SMAR1 repression by pluripotency factors and consequent chemoresistance in breast cancer stem-like cells is reversed by aspirin

Spinal Cord Astroblastoma With EWSR1-BEND2 Fusion in Female Patients : A Report of Four Cases From China and a Comprehensive Literature Review

Astroblastoma is an extremely rare central nervous system tumor characterized by astroblastic pseudorosettes and vascular hyalinization. Despite these histologic hallmarks, its morphology can vary, occasionally resembling other central nervous system tumors such as ependymoma. A novel tumor entity, astroblastoma, meningioma 1 ( MN1 )-altered, has been identified, featuring MN1 gene rearrangements typically involving BEN-domain containing 2 ( BEND2 ) as a fusion partner. Most astroblastomas arise in the cerebral hemisphere. Here, we report 4 cases of spinal cord astroblastoma in female patients, all showing Ewing sarcoma RNA-binding protein 1 fusion with BEND2 , rather than MN1 . These tumors displayed growth patterns akin to traditional intracranial astroblastomas, with three cases demonstrating high-grade histology, including elevated mitotic activity and necrosis. Interestingly, some cases exhibited positive staining for pan-cytokeratin and hormone receptors. DNA methylation profiling clustered three of the four cases with the reference "AB_EWSR," whereas one case exhibited an independent methylation signature near the reference methylation group "AB_EWSR" and "pleomorphic xanthoastrocytoma." Together with the existing literature, we summarized a total of eleven cases, which predominantly affected children and young adults with female predilection. Eight of 10 patients experienced recurrence, underscoring the aggressive nature of this disease. We suggest recognizing a new molecular subgroup of spinal astroblastoma and recommend testing newly diagnosed infratentorial astroblastomas for Ewing sarcoma RNA-binding protein 1-BEND2 fusion.

Biomimetic Nano-Cancer Stem Cell Scavenger for Inhibition of Breast Cancer Recurrence and Metastasis after FLASH-Radiotherapy

Compared to conventional radiotherapy (RT), FLASH-RT delivers ultra-high dose radiation, significantly reducing damage to normal tissue while guaranteeing the effect of cancer treatment. However, cancer recurrence and metastasis frequently occur after all RT due to the existence of intractable cancer stem cells (CSCs). To address this, a biomimetic nanoplatform (named TAFL) of tumor-derived exosome fusion liposomes is designed by co-loading aggregation-induced emission photothermal agents, TPE-BBT, and anti-cancer drugs, aspirin, aiming to clear CSCs for inhibiting cancer recurrence and metastasis after FLASH-RT therapy . Aspirin released in TAFL system triggered by laser irradiation can induce apoptosis and DNA damage of 4T1 CSCs, comprehensively downregulate their stemness phenotype, and inhibit their sphericity. Furthermore, the TPE-BBT mediated mild-photothermal therapy can alleviate the hypoxic tumor microenvironment, inhibit the DNA repair of CSCs, which further amplifies the effect of aspirin against CSCs, therefore reduces the effective dose of aspirin, making TAFL more biologically safe. In vivo experimental results demonstrated that decreased CSCs population mediated by TAFL system treatment significantly inhibited tumor recurrence and metastasis after FLASH-RT therapy. In summary, this TAFL system   provides a new idea for the future clinical application of FLASH-RT therapy.

HDAC9 and miR-512 Regulate CAGE-Promoted Anti-Cancer Drug Resistance and Cellular Proliferation

Histone deacetylase 9 (HDAC9) is known to be upregulated in various cancers. Cancer-associated antigens (CAGEs) are cancer/testis antigens that play an important role in anti-cancer drug resistance. This study aimed to investigate the relationship between CAGEs and HDAC9 in relation to anti-cancer drug resistance. AGSR cells with an anti-cancer drug-resistant phenotype showed higher levels of CAGEs and HDAC9 than normal AGS cells. CAGEs regulated the expression of HDAC9 in AGS and AGSR cells. CAGEs directly regulated the expression of HDAC9. Rapamycin, an inducer of autophagy, increased HDAC9 expression in AGS, whereas chloroquine decreased HDAC9 expression in AGSR cells. The downregulation of HDAC9 decreased the autophagic flux, invasion, migration, and tumor spheroid formation potential in AGSR cells. The TargetScan analysis predicted that miR-512 was a negative regulator of HDAC9. An miR-512 mimic decreased expression levels of CAGEs and HDAC9. The miR-512 mimic also decreased the autophagic flux, invasion, migration, and tumor spheroid forming potential of AGSR cells. The culture medium of AGSR increased the expression of HDAC9 and autophagic flux in AGS. A human recombinant CAGE protein increased HDAC9 expression in AGS cells. AGSR cells displayed higher tumorigenic potential than AGS cells. Altogether, our results show that CAGE-HDAC9-miR-512 can regulate anti-cancer drug resistance, cellular proliferation, and autophagic flux. Our results can contribute to the understanding of the molecular roles of HDAC9 in anti-cancer drug resistance.

