Integrin function in breast carcinoma progression

Fertility-Associated Polymorphism within Bovine ITGβ5 and Its Significant Correlations with Ovarian and Luteal Traits

Simple Summary

The fertility of bovines is essential for cattle husbandry. ITGβ5, which is suggested to be closely related to fertility, is known to mediate cell adhesion and affect a variety of cellular activities. To investigate the relationship between the ITGβ5 gene and the fertility of bovines, 696 ovarian samples were collected and six potential indel (insertion/deletion) within ITGβ5 were analysed, from which a deletion mutation was found to be polymorphic. The genotype frequency and allele frequency of this locus in the investigated population were calculated and the population genetic parameters analyzed. In addition, this locus was found to be significantly correlated with ovarian width and corpus luteum diameter. Considering the importance of ovary and corpus luteum in reproduction, it is tempting to speculate the crucial effects of ITGβ5 on bovine fertility, which still need further validation. The results of our study might provide a theoretical basis for future breeding to enhance bovine reproduction.

Abstract

There is an urgent need to improve bovine fertility, and molecular marker-assisted selection (MAS) can accelerate this process. Genome-wide association studies suggest that Integrin β5 (ITGβ5) might affect fertility in bovines. As a member of the integrins family, ITGβ5 can bind to the extracellular matrix and mediate various cellular processes. In our study, primers spanning six potential insertion/deletion (indel) polymorphisms within the ITGβ5 gene were designed and 696 ovary samples from different individuals, the vast majority not in oestrum were collected for genetic variation detection. A deletion locus, rs522759246, namely P1-D_13-bp, was found to be polymorphic. The allele D frequency was 0.152 and the polymorphism information content (PIC) value was 0.224, indicating a low-degree PIC. This locus did not follow the Hardy–Weinberg equilibrium (p = 1.200E-23). Importantly, associations between P1-D_13-bp and ovarian morphological traits were established. Polymorphisms of this locus had significant correlations with ovarian width (p = 0.015). The corpus luteum is also linked to fertility and P1-D_13-bp was significantly correlated with corpus luteum diameter (p = 0.005). In conclusion, an indel mutation within the bovine ITGβ5 gene was identified, which was significantly associated with several ovarian and luteal traits.

Novel Long Noncoding RNA 005620 Induces Epirubicin Resistance in Triple-Negative Breast Cancer by Regulating ITGB1 Expression

Triple-negative breast cancer (TNBC) is often treated with anthracyclines (e.g., epirubicin or doxorubicin), but very little is known about anthracycline resistance, especially epirubicin resistance in TNBC. To identify novel long noncoding RNAs (lncRNAs) involved in epirubicin resistance in TNBC, we established a new TNBC MDA-MB-231 cell line that was resistant to epirubicin (Epi-R). A total of 12 differentially expressed lncRNAs were identified using RNA sequencing analysis of Epi-R cells. Among these lncRNAs, we found a novel intronic lncRNA, lnc005620, was highly expressed in Epi-R cells and human TNBC tissues. Further gain- and loss-of-function studies demonstrated that lnc005620 played an oncogenic role and partially abrogated the effects of epirubicin on TNBC cells. Using iTRAQ proteomics analysis, we found that three members of the integrin family, integrin β4, integrin β1 and integrin α6, were all upregulated in Epi-R MDA-MB-231 cells. Integrin β1, encoded by the ITGB1 gene, was validated to be a downstream target of lnc005620 in Epi-R MDA-MB-231 cells. Our study demonstrates that novel lnc005620 promotes TNBC progression and chemoresistance to epirubicin via integrin β1 both in vitro and in vivo and provides a promising therapeutic target for TNBC patients in terms of enhancing the benefits of epirubicin treatment.

Integrin-mediated adhesion and mechanosensing in the mammary gland

The mammary gland is dynamically remodelled during its postnatal development and the reproductive cycles. This inherent plasticity has been suggested to increase the susceptibility of the organ to carcinogenesis. Morphological changes in the mammary epithelium involve cell proliferation, differentiation, apoptosis, and migration which, in turn, are affected by cell adhesion to the extracellular matrix (ECM). Integrin adhesion receptors function in the sensing of the biochemical composition, patterning and mechanical properties of the ECM surrounding the cells, and strongly influence cell fate. This review aims to summarize the existing literature on how different aspects of integrin-mediated adhesion and mechanosensing, including ECM composition; stiffness and topography; integrin expression patterns; focal adhesion assembly; dynamic regulation of the actin cytoskeleton; and nuclear mechanotransduction affect mammary gland development, function and homeostasis. As the mechanical properties of a complex tissue environment are challenging to replicate in vitro, emphasis has been placed on studies conducted in vivo or using organoid models. Outright, these studies indicate that mechanosensing also contributes to the regulation of mammary gland morphogenesis in multiple ways.

Carbonic Anhydrase IX (CAIX), Cancer, and Radiation Responsiveness

Carbonic anhydrase IX has been under intensive investigation as a therapeutic target in cancer. Studies demonstrate that this enzyme has a key role in pH regulation in cancer cells, allowing these cells to adapt to the adverse conditions of the tumour microenviroment. Novel CAIX inhibitors have shown efficacy in both in vitro and in vivo pre-clinical cancer models, adversely affecting cell viability, tumour formation, migration, invasion, and metastatic growth when used alone. In co-treatments, CAIX inhibitors may enhance the effects of anti-angiogenic drugs or chemotherapy agents. Research suggests that these inhibitors may also increase the response of tumours to radiotherapy. Although many of the anti-tumour effects of CAIX inhibition may be dependent on its role in pH regulation, recent work has shown that CAIX interacts with several of the signalling pathways involved in the cellular response to radiation, suggesting that pH-independent mechanisms may also be an important basis of its role in tumour progression. Here, we discuss these pH-independent interactions in the context of the ability of CAIX to modulate the responsiveness of cancer to radiation.

Penfluridol: An Antipsychotic Agent Suppresses Metastatic Tumor Growth in Triple-Negative Breast Cancer by Inhibiting Integrin Signaling Axis

Metastasis of breast cancer, especially to the brain, is the major cause of mortality. The inability of anticancer agents to cross the blood-brain-barrier represents a critical challenge for successful treatment. In the current study, we investigated the antimetastatic potential of penfluridol, an antipsychotic drug frequently prescribed for schizophrenia with anticancer activity. We show that penfluridol induced apoptosis and reduced the survival of several metastatic triple-negative breast cancer (TNBC) cell lines. In addition, penfluridol treatment significantly reduced the expression of integrin α6, integrin β4, Fak, paxillin, Rac1/2/3, and ROCK1 in vitro. We further evaluated the efficacy of penfluridol in three different in vivo tumor models. We demonstrate that penfluridol administration to an orthotopic model of breast cancer suppressed tumor growth by 49%. On the other hand, penfluridol treatment inhibited the growth of metastatic brain tumors introduced by intracardiac or intracranial injection of breast cancer cells by 90% and 72%, respectively. Penfluridol-treated tumors from all three models exhibited reduced integrin β4 and increased apoptosis. Moreover, chronic administration of penfluridol failed to elicit significant toxic or behavioral side effects in mice. Taken together, our results indicate that penfluridol effectively reduces the growth of primary TNBC tumors and especially metastatic growth in the brain by inhibiting integrin signaling, and prompt further preclinical investigation into repurposing penfluridol for the treatment of metastatic TNBC.

