The Role of ER Stress and the Unfolded Protein Response in Cancer

Relationship Between ER Stress, UPR, and Tumorigenesis

The ability of cancer cells to spread and metastasize to other tissues exposes them to harsh environmental conditions, including low glucose levels, deficiency in growth factors, lactic acidosis, oxidative stress, hypoxia, and amino acid starvation (13). These conditions pose a threat to proper protein folding in the ER, leading to the accumulation of misfolded proteins and ER stress (13).

The UPR, mediated by PERK, IRE1, and ATF6 pathways (Figure 1), is activated to alleviate ER stress and restore proteostasis. The loss of tumor suppressors and hyperactivation of oncogenes increase protein synthesis to meet the heightened metabolic demands during tumorigenesis (14). PERK activation leads to eIF2α phosphorylation, reducing global protein synthesis while selectively increasing the expression of pro-survival factors like ATF4, which supports metabolic adaptation and cell survival under nutrient-deprived conditions.

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Figure 1.

The endoplasmic reticulum stress pathway. Schematic representation of the unfolded protein response (UPR) pathways activated during endoplasmic reticulum (ER) stress. The figure highlights three major signaling arms: PKR-like ER kinase (PERK), inositol-requiring enzyme 1 (IRE1), and activating transcription factor 6 (ATF6). Activation of these pathways leads to adaptive responses, including ER-associated degradation (ERAD), regulation of protein synthesis, and induction of apoptosis.

Similarly, IRE1 activation initiates the splicing of XBP1 mRNA, producing XBP1s, a transcription factor that upregulates genes involved in protein folding, ER expansion, and ERAD. This helps rapidly dividing cancer cells meet their metabolic and proteostatic demands. ATF6 translocation to the Golgi, followed by cleavage into an active transcription factor, induces the expression of chaperones like BiP and components of the ERAD pathway, further enhancing ER capacity.

Moreover, rapidly multiplying cancer cells require an expanded ER for cell division and distribution to daughter cells (15). Genetic changes within cancer cells, such as aneuploidy, genomic instability, and somatic mutations in proteins related to the secretory pathway, can exacerbate ER stress and sustain UPR activation (13). Persistent UPR activation not only allows cancer cells to survive under these stressful conditions but also contributes to tumor progression and metastasis through signaling pathways that enhance cell migration, invasion, and resistance to apoptosis.

Since cancer cells are highly dependent on the UPR to adapt to these stressful conditions, targeting UPR proteins represents a promising new approach to cancer therapy. Inhibitors of PERK, IRE1, or ATF6 could disrupt the tumor’s adaptive mechanisms, selectively sensitizing cancer cells to stress and promoting apoptosis while sparing normal cells.

BiP in Cancer

GRP78/BiP, also known as heat shock protein family a member 5 (HSPA5), is a molecular chaperone that plays a critical role in the ER stress response. In cancer, BiP overexpression is associated with aggressive behavior, poor prognosis, and resistance to therapy (16-18). Recent studies have shed light on the diverse mechanisms by which BiP promotes cancer progression, making it an attractive therapeutic target.

BiP is overexpressed in various types of human cancers, including breast, liver, gastric, prostate, and colon cancers (19). This overexpression is associated with advanced pathological grade, increased risk of recurrence, and poor patient survival (18). BiP is also present on the surface of cancer cells, where it interacts with other surface proteins to enhance tumor cell survival, proliferation, and invasion (20). For example, cell surface-localized BiP enhances protein kinase B (PKB or AKT) signaling to increase the survival of cancer cells. Additionally, it interacts with Cripto, a multifunctional cell surface protein, to support cancer growth by suppressing the transforming growth factor-β (TGFβ) signaling pathway (21). These findings suggest that BiP could serve as a valuable diagnostic and prognostic biomarker for cancer patients.

In cholangiocarcinoma, inhibiting histone deacetylase 6 (HDAC6), which prevents BiP translocation to the cell surface, was shown to suppress cell proliferation (22). The interaction between E3 ubiquitin ligase 7 in absentia homolog 2 (Siah2) and BiP modulated reactive oxygen species (ROS) levels, promoting cell proliferation in Helicobacter pylori-infected gastric epithelial cancers (23). X-linked inhibitor of apoptosis-associated factor-1 (XAF1) destabilizing BiP led to ER stress response-induced apoptosis in stress-inducible tumors, emphasizing the role of BiP in the adaptive ER stress response (24). Downregulation of BiP reversed pirarubicin resistance in triple-negative breast cancer (TNBC) through the phospho-AKT/mammalian target of rapamycin (mTOR) pathway and miR-495-3p mimics (25). Additionally, BiP repression of ferroptosis promoted colorectal cancer development by stabilizing glutathione peroxidase 4 (GPX4) protein, revealing its protective role in attenuating erastin-induced GPX4 decrease during ferroptosis (26).

