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L1 (LINE1) non-LTR retrotransposons are ubiquitous genomic parasites and the dominant transposable element in humans having generated about 40% of their genomic DNA during their ~ 100 million years (Myr) of activity in primates. L1 replicates in germ line cells and early embryos, causing genetic diversity and defects, but can be active in some somatic stem cells, tumors and during aging. L1 encodes two proteins essential for retrotransposition: ORF2p, a reverse transcriptase that contains an endonuclease domain, and ORF1p, a coiled coil mediated homo trimer, which functions as a nucleic acid chaperone. Both proteins contain highly conserved domains and preferentially bind their encoding transcript to form an L1 ribonucleoprotein (RNP), which mediates retrotransposition. However, the coiled coil has periodically undergone episodes of substantial amino acid replacement to the extent that a given L1 family can concurrently express multiple ORF1s that differ in the sequence of their coiled coils. Here we show that such distinct ORF1p sequences can become entangled forming heterotrimers when co-expressed from separate vectors and speculate on how coiled coil entanglement could affect coiled coil evolution.

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Long interspersed nuclear element 1 (LINE-1) is a dominant autonomous retrotransposon in human genomes which plays a role in affecting the structure and function of somatic genomes, resulting in human disorders including genetic disease and cancer. LINE-1 encoded ORF1p protein which possesses RNA-binding and nucleic acid chaperone activity, and interacts with LINE-1 RNA to form a ribonucleoprotein particle (RNP). ORF1p can be detected in many kinds of tumors and its overexpression has been regarded as a hallmark of histologically aggressive cancers. In this study, we developed an In-Cell Western (ICW) assay in T47D cells to screen the compounds which can decrease the expression of ORF1p. Using this assay, we screened 1,947 compounds from the natural products library of Target Mol and Selleckchem, among which three compounds, Hydroxyprogesterone, 2,2':5',2″-Terthiophene and Ethynyl estradiol displayed potency in diminishing LINE-1 ORF1p expression level. Further mechanistic studies indicated the compounds act by affecting LINE-1 RNA transcription. Notably, we demonstrated that the compounds have an inhibitory effect on the proliferation of several lung and breast cancer cell lines. Taken together, we established a high throughput screening system for ORF1p expression inhibitors and the identified compounds provide some clues to the development of a novel anti-tumor therapeutic strategy by targeting ORF1p.

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Long Interspersed Element-1 (LINE-1) is an oncogenic human retrotransposon that 'copies and pastes' DNA into new locations via reverse transcription. Given that enzymatically active LINE-1 can be exported in extracellular vesicles (EVs), and that LINE-1 mRNA and its two encoded proteins, ORF1p and ORF2p, are required for retrotransposition, the present study examined LINE-1 EV loading patterns relative to reverse transcriptase (RT) activity in vivo and in vitro. Density gradient ultracentrifugation identified conserved patterns of LINE-1 mRNA and protein distribution in EVs, with RT activity readily detected in EV fractions containing both LINE-1 mRNA and protein. Unlike whole cell and tissue lysates, the ORF1p in EVs was detected as a dimer. EVs from ostensibly healthy plasma donors showed variable but consistent ORF1p profiles, with residual levels of LINE-1 mRNA measured in some but not all samples. EVs from cancer cell lines had elevated mean LINE-1 levels and 5-85 times greater RT activity than EVs from normal cells or healthy plasma. EV RT activity was associated with EV LINE-1 mRNA content and was highest in cell lines that also expressed an elevated expression of ORF1p and ORF2p. Given that LINE-1 activation is a hallmark of many cancer types, our findings suggest that an EV LINE-1 'liquid biopsy' may be developed to monitor LINE-1 activity during the course of malignant progression.

