Down-regulation of lncRNA MEG3 promotes chronic low dose cadmium exposure-induced cell transformation and cancer stem cell-like property

The dynamic face of cadmium-induced Carcinogenesis: Mechanisms, emerging trends, and future directions

Cadmium (Cd) is a malleable element with odorless, tasteless characteristics that occurs naturally in the earth's crust, underground water, and soil. The most common reasons for the anthropological release of Cd to the environment include industrial metal mining, smelting, battery manufacturing, fertilizer production, and cigarette smoking. Cadmium-containing products may enter the environment as soluble salts, vapor, or particle forms that accumulate in food, soil, water, and air. Several epidemiological studies have highlighted the association between Cd exposure and adverse health outcomes, especially renal toxicity, and the impact of Cd exposure on the development and progression of carcinogenesis. Also highlighted is the evidence for early-life and even maternal exposure to Cd leading to devastating health outcomes, especially the risk of cancer development in adulthood. Several mechanisms have been proposed to explain how Cd mediates carcinogenic transformation, including epigenetic alteration, DNA methylation, histone posttranslational modification, dysregulated non-coding RNA, DNA damage in the form of DNA mutation, strand breaks, and chromosomal abnormalities with double-strand break representing the most common DNA form of damage. Cd induces an indirect genotoxic effect by reducing p53's DNA binding activity, eventually impairing DNA repair, inducing downregulation in the expression of DNA repair genes, which might result in carcinogenic transformation, enhancing lipid peroxidation or evasion of antioxidant interference such as catalase, superoxide dismutase, and glutathione. Moreover, Cd mediates apoptosis evasion, autophagy activation, and survival mechanisms. In this review, we decipher the role of Cd mediating carcinogenic transformation in different models and highlight the interaction between various mechanisms. We also discuss diagnostic markers, therapeutic interventions, and future perspectives.

Pollutants to pathogens: The role of heavy metals in modulating TGF-β signaling and lung cancer risk

Lung cancer is a malignant tumor that develops in the lungs due to the uncontrolled growth of aberrant cells. Heavy metals, such as arsenic, cadmium, mercury, and lead, are metallic elements characterized by their high atomic weights and densities. Anthropogenic activities, such as industrial operations and pollution, have the potential to discharge heavy metals into the environment, hence presenting hazards to ecosystems and human well-being. The TGF-β signalling pathways have a crucial function in controlling several cellular processes, with the ability to both prevent and promote tumor growth. TGF-β regulates cellular responses by interacting in both canonical and non-canonical signalling pathways. Research employing both in vitro and in vivo models has shown that heavy metals may trigger TGF-β signalling via complex molecular pathways. Experiments conducted in a controlled laboratory environment show that heavy metals like cadmium and arsenic may directly bind to TGF-β receptors, leading to alterations in their structure that enable the receptor to be phosphorylated. Activation of this route sets in motion subsequent signalling cascades, most notably the canonical Smad pathway. The development of lung cancer has been linked to heavy metals, which are ubiquitous environmental pollutants. To grasp the underlying processes, it is necessary to comprehend their molecular effect on TGF-β pathways. With a particular emphasis on its consequences for lung cancer, this abstract delves into the complex connection between exposure to heavy metals and the stimulation of TGF-β signalling.

Epigenetic downregulation of O6-methylguanine-DNA methyltransferase contributes to chronic hexavalent chromium exposure-caused genotoxic effect and cell transformation

