A suppressive role of guanine nucleotide-binding protein subunit beta-4 inhibited by DNA methylation in the growth of anti-estrogen resistant breast cancer cells

BACKGROUND

Breast cancer is the most common malignancy in women worldwide. Although the endocrine therapy that targets estrogen receptor α (ERα) signaling has been well established as an effective adjuvant treatment for patients with ERα-positive breast cancers, long-term exposure may eventually lead to the development of acquired resistance to the anti-estrogen drugs, such as fulvestrant and tamoxifen. A better understanding of the mechanisms underlying antiestrogen resistance and identification of the key molecules involved may help in overcoming antiestrogen resistance in breast cancer.

METHODS

The whole-genome gene expression and DNA methylation profilings were performed using fulvestrant-resistant cell line 182R-6 and tamoxifen-resistant cell line TAMR-1 as a model system. In addition, qRT-PCR and Western blot analysis were performed to determine the levels of mRNA and protein molecules. MTT, apoptosis and cell cycle analyses were performed to examine the effect of either guanine nucleotide-binding protein beta-4 (GNB4) overexpression or knockdown on cell proliferation, apoptosis and cell cycle.

RESULTS

Among 9 candidate genes, GNB4 was identified and validated by qRT-PCR as a potential target silenced by DNA methylation via DNA methyltransferase 3B (DNMT3B). We generated stable 182R-6 and TAMR-1 cell lines that are constantly expressing GNB4 and determined the effect of the ectopic GNB4 on cell proliferation, cell cycle, and apoptosis of the antiestrogen-resistant cells in response to either fulvestrant or tamoxifen. Ectopic expression of GNB4 in two antiestrogen resistant cell lines significantly promoted cell growth and shortened cell cycle in the presence of either fulvestrant or tamoxifen. The ectopic GNB4 induced apoptosis in 182R-6 cells, whereas it inhibited apoptosis in TAMR-1 cells. Many regulators controlling cell cycle and apoptosis were aberrantly expressed in two resistant cell lines in response to the enforced GNB4 expression, which may contribute to GNB4-mediated biologic and/or pathologic processes. Furthermore, knockdown of GNB4 decreased growth of both antiestrogen resistant and sensitive breast cancer cells.

CONCLUSION

GNB4 is important for growth of breast cancer cells and a potential target for treatment.

dsRNA expression in the mouse elicits RNAi in oocytes and low adenosine deamination in somatic cells

Double-stranded RNA (dsRNA) can enter different pathways in mammalian cells, including sequence-specific RNA interference (RNAi), sequence-independent interferon (IFN) response and editing by adenosine deaminases. To study the routing of dsRNA to these pathways in vivo, we used transgenic mice ubiquitously expressing from a strong promoter, an mRNA with a long hairpin in its 3′-UTR. The expressed dsRNA neither caused any developmental defects nor activated the IFN response, which was inducible only at high expression levels in cultured cells. The dsRNA was poorly processed into siRNAs in somatic cells, whereas, robust RNAi effects were found in oocytes, suggesting that somatic cells lack some factor(s) facilitating siRNA biogenesis. Expressed dsRNA did not cause transcriptional silencing in trans. Analysis of RNA editing revealed that a small fraction of long dsRNA is edited. RNA editing neither prevented the cytoplasmic localization nor processing into siRNAs. Thus, a long dsRNA structure is well tolerated in mammalian cells and is mainly causing a robust RNAi response in oocytes.

Sex and tissue-specific differences in low-dose radiation-induced oncogenic signaling

PURPOSE

The possible adverse health effects of low-dose radiation (LDR) exposure constitute a growing concern. Clinically and environmentally relevant exposures occur predominantly under chronic conditions, notwithstanding that most studies of LDR effects have been performed using a single acute exposure. Sex- and tissue-specificity of the LDR-induced changes have not been considered before. We investigated LDR-related expression patterns in muscle, liver and spleen of male and female mice subjected to acute and chronic LDR exposure. Genes involved in oncogenic signaling were of specific interest, as radiation is a well-known carcinogen.

MATERIALS AND METHODS

We analyzed the expression pattern of genes coding for growth factors and growth-factor receptors, cytoplasmic serine/threonine protein kinases, G-proteins and nuclear DNA-binding proteins, and other important components of oncogenic signaling.

