FZD7 has a critical role in cell proliferation in triple negative breast cancer

Identification and characterization of a small-molecule inhibitor of Wnt signaling in glioblastoma cells

Glioblastoma multiforme (GBM) is the most common and prognostically unfavorable form of brain tumor. The aggressive and highly invasive phenotype of these tumors makes them among the most anatomically damaging human cancers with a median survival of less than 1 year. Although canonical Wnt pathway activation in cancers has been historically linked to the presence of mutations involving key components of the pathway (APC, β-catenin, or Axin proteins), an increasing number of studies suggest that elevated Wnt signaling in GBM is initiated by several alternative mechanisms that are involved in different steps of the disease. Therefore, inhibition of Wnt signaling may represent a therapeutically relevant approach for GBM treatment. After the selection of a GBM cell model responsive to Wnt inhibition, we set out to develop a screening approach for the identification of compounds capable of modulating canonical Wnt signaling and associated proliferative responses in GBM cells. Here, we show that the small molecule SEN461 inhibits the canonical Wnt signaling pathway in GBM cells, with relevant effects at both molecular and phenotypic levels in vitro and in vivo. These include SEN461-induced Axin stabilization, increased β-catenin phosphorylation/degradation, and inhibition of anchorage-independent growth of human GBM cell lines and patient-derived primary tumor cells in vitro. Moreover, in vivo administration of SEN461 antagonized Wnt signaling in Xenopus embryos and reduced tumor growth in a GBM xenograft model. These data represent the first demonstration that small-molecule-mediated inhibition of Wnt signaling may be a potential approach for GBM therapeutics.

Modulation of the biliary expression of arylalkylamine N-acetyltransferase alters the autocrine proliferative responses of cholangiocytes in rats

Secretin stimulates ductal secretion by interacting with secretin receptor (SR) activating cyclic adenosine 3',5'-monophosphate/cystic fibrosis transmembrane conductance regulator/chloride bicarbonate anion exchanger 2 (cAMP⇒CFTR⇒Cl(-) /HCO 3- AE2) signaling that is elevated by biliary hyperplasia. Cholangiocytes secrete several neuroendocrine factors regulating biliary functions by autocrine mechanisms. Melatonin inhibits biliary growth and secretin-stimulated choleresis in cholestatic bile-duct-ligated (BDL) rats by interaction with melatonin type 1 (MT1) receptor through down-regulation of cAMP-dependent signaling. No data exist regarding the role of melatonin synthesized locally by cholangiocytes in the autocrine regulation of biliary growth and function. In this study, we evaluated the (1) expression of arylalkylamine N-acetyltransferase (AANAT; the rate-limiting enzyme for melatonin synthesis from serotonin) in cholangiocytes and (2) effect of local modulation of biliary AANAT expression on the autocrine proliferative/secretory responses of cholangiocytes. In the liver, cholangiocytes (and, to a lesser extent, BDL hepatocytes) expressed AANAT. AANAT expression and melatonin secretion (1) increased in BDL, compared to normal rats and BDL rats treated with melatonin, and (2) decreased in normal and BDL rats treated with AANAT Vivo-Morpholino, compared to controls. The decrease in AANAT expression, and subsequent lower melatonin secretion by cholangiocytes, was associated with increased biliary proliferation and increased SR, CFTR, and Cl(-) /HCO 3- AE2 expression. Overexpression of AANAT in cholangiocyte cell lines decreased the basal proliferative rate and expression of SR, CFTR, and Cl(-) /HCO 3- AE2 and ablated secretin-stimulated biliary secretion in these cells.

CONCLUSION

Local modulation of melatonin synthesis may be important for management of the balance between biliary proliferation/damage that is typical of cholangiopathies. (HEPATOLOGY 2013).

