Luyang Sun

LSD1 is a subunit of the NuRD complex and targets the metastasis programs in breast cancer.

Lysine-specific demethylase 1 (LSD1) exerts pathway-specific activity in animal development and has been linked to several high-risk cancers. Here, we report that LSD1 is an integral component of the Mi-2/nucleosome remodeling and deacetylase (NuRD) complex. Transcriptional target analysis revealed that the LSD1/NuRD complexes regulate several cellular signaling pathways including TGFbeta1 signaling pathway that are critically involved in cell proliferation, survival, and epithelial-to-mesenchymal transition. We demonstrated that LSD1 inhibits the invasion of breast cancer cells in vitro and suppresses breast cancer metastatic potential in vivo. We found that LSD1 is downregulated in breast carcinomas and that its level of expression is negatively correlated with that of TGFbeta1. Our data provide a molecular basis for the interplay of histone demethylation and deacetylation in chromatin remodeling. By enlisting LSD1, the NuRD complex expands its chromatin remodeling capacity to include ATPase, histone deacetylase, and histone demethylase.

The Molecular Mechanism Governing the Oncogenic Potential of SOX2 in Breast Cancer

SOX genes encode a family of high-mobility group transcription factors that play critical roles in organogenesis. The functional specificity of different SOX proteins and the tissue specificity of a particular SOX factor are largely determined by the differential partnership of SOX transcription factors with other transcription regulators, many of which have not yet been discovered. Virtually all members of the SOX family have been found to be deregulated in a wide variety of tumors. However, little is known about the cellular and molecular behaviors involved in the oncogenic potential of SOX proteins. Using cell culture experiments, tissue analysis, molecular profiling, and animal studies, we report here that SOX2 promotes cell proliferation and tumorigenesis by facilitating the G1/S transition and through its transcription regulation of the CCND1 gene in breast cancer cells. In addition, we identified β-catenin as the transcription partner for SOX2 and demonstrated that SOX2 andβ-catenin act in synergy in the transcription regulation of CCND1 in breast cancer cells. Our experiments not only determined a role for SOX2 in mammary tumorigenesis but also revealed another activity of the multifunctional protein, β-catenin.

Hypomethylation-linked activation of PAX2 mediates tamoxifen-stimulated endometrial carcinogenesis.

Tamoxifen, a selective oestrogen receptor modulator, has been used in the treatment of all stages of hormone-responsive breast cancer. However, tamoxifen shows partial oestrogenic activity in the uterus and its use has been associated with an increased incidence of endometrial cancer. The molecular explanation for these observations is not known. Here we show that tamoxifen and oestrogen have distinct but overlapping target gene profiles. Among the overlapping target genes, we identify a paired-box gene, PAX2, that is crucially involved in cell proliferation and carcinogenesis in the endometrium. Our experiments show that PAX2 is activated by oestrogen and tamoxifen in endometrial carcinomas but not in normal endometrium, and that this activation is associated with cancer-linked hypomethylation of the PAX2 promoter.

Integration of Estrogen and Wnt Signaling Circuits by the Polycomb Group Protein EZH2 in Breast Cancer Cells

ABSTRACT Essential for embryonic development, the polycomb group protein enhancer of zeste homolog 2 (EZH2) is overexpressed in breast and prostate cancers and is implicated in the growth and aggression of the tumors. The tumorigenic mechanism underlying EZH2 overexpression is largely unknown. It is believed that EZH2 exerts its biological activity as a transcription repressor. However, we report here that EZH2 functions in gene transcriptional activation in breast cancer cells. We show that EZH2 transactivates genes that are commonly targeted by estrogen and Wnt signaling pathways. We demonstrated that EZH2 physically interacts directly with estrogen receptor α and β-catenin, thus connecting the estrogen and Wnt signaling circuitries, functionally enhances gene transactivation by estrogen and Wnt pathways, and phenotypically promotes cell cycle progression. In addition, we identified the transactivation activity of EZH2 in its two N-terminal domains and demonstrated that these structures serve as platforms to connect transcription factors and the Mediator complex. Our experiments indicated that EZH2 is a dual function transcription regulator with a dynamic activity, and we provide a mechanism for EZH2 in tumorigenesis.

Histone demethylase JMJD2B coordinates H3K4/H3K9 methylation and promotes hormonally responsive breast carcinogenesis

It is well-documented that the methylation of histone H3 lysine 4 (H3K4) and of H3K9 are mutually exclusive, an epigenetic phenomenon conserved from yeast to humans. How this opposed methylation modification is accomplished and coordinated in mammalian cells is poorly understood. Here we report that the H3K9 trimethyl demethylase JMJD2B is an integral component of the H3K4-specific methyltransferase, the mixed-lineage leukemia (MLL) 2 complex. We show that the JMJD2B/MLL2 complex is copurified with estrogen receptor α (ERα) and is required for ERα-regulated transcription. We demonstrate that H3K9 demethylation and H3K4 methylation are coordinated in ERα-activated transcription such that H3K9 demethylation is a prerequisite for H3K4 methylation. Significantly, depletion of JMJD2B impairs the estrogen-induced G1/S transition of the cell cycle in vitro and inhibits breast tumorigenesis in vivo. Interestingly, JMJD2B itself is an ERα target gene, and forms a feed-forward regulatory loop in regulation of the hormone response. Our results provide a molecular basis for the coordinated H3K4 methylation/H3K9 demethylation in transcription activation, link the trimethyl demethylase JMJD2B to euchromatin functions, and provide a mechanism for JMJD2B in breast carcinogenesis.