RESULTS

We found sex- and tissue-specific changes in the expression of Ras superfamily members (Nras, Rab2, Rab34, Vav2), protein kinase C (PKC) isoforms (PKCbeta, PKCmu), AP-1 factor components (Jun, JunB and FosB), Wnt signaling pathway members as well as in a variety of other cellular proto-oncogenes and oncogenes. Importantly, Western blot analysis of JunB, PKCmu and Rab2 proteins supported the transcriptomic data.

CONCLUSIONS

Substantially different protein levels were observed in all three tissues (muscle, spleen and liver) of acutely and chronically irradiated female and male animals. Based on the obtained data and available literature, we discuss several possible mechanisms that may contribute to radiation-induced carcinogenesis in various tissues of males and females. From our results we could identify the genes that may serve as sex- and tissue-specific biomarkers of the LDR exposure.

Involvement of microRNA-451 in resistance of the MCF-7 breast cancer cells to chemotherapeutic drug doxorubicin

Many chemotherapy regiments are successfully used to treat breast cancer; however, often breast cancer cells develop drug resistance that usually leads to a relapse and worsening of prognosis. We have shown recently that epigenetic changes such as DNA methylation and histone modifications play an important role in breast cancer cell resistance to chemotherapeutic agents. Another mechanism of gene expression control is mediated via the function of small regulatory RNA, particularly microRNA (miRNA); its role in cancer cell drug resistance still remains unexplored. In the present study, we investigated the role of miRNA in the resistance of human MCF-7 breast adenocarcinoma cells to doxorubicin (DOX). Here, we for the first time show that DOX-resistant MCF-7 cells (MCF-7/DOX) exhibit a considerable dysregulation of the miRNAome profile and altered expression of miRNA processing enzymes Dicer and Argonaute 2. The mechanistic link of miRNAome deregulation and the multidrug-resistant phenotype of MCF-7/DOX cells was evidenced by a remarkable correlation between specific miRNA expression and corresponding changes in protein levels of their targets, specifically those ones that have a documented role in cancer drug resistance. Furthermore, we show that microRNA-451 regulates the expression of multidrug resistance 1 gene. More importantly, transfection of the MCF-7/DOX-resistant cells with microRNA-451 resulted in the increased sensitivity of cells to DOX, indicating that correction of altered expression of miRNA may have significant implications for therapeutic strategies aiming to overcome cancer cell resistance.

Effects of Dicer and Argonaute down-regulation on mRNA levels in human HEK293 cells

RNA interference and the microRNA (miRNA) pathway can induce sequence-specific mRNA degradation and/or translational repression. The human genome encodes hundreds of miRNAs that can post-transcriptionally repress thousands of genes. Using reporter constructs, we observed that degradation of mRNAs bearing sites imperfectly complementary to the endogenous let-7 miRNA is considerably stronger in human HEK293 than HeLa cells. The degradation did not result from the Ago2-mediated endonucleolytic cleavage but it was Dicer- and Ago2-dependent. We used this feature of HEK293 to address the size of a pool of transcripts regulated by RNA silencing in a single cell type. We generated HEK293 cell lines depleted of Dicer or individual Ago proteins. The cell lines were used for microarray analyses to obtain a comprehensive picture of RNA silencing. The 3′-untranslated region sequences of a few hundred transcripts that were commonly up-regulated upon Ago2 and Dicer knock-downs showed a significant enrichment of putative miRNA-binding sites. The up-regulation upon Ago2 and Dicer knock-downs was moderate and we found no evidence, at the mRNA level, for activation of silenced genes. Taken together, our data suggest that, independent of the effect on translation, miRNAs affect levels of a few hundred mRNAs in HEK293 cells.