Hexavalent chromium [Cr(VI)] is a common environmental pollutant and chronic exposure to Cr(VI) causes lung cancer and other types of cancer in humans, although the mechanism of Cr(VI) carcinogenesis remains elusive. Cr(VI) has been considered as a genotoxic carcinogen, but accumulating evidence indicates that Cr(VI) also causes various epigenetic toxic effects that play important roles in Cr(VI) carcinogenesis. However, it is not clear how Cr(VI)-caused epigenetic dysregulations contributes to Cr(VI) carcinogenesis. This study investigates whether Cr(VI) epigenetic toxic effect has an impact on its genotoxic effect. It was found that chronic low dose of Cr(VI) exposure time-dependently down-regulates the expression of a critical DNA damage repair protein O6-methylguanine-DNA methyltransferase (MGMT), leading to the increases of the levels of the highly mutagenic and carcinogenic DNA lesion O6-methylguanine (O6-MeG) in human bronchial epithelial BEAS-2B cells. Moreover, the levels of MGMT and O6-MeG in chronic Cr(VI) exposure-caused human lung cancer tissues are also significantly lower and higher than that in the adjacent normal lung tissues, respectively. It was further determined that chronic low dose of Cr(VI) exposure-transformed BEAS-2B cells display impaired DNA damage repair capacity and a high sensitivity to the toxicity of the alkylating chemotherapeutic drug Temozolomide. In contrast, stably overexpressing MGMT in parental BEAS-2B cells reverses chronic low dose of Cr(VI) exposure-caused DNA damage repair deficiency and significantly reduces cell transformation by Cr(VI). Further mechanistical studies revealed that chronic low dose of Cr(VI) exposure down-regulates MGMT expression through epigenetic mechanisms by increasing DNA methylation and histone H3 repressive modifications. Taken together, these findings suggest that epigenetic down-regulation of a crucial DNA damage repair protein MGMT contributes significantly to the genotoxic effect and cell transformation caused by chronic low dose of Cr(VI) exposure.

Up-regulation of RNA m6A methyltransferase like-3 expression contributes to arsenic and benzo[a]pyrene co-exposure-induced cancer stem cell-like property and tumorigenesis

While arsenic or BaP alone exposure can cause lung cancer, studies showed that arsenic plus BaP co-exposure displays a significantly stronger lung tumorigenic effect. However, the underlying mechanism has not been well understood. Studies showed that RNA molecules are chemically modified. The most frequently occurring RNA modification in eukaryotic messenger RNAs is the N6-methyladenosine (m6A) methylation. This study aimed to determine whether arsenic plus BaP exposure alters RNA m6A methylation and its role in lung tumorigenic effect of arsenic plus BaP exposure. Human bronchial epithelial cells transformed by exposure to arsenic or BaP alone, and arsenic plus BaP and mouse xenograft tumorigenesis models were used in this study. It was found that arsenic plus BaP exposure-transformed cells have significantly higher levels of RNA m6A methylation than arsenic or BaP alone exposure-transformed human bronchial epithelial cells. Western blot analysis showed that arsenic plus BaP exposure greatly up-regulates the m6A writer methyltransferase like-3 (METTL3) expression levels in cultured cells and mouse lung tissues. METTL3 knockdown in cells transformed by arsenic plus BaP exposure drastically reduced their RNA m6A methylation levels. Functional studies revealed that METTL3 knockdown in cells transformed by arsenic plus BaP exposure greatly reduces their anchorage-dependent and -independent growth, cancer stem cell characters and tumorigenesis. The findings from this study suggest that arsenic plus BaP co-exposure causes epitranscriptomic dysregulation, which may contribute significantly to arsenic plus BaP co-exposure-caused synergistic lung tumorigenic effect.

Comprehensive landscape and future perspective of long noncoding RNAs in non-small cell lung cancer: it takes a village

Long noncoding RNAs (lncRNAs) are a distinct subtype of RNA that lack protein-coding capacity but exert significant influence on various cellular processes. In non-small cell lung cancer (NSCLC), dysregulated lncRNAs act as either oncogenes or tumor suppressors, contributing to tumorigenesis and tumor progression. LncRNAs directly modulate gene expression, act as competitive endogenous RNAs by interacting with microRNAs or proteins, and associate with RNA binding proteins. Moreover, lncRNAs can reshape the tumor immune microenvironment and influence cellular metabolism, cancer cell stemness, and angiogenesis by engaging various signaling pathways. Notably, lncRNAs have shown great potential as diagnostic or prognostic biomarkers in liquid biopsies and therapeutic strategies for NSCLC. This comprehensive review elucidates the significant roles and diverse mechanisms of lncRNAs in NSCLC. Furthermore, we provide insights into the clinical relevance, current research progress, limitations, innovative research approaches, and future perspectives for targeting lncRNAs in NSCLC. By summarizing the existing knowledge and advancements, we aim to enhance the understanding of the pivotal roles played by lncRNAs in NSCLC and stimulate further research in this field. Ultimately, unraveling the complex network of lncRNA-mediated regulatory mechanisms in NSCLC could potentially lead to the development of novel diagnostic tools and therapeutic strategies.