BiP, a multifaceted contributor to tumor growth, operates through various mechanisms. One such role involves its facilitation of growth factor maturation and secretion as an endoplasmic reticulum chaperone, supporting the folding and secretion of growth factors and receptors crucial for tumor cell proliferation and survival (27). The essential nature of BiP in cell growth was underscored by inhibiting tumor cell proliferation in BiP heterozygous cells. Recent research has also highlighted the presence of BiP on tumor cell surfaces, enhancing AKT signaling for cell survival and proliferation (28). Additionally, BiP acts as a key player in inhibiting apoptosis, protecting cancer cells from programmed cell death mechanisms. In heterozygous glucose-regulated protein 78 (Grp78 or Grp70) mouse tumor cells, increased Terminal deoxynucleotidyl transferase dUTP Nick-End Labeling (TUNEL) staining and CHOP upregulation indicated the impact of BiP in impeding apoptosis (29). In human breast cancer cells, BiP forms a complex with BCL-2-interacting killer (BIK), inhibiting BIK-mediated apoptosis (30). Furthermore, heightened BiP expression has been associated with reduced sensitivity of cancer cells to the chemotherapy drug temozolomide (TMZ) and radiation therapy (31). Another facet of BiP’s influence on tumor growth involves its facilitation of angiogenesis, crucial for tumor development and metastasis. BiP heterozygosity significantly decreased tumor vasculature without affecting normal organs and tissues (27), emphasizing its pivotal role in tumor angiogenesis. These diverse functions highlight BiP as a central player in promoting tumor growth through intricate cellular processes. BiP plays a multifaceted role in cancer progression, making it a valuable biomarker and a promising therapeutic target. Ongoing research is focused on elucidating the molecular mechanisms by which BiP contributes to cancer and developing novel therapeutic strategies to target this critical protein.

Approaches to target BiP in cancer. Given its diverse roles in cancer progression, BiP has emerged as a promising therapeutic target, with several strategies explored to modulate its activity. Immunotherapy approaches include monoclonal antibodies that block BiP interactions and disrupt its signaling pathways, leading to tumor cell death. For instance, the BiP-specific antibody MAb159 was found to inhibit the PI3K pathway, suppressing both tumor growth and metastasis (32). Additionally, extracellular BiP exposure triggers autoantibody production, particularly in prostate and ovarian cancer patients, where a significant impact on tumor development has been observed (33). Furthermore, chimeric antigen receptor (CAR) T cells targeting cell surface BiP exhibited strong anti-tumor activity against acute myeloid leukemia (AML) cells, stimulating anti-tumor cytokine production and inhibiting tumor progression in preclinical models (34).

Gene therapy strategies have been developed to either silence BiP expression or introduce therapeutic genes counteracting BiP overexpression. Adeno-associated virus (AAV)-based gene therapy has been employed to enhance ER folding capacity by overexpressing BiP. Notably, BiP gene transfer into the Substantia Nigra pars compacta (SNpc) significantly reduced ER stress and decreased apoptosis of dopaminergic neurons induced by alpha-synuclein overexpression (35, 36). In prostate cancer, RNA interference-mediated BiP downregulation in the PC-3 cell line resulted in reduced cell migration and apoptosis induction (37). Another study targeting the VEGF/GRP78 axis with the soluble Fms-like tyrosine kinase-1 (sFLT01) protein in prostate cancer cells (DU145) led to BiP downregulation and reduced expression of matrix metallopeptidases MMP2 and MMP9, while upregulating tissue inhibitors TIMP1 and TIMP2, thereby inhibiting cancer cell proliferation and invasiveness (38). Moreover, BiP knockdown via small interfering RNA (siRNA) led to a decrease in oncogenic KRAS protein levels in lung, colon, and pancreatic cancer cells harboring KRAS mutations, occurring at a post-transcriptional level independent of proteasomal degradation or autophagy (39). Additionally, non-coding RNAs have emerged as potential regulators of BiP-mediated cell fate, as exemplified by the silencing of FR229754, an exonic sense lncRNA, which significantly reduced BiP levels (40).

Small molecule inhibitors targeting BiP’s chaperone activity have demonstrated potential in inhibiting tumor growth and sensitizing cancer cells to chemotherapy and radiation therapy. These inhibitors function by blocking BiP’s ATPase activity, essential for its chaperone function, thereby leading to the accumulation of unfolded proteins in the ER. FL5, a small molecule with strong BiP-binding affinity, exhibited potent antiangiogenic and anticancer effects against human umbilical vein endothelial cells (HUVEC) and renal cancer cells (786-O), with minimal cytotoxicity in normal fibroblasts (Swiss-3T3) (41). Additionally, BiP inhibition through small molecules such as HA15 and YUM70 significantly reduced cancer cell viability in KRAS-driven cancers, underscoring its potential as a therapeutic target (39).

