Mohammad Sultan

Chemoresistance in Cancer Stem Cells and Strategies to Overcome Resistance

In cancers, there exists a subpopulation of cells which are referred to as cancer stem cells (CSCs) or tumor initiating cells that have enhanced tumor-initiating capacity and metastatic potential, and drive tumor progression. Since the initial identification of acute myeloid leukemia CSCs in 1997, CSCs have been found in many types of cancer and have intrinsic resistance to the current chemotherapeutic strategies. With increased levels of detoxifying enzymes, enhanced DNA repair abilities, impressive efflux capacity, and a slower cell-cycle; CSCs present a formidable obstacle against effective chemotherapy. Several methods of specifically targeting CSCs have been developed in recent years, and these compounds have potential as adjuvant therapies. The following is a review of the mechanisms responsible for chemoresistance in CSCs, with an emphasis on potential strategies to overcome this resistance.

Retinoid Signaling in Cancer and its Promise for Therapy

Deregulated signal transduction is a major facet of cancer development and progression. Herein, we review the current paradigm for retinoic acid signaling, its role in cancer and potential therapeutic applications and challenges. Retinoic acid is used with remarkable success in the treatment of one of the most high-risk leukemias, acute promyelocytic leukemia; however, extending its use in the treatment of other cancers has had limited success at best. Functional studies provide clues for the poor performance of retinoic acid as a general cancer therapeutic, connecting retinoic acid signaling to both cell growth arrest and proliferation with tumor suppression and cancer progression consequences. The dualistic role of the retinoic acid signaling pathway in cancer is revealed in its gene transcription targets, cross-talk with other transcription factors, mediation of apoptotic pathways, and influence in the immune system. If the greatest potential benefit of retinoid-based cancer therapeutics is to be achieved, the many physiological roles of retinoic acid need to be considered.

Breast cancer subtype dictates DNA methylation and ALDH1A3-mediated expression of tumor suppressor RARRES1

Breast cancer subtyping, based on the expression of hormone receptors and other genes, can determine patient prognosis and potential options for targeted therapy. Among breast cancer subtypes, tumors of basal-like and claudin-low subtypes are typically associated with worse patient outcomes, are primarily classified as triple-negative breast cancers (TNBC), and cannot be treated with existing hormone-receptor-targeted therapies. Understanding the molecular basis of these subtypes will lead to the development of more effective treatment options for TNBC. In this study, we focus on retinoic acid receptor responder 1 (RARRES1) as a paradigm to determine if breast cancer subtype dictates protein function and gene expression regulation. Patient tumor dataset analysis and gene expression studies of a 26 cell-line panel, representing the five breast cancer subtypes, demonstrate that RARRES1 expression is greatest in basal-like TNBCs. Cell proliferation and tumor growth assays reveal that RARRES1 is a tumor suppressor in TNBC. Furthermore, gene expression studies, Illumina HumanMethylation450 arrays, and chromatin immunoprecipitation demonstrate that expression of RARRES1 is retained in basal-like breast cancers due to hypomethylation of the promoter. Additionally, expression of the cancer stem cell marker, aldehyde dehydrogenase 1A3, which provides the required ligand (retinoic acid) for RARRES1 transcription, is also specific to the basal-like subtype. We functionally demonstrate that the combination of promoter methylation and retinoic acid signaling dictates expression of tumor suppressor RARRES1 in a subtype-specific manner. These findings provide a precedent for a therapeutically-inducible tumor suppressor and suggest novel avenues of therapeutic intervention for patients with basal-like breast cancer.

Hide-and-seek: the interplay between cancer stem cells and the immune system.

The enhanced ability of cancer stem cells (CSCs) to give rise to new tumors suggests that these cells may also have an advantage in evading immune detection and elimination. This tumor-forming ability, combined with the known plasticity of the immune system, which can play both protumorigenic and antitumorigenic roles, has motivated investigations into the interaction between CSCs and the immune system. Herein, we review the interplay between host immunity and CSCs by examining the immune-related mechanisms that favor CSCs and the CSC-mediated expansion of protumorigenic immune cells. Furthermore, we discuss immune cells, such as natural killer cells, that preferentially target CSCs and the strategies used by CSCs to evade immune detection and destruction. An increased understanding of these interactions and the pathways that regulate them may allow us to harness immune system components to create new adjuvant therapies that eradicate CSCs and improve patient survival.

Citral reduces breast tumor growth by inhibiting the cancer stem cell marker ALDH1A3.

