Mutation analysis of the mel-18 gene that shows decreased expression in human breast cancer cell lines

Id1 enhances RING1b E3 ubiquitin ligase activity through the Mel-18/Bmi-1 polycomb group complex

The helix-loop-helix inhibitor of differentiation and DNA binding (Id1) is well known as an oncogene in various tumors. Although it has been reported that Id1 promotes several oncogenic processes, it is still unclear whether Id1 functions through epigenetic transcriptional regulation. In this study, we examined the effect of Id1 on polycomb group (PcG) proteins, which are crucial epigenetic gene silencers, and found that Id1 regulated the expression of Mel-18 and Bmi-1, both of which belong to polycomb repressive complex 1. We also confirmed that Id1 induced Mel-18 downregulation, which was mediated by the Akt pathway, and consequently upregulated the transcription of its target gene, c-Myc. Using a promoter-reporter, we demonstrated that Id1 regulated Bmi-1 transcription through c-Myc binding to its E-box in the promoter. Finally, we examined the activity of E3 ligase RING1b, whose catalytic activity is increased by binding with the RING finger protein Bmi-1, and found that Id1 overexpression enhanced RING1b E3 ligase activity leading to accumulation of H2A ubiquitination and ubiquitin/proteasome-mediated degradation of geminin. Taken together, our study provided a novel link between Id1 and PcG proteins and suggested that Id1 may contribute to tumor development through PcG-mediated epigenetic regulation.

Mel-18 negatively regulates INK4a/ARF-independent cell cycle progression via Akt inactivation in breast cancer

Mel-18, a polycomb group (PcG) protein, has been suggested as a tumor suppressor in human breast cancer. Previously, we reported that Mel-18 has antiproliferative activity in breast cancer cells. However, its functional mechanism has not been fully elucidated. Here, we investigated the role of Mel-18 in human breast cancer. We saw an inverse correlation between Mel-18 and phospho-Akt, which were expressed at low and high levels, respectively, in primary breast tumor tissues from 40 breast cancer patients. The effect of Mel-18 on cell growth was examined in two breast cancer cell lines, SK-BR-3 and T-47D, which express relatively low and high levels of endogenous Mel-18, respectively. On Mel-18 overexpression in SK-BR-3 cells, cell growth was attenuated and G(1) arrest was observed. Likewise, suppression of Mel-18 by antisense expression in T-47D cells led to enhanced cell growth and accelerated G(1)-S phase transition. In these cells, cyclin-dependent kinase (Cdk)-4 and Cdk2 activities were affected by Mel-18, which were mediated by changes in cyclin D1 expression and p27(Kip1) phosphorylation at Thr(157), but not by INK4a/ARF genes. The changes were both dependent on the phosphatidylinositol 3-kinase/Akt signaling pathway. Akt phosphorylation at Ser(473) was reduced by Mel-18 overexpression in SK-BR-3 cells and enhanced by Mel-18 suppression in T-47D cells. Akt-mediated cytoplasmic localization of p27(Kip1) was inhibited by Mel-18 in SK-BR-3 cells. Moreover, Mel-18 overexpression showed reduced glycogen synthase kinase-3beta phosphorylation, beta-catenin nuclear localization, T-cell factor/lymphoid enhancer factor promoter activity, and cyclin D1 mRNA level. Taken together, we established a linear relationship between Mel-18-->Akt-->G(1) phase regulators.

Biology of Polycomb and Trithorax Group Proteins

Cellular phenotypes can be ascribed to different patterns of gene expression. Epigenetic mechanisms control the generation of different phenotypes from the same genotype. Thus differentiation is basically a process driven by changes in gene activity during development, often in response to transient factors or environmental stimuli. To keep the specific characteristics of cell types, tissue-specific gene expression patterns must be transmitted stably from one cell to the daughter cells, also in the absence of the early-acting determination factors. This heritability of patterns of active and inactive genes is enabled by epigenetic mechanisms that create a layer of information on top of the DNA sequence that ensures mitotic and sometimes also meiotic transmission of expression patterns. The proteins of the Polycomb and Trithorax group comprise such a cellular memory mechanism that preserves gene expression patterns through many rounds of cell division. This review provides an overview of the genetics and molecular biology of these maintenance proteins, concentrating mainly on mechanisms of Polycomb group-mediated repression.