The C-terminal region Mesd peptide mimics full-length Mesd and acts as an inhibitor of Wnt/β-catenin signaling in cancer cells

While Mesd was discovered as a specialized molecular endoplasmic reticulum chaperone for the Wnt co-receptors LRP5 and LRP6, recombinant Mesd protein is able to bind to mature LRP5 and LRP6 on the cell surface and acts as a universal antagonist of LRP5/6 modulators. In our previous study, we found that the C-terminal region of Mesd, which is absent in sequences from invertebrates, is necessary and sufficient for binding to mature LRP6 on the cell surface. In the present studies, we further characterized the interaction between the C-terminal region Mesd peptide and LRP5/6. We found that Mesd C-terminal region-derived peptides block Mesd binding to LRP5 at the cell surface too. We also showed that there are two LRP5/6 binding sites within Mesd C-terminal region which contain several positively charged residues. Moreover, we demonstrated that the Mesd C-terminal region peptide, like the full-length Mesd protein, blocked Wnt 3A- and Rspodin1-induced Wnt/β-catenin signaling in LRP5- and LRP6- expressing cells, suppressed Wnt/β-catenin signaling in human breast HS578T cells and prostate cancer PC-3 cells, and inhibited cancer cell proliferation, although the full-length Mesd protein is more potent than its peptide. Finally, we found that treatment of the full-length Mesd protein and its C-terminal region peptide significantly increased chemotherapy agent Adriamycin-induced cytotoxicity in HS578T and PC-3 cells. Together, our results suggest that Mesd C-terminal region constitutes the major LRP5/6-binding domain, and that Mesd protein and its C-terminal region peptide have a potential therapeutic value in cancer.

Tumor-initiating cells and FZD8 play a major role in drug resistance in triple-negative breast cancer

Triple-negative breast cancer (TNBC) studies have shown that neoadjuvant chemotherapy before surgery was effective in the minority of women, whereas the majority who had residual tumor had a relatively poor outcome. To identify the mechanism by which residual cancer cells survive chemotherapy, we initially conducted gene expression profiling using the CRL2335 TNBC cell line derived from a squamous breast carcinoma before and after treatment with cisplatin plus TRAIL. We found a significant increase in the expression of FZD8, one of Wnt receptors, and its downstream targets LEF1 and TCF in residual CRL2335 tumor cells after treatment with cisplatin plus TRAIL. Increased FZD8 levels were further confirmed in other TNBC cell lines. Inhibition of FZD8 by siRNA in CRL2335 cells in the presence of cisplatin plus TRAIL reduced β-catenin and survivin levels and increased apoptosis compared with scrambled siRNA-treated cells. In vivo data show that cisplatin plus TRAIL treatment significantly reduces tumor volume in NOD/SCID mice. However, we found that cisplatin plus TRAIL treatment predominantly eliminated non-tumor-initiating cells, as shown by whole-body fluorescent imaging of mice injected with mammosphere-forming CRL2335 cells stably transfected with DsRed. This led to TIC enrichment in residual tumors, as confirmed by immunostaining for TIC markers. Moreover, an increase in FZD8 expression was observed in residual tumors treated with cisplatin and TRAIL. Taken together, our findings suggest that FZD8-mediated Wnt signaling may play a major role in mediating resistance to chemotherapy, making it a potential target to enhance chemotherapeutic efficacy in patients with TNBCs.

A whole-genome RNA interference screen for human cell factors affecting myxoma virus replication

