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- Herausgeber: Bentham Science Publishers
- Kategorie: Fachliteratur
- Serie: Recent Advances in Signal Transduction Research and Therapy
- Sprache: Englisch
Cancer is driven by numerous genetic and epigenetic changes occurring at the cellular level.These changes drive normal cells to proliferate and escape processes thatusually regulate their survival and migration. Many of these alterations areoften associated with signaling pathways which regulate cell growth anddivision, cell death, survival, invasion and metastasis, and angiogenesis.Almost all cancer cells show high expression of signaling components includinggrowth factor receptor tyrosine kinases (RTKs), small GTPases, serine/threoninekinases, cytoplasmic tyrosine kinases, lipid kinases, estrogen receptor,activation of transcription factors Myc and NF-κB, etc. Updated knowledge aboutthese signaling components is highly desirable for researchers involved indeveloping therapies against cancer.Signal TransductionResearch for Cancer Therapy covers advancements in research on the signalingpathways in the human body, especially in some types of cancers, such asbreast cancer, pancreatic cancer and colon cancer. Keyfeatures of this volume include 8 focused topical reviews on signaling pathwaysin a specific cancer type, coverage of multiple cancer types (breast cancer,colon cancer, hepatocellular cancer, multiple myeloma, acute myeloid leukemia,and pancreatic cancer), and coverage of a wide array of signaling pathways (bothreceptor mediated and non receptor mediated pathways). Thisvolume is essential reading for researchers in pharmaceutical R&D andpostgraduate research programs in pharmacology and allied disciplines.Clinicians involved in oncology will also benefit from the information providedin the chapters.
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Veröffentlichungsjahr: 2020
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PREFACE
Every year, almost 10 million people succumb to death around the world due to one or the other type of cancers. Hundreds of laboratories with thousands of scientists are trying to understand the ever-enigmatic biology of cancer cells, which is the basis for developing new therapies for cancer. Discovery of antifolates aminopterin in 1947 and methotrexate (aka amethopterin) in 1948 as potential anticancer agents by Drs. Yellapragada Subbarow and Sydney Farber not only initiated the era of chemotherapy but the discovery of these synthetic agents also infused the hope that cancers can be treated with synthetic chemicals. In the past more than seven decades, we have witnessed the journey of cancer therapy that started from nonspecific chemotherapy (e.g. antifolates) to targeted biologic agents (e.g. monoclonal antibodies) and now the era of personalized treatment (e.g. CART cells) and immunotherapy. However, deeper understanding of the signal transduction within the cancer cells, and between cancer cells and their surrounding tumor microenvironment has remained central to the development of therapies.
In the present volume 1 titled ‘Advances in Cancer Signal Transduction and Therapy’ of the book series ‘Recent advances in signal transduction research and therapy’, we attempted to review recent advances in select cancer signal transduction pathways that have been targeted or could be potential targets for developing therapeutics for cancers. It would be too exhaustive to cover all the signaling pathways in all cancer types and we do not intend to do so, and due to the very rapidly progressing research on cancer signaling and therapy, we have no doubt that new discoveries will have been made by the time this book is published.
In the first chapter of this book, Fultang et al., dive into the role of Wingless and Int-1 (Wnt) signaling in breast cancer oncogenesis. Moreover, the authors also review the current inhibitors of Wnt signaling that are under investigation. The C-X-C chemokine receptor type 4 (CXCR4) signaling is critical in hematological malignancies, while its role in breast cancer is still unraveling. In the second chapter Raman et al., explain the intersection of CXCR4 signaling with other signaling pathways such as Phosphoinositide 3-kinase (PI3K), mitogen-activated protein kinase (MAPK), cellular Src (c-Src) and Janus kinases-signal transducer and activator of transcription proteins (JAK-STAT) in breast cancer progression and metastasis. A key role of epidermal growth factor receptor (EGFR) signaling in cell proliferation and survival in various cancers is now well known. Here in the third chapter, Meena et al., discuss the role of EGFR in colon cancer and its prospective importance as a target for colon cancer therapy. The critical role of PI3K/AKT (protein kinase B)/mammalian target of rapamycin (mTOR) signaling pathway in various cancers has attracted cancer researchers for a long time. Yadav and Mishra elucidate the PI3K/AKT/mTOR signaling in hepatocellular carcinoma and review the various drugs under investigations that target this pathway in the fourth chapter. The MAPK pathway is one of the key survival pathways that have been targeted to develop cancer therapeutics. In chapter 5, Prasad and Srivastava review MAPK signaling in pancreatic cancer and therapeutic agents under investigation. Aberrant regulation and activation of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) has been implicated in several cancers, inflammatory and autoimmune disorders, and erroneous immune system development. In chapter 6, Arora et al., summarize the role of NF-κB activation specifically in multiple myeloma and various strategies developed for its potential pharmacological intervention to abrogate the process of cancer cell proliferation. Recently, small-molecule inhibitors of Bruton’s tyrosine kinase (BTK) such as ibrutinib have shown an impressive anti-tumor activity in clinical studies in patients with various B cell malignancies. BTK is crucial in B lymphocyte development, differentiation and oncogenic signaling. In chapter 7, Gowda et al., elucidate the role of BTK in B cell malignancies and highlight the current progress in the discovery of small molecule BTK inhibitors. Finally, in the last chapter of this book, Pathania et al., discuss the interconnection between exosomes, tumor microenvironment and cMyc transcription factor, and therapeutic strategies to break that nexus.
