Effects of Low-Dose Diethylstilbestrol Exposure on DNA Methylation in Mouse Spermatocytes

Low doses of DES inhibits the proliferation of GC-2 cells, alters cell cycle progression, triggers cell apoptosis, and induces male reproductive toxicity

2015 Study Abstract

Evidence from previous studies suggests that the male reproductive system can be disrupted by fetal or neonatal exposure to diethylstilbestrol (DES). However, the molecular basis for this effect remains unclear. To evaluate the effects of DES on mouse spermatocytes and to explore its potential mechanism of action, the levels of DNA methyltransferases (DNMTs) and DNA methylation induced by DES were detected.

The results showed that low doses of DES inhibited cell proliferation and cell cycle progression and induced apoptosis in GC-2 cells, an immortalized mouse pachytene spermatocyte-derived cell line, which reproduces primary cells responses to E2. Furthermore, global DNA methylation levels were increased and the expression levels of DNMTs were altered in DES-treated GC-2 cells. A total of 141 differentially methylated DNA sites were detected by microarray analysis. Rxra, an important component of the retinoic acid signaling pathway, and mybph, a RhoA pathway-related protein, were found to be hypermethylated, and Prkcd, an apoptosis-related protein, was hypomethylated.

These results showed that low-dose DES was toxic to spermatocytes and that DNMT expression and DNA methylation were altered in DES-exposed cells. Taken together, these data demonstrate that DNA methylation likely plays an important role in mediating DES-induced spermatocyte toxicity in vitro.

Effects of DES on GC-2 Cell Viability and Proliferation

To explore the potential cytotoxic effects of DES on GC-2 cells, cell proliferation was assessed. GC-2 cells were treated with various concentrations (0~10−4 M) of DES for 24 h, 48 h, or 72 h. DES exposure clearly reduced the viability of GC-2 cells in a dose-dependent manner within a certain concentration and time range. In addition, cell proliferation was significantly decreased when cells were exposed to different DES concentrations. The proportion of newborn (newly divided) cells decreased with increasing DES concentrations, indicating that the DNA replication capacity of GC-2 cells was decreased following DES exposure. Notably, even at a DES concentration of as low as 2×10−7 M, the proportion of newborn cells was lower than that in the DMSO group, suggesting that low doses of DES had adverse effects on mouse spermatocytes.

Effect of DES on GC-2 Cell Cycle Progression

Cell cycle progression is an important factor influencing cell proliferation. We analyzed the influence of DES on cell cycle progression to evaluate its antiproliferative activity. The proportion of cells in S phase increased for the DES-exposed cells compared with that for the DMSO-exposed cells. In particular, the proportion of S phase cells for the DMSO group was 28.97±1.21%, while those for the 2×10−7, 2×10−6, and 2×10−5 M DES-treated groups were 33.72±3.35%, 35.68±3.50%, and 43.44±3.65%, respectively. These results showed that DES changed the proportion of cells in the S cell phase and affected GC-2 cell cell cycle progression.

DES Induced Apoptosis in GC-2 Cells

Apoptosis is another important factor contributing to cell proliferation. Thus, we further examined apoptosis in GC-2 cells treated with or without DES. Apoptotic cells were recognized by their fragmented, degraded nuclei and apoptotic bodies. DES-treated GC-2 cells showed nucleolus pyknosis, and at increasing doses of DES, more nuclear fragmentation was observed. Flow cytometric analysis also produced similar results. After treatment with 0, 2×10−7, 2×10−6, and 2×10−5 M DES, the apoptosis rates were (1.3±0.52)%, (2.4±0.95)%, (2.7±0.68)%, and (16.8±1.34)%, respectively. Altogether, these results demonstrated that DES induced apoptosis in GC-2 cells.

Effects of DES on Global DNA Methylation in GC-2 Cells

Given the important effects of DNA methylation on gene regulation, transcriptional silencing and development, we sought to determine whether the DNA methylation patterns varied between GC-2 cells with and without DES exposure. We performed 5-mC dot blot DNA hybridizations to analyze the methylation statuses of GC-2 cells with and without exposure to this compound. The results of this analysis, with the density of band indicating relative DNA methylation levels, the global DNA methylation level in GC-2 cells was slightly increased following exposure to 2×10−7 and 2×10−6 M DES, and it was significantly increased in GC-2 cells exposed to 2×10−5 M DES. These data indicated that the global DNA methylation level increased with increasing concentrations of DES. They further suggested that DNA methylation might be crucial for the GC-2 cell toxicity observed following low-dose DES exposure.

