Integrins in mammary development

Alpha-6 integrin deletion delays the formation of Brca1/p53-deficient basal-like breast tumors by restricting luminal progenitor cell expansion

BACKGROUND

The aberrant amplification of mammary luminal progenitors is at the origin of basal-like breast cancers associated with BRCA1 mutations. Integrins mediate cell-matrix adhesion and transmit mechanical and chemical signals that drive epithelial stem cell functions and regulate tumor progression, metastatic reactivation, and resistance to targeted therapies. Consistently, we have recently shown that laminin-binding integrins are essential for the expansion and differentiation of mammary luminal progenitors in physiological conditions. As over-expression of the laminin-binding α6 integrin (Itgα6) is associated with poor prognosis and reduced survival in breast cancer, we here investigate the role of Itgα6 in mammary tumorigenesis.

METHODS

We used Blg-Cre; Brca1F/F; Trp53F/F mice, a model that phenocopies human basal-like breast cancer with BRCA1 mutations. We generated mutant mice proficient or deficient in Itgα6 expression and followed tumor formation. Mammary tumors and pretumoral tissues were characterized by immunohistochemistry, flow cytometry, RT-qPCR, Western blotting and organoid cultures. Clonogenicity of luminal progenitors from preneoplastic glands was studied in 3D Matrigel cultures.

RESULTS

We show that Itga6 deletion favors activation of p16 cell cycle inhibitor in the preneoplastic tissue. Subsequently, the amplification of luminal progenitors, the cell of origin of Brca1-deficient tumors, is restrained in Itgα6-deficient gland. In addition, the partial EMT program operating in Brca1/p53-deficient epithelium is attenuated in the absence of Itgα6. As a consequence of these events, mammary tumor formation is delayed in Itgα6-deficient mice. After tumor formation, the lack of Itgα6 does not affect tumor growth but rather alters their differentiation, resulting in reduced expression of basal cell markers.

CONCLUSIONS

Our data indicate that Itgα6 has a pro-tumorigenic role in Blg-Cre; Brca1F/F; Trp53F/F mice developing basal-like mammary tumors. In particular, we reveal that Itgα6 is required for the luminal progenitor expansion and the aberrant partial EMT program that precedes the formation of BRCA1 deficient tumors.

Clinical significance of integrin αV and β superfamily members and focal adhesion kinase activity in oral squamous cell carcinoma: a retrospective observational study

Objectives: Integrins are heterodimeric transmembrane plasma membrane proteins composed of α- and β-chains. They bind to extracellular matrix (ECM) and cytoskeletal proteins as ECM protein receptors. Upon ECM protein binding, integrins activate focal adhesion kinase (FAK) and transduce various signals. Despite their importance, integrin and FAK expression in oral squamous cell carcinoma (OSCC) tissue and the prognosis of patients with OSCC remains elusive. Methods: In a retrospective observational study, we immunohistochemically evaluated integrin αV, β1, β3, β5, β6, FAK, and phosphorylated-FAK (pFAK) expressions as prognostic predictors in 96 patients with OSCC. Patients were classified as positive or negative based on staining intensity, and clinicopathologic characteristics and survival rates of the two groups were compared. The association between above integrin-related proteins and PD-1 or PD-L1 in OSCC tissues was investigated. Results: We observed immunohistochemical integrin αV, β1, β6, β8, and FAK expressions in the cell membrane and cytoplasm but not integrin β3 and β5 in the OSCC tissues. pFAK was expressed in the cytoplasm of OSCC cells. The overall survival rate significantly decreased in pFAK-positive OSCC patients compared to the negative group, and cervical lymph node metastasis significantly increased in integrin β8-positive patients with OSCC (p < 0.05). No association between integrin-related proteins and PD-1 or PD-L1 in OSCC tissues was observed. Conclusion: Our results indicate that pFAK and integrin β8 are prognostic factors for OSCC. Therefore, pFAK- and integrin β8-targeting new oral cancer diagnostic and therapeutic methods hold a promising potential.

