Breast cancer, side population cells and ABCG2 expression

Role of Cancer Side Population Stem Cells in Ovarian Cancer Angiogenesis

Ovarian cancer is one of the most common gynecologic malignancies. Recurrence and metastasis often occur after treatment, and it has the highest mortality rate of all gynecological tumors. Cancer stem cells (CSCs) are a small population of cells with the ability of self-renewal, multidirectional differentiation, and infinite proliferation. They have been shown to play an important role in tumor growth, metastasis, drug resistance, and angiogenesis. Ovarian cancer side population (SP) cells, a type of CSC, have been shown to play roles in tumor formation, colony formation, xenograft tumor formation, ascites formation, and tumor metastasis. The rapid progression of tumor angiogenesis is necessary for tumor growth; however, many of the mechanisms driving this process are unclear as is the contribution of CSCs. The aim of this review was to document the current state of knowledge of the molecular mechanism of ovarian cancer stem cells (OCSCs) in regulating tumor angiogenesis.

Cancer stem cells: advances in knowledge and implications for cancer therapy

Cancer stem cells (CSCs), a small subset of cells in tumors that are characterized by self-renewal and continuous proliferation, lead to tumorigenesis, metastasis, and maintain tumor heterogeneity. Cancer continues to be a significant global disease burden. In the past, surgery, radiotherapy, and chemotherapy were the main cancer treatments. The technology of cancer treatments continues to develop and advance, and the emergence of targeted therapy, and immunotherapy provides more options for patients to a certain extent. However, the limitations of efficacy and treatment resistance are still inevitable. Our review begins with a brief introduction of the historical discoveries, original hypotheses, and pathways that regulate CSCs, such as WNT/β-Catenin, hedgehog, Notch, NF-κB, JAK/STAT, TGF-β, PI3K/AKT, PPAR pathway, and their crosstalk. We focus on the role of CSCs in various therapeutic outcomes and resistance, including how the treatments affect the content of CSCs and the alteration of related molecules, CSCs-mediated therapeutic resistance, and the clinical value of targeting CSCs in patients with refractory, progressed or advanced tumors. In summary, CSCs affect therapeutic efficacy, and the treatment method of targeting CSCs is still difficult to determine. Clarifying regulatory mechanisms and targeting biomarkers of CSCs is currently the mainstream idea.

Polyploidy of MDA-MB-231 cells drives increased extravasation with enhanced cell-matrix adhesion

Metastasis, the leading cause of cancer-related deaths, involves a complex cascade of events, including extravasation. Despite extensive research into metastasis, the mechanisms underlying extravasation remain unclear. Molecular targeted therapies have advanced cancer treatment, yet their efficacy is limited, prompting exploration into novel therapeutic targets. Here, we showed the association of polyploidy in MDA-MB-231 breast cancer cells and their extravasation, using microfluidic systems to reproduce the in vivo microvascular environment. We observed enhanced extravasation in polyploid cells alongside upregulated expression of genes involved in cell-substrate adhesion and cell mechanical dynamics. These findings offer insights into the relationship between polyploidy and extravasation, highlighting potential targets for cancer therapy.

Lau AT, Xu YM. The regulation of microRNAs on chemoresistance in triple-negative breast cancer: a recent update

Triple-negative breast cancer (TNBC) has negative expressions of ER, PR and HER2. Due to the insensitivity to both endocrine therapy and HER2-targeted therapy, the main treatment method for TNBC is cytotoxic chemotherapy. However, the curative effect of chemotherapy is limited because of the existence of acquired or intrinsic multidrug resistance. MicroRNAs (miRNAs) are frequently dysregulated in malignant tumors and involved in tumor occurrence and progression. Interestingly, growing studies show that miRNAs are involved in chemoresistance in TNBC. Thus, targeting dysregulated miRNAs could be a plausible way for better treatment of TNBC. Here, we present the updated knowledge of miRNAs associated with chemoresistance in TNBC, which may be helpful for the early diagnosis, prognosis and treatment of this life-threatening disease.

Breast Cancer Chemoresistance: Insights into the Regulatory Role of lncRNA

Long noncoding RNAs (lncRNAs) are a subclass of noncoding RNAs composed of more than 200 nucleotides without the ability to encode functional proteins. Given their involvement in critical cellular processes such as gene expression regulation, transcription, and translation, lncRNAs play a significant role in organism homeostasis. Breast cancer (BC) is the second most common cancer worldwide and evidence has shown a relationship between aberrant lncRNA expression and BC development. One of the main obstacles in BC control is multidrug chemoresistance, which is associated with the deregulation of multiple mechanisms such as efflux transporter activity, mitochondrial metabolism reprogramming, and epigenetic regulation as well as apoptosis and autophagy. Studies have shown the involvement of a large number of lncRNAs in the regulation of such pathways. However, the underlying mechanism is not clearly elucidated. In this review, we present the principal mechanisms associated with BC chemoresistance that can be directly or indirectly regulated by lncRNA, highlighting the importance of lncRNA in controlling BC chemoresistance. Understanding these mechanisms in deep detail may interest the clinical outcome of BC patients and could be used as therapeutic targets to overcome BC therapy resistance.

Clinical Significance of ABCG2/BCRP Quantified by Fluorescent Nanoparticles in Breast Cancer Patients Undergoing Neoadjuvant Chemotherapy

Breast cancer resistance protein (BCRP), also known as ATP-binding cassette transporter G2 (ABCG2), is associated with chemotherapy resistance. BCRP is also implicated in breast cancer stem cells, and is reported as a poor prognostic factor. However, the relationship of BCRP levels in breast cancer tissues with chemotherapy resistance and prognosis has not been clarified. We aimed to evaluate the correlation between BCRP expression and prognosis in breast cancer using immunohistochemistry with fluorescent phosphor-integrated dots (IHC-PIDs). A total of 37 breast cancer patients with residual cancer in the primary tumor and axillary lymph nodes were evaluated. BCRP levels in breast cancer tissue and metastatic lymph nodes were quantitatively detected after neoadjuvant chemotherapy (NAC). Among these 37 patients, 24 had corresponding core needle biopsies obtained before NAC. Biomarker assay with IHC-PIDs showed high accuracy for the quantitative assessment of BCRP with low expression. High BCRP expression in the primary tumor and metastatic lymph nodes after preoperative chemotherapy was associated with worse overall survival. In conclusion, high BCRP levels may be associated with poor prognosis in patients with breast cancer, having residual tumors within the primary tumor and lymph nodes after preoperative chemotherapy. These findings provide a basis for further appropriate adjuvant therapy in these patients.

DNA Repair and Therapeutic Strategies in Cancer Stem Cells

First-line cancer treatments successfully eradicate the differentiated tumour mass but are comparatively ineffective against cancer stem cells (CSCs), a self-renewing subpopulation thought to be responsible for tumour initiation, metastasis, heterogeneity, and recurrence. CSCs are thus presented as the principal target for elimination during cancer treatment. However, CSCs are challenging to drug target because of numerous intrinsic and extrinsic mechanisms of drug resistance. One such mechanism that remains relatively understudied is the DNA damage response (DDR). CSCs are presumed to possess properties that enable enhanced DNA repair efficiency relative to their highly proliferative bulk progeny, facilitating improved repair of double-strand breaks induced by radiotherapy and most chemotherapeutics. This can occur through multiple mechanisms, including increased expression and splicing fidelity of DNA repair genes, robust activation of cell cycle checkpoints, and elevated homologous recombination-mediated DNA repair. Herein, we summarise the current knowledge concerning improved genome integrity in non-transformed stem cells and CSCs, discuss therapeutic opportunities within the DDR for re-sensitising CSCs to genotoxic stressors, and consider the challenges posed regarding unbiased identification of novel DDR-directed strategies in CSCs. A better understanding of the DDR mediating chemo/radioresistance mechanisms in CSCs could lead to novel therapeutic approaches, thereby enhancing treatment efficacy in cancer patients.

