Notch signaling is a short-range intercellular communication system regulating stemness programs in both physiologic and neoplastic contexts, either by promoting or counteracting tumorigenesis depending on the tumor type. Therefore, Notch represents one of the most promising candidates for therapeutic targeting in several cancers. Over the last two decades, many efforts have been dedicated to antagonize Notch over-activation through the development of different blocking agents that are currently being investigated in preclinical and clinical studies. However, less attention is paid to agonize Notch in cancer contexts in which its restoration would mediate tumor suppression and, even worse, to date potential Notch agonistic agents are lacking.
Aberrant Notch signaling results from genetic and/or epigenetic alterations in the Notch pathway genes or from the deregulated activity of its upstream effectors. However, the understanding of the mechanisms differentially regulating the pathway's components in different cell types remains mostly ambiguous. Accordingly, our proposal aims to unveil the molecular machinery repressing Notch signaling and its relevance in distinct cancer types. Previously, others and we unveiled mechanistic insights into Notch regulation in Notch-dependent T-ALL, demonstrating that the expression and the transcriptional activity of Notch receptors are sustained by the histone demethylase JMJD3. Accordingly, we aim to evaluate whether a similar epigenetic mechanism could regulate Notch signaling in a specular fashion in tumor contexts in which Notch is supposed to act as a tumor suppressor, such as Acute Myeloid Leukemia, Cervical Cancer and Small-cell Lung Cancer. We hypothesize that the histone methyltransferase EZH2, conversely to JMJD3, would be implicated in the repression of Notch genes' expression and anti-tumor function in the above-mentioned cancers, making the selective targeting of EZH2 as a novel "Notch-activating" anti-tumor strategy.
Given the pivotal role of Notch in various cancers, either by promoting or counteracting tumorigenesis in a context-depend fashion, several Notch modulators have been developed in preclinical research, and some of them are under clinical investigation for the treatment of several malignancies. However, the main classes of Notch inhibitors, including the gamma-secretase inhibitors and the blocking antibodies against Notch receptors and ligands, have been shown serious drawbacks that limit their therapeutic use [1]. To date, several additional strategies to inhibit Notch are in development, while less attention is paid to derepress the signaling in the contexts in which it is silenced and the restoration of the signaling would be of significant therapeutic value. In the absence of mutation in Notch genes, defective activation of the pathway results by epigenetic alterations at the Notch genes or deregulated activity of its upstream effectors. However, understanding how different Notch receptors and target genes are activated or repressed in different cell types remains mostly enigmatic.
Accordingly, the proposed research is designed to unveil the molecular mechanisms repressing Notch signaling in distinct cancers to drive the development of novel Notch-based therapeutic strategies. Of note, evidence suggests the epigenetic machinery is a novel target in cancer therapy, and several inhibitors targeting epigenetic regulators have been developed and are ongoing in preclinical and clinical settings. Among them, considerable efforts have been focused on the development of EZH2 inhibitors and results from most of the clinical trials ongoing are showing acceptable efficacy and safety in patients.
In particular, we will focalize on the epigenetic status of the Notch pathway genes and the implication of the histone modifier EZH2 in their transcriptional control in AML, CC and SCLC contexts, in which Notch reactivation would exert a tumor-suppressive function. Given that histone-modifying enzymes, have great potential in being drug targets and some of them are already used in clinical trials, unveiling the epigenetic mechanisms controlling the transcription of the key genes of Notch pathway would be potentially translated into the design of novel efficacy therapeutic strategies. Over the last years, considerable efforts have been focused on the development of EZH2 inhibitors and results from most of the clinical trials ongoing are showing acceptable efficacy and safety of these agents in patients [2]. However, the precise molecular mechanisms by which EZH2 contributes to cancer pathogenesis remain still unclear. Supporting the potential efficacy of the selective Notch reactivation by targeting EZH2 in tumor contexts in which its pathway is silenced, the pharmacological inhibition of JMJD3, the enzymatic counterpart of EZH2, exerted anti-Notch and anti-tumor effects in experimental models of Notch-driven T-ALL and of CRC [3, 4]. Nonetheless, the potential effectiveness of the targeting of the EZH2-Notch interaction is supported by the observation that among the few Notch reactivating agents, the class of them designed to modulate enzymatic activities of the histone modifiers are showing promising results in preclinical trials [5-7].
Nevertheless, despite epigenetic therapy has a rational and solid basis in theory, some issues on its applicability remain to be solved, including the choice of the correct population. In this regard, although the frequency of mutations in epigenetic modulators offers a possibility to develop drugs against biomarker-defined responsive populations, defined biomarkers of the response to the treatment are lacking for most of the epigenetic agents [2]. Therefore, Notch signaling reactivation following EZH2-targeting would be proposed as a potential biomarker for patients' response to the treatment with EZH2 inhibitors. Finally, considering the association of both Notch signaling reactivation and EZH2 inhibition in reversing the resistance to doxorubicin and cysplatin in AML, CC and SCLC [8-12] our study would also investigate gain further mechanistic insights into the role of the Notch/EZH2 interplay relevant in the mechanism sustaining cancer chemoresistance.
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