Coordination between Two Branches of the Unfolded Protein Response Determines Apoptotic Cell Fate.

The kinases PERK and IRE1 alleviate endoplasmic reticulum (ER) stress by orchestrating the unfolded protein response (UPR). If stress mitigation fails, PERK promotes cell death by activating pro-apoptotic genes, including death receptor 5 (DR5). Conversely, IRE1-which harbors both kinase and endoribonuclease (RNase) modules-blocks apoptosis through regulated IRE1-dependent decay (RIDD) of DR5 mRNA. Under irresolvable ER stress, PERK activity persists, whereas IRE1 paradoxically attenuates, by mechanisms that remain obscure. Here, we report that PERK governs IRE1's attenuation through a phosphatase known as RPAP2 (RNA polymerase II-associated protein 2). RPAP2 reverses IRE1 phosphorylation, oligomerization, and RNase activation. This inhibits IRE1-mediated adaptive events, including activation of the cytoprotective transcription factor XBP1s, and ER-associated degradation of unfolded proteins. Furthermore, RIDD termination by RPAP2 unleashes DR5-mediated caspase activation and drives cell death. Thus, PERK attenuates IRE1 via RPAP2 to abort failed ER-stress adaptation and trigger apoptosis.

Disruption of IRE1α through its kinase domain attenuates multiple myeloma.

Multiple myeloma (MM) arises from malignant immunoglobulin (Ig)-secreting plasma cells and remains an incurable, often lethal disease despite therapeutic advances. The unfolded-protein response sensor IRE1α supports protein secretion by deploying a kinase-endoribonuclease module to activate the transcription factor XBP1s. MM cells may co-opt the IRE1α-XBP1s pathway; however, the validity of IRE1α as a potential MM therapeutic target is controversial. Genetic disruption of IRE1α or XBP1s, or pharmacologic IRE1α kinase inhibition, attenuated subcutaneous or orthometastatic growth of MM tumors in mice and augmented efficacy of two established frontline antimyeloma agents, bortezomib and lenalidomide. Mechanistically, IRE1α perturbation inhibited expression of key components of the endoplasmic reticulum-associated degradation machinery, as well as secretion of Ig light chains and of cytokines and chemokines known to promote MM growth. Selective IRE1α kinase inhibition reduced viability of CD138+ plasma cells while sparing CD138- cells derived from bone marrows of newly diagnosed or posttreatment-relapsed MM patients, in both US- and European Union-based cohorts. Effective IRE1α inhibition preserved glucose-induced insulin secretion by pancreatic microislets and viability of primary hepatocytes in vitro, as well as normal tissue homeostasis in mice. These results establish a strong rationale for developing kinase-directed inhibitors of IRE1α for MM therapy.

ATF6 Promotes Colorectal Cancer Growth and Stemness by Regulating the Wnt Pathway.

SIGNIFICANCE: ATF6 intervention reduces colorectal cancer cell and organoid viability by interrupting dysregulated Wnt signaling, identifying a novel facilitator and potential therapeutic target in colorectal cancer.

Misfolded proteins bind and activate death receptor 5 to trigger apoptosis during unresolved endoplasmic reticulum stress.

Disruption of protein folding in the endoplasmic reticulum (ER) activates the unfolded protein response (UPR)-a signaling network that ultimately determines cell fate. Initially, UPR signaling aims at cytoprotection and restoration of ER homeostasis; that failing, it drives apoptotic cell death. ER stress initiates apoptosis through intracellular activation of death receptor 5 (DR5) independent of its canonical extracellular ligand Apo2L/TRAIL; however, the mechanism underlying DR5 activation is unknown. In cultured human cells, we find that misfolded proteins can directly engage with DR5 in the ER-Golgi intermediate compartment, where DR5 assembles pro-apoptotic caspase 8-activating complexes. Moreover, peptides used as a proxy for exposed misfolded protein chains selectively bind to the purified DR5 ectodomain and induce its oligomerization. These findings indicate that misfolded proteins can act as ligands to activate DR5 intracellularly and promote apoptosis. We propose that cells can use DR5 as a late protein-folding checkpoint before committing to a terminal apoptotic fate.

