Research Progress of Breast Cancer Stem Cell Stemness and Breast Cancer Recurrence

Currently, breast cancer is the most common malignant tumour in Chinese women with a high incidence rate, and recurrence and metastasis are the main reasons affecting survival. Breast Cancer Stem Cells (BCSCs) are stem cells capable of continuous regeneration in vivo with strong self-renewal ability and multidirectional differentiation potential, which are highly tumourigenic and insensitive to radiotherapy and chemotherapy, and are highly susceptible to breast cancer recurrence. Therefore, exploring the stemness of BCSCs and their mechanism associated with recurrence is important for developing new therapeutic strategies, improving therapeutic efficacy, and improving patient prognosis.

Role of platelets and breast cancer stem cells in metastasis

The high mortality rate of breast cancer is mainly caused by the metastatic ability of cancer cells,resistance to chemotherapy and radiotherapy,and tumor regression capacity.In recent years,it has been shown that the presence of breast cancer stem cells is closely associated with the migration and metastatic ability of cancer cells,as well as with their resistance to chemotherapy and radiotherapy.The tumor microenvironment is one of the main molecular factors involved in cancer and metastatic processes development,in this sense it is interesting to study the role of platelets,one of the main communicator cells in the human body which are activated by the signals they receive from the microenvironment and can generate more than one response.Platelets can ingest and release RNA,proteins,cytokines and growth factors.After the platelets interact with the tumor microenvironment,they are called"tumor-educated platelets."Tumor-educated platelets transport material from the tumor microenvironment to sites adjacent to the tumor,thus helping to create microenvironments conducive for the development of primary and metastatic tumors.It has been observed that the clone capable of carrying out the metastatic process is a cancer cell with stem cell characteristics.Cancer stem cells go through a series of processes,including epithelial-mesenchymal transition,intravasation into blood vessels,movement through blood vessels,extravasation at the site of the establishment of a metastatic focus,and site colonization.Tumor-educated platelets support all these processes.

Let-7 miRNA and CDK4 siRNA co-encapsulated in Herceptin-conjugated liposome for breast cancer stem cells

Recently,breast cancer stem cells(BCSCs)have rapidly emerged as a novel target for the therapy of breast cancer as they play critical roles in tumor growth,maintenance,metastasis,and recurrence.Let-7 miRNA is known to be downregulated in a variety of cancers,especially BCSCs,whereas CDK4 being overexpressed in human epidermal growth factor receptor 2(HER-2)overexpressing tumor cells.In this study,let-7 miRNA and CDK4-specific siRNA were chosen as therapeutic agents and co-encapsulated in Herceptinconjugated cationic liposomes for breast cancer therapy.Particle size,zeta potential,and encapsulation efficacy of mi/siRNA-loaded PEGylated liposome conjugated with Herceptin(Her-PEG-Lipo-mi/siRNA)were 176 nm,28.1 mV,and 99.7%±0.1%,respectively.Enhanced cellular uptake(86%)was observed by fluorescence microscopy when SK-BR-3 cells were treated with Her-PEG-Lipo-mi/siRNA.Also,the increased amount of let-7a mRNA and decreased amount of cellular CDK4 mRNA were observed by qRT-PCR when SK-BR-3 cells were treated with Her-PEG-Lipo-mi/siRNA,which was even more so when SK-BR-3 stem cells were used(197 vs 768 times increase for let-7a,62%vs 68%decrease for CDK4).Growth inhibition(65%)andmigration arrest(0.5%)of the cellswere achieved by the treatment of the cells with Her-PEG-Lipo-mi/siRNA,but not withmi/siRNA complex or other formulations.In conclusion,an efficient liposomal delivery system for the combination of miRNA and siRNA to target the BCSCs was developed and could be used as an efficacious therapeuticmodality for breast cancer.