Myxoma virus (MYXV) provides an important model for investigating host-pathogen interactions. Recent studies have also highlighted how mutations in transformed human cells can expand the host range of this rabbit virus. Although virus growth depends upon interactions between virus and host proteins, the nature of these interactions is poorly understood. To address this matter, we performed small interfering RNA (siRNA) screens for genes affecting MYXV growth in human MDA-MB-231 cells. By using siRNAs targeting the whole human genome (21,585 genes), a subset of human phosphatases and kinases (986 genes), and also a custom siRNA library targeting selected statistically significant genes ("hits") and nonsignificant genes ("nonhits") of the whole human genome screens (88 genes), we identified 711 siRNA pools that promoted MYXV growth and 333 that were inhibitory. Another 32 siRNA pools (mostly targeting the proteasome) were toxic. The overall overlap in the results was about 25% for the hits and 75% for the nonhits. These pro- and antiviral genes can be clustered into pathways and related groups, including well-established inflammatory and mitogen-activated protein kinase pathways, as well as clusters relating to β-catenin and the Wnt signaling cascade, the cell cycle, and cellular metabolism. The validity of a subset of these hits was independently confirmed. For example, treating cells with siRNAs that might stabilize cells in G(1), or inhibit passage into S phase, stimulated MYXV growth, and these effects were reproduced by trapping cells at the G(1)/S boundary with an inhibitor of cyclin-dependent kinases 4/6. By using 2-deoxy-D-glucose and plasmids carrying the gene for phosphofructokinase, we also confirmed that infection is favored by aerobic glycolytic metabolism. These studies provide insights into how the growth state and structure of cells affect MYXV growth and how these factors might be manipulated to advantage in oncolytic virus therapy.

Up-regulation of miR-182 by β-catenin in breast cancer increases tumorigenicity and invasiveness by targeting the matrix metalloproteinase inhibitor RECK

BACKGROUND

MiR-182 is a member of the miR-183 cluster located at human chromosome 7q32 region and is up-regulated in human cancers. We study the regulation of miR-182 expression and its oncogenic role.

METHODS

MiR-182 level was investigated by real-time reverse transcription-PCR. Chromatin immunoprecipitation assay was used to confirm promoter binding of transcription factors. The correlation between miR-182 and RECK was analyzed by Western blotting, real-time RT-PCR and 3(')-untranslated region reporter assay. Zymography, matrix metalloproteinase activity, invasion and colony formation were used to study the tumorigenic activity.

RESULTS

MiR-182 is over-expressed in human breast tumor tissues and cell lines. Inhibition or knockdown of β-catenin reduced miR-182 level in MDA-MB-231 cells. ChIP assay confirmed the binding of β-catenin on miR-182 promoter. Anti-miR-182 increased the MMP inhibitor RECK protein in MDA-MB-231 cells while pre-miR-182 reduced RECK protein but not mRNA in normal mammary epithelial H184B5F5/M10 cells. Restoration of RECK protein by anti-miR-182 attenuated MMP-9 activity, cell invasion and colony formation. Ectopic expression of miR-182 inhibited restoration of RECK protein by β-catenin inhibitor indicating miR-182 is important for β-catenin-induced down-regulation of RECK. An inverse association between miR-182 and RECK was demonstrated in breast tumor tissues.

CONCLUSIONS

We provide evidence that miR-182 is up-regulated by β-catenin signaling pathway in breast cancer and its up-regulation increases tumorigenicity and invasiveness by repressing RECK.

GENERAL SIGNIFICANCE

Our data demonstrate for the first time that miR-182 expression is controlled by β-catenin. In addition, we identify a new miR-182 target RECK which is important for miR-182-induced tumorigenesis.

SOX9 regulates low density lipoprotein receptor-related protein 6 (LRP6) and T-cell factor 4 (TCF4) expression and Wnt/β-catenin activation in breast cancer

Gene expression profiling has identified breast cancer (BCa) subtypes, including an aggressive basal-like (BL) subtype. The molecular signals underlying the behavior observed in BL-BCa group are largely unknown, although recent results indicate a prevalent increase in Wnt/β-catenin activity. Our immunohistochemistry study confirmed that SOX9, one of the BL-BCa signature genes, was expressed by most BL-BCa, and its expression correlated with indicators of poor prognosis. Importantly, BCa gene expression profiling strongly associated SOX9 with the expression of Wnt/β-catenin pathway components, LRP6 and TCF4. In cancer cell lines, SOX9 silencing reduced cell proliferation and invasion, LRP6 and TCF4 transcription, and decreased Wnt/β-catenin activation. SOX9 expression was also increased by Wnt, indicating that SOX9 is at the center of a positive feedback loop that enhances Wnt/β-catenin signaling. Consistently, SOX9 overexpression in BCa cell lines and transgenic SOX9 expression in breast epithelium caused increased LRP6 and TCF4 expression and Wnt/β-catenin activation. These results identify SOX9-mediated Wnt/β-catenin activation as one of the molecular mechanisms underlying aberrant Wnt/β-catenin activity in BCa, especially in the BL-BCa subgroup.