Up-regulation of integrin α6β4 expression by mitogens involved in dairy cow mammary development

In dairy cows, the extracellular microenvironment varies significantly from the virgin state to lactation. The function of integrin α6β4 is dependent on cell type and extracellular microenvironment, and the precise expression profile of α6β4 and its effects on mammary development remain to be determined. In the present study, real-time PCR and immunohistochemistry were used to analyze the expression and localization of integrin α6β4 in Holstein dairy cow mammary glands. The effects of integrin α6β4 on the proliferation induced by mammogenic mitogens were identified by blocking integrin function in purified dairy cow mammary epithelial cells (DCMECs). The results showed that the localization of β4 subunit and its exclusive partner the α6 subunit were not consistent but were co-localized in basal luminal cells and myoepithelial cells, appearing to prefer the basal surface of the plasma membrane. Moreover, α6 and β4 subunit messenger RNA (mRNA) levels changed throughout the stages of dairy cow mammary development, reflected well by protein levels, and remained higher in the virgin and pregnancy states, with duct/alveolus morphogenesis and active cell proliferation, than during lactation, when growth arrest is essential for mammary epithelial cell differentiation. Finally, the upregulation of integrin expression by both mammogenic growth hormone and insulin-like growth factor-1 and the inhibited growth of DCMECs by function-blocking integrin antibodies confirmed that integrin α6β4 was indeed involved in dairy cow mammary development.

CDK11p58 inhibits ERα-positive breast cancer invasion by targeting integrin β3 via the repression of ERα signaling

Background

CDK11^p58, a Ser/Thr kinase that belongs to the cell division cycle 2-like 1 (CDC2L1) subfamily, is associated with cell cycle progression, tumorigenesis and apoptotic signaling. CDK11^p58 is also involved in the regulation of steroid receptors, such as androgen and estrogen receptors. We previously found that CDK11^p58 was abnormally expressed in prostate cancer. However, its role in breast cancer remains unclear.

Methods

CDK11^p58 expression was evaluated by immunohistochemical staining in a tissue array. A Transwell assay was used to detect invasion and metastasis in breast cancer cells. The TaqMan® Metastasis Gene Expression Assay was used to search for potential downstream factors in the CDK11^p58 signaling pathway. qRT-PCR was used to evaluate mRNA levels, and the dual luciferase array was used to analyze promoter activity. Western blotting was used to detect the protein level.

Results

CDK11^p58 expression was negatively correlated with node status (P = 0.012), relapse status (P = 0.002) and metastasis status (P = 0.023). Kaplan-Meier survival curves indicated that the disease-free survival (DFS) was significantly poor in breast cancer patients with low CDK11 expression. Interestingly, using the breast cancer cell lines ZR-75-30 and MDA-MB-231, we found that CDK11^p58 was capable of repressing the migration and invasion of ERα-positive breast cancer cells, but not ERα-negative breast cancer cells, in a kinase-dependent manner. Gene expression assays demonstrated that integrin β3 mRNA was dramatically repressed by CDK11^p58, and luciferase results confirmed that the integrin β3 promoter was inhibited by CDK11^p58 through ERα repression. The expression of integrin β3 was highly related to ERα signaling; ERα overexpression stimulated integrin β3 expression, whereas siRNA-mediated knockdown of ERα attenuated integrin β3 expression.

Conclusions

These data indicate that CDK11^p58 is an anti-metastatic gene in ERα-positive breast cancer and that the regulation of integrin β3 by CDK11^p58 via the repression of ERα signaling may constitute part of a signaling pathway underlying breast cancer invasion.

NF-κB regulates radioresistance mediated by β1-integrin in three-dimensional culture of breast cancer cells

β1-integrin induction enhances breast cancer cell survival after exposure to ionizing radiation (IR), but the mechanisms of this effect remain unclear. Although NF-κB initiates prosurvival signaling pathways post-IR, the molecular function of NF-κB with other key elements in radioresistance, particularly with respect to extracellular matrix-induced signaling, is not known. We discovered a typical NF-κB-binding site in the β1-integrin promoter region, indicating a possible regulatory role for NF-κB. Using three-dimensional laminin-rich extracellular matrix (3D lrECM) culture, we show that NF-κB is required for β1-integrin transactivation in T4-2 breast cancer cells post-IR. Inhibition of NF-κB reduced clonogenic survival and induced apoptosis and cytostasis in formed tumor colonies. In addition, T4-2 tumors with inhibition of NF-κB activity exhibit decreased growth in athymic mice, which was further reduced by IR with downregulated β1-integrin expression. Direct interactions between β1-integrin and NF-κB p65 were induced in nonmalignant breast epithelial cells, but not in malignant cells, indicating context-specific regulation. As β1-integrin also activates NF-κB, our findings reveal a novel forward feedback pathway that could be targeted to enhance therapy.

Identification of α2β1 integrin inhibitor VP-i with anti-platelet properties in the venom of Vipera palaestinae

Integrins are receptors of the extracellular matrix (ECM), playing a vital role in pathophysiological processes. They bind to ECM ligands like collagens and can mediate wound healing as well as tumor metastasis and thrombosis, thus being a part of cell adhesion and migration as well as platelet aggregation. For this reason, identifying α2β1 integrin-specific antagonists can assist in the development of drugs to treat tumor progression, angiogenesis, and cardiovascular diseases. Snake venoms have been shown to contain antagonists which target collagen-binding integrins. EMS16, rhodocetin, and VP12 are three toxins belonging to the C-type lectin-related protein family and have been proven to inhibit the α2β1 integrin, specifically the α2 integrin A domain. To specifically isolate antagonists targeting the α2β1 integrin A domain, we developed a protocol based on affinity chromatography. Using this novel approach, the toxin VP-i was isolated from Vipera palaestinae venom. We show that VP-i binds to the α2 integrin A domain and that it successfully inhibits adhesion of various cells to type I collagen as well as cell migration. Moreover, our results indicate that VP-i differs structurally from the previously purified VP12, although not functionally, and therefore is a further venom compound which can be utilized for drug development.