Natural compounds have also been identified as BiP modulators, including palmatine and epigallocatechin gallate (EGCG), which impair BiP function, leading to unfolded protein accumulation and UPR activation. EGCG, a green tea polyphenol, induces ER stress-mediated apoptosis in colorectal cancer cells, an effect associated with BiP upregulation (42). Meanwhile, palmatine, a plant-derived compound with diverse protective properties, was shown to reduce GRP78 expression in a streptozotocin-induced diabetic rat model, suggesting its therapeutic potential beyond oncology (43).

PERK in Cancer

PERK serves as a mediator in UPR-related human diseases, encompassing conditions such as tumorigenesis and neurodegenerative disorders. Its involvement is evident in supporting various facets of cancer, including tumor growth, metastasis, autophagy, and resistance to radiation (44-46). As a result, PERK has emerged as a potential therapeutic target for addressing challenges in treatment efficacy and overcoming therapy failure in these diseases.

When PERK is activated, it phosphorylates eIF2α, which reduces global protein synthesis while selectively increasing the translation of ATF4. Several studies have found that the levels of p-eIF2α are increased in a variety of cancer cells, including bronchioloalveolar carcinoma, Hodgkin’s lymphoma, benign and malignant melanoma, colonic epithelial neoplasms, gastrointestinal carcinoma, and breast cancer (46). This suggests that ER stress, particularly the PERK pathway, plays an important role in the initiation and progression of cancer. Further supporting this, a recent study demonstrated that treatment with 17-AAG, a heat shock protein 90 (HSP90) inhibitor, significantly increased apoptosis in breast cancer cells by upregulating PERK/eIF2α signaling. This was accompanied by increased oxidative stress, DNA damage, and disruption of protein homeostasis in the ER, reinforcing the role of PERK in breast cancer cell fate decisions (47). However, one of the studies found that the level of p-eIF2α is decreased in osteosarcoma, a common bone tumor in children and young adults (48). This suggests that the role of PERK in cancer is complex and may vary depending on the type of cancer. In vitro and in vivo studies have documented apoptotic cell death in osteosarcoma cells, accompanied by a subsequent reduction in tumor growth. This effect was observed when the CYT997-mediated activation of the PERK/p-eIF2α/CHOP signaling pathway was upregulated (49). Similarly, Wang et al. have demonstrated the involvement of the PERK/p-eIF2α pathway in apoptotic processes, showing elevated levels of BiP, p-PERK, and p-eIF2α in paraquat-induced apoptosis in human lung epithelial-like A549 cells (50). Furthermore, in human hepatocellular carcinoma cells, increased levels of BiP, PERK, p-eIF2α, ATF4, and CHOP were noted in association with pterostilbene-induced autophagy-dependent cell death (51).

PERK/ATF4 signaling triggers multiple steps in the metastatic cascade, including angiogenesis, migration, colonization, and survival (52). PERK/eIF2α/ATF4 signaling helps cancer cells to survive in the harsh conditions of the tumor microenvironment, including hypoxia (low oxygen levels) and nutrient deprivation. The involvement of the PERK/eIF2α axis in promoting a pro-survival response during hypoxia is underscored by evidence indicating that cancer cells harboring an intact PERK/eIF2α axis exhibit enhanced tolerance to hypoxia and heightened tumorigenicity (53). This heightened resilience and aggressiveness are particularly noteworthy, given that the activation of PERK is hypoxia-inducible factor 1-alpha (HIF-1α)-dependent and more pronounced in the context of normoxia-hypoxia cycles, a common occurrence in tumor masses characterized by irregular vasculature (53). The conferred hypoxia tolerance following PERK activation is predominantly mediated by the downstream effector ATF4. Tumors exhibiting a defective PERK/eIF2α/ATF4 axis showcase apoptotic regions coinciding with hypoxic areas, emphasizing the essential role of a functional PERK/eIF2α/ATF4 axis in mitigating hypoxia-induced damage (53). Furthermore, PERK/eIF2α signaling serves to buffer the escalating ROS levels observed in cyclic hypoxia by upregulating glutathione (GSH) and promoting autophagy. These orchestrated events contribute to the enhanced survival of glioblastoma cells in the face of oxidative damage induced by oscillating hypoxia or radiotherapy (54, 55). In human cervix cancer, the hypoxia-induced activation of PERK/eIF2α/ATF4 signaling upregulates the pro-metastatic protein lysosomal-associated membrane protein 3 (LAMP3) (56), establishing a link between hypoxic activation of PERK and aggressive behavior in cancer cells. Finally, it is worth noting that in the context of nutrient deprivation and hypoxia in the tumor microenvironment, PERK plays a dual role. It facilitates cancer cell survival under low glucose conditions through AKT activation and hexokinase II mitochondrial translocation. In response to hypoxia, PERK induces UPR signaling, contributing to tumor cell survival and migration. Additionally, PERK stimulates metastasis, possibly through the PERK-eIF2α-ATF4-LAMP3 axis, and is linked to the epithelial-to-mesenchymal transition (EMT), enhancing tumor cell invasiveness (57, 58). The PERK/eIF2α/ATF4 axis was found to induce the expression of genes involved in autophagy, such as microtubule-associated protein 1A/1B-light chain 3 (LC3) (59), which is known to be pro-tumorigenic in many cancer cells (60).