PGK1-coupled HSP90 stabilizes GSK3βexpression to regulate the stemness of breast cancer stem cells

Objective:Glycogen synthase kinase-3β(GSK3β)has been recognized as a suppressor of Wnt/β-catenin signaling,which is critical for the stemness maintenance of breast cancer stem cells.However,the regulatory mechanisms of GSK3βprotein expression remain elusive.Methods:Co-immunoprecipitation and mass spectral assays were performed to identify molecules binding to GSK3β,and to characterize the interactions of GSK3β,heat shock protein 90(Hsp90),and co-chaperones.The role of PGK1 in Hsp90 chaperoning GSK3βwas evaluated by constructing 293T cells stably expressing different domains/mutants of Hsp90α,and by performing a series of binding assays with bacterially purified proteins and clinical specimens.The influences of Hsp90 inhibitors on breast cancer stem cell stemness were investigated by Western blot and mammosphere formation assays.Results:We showed that GSK3βwas a client protein of Hsp90.Hsp90,which did not directly bind to GSK3β,interacted with phosphoglycerate kinase 1 via its C-terminal domain,thereby facilitating the binding of GSK3βto Hsp90.GSK3β-bound PGK1 interacted with Hsp90 in the“closed”conformation and stabilized GSK3βexpression in an Hsp90 activity-dependent manner.The Hsp90 inhibitor,17-AAG,rather than HDN-1,disrupted the interaction between Hsp90 and PGK1,and reduced GSK3βexpression,resulting in significantly reduced inhibition ofβ-catenin expression,to maintain the stemness of breast cancer stem cells.Conclusions:Our findings identified a novel regulatory mechanism of GSK3βexpression involving metabolic enzyme PGK1-coupled Hsp90,and highlighted the potential for more effective cancer treatment by selecting Hsp90 inhibitors that do not affect PGK1-regulated GSK3βexpression.

Epithelial-Mesenchymal Transition and Breast Cancer Stem Cells in Breast Cancer Progression

Breast cancer stem cells (BCSCs) are a small subpopulation of cancer cells having the ability of self-renewing and multi-lineage differentiation, which have also been termed as “tumor-initiating cells”. And in recent years, the role of epithelial mesenchymal transition (EMT) in malignant tumors has been valued. This paper will briefly review and discuss the relationship between BCSCs and EMT.

Effects of Notch-1 Down-regulation on Malignant Behaviors of Breast Cancer Stem Cells

This study examined the effect of Notch-1 signaling on malignant behaviors of breast cancer cells by regulating breast cancer stem cells （BCSCs）. BCSCs were enriched by using serum-free me- dium and knocked out of Notch-1 by using a lentiviral vector. Real-time polymerase chain reaction （RT-PCR） and Western blotting were used to detect the Notch-1 expression levels in breast cancer cell lines and BCSCs, and fl0w cytometry to detect the proportion of BCSCs in BCSC spheres. The BCSC self-renewal, migration, invasion, and tumorigenicity were examined by the tumor microsphere-forming assay and transwell assay and after xenotransplantation. The results showed that the Notch-1 silencing reduced the number of BCSC spheres, the proportion of BCSCs, and the number of cells penetrating through the transwell membrane. It also decreased the size of tumors that were implanted in the nude mice. These results suggest that Notch-1 signaling is intimately linked to the behaviors of BCSCs. Blocking Notch-1 signaling can inhibit the malignant behaviors of BCSCs, which may provide a prom- ising therapeutical approach for breast cancer.

STAT3 mediates resistance of CD44^+CD24^(-/low) breast cancer stem cells to tamoxifen in vitro

We sought to determine whether STAT3 mediated tamoxifen resistance of breast cancer stem cells in vitro.The capacities for mammosphere formation and STAT3 expression of CD44＋CD24-/low MCF-7 and MCF-7 were observed.The CD44＋CD24-/low subpopulation ratio and its sensitivity to adriamycin were analyzed in MCF-7 and TAM resistant（TAM-R） cells.Cell cycle,apoptosis,STAT3 and phospho-STAT3 changes were observed af-ter treatment with tamoxifen.Small interference RNA-mediated knockdown of STAT3 in TAM-R cells was also performed.CD44＋CD24-/low MCF-7 showed higher capacities for mammosphere formation and STAT3 expression than total MCF-7.The CD44＋CD24-/low subpopulation was also upregulated in TAM-R cells with less sensitivity to adriamycin than MCF-7.Cell cycle changes,anti-apoptotic effects and STAT3 changes were also found.Mean-while,the knock-down of STAT3 in TAM-R resulted in an increase in sensitivity to tamoxifen.It is concluded that STAT3 plays an essential role in breast cancer stem cells,which correlated with tamoxifen resistance.

