Classically, isolation of CSCs from tumors exploits the detection of cell surface markers associated with normal stem cells. Invariable expression of these cell surface markers in almost all proliferating tumor cells that albeit impart specific functionality, the universality, and clinical credibility of CSC phenotype based on markers is still dubious. Side Population (SP) cells, as defined by Hoechst dye exclusion in flow cytometry, have been identified in many solid tumors and cell lines and the SP phenotype can be considered as an enriched source of stem cells as well as an alternative source for the isolation of cancer stem cells especially when molecular markers for stem cells are unknown. SP cells may be responsible for the maintenance and propagation of tumors and the proportion of SP cells may be a predictor of patient outcome. Several of these markers used in cell sorting have emerged as prognostic markers of disease progression though it is seen that the development of new CSC-targeted strategies is often hindered by poor understanding of their regulatory networks and functions. This review intends to appraise the experimental progress towards enhanced isolation and drug screening based on property of acquired chemoresistance of cancer stem cells.

Integrin αvβ6 is involved in the transition from ductal carcinoma in situ (DCIS) to invasive ductal carcinoma (IDC) of the breast. In addition, integrin β6 (ITGB6) is of prognostic value in invasive breast cancers, particularly in HER2+ subtype. However, pathways mediating the activity of integrin αvβ6 in clinical progression of invasive breast cancers need further elucidation. We have examined human breast cancer specimens (N = 460) for the expression of integrin β6 (ITGB6) mRNA by qPCR. In addition, we have examined a subset (N = 147) for the expression of αvβ6 integrin by immunohistochemistry (IHC). The expression levels of members of Rho–Rac pathway including downstream genes (ACTR2, ACTR3) and effector proteinases (MMP9, MMP15) were estimated by qPCR in the HER2+ subset (N = 59). There is a significant increase in the mean expression of ITGB6 in HER2+ tumors compared to HR+HER2‐ and triple negative (TNBC) subtypes (P = 0.00). HER2+ tumors with the highest levels (top quartile) of ITGB6 have significantly elevated levels of all the genes of the Rho–Rac pathway (P‐values from 0.01 to 0.0001). Patients in this group have a significantly shorter disease‐free survival compared to the group with lower ITGB6 levels (HR = 2.9 (0.9–8.9), P = 0.05). The mean level of ITGB6 expression is increased further in lymph node‐positive tumors. The increased regional and distant metastasis observed in HER2+ tumors with high levels of ITGB6 might be mediated by the canonical Rho–Rac pathway through increased expression of MMP9 and MMP15.

ACE2 a key molecule of the Renin-Angiotensin system has been identified as the receptor for SARS-CoV-2 entry into human cells. In the context of human cancers, there is evidence that ACE2 might function as a tumor suppressor. The expression levels of ACE2 among the different subtypes of breast cancer has not been investigated. We have examined the differential expression of ACE2 and its correlation with prognosis in breast cancer subtypes using the METABRIC (n = 1898) and TCGA (n = 832) cohorts. Correlations were evaluated by Pearsons's correlation co-efficient and Kaplan-Meier analysis was used to estimate differences in disease-free survival between the ACE2 high and ACE2 low groups. There is minimal expression of ACE2 in the luminal classes, but significantly higher levels in the Basal-like and HER2-enriched subclasses. Metastatic biopsies of these tumor types also show enhanced expression of ACE2. High levels of ACE2 correlated with decreased disease-free survival in the HER2-enriched subtype, and it was positively correlated with EGFR expression. These observations suggest ACE2 might function as a context dependent factor driving tumor progression in breast cancer and permit new opportunities for targeted therapy.

Despite the established benefits of long‐term endocrine therapy, women with hormone receptor‐positive breast cancer remain at risk for late relapse. The basis of this is multi‐factorial including genetic, epigenetic, and host factors. In this study we have explored the epigenetic regulation of estrogen receptor (ER)‐dependent molecular and cellular phenotype by hsa‐miR‐18a‐5p using well‐established human ER‐positive (ER+) breast cancer cell lines. miR‐18a was overexpressed in MCF7 and ZR‐75‐1 and this led to an increase in the proliferative ability of the cells and concurrently resulted in decreased expression of luminal markers and higher expression of the basal marker, cytokeratin 14. The cells became more migratory with a significant repression of E‐cadherin and activation of the Wnt noncanonical pathway. We observed an activation of the planar cell polarity (PCP) pathway with increased activation of JNK pathway and eventually change in actin dynamics. There was increased F‐actin polymerization in cells with higher expression of miR‐18a. Examination of miR‐18a expression in a set of human ER+ breast cancer specimens showed a negative correlation between miR‐18a and ESR1 transcripts as well as ER protein. Kaplan‐Meier survival analysis of the cohort stratified by tumor hsa‐miR‐18a‐5p levels produced significant differences in disease‐free survival (log rank P < .05). This observation was independently validated in the METABRIC cohort. These data provide support for a role of hsa‐miR‐18a‐5p in altering the proliferative and migratory behavior of ER+ cells and its potential utility as a prognostic marker in clinical ER+ breast cancers.