An in vitro investigation demonstrated that PERK played a role in enhancing cell proliferation and migration in glioblastoma stem cells (61). Additionally, it was observed to contribute to angiogenesis by interacting with peptidyl glycine α-amidating monooxygenase (PAM) in glioblastoma cell lines (62). The transcription factor CAMP-responsive element-binding protein 3-like 1 (CREB3L1) is identified as a crucial mediator of PERK’s pro-metastatic effects in breast cancer. Operating downstream of PERK, particularly in the mesenchymal subtype of triple-negative tumors, CREB3L1 inhibition through genetic or pharmacological means effectively restrains cancer cell invasion and metastasis (63).

PERK is also selectively critical in inducing ferroptosis in colorectal cancer. PERK inhibition reduces the expression of ATF4, that regulates the expression of the cystine-glutamate transporter solute carrier family 7 member 11 (SLC7A11). SLC7A11 is essential for ferroptosis, and its downregulation makes cancer cells more susceptible to ferroptosis. Loss of PERK function not only makes cancer cells more susceptible to ferroptosis, but it also limits the growth of colorectal tumors in vivo (64). PERK knockout in cancer cells or inhibiting PERK pharmacologically in melanoma-bearing mice led to robust activation of anti-tumor T cell immunity and attenuated tumor growth (65). PERK elimination in ER-stressed malignant cells triggered a type of cell death called paraptosis, which promoted immunogenic cell death (65). Overall, the intricate and context-dependent functions of PERK in cancer underscore its significance as a therapeutic target and warrant further exploration to unravel its precise role in specific cancer types and microenvironments.

Approaches to target PERK in cancer. Several therapeutic strategies have been explored to target the PERK pathway in cancer, including small molecule inhibitors, gene therapy, combination therapies, and natural compounds. Small molecule inhibitors of PERK, such as AMG44, GSK2606414, and GSK2656157, are in preclinical development and have shown promising results (66, 67). GSK2606414, an ATP-competitive PERK inhibitor, was identified through screening a kinase inhibitor library and refined for selectivity, exhibiting an in vitro IC50 of less than 1 nM. However, despite its sub-nanomolar potency, a concentration of 30 nM was required to fully inhibit PERK autophosphorylation under severe ER stress conditions (67). This inhibitor functions by blocking the PERK-eIF2α-ATF4-CHOP axis (68). Similarly, GSK2656157 inhibits PERK with an IC50 ranging from 10 to 30 nmol/l, as demonstrated by its suppression of PERK autophosphorylation, eIF2α phosphorylation, and subsequent reductions in ATF4 and CHOP levels in multiple cancer cell lines (69). In in vivo studies, oral administration of GSK2656157 in mice elicited a dose- and time-dependent response in the pancreas, while twice-daily administration resulted in dose-dependent inhibition of human tumor xenografts (69). Interestingly, a 2017 study (70) revealed that both GSK2606414 and GSK2656157 suppressed TNF-dependent cell death independently of PERK inactivation, emphasizing the need for cautious interpretation when using these inhibitors in studies involving ER stress, cell death, and inflammation (70).

Gene therapy approaches have also been explored to target PERK. Knockdown of PERK in tumor cells using small hairpin RNA (shRNA) induced a G2/M delay, impaired intracellular antioxidant regeneration, led to an accumulation of reactive oxygen species (ROS), and caused oxidative DNA damage (71). PERK deficiency activates the DNA damage checkpoint, disrupting cell cycle progression (71). However, the delivery of siRNA and shRNA to tumors remains a significant challenge.

Combination therapies targeting PERK in conjunction with other cancer pathways have also been investigated. Since PERK signaling interacts with multiple oncogenic pathways, combination strategies may be more effective than PERK inhibition alone. For example, the combination of the proteasome inhibitor bortezomib with GSK2606414 resulted in an additive toxic effect in multiple myeloma plasma cells (72). Additionally, AMG-44, a PERK inhibitor, demonstrated a synergistic anti-tumor effect when combined with an anti-PD-L1 immune checkpoint inhibitor in a B16 tumor-bearing mouse model (73).