We sincerely hope that you will enjoy diving into this book.
LIST OF CONTRIBUTORS
Wnt Signaling in Breast Cancer Oncogenesis, Development and Progression
Norman Fultang,Bela Peethambaran*Abstract
Wnt signaling regulates several cellular processes, including differentiation, proliferation, and stem cell pluripotency. Mutations in Wnt signaling are known to lead to tumor initiation and progression. Wnt/ β-catenin signaling is dysregulated in breast cancer, where it has been shown to mediate oncogenic progression. In this review, the canonical and non-canonical pathways of Wnt/ β-catenin signaling, and their regulation of breast cancer oncogenesis and progression are described. During the last decade, several small molecules and natural compounds have shown to interfere with Wnt signaling and demonstrate potential as Wnt-targeting therapeutic agents. This review also highlights these molecules, some of which are in clinical trials. Finally, strategies of using these molecules in combination therapies with other drug agents are discussed.
INTRODUCTION
Overview
The Wnt/β-catenin pathway is a crucial and highly conserved pathway governing the processes of growth, development, and cell fate [1]. An ever-increasing body of evidence suggests a vital role for Wnt/β-catenin signaling in the oncogenesis, development and progression of cancer [2]. This review will summarize recent findings on the role of this pathway in breast cancer and discuss the emerging therapeutic approaches targeting Wnt signaling.
The Wnt signaling cascade is an evolutionarily conserved pathway. It was first discovered to be responsible for spontaneous mammary hyperplasia and tumor formation in mice after pro-viral insertion in the int-1 locus [3]. A few years later,
the Wingless gene responsible for segment polarity in Drosophila melanogaster was found to be a homolog of int-1. Hence the int/Wingless family was named Wnt [4, 5]. Wnt proteins are encoded by 19 different Wnt genes with sequences sharing a high degree of homology [6]. These proteins are known to bind to cellular receptors during embryonic development and mediate several processes, such as cell proliferation, survival, migration, polarity, cell fate, and self-renewal [7]. Wnt can activate distinct signaling pathways, which include a β-catenin dependent or canonical pathway, and a β-catenin independent or non-canonical pathway [4, 7]. Wnt ligands bind primarily to multiple Frizzled (Fzd) receptors [8], but other Wnt co-receptors include members of the low density lipo-protein related protein 5 and 6 (LRP5/6), receptor-like tyrosine kinase (RYK) and receptor tyrosine-kinase like orphan receptors (ROR) 1 and 2 [8, 9]. These co-receptors are important to regulate downstream signaling either through LRP5/6 co-receptors for the β-catenin dependent or through RYK/ROR1/2 for the β-catenin independent pathway.
Wnt Biosynthesis and Regulation
During its synthesis, Wnt has to undergo a palmitoylation step catalyzed by Porcupine (PORCN), which belongs to the membrane-associated O-acyltransferase family [10, 11]. This step is required for the interaction with Fzd receptors and promotes interaction with the multi-pass transmembrane protein, Wntless, which transports Wnt to the plasma membrane. Since PORCN is required for Wnt secretion, several small molecules have been developed to target it. There are 10 Wnt antagonists that also help mediate Wnt signaling. These antagonists include the secreted Fzd-related proteins (SFRPs), Wnt-inhibitory factor 1 (WIF-1), Wise/SOST, Dickkopf proteins (DKKs), insulin-like growth factor binding protein 4 (IGFBP4), Cereberus, Shisa, Wnt-activated inhibitory factor 1 (Waif1/5 T4), adenomatosis polypsosis coli down regulated 1 (APCDD1) and Tikil [9, 12]. Several of these regulatory factors have potential as therapeutic targets, but so far, only DKK1 has been evaluated clinically for drug development [13]. The challenge in using these regulatory proteins as targets is that they often regulate the activity of other important cellular pathways. For instance, SFRPs and WIF-1 can bind to Wnt in both canonical and non-canonical signaling depending on the cellular need. SFRPS, however, also regulates the Notch and Bone Morphogenic Protein (BMP) signaling cascade, a key developmental pathway [12, 14].
