Chromobox protein homologue 7 protein, with decreased expression in human carcinomas, positively regulates E-cadherin expression by interacting with the histone deacetylase 2 protein

High-mobility group A1 protein inhibits p53-mediated intrinsic apoptosis by interacting with Bcl-2 at mitochondria

The high-mobility group A (HMGA) proteins are a family of non-histone chromatin factors, encoded by the HMGA1 and HMGA2 genes. Several studies demonstrate that HMGA proteins have a critical role in neoplastic transformation, and their overexpression is mainly associated with a highly malignant phenotype, also representing a poor prognostic index. Even though a cytoplasmic localization of these proteins has been previously reported in some highly malignant neoplasias, a clear role for this localization has not been defined. Here, we first confirm the localization of the HMGA1 proteins in the cytoplasm of cancer cells, and then we report a novel mechanism through which HMGA1 inhibits p53-mitochondrial apoptosis by counteracting the binding of p53 to the anti-apoptotic factor Bcl-2. Indeed, we demonstrate a physical and functional interaction between HMGA1 and Bcl-2 proteins. This interaction occurs at mitochondria interfering with the ability of p53 protein to bind Bcl-2, thus counteracting p53-mediated mitochondrial apoptosis. This effect is associated with the inhibition of cytochrome c release and activation of caspases. Consistent with this mechanism, a strong correlation between HMGA1 cytoplasmic localization and a more aggressive histotype of thyroid, breast and colon carcinomas has been observed. Therefore, cytoplasmic localization of HMGA1 proteins in malignant tissues is a novel mechanism of inactivation of p53 apoptotic function.

Tumor suppressor activity of CBX7 in lung carcinogenesis

The generation of knockout mice for the Cbx7 gene validates the tumor suppressor role of CBX7, whose expression is lost in several human malignancies. Indeed, these mice developed liver and lung adenomas and carcinomas. Cyclin E overexpression due to the lack of Cbx7 negative regulation of its expression likely accounts for the phenotype of the Cbx7-KO mice. A similar mechanism is likely involved in human lung carcinogenesis, since cyclin E upregulation associated with the loss of CBX7 expression has been observed in most of the human lung carcinomas analyzed.

CBX7 is a tumor suppressor in mice and humans

The CBX7 gene encodes a polycomb group protein that is known to be downregulated in many types of human cancers, although the role of this protein in carcinogenesis remains unclear. To shed light on this issue, we generated mice null for Cbx7. Mouse embryonic fibroblasts derived from these mice had a higher growth rate and reduced susceptibility to senescence compared with their WT counterparts. This was associated with upregulated expression of multiple cell cycle components, including cyclin E, which is known to play a key role in lung carcinogenesis in humans. Adult Cbx7-KO mice developed liver and lung adenomas and carcinomas. In in vivo and in vitro experiments, we demonstrated that CBX7 bound to the CCNE1 promoter in a complex that included HDAC2 and negatively regulated CCNE1 expression. Finally, we found that the lack of CBX7 protein expression in human lung carcinomas correlated with CCNE1 overexpression. These data suggest that CBX7 is a tumor suppressor and that its loss plays a key role in the pathogenesis of cancer.

miR-193 expression differentiates telocytes from other stromal cells

Telocytes (TCs) are a particular type of interstitial (stromal) cells defined by very long, moniliform telopodes. Their tissue location, between blood vessels and other cells such as cardiomyocytes (CMC) and neurons, suggests a role in intercellular signalling. In order to define a microRNA (miR) signature in cardiac TCs, we have found that miR-193 is differentially expressed between TCs and other interstitial cells. Because miR-193 regulates c-kit, our data support the previous finding that TCs express c-kit in certain circumstances. In addition, the miRs which are specific to CMC and other muscle cells (e.g. miR-133a, miR-208a) are absent in TCs. Overall the data reinforce the view that TCs are a particular type of interstitial (mesenchymal) cells.