ncRNADrug: a database for validated and predicted ncRNAs associated with drug resistance and targeted by drugs

Drug resistance is a major barrier in cancer treatment and anticancer drug development. Growing evidence indicates that non-coding RNAs (ncRNAs), especially microRNAs (miRNAs), long non-coding RNAs (lncRNAs) and circular RNAs (circRNAs), play pivotal roles in cancer progression, therapy, and drug resistance. Furthermore, ncRNAs have been proven to be promising novel therapeutic targets for cancer treatment. Reversing dysregulated ncRNAs by drugs holds significant potential as an effective therapeutic strategy for overcoming drug resistance. Therefore, we developed ncRNADrug, an integrated and comprehensive resource that records manually curated and computationally predicted ncRNAs associated with drug resistance, ncRNAs targeted by drugs, as well as potential drug combinations for the treatment of resistant cancer. Currently, ncRNADrug collects 29 551 experimentally validated entries involving 9195 ncRNAs (2248 miRNAs, 4145 lncRNAs and 2802 circRNAs) associated with the drug resistance of 266 drugs, and 32 969 entries involving 10 480 ncRNAs (4338 miRNAs, 6087 lncRNAs and 55 circRNAs) targeted by 965 drugs. In addition, ncRNADrug also contains associations between ncRNAs and drugs predicted from ncRNA expression profiles by differential expression analysis. Altogether, ncRNADrug surpasses the existing related databases in both data volume and functionality. It will be a useful resource for drug development and cancer treatment. ncRNADrug is available at http://www.jianglab.cn/ncRNADrug.

Breast cancer stem cells generate immune-suppressive T regulatory cells by secreting TGFβ to evade immune-elimination

Cancer stem cells (CSCs), being the primary contributors in tumor initiation, metastasis, and relapse, ought to have seminal roles in evasion of immune surveillance. Tumor-promoting CD4+CD25+FOXP3+ T-regulatory cells (Tregs) have been described to abolish host defense mechanisms by impeding the activities of other immune cells including effector T cells. However, whether CSCs can convert effector T cells to immune-suppressive Treg subset, and if yes, the mechanism underlying CSC-induced Treg generation, are limitedly studied. In this regard, we observed a positive correlation between breast CSC and Treg signature markers in both in-silico and immunohistochemical analyses. Mirroring the conditions during tumor initiation, low number of CSCs could successfully generate CD4+CD25+FOXP3+ Treg cells from infiltrating CD4+ T lymphocytes in a contact-independent manner. Suppressing the proliferation potential as well as IFNγ production capacity of effector T cells, these Treg cells might be inhibiting antitumor immunity, thereby hindering immune-elimination of CSCs during tumor initiation. Furthermore, unlike non-stem cancer cells (NSCCs), CSCs escaped doxorubicin-induced apoptosis, thus constituting major surviving population after three rounds of chemotherapy. These drug-survived CSCs were also able to generate CD4+CD25+FOXP3+ Treg cells. Our search for the underlying mechanism further unveiled the role of CSC-shed immune-suppressive cytokine TGFβ, which was further increased by chemotherapy, in generating tumor Treg cells. In conclusion, during initiation as well as after chemotherapy, when NSCCs are not present in the tumor microenvironment, CSCs, albeit present in low numbers, generate immunosuppressive CD4+CD25+FOXP3+ Treg cells in a contact-independent manner by shedding high levels of immune-suppressive Treg-polarizing cytokine TGFβ, thus escaping immune-elimination and initiating the tumor or causing tumor relapse.

