The requirement of integrins for breast epithelial proliferation

Rac1 controls cell turnover and reversibility of the involution process in postpartum mammary glands

Cell turnover in adult tissues is essential for maintaining tissue homeostasis over a life span and for inducing the morphological changes associated with the reproductive cycle. However, the underlying mechanisms that coordinate the balance of cell death and proliferation remain unsolved. Using the mammary gland, we have discovered that Rac1 acts as a nexus to control cell turnover. Postlactational tissue regression is characterised by the death of milk secreting alveoli, but the process is reversible within the first 48 h if feeding recommences. In mice lacking epithelial Rac1, alveolar regression was delayed. This defect did not result from failed cell death but rather increased cell turnover. Fitter progenitor cells inappropriately divided, regenerating the alveoli, but cell death also concomitantly accelerated. We discovered that progenitor cell hyperproliferation was linked to nonautonomous effects of Rac1 deletion on the macrophageal niche with heightened inflammation. Moreover, loss of Rac1 impaired cell death with autophagy but switched the cell death route to apoptosis. Finally, mammary gland reversibility failed in the absence of Rac1 as the alveoli failed to recommence lactation upon resuckling.

RAC1B function is essential for breast cancer stem cell maintenance and chemoresistance of breast tumor cells

Breast cancer stem cells (BCSC) are presumed to be responsible for treatment resistance, tumor recurrence and metastasis of breast tumors. However, development of BCSC-targeting therapies has been held back by their heterogeneity and the lack of BCSC-selective molecular targets. Here, we demonstrate that RAC1B, the only known alternatively spliced variant of the small GTPase RAC1, is expressed in a subset of BCSCs in vivo and its function is required for the maintenance of BCSCs and their chemoresistance to doxorubicin. In human breast cancer cell line MCF7, RAC1B is required for BCSC plasticity and chemoresistance to doxorubicin in vitro and for tumor-initiating abilities in vivo. Unlike Rac1, Rac1b function is dispensable for normal mammary gland development and mammary epithelial stem cell (MaSC) activity. In contrast, loss of Rac1b function in a mouse model of breast cancer hampers the BCSC activity and increases their chemosensitivity to doxorubicin treatment. Collectively, our data suggest that RAC1B is a clinically relevant molecular target for the development of BCSC-targeting therapies that may improve the effectiveness of doxorubicin-mediated chemotherapy.

Abnormal promoter DNA hypermethylation of the integrin, nidogen, and dystroglycan genes in breast cancer

Cell transmembrane receptors and extracellular matrix components play a pivotal role in regulating cell activity and providing for the concerted integration of cells in the tissue structures. We have assessed DNA methylation in the promoter regions of eight integrin genes, two nidogen genes, and the dystroglycan gene in normal breast tissues and breast carcinomas (BC). The protein products of these genes interact with the basement membrane proteins LAMA1, LAMA2, and LAMB1; abnormal hypermethylation of the LAMA1, LAMA2, and LAMB1 promoters in BC has been described in our previous publications. In the present study, the frequencies of abnormal promoter hypermethylation in BC were 13% for ITGA1, 31% for ITGA4, 4% for ITGA7, 39% for ITGA9, 38% for NID1, and 41% for NID2. ITGA2, ITGA3, ITGA6, ITGB1, and DAG1 promoters were nonmethylated in normal and BC samples. ITGA4, ITGA9, and NID1 promoter hypermethylation was associated with the HER2 positive tumors, and promoter hypermethylation of ITGA1, ITGA9, NID1 and NID2 was associated with a genome-wide CpG island hypermethylated BC subtype. Given that ITGA4 is not expressed in normal breast, one might suggest that its abnormal promoter hypermethylation in cancer is non-functional and is thus merely a passenger epimutation. Yet, this assumption is not supported by our finding that it is not associated with a hypermethylated BC subtype. ITGA4 acquires expression in a subset of breast carcinomas, and methylation of its promoter may be preventive against expression in some tumors. Strong association of abnormal ITGA4 hypermethylation with the HER2 positive tumors (p = 0.0025) suggests that simultaneous presence of both HER2 and integrin α4 receptors is not beneficial for tumor cells. This may imply HER2 and integrin α4 signaling pathways interactions that are yet to be discovered.

Integrin-Rac signalling for mammary epithelial stem cell self-renewal

Background

Stem cells are precursors for all mammary epithelia, including ductal and alveolar epithelia, and myoepithelial cells. In vivo mammary epithelia reside in a tissue context and interact with their milieu via receptors such as integrins. Extracellular matrix receptors coordinate important cellular signalling platforms, of which integrins are the central architects. We have previously shown that integrins are required for mammary epithelial development and function, including survival, cell cycle, and polarity, as well as for the expression of mammary-specific genes. In the present study we looked at the role of integrins in mammary epithelial stem cell self-renewal.

Methods

We used an in vitro stem cell assay with primary mouse mammary epithelial cells isolated from genetically altered mice. This involved a 3D organoid assay, providing an opportunity to distinguish the stem cell- or luminal progenitor-driven organoids as structures with solid or hollow appearances, respectively.

Results

We demonstrate that integrins are essential for the maintenance and self-renewal of mammary epithelial stem cells. Moreover integrins activate the Rac1 signalling pathway in stem cells, which leads to the stimulation of a Wnt pathway, resulting in expression of β-catenin target genes such as Axin2 and Lef1.

Conclusions

Integrin/Rac signalling has a role in specifying the activation of a canonical Wnt pathway that is required for mammary epithelial stem cell self-renewal.

Electronic supplementary material

The online version of this article (10.1186/s13058-018-1048-1) contains supplementary material, which is available to authorized users.

Adhesion, proliferation, and apoptosis in different molecular portraits of breast cancer treated with silver nanoparticles and its pathway-network analysis

Background

Silver nanoparticles (AgNPs) have attracted considerable attention due to the variety of their applications in medicine and other sciences. AgNPs have been used in vitro for treatment of various diseases, such as hepatitis B and herpes simplex infections as well as colon, cervical, and lung cancers. In this study, we assessed the effect on proliferation, adhesion, and apoptosis in breast cancer cell lines of different molecular profiles (MCF7, HCC1954, and HCC70) exposed to AgNPs (2–9 nm).

Methods

Breast cancer cell lines were incubated in vitro; MTT assay was used to assess proliferation. Adhesion was determined by real-time analysis with the xCELLingence system. Propidium iodide and fluorescein isothiocyanate-Annexin V assay were used to measure apoptosis. The transcriptome was assessed by gene expression microarray and Probabilistic Graphical Model (PGM) analyses.

Results

The results showed a decreased adhesion in breast cancer cell lines and the control exposed to AgNPs was noted in 24 hours (p≤0.05). We observed a significant reduction in the proliferation of MCF7 and HCC70, but not in HCC1954. Apoptotic activity was seen in all cell lines exposed to AgNPs, with an apoptosis percentage of more than 60% in cancer cell lines and less than 60% in the control. PGM analysis confirmed, to some extent, the effects of AgNPs primarily on adhesion by changes in the extracellular matrix.

Conclusion

Exposure to AgNPs causes an antiproliferative, apoptotic, and anti-adhesive effect in breast cancer cell lines cultured in vitro. More research is needed to evaluate the potential use of AgNPs to treat different molecular profiles of breast cancer in humans.