Breast Cancer Stem Cells and Tumor Suppressor Genes

Breast Cancer Stem Cells and Sex Steroid Hormones

Breast cancer stem cells (BCSCs) are a small population of tumor-initiating cells that express stem cell-associated markers. In recent years, their properties and mechanisms of regulation have become the focus of intense research due to their intrinsic resistance to conventional cancer therapies. This review describes breast cancer stem cell origin, signaling pathways involved in self-renewal, such as Wnt, Notch and Hedgehog, biomarkers linked to stemness, and the role of sex steroid hormones in BCSC regulation.

Kruppel-like factor 4 signals through microRNA-206 to promote tumor initiation and cell survival

Tumor cell heterogeneity poses a major hurdle in the treatment of cancer. Mammary cancer stem-like cells (MaCSCs), or tumor-initiating cells, are highly tumorigenic sub-populations that have the potential to self-renew and to differentiate. These cells are clinically important, as they display therapeutic resistance and may contribute to treatment failure and recurrence, but the signaling axes relevant to the tumorigenic phenotype are poorly defined. The zinc-finger transcription factor Kruppel-like factor 4 (KLF4) is a pluripotency mediator that is enriched in MaCSCs. KLF4 promotes RAS-extracellular signal-regulated kinase pathway activity and tumor cell survival in triple-negative breast cancer (TNBC) cells. In this study, we found that both KLF4 and a downstream effector, microRNA-206 (miR-206), are selectively enriched in the MaCSC fractions of cultured human TNBC cell lines, as well as in the aldehyde dehydrogenase-high MaCSC sub-population of cells derived from xenografted human mammary carcinomas. The suppression of endogenous KLF4 or miR-206 activities abrogated cell survival and in vivo tumor initiation, despite having only subtle effects on MaCSC abundance. Using a combinatorial approach that included in silico as well as loss- and gain-of-function in vitro assays, we identified miR-206-mediated repression of the pro-apoptotic molecules programmed cell death 4 (PDCD4) and connexin 43 (CX43/GJA1). Depletion of either of these two miR-206-regulated transcripts promoted resistance to anoikis, a prominent feature of CSCs, but did not consistently alter MaCSC abundance. Consistent with increased levels of miR-206 in MaCSCs, the expression of both PDCD4 and CX43 was suppressed in these cells relative to control cells. These results identify miR-206 as an effector of KLF4-mediated prosurvival signaling in MaCSCs through repression of PDCD4 and CX43. Consequently, our study suggests that a pluripotency factor exerts prosurvival signaling in MaCSCs, and that antagonism of KLF4-miR-206 signaling may selectively target the MaCSC niche in TNBC.

Expression distribution of cancer stem cells, epithelial to mesenchymal transition, and telomerase activity in breast cancer and their association with clinicopathologic characteristics

A total of 167 surgically resected primary invasive breast carcinomas and 63 metastatic lymph node lesions were analyzed for immunohistochemical (IHC) localization of the CD44⁺CD24^−low breast cancer stem cell (CSC) markers, epithelial to mesenchymal transition (EMT) markers, and telomerase activity by double-staining IHC technique, in formalin-fixed, paraffin-embedded tissue, the results were validated by double-staining immunofluorescent and flow cytometry techniques. The results showed that CSCs with CD44⁺CD24^−low phenotype were significantly increased in node-positive tumors, high-grade tumors, and ductal carcinoma in situ (DCIS). There was a high incidence of telomerase expression in metastatic lymph node lesion. There were considerably high number of tumor cells with EMT expression in metastatic lymph node lesion, and triple-negative tumor. The occurrence of EMT phenomena was usually accompanied by the co-existence of CSCs of CD44⁺CD24^−low phenotype. There was no association between the existence of CSCs and detection of telomerase activity in tumor cells. Increased numbers of both CSCs of CD44⁺CD24^−low phenotype and cells underwent EMT in DCIS lesion might be an initial step in the stromal invasion and propagation of breast cancer, and occurrence of EMT in the breast tumor associated with high prevalence of CSCs, promoting tumor invasiveness and metastasis.

Effect of bFGF on the MCF-7 Cell Cycle with CD44(+)/CD24(-): Promoting the G0/G1→G2/S Transition

Purpose

Few cells with stem cell characteristics possess capabilities of self-renewal and differentiation, which leads to high tumorigenesis and resistance to standard chemotherapeutic agents. These cells are mostly quiescent, and arrest occurs at the mitotic G0/G1 phase in mitosis. We explored the effects of basic fibroblast growth factor (bFGF) on the MCF-7 cell cycle with CD44⁺/CD24^-.

Methods

Cancer-initiating cells were propagated as mammospheres. The CD44⁺/CD24^- subpopulation was sorted by a fluorescence activating cell sorter-Vantage flow cytometer. A cell cycle analysis was performed with different bFGF concentrations.

