Endoplasmic Reticulum Stress and Metabolism in Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is the most common type of primary liver cancer, accounting for 85% to 90% of all liver cancer cases. It is a hepatocyte-derived primary tumor, causing 550,000 deaths per year, ranking it as one of the most common cancers worldwide. The liver is a highly metabolic organ with multiple functions, including digestion, detoxification, breakdown of fats, and production of bile and cholesterol, in addition to storage of vitamins, glycogen, and minerals, and synthesizing plasma proteins and clotting factors. Due to these fundamental and diverse functions, the malignant transformation of hepatic cells can have a severe impact on the liver’s metabolism. Furthermore, tumorigenesis is often accompanied by activation of the endoplasmic reticulum (ER) stress pathways, which are known to be highly intertwined with several metabolic pathways. Because HCC is characterized by changes in the metabolome and by an aberrant activation of the ER stress pathways, the aim of this review was to summarize the current knowledge that links ER stress and metabolism in HCC, thereby focusing on potential therapeutic targets. Hepatocellular carcinoma (HCC) is the most common type of primary liver cancer, accounting for 85% to 90% of all liver cancer cases. It is a hepatocyte-derived primary tumor, causing 550,000 deaths per year, ranking it as one of the most common cancers worldwide. The liver is a highly metabolic organ with multiple functions, including digestion, detoxification, breakdown of fats, and production of bile and cholesterol, in addition to storage of vitamins, glycogen, and minerals, and synthesizing plasma proteins and clotting factors. Due to these fundamental and diverse functions, the malignant transformation of hepatic cells can have a severe impact on the liver’s metabolism. Furthermore, tumorigenesis is often accompanied by activation of the endoplasmic reticulum (ER) stress pathways, which are known to be highly intertwined with several metabolic pathways. Because HCC is characterized by changes in the metabolome and by an aberrant activation of the ER stress pathways, the aim of this review was to summarize the current knowledge that links ER stress and metabolism in HCC, thereby focusing on potential therapeutic targets. Hepatocellular carcinoma (HCC) is the most common type of liver cancer, accounting for 85% to 90% of all cases of liver cancer.1Llovet J.M. Kelley R.K. Villanueva A. Singal A.G. Pikarsky E. Roayaie S. Lencioni R. Koike K. Zucman-Rossi J. Finn R.S. Hepatocellular carcinoma.Nat Rev Dis Primers. 2021; 7: 6Crossref PubMed Scopus (2128) Google Scholar It is a hepatocyte-derived primary tumor that causes 550,000 deaths per year worldwide, making it the second cause of cancer-related deaths. The incidence of HCC is currently increasing worldwide but varies considerably per region due to differences in the prevalence of underlying risk factors. Risk factors for HCC include: hepatitis B or C virus infections; metabolic disorders such as obesity, type 2 diabetes, and nonalcoholic fatty liver disease; and alcohol abuse or aflatoxin B1 exposure. Most of these risk factors result in chronic liver injury, which leads to fibrosis and cirrhosis, thereby creating a favorable environment for the development of HCC. The main risk factor for HCC in Eastern Asia is hepatitis B virus. However, in the western society, it is currently shifting toward obesity, type 2 diabetes, and nonalcoholic fatty liver disease. HCC remains a major health problem, with a 5-year survival of only 20% due to late detection and limited treatment options. This highlights the need for a better understanding of the pathogenesis of HCC to pave the way for new treatments and improve survival of patients with HCC. Studies over the last decade indicate that endoplasmic reticulum (ER) stress plays an important role in liver cancer.2Pavlović N. Heindryckx F. Exploring the role of endoplasmic reticulum stress in hepatocellular carcinoma through mining of the Human Protein Atlas.Biology (Basel). 