Inorganic arsenic-mediated upregulation of TUG1 promotes apoptosis in human bronchial epithelial cells by activating the p53 signaling pathway

Exposure to arsenic, an environmental contaminant, is known to cause arsenicosis and cancer. Although considerable research has been conducted to understand the underlying mechanism responsible for arsenic-induced cancers, the precise molecular mechanisms remain unknown, especially at the epigenetic regulation level. Long non-coding RNAs (LncRNAs) that have been shown to mediate various biological processes, including proliferation, apoptosis, necrosis, and mutagenesis. There are few studies on LncRNAs and biological damage caused by environmental pollutants. The LncRNAs taurine upregulated gene 1 (TUG1) regulates cell growth both in vitro and in vivo, and contributes its oncogenic role. However, the precise roles and related mechanisms of arsenic-induced cell apoptosis are still not fully understood owing to controversial findings in the literature. In this study, quantitative real-time polymerase chain reaction (qRT-PCR) analysis revealed higher expression levels of TUG1 in people occupationally exposed to arsenic than in individuals living away from the source of arsenic exosure (N = 25). In addition, the results suggested that TUG1 was involved in arsenic-induced apoptosis. Furthermore, knockdown experiments showed that silencing of TUG1 markedly inhibited proliferation, whereas depletion of TUG1 led to increased apoptosis. The TUG1-small interfering RNA (siRNA) combination with arsenic (3 μM/L) slightly increased apoptosis compared with the TUG1-siRNA. Additionally, the knockdown experiments showed that the silencing of TUG1 by siRNA inhibited proliferation and promoted apoptosis by inducing p53, p-p53 (ser392), FAS, BCL2, MDM2, cleaved-caspase7 proteins in 16HBE cells. These results indicated that arsenic mediates the upregulation of TUG1 and induces cell apoptosis via activating the p53 signaling pathway.

A prognostic and immunotherapy effectiveness model for pancreatic adenocarcinoma based on cuproptosis-related lncRNAs signature

Pancreatic adenocarcinoma (PAAD) results in one of the deadliest solid tumors with discouraging clinical outcomes. Growing evidence suggests that long non-coding RNAs (lncRNAs) play a crucial role in altering the growth, prognosis, migration, and invasion of pancreatic cancer cells. Cuproptosis is a novel type of cell death induced by copper (Cu) and is associated with mitochondrial respiration during the tricarboxylic acid cycle. However, the relationship between lncRNAs related to cuproptosis and PAAD is poorly studied. In this study, we investigated the association between a signature of cuproptosis-related lncRNAs and the diagnosis of PAAD. Genomic data and clinical information were obtained using the TCGA dataset, while cuproptosis-related genes (CRGs) from previous studies. Co-expression analysis was utilized to identify lncRNAs associated with cuproptosis. We developed and verified a prognostic risk model following a classification of patients into high- and low-risk categories. The prediction capacity of the risk model was assessed using a number of methods including Kaplan-Meier analysis, receiver operating characteristic (ROC) curves, nomograms, and principal component analysis (PCA). Furthermore, differentially expressed genes (DEGs) were used to perform functional enrichment analyses, and to examine the behaviors of various risk groups in terms of immune-related activities and medication sensitivity. We identified 7 cuproptosis-related lncRNA signatures, including CASC19, FAM83A-AS1, AC074099.1, AC007292.2, AC026462.3, AL358944.1, and AC009019.1, as overall survival (OS) predictors. OS and progression-free survival (PFS) showed significant differences among patients in different risk groups. Independent prognostic analysis revealed that the cuproptosis-related lncRNA signatures can independently achieve patient prognosis. The risk model demonstrated strong predictive ability for patient outcomes, as evidenced by ROC curves, nomograms, and PCA. Higher tumor mutation burden (TMB) and lower tumor immune dysfunction and exclusion (TIDE) scores were observed in the high-risk group. Additionally, the low-risk group was hypersensitive to 3 anti-cancer medications, whereas the high-risk group was hypersensitive to one. A prognostic risk model with a good predictive ability based on cuproptosis-related lncRNAs was developed, providing a theoretical basis for personalized treatment and immunotherapeutic responses in pancreatic cancer.

