Stem Cells and Cellular Origins of Mammary Gland: Updates in Rationale, Controversies, and Cancer Relevance

A review of biological targets and therapeutic approaches in the management of triple-negative breast cancer

BACKGROUND

Triple-negative breast cancer (TNBC) is a heterogeneous, aggressive phenotype of breast cancer with associated chemoresistance. The development of chemo- or radioresistance could be attributed to diverse tumor microenvironments, overexpression of membrane proteins (transporters), epigenetic changes, and alteration of the cell signaling pathways/genes associated with the development of cancer stem cells (CSCs).

AIM OF REVIEW

Due to the diverse and heterogeneous nature of TNBC, therapeutic response to the existing modalities offers limited scope and thus results in reccurance after therapy. To establish landmark therapeutic efficacy, a number of novel therapeutic modalities have been proposed. In addition, reversal of the resistance that developed during treatment may be altered by employing appropriate therapeutic modalities. This review aims to discuss the plethora of investigations carried out, which will help readers understand and make an appropriate choice of therapy directed toward complete elimination of TNBC.

KEY SCIENTIFIC CONCEPTS OF REVIEW

This manuscript addresses the major contributory factors from the tumor microenvironment that are responsible for the development of chemoresistance and poor prognosis. The associated cellular events and molecular mechanism-based therapeutic interventions have been explained in detail. Inhibition of ABC transporters, cell signaling pathways associated with CSCs, and epigenetic modification offers promising results in this regard. TNBC progression, invasion, metastasis and recurrence can also be inhibited by blocking multiple cell signaling pathways, targeting specific receptors/epigenetic targets, disrupting bioenergetics and generating reactive oxygen species (ROS).

Effect, sensitivity, specificity and accuracy of ultrasonic assessment of axillary lymph node-negative breast cancer

Objective

To investigate the diagnostic value of ultrasound for patients with axillary lymph node-negative breast cancer (ALNNBC).

Methods

A retrospective analysis was performed on the clinical data of 204 breast cancer patients who were admitted by Quanzhou First Hospital Affiliated to Fujian Medical University between October 2020 and May 2022. According to the results of axillary lymph node (ALN) examination, the patients were assigned to a positive group(n=102) and a negative group(n=102). All patients underwent diagnosis with color Doppler ultrasound, with pathological diagnosis as the "gold standard" to determine the sensitivity and specificity of ultrasonic diagnosis. A receiver operating characteristic(ROC) curve was established to analyze the efficiency of ultrasonic diagnosis and compare the ultrasonographic features and flow grades between the two groups.

Results

Differences were statistically significant between the two groups in ultrasonographic features of lesions(negative vs positive, all p<0.05), including morphological irregularity(59.8% vs 85.3%), spiky margins(19.6% vs 63.7%), posterior echo attenuation(19.6% vs 44.1%) and microcalcification(40.2% vs 55.89%). The negative group had a lower proportion of patients with grade 2-3 ultrasound blood flow when compared with the positive group(32.4% vs 56.86%), and the difference was statistically significant(p<0.05). Ultrasonic diagnosis of ALNNBC had a sensitivity of 88.24%(90/102), a specificity of 92.16%(94/102), a coincidence rate of 90.20% (184/204), a 95% CI of 0.845-0.928, and an AUC of 0.879.

Conclusions

Ultrasonic diagnosis of ALNNBC is relatively efficient as ultrasonographic features and ultrasound blood flow signals can provide a scientific basis for the diagnosis of ALNNBC.

Macrophage-Secreted CSF1 Transmits a Calorie Restriction-Induced Self-Renewal Signal to Mammary Epithelial Stem Cells

Calorie restriction enhances stem cell self-renewal in various tissues, including the mammary gland. We hypothesized that similar to their intestinal counterparts, mammary epithelial stem cells are insulated from sensing changes in energy supply, depending instead on niche signaling. The latter was investigated by subjecting cultures of mammary epithelial stem cells for 8 days to in vivo paracrine calorie-restriction signals collected from a 4-day-conditioned medium of individual mammary cell populations. Conditioned medium from calorie-restricted non-epithelial cells induced latent cell propagation and mammosphere formation—established markers of stem cell self-renewal. Combined RNA-Seq, immunohistochemistry and immunofluorescence analyses of the non-epithelial population identified macrophages and secreted CSF1 as the energy sensor and paracrine signal, respectively. Calorie restriction-induced pStat6 expression in macrophages suggested that skewing to the M2 phenotype contributes to the sensing mechanism. Enhancing CSF1 signaling with recombinant protein and interrupting the interaction with its highly expressed receptor in the epithelial stem cells by neutralizing antibodies were both affected stem cell self-renewal. In conclusion, combined in vivo, in vitro and in silico studies identified macrophages and secreted CSF1 as the energy sensor and paracrine transmitter, respectively, of the calorie restriction-induced effect on mammary stem cell self-renewal.