Association of polymorphisms in microRNA-binding sites and colorectal cancer in an Iranian population

MicroRNAs (miRNAs) are agents of post-transcriptional gene expression, and they can affect many functions of an individual cell or tissue from extracellular matrix production to inflammatory processes and tumor development. We aimed to determine the possible role of miRNA-binding site polymorphisms located in five cancer-related genes: IL-16, CDKN2A (p16), RAF1, PTGER4, and ITGB4 in colorectal cancer (CRC) risk modification in an Iranian population. This study was performed on 643 individuals (249 CRC cases and 394 healthy controls). We selected five cancer-related genes (IL-16, CDKN2A (p16), RAF1, PTGER4, and ITGB4) and investigated the genotypes of the 3' untranslated region miRNA-binding site polymorphisms in these genes in our study population. The restriction fragment length polymorphism results were confirmed by a direct sequencing method. We found a statistically significant difference between the rs1131445 polymorphism of the IL-16 gene and CRC. The frequencies of the genotypes TT, CT, and CC in controls were 51%, 40.4%, and 8.6%, respectively, and in cases were 41.4%, 44.1%, and 14.5%, respectively, which shows a significant association between the CC genotype of the rs1131445 polymorphism and CRC (P = 0.004). The frequency of the C allele in the CRC group was higher than in the controls, and the C allele of the rs1131445 polymorphism was found to be in association with CRC (P = 0.009). These associations remained significant after Bonferroni's correction for multiple testing. We found that the AA genotype of the rs743554 polymorphism in the ITGB4 gene and the T allele of the rs1051208 polymorphism of the RAF1 gene were associated with the risk of CRC in females; however, after Bonferroni's correction we found that they were non-significant. Finally, we can conclude that a significant relationship exists between the miRNA-binding site polymorphism of the IL-16 gene and CRC risk in the Iranian population.

Immunoexpression of α2β1, α3β1, and α5β1 integrins in pleomorphic adenoma and adenoid cystic carcinoma

The aim of the present study was to compare the expression of α2β1, α3β1, and α5β1 integrins between 28 pleomorphic adenomas (PAs) and 10 adenoid cystic carcinomas (ACCs), and investigate differences in the expression of these integrins according to histologic subtypes of ACCs. It was taken into consideration the presence or absence, distribution, and localization of integrin immunoexpression. There was immunoreactivity in the intercellular contacts of the strands, nests, and solid sheets of PAs, as well as in the luminal and nonluminal cells of the duct-like structures, with a predominant immunoexpression in the luminal cells. The immunoexpression in ACCs varied with histologic subtype of the tumor. It was verified for a tendency of absence and/or reduced expression of all integrins in the solid subtype of ACCs. In general, PAs revealed a more diffuse and remarkable immunoexpression of all studied integrins than ACCs. The reduced integrins expression in ACC may be related to a lesser degree of cell differentiation in this neoplasm. Moreover, the absence and/or reduced expression of the studied integrins in solid ACC suggest a possible role in pathogenesis and more aggressive biological behavior of this histologic subtype.

Integrin signaling in mammary epithelial cells and breast cancer

Cells sense and respond to the extracellular matrix (ECM) by way of integrin receptors, which facilitate cell adhesion and intracellular signaling. Advances in understanding the mammary epithelial cell hierarchy are converging with new developments that reveal how integrins regulate the normal mammary gland. But in breast cancer, integrin signaling contributes to the development and progression of tumors. This paper highlights recent studies which examine the role of integrin signaling in mammary epithelial cells and their malignant counterparts.

β1-integrins signaling and mammary tumor progression in transgenic mouse models: implications for human breast cancer

Consistent with their essential role in cell adhesion to the extracellular matrix, integrins and their associated signaling pathways have been shown to be involved in cell proliferation, migration, invasion and survival, processes required in both tumorigenesis and metastasis. β1-integrins represent the predominantly expressed integrins in mammary epithelial cells and have been proven crucial for mammary gland development and differentiation. Here we provide an overview of the studies that have used transgenic mouse models of mammary tumorigenesis to establish β1-integrin as a critical mediator of breast cancer progression and thereby as a potential therapeutic target for the development of new anticancer strategies.

β1 integrin mediates an alternative survival pathway in breast cancer cells resistant to lapatinib

INTRODUCTION

The overexpression of human epidermal growth factor receptor (HER)-2 in 20% of human breast cancers and its association with aggressive growth has led to widespread use of HER2-targeted therapies, such as trastuzumab (T) and lapatinib (L). Despite the success of these drugs, their efficacy is limited in patients whose tumors demonstrate de novo or acquired resistance to treatment. The β1 integrin resides on the membrane of the breast cancer cell, activating several elements of breast tumor progression including proliferation and survival.

METHODS

We developed a panel of HER2-overexpressing cell lines resistant to L, T, and the potent LT combination through long-term exposure and validated these models in 3D culture. Parental and L/T/LT-resistant cells were subject to HER2 and β1 integrin inhibitors in 3D and monitored for 12 days, followed by quantification of colony number. Parallel experiments were conducted where cells were either stained for Ki-67 and Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) or harvested for protein and analyzed by immunoblot. Results were subjected to statistical testing using analysis of variance and linear contrasts, followed by adjustment with the Sidak method.

RESULTS

Using multiple cell lines including BT474 and HCC1954, we reveal that in L and LT resistance, where phosphorylation of EGFR/HER1, HER2, and HER3 are strongly inhibited, kinases downstream of β1 integrin--including focal adhesion kinase (FAK) and Src--are up-regulated. Blockade of β1 by the antibody AIIB2 abrogates this up-regulation and functionally achieves significant growth inhibition of L and LT resistant cells in 3D, without dramatically affecting the parental cells. SiRNA against β1 as well as pharmacologic inhibition of FAK achieve the same growth inhibitory effect. In contrast, trastuzumab-resistant cells, which retain high levels of phosphorylated EGFR/HER1, HER2, and HER3, are only modestly growth-inhibited by AIIB2.

CONCLUSIONS

Our data suggest that HER2 activity, which is suppressed in resistance involving L but not T alone, dictates whether β1 mediates an alternative pathway driving resistance. Our findings justify clinical studies investigating the inhibition of β1 or its downstream signaling moieties as strategies to overcome acquired L and LT resistance.

Crosstalk between integrin and receptor tyrosine kinase signaling in breast carcinoma progression

This review explored the mechanism of breast carcinoma progression by focusing on integrins and receptor tyrosine kinases (or growth factor receptors). While the primary role of integrins was previously thought to be solely as mediators of adhesive interactions between cells and extracellular matrices, it is now believed that integrins also regulate signaling pathways that control cancer cell growth, survival, and invasion. A large body of evidence suggests that the cooperation between integrin and receptor tyrosine kinase signaling regulates certain signaling functions that are important for cancer progression. Recent developments on the crosstalk between integrins and receptor tyrosine kinases, and its implication in mammary tumor progression, are discussed.