Downregulation of LncRNA GCLC-1 Promotes Microcystin-LR-Induced Malignant Transformation of Human Liver Cells by Regulating GCLC Expression

Microcystin-LR (MCLR) is an aquatic toxin, which could lead to the development of hepatocellular carcinoma (HCC). Long non-coding RNAs (lncRNAs) are considered important regulatory elements in the occurrence and development of cancer. However, the roles and mechanisms of lncRNAs during the process of HCC, induced by MCLR, remain elusive. Here, we identified a novel lncRNA, namely lnc-GCLC-1 (lncGCLC), which is in close proximity to the chromosome location of glutamate-cysteine ligase catalytic subunit (GCLC). We then investigated the role of lncGCLC in MCLR-induced malignant transformation of WRL68, a human hepatic cell line. During MCLR-induced cell transformation, the expression of lncGCLC and GCLC decreased continuously, accompanied with a consistently high expression of miR-122-5p. Knockdown of lncGCLC promoted cell proliferation, migration and invasion, but reduced cell apoptosis. A xenograft nude mouse model demonstrated that knockdown of lncGCLC promoted tumor growth. Furthermore, knockdown of lncGCLC significantly upregulated miR-122-5p expression, suppressed GCLC expression and GSH levels, and enhanced oxidative DNA damages. More importantly, the expression of lncGCLC in human HCC tissues was significantly downregulated in the high-microcystin exposure group, and positively associated with GCLC level in HCC tissues. Together, these findings suggest that lncGCLC plays an anti-oncogenic role in MCLR-induced malignant transformation by regulating GCLC expression.

Long Non-Coding RNA MEG3 in Metal Carcinogenesis

Most transcripts from human genomes are non-coding RNAs (ncRNAs) that are not translated into proteins. ncRNAs are divided into long (lncRNAs) and small non-coding RNAs (sncRNAs). LncRNAs regulate their target genes both transcriptionally and post-transcriptionally through interactions with proteins, RNAs, and DNAs. Maternally expressed gene 3 (MEG3), a lncRNA, functions as a tumor suppressor. MEG3 regulates cell proliferation, cell cycle, apoptosis, hypoxia, autophagy, and many other processes involved in tumor development. MEG3 is downregulated in various cancer cell lines and primary human cancers. Heavy metals, such as hexavalent chromium (Cr(VI)), arsenic, nickel, and cadmium, are confirmed human carcinogens. The exposure of cells to these metals causes a variety of cancers. Among them, lung cancer is the one that can be induced by exposure to all of these metals. In vitro studies have demonstrated that the chronic exposure of normal human bronchial epithelial cells (BEAS-2B) to these metals can cause malignant cell transformation. Metal-transformed cells have the capability to cause an increase in cell proliferation, resistance to apoptosis, elevated migration and invasion, and properties of cancer stem-like cells. Studies have revealed that MEG is downregulated in Cr(VI)-transformed cells, nickel-transformed cells, and cadmium (Cd)-transformed cells. The forced expression of MEG3 reduces the migration and invasion of Cr(VI)-transformed cells through the downregulation of the neuronal precursor of developmentally downregulated protein 9 (NEDD9). MEG3 suppresses the malignant cell transformation of nickel-transformed cells. The overexpression of MEG3 decreases Bcl-xL, causing reduced apoptosis resistance in Cd-transformed cells. This paper reviews the current knowledge of lncRNA MEG3 in metal carcinogenesis.

Epigenetic Regulation in Chromium-, Nickel- and Cadmium-Induced Carcinogenesis

Environmental and occupational exposure to heavy metals, such as hexavalent chromium, nickel, and cadmium, are major health concerns worldwide. Some heavy metals are well-documented human carcinogens. Multiple mechanisms, including DNA damage, dysregulated gene expression, and aberrant cancer-related signaling, have been shown to contribute to metal-induced carcinogenesis. However, the molecular mechanisms accounting for heavy metal-induced carcinogenesis and angiogenesis are still not fully understood. In recent years, an increasing number of studies have indicated that in addition to genotoxicity and genetic mutations, epigenetic mechanisms play critical roles in metal-induced cancers. Epigenetics refers to the reversible modification of genomes without changing DNA sequences; epigenetic modifications generally involve DNA methylation, histone modification, chromatin remodeling, and non-coding RNAs. Epigenetic regulation is essential for maintaining normal gene expression patterns; the disruption of epigenetic modifications may lead to altered cellular function and even malignant transformation. Therefore, aberrant epigenetic modifications are widely involved in metal-induced cancer formation, development, and angiogenesis. Notably, the role of epigenetic mechanisms in heavy metal-induced carcinogenesis and angiogenesis remains largely unknown, and further studies are urgently required. In this review, we highlight the current advances in understanding the roles of epigenetic mechanisms in heavy metal-induced carcinogenesis, cancer progression, and angiogenesis.