2021; 10: 640PubMed Google Scholar,3Pavlović N. Kopsida M. Gerwins P. Heindryckx F. Inhibiting P2Y12 in macrophages induces endoplasmic reticulum stress and promotes an anti-tumoral phenotype.Int J Mol Sci. 2020; 21: 8177Crossref PubMed Scopus (0) Google Scholar ER stress is the accumulation of misfolded or unfolded proteins in the ER lumen, which occurs when the capacity of the ER to correctly fold proteins is exceeded.4Walter P. Ron D. The unfolded protein response: from stress pathway to homeostatic regulation.Science. 2011; 334: 1081-1086Crossref PubMed Scopus (4168) Google Scholar The presence of these proteins in the ER lumen activates the unfolded protein response (UPR), which functions to restore protein homeostasis by slowing down protein translation, increasing the protein folding machinery, and up-regulating the degradation of unfolded proteins.5Kaufman R.J. Orchestrating the unfolded protein response in health and disease.J Clin Invest. 2002; 110: 1389-1398Crossref PubMed Google Scholar However, in case of persistent or excessive ER stress, the UPR can induce pathways leading to cell death.5Kaufman R.J. Orchestrating the unfolded protein response in health and disease.J Clin Invest. 2002; 110: 1389-1398Crossref PubMed Google Scholar,6Hetz C. The unfolded protein response: controlling cell fate decisions under ER stress and beyond.Nat Rev Mol Cell Biol. 2012; 13: 89-102Crossref PubMed Scopus (2722) Google Scholar The proposed key role of the UPR in cancer is to protect tumor cells from apoptosis and support their uncontrolled proliferation.7D’Orazi G. Cirone M. Interconnected adaptive responses: a way out for cancer cells to avoid cellular demise.Cancers (Basel). 2022; 14: 2780Crossref PubMed Scopus (0) Google Scholar Therefore, ER stress and the UPR may have a relevant role in HCC pathogenesis, which could reveal new targets for HCC treatment and diagnosis. Metabolism has recently gained importance in the pathogenesis of HCC due to the pivotal role of the liver in different metabolic processes. Risk factors for HCC include aberrant metabolism disorders such as nonalcoholic fatty liver disease, obesity, and type 2 diabetes. In addition, the deregulation or reprogramming of glucose metabolism is considered a hallmark of solid tumors, as it helps to fuel tumorigenesis and allows cells to thrive under hypoxic circumstances.8Hanahan D. Hallmarks of cancer: new dimensions.Cancer Discov. 2022; 12: 31-46Crossref PubMed Scopus (2111) Google Scholar Lastly, most of the metabolic processes in the liver are performed by hepatocytes, which comprise 85% of the total mass of this organ. Thus, any transformation of hepatocytes during HCC, such as mutations, increased proliferation, or replicative immortality, leads to metabolic alterations. Interestingly, ER stress and metabolic deregulation are functionally intertwined, and both are considered contributing factors to the pathogenesis of HCC (Figure 1). The aim of the current review is to summarize the available knowledge that links ER stress and metabolism in HCC. It first provides an overview on the UPR and its involvement in HCC. Next, the role of the liver in the different metabolic pathways and the alterations found in HCC are presented. Finally, the connections between the disrupted metabolism and ER stress in HCC are disclosed. ER stress is the physiological condition characterized by an accumulation of misfolded or unfolded proteins in the lumen of the ER.5Kaufman R.J. Orchestrating the unfolded protein response in health and disease.J Clin Invest. 