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BACKGROUND: Oral Squamous Cell Carcinoma (OSCC) results from a series of genetic alteration in squamous cells. This particular type of cancer considers one of the most aggressive malignancies to control because of its frequent local invasions to the regional lymph node. Although several biomarkers have been reported, the key marker used to predict the behavior of the disease is largely unknown. Here we report Long INterpersed Element-1 (LINE1 or L1) retrotransposon activity in post-operative oral cancer samples. L1 is the only active retrotransposon occupying around 17% of the human genome with an estimated 500,000 copies. An active L1 encodes two proteins (L1ORF1p and L1ORF2p); both of which are critical in the process of retrotransposition. Several studies report that the L1 retrotransposon is highly active in many cancers. L1 activity is generally determined by assaying L1ORF1p because of its high expression and availability of the antibody. However, due to its lower expression and unavailability of a robust antibody, detection of L1ORF2p has been limited. L1ORF2p is the crucial protein in the process of retrotransposition as it provides endonuclease and reverse transcriptase (RT) activity. METHODS: Immunohistochemistry and Western blotting were performed on the post-operative oral cancer samples and murine tissues. RESULTS: Using in house novel antibodies against both the L1 proteins (L1ORF1p and L1ORF2p), we found L1 retrotransposon is extremely active in post-operative oral cancer tissues. Here, we report a novel human L1ORF2p antibody generated using an 80-amino-acid stretch from the RT domain, which is highly conserved among different species. The antibody detects significant L1ORF2p expression in human oral squamous cell carcinoma (OSCC) samples and murine germ tissues. CONCLUSIONS: We report exceptionally high L1ORF1p and L1ORF2p expression in post-operative oral cancer samples. The novel L1ORF2p antibody reported in this study will serve as a useful tool to understand why L1 activity is deregulated in OSCC and how it contributes to the progression of this particular cancer. Cross-species reactivity of L1ORF2p antibody due to the conserved epitope will be useful to study the retrotransposon biology in mice and rat germ tissues.

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BACKGROUND: Long INterspersed Element-1 (LINE-1) is an autonomous retroelement able to "copy-and-paste" itself into new loci of the host genome through a process called retrotransposition. The LINE-1 bicistronic mRNA codes for two proteins, ORF1p, a nucleic acid chaperone, and ORF2p, a protein with endonuclease and reverse transcriptase activity. Both proteins bind LINE-1 mRNA in cis and are necessary for retrotransposition. While LINE-1 transcription is usually repressed in most healthy somatic cells through a plethora of mechanisms, ORF1p expression has been observed in nearly 50% of tumors, and new LINE-1 insertions have been documented in a similar fraction of tumors, including prostate cancer. RESULTS: Here, we utilized RNA ImmunoPrecipitation (RIP) and the L1EM analysis software to identify ORF1p bound RNA in prostate cancer cells. We identified LINE-1 loci that were expressed in parental androgen sensitive and androgen independent clonal derivatives. In all androgen independent cells, we found higher levels of LINE-1 RNA, as well as unique expression patterns of LINE-1 loci. Interestingly, we observed that ORF1p bound many non-LINE-1 mRNA in all prostate cancer cell lines evaluated, and polyA RNA, and RNA localized in p-bodies were especially enriched. Furthermore, the expression levels of RNAs identified in our ORF1p RIP correlated with RNAs expressed in LINE-1 positive tumors from The Cancer Genome Atlas (TCGA). CONCLUSION: Our results show a significant remodeling of LINE-1 loci expression in androgen independent cell lines when compared to parental androgen dependent cells. Additionally, we found that ORF1p bound a significant amount of non-LINE-1 mRNA, and that the enriched ORF1p bound mRNAs are also amplified in LINE-1 expressing TCGA prostate tumors, indicating the biological relevance of our findings to prostate cancer.

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PURPOSE: Epigenetic dysregulation plays a role in pituitary tumor pathogenesis. Some differences in DNA methylation were observed between invasive and noninvasive nonfunctioning gonadotroph tumors. This study sought to determine the role of DNA methylation changes in repetitive LINE-1 elements in nonfunctioning gonadotroph pituitary tumors. METHODS: We investigated LINE-1 methylation levels in 80 tumors and normal pituitary glands with bisulfite-pyrosequencing. Expression of two LINE-1 open reading frames (L1-ORF1 and L1-ORF2) was analyzed with qRT-PCR in tumor samples and mouse gonadotroph pituitary cells treated with DNA methyltransferase inhibitor. Immunohistochemical staining against L1-ORF1p was also performed in normal pituitary glands and tumors. RESULTS: Hypomethylation of LINE-1 was observed in pituitary tumors. Tumors characterized by invasive growth revealed lower LINE-1 methylation level than noninvasive ones. LINE-1 methylation correlated with overall DNA methylation assessed with HM450K arrays and negatively correlated with L1-ORF1 and L1-ORF2 expression. Treatment of αT3-1 gonadotroph cells with 5-Azacytidine clearly increased the level of L1-ORF1 and L1-ORF2 mRNA; however, its effect on LßT2 cells was less pronounced. Immunoreactivity against L1-ORF1p was higher in tumors than normal tissue. No difference in L1-ORF1p expression was observed in invasive and noninvasive tumors. CONCLUSION: Hypomethylation of LINE-1 is related to invasive growth and influences transcriptional activity of transposable elements.