Hypoxia-Driven TGFβ Modulation of Side Population Cells in Breast Cancer: The Potential Role of ERα

Epithelial-to-mesenchymal transition (EMT) is known to be important in regulating the behaviour of cancer cells enabling them to acquire stem cell characteristics or by enhancing the stem cell characteristics of cancer stem cells, resulting in these cells becoming more migratory and invasive. EMT can be driven by a number of mechanisms, including the TGF-β1 signalling pathway and/or by hypoxia. However, these drivers of EMT differ in their actions in regulating side population (SP) cell behaviour, even within SPs isolated from the same tissue. In this study we examined CoCl2 exposure and TGF-β driven EMT on SP cells of the MDA-MB-231 and MCF7 breast cancer cell lines. Both TGF-β1 and CoCl2 treatment led to the depletion of MDA-MB-231 SP. Whilst TGF-β1 treatment significantly reduced the MCF7 SP cells, CoCl2 exposure led to a significant increase. Single cell analysis revealed that CoCl2 exposure of MCF7 SP leads to increased expression of ABCG2 and HES1, both associated with multi-drug resistance. We also examined the mammosphere forming efficiency in response to CoCl2 exposure in these cell lines, and saw the same effect as seen with the SP cells. We suggest that these contrasting effects are due to ERα expression and the inversely correlated expression of TGFB-RII, which is almost absent in the MCF7 cells. Understanding the EMT-mediated mechanisms of the regulation of SP cells could enable the identification of new therapeutic targets in breast cancer.

Drug resistance of hepatoma cells induced by ATP‑binding cassette transporter G2 by reducing intracellular drug concentration

The side effects and drug resistance during chemotherapy seriously affect the outcome of and may lead to the failure of chemotherapy for patients with hepatoma. The aim of the present study was to investigate the association between the expression of ATP-binding cassette transporter G2 (ABCG2) in hepatoma cells and the drug resistance of hepatoma. An MTT assay was used to determine the half-maximal inhibitory concentration (IC₅₀) of Adriamycin (ADM) in hepatoma HepG2 cells after treatment with ADM for 24 h. An ADM-resistant hepatoma cell subline, HepG2/ADM, was generated from the HepG2 hepatoma cell line through a stepwise selection with ADM doses from 0.01 to 0.1 µg/ml. The HepG2/ABCG2 cell line, an ABCG2-overexpressing hepatoma cell line, was established by transfecting the ABCG2 gene into HepG2 cells. The MTT assay was then used to detect the IC₅₀ of ADM in HepG2/ADM and HepG2/ABCG2 cells after treatment with ADM for 24 h and the resistance index was calculated. The apoptosis, cell cycle and ABCG2 protein expression levels in HepG2/ADM, HepG2/ABCG2 cells, HepG2/PCDNA3.1 and their parental HepG2 cells were detected by flow cytometry. In addition, flow cytometry was used to detect the efflux effect of HepG2/ADM and HepG2/ABCG2 cells after ADM treatment. ABCG2 mRNA expression in cells was detected by reverse transcription-quantitative PCR. After 3 months of ADM treatment, HepG2/ADM cells grew stably in the cell culture medium containing 0.1 µg/ml ADM and the cells were named HepG2/ADM cells. ABCG2 was overexpressed in HepG2/ABCG2 cells. The IC₅₀ of ADM in HepG2, HepG2/PCDNA3.1, HepG2/ADM and HepG2/ABCG2 cells was 0.72±0.03, 0.74±0.01, 11.17±0.59 and 12.75±0.47 µg/ml, respectively. The cell apoptotic rate of HepG2/ADM and HepG2/ABCG2 cells was not significantly different compared with that of HepG2 and HepG2/PCDNA3.1 cells (P>0.05), but the G₀/G₁ phase population of the cell cycle decreased and the proliferation index increased significantly (P<0.05). The expression levels of ABCG2 gene and protein in HepG2/ADM and HepG2/ABCG2 cells were significantly higher than those in HepG2 and HepG2/PCDNA3.1 cells (P<0.01), but there was no significant difference between HepG2 and HepG2/PCDNA3.1 cells (P>0.05). The ADM efflux effect of HepG2/ADM and HepG2/ABCG2 cells was significantly higher than that of parental HepG2 and HepG2/PCDNA3.1 cells (P<0.05). Therefore, the present study demonstrated that ABCG2 expression is highly increased in drug-resistant hepatoma cells and that high expression of ABCG2 is involved in the drug resistance of hepatoma by reducing the intracellular drug concentration.

Analysis of Changes in the Expression of Selected Genes from the ABC Family in Patients with Triple-Negative Breast Cancer

Triple-negative breast cancer (TNBC) is characterized by a lack of expression of hormone receptors (estrogen and progesterone), as cancer cells also do not overexpress the HER2 receptor. Due to their molecular profile, treatments for this type of breast cancer are limited. In some cases, the pharmacotherapy of patients with TNBC is hindered by the occurrence of multidrug resistance, which is largely conditioned by proteins encoded by genes from the ABC family. The aim of our study was to determine the expression profile of 14 selected genes from the ABC family using real-time PCR in 68 patients with TNBC by comparing the obtained results with clinical data and additionally using bioinformatics tools (Ualcan and The Breast Cancer Gene Expression Miner v4.8 (bc -GenExMiner v4.8)), as well as by comparing experimental data with data in the Cancer Genome Atlas (TCGA) database. Based on the conducted studies, we found different levels of gene expression depending on the age of patients, tumor sizes, metastases to lymph nodes, cell infiltration into adipose tissue, tumor stages, or lymphovascularinvasion. The results of the presented studies demonstrate the effect of the expression level of the studied genes on the clinical course and prognosis of patients with TNBC, and suggest how profiling the expression level of genes from the ABC family may be a useful tool in determining personalized TNBC treatment.

'Why is survival with triple negative breast cancer so low? insights and talking points from preclinical and clinical research'

INTRODUCTION

Triple negative breast cancer is typically related to poor prognosis, early metastasis, and high recurrence rate. Intrinsic and extrinsic biological features of TNBC and resistance mechanisms to conventional therapies can support its aggressive behavior, characterizing TNBC how extremely heterogeneous. Novel combination strategies are under investigation, including immunotherapeutic agents, anti-drug conjugates, PARP inhibitors, and various targeting agents, exploring, in the meanwhile, possible predictive biomarkers to correctly select patients for the optimal treatment for their specific subtype.

AREAS COVERED

This article examines the main malignity characteristics across different subtype, both histological and molecular, and the resistance mechanisms, both primary and acquired, to different drugs explored in the landscape of TNBC treatment, that lead TNBC to still has high mortality rate.

EXPERT OPINION

The complexity of TNBC is not only the main reason of its aggressivity, but its heterogeneity should be exploited in terms of therapeutics opportunities, combining agents with different mechanism of action, after a correct selection by biologic or molecular biomarkers. The main goal is to understand what TNBC really is and to act selectively on its characteristics, with a personalized anticancer treatment.

The triple negative breast cancer drugs graveyard: a review of failed clinical trials 2017-2022

INTRODUCTION

Triple-negative breast cancer (TNBC) accounts for 15-20% of breast cancers (BC) and has the worst prognosis. It is characterized by the absence of both hormone receptor (HR) and human epidermal growth factor receptor 2 (HER2). TNBC has more limited therapeutic options compared to other subtypes, meaning that there is still a long way to go to discover target treatments.

AREAS COVERED

Our review aims to summarize phase II/III clinical trials enrolling patients with TNBC that have been published between 2017 and 2022 but failed to reach their primary endpoint. We here try to emphasize the limitations and weaknesses noted in negative studies and to point out unexpected results which might be useful to enhance the therapeutic approach to TNBC disease.

EXPERT OPINION

A deeper understanding of the mechanisms behind TNBC heterogeneity allowed to enhance the knowledge of new prognostic and predictive biomarkers of response. However, it is also through several failed clinical trials that we were able to define new therapeutic approaches which improved TNBC patients' clinical outcomes. Nowadays, we still need to overcome several difficulties to fully recognize different intracellular and extracellular pathways that crosstalk in TNBC and the mechanisms of resistance to identify novel tailored-patients' therapies.

TGF-β, to target or not to target; to prevent thyroid cancer progression?

Thyroid cancer (TC) is a common endocrine cancer with a rising incidence. Current treatment fails to eliminate aggressive thyroid tumours, prompting an investigation into the processes that cause disease progression. In this review, we provide insight into TGF-β driven epithelial to mesenchymal transition (EMT), summarizing the current literature surrounding thyroid carcinogenesis, and discuss the potential for therapeutic strategies targeting the TGF-β signalling pathway. Understanding the underlying mechanisms that regulate cancer stem cell (CSC) growth and TGF-β signalling may provide novel therapeutic approaches for highly resistant TCs.