2002; 110: 1389-1398Crossref PubMed Google Scholar This can be triggered by several stimuli, including a high rate of cell proliferation, hypoxia, nutrient deficiency, and abnormal redox homeostasis. The presence of misfolded or unfolded proteins in the ER lumen is sensed by three transmembrane proteins, namely activating transcription factor 6 (ATF6), inositol-requiring enzyme 1 alpha (IRE1α), and protein kinase RNA-like endoplasmic reticulum kinase (PERK).5Kaufman R.J. Orchestrating the unfolded protein response in health and disease.J Clin Invest. 2002; 110: 1389-1398Crossref PubMed Google Scholar These proteins each activate their own signaling cascade to induce the UPR. During protein homeostasis, binding immunoglobulin protein (BiP), alias Grp78 or heat shock 70 kDa protein 5, is bound to ATF6, IRE1α, and PERK, keeping them in their inactive conformation. Upon ER stress, BiP preferentially binds to unfolded or misfolded proteins in the ER lumen, thereby dissociating from the ER stress sensors and exposing their luminal domains.9Bertolotti A. Zhang Y. Hendershot L.M. Harding H.P. Ron D. Dynamic interaction of BiP and ER stress transducers in the unfolded-protein response.Nat Cell Biol. 2000; 2: 326-332Crossref PubMed Scopus (2186) Google Scholar This facilitates their activation and triggers the UPR, orchestrating a complex signaling network aiming to reduce protein translation, increase the folding capacity of the ER, or, in case of severe or prolonged ER stress, activate proapoptotic pathways. The most prominent and evolutionarily conserved UPR signal transducer is IRE1α, which gets activated as BiP dissociates from its binding site on the receptor. This action results in homodimerization and subsequent autophosphorylation of its kinase domain.9Bertolotti A. Zhang Y. Hendershot L.M. Harding H.P. Ron D. Dynamic interaction of BiP and ER stress transducers in the unfolded-protein response.Nat Cell Biol. 2000; 2: 326-332Crossref PubMed Scopus (2186) Google Scholar Activated IRE1α also exerts an endonuclease function, which enables splicing of X-box binding protein 1 (XBP1) messenger RNA (mRNA) into its active form, called spliced XBP1 (sXBP1). sXBP1 is then able to act as a transcription factor, which induces transcription of a range of proadaptive genes, including ER chaperones and genes involved in ER stress–associated protein degradation. Misfolded or unfolded proteins in ER stress–associated protein degradation are transported to the cytoplasm for ubiquitination and degradation by the proteasome.4Walter P. Ron D. The unfolded protein response: from stress pathway to homeostatic regulation.Science. 2011; 334: 1081-1086Crossref PubMed Scopus (4168) Google Scholar In addition, the endonuclease activity of IRE1α regulates IRE1-dependent decay, a process in which several ER-bound mRNAs or precursor microRNAs are spliced. These two processes reduce the number of misfolded or unfolded proteins and mRNA, decreasing the protein-folding load of the ER and hence ER stress. In addition to its primary proadaptive role, IRE1α induces proapoptotic pathways during prolonged ER stress. This is initiated by the assembly of IRE1α with tumor necrosis factor receptor–associated factor 2 and apoptosis signal-regulating kinase 1. Together, they form a signaling complex, which further activates downstream signaling pathways leading to apoptosis. Additionally, regulated IRE1-dependent decay can contribute to apoptosis in case of severe ER stress. Therefore, IRE1α can activate both proadaptive and proapoptotic pathways, depending on