Expression of microRNA and their gene targets are dysregulated in preinvasive breast cancer

INTRODUCTION

microRNA (miRNA) are short, noncoding RNA that negatively regulate gene expression and may play a causal role in invasive breast cancer. Since many genetic aberrations of invasive disease are detectable in early stages, we hypothesized that miRNA expression dysregulation and the predicted changes in gene expression might also be found in early breast neoplasias.

METHODS

Expression profiling of 365 miRNA by real-time quantitative polymerase chain reaction assay was combined with laser capture microdissection to obtain an epithelium-specific miRNA expression signature of normal breast epithelium from reduction mammoplasty (RM) (n = 9) and of paired samples of histologically normal epithelium (HN) and ductal carcinoma in situ (DCIS) (n = 16). To determine how miRNA may control the expression of codysregulated mRNA, we also performed gene expression microarray analysis in the same paired HN and DCIS samples and integrated this with miRNA target prediction. We further validated several target pairs by modulating the expression levels of miRNA in MCF7 cells and measured the expression of target mRNA and proteins.

RESULTS

Thirty-five miRNA were aberrantly expressed between RM, HN and DCIS. Twenty-nine miRNA and 420 mRNA were aberrantly expressed between HN and DCIS. Combining these two data sets with miRNA target prediction, we identified two established target pairs (miR-195:CCND1 and miR-21:NFIB) and tested several novel miRNA:mRNA target pairs. Overexpression of the putative tumor suppressor miR-125b, which is underexpressed in DCIS, repressed the expression of MEMO1, which is required for ErbB2-driven cell motility (also a target of miR-125b), and NRIP1/RIP140, which modulates the transcriptional activity of the estrogen receptor. Knockdown of the putative oncogenic miRNA miR-182 and miR-183, both highly overexpressed in DCIS, increased the expression of chromobox homolog 7 (CBX7) (which regulates E-cadherin expression), DOK4, NMT2 and EGR1. Augmentation of CBX7 by knockdown of miR-182 expression, in turn, positively regulated the expression of E-cadherin, a key protein involved in maintaining normal epithelial cell morphology, which is commonly lost during neoplastic progression.

CONCLUSIONS

These data provide the first miRNA expression profile of normal breast epithelium and of preinvasive breast carcinoma. Further, we demonstrate that altered miRNA expression can modulate gene expression changes that characterize these early cancers. We conclude that miRNA dysregulation likely plays a substantial role in early breast cancer development.

Polycomb CBX7 directly controls trimethylation of histone H3 at lysine 9 at the p16 locus

BACKGROUND

H3K9 trimethylation (H3K9me3) and binding of PcG repressor complex-1 (PRC1) may play crucial roles in the epigenetic silencing of the p16 gene. However, the mechanism of the initiation of this trimethylation is unknown.

METHODOLOGY/PRINCIPAL FINDINGS

In the present study, we found that upregulating the expression of PRC1 component Cbx7 in gastric cancer cell lines MGC803 and BGC823 led to significantly suppress the expression of genes within the p16-Arf-p15 locus. H3K9me3 formation was observed at the p16 promoter and Regulatory Domain (RD). CBX7 and SUV39H2 binding to these regions were also detectable in the CBX7-stably upregulated cells. CBX7-SUV39H2 complexes were observed within nucleus in bimolecular fluorescence complementation assay (BiFC). Mutations of the chromodomain or deletion of Pc-box abolished the CBX7-binding and H3K9me3 formation, and thus partially repressed the function of CBX7. SiRNA-knockdown of Suv39h2 blocked the repressive effect of CBX7 on p16 transcription. Moreover, we found that expression of CBX7 in gastric carcinoma tissues with p16 methylation was significantly lower than that in their corresponding normal tissues, which showed a negative correlation with transcription of p16 in gastric mucosa.

CONCLUSION/SIGNIFICANCE

These results demonstrated for the first time, to our knowledge, that CBX7 could initiate H3K9me3 formation at the p16 promoter.

