Induction of apurinic endonuclease 1 overexpression by endoplasmic reticulum stress in hepatoma cells

XBP1 as a novel molecular target to attenuate drug resistance in hepatocellular carcinoma

INTRODUCTION

Despite improvements in clinical management of hepatocellular carcinoma (HCC), prognosis remains poor with a 5-year survival rate less than 40%. Drug resistance in HCC makes it challenging to treat; therefore, it is imperative to develop new therapeutic strategies. Higher expression of X-box binding protein 1 (XBP1) in tumor cells is highly correlated with poor prognosis. In tumor cells, XBP1 modulates the unfolded protein response (UPR) to restore homeostasis in endoplasmic reticulum. Targeting XBP1 could be a promising therapeutic strategy to overcome HCC resistance and improve the survival rate of patients.

AREAS COVERED

This review provides the recent evidence that indicates XBP1 is involved in HCC drug resistance via DNA damage response, drug inactivation, and inhibition of apoptosis. In addition, the potential roles of XBP1 in inducing resistance in HCC cells were highlighted, and we showed how its inhibition could sensitize tumor cells to controlled cell death.

EXPERT OPINION

Due to the diversity in molecular mechanism of multidrug-resistance, targeting one specific pathway is inadequate. XBP1 inhibition could be a potential therapeutic target to overcome verity of resistance mechanisms. The main function of this transcription factor in HCC treatment response is an attractive area for further studies and should be discussed more.

Apurinic/apyrimidinic endonuclease 1 regulates palmitic acid-mediated apoptosis in cardiomyocytes via endoplasmic reticulum stress

Cardiomyocyte apoptosis caused by fat metabolism disorder plays an essential role in the pathogenesis of diabetic cardiomyopathy (DCM). Apurinic/apyrimidinic endonuclease 1 (APE1) has multiple functions, including regulating redox and DNA repair. However, the role of APE1 in the pathogenesis of DCM remains unclear. To investigate the mechanism of APE1 on high-fat induced apoptosis in H9C2 cells, we treated H9C2 cells with palmitic acid (PA) as an apoptosis model caused by hyperlipidemia. We found that PA reduced the viability and increased apoptosis of H9C2 cells by inducing up-regulation of APE1 protein and endoplasmic reticulum (ER) stress. APE1 knockdown enhanced PA-induced apoptosis, and ER stress and overexpression of APE1 demonstrated the opposite effect. Furthermore, APE1 regulated PA-induced apoptosis via ER stress. The APE1 mutant (C65A, lack of redox regulation) loses its protective effect against ER stress and apoptosis. These findings indicate that APE1 protects PA-induced H9C2 cardiomyocyte apoptosis through ER stress via its redox-regulated function. This study provided new insights into the therapy for DCM.

A DNA repair-independent role for alkyladenine DNA glycosylase in alkylation-induced unfolded protein response

Stress response pathways, such as the DNA damage response and the UPR, are critical in the etiology and treatment of cancer and other chronic diseases. Knowledge of an interplay between ER stress and genome damage repair is emerging, but evidence linking defective DNA repair and impaired ER stress response is lacking. Here, we show that AAG is necessary for UPR activation in response to alkylating agents. AAG-deficient mice and human cancer cells are impaired in alkylation-induced UPR. Strikingly, this defect can be complemented by an AAG variant defective in glycosylase activity. Our studies suggest that AAG has noncanonical functions and identify AAG as a point of convergence for stress response pathways. This knowledge could be explored to improve cancer treatment.

Alkylating agents damage DNA and proteins and are widely used in cancer chemotherapy. While cellular responses to alkylation-induced DNA damage have been explored, knowledge of how alkylation affects global cellular stress responses is sparse. Here, we examined the effects of the alkylating agent methylmethane sulfonate (MMS) on gene expression in mouse liver, using mice deficient in alkyladenine DNA glycosylase (Aag), the enzyme that initiates the repair of alkylated DNA bases. MMS induced a robust transcriptional response in wild-type liver that included markers of the endoplasmic reticulum (ER) stress/unfolded protein response (UPR) known to be controlled by XBP1, a key UPR effector. Importantly, this response is significantly reduced in the Aag knockout. To investigate how AAG affects alkylation-induced UPR, the expression of UPR markers after MMS treatment was interrogated in human glioblastoma cells expressing different AAG levels. Alkylation induced the UPR in cells expressing AAG; conversely, AAG knockdown compromised UPR induction and led to a defect in XBP1 activation. To verify the requirements for the DNA repair activity of AAG in this response, AAG knockdown cells were complemented with wild-type Aag or with an Aag variant producing a glycosylase-deficient AAG protein. As expected, the glycosylase-defective Aag does not fully protect AAG knockdown cells against MMS-induced cytotoxicity. Remarkably, however, alkylation-induced XBP1 activation is fully complemented by the catalytically inactive AAG enzyme. This work establishes that, besides its enzymatic activity, AAG has noncanonical functions in alkylation-induced UPR that contribute to cellular responses to alkylation.

