Angiogenesis is critical during tumour initiation and malignant progression. Different strategies aimed at blocking vascular endothelial growth factor (VEGF) and its receptors have been developed to inhibit angiogenesis in cancer patients. It has become increasingly clear that in addition to its effect on angiogenesis, other mechanisms including a direct effect of VEGF on tumour cells may account for the efficiency of VEGF-blockade therapies. Cancer stem cells (CSCs) have been described in various cancers including squamous tumours of the skin. Here we use a mouse model of skin tumours to investigate the impact of the vascular niche and VEGF signalling on controlling the stemness (the ability to self renew and differentiate) of squamous skin tumours during the early stages of tumour progression. We show that CSCs of skin papillomas are localized in a perivascular niche, in the immediate vicinity of endothelial cells. Furthermore, blocking VEGFR2 caused tumour regression not only by decreasing the microvascular density, but also by reducing CSC pool size and impairing CSC renewal properties. Conditional deletion of Vegfa in tumour epithelial cells caused tumours to regress, whereas VEGF overexpression by tumour epithelial cells accelerated tumour growth. In addition to its well-known effect on angiogenesis, VEGF affected skin tumour growth by promoting cancer stemness and symmetric CSC division, leading to CSC expansion. Moreover, deletion of neuropilin-1 (Nrp1), a VEGF co-receptor expressed in cutaneous CSCs, blocked VEGF's ability to promote cancer stemness and renewal. Our results identify a dual role for tumour-cell-derived VEGF in promoting cancer stemness: by stimulating angiogenesis in a paracrine manner, VEGF creates a perivascular niche for CSCs, and by directly affecting CSCs through Nrp1 in an autocrine loop, VEGF stimulates cancer stemness and renewal. Finally, deletion of Nrp1 in normal epidermis prevents skin tumour initiation. These results may have important implications for the prevention and treatment of skin cancers.

Adult stem cells (SCs) are at high risk of accumulating deleterious mutations because they reside and self-renew in adult tissues for extended periods. Little is known about how adult SCs sense and respond to DNA damage within their natural niche. Here, using mouse epidermis as a model, we define the functional consequences and the molecular mechanisms by which adult SCs respond to DNA damage. We show that multipotent hair-follicle-bulge SCs have two important mechanisms for increasing their resistance to DNA-damage-induced cell death: higher expression of the anti-apoptotic gene Bcl-2 and transient stabilization of p53 after DNA damage in bulge SCs. The attenuated p53 activation is the consequence of a faster DNA repair activity, mediated by a higher non-homologous end joining (NHEJ) activity, induced by the key protein DNA-PK. Because NHEJ is an error-prone mechanism, this novel characteristic of adult SCs may have important implications in cancer development and ageing.

The maintenance of genome integrity in stem cells (SCs) is critical for preventing cancer formation and cellular senescence. The immortal strand hypothesis postulates that SCs protect their genome by keeping the same DNA strand throughout life by asymmetrical cell divisions, thus avoiding accumulation of mutations that can arise during DNA replication. The in vivo relevance of this model remains to date a matter of intense debate. In this study, we revisited this long-standing hypothesis, by analyzing how multipotent hair follicle (HF) SCs segregate their DNA strands during morphogenesis, skin homeostasis, and SC activation. We used three different in vivo approaches to determine how HF SCs segregate their DNA strand during cell divisions. Double-labeling studies using pulse-chase experiments during morphogenesis and the first adult hair cycle showed that HF SCs incorporate two different nucleotide analogs, contradictory to the immortal strand hypothesis. The co-segregation of DNA and chromatin labeling during pulse-chase experiments demonstrated that label retention in HF SCs is rather a mark of relative quiescence. Moreover, DNA labeling of adult SCs, similar to labeling during morphogenesis, also resulted in label retention in HF SCs, indicating that chromosome segregation occurs randomly in most of these cells. Altogether, our results demonstrate that DNA strand segregation occurs randomly in the majority of HF SCs during development, tissue homeostasis, and following SC activation. STEM CELLS 2008