Molecular complexity of mammary glands development: a review of lactogenic differentiation in epithelial cells

The mammary gland is a dynamic organ with various physiological processes like cellular proliferation, differentiation, and apoptosis during the pregnancy-lactation-involution cycle. It is essential to understand the molecular changes during the lactogenic differentiation of mammary epithelial cells (MECs, the milk-synthesizing cells). The MECs are organized as luminal milk-secreting cells and basal myoepithelial cells (responsible for milk ejection by contraction) that form the alveoli. The branching morphogenesis and lactogenic differentiation of the MECs prepare the gland for lactation. This process is governed by many molecular mediators including hormones, growth factors, cytokines, miRNAs, regulatory proteins, etc. Interestingly, various signalling pathways guide lactation and understanding these molecular transitions from pregnancy to lactation will help researchers design further research. Manipulation of genes responsible for milk synthesis and secretion will promote augmentation of milk yield in dairy animals. Identifying protein signatures of lactation will help develop strategies for persistent lactation and shortening the dry period in farm animals. The present review article discusses in details the physiological and molecular changes occurring during lactogenic differentiation of MECs and the associated hormones, regulatory proteins, miRNAs, and signalling pathways. An in-depth knowledge of the molecular events will aid in developing engineered cellular models for studies related to mammary gland diseases of humans and animals.

HPV-Associated Breast Cancer: Myth or Fact?

Some estimates place the proportion of human malignancies attributable to viruses at between 15 and 20 percent. Viruses including the human papillomavirus are considered an interesting but controversial etiological risk factor for breast cancer. HPV infection is anticipated to be an early trigger in breast cancer carcinogenesis, followed by cumulative alterations over time ("hit and run" mechanism) through synergy with other environmental factors. The association between HPV and breast cancer has not yet been verified. There are very conflicting data on the presence of HPV DNA in breast cancer samples, and we lack a clarified, exact mode of HPV transmission to the breast. In our review article we analyzed the up-to-date knowledge about the association of HPV and breast cancer. Furthermore, we summarized the available original research published since 2010. In conclusion, the complexity and inconsistency of the available results together with the relatively low prevalence of HPV infection requires extensive research with much larger studies and exact and unified diagnostic methods are required to better understand the role of the HPV in breast carcinogenesis.

Parallels between the extracellular matrix roles in developmental biology and cancer biology

Interaction of a tumor with its microenvironment is an emerging field of investigation, and the crosstalk between cancer cells and the extracellular matrix is of particular interest, since cancer patients with abundant and stiff extracellular matrices display a poorer prognosis. At the post-juvenile stage, the extracellular matrix plays predominantly a structural role by providing support to cells and tissues; however, during development, matrix proteins exert a plethora of diverse signals to guide the movement and determine the fate of pluripotent cells. Taking a closer look at the communication between the extracellular matrix and cells of a developing body may bring new insights into cancer biology and identify cancer weaknesses. This review discusses parallels between the extracellular matrix roles during development and tumor growth.

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.

Laminin-binding integrins are essential for the maintenance of functional mammary secretory epithelium in lactation

Integrin dimers α3/β1, α6/β1 and α6/β4 are the mammary epithelial cell receptors for laminins, which are major components of the mammary basement membrane. The roles of specific basement membrane components and their integrin receptors in the regulation of functional gland development have not been analyzed in detail. To investigate the functions of laminin-binding integrins, we obtained mutant mice with mammary luminal cell-specific deficiencies of the α3 and α6 integrin chains generated by the Cre-Lox approach. During pregnancy, mutant mice displayed decreased luminal progenitor activity and retarded lobulo-alveolar development. Mammary glands appeared functional at the onset of lactation in mutant mice, however myoepithelial cell morphology was markedly altered, suggesting cellular compensation mechanisms involving cytoskeleton reorganization. Notably, lactation was not sustained in mutant females, and the glands underwent precocious involution. Inactivation of the p53 gene rescued the growth defects but did not restore lactogenesis in mutant mice. These results suggest that the p53 pathway is involved in the control of mammary cell proliferation and survival downstream of laminin-binding integrins and underline an essential role of cell interactions with laminin for lactogenic differentiation.

Deciphering the Mammary Stem Cell Niche: A Role for Laminin-Binding Integrins

Integrins, which bind laminin, a major component of the mammary basement membrane, are strongly expressed in basal stem cell-enriched populations, but their role in controlling mammary stem cell function remains unclear. We found that stem cell activity, as evaluated in transplantation and mammosphere assays, was reduced in mammary basal cells depleted of laminin receptors containing α3- and α6-integrin subunits. This was accompanied by low MDM2 levels, p53 stabilization, and diminished proliferative capacity. Importantly, disruption of p53 function restored the clonogenicity of α3/α6-integrin-depleted mammary basal stem cells, while inhibition of RHO or myosin II, leading to decreased p53 activity, rescued the mammosphere formation. These data suggest that α3/α6-integrin-mediated adhesion plays an essential role in controlling the proliferative potential of mammary basal stem/progenitor cells through myosin II-mediated regulation of p53 and indicate that laminins might be important components of the mammary stem cell niche.

