X-box binding protein 1 (XBP1): A key protein for renal osmotic adaptation. Its role in lipogenic program regulation.

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.