Molecular pathways: the PERKs and pitfalls of targeting the unfolded protein response in cancer

Garlic Extract Participates in the Proliferation and Apoptosis of Nonsmall Cell Lung Cancer Cells Via Endoplasmic Reticulum Stress Pathway

Objective

To investigate the effect of garlic extract (GE) on the proliferation and apoptosis of cell lines A549 and H1299 in lung cancer (LC).

Methods

A549 and H1299 cells with well-developed logarithmic growth were added with GE at a concentration of 0 μg/ml, 25 μg/ml, 50 μg/M, 75 μg/ml, and 100 μg/ml, respectively. The inhibition of A549 cell proliferation was detected using CCK-8 after cultured for 24 h, 48 h, and 72 h. The apoptosis of A549 cells was analyzed via flow cytometry (FCM) after 24 h of cultivation. In vitro migration of A549 and H1299 cells was determined by cell wound scratch assay after 0 h and 24 h culture. The caspase-3 and caspase-9 protein expression levels in A549 and H1299 cells were evaluated through western blot after 24 h of cultivation.

Results

Colony formation and EdU assays revealed that Z-ajoene could inhibit cell viability and cell proliferation in NSCLC cells. After 24 h culture, there was no significant difference in the proliferation rate of A549 and H1299 cells with different GE concentrations (P > 0.05). A remarkable proliferation rate difference emerged between A549 and H1299 cells with different GE concentrations after 48 and 72 hours of cultivation. The proliferation rate of A549 and H1299 cells in the experiment group was significantly lower than that in the control group. With an elevated level of GE concentration, the proliferation rate of A549 and H1299 cells decreased (P < 0.05) while the apoptotic rate increased continuously.

Conclusion

GE could exert toxic effects on A549 and H1299 cells, inhibit cell proliferation, promote apoptosis, and attenuate cell migration. Meanwhile, it might induce apoptosis of A549 and H1299 cells through the caspase signal pathway, which is positively correlated with the mass action concentration and is expected to be a new drug for LC treatment.

Development and Validation of an Inflammatory Response-Related Gene Signature for Predicting the Prognosis of Pancreatic Adenocarcinoma

Pancreatic adenocarcinoma (PAAD) is a highly dangerous malignant tumor of the digestive tract, and difficult to diagnose, treat, and predict the prognosis. As we all know, tumor and inflammation can affect each other, and thus the inflammatory response in the microenvironment can be used to affect the prognosis. So far, the prognostic value of inflammatory response-related genes in PAAD is still unclear. Therefore, this study aimed to explore the inflammatory response-related genes for predicting the prognosis of PAAD. In this study, the mRNA expression profiles of PAAD patients and the corresponding clinical characteristics data of PAAD patients were downloaded from the public database. The least absolute shrinkage and selection operator (LASSO) Cox analysis model was used to identify and construct the prognostic gene signature in The Cancer Genome Atlas (TCGA) cohort. The PAAD patients used for verification are from the International Cancer Genome Consortium (ICGC) cohort. The Kaplan-Meier method was used to compare the overall survival (OS) between the high- and low-risk groups. Univariate and multivariate Cox analyses were performed to identify the independent predictors of OS. Gene set enrichment analysis (GSEA) was performed to obtain gene ontology (GO) terms and the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways, and the correlation between gene expression and immune infiltrates was investigated via single sample gene set enrichment analysis (ssGSEA). The GEPIA database was performed to examine prognostic genes in PAAD. LASSO Cox regression analysis was used to construct a model of inflammatory response-related gene signature. Compared with the low-risk group, patients in the high-risk group had significantly lower OS. The receiver operating characteristic curve (ROC) analysis confirmed the signature's predictive capacity. Multivariate Cox analysis showed that risk score is an independent predictor of OS. Functional analysis shows that the immune status between the two risk groups is significantly different, and the cancer-related pathways were abundant in the high-risk group. Moreover, the risk score is significantly related to tumor grade, stage, and immune infiltration types. It was also obtained that the expression level of prognostic genes was significantly correlated with the sensitivity of cancer cells to anti-tumor drugs. In addition, there are significant differences in the expression of PAAD tissues and adjacent non-tumor tissues. The novel signature constructed from five inflammatory response-related genes can be used to predict prognosis and affect the immune status of PAAD. In addition, suppressing these genes may be a treatment option.