the level and duration of ER stress.6Hetz C. The unfolded protein response: controlling cell fate decisions under ER stress and beyond.Nat Rev Mol Cell Biol. 2012; 13: 89-102Crossref PubMed Scopus (2722) Google Scholar The second transmembrane ER stress protein is PERK, which becomes activated by dimerization and subsequent auto-phosphorylation upon dissociation of BiP during ER stress.9Bertolotti A. Zhang Y. Hendershot L.M. Harding H.P. Ron D. Dynamic interaction of BiP and ER stress transducers in the unfolded-protein response.Nat Cell Biol. 2000; 2: 326-332Crossref PubMed Scopus (2186) Google Scholar After activation, PERK phosphorylates the alpha subunit of eukaryotic initiation factor 2 (eIF2α), resulting in a general suppression of protein translation. The cell cycle regulator cyclin D1 is one of the many proteins that are down-regulated; this action causes the cell cycle to arrest and offers the cell some time to restore protein homeostasis.10Brewer J.W. Diehl J.A. PERK mediates cell-cycle exit during the mammalian unfolded protein response.Proc Natl Acad Sci U S A. 2000; 97: 12625-12630Crossref PubMed Scopus (372) Google Scholar Simultaneously, phosphorylated eIF2α enhances translation of a specific set of mRNAs, including activating transcription factor 4 (ATF4).11Hamanaka R.B. Bennett B.S. Cullinan S.B. Diehl J.A. PERK and GCN2 contribute to eIF2alpha phosphorylation and cell cycle arrest after activation of the unfolded protein response pathway.Mol Biol Cell. 2005; 16: 5493-5501Crossref PubMed Scopus (208) Google Scholar This protein can induce transcription of various genes involved in the adaptation to ER stress, such as genes involved in autophagy and reactive oxygen species protection. In contrast, ATF4 can induce transcription of CCAAT-enhancer-binding protein homologous protein (CHOP), also known as growth arrest, and DNA damage inducible gene (GADD153), which stimulates expression of proapoptotic genes.12Lai K.-M. Wang J.-H. Lin S.-C. Wen Y. Wu C.-L. Su J.-H. Chen C.-C. Lin C.-C. Crassolide induces G2/M cell cycle arrest, apoptosis, and autophagy in human lung cancer cells via ROS-mediated ER stress pathways.Int J Mol Sci. 2022; 23: 5624Crossref PubMed Scopus (1) Google Scholar Hence, as with IRE1α, PERK induces both proadaptive and proapoptotic pathways. The last transmembrane ER stress protein is ATF6. It is translocated to the Golgi apparatus upon dissociation of BiP, where it is cleaved into its active form by site-1 and site-2 proteases.13Maity S. Komal P. Kumar V. Saxena A. Tungekar A. Chandrasekar V. Impact of ER stress and ER-mitochondrial crosstalk in Huntington’s disease.Int J Mol Sci. 2022; 23: 780Crossref PubMed Scopus (0) Google Scholar Active ATF6 acts as a transcription factor and stimulates the production of ER chaperones, including BiP, which aid in the folding of proteins and hence reduce ER stress.14Lee K. Tirasophon W. Shen X. Michalak M. Prywes R. Okada T. Yoshida H. Mori K. Kaufman R.J. IRE1-mediated unconventional mRNA splicing and S2P-mediated ATF6 cleavage merge to regulate XBP1 in signaling the unfolded protein response.Genes Dev. 2002; 16: 452-466Crossref PubMed Scopus (854) Google Scholar In addition, it induces the production of XBP1 mRNA that can subsequently be spliced by IRE1α, leading to the initiation of downstream signaling pathways. ATF6 is one of the first branches to be activated, creating a time frame for the cell to restore protein homeostasis solely by an increase in ER chaperones before activation of other pathways.15Benjamin I.J. Viewing a stressful episode of ER: is ATF6 the triage nurse?.Circ Res. 2006; 98: 1120-1122Crossref PubMed Scopus (14) Google Scholar This process highlights the importance of ATF6 in the initial stages of ER stress. Several characteristics of solid tumors are known inducers of ER stress, making ER stress a novel hallmark of cancer.7D’Orazi G. Cirone M. Interconnected adaptive responses: a way out for cancer cells to avoid cellular demise.Cancers (Basel). 