Effects of a differentiating therapy on cancer-stem-cell-driven tumors

The growth of many solid tumors has been found to be driven by chemo- and radiotherapy-resistant cancer stem cells (CSCs). A suitable therapeutic avenue in these cases may involve the use of a differentiating agent (DA) to force the differentiation of the CSCs and of conventional therapies to eliminate the remaining differentiated cancer cells (DCCs). To describe the effects of a DA that reprograms CSCs into DCCs, we adapt a differential equation model developed to investigate tumorspheres, which are assumed to consist of jointly evolving CSC and DCC populations. We analyze the mathematical properties of the model, finding the equilibria and their stability. We also present numerical solutions and phase diagrams to describe the system evolution and the therapy effects, denoting the DA strength by a parameter adif. To obtain realistic predictions, we choose the other model parameters to be those determined previously from fits to various experimental datasets. These datasets characterize the progression of the tumor under various culture conditions. Typically, for small values of adif the tumor evolves towards a final state that contains a CSC fraction, but a strong therapy leads to the suppression of this phenotype. Nonetheless, different external conditions lead to very diverse behaviors. For microchamber-grown tumorspheres, there is a threshold in therapy strength below which both subpopulations survive, while high values of adif lead to the complete elimination of the CSC phenotype. For tumorspheres grown on hard and soft agar and in the presence of growth factors, the model predicts a threshold not only in the therapy strength, but also in its starting time, an early beginning being potentially crucial. In summary, our model shows how the effects of a DA depend critically not only on the dosage and timing of the drug application, but also on the tumor nature and its environment.

Immune evasion by cancer stem cells ensures tumor initiation and failure of immunotherapy

Cancer stem cells (CSCs) are a small subpopulation of cells that drive the formation and progression of tumors. However, during tumor initiation, how CSCs communicate with neighbouring immune cells to overcome the powerful immune surveillance barrier in order to form, spread, and maintain the tumor, remains poorly understood. It is, therefore, absolutely necessary to understand how a small number of tumor-initiating cells (TICs) survive immune attack during (a) the “elimination phase” of “tumor immune-editing”, (b) the establishment of regional or distant tumor after metastasis, and (c) recurrence after therapy. Mounting evidence suggests that CSCs suppress the immune system through a variety of distinct mechanisms that ensure the survival of not only CSCs but also non-stem cancer cells (NSCCs), which eventually form the tumor mass. In this review article, the mechanisms via which CSCs change the immune landscape of the tissue of origin, which contains macrophages, dendritic cells (DCs), myeloid-derived suppressor cells (MDSCs), natural killer (NK) cells, and tumor-infiltrating lymphocytes, in favour of tumorigenesis were discussed. The failure of cancer immunotherapy might also be explained by such interaction between CSCs and immune cells. This review will shed light on the critical role of CSCs in tumor immune evasion and emphasize the importance of CSC-targeted immunotherapy as a cutting-edge technique for battling cancer by restricting communication between immune cells and CSCs.

Aspirin Affects MDA-MB-231 Vesicle Production and Their Capacity to Induce Fibroblasts towards a Pro-Invasive State

Long-term administration of aspirin (ASA, acetylsalicylic acid) in oncogenic patients has been related to a reduction in cancer risk incidence, but its precise mechanism of action is unclear. The activation of cancer-associated fibroblasts (CAFs) is a key element in tumor progression and can be triggered by cancer-derived extracellular vesicles (EVs). Targeting the communication between cancer cells and the surrounding tumor microenvironment (TME) may control cancer progression. Our aim was to investigate the effect of ASA on breast cancer cells, focusing on EV secretion and their effect on the biological properties of CAFs. As a result, ASA was shown to reduce the amount and alter the size distribution of EVs produced by MDA-MB-231 tumor cells. Fibroblasts stimulated with EVs derived from MDA-MB-231 treated with ASA (EV-ASA) showed a lower expression of alpha-smooth muscle actin (α-SMA), matrix metalloproteinase-2 (MMP2) but not fibroblast activation protein (FAP) in respect to the ones stimulated with EVs from untreated breast cancer cells (EV-CTR). Furthermore, invasion assays using a three-dimensional (3D) fibroblast spheroid model showed reduced MDA-MB-231 invasion towards fibroblast spheroids pretreated with EV-ASA as compared to spheroids prepared with EV-CTR-stimulated fibroblasts. This suggests that ASA partially inhibits the ability of tumor EVs to stimulate CAFs to promote cancer invasion. In conclusion, ASA can interfere with tumor communication by reducing EV secretion by breast tumor cells as well as by interfering with their capacity to stimulate fibroblasts to become CAFs.