PERK signaling through C/EBPδ contributes to ER stress-induced expression of immunomodulatory and tumor promoting chemokines by cancer cells

Cancer cells experience endoplasmic reticulum (ER) stress due to activated oncogenes and conditions of nutrient deprivation and hypoxia. The ensuing unfolded protein response (UPR) is executed by ATF6, IRE1 and PERK pathways. Adaptation to mild ER stress promotes tumor cell survival and aggressiveness. Unmitigated ER stress, however, will result in cell death and is a potential avenue for cancer therapies. Because of this yin-yang nature of ER stress, it is imperative that we fully understand the mechanisms and dynamics of the UPR and its contribution to the complexity of tumor biology. The PERK pathway inhibits global protein synthesis while allowing translation of specific mRNAs, such as the ATF4 transcription factor. Using thapsigargin and tunicamycin to induce acute ER stress, we identified the transcription factor C/EBPδ (CEBPD) as a mediator of PERK signaling to secretion of tumor promoting chemokines. In melanoma and breast cancer cell lines, PERK mediated early induction of C/EBPδ through ATF4-independent pathways that involved at least in part Janus kinases and the STAT3 transcription factor. Transcriptional profiling revealed that C/EBPδ contributed to 20% of thapsigargin response genes including chaperones, components of ER-associated degradation, and apoptosis inhibitors. In addition, C/EBPδ supported the expression of the chemokines CXCL8 (IL-8) and CCL20, which are known for their tumor promoting and immunosuppressive properties. With a paradigm of short-term exposure to thapsigargin, which was sufficient to trigger prolonged activation of the UPR in cancer cells, we found that conditioned media from such cells induced cytokine expression in myeloid cells. In addition, activation of the CXCL8 receptor CXCR1 during thapsigargin exposure supported subsequent sphere formation by cancer cells. Taken together, these investigations elucidated a novel mechanism of ER stress-induced transmissible signals in tumor cells that may be particularly relevant in the context of pharmacological interventions.

Store-Operated Calcium Entry: Shaping the Transcriptional and Epigenetic Landscape in Pancreatic Cancer

Pancreatic ductal adenocarcinoma (PDAC) displays a particularly poor prognosis and low survival rate, mainly due to late diagnosis and high incidence of chemotherapy resistance. Genomic aberrations, together with changes in the epigenomic profile, elicit a shift in cellular signaling response and a transcriptional reprograming in pancreatic tumors. This endows them with malignant attributes that enable them to not only overcome chemotherapeutic challenges, but to also attain diverse oncogenic properties. In fact, certain genetic amplifications elicit a rewiring of calcium signaling, which can confer ER stress resistance to tumors while also aberrantly activating known drivers of oncogenic programs such as NFAT. While calcium is a well-known second messenger, the transcriptional programs driven by aberrant calcium signaling remain largely undescribed in pancreatic cancer. In this review, we focus on calcium-dependent signaling and its role in epigenetic programs and transcriptional regulation. We also briefly discuss genetic aberration events, exemplifying how genetic alterations can rewire cellular signaling cascades, including calcium-dependent ones.

QRICH1 dictates the outcome of ER stress through transcriptional control of proteostasis

Tissue homeostasis is perturbed in a diversity of inflammatory pathologies. These changes can elicit endoplasmic reticulum (ER) stress, protein misfolding, and cell death. ER stress triggers the unfolded protein response (UPR), which can promote recovery of ER proteostasis and cell survival or trigger programmed cell death. Here, we leveraged single-cell RNA sequencing to define dynamic transcriptional states associated with the adaptive versus terminal UPR in the mouse intestinal epithelium. We integrated these transcriptional programs with genome-scale CRISPR screening to dissect the UPR pathway functionally. We identified QRICH1 as a key effector of the PERK-eIF2α axis of the UPR. QRICH1 controlled a transcriptional program associated with translation and secretory networks that were specifically up-regulated in inflammatory pathologies. Thus, QRICH1 dictates cell fate in response to pathological ER stress.