Breast cancer stem cells: The role of sex steroid receptors

Breast cancer(BC)is the most common cancer among women,and current available therapies often have high success rates.Nevertheless,BC might acquire drug resistance and sometimes relapse.Current knowledge about the most aggressive forms of BC points to the role of specific cells with stem properties located within BC,the so-called“BC stem cells”(BCSCs).The role of BCSCs in cancer formation,growth,invasiveness,therapy resistance and tumor recurrence is becoming increasingly clear.The growth and metastatic properties of BCSCs are regulated by different pathways,which are only partially known.Sex steroid receptors(SSRs),which are involved in BC etiology and progression,promote BCSC proliferation,dedifferentiation and migration.However,in the literature,there is incomplete information about their roles.Particularly,there are contrasting conclusions about the expression and role of the classical BC hormonal biomarkers,such as estrogen receptor alpha(ERα),together with scant,albeit promising information concerning ER beta(ERβ)and androgen receptor(AR)properties that control different transduction pathways in BCSCs.In this review,we will discuss the role that SRs expressed in BCSCs play to BC progression and recurrence and how these findings have opened new therapeutic possibilities.

The p38/MAPK pathway as a therapeutic target to prevent therapeutic escape of breast cancer stem cells

Cancer stem cells(CSCs)play an important role in metastasis development,tumor recurrence,and treatment resistance,and are essential for the eradication of cancer.Currently,therapies fail to eradicate CSCs due to their therapeutic stress-induced cellular escape,which leads to enhanced aggressive behaviors compared with CSCs that have never been treated.However,the underlying mechanisms regulating the therapeutic escape remain unknown.To this end,we established a model to isolate the therapeutic escaped CSCs(TSCSCs)from breast CSCs and performed the transcription profile to reveal the mechanism.Mechanistically,we demonstrated that the behavior of therapeutic escape was regulated through the p38/MAPK signaling pathway,resulting in TSCSCs exhibiting enhanced motility and metastasis.Notably,blocking the p38/MAPK signaling pathway effectively reduced motility and metastasis ability both in vitro and in vivo,which were further supported by downregulated motility-related genes and epithelial-mesenchymal transition(EMT)-related proteins vimentin and N-cadherin.The obtained findings reveal the p38/MAPK pathway as a potential therapeutic target for TSCSCs and would provide profound implications for cancer therapy.

UBE2T mediates the stemness properties of breast cancer cells through the mTOR signaling pathway

Objectives:This study aimed to reveal the role and possible mechanism of the ubiquitin-conjugating enzyme 2T(UBE2T)in the biological activities of breast cancer stem cells(BCSCs).Methods:The specific protein and gene expression were quantified by Western blotting and quantitative real-time polymerase chain reaction,the proportion of BCSCs was examined by flow cytometry,and the self-renewal and proliferation of BCSCs were verified by serial sphere formation and soft agar.Results:Increasing expression of UBE2T was drastically found in breast cancer than that in adjacent tissues.Furthermore,UBE2T overexpression significantly increased the proportion of BCSCs in breast cancer cells and promoted their self-renewal and proliferation.Silent UBE2T exhibited the opposite functions.UBE2T increased the levels of the mammalian target of rapamycin and the phosphorylated mammalian target of rapamycin.Mammalian target of rapamycin(mTOR)inhibitor rapamycin inhibited the function of UBE2T in BCSCs.Conclusion:UBE2T plays a role in BCSCs through mTOR pathway and may suggest a novel therapeutic strategy for breast cancer.

The roles of ncRNAs and histone-modifiers in regulating breast cancer stem cells

Cancer stem cells （CSCs）, a subpopulation of cancer cells with ability of initiating tumorigenesis, exist in many kinds of tumors including breast cancer. Cancer stem cells contribute to treatment resistance and relapse. Conventional treatments only kill differentiated cancer cells, but spare CSCs. Combining conventional treatments with therapeutic drugs targeting to CSCs will eradicate cancer cells more efficiently. Studying the molecular mechanisms of CSCs regulation is essential for developing new therapeutic strategies. Growing evidences showed CSCs are regulated by non-coding RNA （ncRNA） including microRNAs and long non-coding RNAs （IncRNAs）, and histone-modifiers, such as let- 7, miR-93, miR-100, HOTAIR, Bmi-1 and EZH2. Herein we review the roles of microRNAs, IncRNAs and histone- modifiers especially Polycomb family proteins in regulating breast cancer stem cells （BCSCs）.