Impact of transforming growth factor beta 1 on normal and thyroid cancer side population cells

PURPOSE

To determine the impact of exogenous transforming growth factor beta 1 (TGF-β1) on side population (SP) cells isolated from normal, papillary thyroid cancer and anaplastic thyroid cancer cell lines and from human thyroid tissues.

METHODS

All cell populations were stained with Hoechst 33342 and analysed using dual wavelength flow cytometry to identify SP cells. This SP assay was used to assess the impact of TGF-β1 treatment and withdrawal of treatment on SP percentages. Semi-quantitative and quantitative PCR were used for molecular analysis of cells pre and post TGF-β1 treatment.

RESULTS

All cell lines expressed mRNA for both TGFB1 and its receptors, as well as showing variable expression of CDH1 and CDH2, with expressing of CDH1 being highest and CDH2 being lowest in the normal cell line. Exposure to exogenous TGF-β1 resulted in a reduction in mRNA expression of ABCG2 compared to controls which was significant between control and treated cancer cell lines. SP cells were isolated from primary human thyroid tissues, with numbers being significantly higher in papillary thyroid cancers. Exposure to TGF-β1 decreased the SP percentage in both thyroid cancer cell lines and completely abrogated these cells in the primary papillary thyroid cancer cultures. On withdrawal of TGF-β1 the SP phenotype was restored in the cancer cell lines and SP percentages increased to above that of untreated cells.

CONCLUSIONS

TGF-β1 exposure transiently regulates thyroid cancer SP cells, leading to a reduction in SP percentages, while withdrawal of TGF-β1 results in restoration of the SP phenotype.

Analysis of Multiple Drug Resistance Mechanism in Different Types of Soft Tissue Sarcomas: Assessment of the Expression of ABC-Transporters, MVP, YB-1, and Analysis of Their Correlation with Chemosensitivity of Cancer Cells

Chemotherapy of soft tissue sarcomas (STS) is restricted by low chemosensitivity and multiple drug resistance (MDR). The purpose of our study was the analysis of MDR mechanism in different types of STS. We assessed the expression of ABC-transporters, MVP, YB-1, and analyzed their correlation with chemosensitivity of cancer cells. STS specimens were obtained from 70 patients without metastatic disease (2018–2020). Expression level of MDR-associated genes was estimated by qRT-PCR and cytofluorimetry. Mutations in ABC-transporter genes were captured by exome sequencing. Chemosensitivity (SI) of STS to doxorubicin (Dox), ifosfamide (Ifo), gemcitabine (Gem), and docetaxel (Doc) was analyzed in vitro. We found strong correlation in ABCB1, ABCC1, and ABCG2 expression. We demonstrated strong negative correlations in ABCB1 and ABCG2 expression with SI (Doc) and SI (Doc + Gem), and positive correlation of MVP expression with SI (Doc) and SI (Doc + Gem) in undifferentiated pleomorphic sarcoma. Pgp expression was shown in 5 out of 44 STS samples with prevalence of synovial sarcoma relapses and it is strongly correlated with SI (Gem). Mutations in MDR-associated genes were rarely found. Overall, STS demonstrated high heterogeneity in chemosensitivity that makes reasonable in vitro chemosensitivity testing to improve personalized STS therapy, and classic ABC-transporters are not obviously involved in MDR appearance.

Molecular Mechanisms, Biomarkers and Emerging Therapies for Chemotherapy Resistant TNBC

Triple-negative breast cancer (TNBC) is associated with high recurrence rates, high incidence of distant metastases, and poor overall survival (OS). Taxane and anthracycline-containing chemotherapy (CT) is currently the main systemic treatment option for TNBC, while platinum-based chemotherapy showed promising results in the neoadjuvant and metastatic settings. An early arising of intrinsic or acquired CT resistance is common and represents the main hurdle for successful TNBC treatment. Numerous mechanisms were uncovered that can lead to the development of chemoresistance. These include cancer stem cells (CSCs) induction after neoadjuvant chemotherapy (NACT), ATP-binding cassette (ABC) transporters, hypoxia and avoidance of apoptosis, single factors such as tyrosine kinase receptors (EGFR, IGFR1), a disintegrin and metalloproteinase 10 (ADAM10), and a few pathological molecular pathways. Some biomarkers capable of predicting resistance to specific chemotherapeutic agents were identified and are expected to be validated in future studies for a more accurate selection of drugs to be employed and for a more tailored approach, both in neoadjuvant and advanced settings. Recently, based on specific biomarkers, some therapies were tailored to TNBC subsets and became available in clinical practice: olaparib and talazoparib for BRCA1/2 germline mutation carriers larotrectinib and entrectinib for neurotrophic tropomyosin receptor kinase (NTRK) gene fusion carriers, and anti-trophoblast cell surface antigen 2 (Trop2) antibody drug conjugate therapy for heavily pretreated metastatic TNBC (mTNBC). Further therapies targeting some pathologic molecular pathways, apoptosis, miRNAS, epidermal growth factor receptor (EGFR), insulin growth factor 1 receptor (IGF-1R), and androgen receptor (AR) are under investigation. Among them, phosphatidylinositol 3 kinase (PI3K)/protein kinase B (Akt)/mammalian target of rapamycin (mTOR) and EGFR inhibitors as well as antiandrogens showed promising results and are under evaluation in Phase II/III clinical trials. Emerging therapies allow to select specific antiblastics that alone or by integrating the conventional therapeutic approach may overcome/hinder chemoresistance.

Molecular Targets of Triple-Negative Breast Cancer: Where Do We Stand?

Triple-negative breast cancer (TNBC) is a highly aggressive form of breast cancer. Due to its heterogeneity and lack of hormone receptor expression, this subtype is more likely to metastasize and resist treatment attempts than are other forms of breast cancer. Due to the absence of targetable receptors, chemotherapy and breast conserving surgery have been the predominant treatment options for patients. However, resistance to chemotherapy and local recurrence of the tumors is frequent. Emerging immunotherapies have begun to change treatment plans for patients diagnosed with TNBC. In this review, we discuss the various immune pathways identified in TNBC and the role they play as targets for new potential treatment choices. Various therapeutic options that inhibit key pathways in cellular growth cycles, DNA repair mechanisms, epithelial mesenchymal transition, and immunosuppression have been shown to improve survival in patients with this disease. With promising results thus far, continued studies of immunotherapy and neoadjuvant therapy options for TNBC are likely to alter the treatment course for these diagnoses in the future.

Role, molecular mechanism and the potential target of breast cancer stem cells in breast cancer development

Breast cancer (BC) is one of the most common malignant tumors in women globally, and its occurrence has surpassed lung cancer and become the biggest threat for women. At present, breast cancer treatment includes surgical resection or postoperative chemotherapy and radiotherapy. However, tumor relapse and metastasis usually lead to current therapy failure thanks to breast cancer stem cells (BCSCs)-mediated tumorigenicity and drug resistance. Drug resistance is mainly due to the long-term quiescent G0 phase, strong DNA repairability, and high expression of ABC transporter, and the tumorigenicity is reflected in the activation of various proliferation pathways related to BCSCs. Therefore, understanding the characteristics of BCSCs and their intracellular and extracellular molecular mechanisms is crucial for the development of targeted drugs for BCSCs. To this end, we discussed the latest developments in BCSCs research, focusing on the analysis of specific markers, critical signaling pathways that maintain the stemness of BCSCs，such as NOTCH, Wnt/β-catenin, STAT3, Hedgehog, and Hippo-YAP signaling, immunomicroenviroment and summarizes targeting therapy strategies for stemness maintenance and differentiation, which provides a theoretical basis for further exploration of treating breast cancer and preventing relapse derived from BCSCs.

The Role of Breast Cancer Stem Cells in Chemoresistance and Metastasis in Triple-Negative Breast Cancer

Triple negative breast cancer (TNBC) remains an aggressive disease due to the lack of targeted therapies and low rate of response to chemotherapy that is currently the main treatment modality for TNBC. Breast cancer stem cells (BCSCs) are a small subpopulation of breast tumors and recognized as drivers of tumorigenesis. TNBC tumors are characterized as being enriched for BCSCs. Studies have demonstrated the role of BCSCs as the source of metastatic disease and chemoresistance in TNBC. Multiple targets against BCSCs are now under investigation, with the considerations of either selectively targeting BCSCs or co-targeting BCSCs and non-BCSCs (majority of tumor cells). This review article provides a comprehensive overview of recent advances in the role of BCSCs in TNBC and the identification of cancer stem cell biomarkers, paving the way for the development of new targeted therapies. The review also highlights the resultant discovery of cancer stem cell targets in TNBC and the ongoing clinical trials treating chemoresistant breast cancer. We aim to provide insights into better understanding the mutational landscape of BCSCs and exploring potential molecular signaling pathways targeting BCSCs to overcome chemoresistance and prevent metastasis in TNBC, ultimately to improve the overall survival of patients with this devastating disease.

