ER Stress-Induced Sphingosine-1-Phosphate Lyase Phosphorylation Potentiates the Mitochondrial Unfolded Protein Response

The unfolded protein response (UPR) is an elaborate signaling network that evolved to maintain proteostasis in the endoplasmic reticulum (ER) and mitochondria (mt). These organelles are functionally and physically associated, and consequently, their stress responses are often intertwined. It is unclear how these two adaptive stress responses are coordi-nated during ER stress. The inositol-requiring enzyme-1 (IRE1), a central ER stress sensor and prox-imal regulator of the UPRER, harbors dual kinase and endoribonuclease (RNase) activities. IRE1 RNase ac-tivity initiates the transcriptional layer of the UPRER, but IRE1's kinase substrate(s) and their functions are largely unknown. Here, we discovered that sphingo-sine 1-phosphate (S1P) lyase (SPL), the enzyme that degrades S1P, is a substrate for the mammalian IRE1 kinase. Our data show that IRE1-dependent SPL phosphorylation inhibits SPL's enzymatic activity, resulting in increased intracellular S1P levels. S1P has previously been shown to induce the activation of mitochondrial UPR (UPRmt) in nematodes. We determined that IRE1 kinase-dependent S1P induc-tion during ER stress potentiates UPRmt signaling in mammalian cells. Phosphorylation of eukaryotic translation initiation factor 2 alpha (eif2 alpha) is recognized as a critical molecular event for UPRmt activation in mammalian cells. Our data further demonstrate that inhibition of the IRE1-SPL axis abrogates the activa-tion of two eif2 alpha kinases, namely double-stranded RNA-activated protein kinase (PKR) and PKR-like ER kinase upon ER stress. These findings show that the IRE1-SPL axis plays a central role in coordi-nating the adaptive responses of ER and mitochon-dria to ER stress in mammalian cells.