2022; 14: 2780Crossref PubMed Scopus (0) Google Scholar For example, a high rate of cell proliferation, hypoxia, nutrient deficiency, and abnormal redox homeostasis can all trigger the UPR.16Mollinedo F. Gajate C. Direct endoplasmic reticulum targeting by the selective alkylphospholipid analog and antitumor ether lipid edelfosine as a therapeutic approach in pancreatic cancer.Cancers (Basel). 2021; 13: 4173Crossref PubMed Scopus (5) Google Scholar In addition, tumor cells possess numerous mutations that can induce the production of misfolded proteins.17Bamberger C. Diedrich J. Martinez-Bartholome S. Yates 3rd, J.R. Cancer conformational landscape shapes tumorigenesis.J Proteome Res. 2022; 21: 1017-1028Crossref PubMed Scopus (0) Google Scholar Furthermore, ER stress can also be induced by viral infections such as hepatitis B or C virus,18Muroyama R. Nakagawa R. Matsubara Y. Hirata Y. Omata M. Shirasawa H. Kato N. Fusion HBx from HBV integrant affects hepatocarcinogenesis through deregulation of ER stress response.Virus Res. 2022; 315: 198787Crossref PubMed Scopus (1) Google Scholar,19Lin W.-L. Hung J.-H. Huang W. Association of the hepatitis B virus large surface protein with viral infectivity and endoplasmic reticulum stress-mediated liver carcinogenesis.Cells. 2020; 9: 2052Crossref PubMed Scopus (14) Google Scholar which are both known risk factors for HCC. As a result, an increased expression of ATF6, BiP, and sXBP1 has been seen in HCC patients with HCC of different etiologies, thus indicating an up-regulation of the ATF6 and IRE1α branches of the UPR in HCC.2Pavlović N. Heindryckx F. Exploring the role of endoplasmic reticulum stress in hepatocellular carcinoma through mining of the Human Protein Atlas.Biology (Basel). 2021; 10: 640PubMed Google Scholar,20Wu J. Qiao S. Xiang Y. Cui M. Yao X. Lin R. Zhang X. Endoplasmic reticulum stress: multiple regulatory roles in hepatocellular carcinoma.Biomed Pharmacother. 2021; 142: 112005Crossref PubMed Scopus (25) Google Scholar The up-regulation of the UPR can contribute to the survival of tumor cells, leading to the development and progression of cancer. Studies have shown that the PERK branch of the UPR is crucial for inducing tumor cell proliferation and growth by altering redox homeostasis.21Bobrovnikova-Marjon E. Grigoriadou C. Pytel D. Zhang F. Ye J. Koumenis C. Cavener D. Diehl J.A. PERK promotes cancer cell proliferation and tumor growth by limiting oxidative DNA damage.Oncogene. 2010; 29: 3881-3895Crossref PubMed Scopus (225) Google Scholar This process prevents the activation of oxidative DNA checkpoints, allowing tumor cells to survive in hostile tumor microenvironments. Studies have shown that pharmacologically targeting different arms of the UPR can form a promising target to slow down tumor growth.22Jiang W. Chen L. Guo X. Cheng C. Luo Y. Wang J. Wang J. Liu Y. Cao Y. Li P. Wang Z. Ran H. Zhou Z. Ren J. Combating multidrug resistance and metastasis of breast cancer by endoplasmic reticulum stress and cell-nucleus penetration enhanced immunochemotherapy.Theranostics. 2022; 12: 2987-3006Crossref PubMed Scopus (2) Google Scholar,23Pyrczak-Felczykowska A. Reekie T.A. Jąkalski M. Hać A. Malinowska M. Pawlik A. Rys K. Guzow-Krzemińska B. Herman-Antosiewicz A. The isoxazole derivative of usnic acid induces an ER stress response in breast cancer cells that leads to paraptosis-like cell death.Int J Mol Sci. 2022; 23: 1802Crossref PubMed Scopus (1) Google Scholar For instance, inhibition of IRE1α reduces the development of HCC in a diet-induced model for obesity in vivo.20Wu J. Qiao S. Xiang Y. Cui M. Yao X. Lin R. Zhang X. Endoplasmic reticulum stress: multiple regulatory roles in hepatocellular carcinoma.Biomed Pharmacother. 