Proteins are chains of building blocks called amino acids, folded into a flexible 3D shape that is critical for its biological activity. This shape depends on many factors, but one is the chemistry of the amino acids. Because the internal and external environments of cells are mostly water-filled, correctly folded proteins often display so-called hydrophilic (or 'water-loving') amino acids on their surface, while tucking hydrophobic (or 'water-hating') amino acids on the inside. A compartment within the cell called the endoplasmic reticulum folds the proteins that are destined for the outside of the cell. It can handle a steady stream of protein chains, but a sudden increase in demand for production, or issues with the underlying machinery, can stress the endoplasmic reticulum and hinder protein folding. This is problematic because incorrectly folded proteins cannot work as they should and can be toxic to the cell that made them or even to other cells. Many cells handle this kind of stress by activating a failsafe alarm system called the unfolded protein response. It detects the presence of incorrectly shaped proteins and sends signals that try to protect the cell and restore protein folding to normal. If that fails within a certain period of time, it switches to signals that tell the cell to safely self-destruct. That switch, from protection to self-destruction, involves a protein called death receptor 5, or DR5 for short. DR5 typically triggers the cell's self-destruct program by forming molecular clusters at the cell's surface, in response to a signal it receives from the exterior. During a failed unfolded protein response, DR5 seems instead to act in response to signals from inside the cell, but it was not clear how this works. To find out, Lam et al. stressed the endoplasmic reticulum in human cells by forcing it to fold a lot of proteins. This revealed that DR5 sticks to misfolded proteins when they leave the endoplasmic reticulum. In response, DR5 molecules form clusters that trigger the cell's self-destruct program. DR5 directly recognized hydrophobic amino acids on the misfolded protein's surface that would normally be hidden inside. When Lam et al. edited these hydrophobic regions to become hydrophilic, the DR5 molecules could no longer detect them as well. This stopped the cells from dying so easily when they were under stress. It seems that DR5 decides the fate of the cell by detecting proteins that were incorrectly folded in the endoplasmic reticulum. Problems with protein folding occur in many human diseases, including metabolic conditions, cancer and degenerative brain disorders. Future work could reveal whether controlling the activation of DR5 could help to influence if and when cells die. The next step is to understand how DR5 interacts with incorrectly folded proteins at the atomic level. This could aid the design of drugs that specifically target such receptors.

IRE1α Disruption in Triple-Negative Breast Cancer Cooperates with Antiangiogenic Therapy by Reversing ER Stress Adaptation and Remodeling the Tumor Microenvironment.

Cancer cells exploit the unfolded protein response (UPR) to mitigate endoplasmic reticulum (ER) stress caused by cellular oncogene activation and a hostile tumor microenvironment (TME). The key UPR sensor IRE1α resides in the ER and deploys a cytoplasmic kinase-endoribonuclease module to activate the transcription factor XBP1s, which facilitates ER-mediated protein folding. Studies of triple-negative breast cancer (TNBC)-a highly aggressive malignancy with a dismal posttreatment prognosis-implicate XBP1s in promoting tumor vascularization and progression. However, it remains unknown whether IRE1α adapts the ER in TNBC cells and modulates their TME, and whether IRE1α inhibition can enhance antiangiogenic therapy-previously found to be ineffective in patients with TNBC. To gauge IRE1α function, we defined an XBP1s-dependent gene signature, which revealed significant IRE1α pathway activation in multiple solid cancers, including TNBC. IRE1α knockout in TNBC cells markedly reversed substantial ultrastructural expansion of their ER upon growth in vivo. IRE1α disruption also led to significant remodeling of the cellular TME, increasing pericyte numbers while decreasing cancer-associated fibroblasts and myeloid-derived suppressor cells. Pharmacologic IRE1α kinase inhibition strongly attenuated growth of cell line-based and patient-derived TNBC xenografts in mice and synergized with anti-VEGFA treatment to cause tumor stasis or regression. Thus, TNBC cells critically rely on IRE1α to adapt their ER to in vivo stress and to adjust the TME to facilitate malignant growth. TNBC reliance on IRE1α is an important vulnerability that can be uniquely exploited in combination with antiangiogenic therapy as a promising new biologic approach to combat this lethal disease. SIGNIFICANCE: Pharmacologic IRE1α kinase inhibition reverses ultrastructural distension of the ER, normalizes the tumor vasculature, and remodels the cellular TME, attenuating TNBC growth in mice.

Caspase-mediated cleavage of IRE1 controls apoptotic cell commitment during endoplasmic reticulum stress.

Upon detecting endoplasmic reticulum (ER) stress, the unfolded protein response (UPR) orchestrates adaptive cellular changes to reestablish homeostasis. If stress resolution fails, the UPR commits the cell to apoptotic death. Here we show that in hematopoietic cells, including multiple myeloma (MM), lymphoma, and leukemia cell lines, ER stress leads to caspase-mediated cleavage of the key UPR sensor IRE1 within its cytoplasmic linker region, generating a stable IRE1 fragment comprising the ER-lumenal domain and transmembrane segment (LDTM). This cleavage uncouples the stress-sensing and signaling domains of IRE1, attenuating its activation upon ER perturbation. Surprisingly, LDTM exerts negative feedback over apoptotic signaling by inhibiting recruitment of the key proapoptotic protein BAX to mitochondria. Furthermore, ectopic LDTM expression enhances xenograft growth of MM tumors in mice. These results uncover an unexpected mechanism of cross-regulation between the apoptotic caspase machinery and the UPR, which has biologically significant consequences for cell survival under ER stress.

Activation of the proapoptotic death receptor DR5 by oligomeric peptide and antibody agonists.

