Reciprocal changes in gene expression profiles of cocultured breast epithelial cells and primary fibroblasts

The importance of epithelial‐stroma interaction in normal breast development and tumor progression has been recognized. To identify genes that were regulated by these reciprocal interactions, we cocultured a nonmalignant (MCF10A) and a breast cancer derived (MDA‐MB231) basal cell lines, with fibroblasts isolated from breast benign‐disease adjacent tissues (NAF) or with carcinoma‐associated fibroblasts (CAF), in a transwell system. Gene expression profiles of each coculture pair were compared with the correspondent monocultures, using a customized microarray. Contrariwise to large alterations in epithelial cells genomic profiles, fibroblasts were less affected. In MDA‐MB231 highly represented genes downregulated by CAF derived factors coded for proteins important for the specificity of vectorial transport between ER and golgi, possibly affecting cell polarity whereas the response of MCF10A comprised an induction of genes coding for stress responsive proteins, representing a prosurvival effect. While NAF downregulated genes encoding proteins associated to glycolipid and fatty acid biosynthesis in MDA‐MB231, potentially affecting membrane biogenesis, in MCF10A, genes critical for growth control and adhesion were altered. NAFs responded to coculture with MDA‐MB231 by a decrease in the expression of genes induced by TGFβ1 and associated to motility. However, there was little change in NAFs gene expression profile influenced by MCF10A. CAFs responded to the presence of both epithelial cells inducing genes implicated in cell proliferation. Our data indicate that interactions between breast fibroblasts and basal epithelial cells resulted in alterations in the genomic profiles of both cell types which may help to clarify some aspects of this heterotypic signaling. © 2009 UICC     

Human breast adenocarcinoma cell lines promote angiogenesis by providing cells with uPA-PAI-1 and by enhancing their expression

During angiogenesis, endothelial cells use uPA and PAI-1 to migrate and degrade the basement membrane surrounding capillary blood vessels. Invasive tumor cells produce a large amount of uPA that could bind uPAR present at the endothelial cell surface to facilitate their invasion. To verify this hypothesis, endothelial cells were incubated with conditioned medium (CM) from two breast cancer cell lines (MCF7 and MDA MB 231 cells). Within a short incubation period (30 min) with both CM, an increase of uPA, PAI-1 and uPA-PAI-1 complex was detected in endothelial cell layer as assessed by casein zymography, ELISA and uPA immunostaining. The extent of this enhancement was related to the levels of uPA secreted by tumor cells (high in MDA MB 231 cells and low in MCF7 cells). After 2 hr of incubation, the CM from both tumor cells upregulated uPA and PAI-1 mRNA levels in endothelial cells in a time-dependent manner. The uPA increase in the cell layer could not be attributable to an increase of uPAR level. Only the CM from highly invasive MDA MB 231 cells increased the angiogenic morphotype of endothelial cells assessed in a collagen gel. A single addition of amino-terminal fragment of uPA (ATF) was able to abolish the angiogenic effect induced by MDA MB 231 cell CM. Our data demonstrate that the interactions occurring between breast tumor cells and endothelial cells can modulate tumor angiogenesis at least by two mechanisms: an increase of uPA and PAI-1 cell surface-binding and of their expression by endothelial cells.     

JAM‐A expression positively correlates with poor prognosis in breast cancer patients

The cell–cell adhesion protein junctional adhesion molecule‐A (JAM‐A) influences epithelial cell morphology and migration. As migration is required for tumor cell invasion and metastasis, we sought to elucidate the role of JAM‐A in invasive breast cancer. A breast cancer tissue microarray was analyzed for JAM‐A protein expression, in parallel with analysis of JAM‐A gene expression data from a breast cancer clinical dataset. Our data demonstrate a novel association between JAM‐A gene and protein upregulation and poor prognosis in breast cancer. To mechanistically dissect this process, we used lentiviral technology to stably knock down JAM‐A gene expression by shRNA in MCF7 breast cancer cells, which express high‐endogenous levels of JAM‐A. We also antagonized JAM‐A function in wild‐type MCF7 cells using an inhibitory antibody that blocks JAM‐A dimerization. Knockdown or functional antagonism of JAM‐A decreased breast cancer cell migration in scratch‐wound assays. Reductions in β1‐integrin protein levels were observed after JAM‐A‐knockdown in MCF7 cells, suggesting a mechanism for reduced motility after loss of JAM‐A. Consistent with this hypothesis, tissue microarray analysis of β1‐integrin protein expression in invasive breast cancer tissues revealed a trend toward high β1‐integrin protein levels being indicative of poor prognosis. Twenty‐two percent of patients were observed to coexpress high levels of JAM‐A and β1‐integrin protein, and MDA‐MB‐231 breast cells stably overexpressing JAM‐A showed an increase in β1‐integrin protein expression. Our results are consistent with a previously unreported role for JAM‐A overexpression as a possible mechanism contributing to progression in primary breast cancer; and a potential therapeutic target. © 2009 UICC     

