The Role of Serotonin in Breast Cancer Stem Cells
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molecules Review The Role of Serotonin in Breast Cancer Stem Cells William D. Gwynne 1 , Mirza S. Shakeel 2, Adele Girgis-Gabardo 2 and John A. Hassell 2,* 1 Department of Surgery, McMaster University, Hamilton, ON L8S 4L8, Canada; [email protected] 2 Department of Biochemistry and Biomedical Sciences, McMaster University, Hamilton, ON L8S 4L8, Canada; [email protected] (M.S.S.); [email protected] (A.G.-G.) * Correspondence: [email protected] Abstract: Breast tumors were the first tumors of epithelial origin shown to follow the cancer stem cell model. The model proposes that cancer stem cells are uniquely endowed with tumorigenic capacity and that their aberrant differentiation yields non-tumorigenic progeny, which constitute the bulk of the tumor cell population. Breast cancer stem cells resist therapies and seed metastases; thus, they account for breast cancer recurrence. Hence, targeting these cells is essential to achieve durable breast cancer remissions. We identified compounds including selective antagonists of multiple serotonergic system pathway components required for serotonin biosynthesis, transport, activity via multiple 5-HT receptors (5-HTRs), and catabolism that reduce the viability of breast cancer stem cells of both mouse and human origin using multiple orthologous assays. The molecular targets of the selective antagonists are expressed in breast tumors and breast cancer cell lines, which also produce serotonin, implying that it plays a required functional role in these cells. The selective antagonists act synergistically with chemotherapy to shrink mouse mammary tumors and human breast tumor xenografts primarily by inducing programmed tumor cell death. We hypothesize those serotonergic proteins of diverse activity function by common signaling pathways to maintain cancer stem cell viability. Here, we summarize our recent findings and the relevant literature regarding the role of Citation: Gwynne, W.D.; Shakeel, serotonin in breast cancer. M.S.; Girgis-Gabardo, A.; Hassell, J.A. The Role of Serotonin in Breast Cancer Stem Cells. Molecules 2021, 26, Keywords: breast cancer; breast cancer stem cells; serotonin; serotonergic system antagonists; 3171. https://doi.org/10.3390/ G-protein receptors; kinase signaling cascades molecules26113171 Academic Editors: Marcello Leopoldo and Enza Lacivita 1. Introduction Breast cancer is the second most common cancer in women and the leading cause Received: 23 April 2021 of cancer-related deaths worldwide [1]. Despite increased screening and new targeted Accepted: 24 May 2021 therapies, the global incidence of breast cancer continues to increase [2]. Moreover, cytotoxic Published: 26 May 2021 therapies used to treat breast cancer frequently do not achieve long lasting remissions. In fact, roughly 30% of women diagnosed with invasive breast cancer relapse and 90% of Publisher’s Note: MDPI stays neutral these individuals die from metastases. Hence, new therapies are needed that ensure long with regard to jurisdictional claims in lasting remissions. published maps and institutional affil- iations. 1.1. Breast Cancer Clinical and Molecular Subtypes Breast tumors were first classified based on their hormone receptor status (estrogen (ER) and/or progesterone receptors (PR)), whether they overexpressed HER2, or did not express either ER, PR, or HER2, termed triple-negative breast cancer (TNBC) [3]. Global Copyright: © 2021 by the authors. gene expression profiling was subsequently used to molecularly classify breast tumors Licensee MDPI, Basel, Switzerland. into various subtypes: luminal A (ER+ and/or PR+/−); luminal B (ER+ and/or PR+/−) This article is an open access article and either HER2 overexpressing or not; basal-like (triple negative breast cancers (TNBC), distributed under the terms and do not express hormone receptors nor overexpress HER2; and normal-like. Luminal conditions of the Creative Commons A tumors comprise a low fraction of proliferating cells (defined by Ki67-positive cells), Attribution (CC BY) license (https:// whereas luminal B tumors are composed of a high fraction of dividing tumor cells. HER-2 creativecommons.org/licenses/by/ 4.0/). overexpressing breasts tumors do not express ER or PR or express them at low levels. Molecules 2021, 26, 3171. https://doi.org/10.3390/molecules26113171 https://www.mdpi.com/journal/molecules Molecules 2021, 26, x FOR PEER REVIEW 2 of 16 Molecules 2021, 26, 3171 2 of 16 2 overexpressing breasts tumors do not express ER or PR or express them at low levels. Normal-like tumors are similar to luminal A tumors. The prognosis for patients with breastNormal-like cancer is tumors linked are to their similar subtype to luminal in the Aorder tumors. from Thebest prognosisto