As leading cause of cancer death among females, human breast cancer has the features of powerful invasive ability and early metastatic property. Human breast cancer with the incidence rate increasing is the threat to human health. It is significantly meaningful to understand the pathologic mechanism of breast cancer and find treatment target site. Recent researches indicate that not only gene dysfunction but also histone modifications are involved in breast tumorigenesis .
Recent studies have implicated H3K9 modifications in numerous biological phenomena including germ cell development, × chromosome inactivation, DNA damage repair and apoptosis . Recent reports also link deregulated histone methylation to tumorigenesis [15, 16]. An H3K9 histone methyltransferase, Suv39H1, has been shown to function as a tumor suppressor by maintaining H3K9 methylation levels [17, 18]. These data imply that H3K9me3 demethylases JMJD2A protein may take part in tumorigenesis through demethylation of H3K9me3.
Here we hypothesized that down-regulation of JMJD2A expression in MDA-MB-231 cell line would affect breast tumorigenesis and tumor biological characteristics. To test this hypothesis, JMJD2A-specific siRNA was transfected into human breast cancer cell line MDA-MB-231 to observe the effects. It was proved that JMJD2A gene could be silenced efficiently in MDA-MB-231 cell line by transfection with JMJD2A-specific siRNA and HiPerFect Transfection Reagent in this study. According to the results of Quantitative real-time PCR and Western blot analysis, the levels of JMJD2A mRNA and protein expression were both down-regulated based on the transfection. Further, FCM and MTT assay results showed cell cycle changes and proliferation inhibition existed in MDA-MB-231 cell line, and migration and invasion in vitro were both suppressed. These data imply tumor growth and metastasis may be restrained by silencing JMJD2A, and JMJD2A may be associated with breast cancer cell line MDA-MB-231, thus JMJD2A might be the potential therapeutic target in breast cancer.
However, the mechanism of JMJD2A in breast cancer is not very clear, here we discuss the probable role of JMJD2A in breast cancer based on our own recent data and the literature. Local chromatin architecture which is strongly influenced by post-translational modifications of histones like methylation is now generally recognized as an important factor in the regulation of gene expression [19, 20]. The combination of different modifications and the incorporation of different histone variants which have distinct roles in gene regulation, have led to the proposition of a regulatory histone code which determines, at least partly, the transcriptional potential for a specific gene or a genomic region . High endogenous expression of JMJD2A protein catalyzes demethylation of H3K9me3 excessively to break the balance between methylated and demethylated histones. Genome-wide studies show that H3K9me3 is enriched in heterochromatin, especially, as the mark with general repressive nature, H3K9me3 is predominant in coding regions of some active genes [22–25]. The intragenic permissive chromatin regions are flanked by the repressive mark, H3K9me3, and the maintenance of the intragenic chromatin boundary appears to functions as a checkpoint in elongation . These data predict that the H3K9me3 demethylase activities of JMJD2A protein may act as transcriptional activators.
A recent research focusing on another member of JMJD2 family proteins JMJD2B, which is considered to have the similar function as JMJD2A in breast cancer demonstrated that JMJD2B constitutes a key component of the estrogen signaling pathway and the establishment of local epigenetic state and chromatin structure required for proper induction of ER responsive genes. JMJD2B which interacts with ERα and components of the SWI/SNF-B chromatin remodeling complex was recruited to ERα target sites, demethylated H3K9me3 and facilitated transcription of ER responsive oncogenes including MYB, MYC and CCND1, and knockdown of JMJD2B severely impaired estrogen induced cell proliferation and the tumor formation capacity of breast cancer cells as a consequence . Consisting with that research, our data showed that silencing of JMJD2A could suppress the proliferation, migration and invasion of MDA-MB-231 cell line, thereby indicating that JMJD2A may be involved in the estrogen signaling pathway.
Though JMJD2A and 2B exhibited robust interactions with ER, in contrast to depletion of JMJD2B, depletion of JMJD2A caused only a marginal defect in ER target gene induction . There may be another pathway JMJD2A involved in human breast cancer. It was described that JMJD2A has molecular characterization in binding both retinoblastoma protein (pRb) and histone deacetylases (HDACs) . JMJD2A maybe associated with pRb recruits HDACs to the pRB-E2F complex, changes the chromatin structure at the E2F-responsive promoter and induced suppression of target gene E2F expression [29, 30]. E2F1, 4 and their complexes with HDAC play an important role in downregulating the expression of the maternally imprinted tumor suppressor gene ARHI in breast cancer cells. Expression of ARHI is markedly down-regulated in breast cancer, and reactivation of ARHI expression in breast cancer cells is associated with decreased H3K9me3 which is demethylated by JMJD2A [31, 32].
Together, JMJD2A may be, at least in part, involved in human breast cancer by constituting a key component of the estrogen signaling pathway or binding pRb and HDACs to suppress E2F-induced ARHI expression. However, the exact mechanism of JMJD2A in human breast cancer still remains elusive. The role of JMJD2A may be diverse rather than single.
To date, this is the first report highlighting that the suppression of proliferation, invasion and migration in human breast cancer cell line MDA-MB-231, at least in part, results from silencing of JMJD2A. The present study sheds light on the novel role of JMJD2A in breast cancer. However, our results were based on a single cell line. Further researches to determine the differential expression of JMJD2A between normal and cancer breast tissue and the mechanism of JMJD2A in breast cancer are required.