Aberrant methylation and silencing of ARHI,an imprinted tumor suppressor gene in which the function is lost in breast cancers

ARHI is a maternally imprinted tumor suppressor gene that maps to a site on chromosome 1p31 where loss of heterozygosity has been observed in 40% of human breast and ovarian cancers. ARHI is expressed in normal ovarian and breast epithelial cells, but ARHI expression is lost in a majority of ovarian and breast cancers. Expression of ARHI from the paternal allele can be down-regulated by multiple mechanisms in addition to loss of heterozygosity. This article explores the role of DNA methylation in silencing ARHI expression. There are three CpG islands in the ARHI gene. CpG islands I and II are located in the promoter region, whereas CpG island III is located in the coding region. Consistent with imprinting, we have found that all three CpG islands were partially methylated in normal human breast epithelial cells. Additional confirmation of imprinting has been obtained by studying DNA methylation and ARHI expression in murine A9 cells that carry either the maternal or the paternal copy of human chromosome 1. All three CpG islands were methylated, and ARHI was not expressed in A9 cells that contained the maternal allele. Conversely, CpG islands were not methylated and ARHI was expressed in A9 cells that contained the paternal allele of human chromosome 1. Aberrant methylation was found in several breast cancer cell lines that exhibited decreased ARHI expression. Hypermethylation was detected in 67% (6 of 9) of breast cancer cell lines at CpG island I, 33% (3 of 9) at CpG island II, and 56% (5 of 9) at CpG island III. Hypomethylation was observed in 44% (4 of 9) of breast cancer cell lines at CpG island II. When methylation of CpG islands was studied in 20 surgical specimens, hypermethylation was not observed in CpG island I, but 3 of 20 cases exhibited hypermethylation in CpG island II (15%), and 4 of 20 cases had hypermethylation in CpG island III (20%). Treatment with 5-aza-2'-deoxycytidine, a methyltransferase inhibitor, could reverse aberrant hypermethylation of CpG island I, II and III and partially restore ARHI expression in some, but not all of the cell lines. Treatment with 5-aza-2'-deoxycytidine partially reactivated ARHI expression in cell lines with hypermethylation of CpG islands I and II but not in cell lines with partial methylation or hypomethylation of these CpG islands. To test the impact of CpG island methylation on ARHI promoter activity more directly, constructs were prepared with the ARHI promoter linked to a luciferase reporter and transfected into SKBr3 and human embryo kidney 293 cells. Methylation of the entire construct destroyed promoter activity. Selective methylation of CpG island II alone or in combination with CpG island I also abolished ARHI promoter activity. Methylation of CpG I alone partially inhibited promoter activity of ARHI. Thus, hypermethylation of CpG island II in the promoter region of ARHI is associated with the complete loss of ARHI expression in breast cancer cells. Other epigenetic modifications such as hypermethylation in CpG island III may also contribute to the loss of ARHI expression.     

乳腺癌细胞中Syk(L)调节基因转录的生物学机制

背景与目的:全长型脾脏酪氨酸激酶[full-length form of spleen tyrosine kinase,Syk(L)]在乳腺癌中具有抑癌功能,可移位到细胞核内,作为转录抑制因子调节基因的转录.本研究拟探讨Syk(L)行使转录抑制功能的具体分子生物学机制.方法:将pFLAG-CMV-Syk(L)转染人人胚肾细胞HEK293中,采用免疫沉淀方法检测组蛋白去乙酰化酶(histone deacetylases,HDACs)与外源性Syk(L)的结合.采用免疫沉淀方法检测乳腺癌细胞MB468中内源性Syk(L)与HDACl的结合.构建带有FLAG标志的Syk(L)各个功能域质粒,并将其转染人HEK293细胞中,采用免疫沉淀方法检测HDACl与Syk(L)各个功能域之间的结合.采用HDAC活力检测系统检测Syk(L)免疫沉淀复合物中的HDAC活性.结果:HEK293细胞中外源性Syk(L)可与HDAC1、3、6、7结合,Syk(L)通过SH2功能域和KD功能域与HDAC1相互结合.乳腺癌细胞MB468中内源性Syk(L)与HDACl可相互结合.Syk(L)免疫沉淀复合物有明显的HDAC活性.结论:在乳腺癌中,抑癌基因Syk(L)可能通过与HDAC形成复合物调节基因的转录.     