worst prognosis: for patients luminal with A;breast normal-like; cancer is luminal linked to B, their HER2-overexpre subtype in thessing; order and from TNBC. best to Therapies worst prognosis: are tailored luminal for patientsA; normal-like; consistent luminal with their B, HER2-overexpressing; breast tumor subtype. and For TNBC. example, Therapies patients are with tailored HER-2 for overexpressingpatients consistent or ER+ with breast their tumors breast are tumor treated subtype. with monoclonal For example, antibodies patients or with small HER-2 mol- eculesoverexpressing that target or these ER+ receptors, breast tumors or in the are case treated of ER+ with tumors, monoclonal block antibodiesthe synthesis or of small es- trogen.molecules Unfortunately, that target these resistance receptors, to these orin drugs the case often of develops ER+ tumors, in patients. block the synthesis of estrogen. Unfortunately, resistance to these drugs often develops in patients. 1.2. Breast Cancer Stem Cells 1.2. Breast Cancer Stem Cells Breast cancer recurrence has been attributed to breast cancer stem cells [4], which are functionallyBreast cancerdefined recurrence by their capacity has been to attributed initiate tumor to breast growth cancer following stem cells their [4], transplan- which are tationfunctionally into mice defined and are by consequently their capacity also to initiate termed tumor breast growth tumor-initiating following theircells (BTICs) transplanta- [5]. BTICtion intoare micerefractory and are to consequentlycancer therapies, also s termedeed metastases, breast tumor-initiating and account for cells breast (BTICs) cancer [5]. recurrenceBTIC are refractory (Figure 1). to Analyses cancer therapies, of the mamma seedry metastases, tumors from and various account transgenic for breast mouse cancer modelsrecurrence of breast (Figure cancer1). Analyses revealed of that the like mammary human breast tumors tumors, from various these also transgenic follow the mouse can- cermodels stem cell of breast model cancer [6–9]. The revealed model that proposes like human that genetic breast or tumors, epigenetic these changes also follow in tissue- the specificcancer stemcells cellyield model tumor [6 –cells9]. Thewith model stem proposescell-like properties, that genetic including or epigenetic the capacity changes for in limitlesstissue-specific self-renewal cells yield and tumordifferentiation cells with [10]. stem The cell-like cellular properties, heterogene includingity of tumors the capacity is due tofor the limitless aberrant self-renewal differentiation and of differentiation the stem-like tumor [10]. The cells. cellular Hence, heterogeneity breast tumors ofcomprise tumors ais cellular due to hierarchy the aberrant of BTIC differentiation at their apex of theand stem-likenon-tumorigenic tumor cells. proliferating Hence, breastand differen- tumors tiatingcomprise progeny a cellular of these hierarchy cells at of their BTIC base. at their Whether apex BTIC and non-tumorigenicoriginate from normal proliferating breast and differentiating progeny of these cells at their base. Whether BTIC originate from stem cells or whether their frequency differs among breast tumors of distinct subtypes has normal breast stem cells or whether their frequency differs among breast tumors of distinct not yet been unequivocally established. subtypes has not yet been unequivocally established. Figure 1. The breast cancer stem cell model for disease recurrence. Schematic conveying the clinical implications of Figuretherapy-resistant 1. The breast BTIC. cancer Standard stem cell of care model consisting for disease of surgery recurrence. and cytotoxic Schematic therapies conveying principally the clinical eradicate implications non-BTIC. of ther- BTIC apy-resistantremain dormant BTIC. until Standard seeding of local care or consisting distant disease of surgery recurrences. and cytotoxic therapies principally eradicate non-BTIC. BTIC remain dormant until seeding local or distant disease recurrences. 1.3. Epithelial to Mesenchymal Transition and BTIC, Therapeutic Implications 1.3. Epithelial to Mesenchymal Transition and BTIC, Therapeutic Implications Studies during the last decade demonstrate that the induction of an epithelial to Studies during the last decade demonstrate that the induction of an epithelial to mes- mesenchymal transition (EMT) can endow “non-tumorigenic” breast tumor cells with enchymal transition (EMT) can endow “non-tumorigenic” breast tumor cells with BTIC BTIC activity, implying that breast tumor cells transition between non-tumorigenic and activity,tumorigenic implying states that [11 breast–13]. These tumor findings cells transition have therapeutic between non-tumorigenic implications [14 ].and Cytotoxic tumor- igenictherapies states eradicate [11–13].