Natural compounds have also been shown to modulate PERK activity and influence cancer progression. Quercetin, a flavonoid found in fruits and vegetables, alleviates ER stress by activating the PERK/CHOP signaling pathway, increasing eIF2α phosphorylation and CHOP expression (74). Sirtuin 1 (SIRT1) activation is also known to inhibit ER stress (75), and resveratrol, a polyphenol found in red grapes, activates SIRT1 while inhibiting the PERK-eIF2α-ATF4 pathway, ultimately inducing apoptosis in cancer cells (76). In the neuroblastoma cell line SH-SY5Y, curcumin pre-treatment suppressed PERK-dependent eIF2α phosphorylation, inhibited glycogen synthase kinase 3 β (GSK-3β) and ATF4 function, and prevented CHOP-induced apoptosis (77). Similarly, EGCG, a green tea polyphenol, promotes apoptosis in colorectal cancer cells via PERK activation (42). Withaferin A, a major bioactive compound in Withania somnifera, has also demonstrated anti-cancer properties by inducing apoptosis and G2/M cell cycle arrest in glioblastoma multiforme (GBM) cells through the ATF4-ATF3-CHOP axis (78).

IRE1 in Cancer

Elevated IRE1 levels are associated with poor cancer prognosis, as its increased activity can promote tumor survival, metastasis, and chemoresistance (79-81). In colon cancer models, reducing IRE1 expression significantly decreased cell proliferation both in vitro and in vivo (82). Similarly, inhibiting IRE1 RNase activity reduced breast cancer cell proliferation, predominantly through cell cycle arrest in the G1 phase, rather than by inducing cell death (82, 83). These findings underscore IRE1’s role in modulating cell cycle dynamics to support cancer progression. Additionally, the IRE1α-XBP1 pathway activates oncogenic c-MYC signaling (84), a key regulator of cellular growth, division, and metabolism (85). Further supporting the clinical significance of XBP1, a recent study in hepatocellular carcinoma (HCC) revealed that low cytoplasmic XBP1 protein expression was significantly correlated with vascular invasion, poor 5-year overall survival, and reduced disease-specific and disease-free survival. Kaplan-Meier survival analysis confirmed that low XBP1 levels independently predicted worse prognosis, emphasizing its potential as a prognostic biomarker in HCC. These findings suggest that targeting XBP1 could offer new therapeutic strategies for HCC and further reinforce its role in tumor progression and metastasis (86).

The IRE1α pathway plays a critical role in metastasis, particularly in highly invasive breast cancer cells. By interacting with non-muscle myosin IIA (NMIIA), IRE1α modulates cytoskeletal dynamics to enhance metastatic potential (85). IRE1α signaling also promotes Xbp1 mRNA splicing, a process essential for tumorigenicity and cell proliferation (87-89). Its interaction with heat shock protein 47 (HSP47) further intensifies this pathway, regulating mechanisms critical for metastasis (90). These interactions collectively drive tumor growth and enhance cancer cell invasiveness.

Recent research highlights the role of the IRE1α-XBP1s pathway in modulating antitumor immunity by affecting myeloid cells, T cells, and NK cells within the tumor microenvironment. In mouse melanoma models, IRE1α-XBP1s signaling is crucial for NK cell proliferation (91). In metastatic ovarian cancer models, sustained IRE1α-XBP1 activation in tumor-associated dendritic cells enhanced triglyceride biosynthesis and lipid droplet accumulation, impairing antigen presentation (92). Similarly, T cells from human ovarian cancer samples showed increased IRE1α-XBP1s activity, which correlated with reduced T cell infiltration and lower interferon-γ (IFN-γ) expression in tumors (93). Additionally, a study on the impact of microcystin-LR (MC-LR), a carcinogenic cyanobacterial toxin, in colorectal cancer models found that MC-LR activated the IRE1α/XBP1 pathway, promoting cell migration and M2 macrophage polarization. This activation, via the IRE1α/XBP1/HK2 axis, elevated lactate production, influenced macrophage behavior, and facilitated immune evasion, ultimately promoting tumor growth in the tumor microenvironment (94).

IRE1α signaling has been implicated in the development of chemoresistance in various cancers, where its activation promotes survival pathways that enable cancer cells to evade therapeutic effects. Sterol regulatory element-binding proteins (SREBPs), essential for lipid metabolism, also contribute to cancer progression through their involvement in ER stress and apoptosis (95). In non-small cell lung cancer, the IRE1α-XBP1 axis drives chemoresistance by promoting the expression of multidrug resistance-associated protein 1 (MRP1), a drug efflux transporter, through a c-Myc-SREBP1 signaling cascade (96). This pathway not only increases MRP1 levels to enhance drug extrusion but also involves c-Myc-induced SREBP1 binding directly to the MRP1 promoter, regulating its transcription (96). In colon cancer, chemoresistance similarly arises from the overexpression of ATP-binding cassette (ABC) transporters, including ABCB1, ABCC1, and ABCG2, which actively pump chemotherapy agents out of cancer cells (97). Exposure to anticancer drugs like 5-fluorouracil (5-FU) activates the IRE1α-XBP1 pathway, thereby increasing the expression of these transporters and contributing to drug resistance (98).

