Assessing LINE-1 retrotransposition activity in neuroblastoma cells exposed to extremely low-frequency pulsed magnetic fields

A comparative study of long interspersed element-1 protein immunoreactivity in cutaneous malignancies

Background

Skin cancer is the most common cancer worldwide and commonly classified into malignant melanoma (MM) and Nonmelanoma skin cancers (NMSCs), which mainly include basal cell carcinoma (BCC) and squamous cell carcinoma (SCC). The extent to which Long Interspersed Element-1 (LINE-1, L1) ORF1p is expressed in cutaneous malignancies remains to be evaluated. This study aimed to assess LINE-1 ORF1p immunoreactivity in various skin cancer subtypes.

Method

The expression level of LINE-1 ORF1p was evaluated in 95 skin cancer specimens comprising 36 (37.9%) BCC, 28 (29.5%) SCC, and 31 (32.6%) melanoma using the tissue microarray (TMA) technique. Then the association between expression of LINE-1 encoded protein and clinicopathological parameters was analyzed.

Results

We showed that LINE-1 ORF1p expression level was substantially higher in BCC and SCC patients compared with melanoma samples (p < 0.001). BCC cases had a higher LINE-1 histochemical score (H-score) compared with SCC cases (p = 0.004). In SCC samples, a lower level of LINE-1 ORF1p expression was associated with age younger than the mean (p = 0.041). At the same time, no significant correlation was found between LINE-1 ORF1p expression and other clinicopathological parameters (all p > 0.05).

Conclusions

According to our observation, LINE-1 ORF1p immunoreactivity in various skin tumor subtypes extends previous studies of LINE-1 expression in different cancers. LINE-1ORF1p overexpression in NMSCs compared with MM can be considered with caution as a tumor-specific antigen for NMSCs.

Long INterspersed element-1 mobility as a sensor of environmental stresses

Long INterspersed element (LINE-1, L1) retrotransposons are the most abundant transposable elements in the human genome, constituting approximately 17%. They move by a "copy-paste" mechanism, involving reverse transcription of an RNA intermediate and insertion of its cDNA copy at a new site in the genome. L1 retrotransposition (L1-RTP) can cause insertional mutations, alter gene expression, transduce exons, and induce epigenetic dysregulation. L1-RTP is generally repressed; however, a number of observations collected over about 15 years revealed that it can occur in response to environmental stresses. Moreover, emerging evidence indicates that L1-RTP can play a role in the onset of several neurological and oncological diseases in humans. In recent years, great attention has been paid to the exposome paradigm, which proposes that health effects of an environmental factor should be evaluated considering both cumulative environmental exposures and the endogenous processes resulting from the biological response. L1-RTP could be an endogenous process considered for this application. Here, we summarize the current understanding of environmental factors that can affect the retrotransposition of human L1 elements. Evidence indicates that L1-RTP alteration is triggered by numerous and various environmental stressors, such as chemical agents (heavy metals, carcinogens, oxidants, and drugs), physical agents (ionizing and non-ionizing radiations), and experiential factors (voluntary exercise, social isolation, maternal care, and environmental light/dark cycles). These data come from in vitro studies on cell lines and in vivo studies on transgenic animals: future investigations should be focused on physiologically relevant models to gain a better understanding of this topic.

Long INterspersed nuclear Elements (LINEs) in brain and non-brain tissues of the rat

Long INterspersed Element-1 (L1) is a transposable element that can insert copies of itself in new genomic locations causing genomic instability. In somatic cells, L1 retrotransposition activity is usually repressed but somatic L1 retrotransposition has recently been observed during neuronal differentiation. In this study, we evaluate whether L1 elements are differentially active in rat tissues during postnatal development. To this purpose, we quantified L1 in genomic DNA extracted from the olfactory bulb (OB), cerebellum (CE), cortex (CO) and heart (H). Each analysis was repeated on rats aged 7, 21 and 60 days. We found that L1 content in OB and CE tissue was significantly higher than H tissue, in rats of all three ages studied, suggesting that L1 activity could be modulated in postnatal development and neurogenesis.