Traditional Chinese medicine compounds regulate autophagy for treating neurodegenerative disease: A mechanism review

Neurodegenerative diseases (NDs) are common chronic diseases related to progressive damage of the nervous system. Globally, the number of people with an ND is dramatically increasing consistent with the fast aging of society and one of the common features of NDs is the abnormal aggregation of diverse proteins. Autophagy is the main process by which misfolded proteins and damaged organelles are removed from cells. It has been found that the impairment of autophagy is associated with many NDs, suggesting that autophagy has a vital role in the neurodegeneration process. Recently, more and more studies have reported that autophagy inducers display a protective role in different ND experimental models, suggesting that enhancement of autophagy could be a potential therapy for NDs. In this review, the evidence for beneficial effects of traditional Chinese medicine (TCM) regulate autophagy in the models of Alzheimer's disease (AD), Parkinson's disease (PD), and other NDs are presented and common autophagy-related mechanisms are identified. The results demonstrate that TCM which regulate autophagy are potential therapeutic candidates for ND treatment.

The Unfolded Protein Response Mediator PERK Governs Myeloid Cell-Driven Immunosuppression in Tumors through Inhibition of STING Signaling

The primary mechanisms supporting immunoregulatory polarization of myeloid cells upon infiltration into tumors remain largely unexplored. Elucidation of these signals could enable better strategies to restore protective anti-tumor immunity. Here, we investigated the role of the intrinsic activation of the PKR-like endoplasmic reticulum (ER) kinase (PERK) in the immunoinhibitory actions of tumor-associated myeloid-derived suppressor cells (tumor-MDSCs). PERK signaling increased in tumor-MDSCs, and its deletion transformed MDSCs into myeloid cells that activated CD8⁺ T cell-mediated immunity against cancer. Tumor-MDSCs lacking PERK exhibited disrupted NRF2-driven antioxidant capacity and impaired mitochondrial respiratory homeostasis. Moreover, reduced NRF2 signaling in PERK-deficient MDSCs elicited cytosolic mitochondrial DNA elevation and, consequently, STING-dependent expression of anti-tumor type I interferon. Reactivation of NRF2 signaling, conditional deletion of STING, or blockade of type I interferon receptor I restored the immunoinhibitory potential of PERK-ablated MDSCs. Our findings demonstrate the pivotal role of PERK in tumor-MDSC functionality and unveil strategies to reprogram immunosuppressive myelopoiesis in tumors to boost cancer immunotherapy.

ULK1 inhibition as a targeted therapeutic strategy for FLT3-ITD-mutated acute myeloid leukemia

Background

In acute myeloid leukemia (AML), internal tandem duplication mutations in the FLT3 tyrosine kinase receptor (FLT3-ITD) are associated with a dismal outcome. Although uncoordinated 51-like kinase 1 (ULK1), which plays a central role in the autophagy pathway, has emerged as a novel therapeutic target for various cancers, its role in FLT3-ITD AML remains elusive. In this study, we evaluated the effects of ULK1 inhibition on leukemia cell death in FLT3-ITD AML.

Method

We evaluated ULK1 expression and the levels of apoptosis and autophagy following ULK1 inhibition in FLT3-ITD AML cell lines and investigated the mechanism underlying apoptosis induced by ULK1 inhibition. Statistical analysis was performed using GraphPad Prism 4.0 (GraphPad Software Inc).

Results

FLT3-ITD AML cells showed significantly higher ULK1 expression than FLT3-wild-type (WT) AML cells. Two ULK1 inhibitors, MRT 68921 and SBI-0206965, induced apoptosis in FLT3-ITD AML cells, with relatively minimal effects on FLT3-WT AML cells and normal CD34-positive cells. Apoptosis induction by ULK1 inhibition was associated with caspase pathway activation. Interestingly, ULK1 inhibition paradoxically also induced autophagy, showing synergistic interaction with autophagy inhibitors. Hence, autophagy may act as a prosurvival mechanism in FLT3-ITD AML cells. FLT3-ITD protein degradation and inhibition of the ERK, AKT, and STAT5 pathways were also observed in FLT3-ITD AML cells following treatment with ULK1 inhibitors.