Breast cancer stem cells (CSCs) can be identified by increased Aldefluor fluorescence caused by increased expression of aldehyde dehydrogenase 1A3 (ALDH1A3), as well as ALDH1A1 and ALDH2. In addition to being a CSC marker, ALDH1A3 regulates gene expression via retinoic acid (RA) signaling and plays a key role in the progression and chemotherapy resistance of cancer. Therefore, ALDH1A3 represents a druggable anti‐cancer target of interest. Since to date, there are no characterized ALDH1A3 isoform inhibitors, drugs that were previously described as inhibiting the activity of other ALDH isoforms were tested for anti‐ALDH1A3 activity. Twelve drugs (3‐hydroxy‐dl‐kynurenine, benomyl, citral, chloral hydrate, cyanamide, daidzin, DEAB, disulfiram, gossypol, kynurenic acid, molinate, and pargyline) were compared for their efficacy in inducing apoptosis and reducing ALDH1A3, ALDH1A1 and ALDH2‐associated Aldefluor fluorescence in breast cancer cells. Citral was identified as the best inhibitor of ALDH1A3, reducing the Aldefluor fluorescence in breast cancer cell lines and in a patient‐derived tumor xenograft. Nanoparticle encapsulated citral specifically reduced the enhanced tumor growth of MDA‐MB‐231 cells overexpressing ALDH1A3. To determine the potential mechanisms of citral‐mediated tumor growth inhibition, we performed cell proliferation, clonogenic, and gene expression assays. Citral reduced ALDH1A3‐mediated colony formation and expression of ALDH1A3‐inducible genes. In conclusion, citral is an effective ALDH1A3 inhibitor and is able to block ALDH1A3‐mediated breast tumor growth, potentially via blocking its colony forming and gene expression regulation activity. The promise of ALDH1A3 inhibitors as adjuvant therapies for patients with tumors that have a large population of high‐ALDH1A3 CSCs is discussed.

Epigenetic Silencing of TAP1 in Aldefluor+ Breast Cancer Stem Cells Contributes to Their Enhanced Immune Evasion

Avoiding detection and destruction by immune cells is key for tumor initiation and progression. The important role of cancer stem cells (CSCs) in tumor initiation has been well established, yet their ability to evade immune detection and targeting is only partly understood. To investigate the ability of breast CSCs to evade immune detection, we identified a highly tumorigenic population in a spontaneous murine mammary tumor based on increased aldehyde dehydrogenase activity. We performed tumor growth studies in immunocompetent and immunocompromised mice. In immunocompetent mice, growth of the spontaneous mammary tumor was restricted; however, the Aldefluor+ population was expanded, suggesting inherent resistance mechanisms. Gene expression analysis of the sorted tumor cells revealed that the Aldefluor+ tumor cells has decreased expression of transporter associated with antigen processing (TAP) genes and co‐stimulatory molecule CD80, which would decrease susceptibility to T cells. Similarly, the Aldefluor+ population of patient tumors and 4T1 murine mammary cells had decreased expression of TAP and co‐stimulatory molecule genes. In contrast, breast CSCs identified by CD44+CD24− do not have decreased expression of these genes, but do have increased expression of C‐X‐C chemokine receptor type 4. Decitabine treatment and bisulfite pyrosequencing suggests that DNA hypermethylation contributes to decreased TAP gene expression in Aldefluor+ CSCs. TAP1 knockdown resulted in increased tumor growth of 4T1 cells in immunocompetent mice. Together, this suggests immune evasion mechanisms in breast CSCs are marker specific and epigenetic silencing of TAP1 in Aldefluor+ breast CSCs contributes to their enhanced survival under immune pressure. Stem Cells 2018;36:641–654

Cancer Stem Cells and Chemoresistance: Strategies to Overcome Therapeutic Resistance

Cancer stem cells (CSCs) are hypothesized to initiate cancer and give rise to heterogeneous tumors made up of self-renewing CSCs and the differentiated, less tumorigenic non-CSCs, which make up the bulk of the tumor. Importantly, in terms of successful patient treatment, CSCs are also more resistant to commonly used chemotherapeutics. Multiple mechanisms have been identified for CSC-associated chemoresistance. These mechanisms include increased expression of ABC transporter efflux pumps, aldehyde dehydrogenase (ALDH) detoxification enzymes, anti-apoptosis proteins, enhanced DNA repair mechanisms, increased activation of the embryonic signaling pathways (Notch, Wnt and Hedgehog), and quiescence. Identification of these mechanisms has led to development of specific strategies to circumvent CSC-associated chemoresistance (e.g. inhibitors of ABC transporters, ALDH enzymes, and Notch, Wnt, and Hedgehog pathways, and epigenetic modifying drugs). Future clinical evidence will reveal if employing these adjuvant therapies will eradicate CSCs along with the bulk of the tumor, and lead to improved patient outcomes with decreased cancer recurrence.

Abstract B26: Identification of genes that predict response to paclitaxel in breast cancer using an in vivo genome-wide knockdown screen