Celastrol acts as a potent antimetastatic agent targeting beta1 integrin and inhibiting cell-extracellular matrix adhesion, in part via the p38 mitogen-activated protein kinase pathway

Malignant tumors remain a significant health threat, with death often occurring as a result of metastasis. Cell adhesion is a crucial step in the metastatic cascade of tumor cells, and interruption of this step is considered to be a logical strategy for prevention and treatment of tumor metastasis. Celastrol [3-hydroxy-24-nor-2-oxo-1(10),3,5,7-friedelatetraen-29-oic acid], a quinone methide triterpene from the medicinal plant Tripterygium wilfordii, possesses antitumor activities, whereas the underlying mechanism(s) remains elusive. Here, we found that celastrol inhibited cell-extracellular matrix (ECM) adhesion of human lung cancer 95-D and mouse melanoma B16F10 cells. This inhibition was achieved through suppressing beta1 integrin ligand affinity and focal adhesion formation, accompanied by the reduced phosphorylation of focal adhesion kinase (FAK). In understanding the underlying mechanisms, we found that celastrol activated p38 mitogen-activated protein kinase (MAPK) by phosphorylation before the decrement of phosphorylated FAK and that this action was independent of the presence of fibronectin. Using 4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)1H-imidazole (SB203580), a specific inhibitor of p38 MAPK, the effects of celastrol on beta1 integrin function, cell-ECM adhesion, and phosphorylation of FAK were partially attenuated. In addition, focal adhesion-dependent cell migration and invasion were both inhibited by treatment with celastrol. Finally, the antimetastatic activity of celastrol was examined in vivo using the B16F10-green fluorescent protein-injected C57BL/6 mouse model, as indicated by decreased pulmonary metastases in celastrol-administrated mice. Taken together, these data demonstrate for the first time that celastrol exerts potent antimetastatic activity both in vitro and in vivo, and they provide new evidence for the critical roles of p38 MAPK in the regulation of integrin function and cell adhesion.

Expression of estrogen receptor beta increases integrin alpha1 and integrin beta1 levels and enhances adhesion of breast cancer cells

Estrogen effects on mammary gland development and differentiation are mediated by two receptors (ERalpha and ERbeta). Estrogen-bound ERalpha induces proliferation of mammary epithelial and cancer cells, while ERbeta is important for maintenance of the differentiated epithelium and inhibits proliferation in different cell systems. In addition, the normal breast contains higher ERbeta levels compared to the early stage breast cancers, suggesting that loss of ERbeta could be important in cancer development. Analysis of ERbeta-/- mice has consistently revealed reduced expression of cell adhesion proteins. As such, ERbeta is a candidate modulator of epithelial homeostasis and metastasis. Consequently, the aim of this study was to analyze estrogenic effects on adhesion of breast cancer cells expressing ERalpha and ERbeta. As ERbeta is widely found in breast cancer but not in cell lines, we used ERalpha positive T47-D and MCF-7 human breast cancer cells to generate cells with inducible ERbeta expression. Furthermore, the colon cancer cell lines SW480 and HT-29 were also used. Integrin alpha1 mRNA and protein levels increased following ERbeta expression. Integrin beta1-the unique partner for integrin alpha1-increased only at the protein level. ERbeta expression enhanced the formation of vinculin containing focal complexes and actin filaments, indicating a more adhesive potential. This was confirmed by adhesion assays where ERbeta increased adhesion to different extracellular matrix proteins, mostly laminin. In addition, ERbeta expression was associated to less cell migration. These results indicate that ERbeta affects integrin expression and clustering and consequently modulates adhesion and migration of breast cancer cells.

Factors necessary to produce basoapical polarity in human glandular epithelium formed in conventional and high-throughput three-dimensional culture: example of the breast epithelium

Background

Basoapical polarity in epithelia is critical for proper tissue function, and control of proliferation and survival. Cell culture models that recapitulate epithelial tissue architecture are invaluable to unravel developmental and disease mechanisms. Although factors important for the establishment of basal polarity have been identified, requirements for the formation of apical polarity in three-dimensional tissue structures have not been thoroughly investigated.

Results

We demonstrate that the human mammary epithelial cell line-3522 S1, provides a resilient model for studying the formation of basoapical polarity in glandular structures. Testing three-dimensional culture systems that differ in composition and origin of substrata reveals that apical polarity is more sensitive to culture conditions than basal polarity. Using a new high-throughput culture method that produces basoapical polarity in glandular structures without a gel coat, we show that basal polarity-mediated signaling and collagen IV are both necessary for the development of apical polarity.

Conclusion

These results provide new insights into the role of the basement membrane, and especially collagen IV, in the development of the apical pole, a critical element of the architecture of glandular epithelia. Also, the high-throughput culture method developed in this study should open new avenues for high-content screening of agents that act on mammary tissue homeostasis and thus, on architectural changes involved in cancer development.

The Leu33Pro polymorphism in the ITGB3 gene does not modify BRCA1/2-associated breast or ovarian cancer risks: results from a multicenter study among 15,542 BRCA1 and BRCA2 mutation carriers

Integrins containing the beta(3) subunit are key players in tumor growth and metastasis. A functional Leu33Pro polymorphism (rs5918) in the beta(3) subunit of the integrin gene (ITGB3) has previously been suggested to act as a modifier of ovarian cancer risk in Polish BRCA1 mutation carriers. To investigate the association further, we genotyped 9,998 BRCA1 and 5,544 BRCA2 mutation carriers from 34 studies from the Consortium of Investigators of Modifiers of BRCA1/2 for the ITGB3 Leu33Pro polymorphism. Data were analysed within a Cox-proportional hazards framework using a retrospective likelihood approach. There was marginal evidence that the ITGB3 polymorphism was associated with an increased risk of ovarian cancer for BRCA1 mutation carriers (per-allele Hazard Ratio (HR) 1.11, 95% CI 1.00-1.23, p-trend 0.05). However, when the original Polish study was excluded from the analysis, the polymorphism was no longer significantly associated with ovarian cancer risk (HR 1.07, 95% CI 0.96-1.19, p-trend 0.25). There was no evidence of an association with ovarian cancer risk for BRCA2 mutation carriers (HR 1.09, 95% CI 0.89-1.32). The polymorphism was not associated with breast cancer risk for either BRCA1 or BRCA2 mutation carriers. The ITGB3 Leu33Pro polymorphism does not modify breast or ovarian cancer risk in BRCA1 or BRCA2 mutation carriers.

Alpha6beta4 integrin crosslinking induces EGFR clustering and promotes EGF-mediated Rho activation in breast cancer

Background

The α6β4 integrin is overexpressed in the basal subtype of breast cancer and plays an important role in tumor cell motility and invasion. EGFR is also overexpressed in the basal subtype of breast cancer, and crosstalk between α6β4 integrin and EGFR appears to be important in tumor progression.

Methods

We evaluated the effects of α6β4 crosslinking on the distribution and function of EGFR in breast carcinoma cell line MDA-MB-231. Receptor distribution was evaluated by fluorescence microscopy and multispectral imaging flow cytometry, and ligand-mediated EGFR signaling was evaluated using Western blots and a Rho pull-down assay.

Results

Antibody-mediated crosslinking of α6β4 integrin was sufficient to induce cell-surface clustering of not only α6β4 but also EGFR in nonadherent cells. The induced clustering of EGFR was observed minimally after 5 min of integrin crosslinking but was more prominent after 15 min. EGFR clustering had minimal effect on the phosphorylation of Akt or Erk1,2 in response to EGF in suspended cells or in response to HB-EGF in adherent cells. However, EGFR clustering induced by crosslinking α6β4 had a marked effect on Rho activation in response to EGF.

Conclusion

Crosslinking α6β4 integrin in breast carcinoma cells induces EGFR clustering and preferentially promotes Rho activation in response to EGF. We hypothesize that this integrin-EGFR crosstalk may facilitate tumor cell cytoskeletal rearrangements important for tumor progression.

