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Teaser, summary, work performed and final results

Periodic Reporting for period 1 - INTEGRISTEM (Functions of Integrins in Mammary Stem Cell Activity and Tumorigenesis)


Mammary gland is a bilayer of epithelial cells: the milk-producing luminal cell layer and the contractile basal cell layer that allows milk expulsion. Both layers contain stem cells, which are essential for the normal development of the gland during adult life (puberty...


Mammary gland is a bilayer of epithelial cells: the milk-producing luminal cell layer and the contractile basal cell layer that allows milk expulsion. Both layers contain stem cells, which are essential for the normal development of the gland during adult life (puberty, pregnancy, lactation). This bilayer is surrounded by a matrix, called extracellular matrix, whose main component, laminin, controls epithelial cell proliferation and survival. Anomalies of stem cells can lead to their uncontrolled proliferation and the development of cancer. Recent studies indicated that basal-like breast tumors, notably those with BRCA1 mutations, might originate from stem cells of the luminal cell layer. These tumors are aggressive and difficult to treat. Our project aimed to elucidate the role of receptors at the surface of luminal cells that interact with laminin, called integrins, as it might help identifying new targets for breast cancer treatment. To this end, laminin-binding integrins were inactivated specifically in luminal stem cells and consequences on normal mammary development and tumorigenesis studied. Our results demonstrated that laminin-binding integrins are essential for mammary gland development during gestation/lactation. Their impact on luminal stem cell amplification during tumor initiation is still under investigation.

Work performed

1. Transgenic mouse model
First, mice deficient for laminin-binding integrins specifically in the mammary luminal cells were generated; they are called mutant mice. The absence of laminin-binding integrins in the luminal compartment of pregnant and lactating mutant mice was verified.
2. Role in pregnancy
Compared to pregnant control mice, the lobulo-alveolar development was delayed in pregnant mutant mice. Consistently, the level of milk gene expression was decreased in mutant luminal cells. Similar results were obtained using the mammary organoid model, which corresponds to small pieces of mammary gland cultured in 3D in a reconstituted matrix.
3. Role in lactation
In early lactation, histology and milk gene expression in mutant glands were similar to the controls. However, later on, an accumulation of anomalies was observed: perturbed cell polarity and induction of cell death in mutant luminal cells.

The stem cell capacity of mutant luminal progenitors was strongly reduced compared to control ones. In contrast, basal cells were not affected.

The lack of p53, a tumor suppressor gene, leads to the amplification of luminal progenitor cells. Interestingly, the lack of laminin-binding integrins conteracts this effect. Next, a model of basal-like mammary tumorigenesis induced by the loss of p53 and BRCA1 was employed. Tumors reached ethical size in 6 month-old mice and were found positive for several basal markers as in human basal-like cancer, indicating this is pertinent mouse model. Analysis of mammary tumorigenesis in mice deficient for laminin-binding integrins is still ongoing.

Altogether, these results indicate laminin-binding integrins are essential in mammary luminal cells to support lobulo-alveolar development and lactogenic differentiation during pregnancy. They participate in maintaining cell polarity and survival during lactation. Their absence perturbed luminal progenitor functions in both physiological and p53-deficient contexts. These results were presented in several international conferences as posters (European Network of Breast Development and Cancer labs; Gordon Conference on Mammary Gland Development) and oral communication (Annual Meeeting of the French Society for ECM Biology). A manuscript for publication is currently under preparation.

Final results

Breast cancer is still the first cause of cancer-related death in women. Recent studies indicated that basal-like breast tumors might originate from luminal progenitor cells. Basal-like breast tumors represent 15% of breast carcinomas and account for more than 25% breast cancer-related deaths. They are particularly aggressive and difficult to treat, demonstrating the unmet need to identify new targets for the treatment of these patients. This project led to (i) identification of integrin dimers essential to the normal mammary development, (ii) characterization of the mammary stem cell niche, (iii) better understanding of the mechanisms of mammary developmental processes. Work is still ongoing to assess the impact of laminin-binding integrins on basal-like mammary tumorigenesis. This may pave the way to the identification of new targets for breast cancer treatment.

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