2021; 142: 112005Crossref PubMed Scopus (25) Google Scholar Tumor cell proliferation and metastasis is reduced in a chemically induced HCC mouse model when IRE1α endonuclease activity is pharmacologically inhibited.24Pavlović N. Calitz C. Thanapirom K. Mazza G. Rombouts K. Gerwins P. Heindryckx F. Inhibiting IRE1[alpha]-endonuclease activity decreases tumor burden in a mouse model for hepatocellular carcinoma.Elife. 2020; 9: e55865Crossref PubMed Scopus (17) Google Scholar Similarly, a study by Vandewynckel et al25Vandewynckel Y.-P. Laukens D. Bogaerts E. Paridaens A. Van den Bussche A. Verhelst X. Van Steenkiste C. Descamps B. Vanhove C. Libbrecht L. De Rycke R. Lambrecht B.N. Geerts A. Janssens S. Van Vlierberghe H. Modulation of the unfolded protein response impedes tumor cell adaptation to proteotoxic stress: a PERK for hepatocellular carcinoma therapy.Hepatol Int. 2015; 9: 93-104Crossref PubMed Scopus (53) Google Scholar attempted to investigate pharmacologic inhibition of PERK by GSK2656157, which suggested reductions in the expression of ER chaperones and tumor growth in an HCC mouse model. However, a more recent study showed that GSK2656157 and GSK2606414, which are commonly used with the purpose of inhibiting PERK, are in fact not specific for PERK, thereby questioning the accuracy of previous studies using these inhibitors.26Rojas-Rivera D. Delvaeye T. Roelandt R. Nerinckx W. Augustyns K. Vandenabeele P. Bertrand M.J.M. When PERK inhibitors turn out to be new potent RIPK1 inhibitors: critical issues on the specificity and use of GSK2606414 and GSK2656157.Cell Death Differ. 2017; 24: 1100-1110Crossref PubMed Scopus (125) Google Scholar Lastly, not much is known about the effect of inhibition of the ATF6 branch on HCC pathogenesis, which could be interesting for further research due to its suggested up-regulation and/or missense polymorphisms in HCC.27Shuda M. Kondoh N. Imazeki N. Tanaka K. Okada T. Mori K. Hada A. Arai M. Wakatsuki T. Matsubara O. Yamamoto N. Yamamoto M. Activation of the ATF6, XBP1 and grp78 genes in human hepatocellular carcinoma: a possible involvement of the ER stress pathway in hepatocarcinogenesis.J Hepatol. 2003; 38: 605-614Abstract Full Text Full Text PDF PubMed Scopus (417) Google Scholar,28Wu X. Xin Z. Zhang W. Zheng S. Wu J. Chen K. Wang H. Zhu X. Li Z. Duan Z. Li H. Liu Y. A missense polymorphism in ATF6 gene is associated with susceptibility to hepatocellular carcinoma probably by altering ATF6 level.Int J Cancer. 2014; 135: 61-68Crossref PubMed Scopus (32) Google Scholar For instance, there have been cases in which a missense single-nucleotide polymorphism can significantly increase the expression levels of ATF6 mRNA and/or ATF6-regulated genes, such as GRP78, CHOP, and XBP1, contributing to HCC susceptibility. This missense mutation is strongly associated with hepatitis B virus–related HCC, with a reported frequency of 28% of HCC patients carrying this mutation.28Wu X. Xin Z. Zhang W. Zheng S. Wu J. Chen K. Wang H. Zhu X. Li Z. Duan Z. Li H. Liu Y. A missense polymorphism in ATF6 gene is associated with susceptibility to hepatocellular carcinoma probably by altering ATF6 level.Int J Cancer. 