Dual FGF-2 and intergrin alpha5beta1 signaling mediate GRAF-induced RhoA inactivation in a model of breast cancer dormancy

Interactions with the bone marrow stroma regulate dormancy and survival of breast cancer micrometastases. In an in vitro model of dormancy in the bone marrow, we previously demonstrated that estrogen-dependent breast cancer cells are partially re-differentiated by FGF-2, re-express integrin α5β1 lost with malignant transformation and acquire an activated PI3K/Akt pathway. Ligation of integrin α5β1 by fibronectin and activation of the PI3K pathway both contribute to survival of these dormant cells. Here, we investigated mechanisms responsible for the dormant phenotype. Experiments demonstrate that integrin α5β1 controls de novo cytoskeletal rearrangements, cell spreading, focal adhesion kinase rearrangement to the cell perimeter and recruitment of a RhoA GAP known as GRAF. This results in the inactivation of RhoA, an effect which is necessary for the stabilization of cortical actin. Experiments also demonstrate that activation of the PI3K pathway by FGF-2 is independent of integrin α5β1 and is also required for cortical actin reorganization, GRAF membrane relocalization and RhoA inactivation. These data suggest that GRAF-mediated RhoA inactivation and consequent phenotypic changes of dormancy depend on dual signaling by FGF-2-initiated PI3K activation and through ligation of integrin α5β1 by fibronectin.

Polymorphisms in predicted microRNA-binding sites in integrin genes and breast cancer: ITGB4 as prognostic marker

Integrins control the cell attachment to the extracellular matrix and play an important role in mediating cell proliferation, migration and survival. A number of important cancer-associated integrin genes can be regulated by microRNAs (miRNAs) that bind to their target sites in the 3' untranslated regions. We examined the effect of single-nucleotide polymorphisms (SNPs) in predicted miRNA target sites of six integrin genes (ITGA3, ITGA6, ITGAv, ITGB3, ITGB4 and ITGB5) on breast cancer (BC) risk and clinical outcome. Six SNPs were genotyped in 749 Swedish incident BC cases with detailed clinical data and up to 15 years of follow-up together with 1493 matched controls. We evaluated associations between genotypes and BC risk and clinical tumour characteristics. Survival probabilities were compared between different subgroups. As a novel finding, several SNPs seemed to associate with the hormone receptor status. The strongest association was observed between the A allele of the SNP rs743554 in the ITGB4 gene and oestrogen receptor-negative tumours [odds ratio 2.09, 95% confidence intervals (CIs) 1.19-3.67]. The same SNP was associated with survival. The A allele carriers had a worse survival compared with the wild-type genotype carriers (hazard ratio 2.11, 95% CIs 1.21-3.68). The poor survival was significantly associated with the aggressive tumour characteristics: high grade, lymph node metastasis and high stage. None of the SNPs was significantly associated with BC risk. As the ITGB4 SNP seems to influence tumour aggressiveness and survival, it may have prognostic value in the clinic.

The emerging picture of the mouse mammary stem cell

The isolation and characterisation of mammary stem cells is an important step towards elucidating the hierarchy of epithelial cell development in the mammary gland and identifying cells that are targets of breast carcinogenesis. Mammary stem cells have recently been prospectively isolated through the identification of specific cell surface markers and in vivo transplantation into cleared fat pads. These cells were demonstrated to reconstitute an entire mammary gland comprising all mature epithelial cell types and to be capable of self-renewal on serial transplantation, thus possessing the defining features of stem cells. Notably, mouse mammary stem cells were found to share the hallmark properties of the basal subtype of breast cancer. This review will summarize the strategy used in the identification of mouse mammary stem cells and their characterisation.

Evidence for a role of tumor-derived laminin-511 in the metastatic progression of breast cancer

Most studies investigating laminins (LMs) in breast cancer have focused on LM-111 or LM-332. Little is known, however, about the expression and function of alpha5 chain-containing LM-511/521 during metastatic progression. Expression of LM-511/521 subunits was examined in genetically related breast tumor lines and corresponding primary tumors and metastases in a syngeneic mouse model using real-time quantitative polymerase chain reaction, in situ hybridization, and immunohistochemistry. The results from our investigation indicate that LM-511 rather than LM-111, -332, or -521 correlates with metastatic potential in mouse mammary tumors. LM-511 was a potent adhesive substrate for both murine and human breast carcinoma cells and promoted strong haptotactic responses in metastatic lines. Haptotaxis was mediated by alpha3 integrin in both MCF-7 and MDA-MB-231 cells and was strongly inhibited by blocking antibodies against this integrin subunit. However, whereas nonmetastatic MCF-7 cells migrated toward LM-511 primarily via alpha3beta1 integrin, results from antibody perturbation experiments and flow cytometry analysis suggest that this response is mediated by an as yet unidentified alpha3beta integrin heterodimer (other than alpha3beta1) in MDA-MB-231 cells. These results are consistent with earlier reports implicating alpha3 integrins in breast cancer progression and support the role of LM-511 as a functional substrate regulating breast cancer metastasis.

PTH-related protein upregulates integrin alpha6beta4 expression and activates Akt in breast cancer cells

Breast cancer is the most common carcinoma that metastasizes to bone. Tumor-produced parathyroid hormone-related protein (PTHrP), a known stimulator of osteoclastic bone resorption, is a major mediator of the osteolytic process in breast cancer. We have previously shown that PTHrP increases breast cancer cell proliferation, survival, migration, and pro-invasive integrin alpha6beta4 expression. To determine the role of integrin alpha6beta4 in these PTHrP-mediated effects, we utilized two strategies to modulate expression of the alpha6 and beta4 subunits in parental and PTHrP-overexpressing MDA-MB-231 and MCF-7 cells: overexpression of alpha6beta4 by transfection with constructs encoding the alpha6 and beta4 subunits, and suppression of endogenous alpha6beta4 expression by transfection with siRNAs targeting these subunits. We now show that the effects of PTHrP are mediated via upregulation of integrin alpha6beta4 expression. We also show that integrin alpha6beta4 expression is modulated at the mRNA level, indicating a transcriptional and/or post-transcriptional mechanism of action for PTHrP. PTHrP expression also increased the levels of phosphorylated Akt, with a consequent increase in the levels of phosphorylated (inactive) glycogen synthase kinase-3 (GSK-3). The role of PTHrP in breast cancer growth and metastasis may thus be mediated via upregulation of integrin alpha6beta4 expression and Akt activation, with consequent inactivation of GSK-3.