RIG-I Deficiency Promotes Obesity-Induced Insulin Resistance

Inflammation and immunity are linked to the onset and development of obesity and metabolic disorders. Pattern recognition receptors (PRRs) are key regulators of inflammation and immunity in response to infection and stress, and they have critical roles in metainflammation. In this study, we investigated whether RIG-I (retinoic acid-inducible gene I)-like receptors were involved in the regulation of obesity-induced metabolic stress in RIG-I knockout (KO) mice fed a high-fat diet (HFD). RIG-I KO mice fed an HFD for 12 weeks showed greater body weight gain, higher fat composition, lower lean body mass, and higher epididymal white adipose tissue (eWAT) weight than WT mice fed HFD. In contrast, body weight gain, fat, and lean mass compositions, and eWAT weight of MDA5 (melanoma differentiation-associated protein 5) KO mice fed HFD were similar to those of WT mice fed a normal diet. RIG-I KO mice fed HFD exhibited more severely impaired glucose tolerance and higher HOMA-IR values than WT mice fed HFD. IFN-β expression induced by ER stress inducers, tunicamycin and thapsigargin, was abolished in RIG-I-deficient hepatocytes and macrophages, showing that RIG-I is required for ER stress-induced IFN-β expression. Our results show that RIG-I deficiency promotes obesity and insulin resistance induced by a high-fat diet, presenting a novel role of RIG-I in the development of obesity and metabolic disorders.

Ahypoxia-related signature enhances the prediction of the prognosis in hepatocellular carcinoma patients and correlates with sorafenib treatment response

Hepatocellular carcinoma (HCC) is one of the leading cancer death and is the primary malignancy of the liver. Tumor hypoxia is the stressor that is involved in tumorigenesis and significantly increased the aggressiveness of HCC. Here, we systematically analyzed the expression profiles and prognostic values of 84 hypoxia associated genes in HCC. mRNA expression of 84 hypoxia associated genes and clinical parameters of HCC patients were downloaded from TCGA, GSE14520, GSE109211 and ICGC. Consensus clustering analysis was performed for unsupervised classes on the basis of 84 hypoxia associated genes. Univariate and LASSO analysis were used to develop the risk signature. A risk signature was developed, including the expression of APEX1, ATR, CTSA, DNAJC5, ENO1, EPO, HMOX1, LDHA, NDRG1, and PER1, and found to be significantly related with OS and DFS of HCC patients. We stratified HCC patients into the high-risk group and low-risk group by means of the risk signature. Patients of high-risk group had shorter OS and DFS, while that of the low-risk group had longer OS and DFS. The risk signature showed better predictive efficiency than the TNM staging in predicting OS and DFS. Also, macrophage M0 cells, regulatory T cells, and neutrophils were found to be significantly enriched in patients of high-risk group. Next, we validated the discrimination and prognostic value of the risk signature in GSE14520 and the ICGC HCC cohort. Finally, significantly lower risk scores were found in sorafenib treatment responders of GSE109211 cohort, and the AUC for predicting sorafenib treatment response was 0.881. In conclusion, a risk signature developed with the expression of 10 hypoxia associated genes improved the prognosis prediction of HCC and correlated with sorafenib treatment response.