Conclusion

ULK1 is a viable drug target and ULK1 inhibition may represent a promising therapeutic strategy against FLT3-ITD AML.

PERK Inhibition Mitigates Restenosis and Thrombosis: A Potential Low-Thrombogenic Antirestenotic Paradigm

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Drug-eluting stents impede neointimal smooth muscle cell hyperplasia but exacerbate endothelial cell dysfunction and thrombogenicity. It has been a challenge to identify a common target to inhibit both. Findings in this study suggest PERK as such a target.

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A PERK inhibitor administered either via an endovascular (in biomimetic nanocarriers) or perivascular (in hydrogel) route effectively mitigated neointimal hyperplasia in rats.

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Oral gavage of the PERK inhibitor partially preserved the normal blood flow in a mouse model of induced thrombosis.

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Dampening PERK activity inhibited STAT3 while activating SRF in smooth muscle cells, and also reduced prothrombogenic tissue factor and growth impairment of endothelial cells.

Developing endothelial-protective, nonthrombogenic antirestenotic treatments has been a challenge. A major hurdle to this has been the identification of a common molecular target in both smooth muscle cells and endothelial cells, inhibition of which blocks dysfunction of both cell types. The authors’ findings suggest that the PERK kinase could be such a target. Importantly, PERK inhibition mitigated both restenosis and thrombosis in preclinical models, implicating a low-thrombogenic antirestenotic paradigm.

Dual PI3K/mTOR Inhibitor, XL765, suppresses glioblastoma growth by inducing ER stress-dependent apoptosis

Background: Deregulated phosphoinositide 3-kinase (PI3K)/mTOR signaling commonly exists in glioblastoma (GBM), making this axis an attractive target for therapeutic manipulation. A recent dual inhibitor of PI3K/mTOR pathway, XL765, exhibited an attractive suppression effect on GBM tumor growth. However, the exact functional mechanisms of tumor suppression mediated by XL765 have not yet been fully characterized.

Purpose: In this study, we took efforts to assess the effects of PI3K/mTOR blockade by XL765 on GBM growth in vitro and in vivo.

Methods: We analyzed the cytotoxicity of XL765 in three different GBM cell lines, A172, U87MG, and T98G, by using Hoechst 33258 (Invitrogen), Annexin V/propidium iodide (PI), as well as Cell Counting Kit -8 (CCK‐8) assay. We also used A172 xenograft model to study the effect of XL765 in vivo.

Results: We found that XL765 inhibits GBM viability with a wide range of potencies. Importantly, XL765 suppressed GBM cell growth by inducing endoplasmic reticulum (ER) stress dependent apoptosis. The activation of CHOP/DR5 pathway by XL765 induced ER stress is responsible for the induction of apoptosis. Moreover, the inhibition of mTOR signal by XL765 is the major source of ER stress, rather than inhibition of PI3K. At last, we demonstrated that combination of XL765 with GMB chemotherapeutic drug, temozolomide (TMZ), can achieved better therapy effect in vitro and in vivo.

Conclusion: Overall, our data show that targeting PI3K/mTOR by XL765 is a promising therapeutic strategy to relieve tumor burden in GBM patients.

Ilamycin E, a natural product of marine actinomycete, inhibits triple-negative breast cancer partially through ER stress-CHOP-Bcl-2

Breast cancer is the most commonly diagnosed cancer and the leading cause of cancer death among women in the worldwide. Triple-negative breast cancer (TNBC) has a poor clinical outcome. The antitumor efficacy of Ilamycins, natural products with anti-tuberculosis activity isolated from deep sea-derived Streptomyces atratus, in TNBC has not been investigated, and the mechanisms remain elusive. Here, we demonstrated that Ilamycin-E, but not -F, decreases cell viability, inhibits G1/S cell cycle progression, and promotes apoptosis in the TNBC cell lines HCC1937 and MDA-MB-468. Ilamycin E promotes apoptosis via activation of endoplasmic reticulum (ER) stress, increasing the expression of CHOP, and down-regulating the expression of anti-apoptotic protein Bcl-2. Depletion of CHOP or overexpression of Bcl2 significantly rescued Ilamycin E-induced apoptosis. These findings indicate that Ilamycin E has anti-cancer activity in TNBC.