Downregulation of miRNA-21 and cancer stem cells after chemotherapy results in better outcome in breast cancer patients

Epigenetic modifications have been observed as a decline in miRNA-21 expression and breast cancer stem cell(CSC)population after 3 cycles of standard chemotherapy.The epigenetic response(miRNAs expression)and CSCs are also correlated in patients with Breast Cancer.In patients who tolerated chemotherapy well,miRNA-21(non-coding RNA)expression decreased significantly after three cycles of chemotherapy.The miRNA-21 expression in breast cancer tissue was quantified by quantitative PCR(real-time PCR)using the standard protocol.In addition,breast CSCs(CD44+/CD24-)were also decreased in these patients.The miRNA-21 regulates cell division,proliferation,and autophagy of cancerous cells(as it targets phosphatase and tensin homolog/AKT/transcription factor EB/programmed cell death 4/autophagy-related protein 5 and chemotherapy also produces similar effects),thereby contributing to these benefits.Therefore,when all of the targets on genes have been explored by mimic miRNA,chemotherapy combined with anti-miRNA21 therapy may prove useful in the care of cancer patients.

MicroRNAs in breast cancer and breast cancer stem cells and their potential for breast cancer therapy

The discovery of the first miRNA, lin-4, in Caenorhabditis elegans initiated a new era of miRNA biology. Sincethen, thousands of miRNAs annotated, many of which have have been identified and been shown to play roles in a variety of biological processes, including development, differentiation, apoptosis, proliferation, and cell death） Furthermore, growing evidence indicates that miRNA deregulation is a critical cause of cancer formation. The biogenesis, function, and potential application of miRNAs have become active areas of research. With the development of molecular biological technologies, such as northern blotting with radio-labeled probes, cloning, quantitative PCR, serial analysis of gene expression （SAGE）-based techniques, bead-based profiling methods, and oligonucleotide microarrays,2 it is possible to conduct miRNA research precisely and comprehensively. BIOGENESIS OF MICRORNAS MicroRNAs are derived from introns or exons of protein- coding and non-coding genes,3＇4 and are either transcribed by polymerase II as a long primary transcript （primary miRNA） or originate from the introns of mRNAs. Primary miRNAs are further processed by the Drosha microprocessor complex, which recognizes stem-looped secondary structures within primary miRNAs, resulting in the excision and release of-70 nucleotide hairpin precursors termed pre-miRNAs （precursor microRNAs）.5 The mirtron subclass of miRNAs, which are encoded in the introns of genes, generate pre-miRNAs directly from byproducts of intron splicing and disbranching events in the nucleus with the assistance of a ＂debranching enzyme＂.6 After being exported from the nucleus by exportin-5, the pre-miRNAs are subsequently cleaved by Dicer to release a 22-nucleaotide miRNA-miRNA duplex. One strand of this duplex is incorporated into the RNA-induced silencing complex （miRISC）, and eventually serves as a mature microRNA, while the other strand is degraded. The ＂seed＂ region of the mature microRNA （nucleotides 2-8 at the 5＇ end） can bind partially or completely to the 3＇UTR of specific protein-coding gene mRNAs.7＇8 MicroRNAs regulate their targets by directly cleaving mRNAs or inhibiting protein synthesis, depending on the degree of complementarity with the 3＇UTRs of their targets.4

Stem cell therapy: A paradigm shift in breast cancer treatment

As per the latest Globocan statistics,the high prevalence rate of breast cancer in low-and middle-income countries has led to it becoming the most common cancer to be diagnosed,hence posing a major public health challenge.As per this data,more than 11.7%of the estimated new cancer cases in 2020 were due to breast cancer.A small but significant subpopulation of cells with self-renewing ability are present in the tumor stroma and have been given the nomenclature of cancer stem cells(CSCs).These cells display a high degree of plasticity owing to their ability to transition from the slowly cycling quiescent phase to the actively proliferating phenotype.This attribute of CSCs allows them to differentiate into various cell types having diverse functions.Breast CSCs have a pivotal role in development,metastasis,treatment resistance and relapse of breast cancers.This review focuses on the pathways regulating breast CSC maintenance and the current strategies that are being explored for directing the development of novel,targeted,therapeutic approaches for limiting and eradicating this aberrant stem cell population.