Regulation of CD44 expression and focal adhesion by Golgi phosphatidylinositol 4‐phosphate in breast cancer

CD44, a transmembrane receptor, is expressed in the standard or variant form and plays a critical role in tumor progression and metastasis. This protein regulates cell adhesion and migration in breast cancer cells. We previously reported that phosphatidylinositol‐4‐phosphate (PI(4)P) at the Golgi regulates cell migration and invasion in breast cancer cell lines. In this study, we showed that an increase in PI(4)P levels at the Golgi by knockdown of PI(4)P phosphatase SAC1 increased the expression of standard CD44, variant CD44, and ezrin/radixin phosphorylation and enhanced the formation of focal adhesions mediated by CD44 and ezrin/radixin in MCF7 and SK‐BR‐3 cells. In contrast, knockdown of PI 4‐kinase IIIβ in highly invasive MDA‐MB‐231 cells decreased these factors. These results suggest that SAC1 expression and PI(4)P at the Golgi are important in tumor progression and metastasis and are potential prognostic markers of breast cancers.     

Inhibition of human breast cancer xenograft growth by cruciferous vegetable constituent benzyl isothiocyanate

Benzyl isothiocyanate (BITC), a constituent of cruciferous vegetables such as garden cress, inhibits growth of human breast cancer cell lines in culture. The present study was undertaken to determine in vivo efficacy of BITC against MDA‐MB‐231 human breast cancer xenografts. The BITC administration retarded growth of MDA‐MB‐231 cells subcutaneously implanted in female nude mice without causing weight loss or any other side effects. The BITC‐mediated suppression of MDA‐MB‐231 xenograft growth correlated with reduced cell proliferation as revealed by immunohistochemical analysis for Ki‐67 expression. Analysis of the vasculature in the tumors from BITC‐treated mice indicated smaller vessel area compared with control tumors based on immunohistochemistry for angiogenesis marker CD31. The BITC‐mediated inhibition of angiogenesis in vivo correlated with downregulation of vascular endothelial growth factor (VEGF) receptor 2 protein levels in the tumor. Consistent with these results, BITC treatment suppressed VEGF secretion and VEGF receptor 2 protein levels in cultured MDA‐MB‐231 cells. Moreover, the BITC‐treated MDA‐MB‐231 cells exhibited reduced capacity for migration compared with vehicle‐treated control cells. In contrast to cellular data, BITC administration failed to elicit apoptotic response as judged by terminal deoxynucleotidyl transferase‐mediated dUTP nick‐end labeling assay. In conclusion, the present study demonstrates in vivo anti‐cancer efficacy of BITC against MDA‐MB‐231 xenografts in association with reduced cell proliferation and suppression of neovascularization. These preclinical observations merit clinical investigation to determine efficacy of BITC against human breast cancers. © 2010 Wiley‐Liss, Inc.     

miR‐199a‐5p regulates β1 integrin through Ets‐1 to suppress invasion in breast cancer

Increasing evidence has revealed that miR‐199a‐5p is actively involved in tumor invasion and metastasis as well as in the decline of breast cancer tissues. In this research, overexpression of miR‐199a‐5p weakened motility and invasion of breast cancer cells MCF‐7 and MDA‐MB‐231. Upregulation of Ets‐1 increased breast cancer cell invasion, but the mechanism by which miR‐199a‐5p modulates activation of Ets‐1 in breast cancer was not clarified. We investigated the relationship between miR‐199a‐5p and Ets‐1 on the basis of 158 primary breast cancer case specimens, and the results showed that Ets‐1 expression was inversely correlated with endogenous miR‐199a‐5p. Overexpression of miR‐199a‐5p reduced the mRNA and protein levels of Ets‐1 in MCF‐7 and MDA‐MB‐231 cells, whereas anti‐miR‐199a‐5p elevated Ets‐1. siRNA‐mediated Ets‐1 knockdown phenocopied the inhibition invasion of miR‐199a‐5p in vitro. Moreover, luciferase reporter assay revealed that miR‐199a‐5p directly targeted 3′‐UTR of Ets‐1 mRNA. This research revealed that miR‐199a‐5p could descend the levels of β1 integrin by targeting 3′‐UTR of Ets‐1 to alleviate the invasion of breast cancer via FAK/Src/Akt/mTOR signaling pathway. Our results provide insight into the regulation of β1 integrin through miR‐199a‐5p‐mediated Ets‐1 silence and will help in designing new therapeutic strategies to inhibit signal pathways induced by miR‐199a‐5p in breast cancer invasion.     