Approaches to target IRE1 in cancer. Targeting IRE1α signaling has emerged as a promising approach in cancer therapy, with strategies including gene therapy, small-molecule inhibitors, natural compounds, and combination therapies. Gene therapy has demonstrated the potential to modulate IRE1α activity and enhance anti-tumor immunity across various cancer models. In high-grade serous ovarian cancer, in vivo studies have shown that neutrophils within primary tumors activate IRE1α, facilitating tumor progression and immune evasion (99). Genetic deletion of IRE1α in neutrophils, achieved by crossing Ern1 f/f mice with the Mrp8-Cre strain for neutrophil-specific gene deletion, resulted in delayed tumor growth, improved T cell responses, and prolonged survival (99). Similarly, in prostate cancer, elevated IRE1α expression has been linked to immune escape mechanisms. Knockout of IRE1α in Myc-driven prostate cancer (Myc-CaP) cell lines using CRISPR-Cas9 genome editing significantly reduced tumor growth and remodeled the tumor microenvironment (100). In bladder cancer models, gene knockdown of IRE1α revealed its role in regulating tumor-mediated coagulation. Specifically, depletion of microtubule interacting and trafficking domain containing 1 (MITD1), a prognostic gene, led to increased tissue factor expression, promoting hypercoagulation. Inhibiting IRE1α signaling reversed this effect, highlighting its potential as a therapeutic target for modulating both tumor progression and cancer-associated thrombosis (101).

Small-molecule inhibitors targeting IRE1α have also been investigated as potential cancer therapeutics. MKC8866 effectively blocks IRE1α endonuclease activity in prostate cancer models, disrupting its signaling pathway (100). ORIN1001, a selective IRE1α inhibitor, is currently in early-phase clinical trials for treating solid tumors (102, 103). In colon cancer, inhibition of IRE1α RNase activity using 4μ8C suppressed ABC transporter expression and sensitized 5-Fluorouracil (5-FU)-resistant cells to chemotherapy (98). In vivo xenograft models further demonstrated that combining 4μ8C with 5-FU enhanced chemotherapy efficacy, supporting the potential of targeting the IRE1α-XBP1 axis to overcome drug resistance in breast and colon cancers (98). Additionally, inhibition of IRE1α using Kinase-Inhibiting RNase Attenuator 6 (KIRA6) in Drosophila models significantly reduced metastasis induced by environmental carcinogens, providing further evidence of its therapeutic value (104).

Natural compounds have also gained attention for their ability to modulate IRE1α activity in cancer therapy. Chalcone, a polyphenolic compound found in various plants, has demonstrated antitumor effects by inducing apoptosis through ER stress (105). Recent studies suggest that chalcone achieves this effect by sulfonating IRE1α via the Nicotinamide Adenine Dinucleotide Phosphate (NADPH) oxidase 4 pathway, promoting regulated IRE1α-dependent decay of mRNA while bypassing the conventional splicing of XBP1 (106).

Combination therapies incorporating IRE1α inhibition alongside immune checkpoint blockade have shown synergistic effects in both in vitro and in vivo prostate cancer models (98). Administration of MKC8866 in combination with anti-PD-1 therapy reversed immune suppression, enhanced cytotoxic CD8+ T cell activation, and reduced expression of immunosuppressive markers such as PD-1 and lymphocyte activation gene-3 (LAG3), which are associated with T cell exhaustion. This approach significantly improved the anti-tumor response, particularly in tumors with a cold immune profile (98). In high-grade serous ovarian cancer, genetic deletion of IRE1α in neutrophils, combined with anti-PD-1 therapy, further enhanced anti-tumor immunity by reducing immunosuppression and prolonging survival in treated mice. These findings suggest that targeting both the myeloid compartment, through IRE1α inhibition, and the adaptive immune system, via PD-1 blockade, could improve immunotherapy efficacy and provide durable protection against disease progression in ovarian cancer (105).

ATF6 in Cancer

ATF6 plays an essential role in cancer progression, primarily through its regulation of ER stress responses, which impact tumor growth, chemoresistance, and autophagy (107). Elevated levels of active ATF6 have been observed in various cancer types, highlighting its potential involvement in tumor development and progression (108, 109). In hepatocellular carcinoma cells, the activation of ATF6 was found to enhance the expression of genes associated with cell cycle progression and proliferation (110). In patients with colon cancer, increased ATF6 expression correlated with a shorter duration of disease-free survival (111). Additionally, in a triple-negative breast cancer (TNBC) cell model with a missense TP53 mutation, research by Sicari et al. demonstrated that mutant p53-induced activation of ATF6 was essential for maintaining cell viability and promoting invasive behavior (112). In contrast, ATF6 appears to suppress tumor growth in some cancers. In colorectal carcinoma cells, ATF6α and ATF6β knockout models revealed that the loss of ATF6α led to persistent activation of the IRE1 and PERK pathways, which unexpectedly promoted tumor growth (113). In acute myeloid leukemia, ATF6 activation in response to ER stress contributes to apoptosis, suggesting its potential as a pro-death factor under certain conditions (114). These findings suggest the complex role of ATF6 in tumorigenesis, with effects that vary depending on cancer type and context.