Effects of DES on DNMT Expression

Because DNMTs were found to play important roles in establishing and maintaining DNA methylation patterns, we determined the expression levels of Dnmt1, Dnmt3a and Dnmt3b. Compared with the control, Dnmt1 and Dnmt3a expression was slightly increased in GC-2 cells exposed to 2×10−7 M DES and significantly increased in cells exposed to 2×10−6 and 2×10−5 M DES. In contrast, Dnmt3b expression decreased sharply with increasing doses of DES.

Analysis of Differential DNA Methylation following DES Exposure

To further explore DES-induced alterations in methylation, we screened differentially methylated DNA sites using an Affymetrix Mouse Promoter 1.0R Array. Data are available at GEO datasets (GEO number: GSE71311). A total of 141 differentially methylated sites (including 130 hypermethylated and 11 hypomethylated sites) were found in cells with and without 2×10−5 M DES exposure (fold change>3) by microarray analysis. The methylation statuses at some differential methylation sites were verified by MSP, and mRNA expression levels were detected by real-time PCR. In brief, compared with control cells, retinoid X receptor α (rxra) was hypermethylated in cells exposed to 2×10−7, 2×10−6, and 2×10−5 M DES and its mRNA expression was downregulated with increasing doses of DES. Myosin-binding protein H (mybph) was hypermethylated in cells exposed to 2×10−5 M DES, and its expression level was also reduced significantly. Protein kinase C δ (prkcd) was hypomethylated in cells exposed to 2×10−5 M DES, and its mRNA expression was increased. These results indicated that the methylation statuses of these genes were inversely correlated with their mRNA expression levels in DES-exposed GC-2 cells, suggesting that DNA methylation was involved in the regulation of mRNA expression in these cells.

Discussion

The present study has provided several lines of evidence demonstrating that low doses of DES induce spermatocyte toxicity by triggering apoptosis, inhibiting proliferation, and affecting cell cycle progression. We have further found that DNA methylation might play an important role in DES-induced spermatocyte toxicity.

DES has long been known to affect the male reproductive system by causing alterations, such as reproductive organ dysplasia, and germ cell damage. With regard to germ cells, abnormal spermatogenesis is the most common type of DES-induced effect. Some researchers have found that exposure of mice to 5 μg DES results in major morphological alterations to the testes, as reflected by the absence of germ cells in several tubules. Another study has reported that this compound (1.0 mg/kg) induces spermatogenic apoptosis in adult male rats. In our study, the apoptosis rate of GC-2 cells exposed to 2×10−5 M DES was significantly increased compared with that of DMSO-treated cells, and these results are in agreement with those of previous studies. GC-2 cell cycle progression was also altered following exposure to 2×10−5 M DES. Specifically, the percentage of DES-treated cells in the S phase of the cell cycle was greater than that of DMSO-treated cells, indicating that DES induced S phase arrest in spermatocytes. Further analysis using an EDU Cell Proliferation Kit indicated that the percentage of newborn cells was decreased following DES exposure, even at a DES concentration of as low as 2×10−7 M. EDU is readily incorporated into cellular DNA during DNA replication. Mammalian spermatogenesis is a unique process involving successive differentiation steps, including spermatogonial mitosis, spermatocyte meiosis and spermiogenesis. Each primary spermatocyte duplicates its DNA and subsequently undergoes meiosis I to produce two haploid secondary spermatocytes, which later divide once more into haploid spermatids. Interestingly, EDU incorporated into DES-treated spermatocyte cells less frequently than untreated cells. Based on these data, we proposed that low doses of DES can cause spermatocyte toxicity.

DNA methylation has been implicated in the regulation of spermiogenesis. DNA methylation at promoter regions plays a role in gene silencing, and during spermiogenesis, methylation occurs to silence retrotransposons and imprinted genes. Therefore, we proposed that DNA methylation might be involved in DES-induced spermatocyte toxicity. First, we detected the genome-wide methylation statuses of GC-2 cells exposed to 2×10−7, 2×10−6, or 2×10−5 M DES and found a tendency of increased methylation in these cells, even following exposure to low doses of DES. DNMTs, including Dnmt1, Dnmt3a, and Dnmt3b, were found to be involved in DNA methylation. Dnmt1 is responsible for the maintenance of DNA methylation during DNA synthesis, and Dnmt1-deficient embryos have been shown to have 30% less genomic methylation than that found in embryos. This phenomenon was also embodied in our experimental results. Dnmt1 protein expression was increased in GC-2 cells treated with 2×10−7, 2×10−6, or 2×10−5 M DES, consistent with the increase in the global DNA methylation level. Previous studies demonstrated that ERα could upregulate Dnmt1 expression by directly binding to the DNMT1 promoter region in ER-positive human breast cancer cells MFC-7 cells. DES has strong estrogenic activity, can activate ERα, and increase the expression of Dnmt1, which is coincidence with our results. Dnmt3a and Dnmt3b are de novo enzymes that establish methylation patterns. Spermatogonia deficient in Dnmt3a and Dnmt3b display variations in methylation patterns at paternally imprinted regions, which may impair spermatogenesis to an extent. Our results showed that low doses of DES were toxic to spermatocytes in vitro and caused alterations in the Dnmt3a and Dnmt3b protein expression levels. Taken together, our results suggest that DNA methylation plays a role in low-dose DES-induced male reproductive toxicity.