Beta1 integrin inhibitory antibody induces apoptosis of breast cancer cells, inhibits growth, and distinguishes malignant from normal phenotype in three dimensional cultures and in vivo

Current therapeutic approaches to cancer are designed to target molecules that contribute to malignant behavior but leave normal tissues intact. beta(1) integrin is a candidate target well known for mediating cell-extracellular matrix (ECM) interactions that influence diverse cellular functions; its aberrant expression has been implicated in breast cancer progression and resistance to cytotoxic therapy. The addition of beta(1) integrin inhibitory agents to breast cancer cells at a single-cell stage in a laminin-rich ECM (three-dimensional lrECM) culture was shown to down-modulate beta(1) integrin signaling, resulting in malignant reversion. To investigate beta(1) integrin as a therapeutic target, we modified the three-dimensional lrECM protocol to approximate the clinical situation: before treatment, we allowed nonmalignant cells to form organized acinar structures and malignant cells to form tumor-like colonies. We then tested the ability of beta(1) integrin inhibitory antibody, AIIB2, to inhibit tumor cell growth in several breast cancer cell lines (T4-2, MDA-MB-231, BT474, SKBR3, and MCF-7) and one nonmalignant cell line (S-1). We show that beta(1) integrin inhibition resulted in a significant loss of cancer cells, associated with a decrease in proliferation and increase in apoptosis, and a global change in the composition of residual colonies. In contrast, nonmalignant cells that formed tissue-like structures remained resistant. Moreover, these cancer cell-specific antiproliferative and proapoptotic effects were confirmed in vivo with no discernible toxicity to animals. Our findings indicate that beta(1) integrin is a promising therapeutic target, and that the three-dimensional lrECM culture assay can be used to effectively distinguish malignant and normal tissue response to therapy.

Beta4 integrin is a transforming molecule that unleashes Met tyrosine kinase tumorigenesis

Cell multiplication in the absence of integrin-derived adhesive signals (anchorage-independent growth) is the phenotypic hallmark of neoplastic transformation. Therefore, the frequently observed up-regulation of some integrins in tumors has been interpreted as an epiphenomenon and not as a causative factor of oncogenic conversion. beta4 integrin stimulates proliferation and survival of epithelial cells and is overexpressed in human carcinomas, often in concomitance with up-regulation of the Met tyrosine kinase receptor for hepatocyte growth factor. Met is not endowed with transforming ability but can exploit the beta4 cytoplasmic tail as a substrate/adaptor for amplification of mitogenic and antiapoptotic responses, independently of cell adhesion. Here, we show that overexpression of beta4 is sufficient to transform rodent fibroblasts, enhances anchorage-independent growth of breast carcinoma cells, and induces tumorigenesis in nude mice; conversely, RNA interference-mediated depletion abrogates the transformed phenotype of neoplastic cells. These autonomous oncogenic properties are dramatically exacerbated upon Met coexpression, suggesting that the integrin can instigate the latent tumorigenic potential of the kinase. A beta4 nonadhesive variant still cooperates with Met for cellular transformation, confirming the adhesion-independent function of beta4 in magnification of Met biological effects. Conversely, a beta4 signaling-incompetent mutant that cannot be efficiently tyrosine phosphorylated by Met and displays reduced ability to activate phosphatidylinositol 3-kinase-dependent and Ras-dependent pathways aborts transformation. Our findings define beta4 as a signaling accomplice (a "servo-oncogene") of tyrosine kinase proto-oncogenes in primary carcinogenesis, evoke an unorthodox function for a prototypic adhesion molecule in the positive regulation of anchorage-independent growth, and suggest the use of beta4 as a target for anticancer therapy.

Enamel matrix derivative is a potent inhibitor of breast cancer cell attachment to bone

This study examined whether enamel matrix derivative (EMD) inhibits the adhesion of cancer cells to bone. A typical breast cancer cell line, MCF-7, was used. Conditioned human osteosarcoma cell (Saos-2) medium was used as extracellular bone matrix (ECBM) to measure cell attachment. MCF-7 cells were incubated on ECBM-coated culture plates with or without soluble EMD, Arg-Gly-Asp (RGD) sequence blocking peptides, recombinant bone sialoprotein (rBSP), or specific integrin antibodies, and the attached cells were quantified using toluidine blue staining. EMD markedly reduced the attachment of MCF-7 cells to ECBM in a dose-dependent manner. An RGD peptide (GRGDSP) and recombinant BSP inhibited cell attachment to the same degree as EMD. Similarly, anti-alphavbeta3 integrin antibody strongly reduced cell attachment, whereas anti-alphavbeta5 and anti-beta1 integrin antibodies had less marked effects on cell attachment. These results show that EMD inhibits MCF-7 cell attachment to a bone matrix and that it might be useful as an anti-adhesive agent for breast cancer cells to bone in vivo.

Involvement of insulin receptor substrate 2 in mammary tumor metastasis

The insulin receptor substrate (IRS) proteins are adaptor molecules that integrate signals generated by receptors that are implicated in human breast cancer. We investigated the specific contribution of IRS-2 to mammary tumor progression using transgenic mice that express the polyoma virus middle T antigen (PyV-MT) in the mammary gland and IRS-2-null (IRS-2(-/-)) mice. PyV-MT-induced tumor initiation and growth were similar in wild-type (WT) and IRS-2(-/-) mice. However, the latency and incidence of metastasis were significantly decreased in the absence of IRS-2 expression. The contribution of IRS-2 to metastasis is intrinsic to the tumor cells, because IRS-2(-/-) mammary tumor cells did not metastasize when grown orthotopically in the mammary fat pads of WT mice. WT and IRS-2(-/-) tumors contained similar numbers of mitotic cells, but IRS-2(-/-) tumors had a higher incidence of apoptosis than did WT tumors. In vitro, IRS-2(-/-) mammary tumor cells were less invasive and more apoptotic in response to growth factor deprivation than their WT counterparts. In contrast, IRS-1(-/-) tumor cells, which express only IRS-2, were highly invasive and were resistant to apoptotic stimuli. Collectively, our findings reveal an important contribution of IRS-2 to breast cancer metastasis.

PTH-related protein enhances MCF-7 breast cancer cell adhesion, migration, and invasion via an intracrine pathway

Breast cancer is the most common carcinoma that metastasizes to the bone. Parathyroid hormone-related protein (PTHrP), a known stimulator of osteoclastic bone resorption, is a major mediator of the osteolytic process in breast cancer. PTHrP overexpression increases mitogenesis and decreases apoptosis in the human breast cancer cell line MCF-7. In this study, MCF-7 cells were used as a model system to study the effects of PTHrP on breast cancer cell adhesion, migration, and invasion. Clones of MCF-7 cells were established that overexpress wild-type PTHrP or PTHrP mutated in the nuclear localization sequence (NLS). Wild-type PTHrP-overexpressing cells showed significantly higher laminin adhesion and migration, and Matrigel invasion than empty vector-transfectants or cells overexpressing NLS-mutated PTHrP. Wild-type PTHrP also increased the cell surface expression of the pro-invasive integrins alpha6 and beta4; deletion of the NLS negated these effects. Exogenous PTHrP (1-34), (67-86), (107-139), and (140-173) had no effect on integrin expression, or on cell adhesion, migration, and invasion. These results indicate that PTHrP exerts its effects on cell adhesion, migration, invasion, and integrin expression via an intracrine pathway. PTHrP may play a role in breast cancer metastasis by upregulating proinvasive integrin expression, and controlling PTHrP production in breast cancer may provide therapeutic benefit.