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α3- and α6-integrins are required for mammary basal stem cell function

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p53 is activated in mammary basal cells depleted of α3- and α6-integrins

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RHO and myosin II mediate p53 activation in α3- and α6-integrin-depleted cells

Fibulin-2 is required for basement membrane integrity of mammary epithelium

Fibulin-2 (FBLN2) is a secreted extracellular matrix glycoprotein which has been associated with tissue development and remodelling. In the mouse mammary gland, FBLN2 can be detected during ductal morphogenesis in cap cells and myoepithelial cells at puberty and early pregnancy, respectively. In an attempt to assign its function, we knocked down Fbln2 in the mouse mammary epithelial cell line EpH4. FBLN2 reduction led to an increase in the size of spheroidal structures when compared to scrambled control shRNA-transduced cells plated on Matrigel matrix. This phenotype was associated with a disruption of the collagen IV sheath around the epithelial spheroids and downregulation of integrin β1, suggesting a role for FBLN2 in stabilizing the basement membrane (BM). In contrast to mice, in normal adult human breast tissue, FBLN2 was detected in ductal stroma, and in the interlobular stroma, but was not detectable within the lobular regions. In tissue sections of 65 breast cancers FBLN2 staining was lost around malignant cells with retained staining in the neighbouring histologically normal tissue margins. These results are consistent with a role of FBLN2 in mammary epithelial BM stability, and that its down-regulation in breast cancer is associated with loss of the BM and early invasion.

Modular GAG-matrices to promote mammary epithelial morphogenesis in vitro

Matrix systems used to study complex three-dimensional (3D) cellular processes like mammary epithelial tissue morphogenesis and tumorigenesis ex vivo often require ill-defined biological components, which lead to poor reproducibility and a lack of control over physical parameters. In this study, a well-defined, tunable synthetic biohybrid hydrogel composed of the glycosaminoglycan heparin, star-shaped poly(ethylene glycol) (starPEG), and matrix metalloproteinase- (MMP-) cleavable crosslinkers was applied to dissect the biophysical and biochemical signals promoting human mammary epithelial cell (MEC) morphogenesis. We show that compliant starPEG-heparin matrices promote the development of polarized MEC acini. Both the presence of heparin and MMP-cleavable crosslinks are essential in facilitating MEC morphogenesis without supplementation of exogenous adhesion ligands. In this system, MECs secrete and organize laminin in basement membrane-like assemblies to promote integrin signaling and drive acinar development. Therefore, starPEG-heparin hydrogels provide a versatile platform to study mammary epithelial tissue morphogenesis in a chemically defined and precisely tunable 3D in vitro microenvironment. The system allows investigation of biophysical and biochemical aspects of mammary gland biology and potentially a variety of other organoid culture studies.

PGE2 /EP4 Signaling Controls the Transfer of the Mammary Stem Cell State by Lipid Rafts in Extracellular Vesicles

Prostaglandin E₂ (PGE₂ )-initiated signaling contributes to stem cell homeostasis and regeneration. However, it is unclear how PGE₂ signaling controls cell stemness. This study identifies a previously unknown mechanism by which PGE₂ /prostaglandin E receptor 4 (EP₄ ) signaling regulates multiple signaling pathways (e.g., PI3K/Akt signaling, TGFβ signaling, Wnt signaling, EGFR signaling) which maintain the basal mammary stem cell phenotype. A shift of basal mammary epithelial stem cells (MaSCs) from a mesenchymal/stem cell state to a non-basal-MaSC state occurs in response to prostaglandin E receptor 4 (EP₄ ) antagonism. EP₄ antagonists elicit release of signaling components, by controlling their trafficking into extracellular vesicles/exosomes in a lipid raft/caveolae-dependent manner. Consequently, EP₄ antagonism indirectly inactivates, through induced extracellular vesicle/exosome release, pathways required for mammary epithelial stem cell homeostasis, e.g. canonical/noncanonical Wnt, TGFβ and PI3K/Akt pathways. EP₄ antagonism causes signaling receptors and signaling components to shift from non-lipid raft fractions to lipid raft fractions, and to then be released in EP₄ antagonist-induced extracellular vesicles/exosomes, resulting in the loss of the stem cell state by mammary epithelial stem cells. In contrast, luminal mammary epithelial cells can acquire basal stem cell properties following ingestion of EP₄ antagonist-induced stem cell extracellular vesicles/exosomes, and can then form mammary glands. These findings demonstrate that PGE₂ /EP₄ signaling controls homeostasis of mammary epithelial stem cells through regulating extracellular vesicle/exosome release. Reprogramming of mammary epithelial cells can result from EP₄ -mediated stem cell property transfer by extracellular vesicles/exosomes containing caveolae-associated proteins, between mammary basal and luminal epithelial cells. Stem Cells 2017;35:425-444.