Variability in the expression of polycomb proteins in different normal and tumoral tissues. A pilot study using tissue microarrays

In spite of the known function of polycomb group (PcG) genes in stem cell self-renewal, control of cellular proliferation and differentiation, its role in cancer pathogenesis is still poorly understood. We studied the expression by immunohistochemistry of several PcG-maintenance complex proteins (RING1, RNF2, BMI1, MEL18, HPH1 and RYBP) in nontumoral (154 samples) and tumoral (550 samples) human tissues using Tissue Microarrays. For selected genes (BMI1 and RING1) FISH analysis has been also carried out. PcG proteins had a tissue- and cell-type-specific expression pattern. Some of them were highly selectively expressed, such as HPH1, which was detected in germ cells in testis, pituitary and parathyroid glands and Langerhans islets, and RYBP, which was found in placenta, umbilical cord and thyroid gland. By contrast, RING1 was ubiquitously expressed in every normal tissue analyzed. Changes in expression associated with tumoral transformation have been found for BMI1 and RNF2, which exhibited increased expression in a large series of tumors, including gastrointestinal tumors, pituitary and parathyroid adenomas, and lymphomas, compared with their expression in normal-cell counterparts. The high level of expression of BMI1 protein observed in mantle-cell lymphomas and pituitary adenomas is associated in some cases with amplification of BMI1 locus. These findings imply that upregulation of BMI1 may constitute a malignancy marker in different types of cancer, mainly in lymphoid and endocrine tumors. RING1 was lost in a group of renal-cell carcinomas and testicular germ-cell tumors. Lastly, RYBP is anomalously expressed in Hodgkin's lymphomas and oligodendrogliomas, among others tumors. A significant finding of the study is the identification of unique PcG profiles for some tumors, such as testicular germ-cell tumors, which have high levels of HPH1 expression and loss of RING1 and/or BMI1; pituitary adenomas, which expressed every PcG protein analyzed; and clear-cell renal-cell carcinoma, which was the only tumor other than testicular germ-cell tumors that did not express RING1.

The polycomb group gene product Mel-18 interacts with cyclin D2 and modulates its activity

Considerable evidence supports the view that D-type cyclins play a role in G1-S progression. We found that cyclin D2 directly interacts with Mel-18, one of the polycomb group gene products in a yeast two hybrid screen. Further, we have determined the binding domains that are required for interaction between cyclin D2 and Mel-18. The proline/serine-rich domain (P/S domain) of Mel-18 is required to interact with cyclin D2, and the N-terminal region of cyclin D2 is necessary to interact with Mel-18. A co-localization study shows that cyclin D2 and Mel-18 interact within the nucleus. To determine whether Mel-18 affects cyclin D2 activity, we blocked Mel-18 expression using an anti-sense strand system in cyclin D2 over-expressing cells. The results indicate that cells with reduced Mel-18 expression levels show more proliferative activity than the controls. These findings are the first report that Mel-18 directly interacts with cyclin D2 and may inhibit cyclin D2 activity.

Increased expression of the EZH2 polycomb group gene in BMI-1-positive neoplastic cells during bronchial carcinogenesis

Polycomb group (PcG) genes are responsible for maintenance of cellular identity and contribute to regulation of the cell cycle. Recent studies have identified several PcG genes as oncogenes, and a role for PcG proteins in human oncogenesis is suspected. We investigated the expression of BMI-1 and EZH2 PcG oncogenes in human bronchial squamous cell carcinomas (SCCs) and bronchial premalignant precursor lesions (PLs). Whereas normal bronchial epithelium was associated with widespread expression of BMI-1 in resting EZH2-negative cells, neoplastic cells in lung carcinomas displayed altered expression of both BMI-1 and EZH2. Two patterns of abnormal PcG expression were observed: increased expression of BMI-1 in dividing neoplastic cells of PLs and SCCs, and enhanced expression of EZH2 and Ki-67 in BMI-1-positive cells according to severity of the histopathologic stage. We propose that altered expression of BMI-1 and EZH2 is an early event that precedes high rates of proliferation in lung cancer. Because PcG complexes are normally involved in the maintenance of cell characteristics, abnormal PcG expression may contribute to loss of cell identity.