Adipocyte-secreted factors synergistically promote mammary tumorigenesis through induction of anti-apoptotic transcriptional programs and proto-oncogene stabilization

Mammary epithelial cells are embedded in a unique extracellular environment to which adipocytes and other stromal cells contribute. Mammary epithelial cells are critically dependent on this milieu for survival. However, it remains unknown which adipocyte-secreted factors are required for the survival of the mammary epithelia and what role these adipokines play in the process of ductal carcinoma tumorigenesis. Here, we take a systematic molecular approach to investigate the multiple ways adipocytes and adipokines can uniquely influence the characteristics and phenotypic behavior of malignant breast ductal epithelial cells. Microarray analysis and luciferase reporter assays indicate that adipokines specifically induce several transcriptional programs involved in promoting tumorigenesis, including increased cell proliferation (IGF2, FOS, JUN, cyclin D1), invasive potential (MMP1, ATF3), survival (A20, NFkappaB), and angiogenesis. One of the key changes in the transformed ductal epithelial cells associated with the cell cycle involves the induction of NFkappaB (five-fold) and cyclin D1 (three-fold). We show that by regulating the transcription of these molecules, the synergistic activity of adipocyte-derived factors can potentiate MCF-7 cell proliferation. Furthermore, compared to other stromal cell-secreted factors, the full complement of adipokines shows an unparalleled ability to promote increased cell motility, migration, and the capacity for angiogenesis. Adipocyte-secreted factors can affect tumorigenesis by increasing the stabilization of pro-oncogenic factors such as beta-catenin and CDK6 as a result of a reduction in the gene expression of their inhibitors (i.e. p18). An in vivo coinjection system using 3T3-L1 adipocytes and SUM159PT cells effectively recapitulates the host-tumor interactions in primary tumors. Type VI collagen, a soluble extracellular matrix protein abundantly expressed in adipocytes, is further upregulated in adipocytes during tumorigenesis. It promotes GSK3beta phosphorylation, beta-catenin stabilization, and increased beta-catenin activity in breast cancer cells and may critically contribute towards tumorigenesis when not counterbalanced by other factors.

Integrins in mammary gland development and differentiation of mammary epithelium

Integrins are major extracellular matrix (ECM) receptors that can also serve for some cell-cell interactions. They have been identified as important regulators of mammary epithelial cell growth and differentiation. Their ability to promote cell anchorage, proliferation, survival, migration, and the induction of active ECM-degrading enzymes suggests that they play an essential role in normal mammary morphogenesis, but, on the other hand, reveals their potential to promote tumor progression.

Adhesion and migration of extracellular matrix-stimulated breast cancer

BACKGROUND

Extracellular matrix (ECM) components, such as vitronectin and fibronectin, have been shown to enhance the metastatic potential of breast cancer cells. We hypothesized that ECM binding to integrin receptors on breast cancer cells influenced cellular adhesion and migration.

MATERIALS AND METHODS

Adhesion assays were performed using breast cancer cell lines MDA-MB-435 and MDA-MB-231 and various concentrations of vitronectin or fibronectin. Migration assays were performed using the same cell lines and invasion chambers with 8 microm pore polycarbonate membranes. Blocking antibodies and specific peptidomimetic inhibitors to integrin receptors were used to identify the integrin subunits reacting with vitronectin and fibronectin.

RESULTS

While both breast cancer cell lines adhered to and migrated toward vitronectin and fibronectin, MDA-MB-435 had a higher maximum binding to vitronectin and MDA-MB-231 had a higher maximum binding to fibronectin. Anti-beta1 antibody inhibited the adhesion and migration of MDA-MB-231 to fibronectin and the adhesion of MDA-MB-231 to vitronectin but had no effect on vitronectin-induced adhesion or migration of MDA-MB-435. The alpha(v)beta3/alpha(v)beta5 antagonist, SB 265123, inhibited MDA-MB-231 and MDA-MB-435 adhesion and migration to vitronectin but had no effect on migration to fibronectin in either cell line.

CONCLUSIONS

We conclude that the integrin subunits beta1, alpha(v)beta3, and alpha(v)beta5 can be involved in breast cancer cell adhesion and migration to vitronectin and fibronectin. Because more than one integrin inhibitor was required to block adhesion or migration in the cell lines studied, breast cancer therapy based on integrin antagonists would most likely require concomitant use of multiple agents.

Pro-collagen I COOH-terminal trimer induces directional migration and metalloproteinases in breast cancer cells

Breast and prostatic carcinomas, melanoma, and endothelial cell lines are chemoattracted by medium conditioned by mature osteoblasts. The chemoattractant for endothelial cells was identified with C3, carboxyl-terminal trimer of pro-collagen type I. We report that C3 induces directional migration and proliferation, the expression of tissue inhibitor of metalloproteinases-2, pro-metalloproteinase-2 and -9, and their activation in MDA MB231 cells, without changing the expression of tissue inhibitor of metalloproteinases-1 and of metalloproteinase-14. Antiserum against metalloproteinase-2 or -9 or -14, tissue inhibitor of metalloproteinases-1, or GM6001 inhibits the C3-induced migration. Urokinase and its receptor are detected and unchanged upon exposure to C3. The antibody against urokinase or addition of plasminogen activator inhibitor inhibits migration. Blocking antibodies to integrins alpha(2), alpha(6), beta(1), and beta(3) inhibit chemotaxis and do not change urokinase and urokinase receptor expression. Blockage of alpha(2), beta(1), and beta(3) integrins affect differently the induction by C3 of pro-metalloproteinase-2 and -9 and of tissue inhibitor of metalloproteinases-2. Chemotaxis to C3 is also inhibited by genistein, by pertussis toxin, which also inhibits C3-induced pro-metalloproteinase -2 and -9, but not urokinase expression. Wortmannin partially inhibits C3-induced cell migration. Other, but not all, breast carcinoma lines tested responded to C3 with migration and pro-metalloproteinase-2 induction. Presently C3 is the only agent known to induce migration specifically of both endothelial and breast carcinoma cells. The mitogenic and motogenic role of C3 in vitro might prefigure a role in in vivo carcinogenesis and in the establishment of metastasis.

Cell-matrix interactions during development and apoptosis of the mouse mammary gland in vivo

Epithelial cell survival is dependent on extracellular signals provided by both soluble factors and by adhesion. In the mammary gland, extensive apoptosis of epithelial cells occurs rapidly when lactation ceases, but the mechanism of apoptosis induction is not known. In tissue culture, mammary epithelial cells require laminin as a survival ligand and specific beta1 integrins are necessary to suppress apoptosis. To explore the possibility that dynamic changes in cell-matrix interactions contribute to the onset of apoptosis during mammary involution in vivo, a detailed immunohistochemical analysis of the expression of integrin subunits and their extracellular matrix ligands during mouse mammary gland development has been performed. The kinetics of apoptosis were determined by using tissue samples obtained from virgin, pregnant, lactating, and involuting gland. The maximal elevation of apoptosis occurred within 24 hr of weaning as determined by histologic analysis and caspase-3 staining. A wide variety of laminin subunits, together with nidogen-1 and -2, and perlecan were identified within the basement membrane region of epithelial ducts, lobules, and alveoli in both human and mouse mammary gland. However, no change in the distribution of any of the basement membrane proteins or their cognate integrin receptors was observed during the transition from lactation to apoptosis. Instead, we discovered that altered ligand-binding conformation of the beta1 integrin to a nonbinding state coincided with the immediate onset of mammary apoptosis. This finding may provide a novel dynamic mechanism for inhibiting the transduction of extracellular matrix survival signals, thereby contributing to the onset of apoptosis in a developmental context in vivo.