Anticancer effect of curcumin on breast cancer and stem cells

Numerous studies have shown that curcumin,a natural compound,exerts anticancer effects by inhibiting cancer cell proliferation and metastasis and by inducing cell cycle arrest and apoptosis.In particular,curcumin exhibits potent inhibitory effects on breast cancer,the most prevalent type of cancer among women worldwide.It has low maximal inhibitory concentration for breast cancer cell lines that express the hormone receptor ER and sensitizes cell lines to anticancer drugs.Moreover,it can induce apoptosis in cell lines independently of hormone receptor expression.In addition,curcumin inhibits the proliferation of breast cancer stem cells(BCSC),an important factor that influences cancer recurrence.The inhibition of BCSC proliferation suppresses metastasis and reattachment,ultimately limiting tumor formation.A xenograft study similarly showed that curcumin exerts tumor-suppression effects on cancer cells and cancer stem cells.Therefore,curcumin is a potential anticancer compound,and its concurrent application with other anticancer drugs appears promising.

Enrichment of breast cancer stem cells using a keratinocyte serum-free medium

Background Keratinoyte serum-free medium （K-SFM） is a defined medium used to support the growth of primary keratinocytes and embryonic stem cell. The aim of this research was to optimize enrichment of breast cancer stem cells （CSCs） using K-SFM. Methods＇ A K-SFM was used to enrich CSCs from two breast cancer cell lines and a primary culture of breast cancer. RPMI-1640 supplemented with 10% fetal calf serum （FCS） was used as a control. CSCs were identified with flow cytometry using CD44＋/CD24-as molecular markers. The expression of a variety of CSC markers （Oct-4, ABCG2, Nanog, N-cadherin, and E-cadherin） was analyzed with real-time PCR. Results Much higher percentage of CSCs was achieved with K-SFM： 17.3% for MCF-7 cells, 17.4% for SKBR-3, and 20.0% for primary breast cancer culture. Less than 1% CSC was achieved using RPMI-1640 supplemented with 10% FCS. In comparison to the CSCs obtained with RPMI-1640, CSCs in the K-SFM expressed higher levels of Oct-4, ABCG2, Nanog and N-cadherin, and lower level of E-cadherin. Conclusion K-SFM is an optimal culture medium to maintain and to enrich breast CSCs.

Cancer stem-like cells directly participate in vasculogenic mimicry channels in triple-negative breast cancer

Objective: Vasculogenic mimicry(VM) channels that are lined by tumor cells are a functional blood supply in malignant tumors.However, the role of VM-initiating cells remains poorly understood. Cancer stem-like cells(CSCs) are positively correlated with VM. In this study, triple-negative breast cancer(TNBC) enriched with CSCs was used to investigate the relationship between VM and CSCs.Methods: The expression of several CSC markers was detected by immunohistochemistry in 100 human breast cancer samples.The clinical significance of CSC markers and the relationship between VM, CSCs, breast cancer subtypes, and VM-associated proteins were analyzed. CD133+ and ALDH+ human and mouse TNBC cells were isolated by FACS to examine the ability of VM formation and the spatial relationship between VM and CSCs.Results: CSCs were associated with TNBC subtype and VM in human invasive breast cancer. CSCs in TNBC MDA-MB-231 cells formed more VM channels and expressed more molecules promoting VM than the non-TNBC MCF-7 cells in vitro. MDA-MB-231 cells that encircled VM channels on Matrigel expressed CD133. Moreover, CSCs were located near VM channels in the 3D reconstructed blood supply system in human TNBC grafts. The CD133+ and ALDH+ cells isolated from TA2 mouse breast cancer formed more VM channels in vivo.Conclusions: CSCs line VM channels directly. Additionally, CSCs provide more VM-related molecules to synergize VM formation. The signaling pathways that control CSC differentiation may also be potential treatment targets for TNBC.

Establishment and molecular characterization of breast cancer mesenchymal stem cell line derived from human non-metastasis breast cancer tumor

Breast cancer remains a leading cause of morbidity and mortality in women mainly because of the propensity of primary breast tumors to metastasize. It is composed of heterogeneous cell populations with different biological properties. Breast cancer-initiating cells have been recently identified in breast carcinoma as CD44+/CD24-/low cells, which display stem cell like properties. In the present study, we have isolated breast cancer stem cells from non-metastasis tumor tissue, which is presently at passage 18 and designated as human Breast Cancer Mesenchymal Stem Cells (hBCMSCs) line. These cells showed spindle shaped morphology and formed mammos-pheres as well as pluripotency clones indicating their stem cell nature. Molecular marker study confirmed mesenchymal nature as well as pluripotency, plasticity and oncogenicity of these cells. The hBCMSCs cell line may likely contain a heterogeneous population of malignant cells. Interestingly, we also found that these cells exhibit BRCA 2 mutation, which was found in Indian population. Overall, this study revealed that hBCMSCs cell line may represent a suitable in vitro model to study the mechanism of breast cancer which further leads to an identification of molecular targets for future breast cancer targeted therapy.