Klotho suppresses colorectal cancer through modulation of the unfolded protein response

Klotho is an anti-aging transmembrane protein, which can be shed and function as a hormone. Accumulating data indicate klotho as a tumor suppressor in a wide array of malignancies and indicate the subdomain KL1 as the active region of the protein. We aimed to study the role of klotho as a tumor suppressor in colorectal cancer. Bioinformatics analyses of TCGA datasets indicated reduced klotho mRNA levels in human colorectal cancer, along with negative regulation of klotho expression by hypermethylation of the promoter and 1st exon, and hypomethylation of an area within the gene. Overexpression or treatment with klotho or KL1 inhibited proliferation of colorectal cancer cells in vitro. The in vivo activity of klotho and KL1 was examined using two models recapitulating development of tumors in the normal colonic environment of immune-competent mice. Treatment with klotho inhibited formation of colon polyps induced by the carcinogen azoxymethane, and KL1 treatment slowed growth of orthotopically-implanted colorectal tumors. Gene expression array revealed that klotho and KL1 expression enhanced the unfolded protein response (UPR) and this was further established by increased levels of spliced XBP1, GRP78 and phosphorylated-eIF2α. Furthermore, attenuation of the UPR partially abrogated klotho tumor suppressor activity. In conclusion, this study indicates klotho as a tumor suppressor in colorectal cancer and identifies, for the first time, the UPR as a pathway mediating klotho activities in cancer. These data suggest that administration of exogenous klotho or KL1 may serve as a novel strategy for prevention and treatment of colorectal cancer.

Codon misreading tRNAs promote tumor growth in mice

Deregulation of tRNAs, aminoacyl-tRNA synthetases and tRNA modifying enzymes are common in cancer, raising the hypothesis that protein synthesis efficiency and accuracy (mistranslation) are compromised in tumors. We show here that human colon tumors and xenograft tumors produced in mice by two epithelial cancer cell lines mistranslate 2- to 4-fold more frequently than normal tissue. To clarify if protein mistranslation plays a role in tumor biology, we expressed mutant Ser-tRNAs that misincorporate Ser-at-Ala (frequent error) and Ser-at-Leu (infrequent error) in NIH3T3 cells and investigated how they responded to the proteome instability generated by the amino acid misincorporations. There was high tolerance to both misreading tRNAs, but the Ser-to-Ala misreading tRNA was a more potent inducer of cell transformation, stimulated angiogenesis and produced faster growing tumors in mice than the Ser-to-Leu misincorporating tRNA. Upregulation of the Akt pathway and the UPR were also observed. Most surprisingly, the relative expression of both misreading tRNAs increased during tumor growth, suggesting that protein mistranslation is advantageous in cancer contexts. These data highlight new features of protein synthesis deregulation in tumor biology.

Tannic Acid Induces Endoplasmic Reticulum Stress-Mediated Apoptosis in Prostate Cancer

