Proteomic analysis of mouse mammary terminal end buds identifies axonal growth cone proteins

SCA-1 Labels a Subset of Estrogen-Responsive Bipotential Repopulating Cells within the CD24+ CD49fhi Mammary Stem Cell-Enriched Compartment

Estrogen stimulates breast development during puberty and mammary tumors in adulthood through estrogen receptor-α (ERα). These effects are proposed to occur via ERα⁺ luminal cells and not the mammary stem cells (MaSCs) that are ERα^neg. Since ERα⁺ luminal cells express stem cell antigen-1 (SCA-1), we sought to determine if SCA-1 could define an ERα⁺ subset of EpCAM⁺/CD24⁺/CD49f^hi MaSCs. We show that the MaSC population has a distinct SCA-1⁺ population that is abundant in pre-pubertal mammary glands. The SCA-1⁺ MaSCs have less stem cell markers and less in vivo repopulating activity than their SCA-1^neg counterparts. However, they express ERα and specifically enter the cell cycle at puberty. Using estrogen-deficient aromatase knockouts (ArKO), we showed that the SCA-1⁺ MaSC could be directly modulated by estrogen supplementation. Thus, SCA-1 enriches for an ERα⁺, estrogen-sensitive subpopulation within the CD24⁺/CD49f^hi MaSC population that may be responsible for the hormonal sensitivity of the developing mammary gland.

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SCA-1⁺ delineates ER-positive cells in the CD24⁺ CD49f^hi mammary stem population

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SCA-1⁺ cells have lower repopulation activity

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SCA-1⁺ cells are estrogen responsive

Clinical significance of SPRR1A expression in progesterone receptor-positive breast cancer

Small proline-rich repeat protein 1A (SPRR1A) is a marker for terminal squamous cell differentiation. Previous studies showed that SPRR1A expression increases in squamous cell carcinoma of the skin, but decreases in esophageal squamous cell carcinoma. This study focuses on the expression of SPRR1A protein in breast cancers (BCs) in China. A total of 111 patients with histologically confirmed BC, who underwent radical surgery between January 2006 and September 2007 in China Medical University, were enrolled. The relationship between SPRR1A expression and clinicopathological factors as well as BC prognoses was also determined. Overall, SPRR1A expression was detected in more than half of the BC specimens by immunohistochemistry (56/111, 53.8%), but there was no significant difference between age groups (≥50 vs. <50 years) in terms of SPRR1A expression (P = 0.915), as well as no differences between SPRR1A expression and the clinical stage (0-I vs. II-III) or nodal status (P = 0.234 and 0.632, respectively). Moreover, human epidermal growth factor receptor 2 overexpression was not correlated with SPRR1A expression, whereas Ki67 was associated with SPRR1A expression (P = 0.155 and 0.028, respectively). Interestingly, SPRR1A expression was significantly associated with progesterone receptor-positive (P = 0.010) rather than estrogen receptor-positive (0.778) BCs. The 5-year survival rate in patients did not differ with the presence or absence of SPRR1A expression (P = 0.753), whereas the combination of SPRR1A expression, progesterone receptor status, and menopausal status allowed identification of a subgroup of BC patients with a good long-term prognosis. Thus, the SPRR1A status might play an important role in the prognosis of postmenopausal breast carcinoma patients, especially that of progesterone receptor-positive subgroups.

Mammary morphogenesis and regeneration require the inhibition of EMT at terminal end buds by Ovol2 transcriptional repressor

Epithelial cells possess remarkable plasticity, having the ability to become mesenchymal cells through alterations in adhesion and motility (epithelial-to-mesenchymal transition [EMT]). However, how epithelial plasticity is kept in check in epithelial cells during tissue development and regeneration remains to be fully understood. Here we show that restricting the EMT of mammary epithelial cells by transcription factor Ovol2 is required for proper morphogenesis and regeneration. Deletion of Ovol2 blocks mammary ductal morphogenesis, depletes stem and progenitor cell reservoirs, and leads epithelial cells to undergo EMT in vivo to become nonepithelial cell types. Ovol2 directly represses myriad EMT inducers, and its absence switches response to TGF-β from growth arrest to EMT. Furthermore, forced expression of the repressor isoform of Ovol2 is able to reprogram metastatic breast cancer cells from a mesenchymal to an epithelial state. Our findings underscore the critical importance of exquisitely regulating epithelial plasticity in development and cancer.