The cell-extrinsic apoptotic pathway triggers programmed cell death in response to certain ligands that bind to cell-surface death receptors. Apoptosis is essential for normal development and homeostasis in metazoans, and furthermore, selective activation of the cell-extrinsic pathway in tumor cells holds considerable promise for cancer therapy. We used phage display to identify peptides and synthetic antibodies that specifically bind to the human proapoptotic death receptor DR5. Despite great differences in overall size and structure, the DR5-binding peptides and antibodies shared a tripeptide motif, which was conserved within a disulfide-constrained loop of the peptides and the third complementarity determining region of the antibody heavy chains. The X-ray crystal structure of an antibody in complex with DR5 revealed that the tripeptide motif is buried at the core of the interface, confirming its central role in antigen recognition. We found that certain peptides and antibodies exhibited potent proapoptotic activity against DR5-expressing SK-MES-1 lung carcinoma cells. These phage-derived ligands may be useful for elucidating DR5 activation at the molecular level and for creating synthetic agonists of proapoptotic death receptors.

Antitherapeutic antibody-mediated hepatotoxicity of recombinant human Apo2L/TRAIL in the cynomolgus monkey.

Apo2L/TRAIL is a member of the tumor necrosis factor superfamily and an important inducer of apoptosis. Recombinant human (rhu) Apo2L/TRAIL has been attractive as a potential cancer therapeutic because many types of tumor cells are sensitive to its apoptosis-inducing effects. Nonclinical toxicology studies were conducted to evaluate the safety of rhuApo2L/TRAIL for possible use in humans. The cynomolgus monkey was chosen for this safety assessment based on high protein sequence homology between human and cynomolgus Apo2L/TRAIL and comparable expression of their receptors. Although hepatotoxicity was observed in repeat-dose monkey studies with rhuApo2L/TRAIL, all animals that displayed hepatotoxicity had developed antitherapeutic antibodies (ATAs). The cynomolgus ATAs augmented the cytotoxicity of rhuApo2L/TRAIL but not of its cynomolgus counterpart. Of note, human and cynomolgus Apo2L/TRAIL differ by four amino acids, three of which are surface-exposed. In vivo studies comparing human and cynomolgus Apo2L/TRAIL supported the conclusion that these distinct amino acids served as epitopes for cross-species ATAs, capable of crosslinking rhuApo2L/TRAIL and thus triggering hepatocyte apoptosis. We describe a hapten-independent mechanism of immune-mediated, drug-related hepatotoxicity - in this case - associated with the administration of a human recombinant protein in monkeys. The elucidation of this mechanism enabled successful transition of rhuApo2L/TRAIL into human clinical trials.

Engineering an APRIL-specific B cell maturation antigen.

B cell maturation antigen (BCMA) is a tumor necrosis factor receptor family member whose physiological role remains unclear. BCMA has been implicated as a receptor for both a proliferation-inducing ligand (APRIL) and B cell-activating factor (BAFF), tumor necrosis factor ligands that bind to multiple tumor necrosis factor receptor and have been reported to play a role in autoimmune disease and cancer. The results presented herein provide a dual perspective analysis of BCMA binding to both APRIL and BAFF. First, we characterized the binding affinity of monomeric BCMA for its ligands; BAFF binding affinity (IC50 = 8 +/- 5 microm) is about 1000-fold reduced compared with the high affinity interaction of APRIL (IC50 = 11 +/- 3 nm). Second, shotgun alanine scanning of BCMA was used to map critical residues for either APRIL or BAFF binding. In addition to a previously described "DXL" motif (Gordon, N. C., Pan, B., Hymowitz, S. G., Yin, J., Kelley, R. F., Cochran, A. G., Yan, M., Dixit, V. M., Fairbrother, W. J., and Starovasnik, M. A. (2003) Biochemistry 42, 5977-5983), the alanine scanning results predicted four amino acid positions in BCMA (Tyr13, Ile22, Gln25, and Arg27) that could impart ligand specificity. Substitution of Tyr13 was tolerated for BAFF binding but not APRIL binding. Arg27 was required for high affinity binding to APRIL, whereas substitutions of this residue had minimal effect on affinity for BAFF. Further phage display experiments suggested the single mutations of I22K, Q25D, and R27Y as providing the greatest difference in APRIL versus BAFF binding affinity. Incorporation of the Q25D and R27Y substitutions into BCMA produced a dual specificity variant, since it has comparable binding affinity for both APRIL and BAFF, IC50 = 350 and 700 nm, respectively. Binding of the I22K mutant of monomeric BCMA to BAFF was undetectable (IC50 > 100 microm), but affinity for binding to APRIL was similar to wild-type BCMA. Based on these results, a BCMA-Fc fusion with the single I22K mutation was produced that binds APRIL, IC50 = 12 nm, and has no measurable affinity for BAFF. These results suggest that APRIL is the preferred ligand for BCMA and show that specificity can be further modified through amino acid substitutions.