Types of Pathways
The Canonical Wnt Signaling Pathway in Breast Cancer
Canonical Wnt signaling plays an important role in cell fate decisions in early embryogenesis during the development of organs, such as the lungs, kidney, skin, and bone [8]. This pathway is also critical in neural patterning and stem cell renewal [15]. Canonical Wnt signaling is also called the β-catenin dependent pathway as it results in an accumulation of cytoplasmic β-catenin followed by the latter’s translocation to the nucleus [8, 15, 16]. Genetic and biochemical evidence suggests that Wnt binds to Fzd receptors, which have seven transmembrane receptors with a cysteine-rich domain at the N-terminal. Fzd is required for multiple Wnt pathways, but another single-pass transmembrane receptor, LRP6/5 is specifically required for the Wnt/β-catenin canonical pathway [17].
Canonical Wnt signaling results in stabilization and nuclear translocation of ß-catenin [18, 19], which is degraded by a destruction complex consisting of AXIN, Protein phosphatase 2A (PP2A), Casein Kinase 1α (CK1α), and Glycogen synthase kinase 3 (GSK3) [19, 20]. In the absence of Wnt, this destruction complex phosphorylates ß-catenin, tagging it for ubiquitination and subsequent degradation by the proteasome. Wnt ligands, when bound to the Fzd receptor and the LRP 5/6 transmembrane co-receptor, trigger the recruitment of Disheveled (Dvl) to the plasma membrane [8, 21-24]. AXIN is also recruited to the phosphorylated cytoplasmic tail of LRP5/6 [17, 25]. Dvl forms a complex with AXIN, Fzd, and LRP5/6 [26]. Recruitment of AXIN and Dvl prevents the formation of the destruction complex leading to the stabilization of cytoplasmic ß-catenin [18, 19]. Dvl proteins also assemble the signalosome that is responsible for phosphorylation of multiple motifs of LFP5/6, one of which is phosphorylated-PPPSPXS/T. P-PPPSPXS/T acts as a competitive inhibitor of GSK3 [26], a kinase that phosphorylates and tags ß-catenin for degradation. The net result of these events is the accumulation of unphosphorylated β-catenin resulting in its stabilization. The stabilized β-catenin translocates to the nucleus, where it acts as a transcriptional co-activator in combination with the T-cell factor (TCF) and the lymphoid enhancer-binding factor (LEF) family of transcription factors. This leads to the recruitment of transcriptional Kat3 co-activators p300 and CREB binding protein (CBP) to transcribe Wnt target genes (Fig. 1A) [27]. ß-catenin can also interact with other transcriptional co-activators, including BRG-1, a component of the SWI/SNF nuclear remodeling complex, Hsp90 co-chaperone Cdc37, and C-terminal-binding protein (CtBP) [28-31]. In the absence of Wnt signaling and nuclear ß-catenin, TCF forms a complex with Groucho proteins to recruit histone deacetylases (HDACs) and repress the transcription of Wnt target genes [32-34].
Fig. (1))A)Wnt on. When Wnt ligand binds to Fzd and LRP receptor, Dvl is recruited to the plasma membrane, forming a complex with Axin and other β-catenin destruction complex members resulting in β-catenin stabilization. Stabilized β-catenin then translocates to the nucleus for transcriptional activity.B). Wnt off. In the absence of Wnt ligand bound to Fzd and LRP receptor, cytoplasmic β-catenin is phosphorylated and tagged for degradation by members of the β-catenin destruction complex. Phosphorylated β-catenin is ubiquitinated by β-TrCP and degraded by the proteasome.In the absence of Wnt, GSK3 and CK1α, in turn, phosphorylate β-catenin. The phosphorylated β-catenin is recognized by Fbox/WD repeat protein β-TrCP, which is a ubiquitin ligase. This results in β-catenin ubiquitination and degradation in the proteasome (Fig. 1B). A recent study identified YAP/TAZ as novel Wnt regulators. It was shown that in the absence of Wnt ligand binding, they could become part of the destruction complex and recruit β-TrCP [35]. The Hippo signaling pathway YAP/TAZ is a key oncogenic pathway that promotes stemness in breast and other cancers [36]. Recently, wnt signaling has similarly been described as a regulator of breast cancer stemness [37, 38].