To further explore the potential mechanism of action of DES, DNA microarray technology is a useful tool for mapping methylation changes at multiple CpG loci. Microarray analysis performed in this study revealed the presence of thousands of variations in DNA methylation between GC-2 cells with and without DES exposure. The genes that were found to be differentially methylated are involved in the following processes: DNA repair, including mnd1 and nono; cell cycle progression, including hbp1 and ccno; apoptosis and proliferation, including rnf5, prkcd, jtb, nlrx1, mybph and rhoa; male reproductive development, including mybph, cldn11 and fkbp6; and other processes. The MSP assay results confirmed that the methylation statuses of some of the abovementioned genes were associated with low-dose DES-induced GC-2 cell toxicity. Rxra, an important component of the retinoic acid signaling pathway, is a key regulator of embryonic development and has been linked to several birth defects. Rxra knockout animals showed an increase in apoptosis, resulting in abnormal morphogenesis during development, in addition to abnormal cell proliferation, cell differentiation, and cell death processes in adult differentiated tissues. The two zinc fingers of the rxra DNA binding domain fold to form a single structural domain that consists of two perpendicularly oriented helices, which resembles the corresponding regions of ER. What’s more, Angelika Rosenauer et al indicated that transient expression in ER-negative human breast cancer cells MDA-MB-231 of wild-type ER directly stimulated the transcriptional response to RA(retinoic acid). Importantly, this activation was greater than that obtained by transfection of RAR(RA receptor), RXR(retinoid X receptor), or RAR combined with RXR, and the DNA binding domain of ER plays a key role in the response to RA-induced transcription. These researches suggested that ER had relation with rxra, and DES, as a strong ER agonist, had effect on the express of rxra. Mybph directly inhibits rock1 and plays important roles in cell motility and proliferation. In our study, the rxra and mybph promoters were hypermethylated, and their mRNA levels were reduced in low-dose DES-treated GC-2 cells. Accordingly, the viability of DES-treated cells was decreased, suggesting that decreases in the mRNA levels of rxra and mybph due to hypermethylation played important roles in low-dose DES-induced GC-2 cell toxicity. Prkcd is involved in the regulation of a variety of cellular functions, including apoptosis and cell growth and differentiation. Its overexpression has been shown to induce phenotypic changes indicative of apoptosis in several cell types. Our results indicated that prkcd was hypomethylated and that its high expression in DES-exposed cells was correlated with the increased apoptosis rate, similar to the previously reported theoretical results. These findings suggested that DNA methylation played an important role in low-dose DES-induced male reproductive toxicity.

As is known to all, DES has multigenerational effects. Some researches found that there is a high prevalence of hypospadias in the sons of women exposed to DES in utero. A nationwide cohort study in collaboration with a French association of DES-exposed women showed that a significant proportion of boys born to DES daughters exhibited hypospadias with no other molecular defects identified. DES-induced changes in epigenetic background and alteration of DNA methylation could be significant factors in the susceptibility to disease development. Epigenetic changes in the some genes, transmitted through the DES daughter, could explain such a finding. In our study, low dose of DES could change methylation status of many genes in GC-2 cells. Based on these, we deduced that low dose of DES could affect the methylation of germ cells in the same way, and many of the epigenetic changes would transmitted from father to grandson. Therefore, it is necessary to make further study related to low DES exposure and DNA methylation in germ cells.

In conclusion, our results showed that low doses of DES inhibited the proliferation of GC-2 cells, altered cell cycle progression, triggered cell apoptosis, and induced male reproductive toxicity. Through molecular studies, we have found that global DNA methylation and DNMT expression vary in DES–exposed GC-2 cells. Additionally, differentially methylated DNA sites were found in GC-2 cells treated with DES compared with those treated with DMSO. These results suggested that epigenetic modification might be a potential mechanism of low-dose DES-induced male reproductive toxicity.

Sources and more information
DES DIETHYLSTILBESTROL RESOURCES

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