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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.

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Retrotransposon activation occurs in a variety of neurological disorders including multiple sclerosis and Alzheimer's Disease. While the origins of disease-related retrotransposon activation have remained mostly unidentified, this phenomenon may well contribute to disease progression by inducing inflammation, disrupting transcription and, potentially, genomic insertion. Here, we report that the inhibition of mitochondrial respiratory chain complex I by pharmacological agents widely used to model Parkinson's disease leads to a significant increase in expression of the ORF1 protein of the long interspersed nucleotide element 1 (LINE1) retrotransposon in human dopaminergic LUHMES cells. These findings were recapitulated in midbrain lysates from accordingly treated wild-type mice that mimic Parkinson's disease. Retrotransposon activation was paralleled by a loss of DNA cytosine methylation, providing a potential mechanism of retrotransposon mobilization. Loss of DNA methylation as well as retrotransposon activation were suppressed by the mitochondrial antioxidant phenothiazine, indicating that the well-established production of oxidants by inhibited complex I was causing these effects. Retrotransposon activation in some brain disorders may be less of a primary disease trigger rather than a consequence of mitochondrial distress, which is very common in neurodegenerative diseases.

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Oral squamous cell carcinoma (OSCC) is highly predominant in India due to excessive use of tobacco. Here we investigated Long INterpersed Element 1 (LINE or L1) retrotransposon activity in OSCC samples in the same population. There are almost 500,000 copies of L1 occupied around 30%  of the human genome. Although most of them are inactive, around 150-200 copies are actively jumping in a human genome. L1 encodes two proteins designated as ORF1p and ORF2p and expression of both proteins are critical for the process of retrotransposition. Here we have analyzed L1 ORF1p expression in a small cohort (n = 15) of paired cancer-normal tissues obtained from operated oral cancer patients. Immunohistochemistry (IHC) with the human ORF1 antibody showed the presence of ORF1p in almost 60%  cancer samples, and few of them also showed aberrant p53 expression.  Investigating L1 promoter methylation status, showed certain trends towards hypomethylation of the L1 promoter in cancer tissues compared to its normal counterpart. Our data raise the possibility that L1ORF1p expression might have some role in the onset and progression of this particular type of cancer.

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The L1 retrotransposon is the dominant transposable element in mammalian genomes. L1 comprises at least 20% of the human genome. While most L1 regions are inactive, a few still retain the ability to retrotranspose. L1 encodes two proteins, ORF1p and ORF2p, which are required for retrotransposition. During retrotransposition, ORF2p functions as the reverse transcriptase and the endonuclease. ORF1p is a nucleic acid chaperone that binds nucleic acids with high affinity. However, to date, a detailed mechanistic understanding of ORF1p function in L1 retrotransposition is lacking. The single molecule DNA stretching methods described here have been extensively used to understand ORF1p's complex nucleic acid binding properties. By correlating these properties to ORF1p's ability to support L1 retrotransposition in in vivo cell-culture based assays, these studies have significantly contributed to advance the understanding of ORF1p function. Although described in the context of ORF1p, these methods provide a general mechanism to study complex protein-DNA interactions.