Role of Polycomb Group Proteins in Stem Cell Self-Renewal and Cancer

Polycomb group proteins (PcG) form part of a gene regulatory mechanism that determines cell fate during normal and pathogenic development. The mechanism relies on epigenetic modifications on specific histone tails that are inherited through cell divisions, thus behaving de facto as a cellular memory. This cellular memory governs key events in organismal development as well as contributing to the control of normal cell growth and differentiation. Consequently, the dysregulation of PcG genes, such as Bmi1, Pc2, Cbx7, and EZH2 has been linked with the aberrant proliferation of cancer cells. Furthermore, at least three PcG genes, Bmi1, Rae28, and Mel18, appear to regulate self-renewal of specific stem cell types suggesting a link between the maintenance of cellular homeostasis and tumorigenesis. In this review, we will briefly summarize current views on PcG function and the evidence linking specific PcG proteins with the behavior of stem cells and cancer cells.

Poorly differentiated breast carcinoma is associated with increased expression of the human polycomb group EZH2 gene

Polycomb group (PcG) genes contribute to the maintenance of cell identity, cell cycle regulation, and oncogenesis. We describe the expression of five PcG genes (BMI-1, RING1, HPC1, HPC2, and EZH2) innormal breast tissues, invasive breast carcinomas, and their precursors. Members of the HPC-HPH/PRC1 PcG complex, including BMI-1, RING1, HPC1, and HPC2, were detected in normal resting and cycling breast cells. The EED-EZH/PRC2 PcG complex protein EZH2 was only found in rare cycling cells, whereas normal resting breast cells were negative for EZH2. PcG gene expression patterns in ductal hyperplasia (DH), well-differentiated ductal carcinoma in situ (DCIS), and well-differentiated invasive carcinomas closely resembled the pattern in healthy cells. However, poorly differentiated DCIS and invasive carcinomas frequently expressed EZH2 in combination with HPC-HPH/PRC1 proteins. Most BMI-1/EZH2 double-positive cells in poorly differentiated DCIS were resting. Poorly differentiated invasive carcinoma displayed an enhanced rate of cell division within BMI-1/EZH2 double-positive cells. We propose that the enhanced expression of EZH2 in BMI-1(+) cells contributes to the loss of cell identity in poorly differentiated breast carcinomas, and that increased EZH2 expression precedes high frequencies of proliferation. These observations suggest that deregulated expression of EZH2 is associated with loss of differentiation and development of poorly differentiated breast cancer in humans.

Polycomb group genes as epigenetic regulators of normal and leukemic hemopoiesis

Epigenetic modification of chromatin structure underlies the differentiation of pluripotent hemopoietic stem cells (HSCs) into their committed/differentiated progeny. Compelling evidence indicates that Polycomb group (PcG) genes play a key role in normal and leukemic hemopoiesis through epigenetic regulation of HSC self-renewal/proliferation and commitment. The PcG proteins are constituents of evolutionary highly conserved molecular pathways regulating cell fate in several other tissues through diverse mechanisms, including 1) regulation of self-renewal/proliferation, 2) regulation of senescence/immortalization, 3) interaction with the initiation transcription machinery, 4) interaction with chromatin-condensation proteins, 5) modification of histones, 6) inactivation of paternal X chromosome, and 7) regulation of cell death. It is therefore not surprising that PcG genes lead to pleiotropic phenotypes when mutated and have been associated with malignancies in several systems in both mice and humans. Although much remains to be learned regarding the PcG mechanism(s) of action, advances in identifying the functional domains and enzymatic activities of these multimeric protein complexes have provided insights into how PcG proteins accomplish such processes. Some of the new insights into a role for the PcG cellular memory system in regulating normal and leukemic hemopoiesis are reviewed here, with special emphasis on their potential involvement in epigenetic regulation of gene expression through modification of chromatin structure.