Integration of Single-Cell Transcriptomics With a High Throughput Functional Screening Assay to Resolve Cell Type, Growth Kinetics, and Stemness Heterogeneity Within the Comma-1D Cell Line

Cell lines are one of the most frequently implemented model systems in life sciences research as they provide reproducible high throughput testing. Differentiation of cell cultures varies by line and, in some cases, can result in functional modifications within a population. Although research is increasingly dependent on these in vitro model systems, the heterogeneity within cell lines has not been thoroughly investigated. Here, we have leveraged high throughput single-cell assays to investigate the Comma-1D mouse cell line that is known to differentiate in culture. Using scRNASeq and custom single-cell phenotype assays, we resolve the clonal heterogeneity within the referenced cell line on the genomic and functional level. We performed a cohesive analysis of the transcriptome of 5,195 sequenced cells, of which 85.3% of the total reads successfully mapped to the mm10-3.0.0 reference genome. Across multiple gene expression analysis pipelines, both luminal and myoepithelial lineages were observed. Deep differential gene expression analysis revealed eight subclusters identified as luminal progenitor, luminal differentiated, myoepithelial differentiated, and fibroblast subpopulations-suggesting functional clustering within each lineage. Gene expression of published mammary stem cell (MaSC) markers Epcam, Cd49f, and Sca-1 was detected across the population, with 116 (2.23%) sequenced cells expressing all three markers. To gain insight into functional heterogeneity, cells with patterned MaSC marker expression were isolated and phenotypically investigated through a custom single-cell high throughput assay. The comparison of growth kinetics demonstrates functional heterogeneity within each cell cluster while also illustrating significant limitations in current cell isolation methods. We outlined the upstream use of our novel automated cell identification platform-to be used prior to single-cell culture-for reduced cell stress and improved rare cell identification and capture. Through compounding single-cell pipelines, we better reveal the heterogeneity within Comma-1D to identify subpopulations with specific functional characteristics.

Single-cell analysis reveals Comma-1D as a unique cell model for mammary gland development and breast cancer

The mammary epithelial tree contains two distinct populations, luminal and basal. The investigation of how this heterogeneity is developed and how it influences tumorigenesis has been hampered by the need to perform these studies using animal models. Comma-1D is an immortalized mouse mammary epithelial cell line that has unique morphogenetic properties. By performing single-cell RNA-seq studies we found that Comma-1D cultures consist of two main populations with luminal and basal features and a smaller population with mixed lineage and bipotent characteristics. We demonstrated that multiple transcription factors associated with the differentiation of the mammary epithelium in vivo also modulate this process in Comma-1D cultures. Additionally, we found that only cells with luminal features were able to acquire transformed characteristics after an oncogenic HER2 mutant was introduced in their genomes. Overall, our studies characterize at a single-cell level the heterogeneity of the Comma-1D cell line and illustrate how Comma-1D cells can be used as an experimental model to study both the differentiation and the transformation processes in vitro.

Characterization of murine mammary stem/progenitor cells in a D-galactose-induced aging model

Aging plays an important role in many diseases, including breast cancer. Aged mammary stem/progenitor cells are perceived to be the cells of origin in breast tumorigenesis; however, the extensive use of mice who have aged naturally for research is hampered by cost, time, disease complications, and high mortality. In this study, we characterized murine mammary stem/progenitor cells in a D-galactose-induced accelerated aging model and compared them with findings from our earlier study on mice from natural aging. Our results showed that mammary glands in the D-galactose-induced aging model mimic natural aging in terms of pathological changes, epithelial cell composition, and mammary stem/progenitor cell function. These changes are accompanied by elevated inflammatory responses both systemically in the blood and locally in the mammary glands, which is similar in mice who age naturally. Our study for the first time evaluated the mammary glands and mammary stem/progenitor function in a D-galactose-induced aging model in rodents, and our findings suggest that D-galactose treatment can be used as a surrogate to study the role aged stem/progenitor cells play in breast tumorigenesis.