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LINE-1 (L1) retrotransposons are mobile genetic elements capable of "copy-and-pasting" their own sequences into random genomic loci, and one of the proteins it uses to achieve mobility is LINE-1 open reading frame 1 protein (L1ORF1p). L1ORF1p expression is found across many epithelial cancers, including small cohorts of ovarian and endometrial cancers, and is highly expressed in cancers with mutant p53 expressions. Here we aimed to gain insights into L1ORF1p expression levels within specific histotypes of ovarian cancers: high-grade serous (n = 585), low-grade serous (n = 26), clear cell (n = 132), endometrioid (n = 148), and mucinous (n = 32) ovarian cancers, as well as endometrial cancers (n = 607) using tissue microarray (TMA's). We demonstrated that L1ORF1p expression is associated with advanced stage and serous histotype in gynecological cancers. Like previous studies, we found a higher proportion of L1ORF1p expression in cases with aberrant p53 expression. We evaluated the expression of L1ORF1p in serous tubal intraepithelial carcinomas (STICs) (n = 6) and p53 signature lesions (n = 2) in fallopian tubes. Three STIC cases displayed aberrant p53 overexpression with corresponding L1ORF1p expression in the same tissues, but such correlation was not seen in the two p53 signature lesions, suggesting that L1 protein may be expressed after dysplastic transformation. The remaining three STIC cases have TP53 nonsense mutations with absent p53 expression but a strong and clear L1ORF1p expression within the STIC lesions. While L1ORF1p may not be prognostic in gynecological cancers, it may be useful clinically as a diagnostic IHC marker for p53 null STIC lesions and this warrants further investigation.

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INTRODUCTION: Long interspersed nuclear elements-1 (L1), as the only one self-active retrotransposon of the mobile element, was found to be generally activated in tumor cells. The 5'UTR of L1 (L1-5'UTR) contains both sense and antisense bidirectional promoters, transcription products of which can generate double-strand RNA (dsRNA). In addition, L1-ORF1p, a dsRNA binding protein encoded by L1, is considered to engage in some RNA-protein (RNP) formation. Ago2, one of the RISC components, can bind to dsRNA to form RISC complex, but its role in L1 regulation still remains unclear. Due that the 5'UTR of L1 (L1-5'UTR) contains both sense and antisense bidirectional promoters, so the activities in both string were identified. A dsRNA-mediated regulation of L1-5'UTR, with the feedback regulation of L1-ORF1p as well as other key molecules engaged (Ago1-4) in this process, was also investigated. MATERIAL AND METHODS: Genomic DNA was extracted from HEK293 cells and subjected to L1-5'UTR prepa-ration by PCR. Report gene system pIRESneo with SV40 promoter was employed. The promoter activities of different regions in L1-5'UTR were identified by constructing these regions into pIRESneo, which SV40 region was removed prior, to generate different recombinant plasmids. The promoter activities in recombinant plasmids were detected by the luciferase expression assay. Western blot and co-immunoprecipitation were employed to identify proteins expression and protein-protein interaction respectively. RESULTS: Ago2 is a member of Agos family, which usually forms a RISC complex with si/miRNA and is involved in post- transcriptional regulation of many genes. Here L1-ORF1p and Ago2 conducts a regulation as a negative feedback for L1-5'UTR sense promoter. L1-ORF1p could form the immune complexes with Ago1, Ago2 and Ago4, respectively. CONCLUSIONS: L1-5'UTR harbors both sense and antisense promoter activity and a dsRNA-mediated regulation is responsible for L1-5'UTR regulation. Agos proteins and L1-ORF1p were engaged in this process.

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BACKGROUND: Long Interspersed Element 1 (LINE-1) is a retrotransposon that is present in 500,000 copies in the human genome. Along with Alu and SVA elements, these three retrotransposons account for more than a third of the human genome sequence. These mobile elements are able to copy themselves within the genome via an RNA intermediate, a process that can promote genome instability. LINE-1 encodes two proteins, ORF1p and ORF2p. Association of ORF1p, ORF2p and a full-length L1 mRNA in a ribonucleoprotein (RNP) particle, L1 RNP, is required for L1 retrotransposition. Previous studies have suggested that fusion of a tag to L1 proteins can interfere with L1 retrotransposition. RESULTS: Using antibodies detecting untagged human ORF1p, western blot analysis and manipulation of ORF1 sequence and length, we have identified a set of charged amino acids in the C-terminal region of ORF1p that are important in determining its subcellular localization. Mutation of 7 non-identical lysine residues is sufficient to make the resulting ORF1p to be predominantly cytoplasmic, demonstrating intrinsic redundancy of this requirement. These residues are also necessary for ORF1p to retain its association with KPNA2 nuclear pore protein. We demonstrate that this interaction is significantly reduced by RNase treatment. Using co-IP, we have also determined that human ORF1p associates with all members of the KPNA subfamily. CONCLUSIONS: The prediction of NLS sequences suggested that specific sequences within ORF1p could be responsible for its subcellular localization by interacting with nuclear binding proteins. We have found that multiple charged amino acids in the C-terminus of ORF1p are involved in ORF1 subcellular localization and interaction with KPNA2 nuclear pore protein. Our data demonstrate that different amino acids can be mutated to have the same phenotypic effect on ORF1p subcellular localization, demonstrating that the net number of charged residues or protein structure, rather than their specific location, is important for the ORF1p nuclear localization. We also identified that human ORF1p interacts with all members of the KPNA family of proteins and that multiple KPNA family genes are expressed in human cell lines.

