Modulating drug resistance by targeting BCRP/ABCG2 using retrovirus-mediated RNA interference

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.

MICA enhances sensitivity to cisplatin in patients with extensive small cell lung cancer via downregulation of ABCG2

Immunotherapy utilizing natural killer cell-activated receptor natural-killer group-2 member D ligands (NKG2DLs) has had preclinical success in the treatment of small cell lung cancer. The present study aimed to investigate the association between NKG2Ls and chemoresistance. The mRNA expression of six NKG2DLs associated with progression-free survival time (PFS) and first-line chemotherapy were assessed in the present study. Major histocompatibility complex class I polypeptide-related sequence A (MICA)-overexpressing NCI-H446 cell line was constructed, and the mRNA expression levels of 11 genes associated with chemotherapy sensitivity were determined. The results demonstrated that MICA was positively and significantly associated with PFS. Furthermore, MICA expression was 1.6 times higher in patients with prolonged PFS compared with the rapid chemoresistance group. ATP binding cassette subfamily G member 2 (ABCG2) mRNA expression was associated with chemotherapy resistance and significantly downregulated in the cell line overexpressing MICA. Moreover, following addition of nicardipine (an ABCG2 inhibitor), chemotherapeutic sensitivity increased in the MICA-overexpressing cell line. Taken together, the results of the present study suggested that MICA may enhance the chemosensitivity of patients with extensive small cell lung cancer by downregulating ABCG2.

Association between the methylation status of PCDH17 and the efficacy of neoadjuvant chemotherapy in triple-negative breast cancer

The present study aimed to assess whether the methylation status of the protocadherin 17 gene (PCDH17) in triple-negative breast cancer (TNBC) tissues was associated with the efficacy of neoadjuvant chemotherapy (NAC). The present study included 280 patients diagnosed with TNBC using core needle biopsy. Tumor pathological diagnosis was determined via hematoxylin and eosin staining. Immunohistochemical staining was used to determine estrogen receptor, progesterone receptor, human epidermal growth factor receptor-2 and Ki-67 status. PCDH17 methylation status was analyzed using methylation-specific PCR. χ² tests were performed to analyze differences between PCDH17 methylation status and TNBC clinicopathological features. Univariate and multivariate logistic regressions were used to analyze whether PCDH17 methylation status predicted a curative effect of NAC. The multivariate analysis included factors with P<0.2 from the univariate analysis and those that were clinically associated with NAC. A total of 228 patients were positive for PCDH17 methylation, while the remainder 52 were negative. Additionally, 107 patients achieved pathological complete response (pCR) after NAC. The pCR rate was 67.3% among the 52 patients negative for PCDH17 methylation and 31.6% among the 228 patients positive for PCDH17 methylation. Patients who were negative for PCDH17 methylation and had high Ki67 expression exhibited significantly higher pCR rates than their counterparts. The present results demonstrate that PCDH17 methylation status may predict the response to NAC in patients with TNBC. Therefore, this epigenetic characteristic may serve as an indicator of treatment efficacy.

Quantitative Proteomic Profiling Identifies SOX8 as Novel Regulator of Drug Resistance in Gestational Trophoblastic Neoplasia