An insight into the cancer stem cell survival pathways involved in chemoresistance in triple-negative breast cancer

Triple-negative breast cancer (TNBC) is the most complex, aggressive and fatal subtype of breast cancer. Owing to the lack of targeted therapy and heterogenic nature of TNBC, chemotherapy remains the sole treatment option for TNBC, with taxanes and anthracyclines representing the general chemotherapeutic regimen in TNBC therapy. But unfortunately, patients develop resistance to the existing chemotherapeutic regimen, resulting in approximately 90% treatment failure. Breast cancer stem cells (BCSCs) are one of the major causes for the development of chemoresistance in TNBC patients. After surviving the chemotherapy damage, the presence of BCSCs results in relapse and recurrence of TNBC. Several pathways are known to regulate BCSCs' survival, such as the Wnt/β-catenin, Hedgehog, JAK/STAT and HIPPO pathways. Therefore it is imperative to target these pathways in the context of eliminating chemoresistance. In this review we will discuss the novel strategies and various preclinical and clinical studies to give an insight into overcoming TNBC chemoresistance. We present a detailed account of recent studies carried out that open an exciting perspective in relation to the mechanisms of chemoresistance.

Triple Negative Breast Cancer: A Mountain Yet to Be Scaled Despite the Triumphs

Metastatic progression and tumor recurrence pertaining to TNBC are certainly the leading cause of breast cancer-related mortality; however, the mechanisms underlying TNBC chemoresistance, metastasis, and tumor relapse remain somewhat ambiguous. TNBCs show 77% of the overall 4-year survival rate compared to other breast cancer subtypes (82.7 to 92.5%). TNBC is the most aggressive subtype of breast cancer, with chemotherapy being the major approved treatment strategy. Activation of ABC transporters and DNA damage response genes alongside an enrichment of cancer stem cells and metabolic reprogramming upon chemotherapy contribute to the selection of chemoresistant cells, majorly responsible for the failure of anti-chemotherapeutic regime. These selected chemoresistant cells further lead to distant metastasis and tumor relapse. The present review discusses the approved standard of care and targetable molecular mechanisms in chemoresistance and provides a comprehensive update regarding the recent advances in TNBC management.

Pleiotropic Roles of ABC Transporters in Breast Cancer

Chemotherapeutics are the mainstay treatment for metastatic breast cancers. However, the chemotherapeutic failure caused by multidrug resistance (MDR) remains a pivotal obstacle to effective chemotherapies of breast cancer. Although in vitro evidence suggests that the overexpression of ATP-Binding Cassette (ABC) transporters confers resistance to cytotoxic and molecularly targeted chemotherapies by reducing the intracellular accumulation of active moieties, the clinical trials that target ABCB1 to reverse drug resistance have been disappointing. Nevertheless, studies indicate that ABC transporters may contribute to breast cancer development and metastasis independent of their efflux function. A broader and more clarified understanding of the functions and roles of ABC transporters in breast cancer biology will potentially contribute to stratifying patients for precision regimens and promote the development of novel therapies. Herein, we summarise the current knowledge relating to the mechanisms, functions and regulations of ABC transporters, with a focus on the roles of ABC transporters in breast cancer chemoresistance, progression and metastasis.

Trailblazing perspectives on targeting breast cancer stem cells

Breast cancer (BCa) is one of the most prevalent malignant tumors affecting women's health worldwide. The recurrence and metastasis of BCa have made it a long-standing challenge to achieve remission-persistent or disease-undetectable clinical outcomes. Cancer stem cells (CSCs) possess the ability to self-renew and generate heterogeneous tumor bulk. The existence of CSCs has been found to be vital in the initiation, metastasis, therapy resistance, and recurrence of tumors across cancer types. Because CSCs grow slowly in their dormant state, they are insensitive to conventional chemotherapies; however, when CSCs emerge from their dormant state and become clinically evident, they usually acquire genetic traits that make them resistant to existing therapies. Moreover, CSCs also show evidence of acquired drug resistance in synchrony with tumor relapses. The concept of CSCs provides a new treatment strategy for BCa. In this review, we highlight the recent advances in research on breast CSCs and their association with epithelial-mesenchymal transition (EMT), circulating tumor cells (CTCs), plasticity of tumor cells, tumor microenvironment (TME), T-cell modulatory protein PD-L1, and non-coding RNAs. On the basis that CSCs are associated with multiple dysregulated biological processes, we envisage that increased understanding of disease sub-classification, selected combination of conventional treatment, molecular aberration directed therapy, immunotherapy, and CSC targeting/sensitizing strategy might improve the treatment outcome of patients with advanced BCa. We also discuss novel perspectives on new drugs and therapeutics purposing the potent and selective expunging of CSCs.

Apigenin by targeting hnRNPA2 sensitizes triple-negative breast cancer spheroids to doxorubicin-induced apoptosis and regulates expression of ABCC4 and ABCG2 drug efflux transporters

Acquired resistance to doxorubicin is a major hurdle in triple-negative breast cancer (TNBC) therapy, emphasizing the need to identify improved strategies. Apigenin and other structurally related dietary flavones are emerging as potential chemo-sensitizers, but their effect on three-dimensional TNBC spheroid models has not been investigated. We previously showed that apigenin associates with heterogeneous ribonuclear protein A2/B1 (hnRNPA2), an RNA-binding protein involved in mRNA and co-transcriptional regulation. However, the role of hnRNPA2 in apigenin chemo-sensitizing activity has not been investigated. Here, we show that apigenin induced apoptosis in TNBC spheroids more effectively than apigenin-glycoside, owing to higher cellular uptake. Moreover, apigenin inhibited the growth of TNBC patient-derived organoids at an in vivo achievable concentration. Apigenin sensitized spheroids to doxorubicin-induced DNA damage, triggering caspase-9-mediated intrinsic apoptotic pathway and caspase-3 activity. Silencing of hnRNPA2 decreased apigenin-induced sensitization to doxorubicin in spheroids by diminishing apoptosis and partly abrogated apigenin-mediated reduction of ABCC4 and ABCG2 efflux transporters. Together these findings provide novel insights into the critical role of hnRNPA2 in mediating apigenin-induced sensitization of TNBC spheroids to doxorubicin by increasing the expression of efflux transporters and apoptosis, underscoring the relevance of using dietary compounds as a chemotherapeutic adjuvant.

Counteracting Chemoresistance with Metformin in Breast Cancers: Targeting Cancer Stem Cells

Despite the leaps and bounds in achieving success in the management and treatment of breast cancers through surgery, chemotherapy, and radiotherapy, breast cancer remains the most frequently occurring cancer in women and the most common cause of cancer-related deaths among women. Systemic therapeutic approaches, such as chemotherapy, although beneficial in treating and curing breast cancer subjects with localized breast tumors, tend to fail in metastatic cases of the disease due to (a) an acquired resistance to the chemotherapeutic drug and (b) the development of intrinsic resistance to therapy. The existence of cancer stem cells (CSCs) plays a crucial role in both acquired and intrinsic chemoresistance. CSCs are less abundant than terminally differentiated cancer cells and confer chemoresistance through a unique altered metabolism and capability to evade the immune response system. Furthermore, CSCs possess active DNA repair systems, transporters that support multidrug resistance (MDR), advanced detoxification processes, and the ability to self-renew and differentiate into tumor progenitor cells, thereby supporting cancer invasion, metastasis, and recurrence/relapse. Hence, current research is focusing on targeting CSCs to overcome resistance and improve the efficacy of the treatment and management of breast cancer. Studies revealed that metformin (1, 1-dimethylbiguanide), a widely used anti-hyperglycemic agent, sensitizes tumor response to various chemotherapeutic drugs. Metformin selectively targets CSCs and improves the hypoxic microenvironment, suppresses the tumor metastasis and inflammation, as well as regulates the metabolic programming, induces apoptosis, and reverses epithelial-mesenchymal transition and MDR. Here, we discuss cancer (breast cancer) and chemoresistance, the molecular mechanisms of chemoresistance in breast cancers, and metformin as a chemo-sensitizing/re-sensitizing agent, with a particular focus on breast CSCs as a critical contributing factor to acquired and intrinsic chemoresistance. The review outlines the prospects and directions for a better understanding and re-purposing of metformin as an anti-cancer/chemo-sensitizing drug in the treatment of breast cancer. It intends to provide a rationale for the use of metformin as a combinatory therapy in a clinical setting.

