In renal cells, hyperosmolarity can induce cellular stress or differentiation. Both processes require active endoplasmic reticulum (ER)-associated protein synthesis. Lipid biosynthesis also occurs at ER surface. We showed that hyperosmolarity upregulates glycerophospholipid (GP) and triacylglycerol (GL-TG) de novo synthesis. Considering that massive synthesis of proteins and/or lipids may drive to ER stress, herein we evaluated whether hyperosmolar environment induces ER stress and the participation of inositol-requiring enzyme 1α (IRE1α)-XBP1 in hyperosmotic-induced lipid synthesis. Treatment of Madin-Darby canine kidney (MDCK) cells with hyperosmolar medium triggered ER stress-associated unfolded protein response (UPR). Hyperosmolarity significantly increased xbp1 mRNA and protein as function of time; 24 h of treatment raised the spliced form of XBP1 protein (XBP1s) and induced its translocation to nuclear compartment where it can act as a transcription factor. XBP1 silencing or IRE1α ribonuclease (RNAse) inhibition impeded the expression of lipin1, lipin2 and diacylglycerol acyl transferase-1 (DGAT1) enzymes which yielded decreased GL-TG synthesis. The lack of XBP1s also decreased sterol regulatory element binding protein (SREBP) 1 and 2. Together our data demonstrate that hyperosmolarity induces IRE1α → XBP1s activation; XBP1s drives the expression of SREBP1 and SREBP2 which in turn regulates the expression of the lipogenic enzymes lipin1 (LPIN1) and 2 (LPIN2) and DGAT1. We also demonstrated for the first time that tonicity-responsive enhancer binding protein (TonEBP), the master regulator of osmoprotective response, regulates XBP1 expression. Thus, XBP1 acts as an osmoprotective protein since it is activated by high osmolarity and upregulates lipid metabolism, membranes generation and the restoration of ER homeostasis.

中文翻译：

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X-box结合蛋白1（XBP1）：肾脏渗透适应的关键蛋白。它在脂肪生成程序调节中的作用。

在肾细胞中，高渗可以诱导细胞应激或分化。这两个过程都需要活性内质网（ER）相关的蛋白质合成。脂质生物合成也发生在ER表面。我们表明，高渗透压上调了甘油磷脂（GP）和三酰基甘油（GL-TG）从头合成。考虑到蛋白质和/或脂质的大量合成可能会导致内质网应激，本文我们评估了高渗环境是否诱导内质网应激，以及需要肌醇的酶1α（IRE1α）-XBP1参与高渗诱导的脂质合成。用高渗介质处理Madin-Darby犬肾（MDCK）细胞会触发与ER应激相关的未折叠蛋白应答（UPR）。高渗性显着增加xbp1 mRNA和蛋白质随时间的变化；处理24小时引起了XBP1蛋白（XBP1s）的剪接形式并诱导其易位至核室，在核室中它可以充当转录因子。XBP1沉默或IRE1α核糖核酸酶（RNAse）抑制会阻止lipin1，lipin2和二酰基甘油酰基转移酶1（DGAT1）酶的表达，从而降低GL-TG的合成。XBP1s的缺乏也降低了固醇调节元件结合蛋白（SREBP）1和2。我们的数据共同表明，高渗透压可诱导IRE1α→XBP1s的活化。XBP1s驱动SREBP1和SREBP2的表达，而SREBP1和SREBP2则调节脂肪酶lipin1（LPIN1）和2（LPIN2）和DGAT1的表达。我们还首次证明了渗透性反应增强剂结合蛋白（TonEBP）是渗透保护反应的主要调节剂，调节XBP1表达。因此，由于XBP1被高渗透压激活并上调脂质代谢，膜生成和ER稳态的恢复，因此它是一种渗透保护蛋白。