The development of drug resistance remains one of the major challenges to current chemotherapeutic regimens in gestational trophoblastic neoplasia (GTN). Further understanding on the mechanisms of drug resistance would help to develop more effective therapy to treat GTN. Herein, tandem mass tag-based (TMT) quantitative proteomic technique was used to establish drug resistance-related proteomic profiles in chemoresistant GTN cell models (JEG3/MTX, JEG3/VP16, JEG3/5-Fu). In total, we identified 5,704 protein groups, among which 4,997 proteins were quantified in JEG3 and its chemoresistant sublines. Bioinformatics analysis revealed that multiple biological processes/molecular pathways/signaling networks were involved in the regulation of drug resistance in chemoresistant JEG3 sublines. SOX8 was upregulated in all the three chemoresistant sublines, and its function was further investigated. Knockdown of SOX8 significantly reduced cell viability, impaired soft agar clonogenesis, and increased caspase-3 activities after drug treatment in JEG3 chemoresistant sublines. In addition, over-expression of SOX8 promoted cell survival, enhanced soft agar clonogenesis, and attenuated caspase-3 activities after drug treatment in GTN cells. Importantly, SOX8 might be a potential regulator of reactive oxygen species (ROS) homeostasis, as SOX8 regulated the expression of antioxidant enzymes (GPX1, HMOX1) and reduced drug-induced ROS accumulation in GTN cell models. Collectively, SOX8 might promote drug resistance through attenuating the accumulation of ROS induced by chemotherapeutic drugs in GTN cells. Targeting SOX8 might be useful to sensitize GTN cells to chemotherapy.

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.

c-Myc maintains the self-renewal and chemoresistance properties of colon cancer stem cells

Cancer stem cells (CSCs) are responsible for cancer formation, recurrence and drug resistance. c-Myc, one of the core markers for stem cells, has recently been considered to serve as a link between malignancy and ‘stemness’. However, the precise function of c-Myc in colon CSCs is still unclear. In the present study, a subpopulation of colon CSCs expressing a CD133 surface phenotype was isolated from the human HT-29 cell line, which possess greater tumor sphere-forming efficiency and have higher expression of ‘stemness’-associated genes compared with CD133-negative cells. Furthermore, it was demonstrated that c-Myc was highly expressed in CD133⁺ colon CSCs. Knockdown of c-Myc expression with small interfering RNA in colon CSCs can significantly inhibit tumor sphere formation, reduce the invasive and migratory capacity of CD133⁺ cells in vitro, and suppress the tumorigenicity of colon CSCs in vivo. In addition, it was suggested that c-Myc silencing may sensitize colon CSCs to chemotherapy-induced cytotoxicity via the downregulation of ABCG2 and ABCB5. These findings support a central role for c-Myc in maintaining the self-renewing and chemoresistant properties of colon CSCs.

Mechanisms of drug resistance in colon cancer and its therapeutic strategies

Drug resistance develops in nearly all patients with colon cancer, leading to a decrease in the therapeutic efficacies of anticancer agents. This review provides an up-to-date summary on over-expression of ATP-binding cassette (ABC) transporters and evasion of apoptosis, two representatives of transport-based and non-transport-based mechanisms of drug resistance, as well as their therapeutic strategies. Different ABC transporters were found to be up-regulated in colon cancer, which can facilitate the efflux of anticancer drugs out of cancer cells and decrease their therapeutic effects. Inhibition of ABC transporters by suppressing their protein expressions or co-administration of modulators has been proven as an effective approach to sensitize drug-resistant cancer cells to anticancer drugs in vitro. On the other hand, evasion of apoptosis observed in drug-resistant cancers also results in drug resistance to anticancer agents, especially to apoptosis inducers. Restoration of apoptotic signals by BH3 mimetics or epidermal growth factor receptor inhibitors and inhibition of cancer cell growth by alternative cell death pathways, such as autophagy, are effective means to treat such resistant cancer types. Given that the drug resistance mechanisms are different among colon cancer patients and may change even in a single patient at different stages, personalized and specific combination therapy is proposed to be more effective and safer for the reversal of drug resistance in clinics.