Breast carcinoma cells modulate the chemoattractive activity of human bone marrow‐derived mesenchymal stromal cells by interfering with CXCL12

We investigated whether breast tumor cells can modulate the function of mesenchymal stromal cells (MSCs) with a special emphasis on their chemoattractive activity towards hematopoietic stem and progenitor cells (HSPCs). Primary MSCs as well as a MSC line (SCP‐1) were cocultured with primary breast cancer cells, MCF‐7, MDA‐MB231 breast carcinoma or MCF‐10A non‐malignant breast epithelial cells or their conditioned medium. In addition, the frequency of circulating clonogenic hematopoietic progenitors was determined in 78 patients with breast cancer and compared with healthy controls. Gene expression analysis of SCP‐1 cells cultured with MCF‐7 medium revealed CXCL12 (SDF‐1) as one of the most significantly downregulated genes. Supernatant from both MCF‐7 and MDA‐MB231 reduced the CXCL12 promoter activity in SCP‐1 cells to 77% and 47%, respectively. Moreover, the CXCL12 mRNA and protein levels were significantly reduced. As functional consequence of lower CXCL12 levels, we detected a decreased trans‐well migration of HSPCs towards MSC/tumor cell cocultures or conditioned medium. The specificity of this effect was confirmed by blocking studies with the CXCR4 antagonist AMD3100. Downregulation of SP1 and increased miR‐23a levels in MSCs after contact with tumor cell medium as well as enhanced TGFβ1 expression were identified as potential molecular regulators of CXCL12 activity in MSCs. Moreover, we observed a significantly higher frequency of circulating colony‐forming hematopoietic progenitors in patients with breast cancer compared with healthy controls. Our in vitro results propose a potential new mechanism by which disseminated tumor cells in the bone marrow may interfere with hematopoiesis by modulating CXCL12 in protected niches.     

hCLP46基因在不同侵袭转移潜能乳腺癌细胞表达中的差异及其意义

目的 探讨hCLP46基因在MCF-7和MDA-MB-231乳腺癌细胞中表达的差异性及其意义。方法 采用RT-PCR的方法检测2个细胞系(乳腺癌低度侵袭转移细胞系MCF-7、中度侵袭转移细胞系MDA-MB-231)中hCLP46 mRNA的表达差异。取MCF-7和MDA-MB-231细胞株进行培养,分别加入100 μmol/L的转化生长因子-β(TGF-β)并设对照组,培养72 h,用Western-blot方法分析P15蛋白表达。结果 RT-PCR检测hCLP46结果显示,在MCF-7和MDA-MB-231细胞系中,内参基因GAPDH的表达量相近,hCLP46基因在两个细胞系中均表达,其中MDA-MB-231细胞系中的表达高于MCF-7细胞系。两个细胞系分别加入TGF-β培养72 h后,与相应的对照细胞系相比,P15的表达均有降低,hCLP46基因高表达的MDA-MB-231细胞中P15表达较MCF-7细胞显著降低。结论 hCLP46基因过表达可能抑制TGF-β对MDA-MB-231和MCF-7乳腺癌细胞P15基因的上调,hCLP46可能在乳腺癌的发病中起到一定作用。     