Non-canonical Wnt Pathways in Breast Cancer
These pathways are characterized by Wnt-activated cellular signaling but do not result in β-catenin stabilization and transcriptional activity. The two non-canonical pathways, identified so far, are the planar cell polarity (PCP) and the Wnt/Ca2+ pathways [39].
The PCP pathway is an LRP5/6 and ß-catenin independent pathway that regulates cell polarity, organization, and migration through the modulation of actin cytoskeleton elements [40]. Binding of Wnt ligands to Fzd and its co-receptors ROR1/2, RYK, PTK7 or NRH1, leads to recruitment of Dvl, which in turn forms a complex with Dvl-associated activator of morphogenesis 1 (DAAM-1) [41-43]. DAAM1 activates G-protein Rho, which activates Rho-associated kinase, ROCK. Activated ROCK regulates the cytoskeleton and cell polarity [44]. Dvl also interacts with Rac1. Rac-1 activates c-Jun N-terminal Kinase, which can regulate actin polymerization [45]. PCP and β-catenin-dependent Wnt signaling can negatively regulate each other [46].
In the Wnt/Ca2+ pathway, Wnt binds to the Fzd receptors and activates G proteins, which then activate phospholipase C or cGMP-specific PDE, triggering the release of intracellular calcium [47, 48]. Calcium activates target genes, such as protein kinase C (PKC), calcineurin, and calcium/calmodulin-dependent kinase II (CaMKII), which are responsible for cell fate and migration [49].
Several non-canonical elements are aberrantly expressed in breast cancer. Overexpression of planar cell polarity protein VANGL-1, for example, correlates with increased risk of disease relapse and metastasis in breast cancer [50]. VANGL-2 is also highly expressed in breast cancer, where it contributes to increased proliferation and poorer prognosis [51]. Genome amplification of Fzd6 and its non-canonical ligands Wnt11 and Wnt5B were also observed in triple-negative breast cancer [52].
Regulators of Wnt Signaling
Wnt signaling is tightly regulated at several points to ensure proper cell function. An ever-accumulating body of evidence suggests several secreted and intracellular factors regulate various steps of Wnt signaling. As previously mentioned, extracellular factors, such as Dkk, WIF-1, SFRs, Cerberus, Frzb, Wise, SOST, IGFBP-4, and Naked cuticle, can bind to Fzd or LRP5/6 and inhibit Wnt signaling [9, 53-55]. These molecules constitute a regulatorily important class of Wnt antagonists. Other non-Wnt ligands can also activate Wnt signaling upon binding to Fzd. These include Norrin and members of the R-Spondin family [56, 57]. The Wnt pathways can also regulate each other: for example, the non-canonical Wnt/Ca2+ pathway can negatively regulate canonical Wnt/ß-catenin/TCF signaling [19, 49]. Other Wnt regulators include transmembrane E3 ubiquitin ligase ZNRF3/RNF43, which negatively regulates Wnt signaling by promoting the turnover of Fzd and LRP6 receptors [58]. R-spondins also promote Wnt signaling by binding ZNRF3, promoting its interaction with LGR4 and subsequent clearance from the plasma membrane [58].
Wnt and Breast Cancer Stem Cells
Cancer Stem Cells (CSCs) are tumor-initiating cells that possess the characteristics of self-renewal and differentiation [2, 59]. Mutations in long-term stem cells or their progeny can lead to malignancy [2]. In normal tissue, stem cells are found in organ systems, such as hematopoietic, dermal, and intestinal tissue, that regenerate and differentiate [2, 60]. Cancer stem cells similarly regenerate tumors leading to disease recurrence and relapse. Wnt has been shown to play a role in self-renewal of hematopoietic stems cells (HSC) [61, 62]. This is seen to coincide with increases in β-catenin levels with ensuing activation of TCF/LEF-1 promoter activity. Within the mammary gland, Wnt signaling plays a key role in regulating stem and progenitor cells [63]. Aberrant Wnt signaling has been linked to tumor initiation in breast cancer via altered regulation of mammary progenitor/stem cell populations [64, 65]. Mechanistically, key stem cell markers CD44 and ALDH1 are transcriptional targets of Wnt/ß-catenin, which might explain how aberrant Wnt hyperactivation promotes stemness [37, 66, 67].