WNT signalling pathways as therapeutic targets in cancer

Since the initial discovery of the oncogenic activity of WNT1 in mouse mammary glands, our appreciation for the complex roles for WNT signalling pathways in cancer has increased dramatically. WNTs and their downstream effectors regulate various processes that are important for cancer progression, including tumour initiation, tumour growth, cell senescence, cell death, differentiation and metastasis. Although WNT signalling pathways have been difficult to target, improved drug-discovery platforms and new technologies have facilitated the discovery of agents that can alter WNT signalling in preclinical models, thus setting the stage for clinical trials in humans.

Functional consequences of WNT3/Frizzled7-mediated signaling in non-transformed hepatic cells

We have previously demonstrated that WNT3 and Frizzled7 (FZD7) expression levelswere upregulated in hepatocellular carcinoma (HCC) and that they directly interact to activate the canonical Wnt/β–catenin pathway in HCC cell lines. In this study, we investigated the functional consequences of WNT3 and FZD7 expression levels in non-transformed hepatic cells to address the question of whether WNT3/FZD7-mediated signal transduction could be involved in cellular transformation. After stable transfection of WNT3 and FZD7, the activation of the Wnt/β–catenin pathway was confirmed by western blot, immunostaining and quantitative real-time reverse transcriptase–PCR (qRT–PCR) analysis in two non-transformed hepatocyte-derived cell lines. In vitro characteristics of the malignant phenotype were measured, including cell proliferation, migration, invasion and anchorage-independent growth in soft agar. Stable expression of WNT3 and FZD7 in the two cell lines led to cellular accumulation of β-catenin and expression of downstream target genes activated by this pathway. In the stable WNT3/FZD7-expressing clones, hepatic cell proliferation, migration, invasion as well as soft agar colony formation were enhanced compared with the non-transformed control cells. The epithelial–mesenchymal transition (EMT) factors, Twist, Snail and Vimentin, were increased in cells expressing WNT3 and FZD7. However, the WNT3/FZD7-expressing cells did not form tumors in vivo. We conclude that activation of the WNT3/FZD7 canonical pathway has a role in the early stages of hepatocarcinogenesis by promoting the acquisition of a malignant phenotype with features of EMT.

Wnt signaling in mammary glands: plastic cell fates and combinatorial signaling

The mouse mammary gland is an outstanding developmental model that exemplifies the activities of many of the effector pathways known to organize mammalian morphogenesis; furthermore, there are well-characterized methods for the specific genetic manipulation of various mammary epithelial cell components. Among these signaling pathways, Wnt signaling has been shown to generate plasticity of fate determination, expanding the genetic programs available to cells in the mammary lineage. It is responsible first for the appearance of the mammary fate in embryonic ectoderm and then for maintaining bi-potential basal stem cells in adult mammary ductal trees. Recent technical developments have led to the separate analysis of various mammary epithelial cell subpopulations, spurring the investigation of Wnt-dependent interactions. Although Wnt signaling was shown to be oncogenic for mouse mammary epithelium even before being identified as the principle oncogenic driver for gut epithelium, conclusive data implicating this pathway as a tumor driver for breast cancer lag behind, and we examine potential reasons.