乳腺癌细胞系中肿瘤干细胞相关亚群的初步研究

目的 通过乳腺癌细胞系及其干细胞的培养,化疗药干预和流式细胞仪筛选鉴定,探讨不同乳腺癌细胞系中CD44⁺CD24^-/low细胞比例,及富集乳腺癌干细胞相关亚群的方法。方法 通过细胞培养乳腺癌细胞系MCF-7、MDA-MB-231,观察生长曲线,比较化疗药物干预下生长情况;利用流式细胞仪检测两种乳腺癌细胞系中CD44⁺CD24^-/low细胞比例;无血清悬浮培养,化疗药(多西紫杉醇、表阿霉素)干预这两种乳腺癌细胞系,观察其是否形成细胞球。结果 (1)MDA-MB-231细胞系倍增时间短,生长速率高于MCF-7细胞系;(2)MCF-7细胞系中可能存在较大比例肿瘤干细胞,其对化疗抵制,能自我更新;(3)化疗敏感性用两独立样本t检验,MCF-7细胞,差异没有统计学意义;MDA-MB-231细胞,差异有统计学意义,提示MDA-MB-231细胞系对该方案化疗较敏感;(4)无血清悬浮培养,MDA-MB-231细胞系未发现明显细胞球;MCF-7细胞系初次无血清培养约6天出现细胞球。加入化疗药筛选后两种细胞,见大部分肿瘤细胞逐渐死亡,未发现明显细胞球;(5)流式细胞仪检测,MCF-7、MDA-MB-231两种细胞系中主要是CD44⁺CD24⁺亚群和CD44^-CD24⁺亚群,CD44⁺CD24^-/low细胞比例分别2.07%和0.20%。结论 (1)MDA-MB-231细胞系增值较快,恶性度相对较高,其对TA联合化疗药物较敏感;MCF-7细胞系中可能存在少量肿瘤干细胞,对化疗抵制,能自我更新;(2)无血清培养液培养MCF-7细胞系能形成悬浮细胞球;流式细胞仪检测两种细胞系中CD44⁺CD24^-/low细胞比例小;(3)CD44⁺CD24^-/low表型可能不是乳腺癌干细胞唯一特异性的表面标志。     

细胞周期相关因子与乳腺细胞的癌变及生物学特性的相关性研究

目的：研究正常乳腺上皮细胞与乳腺癌细胞之间以及不同恶性程度的乳腺癌细胞之间的细胞周期相关因子的表达异同。方法：采用Western印迹法检测正常乳腺细胞株AG11132A、ER阳性和乳腺癌细胞株MCF－7及ER阴性的乳腺癌细胞株MDA－MB－231之间细胞周期蛋白D1、E及P21蛋白表达的异同及与其生物学特性的关系。结果：（1）正常乳腺上皮细胞株高表达P21蛋白,低表达周期蛋白E。与正常细胞相比,乳腺癌细胞株MCF－7、MDA－MB－231细胞高表达周期蛋白E,其中表达的周期蛋白E中存在异常的你分子量周期蛋白E成分,而正常乳腺上皮细胞不表达这种异常 的周期蛋白E。（2）在乳腺癌细胞株之间,相对ER阳性的MCF－7细胞,ER阴性的MDA－MB－231细胞则高表达周期蛋白E,基本无表达P21蛋白。结论L（1）正常细胞与这间、相对低恶性程度的民相对高恶性和蔼的癌细胞之间的差别是多环节的、质和量异常导致的结果;（2）周期蛋白E及P21均可反映乳腺癌细胞的增殖活性和恶性程度,可能是乳腺癌的临床预后指标。     

EGFRvIII‐induced estrogen‐independence,tamoxifen‐resistance phenotype correlates with PgR expression and modulation of apoptotic molecules in breast cancer

The tumor‐specific, ligand‐independent, constitutively active epidermal growth factor receptor (EGFR) variant, EGFRvIII, remains understudied in breast cancer. Here, we report that expression of EGFRvIII in the ErbB‐2‐overexpressing, estrogen‐dependent MDA‐MB‐361 breast cancer cell line resulted in significant estrogen‐independent tumor growth in ovariectomized, athymic nude mice in comparison to MDA‐MB‐361/wt cells. MDA‐MB‐361/vIII breast cancer cells maintained estrogen‐induced tumor growth, but were tamoxifen‐resistant in the presence of estrogen, while MDA‐MB‐361/wt cells had a significant reduction in tumor growth in the presence of estrogen and tamoxifen. Tamoxifen alone did not have a significant effect on EGFRvIII‐mediated estrogen‐independent tumor growth. Constitutive signaling from the EGFRvIII receptor resulted in an increased activation of both the Akt and MAPK pathways. Compared to estrogen‐dependent, tamoxifen‐sensitive MCF‐7/vIII breast cancer cells, which had unchanged levels of ERα, but an increase in progesterone receptor (PgR) in comparison to MCF‐7/wt cells, MDA‐MB‐361/vIII cells had a reduction in ERα expression as well as a more pronounced reduction in PgR compared with MDA‐MB‐361/wt cells. EGFRvIII expression was also significantly associated with an absence of PgR protein in invasive human breast cancer specimens. Alterations of proapoptotic proteins and antiapoptotic proteins were observed in EGFRvIII transfectants. In conclusion, constitutive signaling through EGFRvIII and its downstream effector proteins crosstalks with the ERα pathway, resulting in loss of PgR expression and alterations in the apoptotic pathway, which may result in the estrogen‐independent, tamoxifen‐resistant phenotype conferred to EGFRvIII‐expressing breast cancer cells. © 2009 UICC     