Dysregulated Wnt signaling has also been linked with increased EMT, multidrug resistance, and immune escape in breast CSC populations contributing to tumor persistence [68-71]. Dysregulation of Wnt signaling was also associated with increased PD-L1 expression in triple-negative breast cancer stem cells, possibly contributing to immune evasion [72]. Wnt signaling is enriched in triple-negative breast cancer compared to other breast cancer subtypes [73]. Perhaps accordingly, triple-negative breast cancer is the most stem-like breast cancer subtype and has the worst clinical outcomes [74-76]. Inhibition of Wnt/ß-catenin signaling has shown promise in repressing breast cancer stemness, concurrently reducing cancer metastasis and chemoresistance [77].
Non-canonical Wnt receptor ROR1 has also been shown to promote stemness and drug resistance in breast cancer [78-80]. Inhibition of ROR1 with monoclonal antibodies or small molecule inhibitors repressed breast cancer stemness and potentiated chemotherapy efficacy in breast cancer [78, 79, 81].
Role of Wnt Signaling in Breast Cancer Tumorigenesis and Progression
Several studies have shown that Wnt signaling is vital in the development of several organs, including mammary glands. A study by Van Genderen et al. provided evidence for LEF1 involvement in normal mammary gland development in mice [82]. LEF1 is a transcription factor of the TCF family, which associates with β-catenin to stimulate the expression of Wnt target genes. Wnt is required for mammary gland morphogenesis as was shown in a study where epithelial buds from Wnt-4 knockout mice were implanted in the post-natal mammary fat pads resulting in the reduction of lobular branching [83]. Overexpression of Wnt-4 induced pregnancy-like growth in the reconstituted mammary gland [84]. This suggests a role for Wnt in development at the bud stage. Wnt10b, also known as Wnt-12, is required for mammary bud development [85].
β-catenin is known to affect target genes, such as c-JUN, FRA-1, C-MYC, and CYCLIN D1, which are all involved in proliferation and development [86]. Consequently, overexpression or mutations in any of the Wnt pathway proteins can lead to malignant growth [87]. In breast cancer, Wnt pathways are frequently deregulated, contributing to malignancy [3]. It has been shown that treatments with Wnt ligands significantly increase breast cancer cell motility [88]. Conversely, blocking the pathway by either knocking down Wnt, Dvl, or β-catenin reduced the aggressiveness of breast cancer [89, 90]. These results have also been replicated in a mouse model of Erb2-driven tumor progression [87, 91]. Small molecule inhibition of ß-catenin as a well as knockdown of Wnt-associated transcription factor, SOX4, had similar effects repressing breast cancer proliferation and migration [92]. Blocking endogenous β-catenin by RNA interference in transgenic mice similarly showed a significant reduction in tumor cell invasion [77].
Loss of β-catenin at the membrane has been shown to be a key feature of invasive ductal carcinoma, the molecular subtype of breast cancer with the worst prognosis [93]. Expression of other components in the Wnt pathway, such as AXIN, CK1α, GSK-3β, and protein phosphatase 2A, has also been shown to be associated with breast cancer progression [94, 95]. The differential expression of co-receptors, LRP5 and LRP6, has also been shown to be associated with mammary gland tumorigenesis [96]. Silencing of these co-receptors reduced Wnt signaling, which led to decreased cancer cell proliferation and in vivo tumor growth. Wnt receptor Fzd7 is also upregulated in breast cancer patients, where its silencing resulted in a significant reduction in tumor growth [97].
The mutations and/or mechanisms underlying anomalous activation of Wnt pathways in breast cancer are not fully understood. Increased Wnt signaling might be a result of several factors, including mutations in ß-catenin, APC, or other members of the destruction complex or impaired activity of wnt regulatory pathways [20, 98, 99]. Modulated epigenetic regulation of several members of the Wnt pathways might also contribute to increased Wnt signaling [100].
