The effects of ER stress on protein secretion by cardiac myocytes are not well understood. In this study, the ER stressor thapsigargin (TG), which depletes ER calcium, induced death of cultured neonatal rat ventricular myocytes (NRVMs) in high media volume but fostered protection in low media volume. In contrast, another ER stressor, tunicamycin (TM), a protein glycosylation inhibitor, induced NRVM death in all media volumes, suggesting that protective proteins were secreted in response to TG but not TM. Proteomic analyses of TG- and TM-conditioned media showed that the secretion of most proteins was inhibited by TG and TM; however, secretion of several ER-resident proteins, including GRP78 was increased by TG but not TM. Simulated ischemia, which decreases ER/SR calcium also increased secretion of these proteins. Mechanistically, secreted GRP78 was shown to enhance survival of NRVMs by collaborating with a cell-surface protein, CRIPTO, to activate protective AKT signaling and to inhibit death-promoting SMAD2 signaling. Thus, proteins secreted during ER stress mediated by ER calcium depletion can enhance cardiac myocyte viability.

中文翻译：

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心肌细胞分泌组的蛋白质组学分析揭示了内质网应激反应的细胞外保护功能。

内质网应激对心肌细胞蛋白质分泌的影响尚不清楚。在这项研究中，消耗 ER 钙的 ER 应激物毒胡萝卜素 (TG) 在高培养基体积下诱导培养的新生大鼠心室肌细胞 (NRVM) 死亡，但在低培养基体积下促进保护。相比之下，另一种 ER 应激物衣霉素 (TM)，一种蛋白质糖基化抑制剂，可在所有培养基体积中诱导 NRVM 死亡，这表明保护性蛋白质是响应 TG 而分泌的，而不是 TM。TG 和 TM 条件培养基的蛋白质组学分析表明，大多数蛋白质的分泌受到 TG 和 TM 的抑制；然而，TG 增加了几种内质网驻留蛋白（包括 GRP78）的分泌，但 TM 没有增加。模拟缺血会降低 ER/SR 钙离子也会增加这些蛋白质的分泌。从机制上讲，分泌的 GRP78 被证明通过与细胞表面蛋白 CRIPTO 合作来激活保护性 AKT 信号并抑制促进死亡的 SMAD2 信号，从而提高 NRVM 的存活率。因此，由内质网钙耗竭介导的内质网应激期间分泌的蛋白质可以增强心肌细胞的活力。