Cold-induced retrotransposition of fish LINEs

Classes of retrotransposons constitute a large portion of metazoan genome. There have been cases reported that genomic abundance of retrotransposons is correlated with the severity of low environmental temperatures. However, the molecular mechanisms underlying such correlation are unknown. We show here by cell transfection assays that retrotransposition (RTP) of a long interspersed nuclear element (LINE) from an Antarctic notothenioid fish Dissostichus mawsoni (dmL1) could be activated by low temperature exposure, causing increased dmL1 copies in the host cell genome. The cold-induced dmL1 propagation was demonstrated to be mediated by the mitogen-activated protein kinases (MAPK)/p38 signaling pathway, which is activated by accumulation of reactive oxygen species (ROS) in cold-stressed conditions. Surprisingly, dmL1 transfected cells showed an increase in the number of viable cells after prolonged cold exposures than non-transfected cells. Features of cold inducibility of dmL1 were recapitulated in LINEs of zebrafish origin both in cultured cell lines and tissues, suggesting existence of a common cold-induced LINE amplification in fishes. The findings reveal an important function of LINEs in temperature adaptation and provid insights into the MAPK/p38 stress responsive pathway that shapes LINE composition in fishes facing cold stresses.

Assessing the combined effect of extremely low-frequency magnetic field exposure and oxidative stress on LINE-1 promoter methylation in human neural cells

Extremely low frequency magnetic fields (ELF-MF) have been classified as "possibly carcinogenic", but their genotoxic effects are still unclear. Recent findings indicate that epigenetic mechanisms contribute to the genome dysfunction and it is well known that they are affected by environmental factors. To our knowledge, to date the question of whether exposure to ELF-MF can influence epigenetic modifications has been poorly addressed. In this paper, we investigated whether exposure to ELF-MF alone and in combination with oxidative stress (OS) can affect DNA methylation, which is one of the most often studied epigenetic modification. To this end, we analyzed the DNA methylation levels of the 5'untranslated region (5'UTR) of long interspersed nuclear element-1s (LINE-1 or L1), which are commonly used to evaluate the global genome methylation level. Human neural cells (BE(2)C) were exposed for 24 and 48 h to extremely low frequency pulsed magnetic field (PMF; 50 Hz, 1 mT) in combination with OS. The methylation levels of CpGs located in L1 5'UTR region were measured by MassARRAY EpiTYPER. The results indicate that exposures to the single agents PMF and OS induced weak decreases and increases of DNA methylation levels at different CpGs. However, the combined exposure to PMF and OS lead to significant decrease of DNA methylation levels at different CpG sites. Most of the changes were transient, suggesting that cells can restore homeostatic DNA methylation patterns. The results are discussed and future research directions outlined.

Cell-host, LINE and environment: Three players in search of a balance

Long interspersed nuclear elements -1 (LINEs, L1s) are retroelements occupying almost 17% of the human genome. L1 retrotransposition can cause deleterious effects on the host-cell and it is generally inhibited by suppressive mechanisms, but it can occur in some specific cells during early development as well as in some tumor cells and in the presence of several environmental factors. In a recent publication we reported that extremely low frequency pulsed magnetic field can affect L1 retrotransposition in neuroblastoma cells. In this commentary we discuss the interaction between environment and L1 activity in the light of the new emerging paradigm of host-LINE relationship.

Effect of heavy metals on silencing of engineered long interspersed element-1 retrotransposon in nondividing neuroblastoma cell line

BACKGROUND

L1 retrotransposons are the most active mobile DNA elements in human genome. Unregulated L1 retrotransposition may have deleterious effect by disrupting vital genes and inducing genomic instabilities. Therefore, human cells control L1 elements by silencing their activities through epigenetic mechanisms. It has been shown that cell division and heavy metals stimulate the frequency of L1 activities. Removal of silencing by L1 motivators may restart L1 element functions. Here, we have proposed that weather neurotoxic environmental heavy metals (as L1 stimulating factors) have a role in removing L1 silencing and restating its activities in nondividing neuronal cells.

