Nickel and cadmium-induced SLBP depletion: A potential pathway to metal mediated cellular transformation

Contribution of histone variants to aneuploidy: a cancer perspective

Histone variants, which generally differ in few amino acid residues, can replace core histones (H1, H2A, H2B, and H3) to confer specific structural and functional features to regulate cellular functions. In addition to their role in DNA packaging, histones modulate key processes such as gene expression regulation and chromosome segregation, which are frequently dysregulated in cancer cells. During the years, histones variants have gained significant attention as gatekeepers of chromosome stability, raising interest in understanding how structural and functional alterations can contribute to tumourigenesis. Beside the well-established role of the histone H3 variant CENP-A in centromere specification and maintenance, a growing body of literature has described mutations, aberrant expression patterns and post-translational modifications of a variety of histone variants in several cancers, also coining the term "oncohistones." At the molecular level, mechanistic studies have been dissecting the biological mechanisms behind histones and missegregation events, with the potential to uncover novel clinically-relevant targets. In this review, we focus on the current understanding and highlight knowledge gaps of the contribution of histone variants to aneuploidy, and we have compiled a database (HistoPloidyDB) of histone gene alterations linked to aneuploidy in cancers of the The Cancer Genome Atlas project.

An increase in polyadenylation of histone isoforms, Hist1h2ah and Hist2h3c2, is governed by 3'-UTR in de-differentiated and undifferentiated hepatocyte

Replication-dependent histones have a stem-loop structure at the 3' end of messenger RNA (mRNA) and are stabilized by stem-loop binding protein (SLBP). Moreover, loss of SLBP and imbalance in the level of ARE (adenylate-uridylate-rich elements)-binding proteins, HuR, and BRF1 are associated with the polyadenylation of canonical histone mRNAs under different physiological conditions. Previous studies from the lab have shown increased protein levels of H2A1H and H3.2 in N-nitrosodiethylamine (NDEA)-induced hepatocellular carcinoma (HCC). In this study, we report that increase in the polyadenylation of histone mRNA contributes to increased levels of H2A1H and H3.2 in NDEA-induced HCC. The persistent exposure to carcinogen with polyadenylation of histone mRNA increases the total histone pool resulting in aneuploidy. The embryonic liver has also shown increased polyadenylated histone isoforms, Hist1h2ah and Hist2h3c2, primarily contributing to their increased protein levels. The increase in polyadenylation of histone mRNA in HCC and e15 are in coherence with the decrease in SLBP and BRF1 with an increase in HuR. Our studies in neoplastic CL38 cell line showed that direct stress on the cells induces downregulation of SLBP with enhanced histone isoform polyadenylation. Moreover, the polyadenylation is related to increase in activated MAP kinases, p38, ERK, and JNK in HCC liver tumor tissues and CL38 cells treated with arsenic. Our data suggest that SLBP degrades under stress, destabilizing the stem-loop, elongating histone isoforms mRNA with 3' polyadenylated tail with increase of HuR and decrease of BRF1. Overall, our results indicate that SLBP may play an essential part in cell proliferation, at least in persistent exposure to stress, by mediating the stabilization of histone isoforms throughout the cell cycle.

Polyadenylation of canonical histone H3.1 in carcinogenesis

Canonical histone messenger RNAs (mRNAs) are transcribed during S phase and do not terminate with a poly(A) tail at the 3' end. Instead, the histone mRNAs display a stem-loop structure at their 3-end. Stem-loop-binding protein (SLBP) binds the stem-loop and regulates canonical histone mRNA metabolism. We previously demonstrated that exposure to arsenic, an environmental carcinogen, induces polyadenylation of canonical histone H3.1 mRNA, causing transformation of human cells in vitro. Arsenic decreased cellular levels of SLBP by inducing its proteasomal degradation and inhibiting SLBP transcription via epigenetic mechanisms. Similarly, we also reported that nickel and arsenic have similar effects on canonical histone mRNA transcription and translation. Most recently, we further demonstrated that bisphenols' exposure increased polyadenylation of canonical histone H3.1 mRNA possibly through down-regulation of SLBP expression. This facilitates the abnormal stability of at least one canonical histone isoform (H3.1), and also increases H3 protein levels. Excess expression of canonical histones have been shown to increase sensitivity to DNA damage as well as increase the frequency of missing chromosomes and induce genomic instability. Thus, polyadenylation of canonical histone mRNA following arsenic, nickel and bisphenols exposure may contribute to metal and bisphenol-induced carcinogenesis.