Knockdown of prolyl‐4‐hydroxylase domain 2 inhibits tumor growth of human breast cancer MDA‐MB‐231 cells by affecting TGF‐β1 processing

The prolyl‐4‐hydroxylase domain 1–3 (PHD1–3) enzymes are regulating the protein stability of the α‐subunit of the hypoxia‐inducible factor‐1 (HIF‐1), which mediates oxygen‐dependent gene expression. PHD2 is the main isoform regulating HIF‐1α hydroxylation and thus stability in normoxia. In human cancers, HIF‐1α is overexpressed as a result of intratumoral hypoxia which in turn promotes tumor progression. The role of PHD2 for tumor progression is in contrast far from being thoroughly understood. Therefore, we established PHD2 knockdown clones of MDA‐MB‐231 breast cancer cells and analyzed their tumor‐forming potential in a SCID mouse model. Tumor progression was significantly impaired in the PHD2 knockdown MDA‐MB‐231 cells, which could be partially rescued by re‐establishing PHD2 expression. In a RNA profile screen, we identified the secreted phosphoprotein 1 (SPP1) as one target, which is differentially regulated as a consequence of the PHD2 knockdown. Knockdown of PHD2 drastically reduced the SPP1 expression in MDA‐MB‐231 cells. A correlation of SPP1 and PHD2 expression was additionally verified in 294 invasive breast cancer biopsies. In subsequent analyses, we identified that PHD2 alters the processing of transforming growth factor (TGF)‐β1, which is highly involved in SPP1 expression. The altered processing capacity was associated with a dislocation of the pro‐protein convertase furin. Thus, our data demonstrate that in MDA‐MB‐231 cells PHD2 might affect tumor‐relevant TGF‐β1 target gene expression by altering the TGF‐β1 processing capacity.     

A novel role for flotillin‐1 in H‐Ras‐regulated breast cancer aggressiveness

Elevated expression and aberrant activation of Ras have been implicated in breast cancer aggressiveness. H‐Ras, but not N‐Ras, induces breast cell invasion. A crucial link between lipid rafts and H‐Ras function has been suggested. This study sought to identify the lipid raft protein(s) responsible for H‐Ras‐induced tumorigenicity and invasiveness of breast cancer. We conducted a comparative proteomic analysis of lipid raft proteins from invasive MCF10A human breast epithelial cells engineered to express active H‐Ras and non‐invasive cells expressing active N‐Ras. Here, we identified a lipid raft protein flotillin‐1 as an important regulator of H‐Ras activation and breast cell invasion. Flotillin‐1 was required for epidermal growth factor‐induced activation of H‐Ras, but not that of N‐Ras, in MDA‐MB‐231 triple‐negative breast cancer (TNBC) cells. Flotillin‐1 knockdown inhibited the invasiveness of MDA‐MB‐231 and Hs578T TNBC cells in vitro and in vivo. In xenograft mouse tumor models of these TNBC cell lines, we showed that flotillin‐1 played a critical role in tumor growth. Using human breast cancer samples, we provided clinical evidence for the metastatic potential of flotillin‐1. Membrane staining of flotillin‐1 was positively correlated with metastatic spread (p = 0.013) and inversely correlated with patient disease‐free survival rates (p = 0.005). Expression of flotillin‐1 was associated with H‐Ras in breast cancer, especially in TNBC (p < 0.001). Our findings provide insight into the molecular basis of Ras isoform‐specific interplay with flotillin‐1, leading to tumorigenicity and aggressiveness of breast cancer.     