Mechanisms of drug resistance in colon cancer and its therapeutic strategies

Drug resistance develops in nearly all patients with colon cancer, leading to a decrease in the therapeutic efficacies of anticancer agents. This review provides an up-to-date summary on over-expression of ATP-binding cassette (ABC) transporters and evasion of apoptosis, two representatives of transport-based and non-transport-based mechanisms of drug resistance, as well as their therapeutic strategies. Different ABC transporters were found to be up-regulated in colon cancer, which can facilitate the efflux of anticancer drugs out of cancer cells and decrease their therapeutic effects. Inhibition of ABC transporters by suppressing their protein expressions or co-administration of modulators has been proven as an effective approach to sensitize drug-resistant cancer cells to anticancer drugs in vitro. On the other hand, evasion of apoptosis observed in drug-resistant cancers also results in drug resistance to anticancer agents, especially to apoptosis inducers. Restoration of apoptotic signals by BH3 mimetics or epidermal growth factor receptor inhibitors and inhibition of cancer cell growth by alternative cell death pathways, such as autophagy, are effective means to treat such resistant cancer types. Given that the drug resistance mechanisms are different among colon cancer patients and may change even in a single patient at different stages, personalized and specific combination therapy is proposed to be more effective and safer for the reversal of drug resistance in clinics.

Silencing of ABCG2 by MicroRNA-3163 Inhibits Multidrug Resistance in Retinoblastoma Cancer Stem Cells

To investigate the function and regulation mechanism of ATP-binding cassette, subfamily G, member 2 (ABCG2) in retinoblastoma cancer stem cells (RCSCs), a long-term culture of RCSCs from WERI-Rb1 cell line was successfully established based on the high expression level of ABCG2 on the surface of RCSCs. To further explore the molecular mechanism of ABCG2 on RCSCs, a microRNA that specifically targets ABCG2 was predicted. Subsequently, miR-3163 was selected and confirmed as the ABCG2-regulating microRNA. Overexpression of miR-3163 led to a significant decrease in ABCG2 expression. Additionally, ABCG2 loss-of-function induced anti-proliferation and apoptosis-promoting functions in RCSCs, and multidrug resistance to cisplatin, carboplatin, vincristine, doxorubicin, and etoposide was greatly improved in these cells. Our data suggest that miR-3163 has a significant impact on ABCG2 expression and can influence proliferation, apoptosis, and drug resistance in RCSCs. This work may provide new therapeutic targets for retinoblastoma.

Natural polyphenols: Influence on membrane transporters

Accumulated evidence has focused on the use of natural polyphenolic compounds as nutraceuticals since they showed a wide range of bioactivities and exhibited protection against variety of age-related disorders. Polyphenols have variable potencies to interact, and hence alter the activities of various transporter proteins, many of them classified as anion transporting polypeptide-binding cassette transporters like multidrug resistance protein and p-glycoprotein. Some of the efflux transporters are, generally, linked with anticancer and antiviral drug resistance; in this context, polyphenols may be beneficial in modulating drug resistance by increasing the efficacy of anticancer and antiviral drugs. In addition, these effects were implicated to explain the influence of dietary polyphenols on drug efficacy as result of food-drug interactions. However, limited data are available about the influence of these components on uptake transporters. Therefore, the objective of this article is to review the potential efficacies of polyphenols in modulating the functional integrity of uptake transporter proteins, including those terminated the effect of neurotransmitters, and their possible influence in neuropharmacology.

MBL-II-141, a chromone derivative, enhances irinotecan (CPT-11) anticancer efficiency in ABCG2-positive xenografts

ABCG2 is responsible for the multidrug resistance (MDR) phenotype, and strongly modulates cancer outcomes. Its high expression at a number of physiological barriers, including blood-brain and intestinal barriers, impacts on drug pharmacokinetics parameters. We characterized MBL-II-141, a specific and potent ABCG2 inhibitor. Combination of 10 mg/kg MBL-II-141 with the anticancer agent CPT-11 completely blocked the growth of 90% freshly implanted ABCG2-positive tumors. Moreover, the same combination slowed the growth of already established tumors. As required for preclinical development, we defined the main pharmacokinetics parameters of MBL-II-141 and its influence on the kinetics of CPT-11 and its active metabolite SN-38 in mice. MBL-II-141 distribution into the brain occurred at a low, but detectable, level. Interestingly, preliminary data suggested that MBL-II-141 is well tolerated (at 50 mg/kg) and absorbed upon force-feeding. MBL-II-141 induced a potent sensitization of ABCG2-positive xenografts to CPT-11 through in vivo ABCG2 inhibition. MBL-II-141 strongly increased CPT-11 levels in the brain, and therefore would be a valuable agent to improve drug distribution into the brain to efficiently treat aggressive gliomas. Safety and other pharmacological data strongly support the reglementary preclinical development of MBL-II-141.