Antagonists of the serotonin receptor 5A target human breast tumor initiating cells

Background

Breast tumor initiating cells (BTIC) are stem-like cells that initiate and sustain tumor growth, and drive disease recurrence. Identifying therapies targeting BTIC has been hindered due primarily to their scarcity in tumors. We previously reported that BTIC frequency ranges between 15% and 50% in multiple mammary tumors of 3 different transgenic mouse models of breast cancer and that this frequency is maintained in tumor cell populations cultured in serum-free, chemically defined media as non-adherent tumorspheres. The latter enabled high-throughput screening of small molecules for their capacity to affect BTIC survival. Antagonists of several serotonin receptors (5-HTRs) were among the hit compounds. The most potent compound we identified, SB-699551, selectively binds to 5-HT5A, a Gα_i/o protein coupled receptor (GPCR).

Methods

We evaluated the activity of structurally unrelated selective 5-HT5A antagonists using multiple orthogonal assays of BTIC frequency. Thereafter we used a phosphoproteomic approach to uncover the mechanism of action of SB-699551. To validate the molecular target of the antagonists, we used the CRISPR-Cas9 gene editing technology to conditionally knockout HTR5A in a breast tumor cell line.

Results

We found that selective antagonists of 5-HT5A reduced the frequency of tumorsphere initiating cells residing in breast tumor cell lines and those of patient-derived xenografts (PDXs) that we established. The most potent compound among those tested, SB-699551, reduced the frequency of BTIC in ex vivo assays and acted in concert with chemotherapy to shrink human breast tumor xenografts in vivo. Our phosphoproteomic experiments established that exposure of breast tumor cells to SB-699551 elicited signaling changes in the canonical Gα_i/o-coupled pathway and the phosphoinositide 3-kinase (PI3K)/AKT/mammalian target of rapamycin (mTOR) axis. Moreover, conditional mutation of the HTR5A gene resulted in the loss of tumorsphere initiating cells and BTIC thus mimicking the effect of SB-699551.

Conclusions

Our data provide genetic, pharmacological and phosphoproteomic evidence consistent with the on-target activity of SB-699551. The use of such agents in combination with cytotoxic chemotherapy provides a novel therapeutic approach to treat breast cancer.

HIF-1ɑ-regulated miR-1275 maintains stem cell-like phenotypes and promotes the progression of LUAD by simultaneously activating Wnt/β-catenin and Notch signaling

Rationale: Cancer stem cells (CSCs) are considered to be essential for tumorigenesis, recurrence, and metastasis and therefore serve as a biomarker for tumor progression in diverse cancers. Recent studies have illustrated that specific miRNAs exhibit novel therapeutic potential by controlling CSC properties. miR-1275 is upregulated in lung adenocarcinoma (LUAD) and enhances its stemness. However, the underlying mechanisms have not been elucidated.

Methods: miRNA expression microarray of LUAD and adjacent nontumor tissues was used to identify miRNAs involved in LUAD malignant progression. miR-1275 expression level was determined using quantitative real-time PCR (RT-qPCR) and in situ hybridization (ISH), and its correlation with clinicopathological characteristics was analyzed in LUAD specimens. The upstream regulator of miR-1275 was validated by chromatin immunoprecipitation (ChIP). The biological functions and underlying mechanisms of miR-1275 were investigated both in vitro and in vivo.

Results: MiR-1275 was highly upregulated in lung cancer cell lines and LUAD tissues. Overexpression of miR-1275 in lung cancer patients was associated with shorter overall- and recurrence-free-survival. Proto-oncogene HIF-1ɑ was identified as the transcription mediator of miR-1275. Activation of Wnt/β-catenin and Notch signaling by miR-1275 was found to enhance the stemness of LUAD cells, while antagonizing miR-1275 or suppressing Wnt/β-catenin and Notch pathways potently reversed miR-1275-induced pathway co-activation and stemness. Enhanced stemness dramatically promoted tumorigenicity, recurrence, and metastasis. miR-1275 directly targeted multiple antagonists of Wnt/β-catenin and Notch pathways, including DKK3, SFRP1, GSK3β, RUNX3, and NUMB, respectively, which resulted in signaling activation.

Conclusions: Our findings identified miR-1275 as a potential oncogene in LUAD that exerts its tumorigenic effect through co-activating Wnt/β-catenin and Notch signaling pathways. Thus, HIF-1ɑ-regulated miR-1275 might be a potential therapeutic target for LUAD.

Breast Cancer Stem Cells as Drivers of Tumor Chemoresistance, Dormancy and Relapse: New Challenges and Therapeutic Opportunities

Breast cancer is the most frequent cancer among women worldwide. Therapeutic strategies to prevent or treat metastatic disease are still inadequate although great progress has been made in treating early-stage breast cancer. Cancer stem-like cells (CSCs) that are endowed with high plasticity and self-renewal properties have been shown to play a key role in breast cancer development, progression, and metastasis. A subpopulation of CSCs that combines tumor-initiating capacity and a dormant/quiescent/slow cycling status is present throughout the clinical history of breast cancer patients. Dormant/quiescent/slow cycling CSCs are a key component of tumor heterogeneity and they are responsible for chemoresistance, tumor migration, and metastatic dormancy, defined as the ability of CSCs to survive in target organs and generate metastasis up to two decades after diagnosis. Understanding the strategies that are used by CSCs to resist conventional and targeted therapies, to interact with their niche, to escape immune surveillance, and finally to awaken from dormancy is of key importance to prevent and treat metastatic cancer. This review summarizes the current understanding of mechanisms involved in CSCs chemoresistance, dissemination, and metastasis in breast cancer, with a particular focus on dormant cells. Finally, we discuss how advancements in the detection, molecular understanding, and targeting of dormant CSCs will likely open new therapeutic avenues for breast cancer treatment.

Mechanisms of Chemotherapy Resistance in Triple-Negative Breast Cancer-How We Can Rise to the Challenge

Triple-negative (TNBC) is the most lethal subtype of breast cancer owing to high heterogeneity, aggressive nature, and lack of treatment options. Chemotherapy remains the standard of care for TNBC treatment, but unfortunately, patients frequently develop resistance. Accordingly, in recent years, tremendous effort has been made into elucidating the mechanisms of TNBC chemoresistance with the goal of identifying new molecular targets. It has become evident that the development of TNBC chemoresistance is multifaceted and based on the elaborate interplay of the tumor microenvironment, drug efflux, cancer stem cells, and bulk tumor cells. Alterations of multiple signaling pathways govern these interactions. Moreover, TNBC's high heterogeneity, highlighted in the existence of several molecular signatures, presents a significant obstacle to successful treatment. In the present, in-depth review, we explore the contribution of key mechanisms to TNBC chemoresistance as well as emerging strategies to overcome them. We discuss novel anti-tumor agents that target the components of these mechanisms and pay special attention to their current clinical development while emphasizing the challenges still ahead of successful TNBC management. The evidence presented in this review outlines the role of crucial pathways in TNBC survival following chemotherapy treatment and highlights the importance of using combinatorial drug strategies and incorporating biomarkers in clinical studies.

Histone methyltransferase NSD2 regulates apoptosis and chemosensitivity in osteosarcoma

Osteosarcoma (OS) is a primary malignant bone tumour. However, the genetic basis for the pathogenesis of OS remains elusive. In this study, we uncovered the role of the histone methyltransferase NSD2 in regulating tumourigenesis and chemosensitivity in OS. We show that NSD2 knockdown leads to increased apoptosis in OS cells in vitro and in vivo. Additionally, NSD2 knockdown significantly enhances the efficacy of cisplatin against OS cells and accordingly inhibits properties associated with cancer stem cells (CSCs). Furthermore, RNA sequencing (RNAseq) and Gene Ontology (GO) analysis revealed that NSD2 promotes transcription of genes associated with negative regulation of apoptotic signalling pathways and CSC properties. The results of chromatin immunoprecipitation quantitative polymerase chain reaction (ChIP-qPCR) assays indicated that NSD2 knockdown leads to decreased H3K36me2 modification at BCL2 and SOX2 loci, thus inhibiting the transcription of these two genes that are closely correlated with apoptosis, CSC properties and chemosensitivity in OS cells. Pathway analysis demonstrated that the ERK and AKT pathways mediate the regulation of OS progression and chemosensitivity by NSD2. Overall, our study is the first to uncover the function of NSD2 in OS chemosensitivity. NSD2 regulates the expression of the apoptosis regulatory proteins BCL2 and SOX2 through the ERK and AKT pathways. Our results suggest that NSD2 is a new target for combined chemotherapy and is a prognostic factor for OS.