Endoplasmic reticulum (ER) stress is an intriguing target with significant clinical importance in chemotherapy. Interference with ER functions can lead to the accumulation of unfolded proteins, as detected by transmembrane sensors that instigate the unfolded protein response (UPR). Therefore, controlling induced UPR via ER stress with natural compounds could be a novel therapeutic strategy for the management of prostate cancer. Tannic acid (a naturally occurring polyphenol) was used to examine the ER stress mediated UPR pathway in prostate cancer cells. Tannic acid treatment inhibited the growth, clonogenic, invasive, and migratory potential of prostate cancer cells. Tannic acid demonstrated activation of ER stress response (Protein kinase R-like endoplasmic reticulum kinase (PERK) and inositol requiring enzyme 1 (IRE1)) and altered its regulatory proteins (ATF4, Bip, and PDI) expression. Tannic acid treatment affirmed upregulation of apoptosis-associated markers (Bak, Bim, cleaved caspase 3, and cleaved PARP), while downregulation of pro-survival proteins (Bcl-2 and Bcl-xL). Tannic acid exhibited elevated G₁ population, due to increase in p18INK4C and p21WAF1/CIP1 expression, while cyclin D1 expression was inhibited. Reduction of MMP2 and MMP9, and reinstated E-cadherin signifies the anti-metastatic potential of this compound. Altogether, these results demonstrate that tannic acid can promote apoptosis via the ER stress mediated UPR pathway, indicating a potential candidate for cancer treatment.

Noncanonical SQSTM1/p62-Nrf2 pathway activation mediates proteasome inhibitor resistance in multiple myeloma cells via redox, metabolic and translational reprogramming

Multiple Myeloma (MM) is a B-cell malignancy characterized by the accumulation of clonal plasma cells in the bone marrow, with drug resistance being a major cause of therapeutic failure. We established a carfilzomib-resistant derivative of the LP-1 MM cell line (LP-1/Cfz) and found that the transcription factor NF-E2 p45-related factor 2 (Nrf2; gene symbol NFE2L2) contributes to carfilzomib resistance. The mechanism of Nrf2 activation involved enhanced translation of Nrf2 as well as its positive regulator, the autophagy receptor sequestosome 1 (SQSTM1)/p62. The eukaryotic translation initiation factor gene EIF4E3 was among the Nrf2 target genes upregulated in LP-1/Cfz cells, suggesting existence of a positive feedback loop. In line with this, we found that siRNA knockdown of eIF4E3 decreased Nrf2 protein levels. On the other hand, elevated SQSTM1/p62 levels were due at least in part to activation of the PERK-eIF2α pathway. LP-1/Cfz cells had decreased levels of reactive oxygen species as well as elevated levels of fatty acid oxidation and prosurvival autophagy. Genetic and pharmacologic inhibition of the Nrf2-EIF4E3 axis or the PERK-eIF2α pathway, disruption of redox homeostasis or inhibition of fatty acid oxidation or autophagy conferred sensitivity to carfilzomib. Our findings were supported by clinical data where increased EIF4E3 expression was predictive of Nrf2 target gene upregulation in a subgroup of patients with chemoresistant minimal residual disease and relapsed/refractory MM. Thus, our data offer a preclinical rationale for including inhibitors of the SQSTM1/p62-Nrf2 pathway to the treatment regimens for certain advanced stage MM patients.

mTOR inhibitors activate PERK signaling and favor viability of gastrointestinal neuroendocrine cell lines

mTOR and Unfolded Protein Response (UPR) are two signaling pathways frequently activated in cancer cells. The mTOR pathway has been shown to be up-regulated in most gastroenteropancreatic neuroendocrine tumors. In contrast, little is known about the UPR status in neoplastic neuroendocrine cells. However, these hormone-producing cells are likely to present distinctive adaptations of this pathway, as other secretory cells. We therefore analyzed the status of the three axes of UPR and their relation to mTOR pathway in two gastrointestinal neuroendocrine tumors (GI-NET) cell lines STC-1 and GluTag. At baseline, pharmacological inducers activate the three arms of UPR: PERK, ATF6 and IRE1. Although hypoxia stimulates the PERK, ATF6 and IRE-1 pathways in both cell lines, glucose depletion activates UPR only in STC-1 cell line. Strikingly, P-p70S6K1 increases concomitantly to P-PERK and BiP in response to thapsigargin treatment, glucose depletion or hypoxia. We found that different mTOR inhibitors activate the PERK signaling pathway. To confirm that mTOR inhibition modulates PERK activation, we inhibited PERK and showed that it decreased cell viability when associated to mTOR inhibition, indicating that mTOR drives a PERK-dependent survival pathway. In conclusion, in GI-NET cell lines, UPR signaling is functional and PERK arm is induced by mTOR inhibition. These observations open up new perspectives for therapeutic strategies: the crosstalk between mTOR and UPR might contribute to the resistance to mTOR inhibitors and could be targeted by mTOR and PERK inhibitors in combination therapy.