Mammalian drug efflux transporters of the ATP binding cassette (ABC) family in multidrug resistance: A review of the past decade

Multidrug resistance (MDR) is a serious phenomenon employed by cancer cells which hampers the success of cancer pharmacotherapy. One of the common mechanisms of MDR is the overexpression of ATP-binding cassette (ABC) efflux transporters in cancer cells such as P-glycoprotein (P-gp/ABCB1), multidrug resistance-associated protein 2 (MRP2/ABCC2), and breast cancer resistance protein (BCRP/ABCG2) that limits the prolonged and effective use of chemotherapeutic drugs. Researchers have found that developing inhibitors of ABC efflux transporters as chemosensitizers could overcome MDR. But the clinical trials have shown that most of these chemosensitizers are merely toxic and only show limited or no benefits to cancer patients, thus new inhibitors are being explored. Recent findings also suggest that efflux pumps of the ABC transporter family are subject to epigenetic gene regulation. In this review, we summarize recent findings of the role of ABC efflux transporters in MDR.

Pharmacokinetic strategies to improve drug penetration and entrapment within solid tumors

Despite the discovery of a large number of anticancer agents, cancer still remains among the leading causes of death since the middle of the twentieth century. Solid tumors possess a high degree of genetic instability and emergence of treatment resistance. Tumor resistance has emerged for almost all approved anticancer drugs and will most probably emerge for newly discovered anticancer agents as well. The use of pharmacokinetic approaches to increase anticancer drug concentrations within the solid tumor compartment and prolong its entrapment might diminish the possibility of resistance emergence at the molecular pharmacodynamic level and might even reverse tumor resistance. Several novel treatment modalities such as metronomic therapy, angiogenesis inhibitors, vascular disrupting agents and tumor priming have been introduced to improve solid tumor treatment outcomes. In the current review we will discuss the pharmacokinetic aspect of these treatment modalities in addition to other older treatment modalities, such as extracellular matrix dissolving agents, extracellular matrix synthesis inhibitors, chemoembolization and cellular efflux pump inhibition. Many of these strategies showed variable degrees of success/failure; however, reallocating these modalities based on their influence on the intratumoral pharmacokinetics might improve their understanding and treatment outcomes.

The ellagic acid-derived gut microbiota metabolite, urolithin A, potentiates the anticancer effects of 5-fluorouracil chemotherapy on human colon cancer cells

The ellagic acid-derived gut microbiota metabolite, urolithin A, at concentrations achievable in the human colorectum, enhances the anticancer effects of 5-FU-chemotherapy on three different colon cancer cells.

P-glycoprotein inhibitors of natural origin as potential tumor chemo-sensitizers: A review

Resistance of solid tumors to treatment is significantly attributed to pharmacokinetic reasons at both cellular and multi-cellular levels. Anticancer agent must be bio-available at the site of action in a cytotoxic concentration to exert its proposed activity. P-glycoprotein (P-gp) is a member of the ATP-dependent membrane transport proteins; it is known to pump substrates out of cells in ATP-dependent mechanism. The over-expression of P-gp in tumor cells reduces the intracellular drug concentrations, which decreases the cytotoxicity of a broad spectrum of antitumor drugs. Accordingly, P-gp inhibitors/blockers are potential enhancer for the cellular bioavailability of several clinically important anticancer drugs such as, anthracyclines, taxanes, vinca alkaloids, and podophyllotoxins. Besides several chemically synthesized P-gp inhibitors/blockers, some naturally occurring compounds and plant extracts were reported for their modulation of multidrug resistance; however, this review will focus only on major classes of naturally occurring inhibitors viz., flavonoids, coumarins, terpenoids, alkaloids and saponins.