2014; 135: 61-68Crossref PubMed Scopus (32) Google Scholar The liver is responsible for maintaining glucose homeostasis by continuously regulating glucose production and storage, and ER stress pathways interfere with these processes (Figure 2). To maintain glucose homeostasis, hepatocytes perform four metabolic reactions: glycolysis, glycogenesis, gluconeogenesis, and glycogenolysis. Glycolysis is a catabolic pathway of enzyme-catalyzed reactions that break down glucose to generate energy. In normal conditions, cells catabolize glucose to pyruvate and later to acetyl coenzyme A through the tricarboxylic acid cycle to finally produce large amounts of adenosine triphosphate via oxidative phosphorylation. However, in hypoxic or anaerobic conditions, cells follow glycolysis instead, and metabolize glucose to lactate, producing smaller amounts of energy per glucose molecule. In the 1920s, Otto Warburg reported that tumorigenic cells tend to shift their main energy source from oxidative phosphorylation to glycolysis, even under normal oxygen conditions.8Hanahan D. Hallmarks of cancer: new dimensions.Cancer Discov. 2022; 12: 31-46Crossref PubMed Scopus (2111) Google Scholar During this metabolic change, known as the Warburg effect, tumor cells consume larger amounts of glucose and produce energy in a faster manner, thus stimulating proliferation.29Liu J. Yang C. Huang X.-M. Lv P.-P. Yang Y.-K. Zhao J.-N. Zhao S.-Y. Sun W.-J. Knockdown of FBI-1 inhibits the Warburg effect and enhances the sensitivity of hepatocellular carcinoma cells to molecular targeted agents via miR-3692/HIF-1[alpha].Front Oncol. 2021; 11: 796839Crossref PubMed Scopus (5) Google Scholar,30Wan H. Tian Y. Zhao J. Su X. LINC00665 targets miR-214-3p/MAPK1 axis to accelerate hepatocellular carcinoma growth and Warburg effect.J Oncol. 2021; 2021: 9046798Crossref PubMed Scopus (5) Google Scholar Therefore, inhibition of enzymes involved in these pathways could potentially limit tumor proliferation. Down-regulation of triosephosphate isomerase 1 (TPI1), which is involved in the glycolytic and gluconeogenic pathway, reportedly inhibits HCC cell growth, migration, invasion, and survival.31Li L. Xu H. Qu L. Xu K. Liu X. Daidzin inhibits hepatocellular carcinoma survival by interfering with the glycolytic/gluconeogenic pathway through downregulation of TPI1.Biofactors. 2022; 48: 883-896Crossref PubMed Scopus (3) Google Scholar Moreover, TPI1 expression is significantly decreased in HCC tissues and is correlated with HCC progression and survival, suggesting that TPI1 might serve as a tumor suppressor.32Jiang H. Ma N. Shang Y. Zhou W. Chen T. Guan D. Li J. Wang J. Zhang E. Feng Y. Yin F. Yuan Y. Fang Y. Qiu L. Xie D. Wei D. Triosephosphate isomerase 1 suppresses growth, migration and invasion of hepatocellular carcinoma cells.Biochem Biophys Res Commun. 2017; 482: 1048-1053Crossref PubMed Scopus (40) Google Scholar ATF4 knockdown reduces the expression of TPI1 in Drosophila, which suggests a role of the UPR in the regulation of this enzyme.33Lee J.E. Oney M. Frizzell K. Phadnis N. Hollien J. Drosophila melanogaster activating transcription factor 4 regulates glycolysis during endoplasmic reticulum stress.G3 (Bethesda). 2015; 5: 667-675Crossref PubMed Scopus (33) Google Scholar Another key glycolytic enzyme involved in tumor proliferation is hexokinase and, more specifically, one of its four isoenzymes, hexokinase 2 (HXK2). HCC exhibits aberrant expression of HXK2, contributing to early recurrence and poorer prognosis of surgically resected HCC.34Peng S.-Y. Lai P.-L. Pan H.-W. Hsiao L.-P. Hsu H.-C. Aberrant expression of the glycolytic enzymes aldolase B and type II hexokinase in hepatocellular carcinoma are predictive markers for advanced stage, early recurrence and poor prognosis.Oncol Rep. 2008; 19: 1045-1053PubMed Google Scholar DeWaal et al35DeWaal D. Nogueira V. Terry A.R. Patra K.C. Jeon S.-M. Guzman G. Au J. Long C.P. Antoniewicz M.R. Hay N. Hexokinase-2 depletion inhibits glycolysis and induces oxidative phosphorylation in hepatocellular carcinoma and sensitizes to metformin.Nat Commun. 