Activating transcription factor 3 promotes loss of the acinar cell phenotype in response to cerulein-induced pancreatitis in mice

Pancreatitis is a debilitating disease of the exocrine pancreas that, under chronic conditions, is a major susceptibility factor for pancreatic ductal adenocarcinoma (PDAC). Although down-regulation of genes that promote the mature acinar cell fate is required to reduce injury associated with pancreatitis, the factors that promote this repression are unknown. Activating transcription factor 3 (ATF3) is a key mediator of the unfolded protein response, a pathway rapidly activated during pancreatic insult. Using chromatin immunoprecipitation followed by next-generation sequencing, we show that ATF3 is bound to the transcriptional regulatory regions of >30% of differentially expressed genes during the initiation of pancreatitis. Of importance, ATF3-dependent regulation of these genes was observed only upon induction of pancreatitis, with pathways involved in inflammation, acinar cell differentiation, and cell junctions being specifically targeted. Characterizing expression of transcription factors that affect acinar cell differentiation suggested that acinar cells lacking ATF3 maintain a mature cell phenotype during pancreatitis, a finding supported by maintenance of junctional proteins and polarity markers. As a result, Atf3-/- pancreatic tissue displayed increased tissue damage and inflammatory cell infiltration at early time points during injury but, at later time points, showed reduced acinar-to-duct cell metaplasia. Thus our results reveal a critical role for ATF3 as a key regulator of the acinar cell transcriptional response during injury and may provide a link between chronic pancreatitis and PDAC.

ATF4 regulates CCL2 expression to promote endometrial cancer growth by controlling macrophage infiltration

Activating transcription factor 4 (ATF4), an endoplasmic reticulum stress-inducible transcription factor, plays important roles in cancer progression and resistance to therapy. However, no report is available about its roles in endometrial cancer (EC). In this study, we found that ATF4 is commonly expressed in EC cell lines. Loss-of-function studies in two EC cell lines showed that ATF4 knockdown suppresses tumor growth of EC in vivo without influencing cell proliferation in vitro. And xenograft tumors derived from ATF4-knockdown cells had reduced M2 macrophage infiltration. In clinical specimens, ATF4-high expressing tumors indeed contained more macrophage infiltration compared to those with lower ATF4 expression. Moreover, we identified that ATF4-mediated chemokine CCL2 expression ultimately results in macrophage infiltration and tumor growth of EC. Taken together, our findings suggest that ATF4 contributes to tumor growth of EC by promoting CCL2 and subsequent recruitment of macrophage, and that ATF4/CCL2 axis might be a potential therapeutic target for EC.

Emerging Role of the Unfolded Protein Response in Tumor Immunosurveillance

Disruption of endoplasmic reticulum (ER) homeostasis results in ER stress and activation of the unfolded protein response (UPR). This response alleviates cell stress, and is activated in both tumor cells and tumor infiltrating immune cells. The UPR plays a dual function in cancer biology, acting as a barrier to tumorigenesis at the premalignant stage, while fostering cancer maintenance in established tumors. In infiltrating immune cells, the UPR has been involved in both immunosurveillance and immunosuppressive functions. This review aims to decipher the role of the UPR at different stages of tumorigenesis and how the UPR shapes the balance between immunosurveillance and immune escape. This knowledge may improve existing UPR-targeted therapies and the design of novel strategies for cancer treatment.