Results

Differences in the CD44⁺/CD24^- cell proliferation under different bFGF concentrations were observed (p=0.001). When the bFGF concentration was increased, the proportion of CD44⁺/CD24^- at G0/G1 decreased (p=0.023).

Conclusion

We conclude that bFGF may sustain CD44⁺/CD24^- cell proliferation and could promote cell progression through the G0/G1→G2/S phase transition.

Biomarkers to Target Heterogeneous Breast Cancer Stem Cells

Breast cancer is the most common cancer and the second leading cause of death in U.S. women. Due to early detection and advanced treatment, the breast cancer death rate has been declining since 1990. However, disease recurrence is still the major obstacle in moving from therapy to truly curative treatments. Recent evidence has indicated that breast cancer recurrence is often caused by a subpopulation of breast cancer cells. This subset of cancer cells, usually referred to as breast cancer stem cells (BCSCs), exhibits stem cell phenotypes. They can self-renew and asymmetrically divide to more differentiated cancer cells. These cells are also highly resistant to conventional therapeutic reagents. Therefore, identifying and characterizing these BCSC subpopulations within the larger population of breast cancer cells is essential for developing new strategies to treat breast cancer and prevent recurrence. In this review article, we discuss the current proposed model for the origin of tumor heterogeneity, summarize the recent findings of cell surface and cytoplasmic markers for BCSC identification, review the regulatory mechanisms by which BCSCs maintain or non-cancer stem cells acquire BCSC characteristics, describe the proposed strategies to eliminate BCSCs, and highlight the current limitations and challenges to translate basic BCSC research to clinical application including establishment of clinical biomarkers and therapeutic treatments specifically targeting BCSCs.

Why does oestrogen-only hormone therapy have such a small impact on breast cancer risk? A hypothesis

There seems to be irrefutable evidence that oestrogen is involved in the pathogenesis of breast cancer. The disease mostly affects women and the epidemiology of breast cancer relates to reproductive markers such as pregnancy, age at menarche and age of menopause. Most breast cancers elaborate oestrogen receptors (ER) and in such cases endocrine therapies such as tamoxifen and aromatase-inhibitors (AIs) are effective adjuvant treatments. However, high-quality randomised controlled trials (RCTs) (such as the WHI study) have shown that oestrogen-only hormone therapy (ET) does not increase breast cancer risk at all. This would seem to be a remarkable paradox. There appears to be at least two reasons for this apparent contradiction. First, it has been known for two decades that the breast itself produces oestrogens locally and the microenvironment around a breast cancer is more important that the impact of systemic-oestrogens. Second, breast cancer stem cells (breast CSC) have been identified and it seems likely that these long-lived, multipotential cells are responsible for the genesis of many breast cancers, as well as their malignant behaviour. Breast CSC usually do not contain sex-hormone receptors, but their offspring often elaborate ER and progesterone receptor (PR). Thus, it appears unlikely that oestrogen per se initiates breast cancer, but rather might stimulate an existing tumour.

Transgelin promotes migration and invasion of cancer stem cells

Recent studies have suggested the existence of a small subset of cancer cells called cancer stem cells (CSCs), which possess the ability to initiate malignancies, promote tumor formation, drive metastasis, and evade conventional chemotherapies. Elucidation of the specific signaling pathway and mechanism underlying the action of CSCs might improve the efficacy of cancer treatments. In this study, we analyzed differentially expressed proteins between tumerigenic and nontumorigenic cells isolated from the human hepatocellular carcinoma (HCC) cell line, Huh7, via proteomic analysis to identify proteins correlated with specific features of CSCs. The expression level of Transgelin was 25-fold higher in tumorigenic cells than nontumorigenic cells. Similar results were also observed in tumorigenic cells derived from colorectal adenocarcinoma and prostate carcinoma. More importantly, the elevated levels of Transgelin significantly increased the invasiveness of tumorigenic cells, whereas reduced levels decreased the invasive potential. Moreover, in tumors derived from Huh7-induced xenografts, Transgelin was also co-expressed with CXCR4, which is responsible for tumor invasion. Taken together, these results indicate that the metastatic potential of CSCs arises from highly expressed Transgelin.

Breast cancer, stem cells and sex hormones: part 1. The impact of fetal life and infancy

Like other organs, the breast contains rare somatic stem cells (SCs) that are long-lived and slowly dividing. In the adult breast, they are closely regulated in areas located along the breast ducts called SC niches. Breast SCs can produce offspring that become ductal, alveoli or myoepithelial cells. In fetal life, SCs form the primitive breast ducts and up to 30 weeks of gestational age, this process appears to be largely independent of estrogen. Early life risk factors for breast cancer include birth weight, rapid growth during infancy and diet. The impact of these risk factors may be mediated through SC number. These somatic breast SCs persist into adult life and so they are exposed to oncogenic influences for much longer than the short-lived differentiated breast ductal and alveolar cells. As such, it is likely that the breast SC is a prominent target for carcinogenesis and so SC number may be an important determinant of breast cancer risk later in life.