2018; 9: 446Crossref PubMed Scopus (277) Google Scholar also observed that HCC cells express HXK2. Inhibition of HXK2 with 3-bromopyruvate promotes tumor cell death by inducing ER stress in human HCC cell lines (Huh-7 and SNU-761)36Yu S.J. Yoon J.-H. Yang J.-I. Cho E.J. Kwak M.S. Jang E.S. Lee J.-H. Kim Y.J. Lee H.-S. Kim C.Y. Enhancement of hexokinase II inhibitor-induced apoptosis in hepatocellular carcinoma cells via augmenting ER stress and anti-angiogenesis by protein disulfide isomerase inhibition.J Bioenerg Biomembr. 2012; 44: 101-115Crossref PubMed Scopus (35) Google Scholar and improves efficacy of sorafenib in in vivo models of HCC.37Yoo J.-J. Yu S.J. Na J. Kim K. Cho Y.Y. Lee Y.B. Cho E.J. Lee J.-H. Kim Y.J. Youn H. Yoon J.-H. Hexokinase-II inhibition synergistically augments the anti-tumor efficacy of sorafenib in hepatocellular carcinoma.Int J Mol Sci. 2019; 20: 1292Crossref PubMed Scopus (28) Google Scholar Similarly, PERK silencing blocks the translocation of HXK2 to mitochondria via inhibition of p-Akt in glioma cells, which inhibits tumor growth.38Hou X. Liu Y. Liu H. Chen X. Liu M. Che H. Guo F. Wang C. Zhang D. Wu J. Chen X. Shen C. Li C. Peng F. Bi Y. Yang Z. Yang G. Ai J. Gao X. Zhao S. PERK silence inhibits glioma cell growth under low glucose stress by blockage of p-AKT and subsequent HK2’s mitochondria translocation.Sci Rep. 2015; 5: 9065Crossref PubMed Scopus (0) Google Scholar This suggests PERK-dependent regulation of HXK2 in tumor cells, linking ER stress to glycolysis. In addition, depletion of HXK2 results in inhibition of glycolysis and induction of oxidative phosphorylation, opposite to what tumor cells usually experience during the Warburg effect.35DeWaal D. Nogueira V. Terry A.R. Patra K.C. Jeon S.-M. Guzman G. Au J. Long C.P. Antoniewicz M.R. Hay N. Hexokinase-2 depletion inhibits glycolysis and induces oxidative phosphorylation in hepatocellular carcinoma and sensitizes to metformin.Nat Commun. 2018; 9: 446Crossref PubMed Scopus (277) Google Scholar This also sensitizes HCC cells to metformin, a compound broadly used in patients with HCC,39Abd El-Fattah E.E. Zakaria A.Y. Metformin modulate immune fitness in hepatocellular carcinoma: molecular and cellular approach.Int Immunopharmacol. 2022; 109: 108889Crossref PubMed Scopus (1) Google Scholar which seems to regulate ER stress response in HCC cells, endothelial cells, and patients with type 2 diabetes.40Chuan L. Zhang L. Fu H. Yang Y. Wang Q. Jiang X. Li Z. Ni K. Ding L. Metformin prevents brain injury after cardiopulmonary resuscitation by inhibiting the endoplasmic reticulum stress response and activating AMPK-mediated autophagy.Scott Med J. 2021; 66: 16-22Crossref PubMed Scopus (2) Google Scholar,41Kapadia P. Bikkina P. Landicho M.A. Parekh S. Haas M.J. Mooradian A.D. Effect of anti-hyperglycemic drugs on endoplasmic reticulum (ER) stress in human coronary artery endothelial cells.Eur J Pharmacol. 2021; 907: 174249Crossref PubMed Scopus (8) Google Scholar More specifically, metformin reportedly reduces activation of the PERK pathway, thus preventing activation of the proapoptotic arm of ER stress signaling.31Li L. Xu H. Qu L. Xu K. Liu X. Daidzin inhibits hepatocellular carcinoma survival by interfering with the glycolytic/gluconeogenic pathway through downregulation of TPI1.Biofactors. 2022; 48: 883-896Crossref PubMed Scopus (3) Google Scholar,40Chuan L. Zhang L. Fu H. Yang Y. Wang Q. Jiang X. Li Z. Ni K. Ding L. Metformin prevents brain injury after cardiopulmonary resuscitation by inhibiting the endoplasmic reticulum stress response and activating AMPK-mediated autophagy.Scott Med J. 2021; 66: 16-22Crossref PubMed Scopus (2) Google Scholar The liver can produce gluco