Targeting of the Eukaryotic Translation Initiation Factor 4A Against Breast Cancer Stemness

Breast cancer stem cells (BCSCs) are intrinsically chemoresistant and capable of self-renewal. Following chemotherapy, patients can develop minimal residual disease due to BCSCs which can repopulate into a relapsed tumor. Therefore, it is imperative to co-target BCSCs along with the bulk tumor cells to achieve therapeutic success and prevent recurrence. So, it is vital to identify actionable molecular targets against both BCSCs and bulk tumor cells. Previous findings from our lab and others have demonstrated that inhibition of the emerging drug target eIF4A with Rocaglamide A (RocA) was efficacious against triple-negative breast cancer cells (TNBC). RocA specifically targets the pool of eIF4A bound to the oncogenic mRNAs that requires its helicase activity for their translation. This property enables specific targeting of tumor cells. The efficacy of RocA against BCSCs is unknown. In this study, we postulated that eIF4A could be a vulnerable node in BCSCs. In order to test this, we generated a paclitaxel-resistant TNBC cell line which demonstrated an elevated level of eIF4A along with increased levels of cancer stemness markers (ALDH activity and CD44), pluripotency transcription factors (SOX2, OCT4, and NANOG) and drug transporters (ABCB1, ABCG2, and ABCC1). Furthermore, genetic ablation of eIF4A resulted in reduced expression of ALDH1A1, pluripotency transcription factors and drug transporters. This pointed out that eIF4A is likely associated with selected set of proteins that are critical to BCSCs, and hence targeting eIF4A may eliminate BCSCs. Therefore, we isolated BCSCs from two TNBC cell lines: MDA-Bone-Un and SUM-159PT. Following RocA treatment, the self-renewal ability of the BCSCs was significantly reduced as determined by the efficiency of the formation of primary and secondary mammospheres. This was accompanied by a reduction in the levels of NANOG, OCT4, and drug transporters. Exposure to RocA also induced cell death of the BCSCs as evaluated by DRAQ7 and cell viability assays. RocA treatment induced apoptosis with increased levels of cleaved caspase-3. Overall, we identified that RocA is effective in targeting BCSCs, and eIF4A is an actionable molecular target in both BCSCs and bulk tumor cells. Therefore, anti-eIF4A inhibitors could potentially be combined synergistically with existing chemo-, radio- and/or immunotherapies.

Do Different Stemness Markers Identify Different Pools of Cancer Stem Cells in Malignancies: A Study on ER+ and ER-Breast Cancer Cell Lines

In view of popularity of cancer stem cell (CSC) model all events in evolution of cancer are being explained in that context. Breast cancer is first solid tumor in which CSCs were identified. We aimed to compare stemness profile of two major subtypes [Estrogen receptor positive (ER⁺) and negative (ER^-)] breast cancer using different sets of markers. Expression of CD44/CD24, CK/Vimentin, E-Cadherin/Fibronectin and percentage of side population (SP) was studied in ER⁺ (T47D) and ER^- (MDA-MB-231) cell lines by flow cytometry. Breast CSCs (BCSCs) were sorted using CD44⁺/CD24^-/low expression and SP analysis and cultured. BCSCs were then compared with Non-CSCs (NCSCs) for response to drugs (Paclitaxel and Cisplatin), Ki67 and ER expression. Results showed higher expression of stemness markers (CD44⁺/CD24^-/low, CK⁺/Vimentin⁺ and E-Cadherin^-/Fibrinectin^F+) in MDA-MB-231 cells. Percentage SP representing BCSCs was found to be significantly more in later (3.20 ± 0.002 cf. T47D 1.25% ± 0.0007). BCSCs were found to be more resistant to drugs as compared to NCSCs in both cell lines. ER expression was weak in BCSCs sorted from T47D as compared to NCSCs. Ki67 was expressed in both BCSCs and NCSCs. Differences in expression of stemness markers help to explain aggressive behavior, higher recurrence rate and metastatic potential of MDA-MB-231 cells. However, no correlation amongst different markers used suggests that they may be identifying varied populations of cells in tumor hierarchy. A weak ER expression in BCSCs may be strategy used by BCSCs to escape effect of hormone therapy in ER⁺ breast cancers.

SDF-1/CXCR4 axis promotes the growth and sphere formation of hypoxic breast cancer SP cells by c-Jun/ABCG2 pathway

ATP-binding cassette sub-family G member 2 (ABCG2) confers to the major phenotypes of side population (SP) cells, the cancer stem-like cells. In this study, the SP cells displayed a distinctly higher ABCG2 expression level, sphere formation efficiency (SFE) and growth rate even under hypoxia condition. CXCR4 overexpression by pcDNA-CXCR4 transfection robustly increased ABCG2 expression, and promoted SFE and growth of hypoxic SP cells, while CXCR4 inhibitor AMD3100 could suppress the promotion. Additionally, we found that CXCR4 promoted the expression of c-Jun, a major gene in the oncogenic JNK/c-Jun pathway. Our data on electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation (ChIP) assays both showed that c-Jun directly bound with the ABCG2 promoter sequence. Moreover, overexpression of JNK/c-Jun promoted ABCG2 expression, SFE, and growth of hypoxic SP cells and the promotion could be rescued by c-Jun inhibitor SP600125. In conclusion, CXCR4 increases the growth and SFE of breast cancer SP cells under hypoxia through c-Jun-mediated transcriptional activation of ABCG2.

Secretome within the bone marrow microenvironment: A basis for mesenchymal stem cell treatment and role in cancer dormancy

The secretome produced by cells within the bone marrow is significant to homeostasis. The bone marrow, a well-studied organ, has multiple niches with distinct roles for supporting stem cell functions. Thus, an understanding of mediators involved in the regulation of stem cells could serve as a model for clinical problems and solutions such as tissue repair and regeneration. The exosome secretome of bone marrow stem cells is a developing area of research with respect to the regenerative potential by bone marrow cell, particularly the mesenchymal stem cells. The bone marrow niche regulates endogenous processes such as hematopoiesis but could also support the survival of tumors such as facilitating the cancer stem cells to exist in dormancy for decades. The bone marrow-derived secretome will be critical to future development of therapeutic strategies for oncologic diseases, in addition to regenerative medicine. This article discusses the importance for parallel studies to determine how the same secretome may compromise safety during the use of stem cells in regenerative medicine.

Clathrin-mediated endocytic uptake of PUFA enriched self-nanoemulsifying lipidic systems (SNELS) of an anticancer drug against triple negative cancer and DMBA induced preclinical tumor model

The current studies envisage unravelling the underlying cellular internalisation mechanism of the systematically developed docetaxel (DTH) polyunsaturated fatty acid (PUFA) enriched self-nanoemulsifying lipidic micellar systems (SNELS). The concentration-, time- and cytotoxicity-related effects of DTH-SNELS on triple negative breast cancer (TNBC) MDA-MB-231 and non-TNBC MCF-7 cell lines were assessed through Presto-blue assay. Subsequently, rhodamine-123 (Rh-123) loaded SNELS were employed for evaluating their internalisation through flow cytometry and fluorescence microscopy, establishing it to be "clathrin-mediated" endocytic pathway. Apoptosis assay (65% cell death) and cell cycle distribution (47% inhibition at G2/M phase) further corroborated the cytotoxicity of DTH-SNELS towards cancerous cells. Biodistribution, histopathology and haematology studies indicated insignificant toxicity of the optimized formulation on vital organs. Preclinical anticancer efficacy studies using 7,12-dimethylbenzantracene (DMBA)-induced model construed significant reduction in breast tumor-volume. Overall, extensive in vitro and in vivo studies indicated the intracellular localization and cytotoxicity, suggesting DTH-SNELS as promising delivery systems for breast tumor therapeutics including TNBC.