Treatment decisions for breast cancer are based upon stage, tumor grade and hormone receptor status, and can include surgical resection, hormone receptor antagonists, radiation, and chemotherapy (e.g. paclitaxel). Breast cancer treatment success depends upon avoidance of chemotherapy resistance (i.e. achieving complete response) and prevention of both over- and under-treatment. Increased understanding of the genes which cause resistance and sensitivity to currently used drugs would lead to development of more effective therapeutic strategies that are specifically tailored to patient groups based on molecular profiling of their tumors (i.e. personalized medicine). Being able to identify the genes which when expressed in a tumor predict sensitivity or resistance to treatment prior to administration of paclitaxel would improve treatment efficacy and patient survival. We performed an in vivo shRNA genome-wide screen with MDA-MB-231 tumors treated with paclitaxel for the purpose of identifying genes which determine breast cancer response to paclitaxel. Completion of 6 replicates of the in vivo screen identified 26 putative paclitaxel sensitivity genes and 14 putative paclitaxel resistance genes (e.g. BCL6) for breast cancer. Screen-identified putative paclitaxel resistance were verified by individual knockdown clone generation and comparison of their sensitivity to paclitaxel-induced decreased cell proliferation, cell-cycle arrest, and apoptosis to a shRNA scramble control clone. Upon individual knockdown of the putative resistance genes (e.g. BCL6), MDA-MB-231 cells were more sensitive to paclitaxel and demonstrated increased apoptosis and decreased paclitaxel IC50 concentrations. Finally, expression of a preliminary gene signature generated from the screen-identified hits was tested for its ability to predict response to paclitaxel in two archived patient data sets. The preliminary gene signature predicted response to paclitaxel in the datasets with an accuracy ranging from 70 to 100%. Further confirmation experiments of the remaining potential resistance and sensitivity genes will help to generate a more robust genetic profile which can be used to identify candidate breast cancer patients who would most benefit from paclitaxel treatment as opposed to treatment with other drugs. Citation Format: Mohammad Sultan, Thomas Tan Huynh, Margaret Lois Thomas, Krysta Mila Coyle, Carman A. Giacomantonio, Paola Marcato. Identification of genes that predict response to paclitaxel in breast cancer using an in vivo genome-wide knockdown screen. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research; Oct 17-20, 2015; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(2_Suppl):Abstract nr B26.

Abstract A18: Expression of the tumor suppressor gene RARRES1 in the differentiation hierarchy of breast cancer is regulated by DNA methylation

The tumor-suppressive function of the retinoic acid (RA)-inducible gene, retinoic acid receptor responder 1 (RARRES1), has been reported in many cancer types; however, in inflammatory breast cancer, RARRES1 is pro-tumorigenic. Furthermore, high transcript levels of RARRES1 are associated with more aggressive triple-negative breast cancers (TNBCs) and poorer patient outcomes. This suggests that RARRES1 may also be oncogenic and requires investigation to determine its role in breast cancer. Expression analyses of 20 breast cancer cell lines (including 18 TNBC and two estrogen-receptor-positive cell lines) revealed that RARRES1 is predominantly expressed in basal-like breast cancer cells, but is often methylated and silenced in claudin-low breast cancer cells. We have identified possible sites of regulation by methylation in RARRES1 using Illumina 450K methylation arrays and 5-methylcytosine ChIP. Basal-like cells express higher levels of the cancer stem cell (CSC) marker ALDH1A3. Expression of RARRES1 is dependent on, and strongly correlates with ALDH1A3 expression in fixed breast cancer patient samples. Immunohistochemistry of the same patient tumor samples revealed RARRES1 expression is localized to the endoplasmic reticulum. Finally, knockdown of RARRES1 in claudin-low MDA-MB-231 and basal-like MDA-MB-468 and HCC1937 significantly increased cell proliferation and tumor growth, suggesting RARRES1 has a tumor suppressive function regardless of position on the differentiation hierarchy. We conclude that RARRES1 is a tumor suppressor in TNBC with methylation and expression profiles distinct to the differentiation hierarchy observed in breast cancer. Citation Format: Krysta Mila Coyle, Dejan Vidovic, Cheryl A. Dean, Margaret Lois Thomas, Mohammad Sultan, Derek Clements, Ahmad Vaghar-Kashani, Melissa Wallace, Ian Weaver, Carman A. Giacomantonio, Lucy Helyer, Paola Marcato. Expression of the tumor suppressor gene RARRES1 in the differentiation hierarchy of breast cancer is regulated by DNA methylation. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research; Oct 17-20, 2015; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(2_Suppl):Abstract nr A18.

Targeting Key Stemness-Related Pathways in Human Cancers

It is increasingly apparent that cancer stem cells (CSCs) play a substantial role in the response of human cancers to therapy. Indeed, the failure of mainstream chemotherapies to reduce the CSC burden may explain the high rates of tumor recurrence and metastasis. The development of new, anti-CSC agents is thus of great importance to reduce cancer-related mortality. One strategy to target CSCs focuses on their dependence on cell-signaling pathways, which differ from the majority of the tumor cells; these pathways include the embryonic Notch, Wingless-related (Wnt), and Hedgehog (Hh) pathways. Recently, there has been a surge in the development and clinical evaluation of targeted anti-Notch, anti-Wnt, and anti-Hh agents. Herein, we discuss the signaling paradigm for each of these pathways, identify druggable targets, and discuss selected pre-clinical and clinical findings with agents targeting each pathway. A number of natural molecules have shown some efficacy in inhibiting these stemness pathways. Importantly, we consider other disease-specific targeted agents to discuss roadblocks to the success of these anti-stemness agents – including financial considerations, the development of resistance, and on-target adverse effects. Novel clinical trial elements are required to adequately assess the success of these agents; however, the future for anti-CSC therapy is promising.