Epigenetic mechanisms in metal carcinogenesis

Many metals exhibit genotoxic and/or carcinogenic effects. These toxic metals can be found ubiquitously - in drinking water, food, air, general use products, in everyday and occupational settings. Exposure to such carcinogenic metals can result in serious health disorders, including cancer. Arsenic, cadmium, chromium, nickel, and their compounds have already been recognized as carcinogens by the International Agency for Research on Cancer. This review summarizes a wide range of epigenetic mechanisms contributing to carcinogenesis induced by these metals, primarily including, but not limited to, DNA methylation, miRNA regulation, and histone posttranslational modifications. The mechanisms are described and discussed both from a metal-centric and a mechanism-centric standpoint. The review takes a broad perspective, putting the mechanisms in the context of real-life exposure, and aims to assist in guiding future research, particularly with respect to the assessment and control of exposure to carcinogenic metals and novel therapy development.

Stem-loop binding protein and metal carcinogenesis

Pre-mRNA processing of the replication-dependent canonical histone mRNAs requires an endonucleolytic cleavage immediately after a conserved stem loop structure which occurs before RNA Pol II encounters any poly(A) signal. Thus, in contrast to all other eukaryotic mRNAs, the canonical histone mRNAs are not polyadenylated in their 3' ends. The binding of stem-loop binding protein (SLBP) to the stem loop structure of the histone mRNAs is required for this process. SLBP is also involved in regulation of histone mRNA nuclear export, degradation, and translation. Depletion of SLBP has been shown to induce polyadenylation of histone mRNAs and alteration of histone protein levels, which are considered to contribute to the observed aberrant cell cycle progress and genomic instability resulting from the loss of SLBP function. Recent studies have demonstrated that some heavy metal carcinogens, including arsenic and nickel, can induce the loss of SLBP and the gain of polyadenylation of canonical histone mRNAs. Polyadenylated canonical histone H3 can result in abnormal transcription, cell cycle arrest, genomic instability, and cell transformation, which links SLBP depletion and subsequent histone mRNA misprocessing to cancer. This review seeks to briefly summarize what is known about regulation of SLBP expression, consequences of SLBP depletion, its roles in cancer-related end points, with particular focus on metal-induced SLBP depletion and the potential of SLBP depletion as a new mechanism for metal-induced carcinogenesis.

Dysregulation of microRNAs in metal-induced angiogenesis and carcinogenesis

MicroRNAs (miRNAs) are small endogenous non-coding RNAs that regulate cancer initiation, development, angiogenesis, and therapeutic resistance. Metal exposure widely occurs through air, water, soil, food, and industrial contaminants. Hundreds of millions of people may have metal exposure associated with toxicity, serious health problems, and cancer occurrence. Metal exposure is found to induce oxidative stress, DNA damage and repair, and activation of multiple signaling pathways. However, molecular mechanisms of metal-induced carcinogenesis remain to be elucidated. Recent studies demonstrated that the exposure of metals such as arsenic, hexavalent chromium, cadmium, and nickel caused dysregulation of microRNAs that are implicated to play an important role in cell transformation, tumor growth and angiogenesis. This review focuses on the recent studies that show metal-induced miRNA dysregulation and underlined mechanisms in cell malignant transformation, angiogenesis and tumor growth.

LncRNA-ENST00000446135 is a novel biomarker of cadmium toxicity in 16HBE cells, rats, and Cd-exposed workers and regulates DNA damage and repair

Cadmium (Cd) and its compounds are well-known human carcinogens, but the mechanisms underlying the carcinogenesis are not well understood. This study aimed to investigate whether long noncoding RNA (LncRNA)-ENST00000446135 could serve as a novel biomarker of Cd toxicity in cells, animals, and Cd-exposed workers and regulate DNA damage and repair. LncRNA-ENST00000446135 expression increased gradually in cadmium chloride-transformed 16HBE cells. Small interfering RNA-mediated knockdown of LncRNA-ENST00000446135 inhibited the growth of DNA-damaged cells and decreased the expressions of DNA damage-related genes (ATM, ATR, and ATRIP), whereas increased the expressions of DNA repair-related genes (DDB1, DDB2, OGG1, ERCC1, MSH2, XRCC1, and BARD1). Chromatin immunoprecipitation-sequencing showed that MSH2 is a direct transcriptional target of lncRNA-ENST00000446135. Cadmium increased lncRNA-ENST00000446135 expression in the lung of Cd-exposed rats in a dose-dependent manner. A significant positive correlation was observed between blood ENST00000446135 expression and urinary/blood Cd concentrations, and there were significant correlations of LncRNA-ENST00000446135 expression with the DNA damage cell and the expressions of target genes in the lung of Cd-exposed rats and the blood of Cd-exposed workers and significantly correlated with liver and renal function in Cd-exposed workers. These results indicate that the expression of LncRNA-ENST00000446135 is upregulated and may serve as a signature for DNA damage and repair related to the epigenetic mechanisms underlying the cadmium toxicity and become a novel biomarker of cadmium toxicity.

