TMIC-35. CROSSTALK BETWEEN TAMS AND GLIOBLASTOMA STEM-LIKE CELLS IN A HYPOXIC MICROENVIRONMENT INDUCES SENESCENCE ARREST AND ENHANCED RADIORESISTANCE

Patients with glioblastoma (GBM) have an overall survival of 15 months. These catastrophic survival rates are correlated with systematic relapses that might arise from remaining glioblastoma stem cells (GSCs) left behind after treatments. Our goal was identifying whether radioresistance of GBM is caused by GSCs and how hypoxia and TAMs affect this process. First, we established a glioblastoma-brain cortical organoid (GBM-BCOs) to model human GBM. We co-cultured GFP-labeled GBM neurosphere with BCOs for 14 days and we observed that tumor take rate was 100% for all GBM neurosphere tested. Next, we evaluated whether hypoxia enhanced TAMs migration into GBM-BCOs. We incorporated THP-1 cells into GFP-GBM-BCOS for 14 days and incubated for 24 h under normoxic or hypoxic conditions. Hypoxia increased THP-1 cells influx into GBM-BCOs. Further, we investigated whether TAMs enhanced the enrichment of GSCs in GBM neurosphere and in GBM-BCOs. We found that, co-culture of THP-1 cells with GBM neurosphere or with GBM-BCOs system, enhanced CD133+ population after 72 h hypoxic condition. In addition, IF of GBM human tissue revealed increased of CD133+ and IBA1+ cells in peri-necrotic area compared to non-necrotic area and non-necrotic tumor. To examine whether TAMs enhanced the radioresistant phenotype and the quiescent population, we irradiated GBM neurosphere grown under hypoxic and normoxic conditions with different doses of γ-rays. After irradiation, these cells were co-cultured with TAMs for 72 h under normoxic and hypoxic conditions. MTT assay showed that TAMs enhanced the radioresistant phenotype promoted by hypoxia in GBM neurosphere and an increase in quiescent markers (γ-H2AX and IL-6) were found in our co-culture model after hypoxia exposure. These results indicate, not only hypoxic condition play a key role in the GSCs enrichment and radioresistance, but also crosstalk between TAMs and GSCs promote tumor oncogenesis and contribute to an ineffective anti-tumor response.

TMIC-21. THE ROLE OF CLEC5A ON M2-LIKE TUMOR-ASSOCIATED MACROPHAGES POLARIZATION AND DISEASE PROGRESSION IN GLIOBLASTOMA

Glioblastoma, IDH-wildtype (GBM, WHO grade 4) is the most common primary intracranial malignant tumor of adults and is characterized by an immunosuppressive tumor micro-environment (TME) and poor clinical outcomes. CLEC5A is a protein-coding gene involved in inflammatory and infectious diseases, where its activation enhances immune response. However, its role in GBM immune regulation is unclear. Seurat analysis of scRNA-seq data and CIBERSORTx analysis of GBM RNA-seq data showed that M2-like TAMs accounted for the largest proportion of immune cells in the GBM TME. Among CD163+ cells (M2-like TAM marker) in GBM, CLEC5A expression was most highly associated with overall patient survival. IHC on tumor microarrays (TMA) and analysis of Ivy GAP data indicated that CLEC5A and M2-like TAMs were preferentially expressed in tumor peri-necrotic zone. THP-1 cells exposed to GBM conditioned media showed increased CLEC5A and CD163 expression by RT-qPCR and Western blot compared to other immune-activating factors (IL-4, IL-13). CLEC5A overexpression in THP-1 cells enhanced their migration toward glioma cells in vitro, and also led to increased M2-like TAM biomarker expression. Cytokine antibody microarray results revealed that overexpressing CLEC5A in THP-1 cells also significantly increased the expression of IL-1β, IL-4, IL-6, IL-8, IL-10, IL-13, GM-CSF, and other cytokines, which all contribute to the immunosuppressive TME. In TCGA GBM data, tumors with higher CLEC5A expression were enriched in IL6-JAK-STAT3 signaling. In vivo, silencing CLEC5A delayed glioma growth and prolonged survival of tumor-bearing mice, and analysis of tumors showed reduced expression of CLEC5A, CD163 and Ki-67. Collectively, CLEC5A is expressed by M2-like TAMs and enhances M2-like TAM polarization, likely through IL6-JAK-STAT3 signaling.

