Abstract SY01-02: Targeting stem cell pathways in human cancer
Authors: De Maria Ruggero , Maugeri-Saccà Marcello 
Published: July 2016

Abstract
Background
Tumors contain a specialized subset of phenotypically and functionally distinct cells with unique properties, namely self-renewal, differentiation ability and tumorigenicity (1). Upon seminal characterization efforts, this rare cellular compartment, usually referred to as cancer stem cells (CSCs), was designated as the ideal target for the development of more effective anticancer therapies. The message conveyed by recent investigations is that CSCs are not fixed in an undifferentiated state, but rather that a number of signals can alter this condition. The most dramatic implication is that CSC-related properties can be installed in non-CSCs. The partial elucidation of the multifaceted mechanisms that drive the conversion of cancer cells into CSCs, along with the characterization of intracellular mechanisms accounting for their insensitivity to current anticancer drugs, has raised novel enthusiasm towards the development of CSC-directed therapies.
Discussion
Exposure of cancer cells to a variety of stimuli dispatched by the tumor microenvironment was found to fuel the conversion of non-CSCs into cells with stem-like traits. Epithelial-mesenchymal transition (EMT) was the first mechanism described to act as an inducer of CSCs (2). Afterwards, the same properties were attributed to hypoxia (3), low pH (4), and cytokines (5). We have recently described CD44v6 as a CSC marker required for the metastatic dissemination of colorectal cancer stem cells (CR-CSCs) (5). Reprogramming of CD44v6− progenitors into CD44v6+ CSCs occurs after prolonged exposure to a variety of cytokines produced by tumor-associated cells, such as the hepatocyte growth factor (HGF), osteopontin (OPN), and stromal-derived factor 1α (SDF-1), through the Wnt/β-catenin pathway. Importantly, selective targeting of CD44v6-expressing CR-CSCs was obtained through phosphatidylinositol 3-kinase (PI3K) inhibition, raising the hypothesis that PI3K inhibitors may be active in preventing the onset of distant lesions (6). When referring to CSC pathways, the attention turns to the Hedgehog, Notch, Wnt and TGFβ, which were originally placed at the center of the signaling network governing CSCs (1). More recently, the developmental Hippo pathway was described as a further signal involved in the acquisition/retention of CSC properties (7). Core components of the Hippo pathway are operatively grouped into a regulatory module, which contains kinases and adaptors (MST1/2, LATS1/2, SAV1, MOB1A/B), and a transcriptional module composed by the two closely related oncoproteins TAZ and YAP (7). When the kinase core module is inactive or its regulatory activity is suppressed, TAZ/YAP interact with transcription factors, ultimately activating a tumor-promoting genetic program. Breast cancer is serving as a benchmark for studying Hippo at the CSC level. TAZ was originally connected to self-renewal ability via EMT. This model envisions that EMT, by delocalizing the cell polarity determinant Scribble from the cell membrane, alleviates the Scribble-mediated inhibition of TAZ (8). After this first report, we demonstrated that TAZ is also required for metastatic dissemination of breast CSCs (BCSCs) (9). Moreover, we showed that low TAZ expression efficiently predicts pathological complete response in Luminal B, HER2-positive breast cancer patients who received neoadjuvant chemotherapy and trastuzumab (10). A new wave of studies clarified that both hypoxia and the extracellular matrix are important regulators of the TAZ-driven expansion of the CSC compartment (11, 12). Thus, Hippo transducers are emerging as crucial regulators of BCSCs, by sensing a variety of extracellular cues, such as those connected with EMT, hypoxia, ECM rigidity and cell density.
A key tenet of CSC biology refers to the ability of CSCs to adapt to stressful conditions, such as the lesions inflicted on DNA by chemotherapeutic agents (13). Genome integrity is safeguarded by a complex intracellular network, the DNA damage response (DDR) machinery, requiring the cooperation between cell cycle checkpoints, DNA repair mechanisms, and apoptotic pathways. In CSCs, a combination of increased DNA damage repair ability and decreased programmed cell death contributes to their insensitivity to chemotherapy and radiotherapy (13). Crosstalking between ATM-Chk2 and ATR-Chk1 pathways is essential in initiating DNA repair upon exposure to chemotherapy or radiotherapy. The first link between Chk1 and CSCs stemmed from a study of glioblastoma stem cells (GBM-SCs) (14). In this setting, the CD133+ population was characterized by increased radioresistance compared to the CD133− counterpart, as a result of a more rapid activation of the ATM-Chk2 and ATR-Chk1 cascades. Importantly, the targeting of Chk1 rendered GBM-SCs vulnerable to radiation. Similar observations were made by our group using CSCs isolated from non-small cell lung cancer patients (15). This intrinsically chemoresistant population was efficiently killed when chemotherapy was combined with the pharmacological inhibition of Chk1. The therapeutic potential of novel agents targeting cell cycle checkpoints was later extended to CR-CSCs (16). Moreover, after having challenged a collection of patient-derived CR-CSCs with a library of agents targeting the principal signal transduction pathways, we observed that a novel Chk1 inhibitor was one of the most potent compound able to kill the CSC compartment as monotherapy (unpublished results). This anti-CSC activity was evident is specific molecular contexts, such as TP53 mutated tumors and tumors carrying hyperdiploid DNA content.
Future Directions
The lesson we have learned in recent years is that CSC content can be modified, and increased, by a multiplicity of cues. Achieving a deeper understanding of these signals is therefore essential for more focused clinical investigations with agents supposedly endowed with anti-CSC activity. The same considerations can be extended to DDR-targeting agents, considering the number of Chk1 inhibitors that have entered clinical development (17). Overall, preclinical investigations conducted so far provided a robust background for clinical trials with established and investigational agents, envisioning the assessment of specific endpoints measuring CSC function/content.
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Citation Format: Ruggero De Maria, Marcello Maugeri-Saccà. Targeting stem cell pathways in human cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr SY01-02.