The interest of the Van Meir laboratory lies in understanding the molecular basis for human tumor development and how we can use this knowledge to devise new therapeutics that will improve patient survival. We examine how genetic alterations and hypoxia induce changes in cell biology that promote tumor formation with particular emphasis on cell-cell signaling, tumor hypoxia/ angiogenesis and resistance to therapy. We have developed novel therapeutic approaches for cancer using viral therapies, epigenetic agents and small molecule HIF inhibitors. We aim to translate these novel agents to testing in clinical trials with the hope to develop novel medicines for cancer treatment. The principal model systems we use are malignant brain tumors (glioblastoma and medulloblastoma), but we study additional cancers as well such as uveal melanoma, a malignant eye tumor that metastasizes to the liver.
The Role of Adhesion Receptor ADGRB1/BAI1 in Glioblastoma
New therapies are urgently needed for patients with malignant gliomas, which are highly invasive and lethal brain tumors. Patients with glioblastoma (GBM) die within 1-2 years of diagnosis despite current conventional therapies, including surgery, radiation and chemotherapy. A major driver of tumor recurrence is the infiltrative nature of the glioma cells into adjacent normal brain, which is driven by the activation of a mesenchymal transcription program. This mesenchymal transition is activated through extracellular stimulation of cell surface receptors by growth factors such as TGFβ1. The overall purpose of the present project is to investigate the role of adhesion G protein-coupled receptor B1 (ADGRB1/BAI1) in the mesenchymal switch and invasion and explore new therapies for GBM based on the related mechanisms. ADGRB1 is an orphan adhesion GPCR specifically expressed in the brain. We previously showed that ADGRB1 expression is significantly reduced in patients with GBM through epigenetic silencing, suggesting that ADGRB1 loss may facilitate tumor formation. Our new preliminary data show that low ADGRB1 expression correlates with invasion and poor outcome in glioma patients. Restoration of ADGRB1 expression in GBM cells suppresses the mesenchymal phenotype in culture and mice xenografts. Our pilot studies further suggest ADGRB1 can inhibit TGFβ1-driven mesenchymal transition through a WxLWxLW motif in its first thrombospondin type 1 repeat (TSR1). This motif mediates ADGRB1 binding to the latent TGFβ1 complex and prevents TGFβ1 maturation. Based on these results, we hypothesize that ADGRB1 acts as a brain tumor suppressor by blocking the TGFβ1-mediated mesenchymal switch and that restoration of its expression with epigenetic therapy will represent a novel therapeutic intervention for GBM. To test our hypothesis, we propose the following aims: (i) define how ADGRB1 negatively regulates the mesenchymal transition and glioma invasion, (ii), determine how BAI1 antagonizes TGFβ1 pro-mesenchymal signaling and (iii) evaluate whether epigenetic restoration of BAI1 expression can inhibit glioma cell invasion in vitro and in vivo, and augment survival post-operation. These studies are important as we identified a specific region in the extracellular domain of BAI1 that antagonizes TGFβ1 maturation and the glioma mesenchymal switch, providing a new mechanism for antagonizing this oncogenic pathway that can be exploited therapeutically. These findings support targeting this new pathway in patients whose cancers are driven by mesenchymal transition.
Targeting Mechanisms of Medulloblastoma Formation
There is an urgent need to develop novel therapies for patients with medulloblastoma (MB), the most common malignant central nervous system (CNS) tumor in children. Current treatments include surgery, radiotherapy, and chemotherapy and result in 5-year survival rates of 40-90% depending on subtype. However, children suffer important morbidity secondary to treatment, including neurological, intellectual and physical disabilities. The overall purpose of the present project is to investigate the role of the ADGRB3 receptor in the susceptibility of cerebellar transformation and explore new therapies for MB based on the related mechanisms. ADGRB3 is an orphan seven-transmembrane G protein-coupled receptor (GPCR) specifically expressed in the brain, and belonging to the adhesion-type sub-family. Our new preliminary data show that ADGRB3 expression is significantly reduced in patients with MBs of the WNT group, and the promoter is epigenetically silenced, suggesting that ADGRB3 loss may facilitate WNT-MB formation. We present evidence for the involvement of methylated CpG binding protein MBD2 and histone methyltransferase EZH2 in switch to a silent chromatin. Moreover, we show that reactivation of ADGRB3 can reduce cell proliferation and tumor growth, supporting a tumor-suppressive role. To test this in the physiological setting, we generated ADGRB3 knockout (KO) mice, which we plan to cross with mice expressing mutant b-catenin in neural progenitors of the rhombic lip and dorsal brainstem, which are the cells of origin of WNT-MB. Based on these results, we hypothesize that ADGRB3 is a tumor suppressor in the cerebellum and that restoration of its expression with epigenetic therapy may represent a novel therapeutic intervention for children with WNT-MB. To test our hypothesis, we propose the following aims: (i) identify and target the epigenetic mechanism(s) underlying ADGRB3 gene silencing in WNT-MB, (ii) determine whether and how the restoration of ADGRB3 expression can inhibit MB cell growth, oncogenic signaling and tumorigenic properties, and (iii) determine whether loss of ADGRB3 gene expression in the background of oncogenic Ctnnb1 activation predisposes mice to cerebellar transformation and MB tumor development. These studies are important as they increase our knowledge about developmental neurobiology in the CNS, and may lead to the development of novel therapeutic approaches for patients with medulloblastoma.
Studying Mechanisms of Cancer Resistance to Therapy
Glioblastoma (GBM) is composed of heterogeneous tumor cell populations, including those with stem cell properties, termed glioma stem cells (GSCs). GSCs are innately less radiation sensitive than the tumor bulk and are believed to drive GBM formation and recurrence after repeated irradiation. However, it is unclear how GSCs adapt to escape the toxicity of repeated irradiation used in clinical practice. To identify important mediators of adaptive radioresistance in GBM, we generated radioresistant human and mouse GSCs by exposing them to repeat cycles of irradiation. Surviving subpopulations acquired strong radioresistance in vivo, which was accompanied by a reduction in cell proliferation and an increase in cell-cell adhesion and N-cadherin expression. Increasing N-cadherin expression rendered parental GSCs radioresistant, reduced their proliferation, and increased their stemness and intercellular adhesive properties. Conversely, radioresistant GSCs lost their acquired phenotypes upon CRISPR/Cas9-mediated knockout of N-cadherin. Mechanistically, elevated N-cadherin expression resulted in the accumulation of β-catenin at the cell surface, which suppressed Wnt/β-catenin proliferative signaling, reduced neural differentiation, and protected against apoptosis through Clusterin secretion. N-cadherin upregulation was induced by radiation-induced IGF1 secretion, and the radiation resistance phenotype could be reverted with picropodophyllin, a clinically applicable blood-brain-barrier permeable IGF1 receptor inhibitor, supporting clinical translation
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Osuka, S., Zhu, D., Zhang, Z., Li, C., Stackhouse, C. T., Sampetrean, O., Olson, J. J., Gillespie, G. Y., Saya, H., Willey, C. D., & Van Meir, E. G. (2021). N-cadherin upregulation mediates adaptive radioresistance in glioblastoma. The Journal of clinical investigation, 131(6), e136098. https://doi.org/10.1172/JCI136098