Blockade of XBP1 splicing by inhibition of IRE1α is a promising therapeutic option in multiple myeloma

Abstract

Multiple myeloma (MM) cells are characterized by high protein synthesis resulting in chronic endoplasmic reticulum (ER) stress, which is adaptively managed by the unfolded protein response. Inositolrequiring enzyme 1␣ (IRE1␣) is activated to splice X-box binding protein 1 (XBP1) mRNA, thereby increasing XBP1s protein, which in turn regulates genes responsible for protein folding and degradation during the unfolded protein response. In this study, we examined whether IRE1␣- IntroductionTreatment for multiple myeloma (MM) has remarkably improved because of novel agents, such as bortezomib, thalidomide, and lenalidomide. [1][2][3] However, MM remains incurable, and nextgeneration novel agents are urgently needed. Because of high levels of endoplasmic reticulum (ER) stress and adaptation by the unfolded protein response (UPR), targeting signaling by the UPR and blocking this key survival pathway represent a new therapeutic strategy. In mammalian cells, protein folding is proportionally fine-tuned to the metabolic state of the cell within its microenvironment. Extracellular insults, such as low nutrients, hypoxia, and multiple drugs, result in the accumulation of misfolded proteins in the ER, thereby causing ER stress and initiating the UPR. 4 The UPR in turn increases the biosynthetic capacity and decreases the biosynthetic burden of the ER, to maintain cellular homeostasis. However, when the stress cannot be compensated by the UPR, cellular apoptosis occurs. 5 The UPR consists of 3 branches of signaling pathways, which initiate from 3 ER transmembrane proteins: inositol-requiring enzyme 1␣ (IRE1␣), PKR-like ER kinase (PERK), and activating transcription factor 6 (ATF6). In the resting state, these proteins are associated with molecular chaperone BiP/GRP78 in the ER. However, when unfolded proteins accumulate in the ER, BiP/GRP78 dissociates from them, thereby inducing UPR signaling. 6 In the UPR, IRE1␣ is activated by oligomerization and autophosphorylation, resulting in activation of its endoribonuclease to cleave and initiate splicing of the X-box binding protein 1 (XBP1) mRNA. A 26-nucleotide intron from XBP1 is removed by activated IRE1␣ endoribonuclease, resulting in a translational frame-shift to modify unspliced XBP1 (XBP1u: inactive) into spliced XBP1 (XBP1s: active). 7 XBP1 is a unique transcription factor that regulates genes responsible for ER-associated degradation (ERAD), such as EDEM, and those responsible for promoting protein folding, such as p58IPK and other ER chaperones. 8 Thus, IRE1␣-XBP1 pathway has a prosurvival role in the UPR. However, under conditions of prolonged and uncompensated stress, the UPR leads to cellular apoptosis, known as the terminal UPR. The proapoptotic transcription factor CHOP, also known as GADD153, is induced via PERK and ATF6 pathways. CHOP causes downregulation of BCL2, thereby leading to caspase-dependent apoptosis. 9 IRE1␣ also has a proapoptotic role: it binds TRAF2 and activates ASK1, which causes JNK activation, thereby leading to caspase-dependent apoptosis. 10 ...

Targeting IRE1 with small molecules counteracts progression of atherosclerosis

“…Although limited pharmacodynamic data are available for the IRE1 inhibitors that we used, target engagement has been shown in cells and tissues (21,39). We based our in vivo drug dosage and delivery on earlier in vivo studies that successfully administered the IRE1 modulators without toxicity (21,24,31). Furthermore, we confirmed target engagement in tissues by assessing drug-induced suppression of Xbp1 mRNA splicing and RIDD activities by observing a significant reduction in the spliced Xbp1 mRNA (P < 0.05) and a modest increase in canonical RIDD target mRNAs (P < 0.05) (Fig.…”

“…This quest has led to the identification of several new small molecules that target the enzymatic activities of specific UPR regulators (19)(20)(21)(22)(23). Specifically, blocking IRE1 or XBP1s function has been shown to be beneficial for restraining tumor progression in mouse models (21,24), highlighting that the specific targeting of the UPR can have beneficial impact in disease.…”

“…Third, experimentally sustained Xbp1 mRNA splicing in the vessel wall promotes atherosclerosis, whereas its ablation ameliorates hypercholesterolemia in obese or apolipoprotein E-deficient (ApoE −/− ) mice (10,30). Two small molecules, STF-083010 and 4μ8C, which selectively inhibit IRE1's RNase function, have been used in cells and animals to produce favorable therapeutic outcomes in other disease settings: STF-083010 reduced growth of multiple myeloma (21,24,31,32), and 4μ8c suppressed inflammation in a murine arthritis model (33). The action of both compounds is wellunderstood mechanistically.…”

Protein misfolding and dysregulated protein homeostasis in autoinflammatory diseases and beyond

“…Therefore, limiting succinate accumulation has marked potential for management of a range of conditions, including autoinflammatory diseases resistant to standard therapies, as well as more widespread conditions, where dysregulation of the UPR underlies pathogenesis. Blockade of XBP1 splicing by inhibition of IRE1 has shown promise in the treatment of myeloma [159]. Collectively, these studies reveal that the effects of Xbp1s are very context dependent and that the UPR may play a key role to protect cells under stress in addition to the more publicized contribution to causing disease.…”

Understanding the multiple biological aspects leading to myeloma

“…Conflicting data regarding the suppression of the Ire1-Xbp1 pathway suggests that it could promote either resistance 108 or sensitivity to proteasome inhibition. 109 haematologica | 2014; 99(4)…”