METHODS

L1-RP green fluorescent protein (GFP)-tagged knock-in human neuroblastoma clones were prepared. Single-cell clone was treated with mitomycin-c combined with nontoxic and toxic concentrations of iron (Fe), copper (Cu), and mercury (Hg). Silencing status of engineered L1 elements in dividing and nondividing cells was determined through measuring the amount of GFP expressing cells with flow cytometry. The cytotoxic effect of mitomycin-c combined with metals was measured by MTT assay.

RESULTS

Hg in nondividing cells and Fe, Cu, and Hg in dividing neuroblastoma cells could significantly remove L1 silencing. Also, mitomycin-c treatment did not have any effect on metal toxicity status in neuroblastoma cells.

CONCLUSION

Totally, our findings have shown that cell division has a role in removing L1 silencing as well as L1 retrotransposition induced by environmental heavy metals. It has been also indicated that Hg at all concentrations could remove silencing of engineered L1 element regardless of cell cycle state.

Evaluating the extent of LINE-1 mobility following exposure to heavy metals in HepG2 cells

The long interspersed elements-1 (LINE1 or L1 retrotransposon) constitute 17% of the human genome and retain mobility properties within the genome. At present, 80-100 human L1 elements are thought to be active in the genome. The mobilization of these active elements may be influenced upon exposure to the heavy metals. In the present study, we evaluated the association of aluminum, lead, and copper exposure with L1 retrotransposition in human hepatocellular carcinoma (HepG2) cell line. An in vitro retrotransposition assay using an enhanced green fluorescent protein (EGFP)-tagged L1RP cassette was established to track EGFP shining as the mark of retrotransposition. Following determination of noncytotoxic concentrations of these metals, pL1RP-EGFP-transfected HepG2 cells were subjected to long-term treatment. Flow cytometry analysis of cells treated with various concentrations of these metals along with quantitative real-time PCR was used to quantify L1 retrotransposition frequencies. Aluminum significantly increased L1 retrotransposition frequency, while no significant association was found concerning lead exposure and L1 retrotransposition. Copper treatment downregulated L1 retrotransposition as a result of EGFP-tagged L1RP expression. Our findings suggest that aluminum might have the potential to cause genomic instability by the enhancement of L1 mobilization. Thus, the risk of induced L1 retrotransposition should be considered during drug safety evaluation and risk assessments of exposure to toxic environmental agents. Further studies are needed for a more robust assay to evaluate any associations between long-term lead exposure and L1 mobility in cell culture assay.

Mercury specifically induces LINE-1 activity in a human neuroblastoma cell line

L1 retro-elements comprise 17% of the human genome. Approximately 100 copies of these autonomous mobile elements are active in our DNA and can cause mutations, gene disruptions, and genomic instability. Therefore, human cells control the activities of L1 elements, in order to prevent their deleterious effects through different mechanisms. However, some toxic agents increase the retrotransposition activity of L1 elements in somatic cells. In order to identify specific effects of neurotoxic metals on L1 activity in neuronal cells, we studied the effects of mercury and cobalt on L1-retroelement activity by measuring levels of cellular transcription, protein expression, and genomic retrotransposition in a neuroblastoma cell line compared with the effects in three non-neuronal cell lines. Our results show that mercury increased the expression of L1 RNA, the activity of the L1 5'UTR, and L1 retrotransposition exclusively in the neuroblastoma cell line but not in non-neuronal cell lines. However, cobalt increased the expression of L1 RNA in neuroblastoma cells, HeLa cells, and wild-type human fibroblasts, and also increased the activity of the L1 5'UTR as well as the SV40 promoter in HeLa cells but not in neuroblastoma cells. Exposure to cobalt did not result in increased retrotransposition activity in HeLa cells or neuroblastoma cells. We conclude that non-toxic levels of the neurotoxic agent mercury could influence DNA by increasing L1 activities, specifically in neuronal cells, and may make these cells susceptible to neurodegeneration over time.