Adipocyte activation of cancer stem cell signaling in breast cancer

Signaling within the tumor microenvironment has a critical role in cancer initiation and progression. Adipocytes, one of the major components of the breast microenvironment,have been shown to provide pro-tumorigenic signals that promote cancer cell proliferation and invasiveness in vitro and tumorigenicity in vivo. Adipocyte secreted factors such as leptin and interleukin-6(IL-6) have a paracrine effect on breast cancer cells. In adipocyte-adjacent breast cancer cells, the leptin and IL-6 signaling pathways activate janus kinase 2/signal transducer and activatorof transcription 5, promoting the epithelial-mesenchymal transition, and upregulating stemness regulators such as Notch, Wnt and the Sex determining region Y-box 2/octamer binding transcription factor 4/Nanog signaling axis. In this review we will summarize the major signaling pathways that regulate cancer stem cells in breast cancer and describe the effects that adipocyte secreted IL-6 and leptin have on breast cancer stem cell signaling. Finally we will introduce a new potential treatment paradigm of inhibiting the adipocyte-breast cancer cell signaling via targeting the IL-6 or leptin pathways.

HIF-2α regulates CD44 to promote cancer stem cell activation in triple-negative breast cancer via PI3K/AKT/mTOR signaling

BACKGROUND Breast cancer is a common malignant tumor that seriously threatens women’s health.Breast cancer stem cell(CSC)-like cell population may be the main factor for breast cancer metastasis.Therefore,targeted therapy for CSCs has great potential significance.Hypoxia-inducible factor is a transcription factor widely expressed in tumors.Studies have shown that down-regulation of the hypoxia signaling pathway inhibits tumor stem cell self-renewal and increases the sensitivity of stem cells to radiotherapy and chemotherapy mediated by hypoxiainducible factor-2α(HIF-2α).However,the specific mechanism remains unclear and further research is necessary.AIM To investigate the effect of HIF-2αdown-regulation on stem cell markers,microsphere formation,and apoptosis in breast cancer cell line MDA-MB-231 under hypoxia and its possible mechanism.METHODS Immunohistochemistry was used to detect the expression of HIF-2αand CD44 in triple-negative breast cancer(TNBC)and non-TNBC tissues.Double-labeling immunofluorescence was applied to detect the co-expression of HIF-2αand CD44 in MDA-MB-231 cells and MCF-7 cells.HIF-2αwas silenced by RNA interference,and the expression of CD44 and transfection efficiency were detected by real-time fluorescent quantitative PCR.Further,flow cytometry,TdT-mediated X-dUTP nick end labeling,and mammosphere formation assays were used to evaluate the effect of HIF-2αon CSCs and apoptosis.The possible mechanisms were analyzed by Western blot.RESULTS The results of immunohistochemistry showed that HIF-2αwas highly expressed in both TNBC and non-TNBC,while the expression of CD44 in different molecular types of breast cancer cells was different.In in vitro experiments,it was found that HIF-2αand CD44 were expressed almost in the same cell.Compared with hypoxia+negative-sequence control,HIF-2αsmall interfering ribonucleic acid transfection can lower the expression of HIF-2αand CD44 mRNA(P<0.05),increase the percentage of apoptotic cells(P<0.05),and resulted in a reduction of CD44+/CD24−population(P<0.05)and mammosphere formation(P<0.05)in hypoxic MDA-MB-231 cells.Western blot analysis revealed that phosphorylated protein-serine-threonine kinase(p-AKT)and phosphorylated mammalian target of rapamycin(p-mTOR)levels in MDA-MB-231 decreased significantly after HIF-2αsilencing(P<0.05).CONCLUSION Down-regulation of HIF-2αexpression can inhibit the stemness of human breast cancer MDA-MB-231 cells and promote apoptosis,and its mechanism may be related to the CD44/phosphoinosmde-3-kinase/AKT/mTOR signaling pathway.