content.
目的: 探究肌醇依赖性激酶1α（IRE1α）通过调控内质网钙稳态蛋白（CHERP）影响软骨细胞自噬功能的相关机制。
方法: 衣霉素（TM）处理人软骨细胞C28/I2，Western blot检测不同时间点的内质网应激和自噬相关蛋白指标。4μ8c、雷帕霉素（Rapa）处理C28/I2（分为：NC组、4μ8c组、Rapa组、4μ8c+Rapa组），Western blot检测自噬相关蛋白指标。分别从软骨组织特异性ERN1基因敲除小鼠（ERN1 CKO）和对照小鼠（Control）软骨组织分离原代软骨细胞，qPCR检测ERN1及自噬相关蛋白ATG5、ATG7的mRNA水平，Western blot检测IRE1α/p-IRE1α、CHERP表达水平，流式细胞术检测胞内钙离子含量。巴佛洛霉素A1（Bafilomycin A1）处理原代软骨细胞（分为Control组、Control+Bafilomycin A1组、ERN1 CKO组、ERN1 CKO+Bafilomycin A1组），Western blot检测LC3Ⅱ、LC3Ⅰ的表达。自噬双荧光病毒在Rapa处理下检测自噬流（分为Control+Rapa组、ERN1 CKO+ Rapa组）。C28/I2中过表达CHERP与IRE1α，免疫荧光检测自噬水平。
结果: TM处理C28/I2细胞后，内质网应激相关标志蛋白表达上调，LC3Ⅱ/LC3Ⅰ比值上调（P＜0.05），p62表达下调（P＜0.05）。相较对照组，Rapa上调LC3Ⅱ/LC3Ⅰ比值（P＜0.001），4μ8c+ Rapa组相较Rapa组的LC3Ⅱ/LC3Ⅰ比值降低（P＜0.05）。相较于对照组，原代软骨细胞中敲除ERN1/IRE1α后，ERN1（P＜0.01）、ATG5（P＜0.001）、ATG7（P＜0.001）mRNA水平显著下调。IRE1α/p-IRE1α蛋白表达缺失或显著下调（P＜0.01），CHERP蛋白表达上调（P＜0.05），胞内钙离子含量显著上升（P＜0.001）。巴佛洛霉素A1处理原代软骨细胞后，Control+Bafilomycin A1组较Control组的LC3Ⅱ/LC3Ⅰ比值上调（P＜0.01），ERN1 CKO+Bafilomycin A1组较ERN1 CKO组的LC3Ⅱ/LC3Ⅰ比值上调（P＜0.05）、较Control+Bafilomycin A1组的LC3Ⅱ/LC3Ⅰ比值下降（P＜0.05）。自噬双荧光病毒处理后，ERN1 CKO+Rapa组较Control+Rapa组未淬灭LC3-GFP荧光增强（P＜0.05）。C28/I2细胞中过表达CHERP降低LC3荧光强度，过表达IRE1α增强LC3荧光表达并能部分挽救CHERP导致的荧光降低情况。
结论: IRE1α缺陷通过上调内质网钙稳态蛋白，升高胞内钙离子含量，进一步导致软骨细胞的自噬功能受损。
Keywords: IRE1α; autophagic flux; calcium homeostasis endoplasmic reticulum protein; calcium ions; chondrocytes.