Notch in cancer and cancer metabolism : six degrees of intracellular turbulence

Abstract: Notch signaling is an evolutionarily conserved cell-to-cell contact-dependent signaling mechanism in multicellular organisms directing cellular fates both in early development and adult tissues. In metazoans the Notch pathway consists of multiple paralogs of receptors and ligands constituting a complex juxtacrine communications network orchestrating organismal homeostasis. Binding of receptors on signal-receiving cells to the ligands on signal-sending cells leads to proteolytic cleavage and release of the intracellular domain of Notch (NICD). NICD subsequently translocates to the cell nucleus to activate Notch downstream gene expression machinery by binding to the Notch-dependent transcriptional regulator CSL. Notch is highly context-dependent, and the nature of Notch-mediated outcomes is governed by multiple factors such as crosstalk with other signaling pathways, post-translational modifications, and CSL- binding type preference. Notch is ultimately a cell fate decider with a temporal specificity, where context and time can determine whether Notch inhibits or promotes a cellular outcome. The importance of the Notch pathway is further emphasized by the dramatic effects of dysregulated Notch signaling, which often leads to life-threatening diseases and cancer, such as CADASIL and T-ALL. In this thesis I have glimpsed behind the veil into the unknowns of Notch signaling and investigated several novel aspects and peculiarities relating to Notch deregulation in cancer, and to Notch regulation via post-translational modifications. “When Notch and Pim Unite”, Notch1 ICD undergoes post-translational phosphorylation by Pim kinases occurring at the nuclear localization signal within the PPD-domain, thus modulating the nuclear transport and transactivation of N1ICD. This impacts tumor growth and metabolism in breast cancer, and migration in prostate cancer. In “A Metabolic Turn of Events” we discover that Notch signaling is able to reprogram the metabolism in breast cancer where high Notch levels induce the PI3K/Akt pathway leading to a shift towards aerobic glycolysis, while low Notch leads to a forced switch to glycolysis following mitochondrial oxidative phosphorylation defects. The Notch deficiency subsequently sensitizes the cancer cells for low glucose conditions. Next we unleash “Systematic KOs”, when we knockout CSL in MDA-MB-231 breast cancer cells which leads to increased tumor growth and an activated hypoxic response. Furthermore, comparison of the Notch wild-type and CSL knock-out transcriptomic signatures reveals an upregulation of over 1700 genes not part of the Notch gene signature, suggesting that CSL transcriptionally controls a number of genes not part of the canonical Notch signature. Lastly, we are “Falling Into Hypoxity” as canonical Notch1 is shown to induce HIF2α and trigger a HIF1α-to-HIF2α switch in medulloblastoma. However, Notch1 remains tumor suppressive in CAM-xenographs and the genetic removal of HIF2α increases tumor growth. Taken together, this thesis contributes new puzzle pieces to building a complete picture of the Notch signaling pathway, its role in cancer, and provides new vistas for future anti-Notch therapies.