The loss of the CBX7 gene expression represents an adverse prognostic marker for survival of colon carcinoma patients

We have previously shown that CBX7 expression is associated with a more malignant phenotype in thyroid cancer. On this basis, we decided to investigate its possible prognostic value in colorectal cancer (CRC). CBX7 expression has been analysed by immunohistochemistry in tissue microarray (TMA) specimens obtained from a large series of sporadic CRC resections (n=1420). The CBX7 expression data have been correlated with several clinico-pathological parameters. CBX7 expression is reduced or absent in a significant number of CRC samples in comparison to the normal colonic mucosa and the loss of CBX7 expression correlates with a poor outcome of CRC (p<0.001). The block of CBX7 expression seems to occur at a transcriptional level since quantitative RT-PCR analysis showed a reduced CBX7-specific mRNA levels in CRC samples versus normal counterpart tissue (up to more than 50-fold). Finally, the restoration of CBX7 expression in two CRC cell lines reduces their proliferation rate suggesting a role of the loss of CBX7 expression in the progression step of colon carcinogenesis. Therefore, the data reported here indicate that the evaluation of CBX7 expression may represent a valid tool in the prognosis of colon cancer since a reduced survival of CRC patients is associated with the loss of CBX7 expression.

Identification of a New Pathway for Tumor Progression: MicroRNA-181b Up-Regulation and CBX7 Down-Regulation by HMGA1 Protein

High mobility group A (HMGA) overexpression plays a critical role in neoplastic transformation. To investigate whether HMGA acts by regulating the expression of microRNAs, we analyzed the microRNA expression profile of human breast adenocarcinoma cells (MCF7) transfected with the HMGA1 gene, which results in a highly malignant phenotype. Among the microRNAs induced by HMGA1, we focused on miR-181b, which was overexpressed in several malignant neoplasias including breast carcinomas. We show that miR-181b regulates CBX7 protein levels, which are down-regulated in cancer, and promotes cell cycle progression. We also demonstrate that CBX7, being negatively regulated by HMGA, is able to negatively regulate miR-181b expression. Finally, there was a direct correlation between HMGA1 and miR-181b expression and an inverse correlation between HMGA1 and CBX7 expression in human breast carcinomas. These data indicate the presence of a novel pathway involving HMGA1, miR-181b, and CBX7, which leads to breast cancer progression.

Loss of the CBX7 protein expression correlates with a more aggressive phenotype in pancreatic cancer

Polycomb group (PcG) proteins function as multiprotein complexes and are part of a gene regulatory mechanism that determines cell fate during normal and pathogenic development. Several studies have implicated the deregulation of different PcG proteins in neoplastic progression. Pancreatic ductal adenocarcinoma is an aggressive neoplasm that follows a multistep model of progression through precursor lesions called pancreatic intraepithelial neoplasia (PanIN). Aim of this study was to investigate the role of PcG protein CBX7 in pancreatic carcinogenesis and to evaluate its possible diagnostic and prognostic significance. We analysed by immunohistochemistry the expression of CBX7 in 210 ductal pancreatic adenocarcinomas from resection specimens, combined on a tissue microarray (TMA) including additional 40 PanIN cases and 40 normal controls. The results were evaluated by using receiver operating characteristic (ROC) curve analysis for the selection of cut-off scores and correlated to the clinicopathological parameters of the tumours and the outcome of the patients. Expression of E-cadherin, a protein positively regulated by CBX7, was also assessed. A significantly differential, and progressively decreasing CBX7 protein expression was found between normal pancreatic tissue, PanINs and invasive ductal adenocarcinoma. Loss of CBX7 expression was associated with increasing malignancy grade in pancreatic adenocarcinoma, whereas the maintenance of CBX7 expression showed a trend toward a longer survival. Moreover, loss of E-cadherin expression was associated with loss of CBX7 and with a trend towards worse patient survival. These results suggest that CBX7 plays a role in pancreatic carcinogenesis and that its loss of expression correlates to a more aggressive phenotype.