TMIC-48. HYPOXIC AND PERI-NECROTIC ZONES IN GLIOBLASTOMA CAUSE HIPPO-OFF SIGNALING, POTENTIALLY SUPPORTING GLIOMA STEM CELLS

Glioblastoma (GBM) is the most common and malignant primary brain tumor with pre-necrotic and post-necrotic growth phases. The development of necrosis is associated with extensive tumor micro-environmental (TME) rearrangement and rapid biologic advancement. Compared to pre-necrotic GBMs, post-necrotic GBMs are heterogenous and contain multi-focal regions of necrosis surrounded by pseudopalisading cells and characterized by severe hypoxia. Hypoxic, peri-necrotic zones are highly enriched for glioma stem cells (GSCs) and tumor associated macrophages (TAMs), the latter accounting for 30-40% total tumor cellularity. Using Ivy-GAP data, we performed gene set enrichment analysis (GSEA) for stem-cell associated markers, comparing peri-necrotic to non-necrotic regions. We also used TCGA/CGGA data to identify correlations between hypoxia markers and classic stem-cell associated markers (SOX2, SOX9, nestin, STAT3, prominin). None of these showed enrichment in the peri-necrotic zone or association with hypoxia markers. However, we uncovered strong associations of Hippo-off signaling with peri-necrotic regions, signifying activation of YAP/TAZ transcriptional activation. We therefore hypothesized that Hippo-off (YAP/TAZ transcriptional activation) in hypoxic peri-necrotic zones promotes GSC self-renewing division. To support this, we performed GSEA for Hippo pathway readout genes and found peri-necrotic enrichment of Hippo regulated genes CYR61, BIRC2, SERPINE1 and AREG. Using patient-derived GBM neurosphere cultures, we exposed glioma cells to 1% hypoxia and found that p-YAP1 and p-TAZ protein levels were downregulated compared to normoxia (21%) at 24 hrs, correlating with Hippo-on. Hippo pathway gene targets were also upregulated, indicating activation of YAP/TAZ transcription. Using Ivy-GAP, TCGA and CGGA data, we also found peri-necrotic/hypoxic enrichment of two cytokines, IL-1β and IL-6, known to trigger YAP/TAZ activation. Using cytokine multiplex analysis, we observed an upregulation in both IL-1β and IL-6 secretion from hypoxic monocytes and microglia, raising the possibility that these cytokines directly stimulate Hippo-off and promote GSC self-renewing division in the GBM peri-necrotic niche.

Endothelial protein C receptor function in murine and human breast cancer development

Several markers identify cancer stem cell-like populations, but little is known about the functional roles of stem cell surface receptors in tumor progression. Here, we show that the endothelial protein C receptor (EPCR), a stem cell marker in hematopoietic, neuronal and epithelial cells, is crucial for breast cancer growth in the orthotopic microenvironment of the mammary gland. Mice with a hypomorphic allele of EPCR show reduced tumor growth in the PyMT-model of spontaneous breast cancer development and deletion of EPCR in established PyMT tumor cells significantly attenuates transplanted tumor take and growth. We find expansion of EPCR⁺ cancer stem cell-like populations in aggressive, mammary fat pad-enhanced human triple negative breast cancer cells. In this model, EPCR-expressing cells have markedly increased mammosphere- and tumor-cell initiating activity compared to another stable progenitor-like subpopulation present at comparable frequency. We show that receptor blocking antibodies to EPCR specifically attenuate in vivo tumor growth initiated by either EPCR⁺ cells or the heterogenous mixture of EPCR⁺ and EPCR^- cells. Furthermore, we have identified tumor associated macrophages as a major source for recognized ligands of EPCR, suggesting a novel mechanism by which cancer stem cell-like populations are regulated by innate immune cells in the tumor microenvironment.