Characterization of Gene Expression Signatures for the Identification of Cellular Heterogeneity in the Developing Mammary Gland

The developing mammary gland depends on several transcription-dependent networks to define cellular identities and differentiation trajectories. Recent technological advancements that allow for single-cell profiling of gene expression have provided an initial picture into the epithelial cellular heterogeneity across the diverse stages of gland maturation. Still, a deeper dive into expanded molecular signatures would improve our understanding of the diversity of mammary epithelial and non-epithelial cellular populations across different tissue developmental stages, mouse strains and mammalian species. Here, we combined differential mammary gland fractionation approaches and transcriptional profiles obtained from FACS-isolated mammary cells to improve our definitions of mammary-resident, cellular identities at the single-cell level. Our approach yielded a series of expression signatures that illustrate the heterogeneity of mammary epithelial cells, specifically those of the luminal fate, and uncovered transcriptional changes to their lineage-defined, cellular states that are induced during gland development. Our analysis also provided molecular signatures that identified non-epithelial mammary cells, including adipocytes, fibroblasts and rare immune cells. Lastly, we extended our study to elucidate expression signatures of human, breast-resident cells, a strategy that allowed for the cross-species comparison of mammary epithelial identities. Collectively, our approach improved the existing signatures of normal mammary epithelial cells, as well as elucidated the diversity of non-epithelial cells in murine and human breast tissue. Our study provides a useful resource for future studies that use single-cell molecular profiling strategies to understand normal and malignant breast development.

Expression of NR5A2, NUP153, HNF4A, USP15 and FNDC3B is consistent with their use as novel biomarkers for bovine mammary stem/progenitor cells

Mammary stem cells (MaSC) are essential for growth and maintenance of mammary epithelium. Previous studies have utilized morphological characteristics or retention of bromodeoxyuridine (BrdU) label to identify MaSC and progenitor cells, these approaches may not be feasible or may not identify all resident stem cells. Alternatively, these special cells may be identified by assessing protein and mRNA expression of appropriate markers. The focus of this study was to assess the staining patterns and in situ quantification of novel candidate markers for bovine MaSC/progenitor cells. The candidate markers for MaSC/progenitor cells for immunohistochemical analysis were: NR5A2, NUP153, HNF4A, USP15 and FNDC3B and for in situ transcripts quantification were HNF4A and NUP153. We also evaluated protein expression pattern of presumptive MaSC markers known from the literature namely, ALDH1, MSI1 and Notch3. We found that NR5A2, NUP153, HNF4A and USP15-labeled cells represented 2.5-6% of epithelial cells prepubertally and were distributed in a fashion consistent with the location and abundance of MaSC/progenitor cells. A transient increase (10-37%) in expression of these markers was observed at peak lactation. FNDC3B was localized mainly in the nucleus prepubertally and in the cytoplasm of myoepithelial cells and nuclei of a limited number of alveolar cells during lactation. Abundant expression (~ 48%) and luminal localization of ALDH1 precludes its use as a bovine MaSC marker but may include transamplifying progenitor cells. MSI1 staining was consistent with MaSC localization. Onset of lumen formation in mammary ducts of prepubertal gland was associated with Notch 3 expression in the apical surface of luminal cells. RNAscope analysis of HNF4A and NUP153 transcripts in calf mammary gland showed very low copy numbers in a few epithelial cells, supporting the idea that these markers are expressed by fewer cells of epithelial origin. This study suggests that NR5A2, NUP153, HNF4A, USP15 and FNDC3B are likely markers for bovine MaSC/progenitor cells. Quantification of RNA transcripts of HNF4A and NUP153 in bovine MEC as potential MaSC markers are novel. Further studies to correlate protein expression of these markers with their transcripts level using single cell analysis in larger samples in lactating cow at different physiological stages are warranted.

Correlation of Clinicopathological Features and LGR5 Expression in Triple-Negative Breast Cancer

LGR5 is the most robust known stem cell marker for gastrointestinal tumors, but there are few reports in breast cancer. Triple negative breast cancer (TNBC) is the most malignant subtype of breast cancer, and thus identification of new cancer stem cell populations in TNBC may help to identify targeted therapies. LGR5 expression was evaluated by RNAscope, a newly developed RNA in situ hybridization technique, using a tissue microarray consisting of 43 patient samples of TNBC selected from the medical archives at our hospital. Patients were stratified into negative and positive LGR5 expression groups. Tumor necrosis was greater in the LGR5-positive group compared with the LGR5-negative group (P = .026). Mitosis tended to show a high value in the LGR5-positive group compared with the LGR5-negative group (P = .0831), while stage tended to show a high stage in the LGR5-positive group compared with the LGR5-negative group (P = .0617). Cox proportional hazards models revealed that the LGR5-positive group (overall survival (OS) = 2.12; 95% CI: 2.12-2.12; P = 0.1575) had no relationship with OS. LGR5 expression is associated with tumor necrosis of TNBC and suggested higher malignant potential.