Epigenetic and epitranscriptomic mechanisms of chromium carcinogenesis

Dysregulation of Long Non-coding RNAs-the Novel lnc in Metal Toxicity and Carcinogenesis

PURPOSE OF REVIEW

Metals are common environmental pollutants. Acute and chronic exposures to non-essential toxic metals or excessive essential metals cause various diseases including cancer in humans. However, the underlying mechanisms have not been well understood. Long non-coding RNAs (lncRNAs) refer to RNA transcripts that have more than 200 nucleotides but do not have significant protein coding capacities. While lncRNAs were once considered transcription noise, they have become increasingly recognized as crucial players in various physiological and pathogenesis processes. The goal of this article is to review and discuss recent studies that show important roles of lncRNA dysregulations in metal toxicity and carcinogenesis.

RECENT FINDINGS

Recent studies showed that metal exposures dysregulate expression of lncRNAs in cultured cells, animals and humas. However, only a few studies determined the mechanisms of how metal exposure dysregulated expression of lncRNAs. The majority of the studies reported the association of abnormally expressed lncRNAs with various toxic effects of metal exposures, only limited studies established causal relationships demonstrating causal roles of dysregulated lncRNAs in metal toxicity and carcinogenesis. Mechanistically, most studies reported that dysregulated lncRNAs functioned as microRNA sponges to regulate gene expression, much less studies explored other mechanisms of lncRNA actions. It is evident that metal exposures dysregulate expression of lncRNAs, which may serve as novel mediators in metal toxicity and carcinogenesis. Further studies are needed to establish dysregulated lncRNAs as potential diagnostic biomarkers and therapeutic targets for metal exposure-associated diseases.

Stress granules play a critical role in hexavalent chromium-induced malignancy in a G3BP1 dependent manner

Stress granules (SGs) are dynamic membraneless organelles influencing multiple cellular pathways including cell survival, proliferation, and malignancy. Hexavalent chromium [Cr(VI)] is a toxic heavy metal associated with severe environmental health risks. Low-level environmental exposure to Cr(VI) has been reported to cause cancer, but the role of SGs in Cr(VI)-induced health effects remains unclear. This study was intended to elucidate the impact of Cr(VI) exposure on SG dynamics and the role of SGs in Cr(VI)-induced malignancy. Results showed that both acute exposure to high concentration of Cr(VI) and prolonged exposure to low concentration of Cr(VI)-induced SG formation in human bronchial epithelium BEAS-2B cells. Cells pre-exposed to Cr(VI) exhibited a more robust SG response compared to cells without pre-exposure. An up-regulated SG response was associated with increased malignant properties in cells exposed to low concentration Cr(VI) for an extended period of time up to 12 months. Knocking out the SG core protein G3BP1 in Cr(VI)-transformed (CrT) cells reduced SG formation and malignant properties, including proliferation rate, sphere formation, and malignant markers. The results support a critical role for SGs in mediating Cr(VI)-induced malignancy in a G3BP1-dependent manner, representing a novel mechanism and a potential therapeutic target.

Hexavalent chromium exposure activates the non-canonical nuclear factor kappa B pathway to promote immune checkpoint protein programmed death-ligand 1 expression and lung carcinogenesis

Lung cancer is the leading cause of cancer-related death worldwide; however, the mechanism of lung carcinogenesis has not been clearly defined. Chronic exposure to hexavalent chromium [Cr(VI)], a common environmental and occupational pollutant, causes lung cancer, representing an important lung cancer etiology factor. The mechanism of how chronic Cr(VI) exposure causes lung cancer remains largely unknown. By using cell culture and mouse models and bioinformatics analyses of human lung cancer gene expression profiles, this study investigated the mechanism of Cr(VI)-induced lung carcinogenesis. A new mouse model of Cr(VI)-induced lung carcinogenesis was developed as evidenced by the findings showing that a 16-week Cr(VI) exposure (CaCrO4, 100 μg per mouse once per week) via oropharyngeal aspiration induced lung adenocarcinomas in male and female A/J mice, whereas none of the sham-exposed control mice had lung tumors. Mechanistic studies revealed that chronic Cr(VI) exposure activated the non-canonical NFκB pathway through the long non-coding RNA (lncRNA) ABHD11-AS1/deubiquitinase USP15-mediated tumor necrosis factor receptor-associated factor 3 (TRAF3) down-regulation. The non-canonical NFκB pathway activation increased the interleukin 6 (IL-6)/Janus kinase (Jak)/signal transducer and activator of transcription 3 (Stat3) signaling. The activation of the IL-6/Jak signaling axis by Cr(VI) exposure not only promoted inflammation but also stabilized the immune checkpoint molecule programmed death-ligand 1 (PD-L1) protein in the lungs, reducing T lymphocyte infiltration to the lungs. Given the well-recognized critical role of PD-L1 in inhibiting anti-tumor immunity, these findings suggested that the lncRNA ABHD11-AS1-mediated non-canonical NFκB pathway activation and PD-L1 up-regulation may play important roles in Cr(VI)-induced lung carcinogenesis.