Expression of the tumor suppressor gene ARHI in epithelial ovarian cancer is associated with increased expression of p21WAF1/CIP1 and prolonged progression-free survival.

PURPOSE: ARHI, an imprinted putative tumor suppressor gene, is expressed in normal ovarian epithelial cells, but its expression is down-regulated or lost in most ovarian cancer cell lines. Reexpression of ARHI in cancer cells induces p21(WAF1/CIP1), down-regulates cyclin D1 promoter activity and inhibits growth in cell culture and in heterografts. To determine the relevance of these observations to clinical cancer, we have now measured ARHI expression in normal, benign and malignant ovarian tissues using immunohistochemistry and in situ hybridization. EXPERIMENTAL DESIGN: Paraffin embedded tissues from 7 normal ovaries, 22 cystadenomas and 42 borderline lesions were analyzed using standard immunoperoxidase and in situ hybridization techniques to assess ARHI expression. In addition, immunohistochemistry against ARHI was performed on a tissue microarray containing 441 consecutive cases of ovarian carcinoma. RESULTS: Strong ARHI expression was found in normal ovarian surface epithelial cells, cysts and follicles using immunohistochemistry and in situ hybridization. Reduced ARHI expression was observed in tumors of low malignant potential as well as in invasive cancers. ARHI expression was down-regulated in 63% of invasive ovarian cancer specimens and could not be detected in 47%. When immunohistochemistry and in situ hybridization were compared, ARHI protein expression could be down-regulated in the presence of ARHI mRNA. ARHI expression was correlated with expression of p21(WAF1/CIP1) (P = 0.0074) but not with cyclin D1 and associated with prolonged disease free survival (P = 0.001). On multivariate analysis, ARHI expression, grade and stage were independent prognostic factors. ARHI expression did not correlate with overall survival. CONCLUSIONS: Persistence of ARHI expression in epithelial ovarian cancers correlated with prolonged disease free survival and expression of the cyclin dependent kinase inhibitor p21(WAF1/CIP1).     

Reexpression of the tumor suppressor gene ARHI induces apoptosis in ovarian and breast cancer cells through a caspase-independent calpain-dependent pathway

ARHI, an imprinted putative tumor suppressor gene, encodes a M(r) 26,000 GTP-binding protein that is 60% homologous to ras and rap but has a dramatically different function. ARHI expression is down-regulated in a majority of breast and ovarian cancers. Using a dual adenovirus system, we have reexpressed ARHI in ovarian cancer and breast cancer cells that have lost ARHI expression. Reexpression of ARHI inhibited growth, decreased invasiveness, and induced apoptosis. At 5 days after infection with ARHI adenovirus, 30-45% of MDA-MB-231 breast cancer cells and 5-11% of SKOv3 ovarian cancer cells were apoptotic as judged by a terminal deoxynucleotidyl transferase-mediated nick end labeling assay and by Annexin V staining with flow cytometric analysis. Although poly(ADP-ribose) polymerase could be detected immunohistochemically in the nuclei of apoptotic cells, no activation of the effector caspases (caspase 3, 6, 7, or 12) or the initiator caspases (caspase 8 or 9) could be detected in cell lysates using Western blotting. When gene expression was analyzed on a custom cDNA array that contained 2304 known genes, infection with ARHI adenovirus up-regulated 15 genes relative to control cells infected with LacZ adenovirus. The greatest degree of mRNA up-regulation was observed in a Homo sapiens calpain-like protease. On Western blot analysis, calpain protein was increased 2-3-fold at 3-5 days after infection with ARHI adenovirus. No increase in calpain protein was observed after LacZ adenovirus infection. Calpain cleavage could be detected after ARHI reexpression, and inhibitors of calpain, but not inhibitors of caspase, partially prevented ARHI-induced apoptosis. Consequently, reexpression of ARHI in breast and ovarian cancer cells appears to induce apoptosis through a caspase-independent, calpain-dependent mechanism.     

