XBP-1 regulation of arachidonic acid and glicerolipids metabolism in renal epithelial cells under osmotic stress

Abstract

Hyperosmolarity is a key controversial signal for renal cells. Under physiological conditions, it induces renal celldifferentiation and maturation of urine concentrating system. However, abrupt changes in environmental osmolarity may also induce cell stress that can lead to death. To adapt and survive in such adverse conditions, renal cells implement differentosmoprotective mechanisms that includes both the upregulation of cyclooxygenase-2 (COX-2) expression and prostaglandins(PGs) synthesis from arachidonic acid (AA), and a coordinated increase in phospholipids (PL) and triacylglycerides (TAG)biosynthesis. We previously shown that hyperosmolarity induces ER stress and activates the unfolded protein response (UPR)in Madyn Darby Canine Kidney Cells (MDCK) through IRE1α-XBP1s pathway, and that XBP1s modulates lipid synthesisregulating lipogenic enzymes expression. In the present work we evaluated how XBP1s modulates phospholipase A2(PLA2)/COX-2/PGs pathway and its relationship with lipid synthesis induction under osmotic stress. MDCK cells weresubjected to hyperosmolarity (298-512 mOsm/kg H2O) for different periods of time (0, 12, 24 and 48 h) and treated withdifferent PLA2 (cPLA2, iPLA2 and sPLA2) and IRE1α inhibitors. RT-PCR studies showed that hyperosmolarity increasedcPLA2 expression at 24 and 48 h but did not upregulate iPLA2 expression. Inhibition of cPLA2 but not iPLA2 nor sPLA2prevented hyperosmolarity-induced lipid synthesis and lipid droplets accumulation. Furthermore, IRE1α RNase activityinhibition was accompanied by a decrease in cPLA2 and COX-2 but not in iPLA2 expression evaluated by RT-PCR. Instead,western blot analysis showed a significant increase in COX-2 protein levels when xbp1 (u) splicing was blocked by IRE1αinhibitor. Our findings suggest that the UPR modulates glycerolipids metabolism under osmotic stress by regulatingcPLA2/COX-2/PGs axis.