(622ab) Role of Tryptophan in the Activity of IRE1Î± Protein Activation during ER Stress and the Induction of Epithelial-Mesenchymal Transition (EMT)
The endoplasmic reticulum (ER) is an organelle involved in protein synthesis, folding and post-translational modifications. Inositol requiring enzyme -1α (IRE1α), a type I transmembrane (TM) protein possessing both kinase and endonuclease functions, is the most conserved and well characterized ER stress sensor protein. IRE1α plays an essential role in maintaining ER homeostasis and its failure to reestablish homeostasis is associated with diverse diseases. ER stress is known to activate IRE1α by leading to dissociation of the BiP protein from the luminal domain of IRE1α.
Long chain free fatty acids (FFAs), especially palmitate (PA), are known to induce ER stress in many types of cells and contribute to diseases, including cancers. Cancer cells frequently exhibit alterations in fatty acid metabolism to sustain growth and proliferation, fulfill energy requirements and provide metabolites for anabolic processes. It has been observed that FFA levels are significantly elevated in the plasma of hepatocellular carcinoma (HCC) patients.
Recent studies implicate an involvement of PA in ER stress and diseases such as cancer, however the mechanisms that regulate ER stress and the unfolded protein response (UPR) (e.g. the IRE1α signaling) and, in particular, the role PA plays in this process is unclear. Recently we found that PA induces epithelial to mesenchymal transition (EMT), i.e. cells start to display more mesenchymal markers and phenotypes. In this study, we investigated wild-type (WT) IRE1α and IRE1α with a W457A mutation with respect to its splicing activity and migration ability to determine the functional importance of the tryptophan residue in activation of the ER stress pathway and its impact on EMT. The cells expressing WT IRE1α are more invasive and aggressive with an elevated resistance to apoptosis as compared with cells expressing the mutant IRE1α. Wound healing studies in mouse embryonic cells (MEF) IRE1-/- knockout cells transfected with IRE1α showed increased migration ability on treatment with PA as compared to IRE1α with a W457A mutation.
Studying the effect of fatty acids on the invasiveness and metastatic ability of cells through ER stress sensors like IRE1α provide insights into the biochemical changes induced by fatty acids that contribute to cancer progression.