HDAC11 is a novel drug target in carcinomas

Inhibition of histone deacetylase (HDAC) activity as stand‐alone or combination therapy represents a promising therapeutic approach in oncology. The pan‐ or class I HDAC inhibitors (HDACi) currently approved or in clinical studies for oncology give rise to dose‐limiting toxicities, presumably because of the inhibition of several HDACs. This could potentially be overcome by selective blockade of single HDAC family members. Here we report that HDAC11, the most recently identified zinc‐dependent HDAC, is overexpressed in several carcinomas as compared to corresponding healthy tissues. HDAC11 depletion is sufficient to cause cell death and to inhibit metabolic activity in HCT‐116 colon, PC‐3 prostate, MCF‐7 breast and SK‐OV‐3 ovarian cancer cell lines. The antitumoral effect induced can be mimicked by enforced expression of a catalytically impaired HDAC11 variant, suggesting that inhibition of the enzymatic activity of HDAC11 by small molecules could trigger the desired phenotypic changes. HDAC11 depletion in normal cells causes no changes in metabolic activity and viability, strongly suggesting that tumor‐selective effects can be achieved. Altogether, our data show that HDAC11 plays a critical role in cancer cell survival and may represent a novel drug target in oncology.     

Effects of ARHI on breast cancer cell biological behavior regulated by microRNA-221

The aplysia ras homolog member I (ARHI) is a tumor suppressor gene and is downregulated in various cancers. The downregulation of ARHI was regulated by miR-221 in prostate cancer cell lines. However, it has not been reported whether ARHI is regulated by miR-221 in breast cancer. Here, we reported that the ARHI protein level was downregulated in breast cancer tissues and breast cancer cell lines. The overexpression of ARHI could inhibit cell proliferation and invasion and induce cell apoptosis. To address whether ARHI is regulated by miR-221 in breast cancer cell lines, the results in this study showed that a significant inverse correlation existed between ARHI and miR-221. MiR-221 displayed an upregulation in breast cancer tissues and breast cancer cell lines. The inhibition of miR-221 induced a significant upregulation of ARHI in MCF-7 cells. To prove a direct interaction between miR-221 and ARHI mRNA, ARHI 3′UTR, which includes the potential target site for miR-221, was cloned downstream of the luciferase reporter gene of the pMIR-REPORT vector to generate the pMIR-ARHI-3′UTR vector. The results confirmed a direct interaction of miR-221 with a target site on the 3′UTR of ARHI. In conclusion, ARHI is a tumor suppressor gene that is downregulated in breast cancer. The overexpression of ARHI could inhibit breast cancer cell proliferation and invasion and induce cell apoptosis. This study demonstrated for the first time that the downregulation of ARHI in breast cancer cells could be regulated by miR-221.     

抑癌基因ARHI与全身各系统肿瘤相关性及机制的研究进展

抑癌基因 ARHI（aplasia ras homologue I）是1999年新发现的母源性印迹基因。最近研究发现ARHI 具有抑制肿瘤细胞增殖、转移、侵袭,调控肿瘤细胞自噬等作用。另外,学者们还发现 ARHI 不仅与卵巢癌、乳腺癌、胰腺癌密切相关,而且还与其他生殖、消化、内分泌等全身各系统肿瘤具有相关性。本文就最近几年国内外对 ARHI 与全身各系统肿瘤的相关性及其机制的最新研究进展进行综述。     

Re-expression of ARHI (DIRAS3) induces autophagy in breast cancer cells and enhances the inhibitory effect of paclitaxel

Background  

ARHI is a Ras-related imprinted gene that inhibits cancer cell growth and motility. ARHI is downregulated in the majority of breast cancers, and loss of its expression is associated with its progression from ductal carcinoma in situ (DCIS) to invasive disease. In ovarian cancer, re-expression of ARHI induces autophagy and leads to autophagic death in cell culture; however, ARHI re-expression enables ovarian cancer cells to remain dormant when they are grown in mice as xenografts. The purpose of this study is to examine whether ARHI induces autophagy in breast cancer cells and to evaluate the effects of ARHI gene re-expression in combination with paclitaxel.