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BACKGROUND: LINE-1 ORF-1p is encoded by the human pro-oncogene LINE-1. Our previous work showed that LINE-1 ORF-1p could enhance the resistance of hepatocellular carcinoma (HCC) cells to antitumor agents. However, the mechanisms involved in LINE-1 ORF-1p-mediated drug resistance remain largely unknown. MATERIALS AND METHODS: The endogenous mRNA level of LINE-1 ORF-1p in clinical HCC specimens was examined using quantitative PCR (qPCR). The prognosis of HCC patients was assessed using time to progression and overall survival. The transcription factor activity of pregnenolone X receptor (PXR) was examined using luciferase gene reporter assays, qPCR, chromatin immunoprecipitation assays and cellular subfraction assays. Protein interaction between LINE-1 ORF-1p and PXR was detected by co-immunoprecipitation. The effect of LINE-1 ORF-1p on sorafenib resistance in HCC cells was studied using in vitro and in vivo models. RESULTS: A high level of LINE-1 ORF-1p in clinical specimens was related to poor prognosis in patients who received sorafenib treatment. LINE-1 ORF-1p increased the transcription factor activity of PXR by interacting with PXR and enhancing its cytoplasmic/nuclear translocation, and recruiting PXR to its downstream gene promoter, in turn enhancing the expression of the sorafenib resistance-related genes, CYP3A4 and mdr-1. LINE-1 ORF-1p enhanced the resistance to and clearance of sorafenib in HCC cells. CONCLUSION: LINE-1 ORF-1p enhances the transcription factor activation of PXR and promotes the clearance of and resistance to sorafenib in HCC cells.

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PIWI proteins and their associated small RNAs, called PIWI-interacting RNAs (piRNAs), restrict transposon activity in animal gonads to ensure fertility. Distinct biogenesis pathways load piRNAs into the PIWI proteins MILI and MIWI2 in the mouse male embryonic germline. While most MILI piRNAs are derived via a slicer-independent pathway, MILI slicing loads MIWI2 with a series of phased piRNAs. Tudor domain-containing 12 (TDRD12) and its interaction partner Exonuclease domain-containing 1 (EXD1) are required for loading MIWI2, but only Tdrd12 is essential for fertility, leaving us with no explanation for the physiological role of Exd1. Using an artificial piRNA precursor, we demonstrate that MILI-triggered piRNA biogenesis is greatly reduced in the Exd1 mutant. The situation deteriorates in the sensitized Exd1 mutant (Exd1-/-;Tdrd12+/-), where diminished MIWI2 piRNA levels de-repress LINE1 retrotransposons, leading to infertility. Thus, EXD1 enhances MIWI2 piRNA biogenesis via a functional interaction with TDRD12.

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LINE-1 retrotransposition is tightly restricted by layers of regulatory control, with epigenetic pathways being the best characterized. Looking at post-transcriptional regulation, we now show that LINE-1 mRNA 3' ends are pervasively uridylated in various human cellular models and in mouse testes. TUT4 and TUT7 uridyltransferases catalyze the modification and function in cooperation with the helicase/RNPase MOV10 to counteract the RNA chaperone activity of the L1-ORF1p retrotransposon protein. Uridylation potently restricts LINE-1 retrotransposition by a multilayer mechanism depending on differential subcellular localization of the uridyltransferases. We propose that uridine residues added by TUT7 in the cytoplasm inhibit initiation of reverse transcription of LINE-1 mRNAs once they are reimported to the nucleus, whereas uridylation by TUT4, which is enriched in cytoplasmic foci, destabilizes mRNAs. These results provide a model for the post-transcriptional restriction of LINE-1, revealing a key physiological role for TUT4/7-mediated uridylation in maintaining genome stability.