Epigenetic influence of environmentally neurotoxic metals

Continuous globalization and industrialization have ensured metals are an increasing aspect of daily life. Their usefulness in manufacturing has made them vital to national commerce, security and global economy. However, excess exposure to metals, particularly as a result of environmental contamination or occupational exposures, has been detrimental to overall health. Excess exposure to several metals is considered environmental risk in the aetiology of several neurological and neurodegenerative diseases. Metal-induced neurotoxicity has been a major health concern globally with intensive research to unravel the mechanisms associated with it. Recently, greater focus has been directed at epigenetics to better characterize the underlying mechanisms of metal-induced neurotoxicity. Epigenetic changes are those modifications on the DNA that can turn genes on or off without altering the DNA sequence. This review discusses how epigenetic changes such as DNA methylation, post translational histone modification and noncoding RNA-mediated gene silencing mediate the neurotoxic effects of several metals, focusing on manganese, arsenic, nickel, cadmium, lead, and mercury.

Polyadenylation of Histone H3.1 mRNA Promotes Cell Transformation by Displacing H3.3 from Gene Regulatory Elements

Replication-dependent canonical histone messenger RNAs (mRNAs) do not terminate with a poly(A) tail at the 3' end. We previously demonstrated that exposure to arsenic, an environmental carcinogen, induces polyadenylation of canonical histone H3.1 mRNA, causing transformation of human cells in vitro. Here we report that polyadenylation of H3.1 mRNA increases H3.1 protein, resulting in displacement of histone variant H3.3 at active promoters, enhancers, and insulator regions, leading to transcriptional deregulation, G2/M cell-cycle arrest, chromosome aneuploidy, and aberrations. In support of these observations, knocking down the expression of H3.3 induced cell transformation, whereas ectopic expression of H3.3 attenuated arsenic-induced cell transformation. Notably, arsenic exposure also resulted in displacement of H3.3 from active promoters, enhancers, and insulator regions. These data suggest that H3.3 displacement might be central to carcinogenesis caused by polyadenylation of H3.1 mRNA upon arsenic exposure. Our findings illustrate the importance of proper histone stoichiometry in maintaining genome integrity.

Metals and molecular carcinogenesis

Many metals are essential for living organisms, but at higher doses they may be toxic and carcinogenic. Metal exposure occurs mainly in occupational settings and environmental contaminations in drinking water, air pollution and foods, which can result in serious health problems such as cancer. Arsenic (As), beryllium (Be), cadmium (Cd), chromium (Cr) and nickel (Ni) are classified as Group 1 carcinogens by the International Agency for Research on Cancer. This review provides a comprehensive summary of current concepts of the molecular mechanisms of metal-induced carcinogenesis and focusing on a variety of pathways, including genotoxicity, mutagenesis, oxidative stress, epigenetic modifications such as DNA methylation, histone post-translational modification and alteration in microRNA regulation, competition with essential metal ions and cancer-related signaling pathways. This review takes a broader perspective and aims to assist in guiding future research with respect to the prevention and therapy of metal exposure in human diseases including cancer.

Metals and Mechanisms of Carcinogenesis

Metal exposure is pervasive and not limited to sporadic poisoning events or toxic waste sites. Hundreds of millions of people around the globe are affected by chronic metal exposure, which is associated with serious health concerns, including cancer, as demonstrated in a variety of studies at the molecular, systemic, and epidemiologic levels. Metal-induced toxicity and carcinogenicity are sophisticated and complex in nature. This review provides a broad context and holistic view of currently available studies on the mechanisms of metal-induced carcinogenesis. Specifically, we focus on the five most prevalent carcinogenic metals, arsenic, nickel, cadmium, chromium, and beryllium, and their potential to drive carcinogenesis in humans. A comprehensive understanding of the mechanisms behind the development of metal-induced cancer can provide valuable insights for therapeutic intervention involving molecular targets in metal-induced carcinogenesis.

Toxicity of organic and inorganic nickel in pancreatic cell cultures: Comparison to cadmium

Nickel compounds are Group 1 carcinogens and possibly cancer-causing in the pancreas. We examined the toxicity of nickel in both 2-D and 3-D pancreatic cell cultures, to determine the LD50 for organic and inorganic nickel in normal and cancerous cells. Assays with cadmium chloride were performed to be a comparison to potential nickel-induced toxicity. Cells were exposed to twelve concentrations of NiCl2 or Ni-(Ac)2 for 48h (2-D), or six concentrations for 48 hours (3-D). There was a significant (P=0.0016) difference between HPNE and AsPC-1 LD50 values after cadmium exposure, at 69.9 µM and 29.2 µM, respectively. Neither form of nickel exhibited toxicity in 2-D or 3-D cultures, but after 48h, changes in spheroid morphology were observed. The inability of Ni to reduce viable cell numbers suggests a toxic mechanism that differs from cadmium, also a Group 1 carcinogen. The cell microenvironment was not a factor in nickel toxicity with no changes in viable cells in either 2-D or 3-D cultures. These studies only examined cytotoxicity, and not genotoxicity, a potential mechanism of nickel carcinogenicity. Alterations in DNA function or the expression of apoptotic proteins/processes would take longer to manifest. Current work focuses on cellular changes following extended nickel exposure.