Despite in depth characterization of the role of IRE1 alpha (inositol-requiring enzyme 1 alpha) in activating the unfolded protein response (UPR) in the ER - little is known about the molecular mechanisms by which this ER protein has shown to regulate intracellular calcium levels and subsequent apoptosis. Intracellular calcium homeostasis is fundamental to many physiological processes, and an increase in Ca2+ is associated with both the early and late stages of apoptosis. A Nature article fleshed out a pro-apoptotic IRE1alpha –TRAF2–JNK pathway that has potential to be activated by prolonged ER stress. Specifically, prolonged IRE1-mediated activation may promote apoptosis by degrading the mRNAs that encode essential cell-survival proteins. Additional research studies have proposed similar pathways that bring in other pro apoptotic proteins and signaling pathways.
One model for how IRE1alpha moderates Ca2+ through InsP3R was investigated using both an IRE1 alpha antibody and an InsP3R antibody. Inositol-1,4,5-trisphosphate receptors (InsP3Rs) are ion channels responsible for cytosolic Ca2+ signaling, and thus in turn also essential for a wide range of cellular processes. Preliminary findings did not show any interaction with IRE1alpha and InsP3R, however further Co-IP studies in IRE1alpha KD cells, utilizing an IRE1 alpha antibody (alongside CIB1 and ASK1 antibodies) showed that in IRE1 alpha KD cells – CIB1/ASK1 form a complex. What’s more is that treatment with InsP3R blockers inhibit Ca2+ increase and later cell death in the IRE1 alpha-KD cells as well. A similar elucidated by Nishitoh et al suggests that ER stress activates ASK1 through formation of an IRE1alpha -TRAF2-ASK1 complex, leading to apoptotic neuronal cell death. Nishitoh’s group used IRE1 alpha antibody to show that IRE1 alpha alone is not sufficient to activate the JNK pathway, however with the addition of ASK1 positive MEF cells resulted in proteasome inhibitor induced JNK activation. A related study was performed by Hetz et al, which showed that Bim and PUMA engage the IRE1alpha/TRAF2/JNK pathway via interaction between IRE1alpha and Bak using an IRE1 alpha antibody. Bim and PUMA are BH3-only proteins that are thought to act downstream of ER stress, making this a viable hypothesis and finding.
These compiled studies suggest that signal transduction between IRE1alpha–TRAF2 and phosphorylation of JNK induced apoptosis involves pro-apoptotic Bcl-2 family members, increases intracellular calcium, and results in cytotoxicity and subsequent cell death. Furthermore, other research has also implicated ASK1 in addition to the IRE1alpha –TRAF2-JNK pathway. Combined these results point to a promising lead in unwrapping deeper functions of the ER and specifically IRE1alpha and its role in more than a prominent stress sensor.