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.

Downregulation of β3 integrin by miR-30a-5p modulates cell adhesion and invasion by interrupting Erk/Ets‑1 network in triple-negative breast cancer

Integrins are adhesion receptors involved in bidirectional signaling and are crucial for various cellular responses during normal homeostasis and pathological conditions, such as cancer progression and metastasis. In the present study, we demonstrated that blockage of β3 integrin-mediated cell- extracellular matrix interactions restrained triple-negative breast cancer (TNBC) growth, and elevated β3 integrin can trigger the rewiring of Erk/Ets-1 signaling pathways, thereby enhancing cell growth and invasion. Ectopic expression of miRNA has been implicated in the deregulation of integrin expression and activity, blocking of cancer tumor development and progression, and acquisition of metastatic phenotype. miR-30a-5p expression has been implicated in the progression of breast cancer. Overexpression of miR-30a-5p suppressed the proliferation, migration and invasion of breast cancer cells. On the contrary, inhibition of miR-30a-5p promoted the proliferation, migration, and invasion of TNBC cells by suppressing the expression of ERK/Ets-1 signal. An inverse correlation was found between the mRNA expressions of miR-30a-5p and β3 integrin in TNBC samples. Furthermore, bioinformatics analysis revealed the putative miR-30 binding sites in the 3'-UTR of β3 integrin. Results of luciferase assay revealed a strong repression of luciferase activity after transfection with miR‑30a-5p and wild-type 3'-UTR of β3 integrin. In TNBC cells, miR-30a-5p promoted an epithelial phenotype and suppressed invasion by specifically targeting β3 integrin subunit to subsequently interdict the β3 integrin/Erk/Ets-1 network.

Phenotypic and functional characterization of two bovine mammary epithelial cell lines in 2D and 3D models

Immortalized bovine mammary epithelial cells (BME-UV1) and immortalized bovine mammary alveolar cells (MAC-T) have been extensively used as in vitro cell models to understand milk production in dairy cows. Precise knowledge about their phenotype and performance remains, however, unknown. This study aims to characterize MAC-T and BME-UV1 profiles when cultured in two-dimensional adherent, three-dimensional adherent (Matrigel), and three-dimensional no adherent [ultralow attachment (ULA)] supports. MAC-T and BME-UV1 were compared according to their proliferation capacities and to specific cell surface markers CD24, CD326 [epithelial cell adhesion molecule (EpCAM)], CD10, and integrin CD49f (α-6). Cytokeratin (CK14 and CK19), signal transducer and activator of transcription 5, and other proteins (occludin and cadherin-1) were analyzed. BME-UV1 in ULA support expressed higher CD49f marker. A different intensity of CD49 staining allowed the discrimination between the two cell lines in adherent condition. CD10, EpCAM, and CK19 expressions show that BME-UV1 cells have luminal capacity, while MAC-T has a myoepithelial profile with a high expression of CK14. BME-UV1 cells possess a closer committed progenitor profile due to their higher expression in aldehyde dehydrogenase and EpCAM. We observed that BME-UV1 cells have a better capacity to form spherical structures, mammospheres, in Matrigel than MAC-T, which was confirmed by the higher mammosphere area. In the ULA condition, BME-UV1 proliferated over the 6 days of culture. Taken together, our results clearly confirm the BME-UV1 luminal profile and MAC-T ductal/myoepithelial-like phenotype.