MiR‐200c inhibits autophagy and enhances radiosensitivity in breast cancer cells by targeting UBQLN1

Radioresistance is a major challenge during the treatment of breast cancer. A further understanding of the mechanisms of radioresistance could provide strategies to address this challenge. In our study, we compared the expression of miR‐200c in four distinct breast cancer cell lines: two representative basal cancer cells (MDA‐MB‐231 and BT549) vs. two representative luminal cancer cells (MCF‐7 and BT474). The results revealed practically lower expression of miR‐200c in the two basal cancer cell lines and higher expression of miR‐200c in luminal cancer cells compared to the normal breast epithelial cell line MCF‐10A. Ectopic expression of miR‐200c in MDA‐MB‐231 cells inhibited irradiation‐induced autophagy and sensitized the breast cancer cells to irradiation. We also identified UBQLN1 as a direct functional target of miR‐200c involved in irradiation‐induced autophagy and radioresistance. In 35 human breast cancer tissue samples, we detected an inverse correlation between the expression of miR‐200c vs. UBQLN1 and LC3. These results indicate that the identified miR‐200c/UBQLN1‐mediated autophagy pathway may help to elucidate radioresistance in human breast cancer and might represent a therapeutic strategy.     

人乳腺癌细胞株WAF1/CIP1/p21基因的表达及其与细胞生物学特性的关系

目的研究人乳腺癌细胞株ＷＡＦ１／ＣＩＰ１基因的ＤＮＡ状况、ｍＲＮＡ和蛋白的表达水平及其意义。方法应用细胞培养、分子生物学Ｓｏｕｔｈｅｒｎｂｌｏｔ和Ｎｏｒｔｈｅｒｎｂｌｏｔ杂交以及免疫组化染色等技术，检测人乳腺癌表达野生型ｐ５３（ｗｔｐ５３）的ＭＣＦ７细胞和表达突变型ｐ５３（ｍｔｐ５３）的ＭＤＡＭＢ２３１细胞中ＷＡＦ１／ＣＩＰ１基因ＤＮＡ状况、ｍＲＮＡ和蛋白质的表达水平，研究其与ｍｄｍ２、ｐ５３蛋白的表达和细胞生物学特性的关系。结果比较ＭＣＦ７细胞与ＭＤＡＭＢ２３１细胞：（１）两者ＷＡＦ１／ＣＩＰ１基因ＤＮＡ状况无明显差异，前者ｍＲＮＡ和蛋白质的表达水平明显高于后者（Ｐ＜０．０５）；（２）两者ｐ５３蛋白的性质和分布不同，前者ｍｄｍ２蛋白的表达水平明显高于后者（Ｐ＜０．０５）；（３）前者生物学特性好于后者。结论人乳腺癌细胞株ＷＡＦ１／ＣＩＰ１基因ｍＲＮＡ和蛋白质的表达水平与ｐ５３基因表型和细胞生物学特性有关。     

Expression of MAS1 in breast cancer

MAS1 is a receptor for angiotensin 1‐7 (A1‐7), which is derived from angiotensin II (A‐II) by the action of angiotensin converting enzyme (ACE) 2. MAS1 induces anti‐A‐II phenotypes, such as vessel dilation and depression of blood pressure. Using immunohistochemistry, we examined the role of MAS1 in 132 cases of invasive ductal carcinoma (IDC) of the breast. While benign mammary tissues expressed MAS1 at high levels, MAS1 expression was attenuated in all IDC, especially in scirrhous IDC. The decrease in MAS1 expression was associated with tumor growth, lymph node metastasis, and grade. MAS1 expression was inversely associated with the proliferation index and epidermal growth factor receptor and human epidermal growth factor receptor‐2 expression. Of the 132 cases, 12 (9.1%) were triple‐negative breast cancer (TNBC) cases. All TNBC cases (the 12 cases and the additional 36 cases using a tissue array) expressed MAS1. Using the TNBC cell lines 4T1 and MDA‐MB‐468, which expresses MAS1, we found that cell growth, anti‐apoptotic survival and invasion were suppressed by MAS1 activation with A1‐7 treatment and enhanced by MAS1 knockdown. In contrast, synergic effect was found between tamoxifen and A1‐7 in a luminal A breast cancer cell line, MCF‐7. Combination treatment with cisplatin, an ACE2 activator, and an A‐II type 1 receptor blocker showed synergic effects on tumor growth inhibition of 4T1 tumors in a syngeneic mouse model. These findings suggest that MAS1 might act as an inhibitory regulator of breast cancer and may be a possible molecular target for this malignancy.