The PERK arm of the unfolded protein response regulates satellite cell-mediated skeletal muscle regeneration

Regeneration of skeletal muscle in adults is mediated by satellite stem cells. Accumulation of misfolded proteins triggers endoplasmic reticulum stress that leads to unfolded protein response (UPR). The UPR is relayed to the cell through the activation of PERK, IRE1/XBP1, and ATF6. Here, we demonstrate that levels of PERK and IRE1 are increased in satellite cells upon muscle injury. Inhibition of PERK, but not the IRE1 arm of the UPR in satellite cells inhibits myofiber regeneration in adult mice. PERK is essential for the survival and differentiation of activated satellite cells into the myogenic lineage. Deletion of PERK causes hyper-activation of p38 MAPK during myogenesis. Blocking p38 MAPK activity improves the survival and differentiation of PERK-deficient satellite cells in vitro and muscle formation in vivo. Collectively, our results suggest that the PERK arm of the UPR plays a pivotal role in the regulation of satellite cell homeostasis during regenerative myogenesis.

Unfolding anti-tumor immunity: ER stress responses sculpt tolerogenic myeloid cells in cancer

Established tumors build a stressful and hostile microenvironment that blocks the development of protective innate and adaptive immune responses. Different subsets of immunoregulatory myeloid populations, including dendritic cells, myeloid-derived suppressor cells (MDSCs) and macrophages, accumulate in the stressed tumor milieu and represent a major impediment to the success of various forms of cancer immunotherapy. Specific conditions and factors within tumor masses, including hypoxia, nutrient starvation, low pH, and increased levels of free radicals, provoke a state of “endoplasmic reticulum (ER) stress” in both malignant cells and infiltrating myeloid cells. In order to cope with ER stress, cancer cells and tumor-associated myeloid cells activate an integrated signaling pathway known as the Unfolded Protein Response (UPR), which promotes cell survival and adaptation under adverse environmental conditions. However, the UPR can also induce cell death under unresolved levels of ER stress. Three branches of the UPR have been described, including the activation of the inositol-requiring enzyme 1 (IRE1), the pancreatic ER kinase (PKR)-like ER kinase (PERK), and the activating transcription factor 6 (ATF6). In this minireview, we briefly discuss the role of ER stress and specific UPR mediators in tumor development, growth and metastasis. In addition, we describe how sustained ER stress responses operate as key mediators of chronic inflammation and immune suppression within tumors. Finally, we discuss multiple pharmacological approaches that overcome the immunosuppressive effect of the UPR in tumors, and that could potentially enhance the efficacy of cancer immunotherapies by reprogramming the function of tumor-infiltrating myeloid cells.

Endoplasmic reticulum-mediated unfolded protein response and mitochondrial apoptosis in cancer

Abrogation of endoplasmic reticulum (ER) protein folding triggered by exogenous or endogenous factors, stimulates a cellular stress response, termed ER stress. ER stress re-establishes ER homeostasis through integrated signaling termed the ER-unfolded protein response (UPR^ER). In the presence of severe toxic or prolonged ER stress, the pro-survival function of UPR^ER is transformed into a lethal signal transmitted to and executed through mitochondria. Mitochondria are key for both apoptotic and autophagic cell death. Thus ER is vital in sensing and coordinating stress pathways to maintain overall physiological homeostasis. However, this function is deregulated in cancer, resulting in resistance to apoptosis induction in response to various stressors including therapeutic agents. Here we review the connections between ER stress and mitochondrial apoptosis, describing potential cancer therapeutic targets.

Getting the better of ER stress

Research over the past few years has highlighted the ability of the unfolded protein response (UPR) to minimize the deleterious effects of accumulated misfolded proteins under both physiological and pathological conditions. The endoplasmic reticulum (ER) adapts to endogenous and exogenous stressors by expanding its protein-folding capacity and by stimulating protective processes such as autophagy and antioxidant responses. Although it is clear that severe ER stress can elicit cell death, several recent studies have shown that low levels of ER stress may actually be beneficial to cells by eliciting an adaptive UPR that 'preconditions' the cell to a subsequent lethal insult; this process is called ER hormesis. The findings have important implications for the treatment of a wide variety of diseases associated with defective proteostasis, including neurodegenerative diseases, diabetes, and cancer. Here, we review the physiological and pathological functions of the ER, with a particular focus on the molecular mechanisms that lead to ER hormesis and cellular protection, and discuss the implications for disease treatment.