TRIM28 multi-domain protein regulates cancer stem cell population in breast tumor development

The expression of Tripartite motif-containing protein 28 (TRIM28)/Krüppel-associated box (KRAB)-associated protein 1 (KAP1), is elevated in at least 14 tumor types, including solid and hematopoietic tumors. High level of TRIM28 is associated with triple-negative subtype of breast cancer (TNBC), which shows higher aggressiveness and lower survival rates. Interestingly, TRIM28 is essential for maintaining the pluripotent phenotype in embryonic stem cells. Following on that finding, we evaluated the role of TRIM28 protein in the regulation of breast cancer stem cells (CSC) populations and tumorigenesis in vitro and in vivo. Downregulation of TRIM28 expression in xenografts led to deceased expression of pluripotency and mesenchymal markers, as well as inhibition of signaling pathways involved in the complex mechanism of CSC maintenance. Moreover, TRIM28 depletion reduced the ability of cancer cells to induce tumor growth when subcutaneously injected in limiting dilutions. Our data demonstrate that the downregulation of TRIM28 gene expression reduced the ability of CSCs to self-renew that resulted in significant reduction of tumor growth. Loss of function of TRIM28 leads to dysregulation of cell cycle, cellular response to stress, cancer cell metabolism, and inhibition of oxidative phosphorylation. All these mechanisms directly regulate maintenance of CSC population. Our original results revealed the role of the TRIM28 in regulating the CSC population in breast cancer. These findings may pave the way to novel and more effective therapies targeting cancer stem cells in breast tumors.

Pharmacological targets of breast cancer stem cells: a review

Breast cancers contain small population of tumor-initiating cells called breast cancer stem cells (BCSCs), which are spared even after chemotherapy. Recently, BCSCs are implicated to be a cause of metastasis, tumor relapse, and therapy resistance in breast cancer. BCSCs have unique molecular mechanisms, which can be targeted to eliminate them. These include surface biomarkers, proteins involved in self-renewal pathways, drug efflux transporters, apoptotic/antiapoptotic proteins, autophagy, metabolism, and microenvironment regulation. The complex molecular mechanisms behind the survival of BCSCs and pharmacological targets for elimination of BCSCs are described in this review.

A role for ABCG2 beyond drug transport: Regulation of autophagy

The ABC drug transporters, including ABCG2, are well known for their ability to efflux a wide spectrum of chemotherapeutic agents, thereby conferring a multidrug-resistant phenotype. However, studies over the past several years suggest that the ABC transporters may play additional role(s) in cell survival in the face of stress inducers that are not ABCG2 substrates (i.e., nutrient deprivation, ionizing radiation, rapamycin). The mechanism by which this occurs is largely unknown. In the present study, using several cancer cell lines and their ABCG2-overexpressing sublines, we show that cells overexpressing ABCG2 were more resistant to these stressors. This resistance was associated with an elevated level of autophagy flux, as measured by a higher rate of SQSTM1/p62 degradation and greater accumulation of LC3-II when compared to parental cells. Knockdown of ABCG2 reduced autophagic activity in resistant cells to a level similar to that observed in parental cells, confirming that the enhanced autophagy was ABCG2-dependent. Moreover, using cell viability, apoptosis, and clonogenic assays, we demonstrated that the ABCG2-expressing cells were more resistant to amino acid starvation and radiation-induced cell death. Importantly, knockdown of the critical autophagy factors ATG5 and ATG7 greatly reduced cell survival, verifying that enhanced autophagy was critical for this effect. Taken together, these data indicate that autophagy induced by various stressors is enhanced/accelerated in the presence of ABCG2, resulting in delayed cell death and enhanced cell survival. This defines a new role for this transporter, one with potential clinical significance.

Identification of side population cells in human lung adenocarcinoma A549 cell line and elucidation of the underlying roles in lung cancer

The present study aimed to isolate and characterize side population (SP) cells in the human lung cancer A549 cell line, and elucidate the molecular mechanism of SP cells underlying lung cancer. The SP and non-SP (NSP) cells in A549 cells were isolated and their differentiation was analyzed by fluorescence-activated cell sorting. An in vitro plate clone assay, Matrigel^® Transwell assay and chemoresistance analysis of the sorted SP and NSP cells were performed. In addition, the sorted SP and NSP cells were injected into BALB/c nude mice to detect their tumorigenic potential in vivo. The expression of ATP-binding cassette sub-family G member 2 (ABCG2) in transplanted tumors was detected by immunohistochemistry. The SP and NSP cells were successfully isolated. The results demonstrated that SP cells accounted for 1.09% of live A549 cells. SP cells produced SP and NSP cells, while NSP cells only produced NSP cells. In addition, SP cells formed more colonies, exhibited improved invasive ability and increased levels of chemoresistance compared with NSP cells in vitro. SP cells demonstrated a higher tumorigenic potential in BALB/c nude mice, and the number of ABCG2-positive cells in the SP xenograft tumors were significantly increased compared with that in the NSP xenograft tumors. The present study indicated that SP cells isolated from the human lung cancer A549 cell line demonstrated increased tumorigenicity, and improved invasive ability and chemoresistance compared with NSP cells. In addition, detection of ABCG2 expression may assist in predicting the chemotherapeutic outcome of patients, and serve as a target for treating lung cancer.

Oxaliplatin resistance in colorectal cancer cells is mediated via activation of ABCG2 to alleviate ER stress induced apoptosis

Oxaliplatin (OXA), is a third generation platinum drug used as first-line chemotherapy in colorectal cancer (CRC). Cancer cells acquires resistance to anti-cancer drug and develops resistance. ATP-binding cassette (ABC) drug transporter ABCG2, one of multidrug resistance (MDR) protein which can effectively discharge a wide spectrum of chemotherapeutic agents out of cancer cells and subsequently reduce the intracellular concentration of these drugs. Role of ABCG2 and plausible molecular signaling pathways involved in Oxaliplatin-Resistant (OXA-R) colon cancer cells was evaluated in the present study. OXA resistant LoVo cells was developed by exposing the colon cells to OXA in a dose-dependent manner. Development of multi drug resistance in OXA-R cells was confirmed by exposing the resistance cells to oxaliplatin, 5-FU, and doxorubicin. OXA treatment resulted in G2 phase arrest in parental LoVo cells, which was overcome by OXA-R LoVo cells. mRNA and protein expression of ABCG2 and phosphorylation of NF-κB was significantly higher in OXA-R than parental cells. Levels of ER stress markers were downregulated in OXA-R than parental cells. OXA-R LoVo cells exposed to NF-κB inhibitor QNZ effectively reduced the ABCG2 and p-NF-κB expression and increased ER stress marker expression. On other hand, invasion and migratory effect of OXA-R cells were found to be decreased, when compared to parental cells. Metastasis marker proteins also downregulated in OXA-R cells. ABCG2 inhibitor verapamil, downregulate ABCG2, induce ER stress markers and induces apoptosis. In vivo studies in nude mice also confirms the same.

Isolation and identification of tumor-initiating cell properties in human gallbladder cancer cell lines using the marker cluster of differentiation 133

The present study aimed to isolate and identify the properties of the cluster of differentiation (CD)133⁺ subset in human gallbladder cancer cells. The CD133⁺ and CD133⁻ subpopulations of the GBC-SD cell line were separated using immunomagnetic separation, and the biological features of the two subpopulations were analyzed in vitro and in vivo. In particular, the present study aimed to determine whether the two subpopulations were resistant to anti-tumor reagents and to identify the underlying molecular mechanisms involved. Following cell sorting of GBC-SD cells using immunomagnetic beads, 90.2±2% of cells were identified as CD133⁺. Immunofluorescence confirmed that CD133 was expressed at higher levels in the Cd133⁺ group compared with the CD133⁻ group. The proliferation of the CD133⁺ group was significantly increased compared with the CD133⁻ group in vitro and in vivo. Following treatment with fluorouracil or gemcitabine, cells in the CD133⁺ group exhibited a decreased sensitivity to these drugs. The number of transmembrane cells was significantly increased in the CD133⁺ group compared with the CD133⁻ group. In addition, the expression levels of ATP binding cassette subfamily G member 2, CD44, C-X-C motif chemokine receptor 4 (CXCR4), phosphorylated-protein kinase B (Akt) and CD133 in the CD133⁺ group were significantly increased, compared with those in the CD133⁻ group. In CD133⁺ GBC-SD cells, stromal cell-derived factor 1α (SDF-1α) or treatment with AMD3100, an inhibitor of CXCR4, promotes or suppresses the SDF-1α/CXCR4 axis, respectively, resulting in increased or decreased CD133 expression through the Akt signaling pathway. Inhibition of the Akt signaling pathway resulted in decreased CD133 expression in GBC-SD cells. Immunomagnetic beads were successfully used for isolation of the CD133⁺ subset from GBC-SD cells. Furthermore, the CD133⁺ subset revealed an increased potential for tumor formation, cell proliferation, invasion and resistance to chemotherapeutic agents with expression of stem cell-associated genes. Therefore, in GBC-SD cells, the CXCR4/Akt/CD133 signaling pathways may be activated.