CD151 represses mammary gland development by maintaining the niches of progenitor cells

Tetraspanin CD151 interacts with laminin-binding integrins (i.e., α3β1, α6β1 and α6β4) and other cell surface molecules to control diverse cellular and physiological processes, ranging from cell adhesion, migration and survival to tissue architecture and homeostasis. Here, we report a novel role of CD151 in maintaining the branching morphogenesis and activity of progenitor cells during the pubertal development of mammary glands. In contrast to the disruption of laminin-binding integrins, CD151 removal in mice enhanced the tertiary branching in mammary glands by 2.4-fold and the number of terminal end buds (TEBs) by 30%, while having minimal influence on either primary or secondary ductal branching. Consistent with these morphological changes are the skewed distribution of basal/myoepithelial cells and a 3.2-fold increase in proliferating Ki67-positive cells. These novel observations suggest that CD151 impacts the branching morphogenesis of mammary glands by upregulating the activities of bipotent progenitor cells. Indeed, our subsequent analyses indicate that upon CD151 removal the proportion of CD24(Hi)CD49f(Low) progenitor cells in the mammary gland increased by 34%, and their proliferating and differentiating activities were significantly upregulated. Importantly, fibronectin, a pro-branching extracellular matrix (ECM) protein deposited underlying mammary epithelial or progenitor cells, increased by >7.2-fold. Moreover, there was a concomitant increase in the expression and nuclear distribution of Slug, a transcription factor implicated in the maintenance of mammary progenitor cell activities. Taken together, our studies demonstrate that integrin-associated CD151 represses mammary branching morphogenesis by controlling progenitor cell activities, ECM integrity and transcription program.

Remodeling of keratin-coupled cell adhesion complexes

Epithelial cells constitute the main barrier between the inside and outside of organs, acting as gatekeepers of their structure and integrity. Hemidesmosomes and desmosomes are respectively cell-matrix and cell-cell adhesions coupled to the intermediate filament cytoskeleton. These adhesions ensure mechanical integrity of the epithelial barrier. Although desmosomes and hemidesmosomes are essential in maintaining strong cell-cell and cell-matrix adhesions, there is an emerging view that they should be remodeled in order to maintain epithelial homeostasis. Here we review the adhesion properties of desmosomes and hemidesmosomes, as well as the mechanisms driving their remodeling. We also discuss recent data suggesting that keratin-coupled adhesion complexes can sense the biomechanical cellular environment and participate in the cellular response to such external cues.

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.

Progression of luminal breast tumors is promoted by ménage à trois between the inflammatory cytokine TNFα and the hormonal and growth-supporting arms of the tumor microenvironment

Breast cancer progression is strongly linked to inflammatory processes, aggravating disease course. The impacts of the inflammatory cytokine TNFα on breast malignancy are not fully substantiated, and they may be affected by cooperativity between TNFα and other protumoral mediators. Here, we show that together with representatives of other important arms of the tumor microenvironment, estrogen (hormonal) and EGF (growth-supporting), TNFα potently induced metastasis-related properties and functions in luminal breast tumor cells, representing the most common type of breast cancer. Jointly, TNFα + Estrogen + EGF had a stronger effect on breast cancer cells than each element alone, leading to the following: (1) extensive cell spreading and formation of FAK/paxillin-enriched cellular protrusions; (2) elevated proportion of tumor cells coexpressing high levels of CD44 and β1 and VLA6; (3) EMT and cell migration; (4) resistance to chemotherapy; (5) release of protumoral factors (CXCL8, CCL2, MMPs). Importantly, the tumor cells used in this study are known to be nonmetastatic under all conditions; nevertheless, they have acquired high metastasizing abilities in vivo in mice, following a brief stimulation by TNFα + Estrogen + EGF. These dramatic findings indicate that TNFα can turn into a strong prometastatic factor, suggesting a paradigm shift in which clinically approved inhibitors of TNFα would be applied in breast cancer therapy.

Signaling events mediated by α3β1 integrin are essential for mammary tumorigenesis

The constitutive activation of β-catenin signaling in the mammary basal epithelial cell layer in transgenic K5ΔNβcat mice leads to basal-type tumor development. Integrins of the β1 family and integrin-mediated signaling events have an important role in breast tumor growth and progression. We show here that the deletion of α3β1 integrin, a major laminin receptor, from the basal layer of the mammary epithelium of K5ΔNβcat mice completely prevented the tumorigenesis induced by β-catenin signaling. Moreover, the depletion of α3β1 integrin from a spontaneously transformed mouse mammary basal epithelial cell line (MEC) prevented the cells from forming colonies in soft agar and greatly reduced tumor development in orthotopic grafts. Inhibition of the integrin signaling intermediates Rac1 or PAK1 (P21-activated Kinase 1) in MEC affected tumor cell growth in soft agar, whereas the expression of activated forms of these effectors in α3-depleted cells rescued the capacity of these cells to grow in non-adherent conditions. Similarly, the tumorigenic potential of α3-depleted cells was restored by the expression of activated PAK1, as assessed by orthotopic transplantation assay. In three-dimensional Matrigel culture, MEC survival and proliferation were affected by the depletion of α3β1 integrin, which also significantly decreased the activation of focal adhesion kinase (FAK), mitogen-activated protein kinase (MAPK) and c-Jun NH2-terminal kinase (JNK). Our data suggest that the activation of signaling cascades downstream from α3β1 and involving the Rac1/PAK1 pathway, MAPK and JNK, promotes prosurvival and proproliferative signals required for the malignant growth of basal mammary epithelial cells, providing further insight into the molecular mechanisms underlying breast cancer initiation and progression.