Animal Models and Clinical Significance of Wnt Signaling in Breast Cancer
The first Wnt transgenic mouse model was constructed by a proviral insertion of mouse mammary tumor virus (MMTV) within the int-1/Wnt1 locus leading to tumorigenesis [101]. Nusse et al. observed that the mice developed lobuloalveolar hyperplasia and then cancer. Other models leverage proviral activation of Wnt10B, the earliest expressed Wnt ligand, to induce hyperplasia and mammary tumors [102]. Important nodes in the Wnt pathway have also been targeted. GSK3β was repressed in a mouse model by creating a kinase inactivated GSK3ß mutant [94]. The mice were shown to have upregulated β-catenin, Cyclin-D1, and developed mammary tumors. Other GSK3ß null models with increased Wnt signaling have been used [103]. Many models targeting proteins in the destruction complex are also being used. Mice with truncated APC, for example, develop tumors with constitutively active Wnt/ß-catenin signaling [104]. Casein Kinase 2 (CK2) overexpression models that lead to mammary adenocarcinoma are also used [95, 105]– CK2 is a serine/threonine kinase, downstream of Dvl which promotes Wnt/ß-catenin signaling [106].
Wnt Signaling in Triple-Negative Breast Cancer
Triple-negative breast cancer (TNBC) is an aggressive subtype of breast cancer with poor prognosis and is characterized by the absence of estrogen receptors (ER), progesterone (PR), and Human epidermal growth factor 2 (HER2) [107, 108]. The lack of these proteins reduces the efficacy of targeted agents, such as Herceptin (targets HER2) and Tamoxifen (targets ER). Wnt signaling is highly upregulated in TNBC compared to other subtypes of breast cancer [90, 108]. It has been demonstrated that β-catenin is a marker for poor outcomes in TNBC patients [16]. Deregulated expression of components of the Wnt signaling pathway has been shown to significantly increase the risk of brain and lung metastasis in TNBC patients [109]. The gene encoding β-catenin (CTNNB1) is not mutated in TNBC patients but is seen to be over-expressed, leading to continuous activation of Wnt signaling [110, 111]. Studies have shown that β-catenin nuclear accumulation promotes cell migration and resistance to chemotherapy in TNBC cells both in vitro and in vivo [108, 112]. Dysregulation of non-canonical Wnt signaling can also promote metastasis of TNBC cells and CSCs through c-Jun N-terminal kinase activation. CSCs are thought to be responsible for the relapse of TNBC [113]. Non-canonical Wnt receptor ROR1 has also been shown to be enriched in ER-negative breast cancer compared to ER-positive breast cancer, where it interacts with CK1ε to promote cell proliferation, survival, and metastasis [80]. Other canonical and non-canonical Wnt receptors are similarly deregulated in TNBC [114].
A study by Lehmann et al., based on 587 TNBC cases from 21 breast cancer data sets, identified 6 molecularly distinct sub-types of TNBC [115]. These sub-types include mesenchymal stem-like (MSL), mesenchymal (M), basal-like 1, basal-like2, immunomodulatory, and luminal androgen receptor (LAR). The LAR sub-type accounts for 10% of TNBCs and has characteristic amplification of CCND1/Cyclin D1, a gene regulated by the Wnt/ β-catenin pathway [116, 117]. The basal-like subtypes have deregulated expression of Wnt associated genes CHEK1, FANCA, FANCG, MSH2, RAD21, AURKB, PLK1, CENPA, BUB1, CCNA2, MYC, NRAS and PRC1 [114, 118-125]. These are involved in DNA damage signaling and cell cycle regulation. In both Lehmann’s M and MSL subtypes, several genes involved in Wnt/ β-catenin, such as CTNNB1, DKK2, DKK3, SFRP4, TCF4, and FZD4, are found to be up-regulated [115]. A subsequent RNA profiling analysis by Burstein et al. narrowed down TNBC cases to four stable, molecularly distinct subtypes [116]. These include LAR, mesenchymal, basal-like immune suppressed, and basal-like immune activated subtypes. The basal-like immune-suppressed subtypes exhibit overexpression of proliferation genes, including SOX transcription factors, which are known regulators of Wnt/ß-catenin signaling [126]. The basal-like immune-activated subtype has increased expression of CDK1, which phosphorylates and modulates Wnt regulators BCL9 and TAZ [127, 128].
Natural Compounds Targeting Wnt Pathways as Cancer Therapeutics
Several naturally occurring molecules have been shown to modulate the activity of one or more components of the Wnt pathway (Table 1).