Double-strand break repair in plants is developmentally regulated

In this study, we analyzed double-strand break (DSB) repair in Arabidopsis (Arabidopsis thaliana) at various developmental stages. To analyze DSB repair, we used a homologous recombination (HR) and point mutation reversion assays based on nonfunctional beta-glucuronidase reporter genes. Activation of the reporter gene through HR or point mutation reversion resulted in the appearance of blue sectors after histochemical staining. Scoring of these sectors at 3-d intervals from 2 to 31 d post germination (dpg) revealed that, although there was a 100-fold increase in the number of genomes per plant, the recombination frequency only increased 30-fold. This translates to a recombination rate at 31 dpg (2.77 x 10(-8)) being only 30% of the recombination rate at 2 dpg (9.14 x 10(-8)). Conversely, the mutation frequency increased nearly 180-fold, resulting in a 1.8-fold increase in mutation rate from 2 to 31 dpg. Additional analysis of DSBs over the early developmental stages revealed a substantial increase in the number of strand breaks per unit of DNA. Furthermore, RNA analysis of Ku70 and Rad51, two key enzymes in two different DSB repair pathways, and further protein analysis of Ku70 revealed an increase in Ku70 levels and a decrease of Rad51 levels in the developing plants. These data suggest that DSB repair mechanisms are developmentally regulated in Arabidopsis, whereby the proportion of breaks repaired via HR substantially decreases as the plants mature.

Homologous recombination in plants is organ specific

In this paper we analysed the genome stability of various Arabidopsis thaliana plant organs using a transgenic recombination system. The system was based on two copies of non-functional GUS (lines #651 and #11) or LUC (line #15D8) reporter genes serving as a recombination substrate. Both reporter assays showed that recombination in flowers or stems were rare events. Most of the recombination sectors were found in leaves and roots, with leaves having over 2-fold greater number of the recombination events per single cell genome as compared to roots. The recombination events per single genome were 9.7-fold more frequent on the lateral half of the leaves than on the medial halves. This correlated with a 2.5-fold higher metabolic activity in the energy source (lateral) versus energy sink (medial) of leaves. Higher metabolic activity was paralleled by a higher anthocyanin production in lateral halves. The level of double strand break (DSB) occurrence was also different among plant organs; the highest level was observed in roots and the lowest in leaves. High level of DSBs strongly positively correlated with the activity of the key repair enzymes, AtKU70 and AtRAD51. The ratio of AtRAD51 to AtKU70 expression was the highest in leaves, supporting the more active involvement of homologous recombination pathway in the repair of DSBs in this organ. Western blot analysis confirmed the real time PCR expression data for AtKU70 gene.

Homologous recombination in plants is temperature and day-length dependent

Homologous recombination (HR) as a strand break repair mechanism was shown to be influenced by various factors. The balance of different vitamins, macro- and microelements, light spectrum, sodium chloride concentration as well as various environmental mutagens were shown to influence the frequency of HR. In this paper we analysed the influence of temperature (4, 22, and 32 degrees C) and day/night duration on the genome stability of plants. We analyzed the HR frequency in transgenic Arabidopsis thaliana plants carrying beta-glucuronidase based homologous recombination substrate. To find the recombination rate (RR), we related the HR frequency to the number of genomes present in plants grown under different conditions. The RR was also standardized to the transcription activity of the transgene. We found RR to be higher in plants grown at suboptimal temperatures (either 4 or 32 degrees C) as compared to plants grown at 22 degrees C. This negatively correlated with the plant metabolic rate and positively correlated with concentration of peroxide produced by plant. In contrast, the RR in plants grown at different day length (8-24 h) was the lowest in plants grown at the longest day (24 h) and highest in the plants grown at the shortest day (8 h). Over 15-fold difference in the RR between plants grown at the shortest and the longest day was observed. Such a difference in recombination rate was primarily due to the higher transgene activity and higher endoreduplication levels in plants grown at longer days. Our data suggests that even "moderate" changes of environmental conditions may have a significant effect on plant genome stability.

Dissimilar genome response to acute and chronic low-dose radiation in male and female mice

The long-term genetic consequences of chronic exposure to low-dose irradiation constitutes a major concern to the general public and research community, especially as chronic radiation has recently been proven to be much more mutagenic and carcinogenic than previously thought. Here we report the results of the first ever comparison of the effects of acute and chronic whole body low-dose radiation exposure on global gene expression. We found a substantial difference between males and females in the expression of genes involved in signaling, growth control, transcription and other pathways upon acute and chronic radiation exposure. Specifically, we found sex differences in the expression of genes coding for G protein-coupled receptors and nuclear receptors. We also found different induction of PKCdelta, PKCbeta and PKCmu, members of PKC signaling pathway as well as in TGF and WNT signaling in males and females. Very pronounced difference, that was confirmed on the level of protein, was observed in the expression of WNT5A that plays an important role in carcinogenesis and muscle regeneration. WNT5A expression was significantly elevated only in chronically exposed females. We also provide the first evidence of the effect of ionizing radiation on the estrogen receptor in females. Repetitive irradiation of muscle tissue has been linked to development of rhabdomyosarcoma (RMS), which, enigmatically, occurs more frequently in males. Our data may be used to study possible mechanisms of RMS development upon chronic radiation exposure. They may provide some clues about the molecular background of the sex differences of RMS occurrence and may in the future lead to the discovery of new biomarkers for RMS predisposition in the irradiated tissue. Overall, differences in male and female responses to acute and chronic low-dose radiation obtained by this study were more drastic than we could have predicted. If confirmed in other experimental systems, these findings could potentially lead to fundamental changes in radiation safety regulations.