Emerging research underscores the pivotal role of ATF6 in autophagy (115, 116). For instance, chicoric acid induces autophagy in gastric cancer cells by upregulating LC3II and activating AMP-activated protein kinase (AMPK) signaling. This effect is notably associated with enhanced ER stress signals, including ATF6 activation (117). Similarly, fisetin activates ATF6 in pancreatic cancer cells through a p8-dependent mechanism (also known as the nuclear protein 1-dependent mechanism), highlighting the importance of ATF6 for the autophagy response under ER stress conditions (118). This suggests that targeting ATF6 could enhance therapeutic efficacy by modulating autophagy.

Beyond autophagy, ATF6 is implicated in shaping the immune microenvironment, impacting macrophage polarization and immune evasion. In oral squamous cell carcinoma, high ER stress markers, including ATF6, correlate with poor survival, as they promote exosomal PD-L1 secretion and M2 macrophage polarization, further enabling immune evasion (119). Some anticancer therapies affect ATF6, which in turn modulates these effects on the immune system. For instance, the nanoplatform PAZ@Fe-MOF, incorporating pazopanib, a multi-tyrosine kinase inhibitor used in breast cancer therapy, suppresses pro-tumorigenic pathways like ATF6-TGFBR1-SMAD3 (transforming growth factor beta receptor 1- mothers against decapentaplegic homolog 3), leading to a reduction in M2-like macrophages and limiting immune evasion (120). In a mouse model, the activation of ATF6 in colon epithelial cells led to intestinal dysbiosis and an innate immune response, which in turn facilitated tumor development (111).

ATF6 also plays a significant role in mediating chemoresistance (121). Interestingly, recent findings suggest that ATF6 plays a critical role in the degradation of mutant (MUT) TP53 in response to ER stress. ER stressors such as thapsigargin or tunicamycin were found to promote the lysosomal degradation of MUT TP53, an effect that was counteracted by ATF6 inhibition but not by targeting IRE1 or PERK. ATF6 activation was essential for maintaining lysosomal function and enabling both chaperone-mediated autophagy (CMA) and macroautophagy, key processes involved in MUT TP53 degradation under ER stress conditions. Mechanistically, ATF6 inhibition led to mTOR activation and downregulation of TFEB and LAMP1, further impairing lysosomal function and stabilizing MUT TP53 levels. These findings highlight the need to consider TP53 status when designing cancer therapies that combine ER stress-inducing agents with ATF6 inhibitors, as such strategies may be more effective in tumors harboring wild-type TP53 while potentially sustaining oncogenic properties of MUT TP53-expressing cancers (122). Additionally, the knockdown of ATF6 increased the sensitivity of osteosarcoma cells to chemotherapy-induced cell death. The activation of ATF6 was found to protect osteosarcoma cells from drug-induced apoptosis via pro-survival effectors like BiP, protein disulfide isomerase (PDI), and endoplasmic reticulum oxidoreductase 1. Analysis of primary osteosarcoma tumors revealed that patients with high levels of nuclear ATF6 had poorer chemotherapy responses, linking ATF6 expression directly to chemoresistance and poorer prognosis (123). In ovarian cancer, inhibition of the inhibitor of DNA binding 1 (ID1) was linked to the activation of ATF6, which promoted autophagy and contributed to chemoresistance against cisplatin and paclitaxel treatments (124).

ATF6 inhibition has been shown to impact cancer cell survival, particularly in dormant tumor cells. For example, ATF6 activation was critical for the survival of dormant squamous carcinoma cells; knockdown of ATF6 extended the survival of nude mice implanted with these quiescent tumor cells through the ATF6–Ras homolog enriched in brain (Rheb)– mTOR pathway (125).

Approaches to target ATF6 in cancer. Strategies to target ATF6 signaling in cancer have gained attention, with approaches including gene therapy, small-molecule inhibitors, natural compounds, and combination therapies. Gene therapy targeting ATF6 has shown promise in enhancing the efficacy of anticancer treatments. In primary liver cancer cells, the natural compound dihydroartemisinin (DHA), derived from artemisinin, exhibited potent anticancer activity, which was significantly enhanced upon ATF6 knockdown using siRNA (126). Similarly, in chemoresistant non-small cell lung cancer models, ATF6 signaling has been implicated in upregulating pro-inflammatory cytokines, creating a paracrine loop that recruits cancer-associated fibroblasts, thereby promoting tumor growth and chemoresistance. Suppression of ATF6 using siRNA disrupted this microenvironment by reducing cytokine production and fibroblast recruitment, ultimately enhancing chemotherapy sensitivity (127). Liposome-encapsulated ATF6 siRNA further suppressed xenograft tumor growth, diminished fibroblast recruitment, and increased cleaved caspase-3 levels, suggesting enhanced apoptosis (127). ATF6 knockdown has also been linked to reduced tumor proliferation and invasion. In triple-negative breast cancer, silencing ATF6 with siRNA decreased the expression of actin filament-associated protein 1 - antisense RNA 1 (AFAP1-AS1), a long non-coding RNA associated with tumorigenesis, indicating a potential regulatory interaction between ATF6 and AFAP1-AS1 in cancer progression (128, 129).