Benzyl butyl phthalate promotes breast cancer stem cell expansion via SPHK1/S1P/S1PR3 signaling

Understanding the regulatory mechanisms unique to breast cancer stem cells (BCSCs) is required to control breast cancer metastasis. We found that phthalates promote BCSCs in human breast cancer cell cultures and xenograft tumors. A toxic phthalate, benzyl butyl phthalate (BBP), activated aryl hydrocarbon receptor in breast cancer cells to stimulate sphingosine kinase 1 (SPHK1)/sphingosine 1-phosphate (S1P)/sphingosine-1-phosphate receptor 3 (S1PR3) signaling and enhance formation of metastasis-initiating BCSCs. BBP induced histone modifications in S1PR3 in side population (SP) cells, but not in non-SP cells. SPHK1 or S1PR3 knockdown in breast cancer cells effectively reduced tumor growth and lung metastasis in vivo. Our findings suggest S1PR3 is a determinant of phthalate-driven breast cancer metastasis and a possible therapeutic target for regulating BCSC populations. Furthermore, the association between breast carcinogenesis and environmental pollutants has important implications for public health.

Targeting long non-coding RNA DANCR inhibits triple negative breast cancer progression

Triple negative breast cancer (TNBC) is non-responsive to conventional anti-hormonal and Her2-targeted therapies, making it necessary to identify new molecular targets for therapy. Long non-coding RNA anti-differentiation ncRNA (lncRNA DANCR) was identified participating in carcinogenesis of hepatocellular carcinoma, but its expression and potential role in TNBC progression is still unclear. In the present study, our results showed that DANCR expression was increased in TNBC tissues compared with the adjacent normal tissues using quantitative real-time PCR (qRT-PCR) in 63 TNBC specimens. Patients with higher DANCR expression correlated with worse TNM stages as well as a shorter overall survival (OS) using Kaplan–Meier analysis. When the endogenous DANCR was knocked-down via specific siRNA, cell proliferation and invasion were decreased obviously in the MDA-MB-231 cells. In vivo xenograft experiments showed that knockdown of the DANCR in MDA-MB-231 cells reduced the tumor growth significantly. Furthermore, a compendium of TNBC cancer stem cell markers such as CD44, ABCG2 transporter and aldehyde dehydrogenase (ALDH1) were greatly downregulated in the MDA-MB-231 cells with DANCR knockdown. Molecular mechanistic studies revealed that knockdown of DANCR was associated with increased binding of EZH2 on the promoters of CD44 and ABCG2, and concomitant reduction of expression of these genes suggested that they may be DANCR targets in TNBC. Thus, our study demonstrated that targeting DANCR expression might be a viable therapeutic approach to treat triple negative breast cancer.

Summary: Targeting LncRNA DANCR obstructs triple negative breast cancer progression by down-regulating cancer stem cell marker CD44 and ABCG2.

Molecular targeting of the Aurora-A/SMAD5 oncogenic axis restores chemosensitivity in human breast cancer cells

Although the majority of breast cancers initially respond to the cytotoxic effects of chemotherapeutic agents, most breast cancer patients experience tumor relapse and ultimately die because of drug resistance. Breast cancer cells undergoing epithelial to mesenchymal transition (EMT) acquire a CD44⁺/CD24^-/ALDH1⁺ cancer stem cell-like phenotype characterized by an increased capacity for tumor self-renewal, intrinsic drug resistance and high proclivity to develop distant metastases. We uncovered in human breast tumor xenografts a novel non-mitotic role of Aurora-A kinase in promoting breast cancer metastases through activation of EMT and expansion of breast tumor initiating cells (BTICs). In this study we characterized the role of the Aurora-A/SMAD5 oncogenic axis in the induction of chemoresistance. Breast cancer cells overexpressing Aurora-A showed resistance to conventional chemotherapeutic agents, while treatment with alisertib, a selective Aurora-A kinase inhibitor, restored chemosensitivity. Significantly, SMAD5 expression was required to induce chemoresistance and maintain a breast cancer stem cell-like phenotype, indicating that the Aurora-A/SMAD5 oncogenic axis promotes chemoresistance through activation of stemness signaling. Taken together, these findings identified a novel mechanism of drug resistance through aberrant activation of the non-canonical Aurora-A/SMAD5 oncogenic axis in breast cancer.

Nanocarrier based approaches for targeting breast cancer stem cells

Breast cancer stem cells (BCSCs) are heterogeneous subpopulation of tumour initiating cells within breast tumours. They are spared even after chemotherapy and responsible for tumour relapse. Targeting BCSCs is, therefore, necessary to achieve radical cure in breast cancer. Despite the availability of agents targeting BCSCs, their clinical application is limited due to their off-target effects and bioavailability issues. Nanotechnology based drug carriers (nanocarriers) offer various advantages to deliver anti-BCSCs agents specifically to their target sites by overcoming their bioavailability issues. In this review, we describe various strategies for targeting BCSCs using nanocarriers.

Immune modulation by a cellular network of mesenchymal stem cells and breast cancer cell subsets: Implication for cancer therapy

The immune modulatory properties of mesenchymal stem cells (MSCs) are mostly controlled by the particular microenvironment. Cancer stem cells (CSCs), which can initiate a clinical tumor, have been the subject of intense research. This review article discusses investigative studies of the roles of MSCs on cancer biology including on CSCs, and the potential as drug delivery to tumors. An understanding of how MSCs behave in the tumor microenvironment to facilitate the survival of tumor cells would be crucial to identify drug targets. More importantly, since CSCs survive for decades in dormancy for later resurgence, studies are presented to show how MSCs could be involved in maintaining dormancy. Although the mechanism by which CSCs survive is complex, this article focus on the cellular involvement of MSCs with regard to immune responses. We discuss the immunomodulatory mechanisms of MSC-CSC interaction in the context of therapeutic outcomes in oncology. We also discuss immunotherapy as a potential to circumventing this immune modulation.

The Implications and Future Perspectives of Nanomedicine for Cancer Stem Cell Targeted Therapies

Cancer stem cells (CSCs) are believed to exhibit distinctive self-renewal, proliferation, and differentiation capabilities, and thus play a significant role in various aspects of cancer. CSCs have significant impacts on the progression of tumors, drug resistance, recurrence and metastasis in different types of malignancies. Due to their primary role, most researchers have focused on developing anti-CSC therapeutic strategies, and tremendous efforts have been put to explore methods for selective eradication of these therapeutically resistant CSCs. In recent years, many reports have shown the use of CSCs-specific approaches such as ATP-binding cassette (ABC) transporters, blockade of self-renewal and survival of CSCs, CSCs surface markers targeted drugs delivery and eradication of the tumor microenvironment. Also, various therapeutic agents such as small molecule drugs, nucleic acids, and antibodies are said to destroy CSCs selectively. Targeted drug delivery holds the key to the success of most of the anti-CSCs based drugs/therapies. The convention CSCs-specific therapeutic agents, suffer from various problems. For instance, limited water solubility, small circulation time and inconsistent stability of conventional therapeutic agents have significantly limited their efficacy. Recent advancement in the drug delivery technology has demonstrated that specially designed nanocarrier-based drug delivery approaches (nanomedicine) can be useful in delivering sufficient amount of drug molecules even in the most interiors of CSCs niches and thus can overcome the limitations associated with the conventional free drug delivery methods. The nanomedicine has also been promising in designing effective therapeutic regime against pump-mediated drug resistance (ATP-driven) and reduces detrimental effects on normal stem cells. Here we focus on the biological processes regulating CSCs' drug resistance and various strategies developed so far to deal with them. We also review the various nanomedicine approaches developed so far to overcome these CSCs related issues and their future perspectives.