Resveratrol - Resveratrol is a polyphenol found in red grapes [129]. Resveratrol is known for its anti-cancer properties and activity, notably in breast and colon cancer [129-131]. In both colon and breast cancer, it has been shown to inhibit Wnt signaling [129, 130]. It has also been shown to decrease ß-catenin nuclear localization and disrupt ß-catenin/TCF4 interaction [130, 131]. Resveratrol is a phyto-estrogen that can be both antagonistic and agonistic to the estrogen receptors contributing to its bioactivity in breast cancer [132, 133]. Resveratrol, as an inhibitor of Wnt signaling in colon cancer, was explored in a clinical trial (NCT00256334). Results suggested that resveratrol failed to significantly inhibit Wnt signaling in colon cancer tissue but was effective in inhibiting Wnt in normal colonic mucosa [134]. Resveratrol also inhibits oncogenic growth factor IGF2 [135]– IGF2 potentially inhibits ß-catenin inhibitor GSK3ß via activation of P13K/AKT [136, 137]. In an on-going clinical trial (NCT04266353), the inhibitory effect of resveratrol on serum IGF2 in African American women with breast cancer is being evaluated.
Vitamin D- Vitamin D was shown to suppress Wnt-signaling in breast cancer, inducing apoptosis, and promoting tamoxifen sensitivity [138]. It has also been shown to be a chemopreventive agent in breast cancer animal models [139, 140]. In colon cancer cells, vitamin D treatment resulted in vitamin D receptors binding to β-catenin, reducing its localization to the nucleus [139, 141], and in turn, reducing colon cancer proliferation and survival.
Curcumin- Curcumin (diferuloylmethane [1,7-bis(4-hydoxy-3-methoxypheyl)- 1, 6-helptadiene-3,5-dione] is a bioactive compound in turmeric [142]. Curcumin is known to affect different kinds of cancers, such as colon, breast, and skin. Curcumin inhibits breast cancer stem cells via inhibition of both Wnt/ß-catenin and sonic hedgehog pathways [143]. It was also shown to inhibit VEGF in breast cancer cells via inhibition of ß-catenin [144]. Curcumin also targets the transcription of Wnt target genes, such as c-myc, c-fos, c-jun, and iNOS in a variety of cancer cells [145]. In another study, curcumin reduced the transcription of β-catenin in colon cancer [146, 147]. Treatment with 40µM of curcumin resulted in the degradation of β-catenin and a decrease in nuclear β-catenin levels [147].
Retinoids- Vitamin A is converted to a number of metabolites called retinoids [148]. Retinoids have been demonstrated to be potent anti-cancer compounds in vitro, in vivo and in clinical trials [149]. The mechanism by which retinoids inhibit cancer cell progression is by inhibiting the activity of oncogenic AP-1 and Wnt/ β-catenin signaling [150]. Retinoids also play a key role in the degradation of Cyclin D1 in cancer cells, which is mediated by GSK3β- an important component of the Wnt/ β-catenin signaling pathway [151].
Genistein- Genistein is an isoflavone abundantly found in soy [152]. Genistein has shown potency in several cancers, where it promotes sFRP2 and GSK3ß activity and represses ß-catenin [153-156]. Genistein has also shown to inhibit Wnt-1 induced proliferation and repress the transcription of wnt-target genes, c-myc and Cyclin D1 [157, 158].
EGCG and other catechins- Catechins are primarily found in tea derived from Camellia sinesis [159, 160]. The most characterized catechins are epigallocatechin-3-gallate (EGCG), epigallocatechin, epicatechin-3-gallate, and epicatechin. Wnt/β-catenin signaling is inhibited by EGCG in breast cancer [161]. EGCG was also shown to reduce both proliferation and invasiveness of breast cancer through induction of HMG-box containing protein 1 and repression of Wnt/ β-catenin signaling [162, 163]. Extracts from green tea were also found to inhibit aberrant colonic crypts in rats and intestinal polyps in the APC-Min mouse [164]. The decrease in tumor burden was associated with the downregulation of β-catenin in the intestine and lower expression of c-jun and cyclin D1.
Lupeol- Lupeol is found in several fruits, such as olives, figs, strawberries, and mangoes [165]. Lupeol has several medicinal benefits, including activity against cancer [165, 166]. Lupeol can inhibit the growth of melanoma cells that have constitutive Wnt/ β-catenin signaling [167]. In these cells, Lupeol prevents nuclear localization of β-catenin by decreasing the latter’s phosphorylation at serine 552 and serine 675, which is necessary for its translocation into the nucleus [167]. This also modulates the expression of several genes in the Wnt signaling pathway. Lupeol also modulates both NF-ƙB and P13K/Akt pathways, which interact with Wnt signaling pathways [166].