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LINE-1 (L1) is a non-long terminal repeat (LTR) retrotransposon inserted throughout the human genome. APOBEC3 (A3) proteins are part of a network of host intrinsic defenses capable of restricting retroviruses and the replication of L1 retroelements. These enzymes inactivate retroviruses primarily through deamination of single-stranded viral DNA. In contrast, only A3A deaminates L1 DNA, while the other six A3 proteins restrict L1 to varying degrees through yet poorly defined mechanisms. Here we provide further insight into the molecular attributes of L1 restriction by A3 proteins. We specifically investigated the roles of A3 protein oligomerization, interactions with RNA and their binding to the various L1 proteins. Our results show that compromising the ability of A3 proteins to oligomerize or interact with a nucleic acid substrate diminished L1 restriction to varying degrees. However the efficiency of their binding to L1 proteins did not predict restriction or the potency of the restriction.

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BACKGROUND/AIM: Reactivation of long interspersed nuclear element-1 (LINE-1) and oxidative stress are suggested to have oncogenic potential to drive tumorigenesis and cancer progression. We previously demonstrated that reactive oxygen species (ROS) caused hypomethylation of LINE-1 elements in bladder cancer cells. In this study, we investigated the expression of LINE-1-encoded protein (ORF1p) and oxidative stress marker 4-hydroxynonenal (4-HNE) in human bladder cancer tissues, as well as induction of ORF1p expression by ROS in bladder cancer cell lines. MATERIALS AND METHODS: Thirty-six cancerous and 15 non-cancerous adjacent tissues were immunohistochemically stained for ORF1p and 4-HNE. ORF1p expression and cell migration were determined in bladder cancer cells exposed to H2O2 Results: ORF1p and 4-HNE expression was higher in cancerous than non-cancerous tissues. Elevated ORF1p expression was associated with increased 4-HNE expression and with advanced tumors. H2O2 provoked oxidative stress and up-regulated ORF1p expression in VM-CUB-1 compared to the untreated control, and to a lesser degree in TCCSUP. H2O2 exposure enhanced cell migration in UM-UC-3, TCCSUP and VM-CUB-1. CONCLUSION: Elevated ORF1p expression is associated with tumor progression. ROS experimentally induce ORF1p expression and promote migration in bladder cancer cells.

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The mechanisms by which a retrotransposon called LINE-1 duplicates itself and spreads through the human genome are becoming clearer.

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BACKGROUND: Recent reports indicate that retrotransposons - a type of mobile DNA - can contribute to neuronal genetic diversity in mammals. Retrotransposons are genetic elements that mobilize via an RNA intermediate by a "copy-and-paste" mechanism termed retrotransposition. Long Interspersed Element-1 (LINE-1 or L1) is the only active autonomous retrotransposon in humans and its activity is responsible for ~ 30% of genomic mass. Historically, L1 retrotransposition was thought to be restricted to the germline; however, new data indicate L1 s are active in somatic tissue with certain regions of the brain being highly permissive. The functional implications of L1 insertional activity in the brain and how host cells regulate it are incomplete. While deep sequencing and qPCR analysis have shown that L1 copy number is much higher in certain parts of the human brain, direct in vivo studies regarding detection of L1-encoded proteins is lacking due to ineffective reagents. RESULTS: Using a polyclonal antibody we generated against the RNA-binding (RRM) domain of L1 ORF1p, we observe widespread ORF1p expression in post-mortem human brain samples including the hippocampus which has known elevated rates of retrotransposition. In addition, we find that two brains from different individuals of different ages display very different expression of ORF1p, especially in the frontal cortex. CONCLUSIONS: We hypothesize that discordance of ORF1p expression in parts of the brain reported to display elevated levels of retrotransposition may suggest the existence of factors mediating post-translational regulation of L1 activity in the human brain. Furthermore, this antibody reagent will be useful as a complementary means to confirm findings related to retrotransposon biology and activity in the brain and other tissues in vivo.