The accurate maintenance of genomic integrity is essential for tissue homeostasis. Deregulation of this process leads to cancer and aging. BRCA1 is a critical mediator of this process. Here, we performed conditional deletion of Brca1 during epidermal development and found that BRCA1 is specifically required for hair follicle (HF) formation and for development of adult HF stem cells (SCs). Mice deficient for Brca1 in the epidermis are hairless and display a reduced number of HFs that degenerate progressively. Surprisingly, the interfollicular epidermis and the sebaceous glands remain unaffected by Brca1 deletion. Interestingly, HF matrix transient amplifying progenitors present increased DNA damage, p53 stabilization, and caspase-dependent apoptosis compared with the interfollicular and sebaceous progenitors, leading to hyperproliferation, apoptosis, and subsequent depletion of the prospective adult HF SCs. Concomitant deletion of p53 and Brca1 rescues the defect of HF morphogenesis and loss of HF SCs. During adult homeostasis, BRCA1 is dispensable for quiescent bulge SCs, but upon their activation during HF regeneration, Brca1 deletion causes apoptosis and depletion of Brca1-deficient bulge SCs. Our data reveal a major difference in the requirement of BRCA1 between different types of epidermal SCs and progenitors and during the different activation stages of adult HF SCs.

No abstract

Many cancers harbor mutations in the Hippo pathway that lead to constitutive activation of the transcriptional co-activators YAP/TAZ that then bind the transcription factor TEAD and drive aberrant transcription of target genes involved in cell proliferation and tumor progression. Hyperactivation of YAP/TAZ has also been associated with resistance to targeted therapies, including MAPK pathway inhibitors. To target cancers that bear mutations in the Hippo pathway or are resistant to therapies due to YAP/TAZ activation, we developed SW-682, a pan-TEAD small molecule inhibitor that blocks TEAD-dependent transcription by binding to the palmitoylation pocket of all TEAD isoforms. In vitro, SW-682 inhibited the proliferation of human Hippo-mutant mesothelioma cells with nanomolar potency, with little to no effect on Hippo wild-type tumor cells. SW-682 down-regulated TEAD-dependent reporter gene expression in a dose-dependent manner, while having no effect on reporters monitoring other pathways. In vivo, daily oral administration of SW-682 to adult mice resulted in tumor regression in Hippo-mutant mesothelioma models and caused down-regulation of expression of the TEAD-dependent genes CCN1 and CCN2 and a YAP gene signature, as measured by qPCR or RNA-seq analysis. SW-682 has a favorable PK profile with good oral bioavailability in the mouse and was well tolerated with no signs of body weight loss. To test the hypothesis that TEAD inhibition can overcome YAP-driven resistance mechanisms, we explored SW-682 in combination with MEK inhibitors in several in vitro and ex vivo patient-derived tumor models including BRAF and NRAS mutated melanoma. Moreover, to identify new indications that may benefit from TEAD inhibition, we screened patient-derived 3D organoid tumor cells and matching patient-derived xenograft models that have been molecularly profiled. In summary, SW-682 is a potent and selective investigational TEAD inhibitor which demonstrated anti-tumor effects in models harboring aberrant expression of the Hippo pathway, suggesting therapeutic potential in multiple Hippo-mutant solid tumors. Citation Format: Lei Chen, Paula Milani de Marval, Kendall Powell, Mark Johnson, Greg Falls, Brian Lawhorn, Aurélie Candi, Amuri Kilonda, Bart Vanderhoydonck, Arnaud Marchand, Matthias Versele, Georg Halder, Stephen L. Gwaltney, Adeela Kamal. SW-682: A novel TEAD inhibitor for the treatment of cancers bearing mutations in the Hippo signaling pathway. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4964.