Long noncoding RNA ABHD11-AS1 interacts with SART3 and regulates CD44 RNA alternative splicing to promote lung carcinogenesis

Hexavalent chromium [Cr(VI)] is a common environmental pollutant and chronic exposure to Cr(VI) causes lung cancer in humans, however, the mechanism of Cr(VI) carcinogenesis has not been well understood. Lung cancer is the leading cause of cancer-related death, although the mechanisms of how lung cancer develops and progresses have been poorly understood. While long non-coding RNAs (lncRNAs) are found abnormally expressed in cancer, how dysregulated lncRNAs contribute to carcinogenesis remains largely unknown. The goal of this study is to investigate the mechanism of Cr(VI)-induced lung carcinogenesis focusing on the role of the lncRNA ABHD11 antisense RNA 1 (tail to tail) (ABHD11-AS1). It was found that the lncRNA ABHD11-AS1 expression levels are up-regulated in chronic Cr(VI) exposure-transformed human bronchial epithelial cells, chronically Cr(VI)-exposed mouse lung tissues, and human lung cancer cells as well. Bioinformatics analysis revealed that ABHD11-AS1 levels are up-regulated in lung adenocarcinomas (LUADs) tissues and associated with worse overall survival of LUAD patients but not in lung squamous cell carcinomas. It was further determined that up-regulation of ABHD11-AS1 expression plays an important role in chronic Cr(VI) exposure-induced cell malignant transformation and tumorigenesis, and the stemness of human lung cancer cells. Mechanistically, it was found that ABHD11-AS1 directly binds SART3 (spliceosome associated factor 3, U4/U6 recycling protein). The interaction of ABHD11-AS1 with SART3 promotes USP15 (ubiquitin specific peptidase 15) nuclear localization. Nuclear localized USP15 interacts with pre-mRNA processing factor 19 (PRPF19) to increase CD44 RNA alternative splicing activating β-catenin and enhancing cancer stemness. Together, these findings indicate that lncRNA ABHD11-AS1 interacts with SART3 and regulates CD44 RNA alternative splicing to promote cell malignant transformation and lung carcinogenesis.

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.

The Oncogenic and Tumor Suppressive Long Non-Coding RNA-microRNA-Messenger RNA Regulatory Axes Identified by Analyzing Multiple Platform Omics Data from Cr(VI)-Transformed Cells and Their Implications in Lung Cancer

Chronic exposure to hexavalent chromium (Cr(VI)) causes lung cancer in humans, however, the underlying mechanism has not been well understood. Long non-coding RNAs (lncRNAs) and microRNAs (miRNAs) are commonly studied non-coding RNAs. miRNAs function mainly through interaction with the 3'-untranslated regions of messenger RNAs (mRNAs) to down-regulate gene expression. LncRNAs have been shown to function as competing endogenous RNAs (ceRNAs) to sponge miRNAs and regulate gene expression. It is now well accepted that lncRNAs and miRNAs could function as oncogenes or tumor suppressors. Dysregulations of lncRNAs and miRNAs have been shown to play important roles in cancer initiation, progression, and prognosis. To explore the mechanism of Cr(VI) lung carcinogenesis, we performed lncRNA, mRNA, and miRNA microarray analysis using total RNAs from our previously established chronic Cr(VI) exposure malignantly transformed and passage-matched control human bronchial epithelial BEAS-2B cells. Based on the differentially expressed lncRNAs, miRNAs, and mRNAs between the control (BEAS-2B-Control) and Cr(VI)-transformed (BEAS-Cr(VI)) cells and by using the lncRNA-miRNA interaction and miRNA target prediction algorithms, we identified three oncogenic (HOTAIRM1/miR-182-5p/ERO1A, GOLGA8B/miR-30d-5p/RUNX2, and PDCD6IPP2/miR-23a-3p/HOXA1) and three tumor suppressive (ANXA2P1/miR-20b-5p/FAM241A (C4orf32), MIR99AHG/miR-218-5p/GPM6A, and SH3RF3-AS1/miR-34a-5p/HECW2) lncRNA-miRNA-mRNA regulatory axes. Moreover, the relevance of these three oncogenic and three tumor suppressive lncRNA-miRNA-mRNA regulatory axes in lung cancer was explored by analyzing publicly available human lung cancer omics datasets. It was found that the identified three oncogenic lncRNA-miRNA-mRNA regulatory axes (HOTAIRM1/miR-182-5p/ERO1A, GOLGA8B/miR-30d-5p/RUNX2, and PDCD6IPP2/miR-23a-3p/HOXA1) and the three tumor suppressive lncRNA-miRNA-mRNA regulatory axes (ANXA2P1/miR-20b-5p/FAM241A (C4orf32), MIR99AHG/miR-218-5p/GPM6A, and SH3RF3-AS1/miR-34a-5p/HECW2) have significant diagnostic and prognosis prediction values in human lung cancer. In addition, our recent studies showed that Cr(VI)-transformed cells display cancer stem cell (CSC)-like properties. Further bioinformatics analysis identified the oncogenic lncRNA-miRNA-mRNA regulatory axes as the potential regulators of cancer stemness. In summary, our comprehensive analysis of multiple platform omics datasets obtained from Cr(VI)-transformed human bronchial epithelial cells identified several oncogenic and tumor suppressive lncRNA-miRNA-mRNA regulatory axes, which may play important roles in Cr(VI) carcinogenesis and lung cancer in general.