Methylation changes in muscle and liver tissues of male and female mice exposed to acute and chronic low-dose X-ray-irradiation

The biological and genetic effects of chronic low-dose radiation (LDR) exposure and its relationship to carcinogenesis have received a lot of attention in the recent years. For example, radiation-induced genome instability, which is thought to be a precursor of tumorogenesis, was shown to have a transgenerational nature. This indicates a possible involvement of epigenetic mechanisms in LDR-induced genome instability. Genomic DNA methylation is one of the most important epigenetic mechanisms. Existing data on radiation effects on DNA methylation patterns is limited, and no one has specifically studied the effects of the LDR. We report the first study of the effects of whole-body LDR exposure on global genome methylation in muscle and liver tissues of male and female mice. In parallel, we evaluated changes in promoter methylation and expression of the tumor suppressor gene p16(INKa) and DNA repair gene O(6)-methylguanine-DNA methyltransferase (MGMT). We observed different patterns of radiation-induced global genome DNA methylation in the liver and muscle of exposed males and females. We also found sex and tissue-specific differences in p16(INKa) promoter methylation upon LDR exposure. In male liver tissue, p16(INKa) promoter methylation was more pronounced than in female tissue. In contrast, no significant radiation-induced changes in p16(INKa) promoter methylation were noted in the muscle tissue of exposed males and females. Radiation also did not significantly affect methylation status of MGMT promoter. We also observed substantial sex differences in acute and chronic radiation-induced expression of p16(INKa) and MGMT genes. Another important outcome of our study was the fact that chronic low-dose radiation exposure proved to be a more potent inducer of epigenetic effects than the acute exposure. This supports previous findings that chronic exposure leads to greater genome destabilization than acute exposure.

Systemic plant signal triggers genome instability

Previously, we have shown that infection of tobacco plants with a viral pathogen triggers local and systemic induction of homologous recombination (HR). Here, we have tested the hypothesis of whether free radicals are potentially involved in the induction of the systemic effect. We report a significant induction of HR in tobacco plants treated with radical-generating agents, UVC or rose Bengal (RB). Importantly, the recombination increase was observed in local (treated) as well as systemic (non-treated) tissue. The systemic increase in recombination implies the existence of a signal that is transmitted to non-treated tissue. Several sets of grafting experiments proved the generation of said signal by both RB and UVC exposure. A statistically significant increase in HR was observed in tissue that received a systemic signal via a grafted leaf. Similar data were obtained from transgenic plants naphthalene degrading salicylate 1-hydroxylase (NahG) unable to accumulate salicylic acid (SA). Interestingly, pre-treatment of plants with the radical-scavenging compound N-acetyl-l-cysteine (NAC) led to a significantly lower recombination increase upon grafting after treatment with UVC and RB. Moreover, leaves taken for grafting from NAC-pre-treated plants exhibited a lower level of oxidized organic compounds. Our data suggest the involvement of free radical production in either generation or maintenance of the recombination signal. We discuss potential mechanisms for generation of the signal and possible adaptive advantages of enhanced genomic flexibility following exposure to DNA-damaging agents.