Developmental changes in the in vitro activated regenerative activity of primitive mammary epithelial cells

Mouse fetal mammary cells display greater regenerative activity than do adult mammary cells when stimulated to proliferate in a new system that supports the production of transplantable mammary stem cells ex vivo.

Many normal adult tissues contain rare stem cells with extensive self-maintaining regenerative potential. During development, the stem cells of the hematopoietic and neural systems undergo intrinsically specified changes in their self-renewal potential. In the mouse, mammary stem cells with transplantable regenerative activity are first detectable a few days before birth. They share some phenotypic properties with their adult counterparts but are enriched in a subpopulation that displays a distinct gene expression profile. Here we show that fetal mammary epithelial cells have a greater direct and inducible growth potential than their adult counterparts. The latter feature is revealed in a novel culture system that enables large numbers of in vitro clonogenic progenitors as well as mammary stem cells with serially transplantable activity to be produced within 7 days from single fetal or adult input cells. We further show that these responses are highly dependent on novel factors produced by fibroblasts. These findings provide new avenues for elucidating mechanisms that regulate normal mammary epithelial stem cell properties at the single-cell level, how these change during development, and how their perturbation may contribute to transformation.

Many adult tissues are maintained by a rare subset of undifferentiated stem cells that can self-renew and give rise to specialized daughter cells that have a more limited regenerative ability. The recent identification of cells in the fetal and adult mammary gland that display the properties of stem cells provides a foundation for investigating their self-renewal and differentiation control. We now show that these stem cell properties can be elicited from single mouse mammary cells placed in 3D cultures if novel factors produced by fibroblasts are present. Moreover, a comparison of the clonal outputs of fetal and adult mammary cells in this in vitro system shows that the fetal mammary cells have superior regenerative activity relative to their adult counterparts. The ability to activate and quantify the regenerative capacity of single mouse mammary epithelial cells in vitro sets the stage for further investigations of the timing and mechanisms that alter their stem cell properties during development, the potential relevance of these events to other normal epithelial tissues, and how these processes might be involved in the genesis of breast cancer.

How integrins control breast biology

This article explores new ideas about how the ECM-integrin axis controls normal and malignant breast biology. We discuss the role of integrins in mammary stem cells, and how cell-matrix interactions regulate ductal and alveolar development and function. We also examine the contribution of integrins to tissue disorganisation and metastasis, and how an altered stromal and ECM tumour microenvironment affects the cancer cell niche both within primary tumours and at distant sites. Finally, we mention novel strategies for integrin-directed breast cancer treatment.

Overexpression of miR-30b in the developing mouse mammary gland causes a lactation defect and delays involution

Background

MicroRNA (miRNA) are negative regulators of gene expression, capable of exerting pronounced influences upon the translation and stability of mRNA. They are potential regulators of normal mammary gland development and of the maintenance of mammary epithelial progenitor cells. This study was undertaken to determine the role of miR-30b on the establishment of a functional mouse mammary gland. miR-30b is a member of the miR-30 family, composed of 6 miRNA that are highly conserved in vertebrates. It has been suggested to play a role in the differentiation of several cell types.

Methodology/Principal Findings

The expression of miR-30b was found to be regulated during mammary gland development. Transgenic mice overexpressing miR-30b in mammary epithelial cells were used to investigate its role. During lactation, mammary histological analysis of the transgenic mice showed a reduction in the size of alveolar lumen, a defect of the lipid droplets and a growth defect of pups fed by transgenic females. Moreover some mammary epithelial differentiated structures persisted during involution, suggesting a delay in the process. The genes whose expression was affected by the overexpression of miR-30b were characterized by microarray analysis.

Conclusion/Significance

Our data suggests that miR-30b is important for the biology of the mammary gland and demonstrates that the deregulation of only one miRNA could affect lactation and involution.