Contribution of proteomics to the study of the role of cytokeratins in disease and physiopathology

Cytokeratins (CKs), the most abundant group of cytoskeletal intermediate filaments, and proteomics are strongly connected. On the one hand, proteomics has been extremely useful to uncover new features and functions of CKs, on the other, the highly abundant CKs serve as an exceptional tool to test new technological developments in proteomics. As a result, proteomics has contributed to finding valuable associations of CKs with diseases as diverse as cancer, cystic fibrosis, steatohepatitis, viral and bacterial infection, keratoconus, vitreoretinopathy, preeclampsia or the chronic fatigue syndrome, as well as to characterizing their participation in a number of physiopathological processes, including drug resistance, response to toxicants, inflammation, stem cell differentiation, embryo development, and tissue repair. In some cases, like in cystic fibrosis, CKs have been described as potential therapeutic targets. The development of a specific field of proteomics where CKs become the main subject of research aims and hypotheses is suggested.

Genetic ablation of caveolin-1 drives estrogen-hypersensitivity and the development of DCIS-like mammary lesions

Caveolin-1 (Cav-1) loss-of-function mutations are exclusively associated with estrogen receptor-positive (ER(+)) human breast cancers. To dissect the role of Cav-1 loss-of-function in the pathogenesis of human breast cancers, we used Cav-1(-/-) null mice as a model system. First, we demonstrated that Cav-1(-/-) mammary epithelia overexpress two well-established ER co-activator genes, CAPER and Foxa1, in addition to ER-alpha. Thus, the functional loss of Cav-1 may be sufficient to confer estrogen-hypersensitivity in the mammary gland. To test this hypothesis directly, we subjected Cav-1(-/-) mice to ovariectomy and estrogen supplementation. As predicted, Cav-1(-/-) mammary glands were hyper-responsive to estrogen and developed dysplastic mammary lesions with adjacent stromal angiogenesis that resemble human ductal carcinoma in situ. Based on an extensive biomarker analysis, these Cav-1(-/-) mammary lesions contain cells that are hyperproliferative and stain positively with nucleolar (B23/nucleophosmin) and stem/progenitor cell markers (SPRR1A and beta-catenin). Genome-wide transcriptional profiling identified many estrogen-related genes that were over-expressed in Cav-1(-/-) mammary glands, including CAPER--an ER co-activator gene and putative stem/progenitor cell marker. Analysis of human breast cancer samples revealed that CAPER is overexpressed and undergoes a cytoplasmic-to-nuclear shift during the transition from pre-malignancy to ductal carcinoma in situ. Thus, Cav-1(-/-) null mice are a new preclinical model for studying the molecular paradigm of estrogen hypersensitivity and the development of estrogen-dependent ductal carcinoma in situ lesions.

Proteomic analysis of the mouse mammary gland is a powerful tool to identify novel proteins that are differentially expressed during mammary development