Toxicoepigenetics and Environmental Health: Challenges and Opportunities

The rapidly growing field of toxicoepigenetics seeks to understand how toxicant exposures interact with the epigenome to influence disease risk. Toxicoepigenetics is a promising field of environmental health research, as integrating epigenetics into the field of toxicology will enable a more thorough evaluation of toxicant-induced disease mechanisms as well as the elucidation of the role of the epigenome as a biomarker of exposure and disease and possible mediator of exposure effects. Likewise, toxicoepigenetics will enhance our knowledge of how environmental exposures, lifestyle factors, and diet interact to influence health. Ultimately, an understanding of how the environment impacts the epigenome to cause disease may inform risk assessment, permit noninvasive biomonitoring, and provide potential opportunities for therapeutic intervention. However, the translation of research from this exciting field into benefits for human and animal health presents several challenges and opportunities. Here, we describe four significant areas in which we see opportunity to transform the field and improve human health by reducing the disease burden caused by environmental exposures. These include (1) research into the mechanistic role for epigenetic change in environment-induced disease, (2) understanding key factors influencing vulnerability to the adverse effects of environmental exposures, (3) identifying appropriate biomarkers of environmental exposures and their associated diseases, and (4) determining whether the adverse effects of environment on the epigenome and human health are reversible through pharmacologic, dietary, or behavioral interventions. We then highlight several initiatives currently underway to address these challenges.

Identification of Candidate lncRNA and Pseudogene Biomarkers Associated with Carbon-Nanotube-Induced Malignant Transformation of Lung Cells and Prediction of Potential Preventive Drugs

Mounting evidence has linked carbon nanotube (CNT) exposure with malignant transformation of lungs. Long non-coding RNAs (lncRNAs) and pseudogenes are important regulators to mediate the pathogenesis of diseases, representing potential biomarkers for surveillance of lung carcinogenesis in workers exposed to CNTs and possible targets to develop preventive strategies. The aim of this study was to screen crucial lncRNAs and pseudogenes and predict preventive drugs. GSE41178 (small airway epithelial cells exposed to single- or multi-walled CNTs or dispersant control) and GSE56104 (lung epithelial cells exposed to single-walled CNTs or dispersant control) datasets were downloaded from the Gene Expression Omnibus database. Weighted correlation network analysis was performed for these two datasets, and the turquoise module was preserved and associated with CNT-induced malignant phenotypes. In total, 24 lncRNAs and 112 pseudogenes in this module were identified as differentially expressed in CNT-exposed cells compared with controls. Four lncRNAs (MEG3, ARHGAP5-AS1, LINC00174 and PVT1) and five pseudogenes (MT1JP, MT1L, RPL23AP64, ZNF826P and TMEM198B) were predicted to function by competing endogenous RNA (MEG3/RPL23AP64-hsa-miR-942-5p-CPEB2/PHF21A/BAMBI; ZNF826P-hsa-miR-23a-3p-SYNGAP1, TMEM198B-hsa-miR-15b-5p-SYNGAP1/CLU; PVT1-hsa-miR-423-5p-PSME3) or co-expression (MEG3/MT1L/ZNF826P/MT1JP-ATM; ARHGAP5-AS1-TMED10, LINC00174-NEDD4L, ARHGAP5-AS1/PVT1-NIP7; MT1L/MT1JP-SYNGAP1; MT1L/MT1JP-CLU) mechanisms. The expression levels and prognosis of all genes in the above interaction pairs were validated using lung cancer patient samples. The receiver operating characteristic curve analysis showed the combination of four lncRNAs, five pseudogenes or lncRNAs + pseudogenes were all effective for predicting lung cancer (accuracy >0.8). The comparative toxicogenomics database suggested schizandrin A, folic acid, zinc or gamma-linolenic acid may be preventive drugs by reversing the expression levels of lncRNAs or pseudogenes. In conclusion, this study highlights lncRNAs and pseudogenes as candidate diagnostic biomarkers and drug targets for CNT-induced lung cancer.