Mitochondrial apurinic/apyrimidinic endonuclease 1 enhances mtDNA repair contributing to cell proliferation and mitochondrial integrity in early stages of hepatocellular carcinoma

BACKGROUND

Hepatocellular carcinoma (HCC) is the leading cause of primary liver cancers. Surveillance of individuals at specific risk of developing HCC, early diagnostic markers, and new therapeutic approaches are essential to obtain a reduction in disease-related mortality. Apurinic/apyrimidinic endonuclease 1 (APE1) expression levels and its cytoplasmic localization have been reported to correlate with a lower degree of differentiation and shorter survival rate. The aim of this study is to fully investigate, for the first time, the role of the mitochondrial form of APE1 in HCC.

METHODS

As a study model, we analyzed samples from a cohort of patients diagnosed with HCC who underwent surgical resection. Mitochondrial APE1 content, expression levels of the mitochondrial import protein Mia40, and mtDNA damage of tumor tissue and distal non-tumor liver of each patient were analyzed. In parallel, we generated a stable HeLa clone for inducible silencing of endogenous APE1 and re-expression of the recombinant shRNA resistant mitochondrially targeted APE1 form (MTS-APE1). We evaluated mtDNA damage, cell growth, and mitochondrial respiration.

RESULTS

APE1's cytoplasmic positivity in Grades 1 and 2 HCC patients showed a significantly higher expression of mitochondrial APE1, which accounted for lower levels of mtDNA damage observed in the tumor tissue with respect to the distal area. In the contrast, the cytoplasmic positivity in Grade 3 was not associated with APE1's mitochondrial accumulation even when accounting for the higher number of mtDNA lesions measured. Loss of APE1 expression negatively affected mitochondrial respiration, cell viability, and proliferation as well as levels of mtDNA damage. Remarkably, the phenotype was efficiently rescued in MTS-APE1 clone, where APE1 is present only within the mitochondrial matrix.

CONCLUSIONS

Our study confirms the prominent role of the mitochondrial form of APE1 in the early stages of HCC development and the relevance of the non-nuclear fraction of APE1 in the disease progression. We have also confirmed overexpression of Mia40 and the role of the MIA pathway in the APE1 import process. Based on our data, inhibition of the APE1 transport by blocking the MIA pathway could represent a new therapeutic approach for reducing mitochondrial metabolism by preventing the efficient repair of mtDNA.

When Endoplasmic Reticulum Proteostasis Meets the DNA Damage Response

Sustaining both proteome and genome integrity (GI) requires the integration of a wide range of mechanisms and signaling pathways. These comprise, in particular, the unfolded protein response (UPR) and the DNA damage response (DDR). These adaptive mechanisms take place respectively in the endoplasmic reticulum (ER) and in the nucleus. UPR and DDR alterations are associated with aging and with pathologies such as degenerative diseases, metabolic and inflammatory disorders, and cancer. We discuss the emerging signaling crosstalk between UPR stress sensors and the DDR, as well as their involvement in cancer biology.

Architecture of The Human Ape1 Interactome Defines Novel Cancers Signatures

APE1 is essential in cancer cells due to its central role in the Base Excision Repair pathway of DNA lesions and in the transcriptional regulation of genes involved in tumor progression/chemoresistance. Indeed, APE1 overexpression correlates with chemoresistance in more aggressive cancers, and APE1 protein-protein interactions (PPIs) specifically modulate different protein functions in cancer cells. Although important, a detailed investigation on the nature and function of protein interactors regulating APE1 role in tumor progression and chemoresistance is still lacking. The present work was aimed at analyzing the APE1-PPI network with the goal of defining bad prognosis signatures through systematic bioinformatics analysis. By using a well-characterized HeLa cell model stably expressing a flagged APE1 form, which was subjected to extensive proteomics analyses for immunocaptured complexes from different subcellular compartments, we here demonstrate that APE1 is a central hub connecting different subnetworks largely composed of proteins belonging to cancer-associated communities and/or involved in RNA- and DNA-metabolism. When we performed survival analysis in real cancer datasets, we observed that more than 80% of these APE1-PPI network elements is associated with bad prognosis. Our findings, which are hypothesis generating, strongly support the possibility to infer APE1-interactomic signatures associated with bad prognosis of different cancers; they will be of general interest for the future definition of novel predictive disease biomarkers. Future studies will be needed to assess the function of APE1 in the protein complexes we discovered. Data are available via ProteomeXchange with identifier PXD013368.