Transforming growth factor-β signalling in tumour resistance to the anti-PD-(L)1 therapy: Updated

Low frequency of durable responses in patients treated with immune checkpoint inhibitors (ICIs) demands for taking complementary strategies in order to boost immune responses against cancer. Transforming growth factor-β (TGF-β) is a multi-tasking cytokine that is frequently expressed in tumours and acts as a critical promoter of tumour hallmarks. TGF-β promotes an immunosuppressive tumour microenvironment (TME) and defines a bypass mechanism to the ICI therapy. A number of cells within the stroma of tumour are influenced from TGF-β activity. There is also evidence of a relation between TGF-β with programmed death-ligand 1 (PD-L1) expression within TME, and it influences the efficacy of anti-programmed death-1 receptor (PD-1) or anti-PD-L1 therapy. Combination of TGF-β inhibitors with anti-PD(L)1 has come to the promising outcomes, and clinical trials are under way in order to use agents with bifunctional capacity and fusion proteins for bonding TGF-β traps with anti-PD-L1 antibodies aiming at reinvigorating immune responses and promoting persistent responses against advanced stage cancers, especially tumours with immunologically cold ecosystem.

SMAR1 inhibits proliferation, EMT and Warburg effect of bladder cancer cells by suppressing the activity of the Wnt/β-catenin signaling pathway

This study aimed to investigate the effects of scaffold matrix attachment region binding protein 1 (SMAR1) on the development of bladder cancer (BCa). SMAR1 expression in paired tumor and corresponding adjacent normal tissues from 55 BCa patients was detected by quantitative reverse transcription-polymerase chain reaction. BCa cells were transfected to regulate SMAR1 expression. BCa cells were treated with XAV-939, LiCl and 2-deoxyglucose. The effect of SMAR1 on the viability, proliferation, migration, invasion and Warburg effect of BCa cells was researched by counting kit-8, colony formation assay, Transwell and aerobic glycolysis assays. Western blot was performed to detect protein expression. BCa cell growth in vivo was recorded in nude mice. Immunohistochemical staining was performed for clinical and xenografted tumor tissue specimens. SMAR1 expression was down-regulated in BCa patients, associating with worse prognoses. SMAR1 knockdown enhanced the viability, proliferation, migration, invasion, EMT and Warburg effect of BCa cells. The opposite effect was found in the SMAR1 overexpression BCa cells. XAV-939 treatment reversed the elevation of β-catenin, c-Myc and Cyclin D1 proteins expression and Warburg effect in Bca cells post-SMAR1 knockdown. LiCl treatment abrogated the inhibition of β-catenin, c-Myc and Cyclin D1 proteins expression and Warburg effect proteins due to SMAR1 overexpression in BCa cells. SMAR1 overexpression inhibited the growth of BCa cells in vivo. SMAR1 might suppress the Wnt/β-catenin signaling pathway activity to inhibit the progression of BCa. It might be an effective treatment target for BCa.

Epithelial-mesenchymal transition in cancer stemness and heterogeneity: updated

Epithelial-mesenchymal transition (EMT) as a trans-differentiation program and a key process in tumor progression is linked positively with increased expansion of cancer stem cells and cells with stem-like properties. This is mediated through modulation of critical tumorigenic events and is positively correlated with hypoxic conditions in tumor microenvironment. The presence of cells eliciting diverse phenotypical states inside tumor is representative of heterogeneity and higher tumor resistance to therapy. In this review, we aimed to discuss about the current understanding toward EMT, stemness, and heterogeneity in tumors of solid organs, their contribution to the key tumorigenic events along with major signaling pathway involved, and, finally, to suggest some strategies to target these critical events.

The paradigm of drug resistance in cancer: an epigenetic perspective

Innate and acquired resistance towards the conventional therapeutic regimen imposes a significant challenge for the successful management of cancer for decades. In patients with advanced carcinomas, acquisition of drug resistance often leads to tumor recurrence and poor prognosis after the first therapeutic cycle. In this context, cancer stem cells (CSCs) are considered as the prime drivers of therapy resistance in cancer due to their 'non-targetable' nature. Drug resistance in cancer is immensely influenced by different properties of CSCs such as epithelial-to-mesenchymal transition (EMT), a profound expression of drug efflux pump genes, detoxification genes, quiescence, and evasion of apoptosis, has been highlighted in this review article. The crucial epigenetic alterations that are intricately associated with regulating different mechanisms of drug resistance, have been discussed thoroughly. Additionally, special attention is drawn towards the epigenetic mechanisms behind the interaction between the cancer cells and their microenvironment which assists in tumor progression and therapy resistance. Finally, we have provided a cumulative overview of the alternative treatment strategies and epigenome-modifying therapies that show the potential of sensitizing the resistant cells towards the conventional treatment strategies. Thus, this review summarizes the epigenetic and molecular background behind therapy resistance, the prime hindrance of present day anti-cancer therapies, and provides an account of the novel complementary epi-drug-based therapeutic strategies to combat drug resistance.