After lactation, the mouse mammary gland undergoes apoptosis and tissue remodelling as the gland reverts to its prepregnant state. This complex change was investigated using 2-DE. An integrated database was produced from lactation and involution proteomes. Forty-four molecular cluster indexes (MCIs) that showed altered expression from lactation to involution were selected for MS analysis. Of these, 32 gave protein annotations, 18 of which were unequivocal proteins. Selected proteins were then studied across all of development, including pregnancy, using data integrated from another proteome database. Two proteins, the RNA polymerase B transcription factor 3 (BTF3) and the minichromosome maintenance protein 3 (MCM3), although initially selected on the basis of the lactation/involution criteria, had expression profiles that indicated an additional role in mammary development and were further analysed. BTF3, a transcription factor previously not described in the mammary gland, was up-regulated strongly in pregnancy, indicating an involvement in alveolar growth. MCM3's expression was greatest in pregnancy and late involution, decreasing through lactation. Immunohistochemistry localised MCM3 to the mammary epithelium, where a greater proportion of cells stained than for the proliferation marker Ki67. MCM3 expression during lactation may identify cells that are licensed to repopulate the gland during cell loss in lactation and following involution.

Involvement of axonal guidance proteins and their signaling partners in the developing mouse mammary gland

Mammary morphogenesis in the mouse is driven by specialized structures at the ends of the developing ducts, the terminal end buds (TEB). The mechanisms controlling the precise branching and spacing of the ducts are, as yet, unknown. To identify genes that are associated with migration of TEB and differentiation of the subtending ducts, we developed a novel method of isolating TEB and ducts free of stroma, and compared the gene expression profiles of these two isolates using oligonucleotide microarrays. Ninety one genes were upregulated in TEB compared to ducts. Three of these genes, Sprr1A, Sema3B, and BASP1, are associated with axonal growth and guidance. Two additional members of the Sprr family, Sprr2A and 2B, not previously associated with axonal growth, were also highly expressed in TEB. Expression of these genes was confirmed by RT-PCR and Western blotting, and the cellular distribution of Sprr1A and BASP1 was demonstrated by immunohistochemistry. Other semaphorins, including Sema3C, 4A, 4F and the cancer invasion associated Sema 4D were also expressed in the mouse mammary gland along with the semaphorin receptors, Plexins A2, A3, B2, and D1, and Neuropilins 1 and 2. These results are discussed in the context of other proteins expressed in the developing gland that are known to be downstream effectors of these signaling molecules. We suggest that these genes may influence ductal growth and morphogenesis in the developing mammary gland.

Annexin A8 is up-regulated during mouse mammary gland involution and predicts poor survival in breast cancer

PURPOSE

Microarray studies have linked Annexin A8 RNA expression to a "basal cell-like" subset of breast cancers, including BRCA1-related cancers, that are characterized by cytokeratin 5 (CK5) and CK17 expression and show poor prognosis. We assessed Annexin A8's contribution to the overall prognosis and its expression in normal, benign, and cancerous tissue and addressed Annexin A8's physiologic role in the mammary gland.

EXPERIMENTAL DESIGN

Using microarrays and reverse transcription-PCR, the Annexin A8 expression was studied during mouse mammary gland development and in isolated mammary structures. Reverse transcription-PCR on cultured human luminal and basal cells, along with immunocytochemistry on normal and benign breast tissues, was used for cellular localization. Annexin A8's prognostic relevance and its coexpression with CK5 were assessed on tissue arrays of 1,631 cases of invasive breast cancer. Coexpression was further evaluated on a small cohort of 14 BRCA1-related breast cancers.

RESULTS

Annexin A8 was up-regulated during mouse mammary gland involution and in pubertal ductal epithelium. Annexin A8 showed preferred expression in cultured basal cells but predominant luminal expression in normal human breast tissue in vivo. Hyperplasias and in situ carcinomas showed a strong staining of basal cells. Annexin A8 expression was significantly associated with grade (P < 0.0001), CK5 (P < 0.0001), and estrogen receptor status (P < 0.0001); 85.7% BRCA1-related breast tumors coexpressed Annexin A8 and CK5.

CONCLUSION

Annexin A8 is involved in mouse mammary gland involution. In humans, it is a luminally expressed protein with basal expression in cell culture and in hyperplasia/ductal carcinoma in situ. Expression in invasive breast carcinomas has a significant effect on survival (P = 0.03) but is not independent of grade or CK5.