SET8 promotes epithelial-mesenchymal transition and confers TWIST dual transcriptional activities.

SET8 is implicated in transcriptional regulation, heterochromatin formation, genomic stability, cell‐cycle progression, and development. As such, it is predicted that SET8 might be involved in the development and progression of tumour. However, whether and how SET8 might be implicated in tumourigenesis is currently unknown. Here, we report that SET8 is physically associated with TWIST, a master regulator of epithelial–mesenchymal transition (EMT). We demonstrated that SET8 and TWIST are functionally interdependent in promoting EMT and enhancing the invasive potential of breast cancer cells in vitro and in vivo. We showed that SET8 acts as a dual epigenetic modifier on the promoters of the TWIST target genes E‐cadherin and N‐cadherin via its H4K20 monomethylation activity. Significantly, in breast carcinoma samples, SET8 expression is positively correlated with metastasis and the expression of TWIST and N‐cadherin and negatively correlated with E‐cadherin. Together, our experiments revealed a novel role for SET8 in tumour invasion and metastasis and provide a molecular mechanism underlying TWIST‐promoted EMT, suggesting SET8 as a potential target for intervention of the metastasis of breast cancer.

Coordinated Regulation of AIB1 Transcriptional Activity by Sumoylation and Phosphorylation

AIB1, a member of the steroid receptor coactivator (SRC) family that participates in gene transcriptional activation by nuclear receptors and other transcription factors, is required for animal growth and reproductive development and implicated in breast carcinogenesis. The mechanisms underlying the AIB1 pleiotropic functions are not fully understood and neither is the regulation of its activity. Here, we showed that AIB1 was a sumoylated protein and the sumoylation attenuated the transactivation activity of AIB1, which is in contrast to the sumoylation of its paralogs, GRIP1 and SRC-1. The transactivation activity of AIB1 is enhanced by its phosphorylation by several kinases, including mitogen-activated protein kinase. We demonstrated in this report that estrogen treatment led to an increased phosphorylation and decreased sumoylation of AIB1 and that the sumoylation coordinated with phosphorylation in regulating the transcriptional activity of AIB1, providing a mechanism for post-translational modifications in regulating the transcriptional output of AIB1.

The catalytic subunit of the proteasome is engaged in the entire process of estrogen receptor‐regulated transcription

The ubiquitin–proteasome system plays an important role in a variety of cellular functions by means of its proteolytic activity. Interestingly, recent studies have indicated that the proteasome components are also integral parts of transcription complexes. In genome‐wide screening for steroid receptor coactivator (SRC)‐interacting proteins using yeast two‐hybrid system, we found that the 20S proteasome β subunit LMP2 (Low Molecular mass Polypeptide 2) interacts directly with the SRC coactivators. We showed that LMP2 is required for estrogen receptor (ER)‐mediated gene transcription and for estrogen‐stimulated cell cycle progression. We found that LMP2‐associated proteasome is recruited to the entire sequence of ER target genes, implicating a role for the proteasome in both transcription initiation and elongation. We demonstrated that the recruitment of LMP2 by SRC coactivators is necessary for cyclic association of ER‐regulated transcription complexes on ER targets. These results revealed a mechanism by which the proteasome machinery is recruited in ER‐mediated gene transcription. Our experiments also provided evidence implicating SRC coactivators in gene transcription elongation.

SIRT7 is a histone desuccinylase that functionally links to chromatin compaction and genome stability

Although SIRT7 is a member of sirtuin family proteins that are described as NAD+-dependent class III histone deacetylases, the intrinsic enzymatic activity of this sirtuin protein remains to be investigated and the cellular function of SIRT7 remains to be explored. Here we report that SIRT7 is an NAD+-dependent histone desuccinylase. We show that SIRT7 is recruited to DNA double-strand breaks (DSBs) in a PARP1-dependent manner and catalyses desuccinylation of H3K122 therein, thereby promoting chromatin condensation and DSB repair. We demonstrate that depletion of SIRT7 impairs chromatin compaction during DNA-damage response and sensitizes cells to genotoxic stresses. Our study indicates SIRT7 is a histone desuccinylase, providing a molecular basis for the understanding of epigenetic regulation by this sirtuin protein. Our experiments reveal that SIRT7-catalysed H3K122 desuccinylation is critically implemented in DNA-damage response and cell survival, providing a mechanistic insight into the cellular function of SIRT7.

JFK, a Kelch domain-containing F-box protein, links the SCF complex to p53 regulation

The p53 tumor suppressor plays a central role in integrating cellular responses to various stresses. Tight regulation of p53 is thus essential for the maintenance of genome integrity and normal cell proliferation. Currently, several ubiquitin ligases, including the single-subunit RING-finger types—MDM2, Pirh2, and COP1—and the HECT-domain type—ARF-BP1—have been reported to target p53 for degradation. Here, we report the identification of a human Kelch domain-containing F-box protein, JFK. We showed that JFK promotes ubiquitination and degradation of p53. But unlike MDM2, Pirh2, COP1, and ARF-BP1, all of which possess an intrinsic ubiquitin ligase activity, JFK destabilizes p53 through the assembly of a Skp1-Cul1-F-box complex. Significantly, JFK inhibits p53-dependent transcription, and depletion of JFK stabilizes p53, promotes cell apoptosis, arrests cells in the G1 phase, and sensitizes cells to ionizing radiation-induced cell death. These data indicate that JFK is a critical negative regulator of p53 and represents a pathway for the maintenance of p53 levels in unstressed cells. Our experiments link the Skp1-Cul1-F-box system to p53 regulation.