The Hippo pathway is a highly conserved signaling pathway across higher-order vertebrates and a key modulator of developmental biology. Both genetic aberrations as well as non-genetic dysregulation of the pathway lead to constitutive nuclear localization and transcriptional activity of the YAP/TAZ-TEAD complex in multiple solid tumor types, including mesothelioma, uveal melanoma, squamous cell cancer, liver cancer, lung cancer, etc. Genetic aberrations are manifested as gene amplifications and gene fusions of the core transcriptional YAP/TAZ-TEAD complex subunits, and, more commonly, as deletions or loss-of-function mutations in the upstream negative regulators of the Hippo pathway such as NF2, LATS1/2 or FAT1. More recently, constitutive activation of YAP/TAZ-TEAD has been implicated in cancer therapy resistance and in immune evasion. Multiple efforts have been devoted to identify small-molecule inhibitors of the YAP/TAZ-TEAD protein-protein interaction, yet with limited success reported to date. Based on the identification of an auto-palmitoylation pocket centrally located in TEAD, and its reported role to sustain YAP/TAZ-TEAD transcriptional activity, we set up a biophysical assay to detect selective small-molecule binding into the palmitoylation pocket of TEAD1. Based on screening a rationally designed compound collection in this assay and iterations of analoging, we identified several novel chemical series of TEAD-palmitoylation pocket binders. Hits were confirmed as specific allosteric inhibitors of YAP/TAZ-TEAD transcription in cell-based assays (Q-PCR and reporter gene assays). Soaking compounds in TEAD crystals revealed structural information enabling hit-to-lead optimization of two different chemical series. Best allosteric inhibitors in the series display single-digit nM potency in transcriptional assays, and translate to low nM inhibition of Hippo mutant (but not WT, >1000x selectivity window) mesothelioma proliferation. These molecules are well suited to probe for additional Hippo-dependent solid cancer types using in vitro cancer cell panels, selected based on genetics and/or a YAP/TAZ-TEAD gene signature. Furthermore, optimization towards orally bioavailable compounds is in progress and an update on in vivo efficacy in various solid tumor models will be presented. Citation Format: Matthias Versele, Aurélie Candi, Marnik Nijs, Wanda Haeck, Hugo Klaassen, Wim Smets, Stéphane Spieser, Bart Vanderhoydonck, Arnaud Marchand, Patrick Chaltin, Leticia Sansores, Georg Halder. Discovery of novel potent allosteric inhibitors of YAP/TAZ-TEAD transcription for the treatment of multiple solid tumor types addicted to Hippo signaling [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5229.

Background: The Hippo pathway is an evolutionarily conserved signaling cascade whose deregulation can promote excessive cell proliferation and tumor development. Pathway output is mediated by the YAP and TAZ transcriptional co-activators, which bind to TEAD family transcription factors to drive target gene expression. Genomic aberrations in Hippo pathway components result in constitutive activation of YAP/TAZ, as seen with NF2 mutations in subsets of mesothelioma and other cancers. Hyperactivation of YAP/TAZ has also been associated with resistance to a variety of targeted agents, including EGFR and CDK4/6 inhibitors, suggesting that targeting the pathway may have utility as part of rationally selected combinations, in addition to genomically-informed monotherapy applications. Activity of the YAP/TAZ-TEAD complex thus represents a compelling pharmacologic target, due to its essential role in the pathway, and the presence of a conserved druggable site in TEAD that is required for transcriptional function. Results and Discussion: Using biophysical techniques, we identified novel small molecules that bind to the TEAD auto-palmitoylation pocket. Initial hits were optimized for antagonism of TEAD-based transcription and drug-like properties, ultimately producing highly potent and orally bioavailable TEAD inhibitors. These compounds selectively inhibited the proliferation of cancer cell lines harboring genomic alterations in the Hippo pathway with low nM potency. In vivo models of Hippo pathway-altered xenografts showed consistent monotherapy activity, with dose-dependent and durable tumor regressions achieved at well-tolerated doses. Further characterization of these compounds as monotherapies and as part of rationally-designed combination regimens is ongoing. Citation Format: Adeela Kamal, Aurélie Candi, Matthias Versele, Bart Vanderhoydonck, Arnaud Marchand, Ron de Jong, Thuy Hoang, Georg Halder, Patrick Chaltin, Stephen L. Gwaltney, Mike Burgess. Novel antagonists of TEAD palmitoylation inhibit the growth of Hippo-altered cancers in preclinical models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3945.