Effect of Hepatitis Viruses on the Nrf2/Keap1-Signaling Pathway and Its Impact on Viral Replication and Pathogenesis

With respect to their genome and their structure, the human hepatitis B virus (HBV) and hepatitis C virus (HCV) are complete different viruses. However, both viruses can cause an acute and chronic infection of the liver that is associated with liver inflammation (hepatitis). For both viruses chronic infection can lead to fibrosis, cirrhosis and hepatocellular carcinoma (HCC). Reactive oxygen species (ROS) play a central role in a variety of chronic inflammatory diseases. In light of this, this review summarizes the impact of both viruses on ROS-generating and ROS-inactivating mechanisms. The focus is on the effect of both viruses on the transcription factor Nrf2 (nuclear factor erythroid 2 (NF-E2)-related factor 2). By binding to its target sequence, the antioxidant response element (ARE), Nrf2 triggers the expression of a variety of cytoprotective genes including ROS-detoxifying enzymes. The review summarizes the literature about the pathways for the modulation of Nrf2 that are deregulated by HBV and HCV and describes the impact of Nrf2 deregulation on the viral life cycle of the respective viruses and the virus-associated pathogenesis.

APEX1 Expression as a Potential Diagnostic Biomarker of Clear Cell Renal Cell Carcinoma and Hepatobiliary Carcinomas

Apurinic/apyrimidinic endonuclease 1/redox effector factor 1 (APEX1) has been known to play key roles in DNA repair, the regulation of diverse transcriptional activity, and cellular responses to redox activity. This study aimed to examine serum APEX1 (s-APEX1) expression as a possible screening biomarker for clear cell renal cell carcinoma (ccRCC), hepatocellular carcinoma (HCC), and proximal and distal cholangiocarcinoma (CC). A total of 216 frozen serum samples were collected from 39 healthy control cases, 32 patients with ≥58 copies/mL of hepatitis B viral DNA (HBV DNA (+)), 40 ccRCC cases, 59 HCC cases, and 46 CC cases. The serum samples were examined for s-APEX1 concentration by enzyme-linked immunosorbent assay. The association of APEX1 expression with clinicopathological characteristics was also studied by immunohistochemical staining in 106 ccRCC, 131 HCC, and 32 intrahepatic CC cases. The median s-APEX1 concentrations of the HCC, CC, ccRCC, healthy control, and HBV DNA (+) groups were 0.294, 0.710, 0.474, 0.038, and 2.384 ng/mL, respectively (p < 0.001). Univariate and multivariate analyses revealed that increased cytoplasmic APEX1 expression led to a shorter disease-free survival period in HCC and CC cases. We suggest that the s-APEX1 level could be a potential diagnostic biomarker of ccRCC, HCC, and CC. Additionally, cytoplasmic APEX1 expression in cancer cells could be used to predict relapses in patients with HCC or CC.

Phosphoglucomutase 1 inhibits hepatocellular carcinoma progression by regulating glucose trafficking

Glycogen metabolism commonly altered in cancer is just beginning to be understood. Phosphoglucomutase 1 (PGM1), the first enzyme in glycogenesis that catalyzes the reversible conversion between glucose 1-phosphate (G-1-P) and glucose 6-phosphate (G-6-P), participates in both the breakdown and synthesis of glycogen. Here, we show that PGM1 is down-regulated in hepatocellular carcinoma (HCC), which is associated with the malignancy and poor prognosis of HCC. Decreased PGM1 expression obstructed glycogenesis pathway, which leads to the increased flow of glucose into glycolysis, thereby promoting tumor cell proliferation and HCC development. The loss of forkhead box protein J2 (FOXJ2), at least partly due to low genomic copy number in HCC, releases cellular nucleic acid-binding protein (CNBP), a nucleic acid chaperon, to bind to and promote G-quadruplex formation in PGM1 promoter and therefore decreases PGM1 expression. In addition, integrated analyses of PGM1 and FOXJ2 expression provide a better prediction for the malignance and prognosis of HCC. This study establishes a tumor-suppressive role of PGM1 by regulating glucose trafficking and uncovers a novel regulatory mechanism of PGM1 expression.