RING finger protein TOPORS modulates the expression of tumor suppressor SMAR1 in colorectal cancer via the TLR4-TRIF pathway

TOP1‐binding arginine/serine‐rich protein (TOPORS), a really interesting new gene finger protein, has the ability to bind to a palindromic consensus DNA sequence that enables it to function as a potential transcriptional regulator. However, its role in regulating the transcription of cancer‐associated genes is yet to be explored. As Toll‐like receptor 4 (TLR4) agonists are known to regress solid tumors, we observed that lipopolysaccharide (LPS) induces TOPORS via a TLR4‐TIR domain‐containing adapter‐inducing interferon‐β‐dependent pathway, which in turn modulates the transcription of tumor suppressor scaffold/matrix attachment region‐binding protein 1 (SMAR1, also known as BANP). ChIP analysis showed that TOPORS binds on the SMAR1 promoter and its occupancy increases upon LPS treatment. A previous study from our laboratory revealed that SMAR1 acts as a repressor of signal transducer and activator of transcription 3 (STAT3) transcription. Tumor growth, as well as tumor‐associated macrophage polarization, depends on the status of the STAT1:STAT3 ratio. LPS‐induced SMAR1 expression decreases STAT3 expression and also skews the macrophage polarization toward M1 phenotype. In contrast, LPS failed to polarize tumor‐associated macrophages to M1 phenotype in a SMAR1‐silenced condition, which shows the involvement of SMAR1 in dictating the fate of colorectal cancer progression. Identification of the molecular mechanism behind LPS‐mediated tumor regression would be crucial for designing cancer treatment strategies involving bacterial components.

Mechanisms Involved in the Promoting Activity of Fibroblasts in HTLV-1-Mediated Lymphomagenesis: Insights into the Plasticity of Lymphomatous Cells

Among the mechanisms leading to progression to Adult T-cell Leukaemia/Lymphoma in Human T-cell Leukaemia Virus type 1 (HTLV-1)-infected subjects, the contribution of stromal components remains poorly understood. To dissect the role of fibroblasts in HTLV-1-mediated lymphomagenesis, transcriptome studies, cytofluorimetric and qRT-PCR analyses of surface and intracellular markers linked to plasticity and stemness in coculture, and in vivo experiments were performed. A transcriptomic comparison between a more lymphomagenic (C91/III) and the parental (C91/PL) cell line evidenced hyperactivation of the PI3K/Akt pathway, confirmed by phospho-ELISA and 2-DE and WB analyses. C91/III cells also showed higher expression of mesenchymal and stemness genes. Short-term coculture with human foreskin fibroblasts (HFF) induced these features in C91/PL cells, and significantly increased not only the cancer stem cells (CSCs)-supporting CD10+GPR77+ HFF subpopulation, but also the percentage of ALDH1bright C91/PL cells. A non-cytotoxic acetylsalicylic acid treatment decreased HFF-induced ALDH1bright C91/PL cells, downregulated mesenchymal and stemness genes in cocultured cells, and delayed lymphoma growth in immunosuppressed mice, thus hindering the supportive activity of HFF on CSCs. These data suggest that crosstalk with HFF significantly intensifies the aggressiveness and plasticity of C91/PL cells, leading to the enrichment in lymphoma-initiating cells. Additional research is needed to better characterize these preliminary findings.

Cellular Metabolomics Profiles Associated With Drug Chemosensitivity in AML

Background

Acute myeloid leukemia (AML) is a hematological malignancy with a dismal prognosis. For over four decades, AML has primarily been treated by cytarabine combined with an anthracycline. Although a significant proportion of patients achieve remission with this regimen, roughly 40% of children and 70% of adults relapse. Over 90% of patients with resistant or relapsed AML die within 3 years. Thus, relapsed and resistant disease following treatment with standard therapy are the most common clinical failures that occur in treating this disease. In this study, we evaluated the relationship between AML cell line global metabolomes and variation in chemosensitivity.