Frizzled homolog proteins, microRNAs and Wnt signaling in cancer

Wnt signaling pathways play important roles in tumorigenesis and are initiated by binding of Wnt to various receptors including frizzleds (FZDs). FZDs are one of several families of receptors comprised of FZD/LRP/ROR2/RYK in the Wnt signaling pathway. Expression of some FZD receptors are up regulated, thereby activating the Wnt signaling pathway and is correlated with cancer malignancy and patient outcomes (recurrence and survival) in many cancers. The FZD family contains ten genes in humans and their function has not been completely examined including the regulatory mechanisms of FZD genes in cancer. Knockdown of FZDs may suppress the Wnt signaling pathway resulting in decreased cell growth, invasion, motility and metastasis of cancer cells. Recently a number of microRNAs (miRNAs) have been identified and reported to be important in several cancers. MiRNAs regulate target gene expression at both the transcription and translation levels. The study of miRNA is a newly emerging field and promises to be helpful in understanding the pathogenesis of FZDs in cancer. In addition, miRNAs may be useful in regulating FZDs in cancer cells. Therefore, the aim of this review is to discuss current knowledge of the functional mechanisms of FZDs in cancer, including regulation by miRNAs and the potential for possible use of miRNAs and FZDs in future clinical applications.

Silibinin inhibits Wnt/β-catenin signaling by suppressing Wnt co-receptor LRP6 expression in human prostate and breast cancer cells

Silibinin is a natural compound isolated from milk thistle seed extracts, and has traditionally been used as a hepatoprotectant. A number of studies have also established the cancer therapeutic and chemopreventive role of silibinin in both in vitro and in vivo models. The low density lipoprotein receptor-related protein-6 (LRP6) is an essential Wnt co-receptor for the Wnt/β-catenin pathway and represents a promising target for cancer prevention and therapy. In the present study, we found that silibinin was able to repress endogenous LRP6 expression and block Wnt3A-induced LRP6 phosphorylation and Wnt/β-catenin signaling activation in HEK293 cells. Importantly, silibinin was also able to suppress endogenous LRP6 expression and phosphorylation and block Wnt/β-catenin signaling in prostate cancer PC-3 and DU-145 cells and breast cancer MDA-MB-231 and T-47D cells. Mechanistically, silibinin inhibited LRP6 promoter activity and decreased LRP6 mRNA levels in prostate and breast cancer cells. Finally, we demonstrated that silibinin displayed anticancer activity with IC(50) values comparable to those shown to suppress LRP6 expression and Wnt/β-catenin signaling activities in prostate and breast cancer cells. Our data indicate that silibinin is a novel small molecule Wnt/β-catenin signaling inhibitor by suppressing Wnt co-receptor LRP6 expression at the transcription level, and that the anti-cancer activity of silibinin is associated with its inhibitory effect on Wnt/LRP6 signaling.

Drugging Wnt signalling in cancer

Aberrant regulation of the Wnt signalling pathway has emerged as a prevalent theme in cancer biology. This chapter summarizes the research that provides a proof of concept for inhibiting Wnt signalling in cancer, the potential means by which this could be achieved, and some recent advances towards this goal. A brief discussion of molecular diagnostics and possible safety concerns is also provided.

Wnt signaling in cancer

Aberrant regulation of the Wnt signaling pathway is a prevalent theme in cancer biology. From the earliest observation that Wnt overexpression could lead to malignant transformation of mouse mammary tissue to the most recent genetic discoveries gleaned from tumor genome sequencing, the Wnt pathway continues to evolve as a central mechanism in cancer biology. This article summarizes the evidence supporting a role for Wnt signaling in human cancer. This includes a review of the genetic mutations affecting Wnt pathway components, as well as some of epigenetic mechanisms that alter expression of genes relevant to Wnt. I also highlight some research on the cooperativity of Wnt with other signaling pathways in cancer. Finally, some emphasis is placed on laboratory research that provides a proof of concept for the therapeutic inhibition of Wnt signaling in cancer.