Small-molecule inhibitors of ATF6 have demonstrated potential in improving cancer treatment efficacy. In pancreatic ductal adenocarcinoma (PDAC), the ATF6 inhibitor Ceapin-A7 significantly enhanced radiosensitivity in radioresistant PDAC cells when combined with radiotherapy, leading to increased apoptosis and G1 phase cell cycle arrest, underscoring the role of the UPR in mediating radioresistance (130). Additionally, Ceapin-A7 increased the sensitivity of human bone osteosarcoma U2-OS cells to the ER stressor Tunicamycin, further supporting the potential of ATF6 inhibition in enhancing chemotherapy effectiveness (131). In chemoresistant lung cancer models, pharmacological inhibition of ATF6 using ABESF effectively suppressed cytokine transcription and prevented its nuclear translocation, disrupting its pro-tumorigenic role (127).

Natural compounds that modulate ATF6 activity have also demonstrated anticancer effects. Several natural compounds, including Norcantharidin, (−)-Agelasidine A, β-asarone, and dihydroartemisinin (DHA), have been shown to activate ATF6, contributing to their therapeutic potential. Norcantharidin (NCTD), known for its antitumor properties, inhibits cervical cancer cell viability and induces cell cycle arrest through ER stress activation. Its cytotoxic effects are reversed by the apoptosis inhibitor z-VAD-FMK and the ER stress inhibitor 4-phenylbutyric acid (4-PBA), confirming its reliance on ER stress pathways. Additionally, NCTD increases reactive oxygen species (ROS) production, reduces mitochondrial membrane potential, and upregulates apoptosis-related proteins, including ATF6 (132). (−)-Agelasidine A, a bioactive compound derived from the sea sponge Agelas nakamurai, exhibits potent anticancer effects in hepatocellular carcinoma (HCC) by inducing apoptosis through intrinsic and extrinsic pathways. It reduces cell viability in Hep3B and HepG2 liver cancer cells by activating caspases, disrupting mitochondrial function, and upregulating ER stress markers, including ATF6. Treatment with 4-PBA confirms that its anticancer effects depend on ER stress activation (133). β-asarone (β-as), extracted from Acorus calamus, exhibits anticancer effects in bladder cancer by inhibiting metastasis and epithelial-to-mesenchymal transition (EMT). β-as treatment activates ATF6, leading to enhanced Golgi cleavage and nuclear localization of ATF6. Silencing ATF6 reverses the anti-metastatic effects of β-as, highlighting its role in cancer progression (134). DHA, an artemisinin derivative, induces ferroptosis in primary liver cancer by activating all three branches of the UPR, including ATF6. Knockdown of ATF6, ATF4, and XBP1 suppresses DHA-induced ferroptosis, confirming ATF6’s essential role in mediating its cytotoxic effects (126).

Combination therapies integrating ATF6-targeting approaches with immunomodulatory strategies have shown potential in cancer treatment. A recent preclinical study combined the Valosin-Containing Protein (VCP)/p97 inhibitor CB-5083 with miR-142 (targeting PD-L1) and resiquimod (R848), an immunoadjuvant, delivered via solid lipid nanoparticles. This combination effectively targeted the VCP/BiP/ATF6 signaling pathway, demonstrating in vitro and in vivo efficacy against PDAC. The treatment enhanced T cell infiltration, promoted dendritic cell maturation, and reduced immunosuppressive cell populations, including regulatory T cells and tumor-associated macrophages, thereby improving the anti-tumor immune response (135). These different treatment strategies highlight how the main parts of the UPR can be targeted in cancer, as shown in Figure 2.

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Figure 2.

Targeting the unfolded protein response pathways for cancer therapy. Overview of pharmacological inhibitors targeting key components of the unfolded protein response (UPR) pathways activated during endoplasmic reticulum (ER) stress. The figure illustrates small-molecule inhibitors and monoclonal antibodies designed to disrupt signaling through PKR-like ER Kinase (PERK), inositol-requiring enzyme 1 (IRE1), and activating transcription factor 6 (ATF6) pathways. These inhibitors represent promising therapeutic strategies for modulating ER stress responses in cancer.