Methods

We performed global metabolomics on seven AML cell lines with varying chemosensitivity to cytarabine and the anthracycline doxorubicin (MV4.11, KG-1, HL-60, Kasumi-1, AML-193, ME1, THP-1) using ultra-high performance liquid chromatography - mass spectrometry (UHPLC-MS). Univariate and multivariate analyses were performed on the metabolite peak intensity values from UHPLC-MS using MetaboAnalyst to identify cellular metabolites associated with drug chemosensitivity.

Results

A total of 1,624 metabolic features were detected across the leukemic cell lines. Of these, 187 were annotated to known metabolites. With respect to doxorubicin, we observed significantly greater abundance of a carboxylic acid (1-aminocyclopropane-1-carboxylate) and several amino acids in resistant cell lines. Pathway analysis found enrichment of several amino acid biosynthesis and metabolic pathways. For cytarabine resistance, nine annotated metabolites were significantly different in resistance vs. sensitive cell lines, including D-raffinose, guanosine, inosine, guanine, aldopentose, two xenobiotics (allopurinol and 4-hydroxy-L-phenylglycine) and glucosamine/mannosamine. Pathway analysis associated these metabolites with the purine metabolic pathway.

Conclusion

Overall, our results demonstrate that metabolomics differences contribute toward drug resistance. In addition, it could potentially identify predictive biomarkers for chemosensitivity to various anti-leukemic drugs. Our results provide opportunity to further explore these metabolites in patient samples for association with clinical response.

SMAR1 attenuates the stemness of osteosarcoma cells via through suppressing ABCG2 transcriptional activity

The promoting roles of the transcriptional regulator SMAR1 have been revealed in several tumors, such as colorectal and breast cancer, however, its roles in osteosarcoma (OS) progression are still confusing. Here, we find that SMAR1 expression is positively correlated with the overall survival of OS patients and negatively correlated with the expression of stemness markers by analyzing the online datasets. Through analyzing different Gene Expression Omnibus (GEO) datasets, SMAR1 is found to be lowly expressed in OS tissues relative to that in adjacent tissues. Functional experiments indicate that SMAR1 overexpression attenuates the stemness of OS cells, characterized as the decrease of stemness marker expression, sphere-formation ability and ALDH activity. Mechanistically, it is shown that SMAR1 increases the deacetylation level of the drug efflux pump ABCG2 via recruiting HDAC2 to the promoter of the gene coding ABCG2, and thus decreases ABCG2 transcriptional activity. Additionally, overexpression of ABCG2 rescues the inhibition of SMAR1 overexpression on the stemness of OS cells. Moreover, this SMAR1/ABCG2 axis positively regulates the chemotherapeutic sensitivity of OS cells. This work indicates that SMAR1 is a critical suppressor for OS progression through transcriptionally regulating ABCG2 expression.

The Tumor Microenvironment as a Driving Force of Breast Cancer Stem Cell Plasticity

Simple Summary

Breast cancer stem cells are a subset of transformed cells that sustain tumor growth and can metastasize to secondary organs. Since metastasis accounts for most cancer deaths, it is of paramount importance to understand the cellular and molecular mechanisms that regulate this subgroup of cells. The tumor microenvironment (TME) is the habitat in which transformed cells evolve, and it is composed by many different cell types and the extracellular matrix (ECM). A body of evidence strongly indicates that microenvironmental cues modulate stemness in breast cancer, and that the coevolution of the TME and cancer stem cells determine the fate of breast tumors. In this review, we summarize the studies providing links between the TME and the breast cancer stem cell phenotype and we discuss their specific interactions with immune cell subsets, stromal cells, and the ECM.

Abstract

Tumor progression involves the co-evolution of transformed cells and the milieu in which they live and expand. Breast cancer stem cells (BCSCs) are a specialized subset of cells that sustain tumor growth and drive metastatic colonization. However, the cellular hierarchy in breast tumors is rather plastic, and the capacity to transition from one cell state to another depends not only on the intrinsic properties of transformed cells, but also on the interplay with their niches. It has become evident that the tumor microenvironment (TME) is a major player in regulating the BCSC phenotype and metastasis. The complexity of the TME is reflected in its number of players and in the interactions that they establish with each other. Multiple types of immune cells, stromal cells, and the extracellular matrix (ECM) form an intricate communication network with cancer cells, exert a highly selective pressure on the tumor, and provide supportive niches for BCSC expansion. A better understanding of the mechanisms regulating these interactions is crucial to develop strategies aimed at interfering with key BCSC niche factors, which may help reducing tumor heterogeneity and impair metastasis.