A unique combination of male germ cell miRNAs coordinates gonocyte differentiation

The last 100 years have seen a concerning decline in male reproductive health associated with decreased sperm production, sperm function and male fertility. Concomitantly, the incidence of defects in reproductive development, such as undescended testes, hypospadias and testicular cancer has increased. Indeed testicular cancer is now recognised as the most common malignancy in young men. Such cancers develop from the pre-invasive lesion Carcinoma in Situ (CIS), a dysfunctional precursor germ cell or gonocyte which has failed to successfully differentiate into a spermatogonium. It is therefore essential to understand the cellular transition from gonocytes to spermatogonia, in order to gain a better understanding of the aetiology of testicular germ cell tumours. MicroRNA (miRNA) are important regulators of gene expression in differentiation and development and thus highly likely to play a role in the differentiation of gonocytes. In this study we have examined the miRNA profiles of highly enriched populations of gonocytes and spermatogonia, using microarray technology. We identified seven differentially expressed miRNAs between gonocytes and spermatogonia (down-regulated: miR-293, 291a-5p, 290-5p and 294*, up-regulated: miR-136, 743a and 463*). Target prediction software identified many potential targets of several differentially expressed miRNA implicated in germ cell development, including members of the PTEN, and Wnt signalling pathways. These targets converge on the key downstream cell cycle regulator Cyclin D1, indicating that a unique combination of male germ cell miRNAs coordinate the differentiation and maintenance of pluripotency in germ cells.

Frizzled7 as an emerging target for cancer therapy

Wnt proteins are secreted glycoproteins that bind to the N-terminal extra-cellular cysteine-rich domain of the Frizzled (Fzd) receptor family. The Fzd receptors can respond to Wnt proteins in the presence of Wnt co-receptors to activate the canonical and non-canonical Wnt pathways. Recent studies indicated that, among the Fzd family, Fzd7 is the Wnt receptor most commonly upregulated in a variety of cancers including colorectal cancer, hepatocellular carcinoma and triple negative breast cancer. Fzd7 plays an important role in stem cell biology and cancer development and progression. In addition, it has been demonstrated that siRNA knockdown of Fzd7, the anti-Fzd7 antibody or the extracellular peptide of Fzd7 (soluble Fzd7 peptide) displayed anti-cancer activity in vitro and in vivo mainly due to the inhibition of the canonical Wnt signaling pathway. Furthermore, pharmacological inhibition of Fzd7 by small interfering peptides or a small molecule inhibitor suppressed β-catenin-dependent tumor cell growth. Therefore, targeted inhibition of Fzd7 represents a rational and promising new approach for cancer therapy.

GPCRs and cancer

G-protein-coupled receptors (GPCRs), which represent the largest gene family in the human genome, play a crucial role in multiple physiological functions as well as in tumor growth and metastasis. For instance, various molecules like hormones, lipids, peptides and neurotransmitters exert their biological effects by binding to these seven-transmembrane receptors coupled to heterotrimeric G-proteins, which are highly specialized transducers able to modulate diverse signaling pathways. Furthermore, numerous responses mediated by GPCRs are not dependent on a single biochemical route, but result from the integration of an intricate network of transduction cascades involved in many physiological activities and tumor development. This review highlights the emerging information on the various responses mediated by a selected choice of GPCRs and the molecular mechanisms by which these receptors exert a primary action in cancer progression. These findings provide a broad overview on the biological activity elicited by GPCRs in tumor cells and contribute to the identification of novel pharmacological approaches for cancer patients.

The Wnt/β-catenin signaling pathway: a potential therapeutic target in the treatment of triple negative breast cancer

Breast cancer continues to be a serious health problem particularly in developed countries. Of particular concern is triple negative breast cancer (TNBC) which does not respond well to standard hormone therapy and is associated with poor overall patient prognosis. Recent studies indicate that Wnt/β-catenin signaling is particularly activated in TNBC, such that the Wnt receptor frizzled-7 (FZD7) and the Wnt co-receptor LRP6 were found to be up regulated in TNBC. In addition, it has been demonstrated that transcriptional knockdown of LRP6 or FZD7 in TNBC cells suppressed tumor growth in vivo. Furthermore, salinomycin, a selective breast cancer stem cell killer, was recently demonstrated to be an inhibitor of Wnt/β-catenin signaling by inducing LRP6 degradation. Therefore, the Wnt/β-catenin signaling pathway and particularly the Wnt receptors on the cell surface may serve as novel therapeutic targets for the treatment of TNBC.