Hepatocellular carcinoma (HCC) is the most common type of primary liver cancer in adults. Sorafenib is the only clinically approved systemic drug for patients with advanced HCC. Identification of novel targets and biomarkers will provide new therapeutic strategies for advanced HCC and better prognostic prediction. Phosphoglucomutase (PGM) is an evolutionarily conserved enzyme that regulates one of the most important pathways in glucose metabolis—catalyzing the bidirectional interconversion of glucose 1-phosphate (G-1-P) and glucose 6-phosphate (G-6-P). In this study, we identify PGM1 as a metabolic tumor suppressor. Its expression allocates more glucose to glycogenesis, which reduces the glycolytic intermediates for biosynthesis, thereby impairing HCC progression. We delineate the mechanism of PGM1 down-regulation in HCC, finding that forkhead box protein J2 (FOXJ2) loss releases cellular nucleic acid-binding protein (CNBP) to bind to and modify the DNA structure of PGM1 promoter, thereby inhibiting PGM1 expression. Immunohistochemical analyses of human HCC tumors indicate that low FOXJ2 and PGM1 expression correlates with the malignancy and poor progression of human HCC. These results also suggest that the activation of residual PGM1 may impair HCC development through switching glycolysis to glycogenesis.

Establishment of a detection assay for DNA endonuclease activity and its application in the screening and prognosis of malignant lymphoma

BACKGROUND

Endonucleases play critical roles in maintaining genomic stability and regulating cell growth. The purpose of this study was to evaluate detection of endonuclease activity as an indicator in the early diagnosis and prognosis of lymphoma.

RESULTS

The method of endonuclease activity determination was successfully established and applied to compare cancer patient and control cohorts. Endonuclease activities of cancer tissues were significantly higher than those of adjacent control tissues (P < 0.001). We next investigated endonuclease activity in peripheral blood of enrolled patients and the controls, which were also significantly higher in patients than in controls (P = 0.015). Additionally, endonuclease activities were elevated in the metastasis subgroup compared with the non-metastasis subgroup(P = 0.038), whereas no significant difference was found between age(≤ 56y, > 56y) and gender (P = 0.736 > 0.05 and P = 0.635 > 0.05, respectively). Although there was no significant difference between control group with the non-metastatic cancer patients (P = 0.800 > 0.05), endonuclease activities were lower in the control group compared with the non-metastatic cancer patients with lymphoma (P = 0.033). The progression-free survival probability of patients with elevated R ratios(R ratio ≥ 1.4) was significantly lower than that of patients with lower R ratios (R ratio < 1.4).

CONCLUSIONS

An assay was established to detect the endonuclease activity,which might be useful for the prognosis of cancers, especially lymphoma.

Nuclear factor erythroid-2-related factor regulates LRWD1 expression and cellular adaptation to oxidative stress in human embryonal carcinoma cells

Leucine-rich repeats and WD repeat domain-containing protein 1 (LRWD1) is implicated in the regulation of signal transduction, transcription, RNA processing and tumor development. However, LRWD1 transcriptional regulation is not fully understood. This study aimed to investigate the relationship between LRWD1 expression and reactive oxygen species (ROS) level in human embryonal carcinoma cell line, NT2/D1 cells, which will help in understanding the transcriptional regulatory role of ROS in cells. Results showed that the exposure of NT2/D1 cells to various concentrations of hydrogen peroxide (H₂O₂) and the nitric oxide (NO) donor sodium nitroprusside (SNP) caused a significant increase in the mRNA and protein expression of LRWD1. In addition, LRWD1 promoter luciferase reporter assay, and Chromatin Immunoprecipitation assay (CHIP assay) showed that nuclear factor erythroid-2-related factor (Nrf2) was involved in the regulation of LRWD1 expression in response to oxidative stress. The involvement of Nrf2 was confirmed by shRNA-mediated knockdown of Nrf2 in NT2/D1 cells, which caused a significant decrease in LRWD1 expression in response to oxidative stress. Similarly, LRWD1 knockdown resulted in the accumulation of H₂O₂ and superoxide anion radical (O2-). Blocking ROS production by N-acetyl cysteine (NAC) protected NT2/D1 shLRWD1cells from H₂O₂-induced cell death. Collectively, oxidative stress increased LRWD1 expression through a Nrf2-dependent mechanism, which plays an important role in cellular adaptation to oxidative stress. These results highlight an evidence, on the molecular level, about LRWD1 transcriptional regulation under oxidative stress.