Atrazine induces homologous recombination but not point mutation in the transgenic plant-based biomonitoring assay

Herbicides, such as atrazine, are extensively used in agriculture in order to suppress growth of weeds. From the soil they inevitably find their way to water supplies, leading to human exposure via drinking water. Therefore, it is extremely important to know whether those chemicals pose any hazard to public health. The genotoxicity of atrazine has been a subject of studies in recent years. However, the data that are currently available are inconclusive. There is a need to examine the genotoxicity of low, environmentally relevant concentrations that are currently assumed to be safe. Up to date, studying the genotoxicity of low concentrations of atrazine has constituted a great challenge due to the lack of appropriate, sensitive test systems. In the present work, we used a new sensitive transgenic plant-based system to study the genotoxicity and mutagenicity of atrazine present at minute concentrations in the liquid media. This system gave us an opportunity to monitor the two main types of rearrangements, the frequency of homologous recombination and point mutations, which are indicators of the genotoxicity of atrazine. Atrazine present at low concentrations was found to be a strong inducer of homologous recombination. On the other hand, it did not have a significant influence on the levels of A --> G and T --> G mutations. These results suggest that the transgenic plant-based biomonitoring system is a useful tool for analyzing the genotoxicity of water contaminated by atrazine. In the future this system can be used to study molecular mechanisms of genotoxicity and mutagenicity atrazine and other triazine herbicides.

Genome stability of vtc1, tt4, and tt5 Arabidopsis thaliana mutants impaired in protection against oxidative stress

Reactive oxygen species (ROS) are formed upon normal cellular metabolism or influence of environmental factors and, at normal levels, they play an important physiological role. However, at elevated levels, radicals are toxic and extremely dangerous to all cellular components, including DNA. To efficiently protect themselves, plants have developed sophisticated mechanisms for radical screening and scavenging. In this paper, we analyzed the genome stability of several plant mutants impaired in the protection against free radicals. We crossed the well-known uidA recombination reporter line 651 to flavonoid (tt4 and tt5) and Vitamin C (vtc1)-deficient plants. We found that in all lines tested, both spontaneous and induced (UVC and Rose Bengal (RB)) recombination was higher than in the original 651 parental line. The mRNA expression levels of various DNA repair (RAD1, RAD54-like, MSH3) as well as radical scavenging genes (GPx1, CAT, FSD3) exhibited substantial differences in both control and induced conditions. Our data show that plants impaired in certain aspects of the protection against elevated levels of free radicals induce the production of scavenging enzymes earlier than wild-type (wt) plants, and the higher level of radical species results in the increased incidence of spontaneous double-strand breaks resulting in a higher expression of DNA repair genes.

Genotoxicity of 2,4-D and dicamba revealed by transgenic Arabidopsis thaliana plants harboring recombination and point mutation markers

The phenoxy herbicides 2,4-D and dicamba are released daily into the environment in large amount. The mechanisms of genotoxicity and mutagenicity of these herbicides are poorly understood, and the available genotoxicity data is controversial. There is a cogent need for a novel genotoxicity monitoring system that could provide both reliable information at the molecular level, and complement existing systems.We employed the transgenic Arabidopsis thaliana 'point mutation' and 'recombination' plants to monitor the genetic effects of the herbicides 2,4-D and dicamba. We found that both herbicides had a significant effect on the frequency of homologous recombination A-->G mutation. Neither herbicides affected the T-->G mutation frequency. Interestingly, these transgenic biomonitoring plants were able to detect the presence of phenoxy herbicides at concentrations that were lower than the guideline levels for Drinking Water Quality. The results of our studies suggest that our transgenic system may be ideal for the evaluation of the genotoxicity of herbicide-contaminated water. Moreover, the unique ability of the plants to detect both double-strand breaks (homologous recombination) and point mutations provides tremendous potential in the study of molecular mechanisms of genotoxicity and mutagenicity of phenoxy herbicides.

Pathogen-induced systemic plant signal triggers DNA rearrangements

Plant genome stability is known to be affected by various abiotic environmental conditions, but little is known about the effect of pathogens. For example, exposure of maize plants to barley stripe mosaic virus seems to activate transposable elements and to cause mutations in the non-infected progeny of infected plants. The induction by barley stripe mosaic virus of an inherited effect may mean that the virus has a non-cell-autonomous influence on genome stability. Infection with Peronospora parasitica results in an increase in the frequency of somatic recombination in Arabidopsis thaliana; however, it is unclear whether effects on recombination require the presence of the pathogen or represent a systemic plant response. It is also not clear whether the changes in the frequency of somatic recombination can be inherited. Here we report a threefold increase in homologous recombination frequency in both infected and non-infected tissue of tobacco plants infected with either tobacco mosaic virus or oilseed rape mosaic virus. These results indicate the existence of a systemic recombination signal that also results in an increased frequency of meiotic and/or inherited late somatic recombination.