Niclosamide suppresses cancer cell growth by inducing Wnt co-receptor LRP6 degradation and inhibiting the Wnt/β-catenin pathway

The Wnt/β-catenin signaling pathway is important for tumor initiation and progression. The low density lipoprotein receptor-related protein-6 (LRP6) is an essential Wnt co-receptor for Wnt/β-catenin signaling and represents a promising anticancer target. Recently, the antihelminthic drug, niclosamide was found to inhibit Wnt/β-catenin signaling, although the mechanism was not well defined. We found that niclosamide was able to suppress LRP6 expression and phosphorylation, block Wnt3A-induced β-catenin accumulation, and inhibit Wnt/β-catenin signaling in HEK293 cells. Furthermore, the inhibitory effects of niclosamide on LRP6 expression/phosphorylation and Wnt/β-catenin signaling were conformed in human prostate PC-3 and DU145 and breast MDA-MB-231 and T-47D cancer cells. Moreover, we showed that the mechanism by which niclosamide suppressed LRP6 resulted from increased degradation as evident by a shorter half-life. Finally, we demonstrated that niclosamide was able to induce cancer cell apoptosis, and displayed excellent anticancer activity with IC₅₀ values less than 1 µM for prostate PC-3 and DU145 and breast MDA-MB-231 and T-47D cancer cells. The IC₅₀ values are comparable to those shown to suppress the activities of Wnt/β-catenin signaling in prostate and breast cancer cells. Our data indicate that niclosamide is a unique small molecule Wnt/β-catenin signaling inhibitor targeting the Wnt co-receptor LRP6 on the cell surface, and that niclosamide has a potential to be developed a novel chemopreventive or therapeutic agent for human prostate and breast cancer.

Anti-spasmogenic effect of cyproheptadine on guinea-pig ileum

Triple-negative breast cancer (TNBC) is a highly aggressive form of breast cancer that lacks targeted therapy options, and patients diagnosed with TNBC have poorer outcomes than patients with other breast cancer subtypes. Emerging evidence suggests that breast cancer stem cells (BCSCs), which have tumor-initiating potential and possess self-renewal capacity, may be responsible for this poor outcome by promoting therapy resistance, metastasis, and recurrence. TNBC cells have been consistently reported to display cancer stem cell (CSC) signatures at functional, molecular, and transcriptional levels. In recent decades, CSC-targeting strategies have shown therapeutic effects on TNBC in multiple preclinical studies, and some of these strategies are currently being evaluated in clinical trials. Therefore, understanding CSC biology in TNBC has the potential to guide the discovery of novel therapeutic agents in the future. In this review, we focus on the self-renewal signaling pathways (SRSPs) that are aberrantly activated in TNBC cells and discuss the specific signaling components that are involved in the tumor-initiating potential of TNBC cells. Additionally, we describe the molecular mechanisms shared by both TNBC cells and CSCs, including metabolic plasticity, which enables TNBC cells to switch between metabolic pathways according to substrate availability to meet the energetic and biosynthetic demands for rapid growth and survival under harsh conditions. We highlight CSCs as potential key regulators driving the aggressiveness of TNBC. Thus, the manipulation of CSCs in TNBC can be a targeted therapeutic strategy for TNBC in the future.

Conservative surgery in the Zollinger-Ellison syndrome

Breast cancer stem cells have been known to contribute immensely to the carcinogenesis of the breast and therapeutic resistance in the clinic. Current studies show that the population of breast cancer stem cells is heterogeneous, involving various cellular markers and regulatory signaling pathways. In addition, different subtypes of breast cancer exhibit distinct subtypes and frequencies of breast cancer stem cells. In this review, we provide an overview of the characteristics of breast cancer stem cells, including their various molecular markers, prominent regulatory signaling, and complex microenvironment. The cellular origins of breast cancer are discussed to understand the heterogeneity and diverse differentiations of stem cells. Importantly, we also outline the recent advances and controversies in the therapeutic implications of breast cancer stem cells in different subtypes of breast cancer.