SMALL MOLECULES AND NOVEL THERAPEUTICS THAT TARGET WNT SIGNALING
Several small molecules that either inhibit or activate Wnt signaling have been identified (Table 2). Compounds, such as IWR [169] and XAV939 [170], that impact the stability of Axin, which is regulated by tankyrase-mediated ADP-ribosylation, have been characterized. These compounds inhibit tankyrase, increasing Axin levels, and lowering β-catenin. There are also compounds that block Porcupine, the enzyme that catalyzes the acylation of Wnt proteins. These molecules are IWP2 [171], C59 [172], and LGK974 [173] (Table 2). Acylation of Wnt is required for Wnt transport. LGK974 is currently undergoing clinical trials for a variety of solid tumors.
Wnt signaling can also be interrupted at the receptor level. There was a recent screen for RNF43 mutations that sensitize pancreatic tumor cells to therapy and make them dependent on the Wnt ligand [177]. Several mutants found in this screen, that suppressed the RNF43 growth phenotype, were due to aberrations in FZD5, indicating that the tumor cells are dependent on Wnt-FZD signaling [177]. The growth of these tumors could be reduced by the use of antibodies against FZD (OMP-18R5), which has been shown to bind to several FZD family members [174]. Wnt protein is highly hydrophobic and because of this, it is challenging to use this as a drug. Recently soluble Wnt agonists have been shown to activate Wnt signaling in vivo [178]. Compounds, such as L807mts, Bio, CHIR, SB-216763, are known to interfere with Wnt signaling, specifically GSK3 [175]. A formylated 6 amino acid fragment Foxy-5, which mimics the effects of Wnt-5a by binding Fzd to impair migration in epithelial cells and Wnt-5a-low cancer cells, was recently investigated in a clinical trial for metastatic breast, colorectal, and prostate cancers (NCT02020291) [176]. A PTK7-antibody drug conjugate (PTK7-ADC) is also being used to treat metastatic and triple-negative breast cancers (NCT03243331) [179].
Perhaps the most effective therapeutic target would be the TCF/β-catenin complex. However, to date, there is no known molecule identified that could disrupt this complex. The binding affinity of these two proteins is very high, making the development of effective small molecule inhibitors of the complex, difficult [7, 180].
Conclusion and Future Directions
In this review, several studies that provide a deeper understanding of the role of Wnt signaling in breast cancer are highlighted. It is well established that non-canonical Wnt signaling is similar in development and cancer but yields two distinct outcomes. Non-canonical Wnt signaling results in tissue mobility in development but metastasis in cancer. The interaction of canonical and non-canonical Wnt pathways and the mechanisms underlying their tissue-specific regulation are also highlighted. Several Wnt-targeted agents are now undergoing clinical trials Table 2, but historically, Wnt signaling has been difficult to target clinically. Wnt signaling is a ubiquitous developmental and regulatory pathway present in several cells and tissue types in the body. This makes non-specific targeting of non-malignant tissue an issue. Another concern is Wnt’s significant role in stem cell regulation and tissue/organ regeneration. Wnt inhibitors/modulators could drastically affect normal stem cell populations and be detrimental to tissue regeneration. To date, no Wnt-inhibitors have been approved by the FDA.
Wnt inhibition can also synergize with other therapies for cancer. Modulating Wnt pathway activity can increase cancer cell sensitivity to chemotherapy [68]. Novel Wnt inhibitor, OMP-18RF, when combined with paclitaxel in lung and breast cancer models showed an enhanced response [181]. Similarly, other Wnt inhibitors, such as PRI-724, in combination with gemcitabine, showed a better response in pancreatic cancer [182]. In a recent study, we showed that inhibition of non-canonical Wnt receptor, ROR1, with Strictinin, a small molecule, increases breast cancer cell sensitivity to Doxorubicin and Cisplatin [78]. ROR1 inhibition has similarly been shown to potentiate Paclitaxel efficacy in breast cancer [79]. In tumors that proliferate independently of Wnt signaling, Wnt inhibition may be able to render hematopoietic stem cells and gut epithelial cells non-proliferative. Because chemotherapeutic agents typically target hyperproliferative cells, this would protect the cells from side effects that are cell cycle-specific. Higher doses of chemotherapy could then be administered without fear of excess gastrointestinal toxicity or myelosuppression.
There is also significant immunomodulation in Wnt activated tumors [70