Mammary epithelial-specific expression of the integrin-linked kinase (ILK) results in the induction of mammary gland hyperplasias and tumors in transgenic mice

The integrin linked kinase (ILK) is a cytoplasmic effector of integrin receptors, involved in the regulation of integrin binding properties as well as the activation of cell survival and proliferative pathways, including those involving MAP kinase, PKB/Akt and GSK-3beta. Overexpression of ILK in cultured intestinal and mammary epithelial cells has been previously shown to induce changes characteristic of oncogenic transformation, including anchorage-independent growth, invasiveness, suppression of anoikis and tumorigenicity in nude mice. In order to determine if ILK overexpression can result in the formation of mammary tumors in vivo, we generated transgenic mice expressing ILK in the mammary epithelium, under the transcriptional control of the mouse mammary tumor virus (MMTV) long terminal repeat (LTR). By the age of 6 months, female MMTV/ILK mice developed a hyperplastic mammary phenotype, which was accompanied by the constitutive phosphorylation of PKB/Akt, GSK-3beta and MAP kinase. Focal mammary tumors subsequently appeared in 34% of the animals at an average age of 18 months. Given the focal nature and long latency of the tumors, however, additional genetic events are likely required for tumor induction in the MMTV/ILK mice. These results provide the first direct demonstration of a potential oncogenic role for ILK, which is upregulated in human tumors and tumor cell lines.

Human MCF10A mammary epithelial cells undergo apoptosis following actin depolymerization that is independent of attachment and rescued by Bcl-2

Many tumor cells are impaired in adhesion-regulated apoptosis, which contributes to their metastatic potential. However, suppression of this apoptotic pathway in untransformed cells is not mediated only by adhesion to the extracellular matrix but also through the resulting ability to spread and adopt a distinct morphology. Since cell spreading is dependent on the integrity of the actin microfilament cytoskeleton, we sought to determine if actin depolymerization was sufficient to induce apoptosis, even in the presence of continuous attachment. For this study, we used a human mammary epithelial cell line (MCF10A), which is immortalized but remains adhesion dependent for survival. Treatment of MCF10A cells with latrunculin-A (LA), an inhibitor of actin polymerization, rapidly led to disruption of the actin cytoskeleton and caused cell rounding but preserved attachment. Initiation of apoptosis in LA-treated MCF10A cells was detected by mitochondrial localization of the Bax apoptotic protein, which was prevented by overexpression of Bcl-2. DNA fragmentation and poly(ADP-ribose) polymerase (PARP) cleavage in LA-treated MCF10A cells indicated progression to the execution phase of apoptosis. The MDA-MB-453 cell line, which was derived from a metastatic human mammary tumor, was resistant to PARP cleavage and loss of viability in response to actin depolymerization. Stable overexpression of Bcl-2 in the untransformed MCF10A cells was able to recapitulate the resistance to apoptosis found in the tumor cell line. We demonstrate that inhibition of actin polymerization is sufficient to stimulate apoptosis in attached MCF10A cells, and we present a novel role for Bcl-2 in cell death induced by direct disruption of the actin cytoskeleton.

Integrin laminin receptors and breast carcinoma progression

This review explores the mechanistic basis of breast carcinoma progression by focusing on the contribution of integrins. Integrins are essential for progression not only for their ability to mediate physical interactions with extracellular matrices but also for their ability to regulate signaling pathways that control actin dynamics and cell movement, as well as for growth and survival. Our comments center on the alpha6 integrins (alpha6beta1 and alpha6beta4), which are receptors for the laminin family of basement membrane components. Numerous studies have implicated these integrins in breast cancer progression and have provided a rationale for studying the mechanistic basis of their contribution to aggressive disease. Recent work by our group and others on mechanisms of breast carcinoma invasion and survival that are influenced by the alpha6 integrins are discussed.

Epithelial development and differentiation in the mammary gland is not dependent on alpha 3 or alpha 6 integrin subunits

In the mammary gland, both laminin and integrins have been shown to be required for normal ductal morphogenesis during development in vivo, and for functional differentiation in culture models. Major integrin receptors for laminins in the mammary gland are alpha 3 beta 1, alpha 6 beta 1, and alpha 6 beta 4. However, the specific subunits that contribute to laminin-mediated mammary cell function and development have not been identified. In this study, we use a genetic approach to test the hypothesis that laminin-binding integrins are required for the function of the mammary gland in vivo. Rudiments of embryonic mammary gland were shown to develop in the absence of these integrin subunits. Postnatal development of the mammary gland was studied in integrin null tissue that had been transplanted into the mammary fat pads of syngeneic hosts. In mammary epithelium lacking alpha 6 integrin, the beta 4 subunit was not apparent and hemidesmosome formation was only rudimentary. However, despite this deficiency, normal ductal morphogenesis and branching of the mammary gland occurred and myoepithelial cells were distributed normally with respect to luminal cells. Mammary alveoli devoid of alpha 3 or alpha 6 integrin formed in pregnancy and were histologically and functionally identical to those in wild-type mammary gland. The tissue underwent full morphological differentiation, and the epithelial cells retained the ability to synthesize beta-casein. This work demonstrates that mammary tissue genetically lacking major laminin-binding integrin receptors is still able to develop and function.

Architectural transcription factor HMGI(Y) promotes tumor progression and mesenchymal transition of human epithelial cells

Numerous studies have demonstrated that overexpression or aberrant expression of the HMGI(Y) family of architectural transcription factors is frequently associated with both neoplastic transformation of cells and metastatic tumor progression. Little is known, however, about the molecular roles played by the HMGI(Y) proteins in these events. Here we report that human breast epithelial cells harboring tetracycline-regulated HMGI(Y) transgenes acquire the ability to form both primary and metastatic tumors in nude mice only when the transgenes are actively expressed. Unexpectedly, the HMG-Y, rather than the HMG-I, isoform of these proteins is the most effective elicitor of both neoplastic transformation and metastatic progression in vivo. Furthermore, expression of either antisense or dominant-negative HMGI(Y) constructs inhibits both the rate of proliferation of tumor cells and their ability to grow anchorage independently in soft agar. Array analysis of transcription profiles demonstrates that the HMG-I and HMG-Y isoform proteins each modulate the expression of distinctive constellations of genes known to be involved in signal transduction, cell proliferation, tumor initiation, invasion, migration, induction of angiogenesis, and colonization. Immunohistochemical analyses of tumors formed in nude mice indicate that many have undergone an epithelial-mesenchymal transition in vivo. Together, these findings demonstrate that overexpression of the HMGI(Y) proteins, more specifically, the HMG-Y isoform protein, is causally associated with both neoplastic transformation and metastatic progression and suggest that induction of integrins and their signaling pathways may play significant molecular roles in these biological events.