Summary Glioblastomas display hierarchies with self-renewing cancer stem-like cells (CSCs). RNA sequencing and enhancer mapping revealed regulatory programs unique to CSCs causing upregulation of the iron transporter transferrin, the top differentially expressed gene compared to tissue-specific progenitors. Direct interrogation of iron uptake demonstrated CSCs potently extract iron from the microenvironment more effectively than other tumor cells. Systematic interrogation of iron flux determined that CSCs preferentially require transferrin receptor and ferritin - two core iron regulators - to propagate and form tumors in vivo. Depleting ferritin disrupted CSC mitotic progression, through the STAT3-FoxM1 regulatory axis, revealing an iron-regulated CSC pathway. Iron is a unique, primordial metal fundamental for earliest life forms, and on which CSCs have an epigenetically programmed, targetable dependence.

Shifting the balance away from tumor-mediated immune suppression toward tumor immune rejection is the conceptual foundation for a variety of immunotherapy efforts currently being tested. These efforts largely focus on activating antitumor immune responses but are confounded by multiple immune cell populations, including myeloid-derived suppressor cells (MDSCs), which serve to suppress immune system function. We have identified immune-suppressive MDSCs in the brains of GBM patients and found that they were in close proximity to self-renewing cancer stem cells (CSCs). MDSCs were selectively depleted using 5-flurouracil (5-FU) in a low-dose administration paradigm, which resulted in prolonged survival in a syngeneic mouse model of glioma. In coculture studies, patient-derived CSCs but not nonstem tumor cells selectively drove MDSC-mediated immune suppression. A cytokine screen revealed that CSCs secreted multiple factors that promoted this activity, including macrophage migration inhibitory factor (MIF), which was produced at high levels by CSCs. Addition of MIF increased production of the immune-suppressive enzyme arginase-1 in MDSCs in a CXCR2-dependent manner, whereas blocking MIF reduced arginase-1 production. Similarly to 5-FU, targeting tumor-derived MIF conferred a survival advantage to tumor-bearing animals and increased the cytotoxic T cell response within the tumor. Importantly, tumor cell proliferation, survival, and self-renewal were not impacted by MIF reduction, demonstrating that MIF is primarily an indirect promoter of GBM progression, working to suppress immune rejection by activating and protecting immune suppressive MDSCs within the GBM tumor microenvironment.

Glioblastoma (GBM) contains a self-renewing, tumorigenic cancer stem cell (CSC) population which contributes to tumor propagation and therapeutic resistance. While the tumor microenvironment is essential to CSC self-renewal, the mechanisms by which CSCs sense and respond to microenvironmental conditions are poorly understood. Scavenger receptors are a broad class of membrane receptors that are well characterized on immune cells and instrumental in sensing apoptotic cellular debris and modified lipids. Here we provide evidence that CSCs selectively utilize the scavenger receptor CD36 to promote their maintenance using patient-derived CSCs and in vivo xenograft models. We detected CD36 expression in GBM cells in addition to previously described cell types including endothelial cells, macrophages and microglia. CD36 was enriched in CSCs and was able to functionally distinguish self-renewing cells. CD36 was co-expressed with integrin alpha 6 and CD133, previously described CSC markers, and CD36 reduction resulted in concomitant loss of integrin alpha 6 expression, self-renewal and tumor initiation capacity. We confirmed that oxidized phospholipids, ligands of CD36, were present in GBM and found that the proliferation of CSCs, but not non-CSCs, increased with exposure to oxidized low-density lipoprotein. CD36 was an informative biomarker of malignancy and negatively correlated to patient prognosis. These results provide a paradigm for CSCs to thrive by the selective enhanced expression of scavenger receptors, providing survival and metabolic advantages.

No abstract

SUMMARY The coordination of complex tumor processes requires cells to rapidly modify their phenotype and is achieved by direct cell-cell communication through gap junction channels composed of connexins. Previous reports have suggested that gap junctions are tumor suppressive based on connexin43 (Cx43), but this does not take into account differences in connexin-mediated ion selectivity and intercellular communication rate that drive gap junction diversity. We find that glioblastoma cancer stem cells (CSCs) possess functional gap junctions that can be targeted using clinically relevant compounds to reduce self-renewal and tumor growth. Our analysis reveals that CSCs express Cx46, while Cx43 is predominantly expressed in non-CSCs. During differentiation, Cx46 is reduced, while Cx43 is increased, and targeting Cx46 compromises CSC maintenance. The difference between Cx46 and Cx43 is reflected in elevated cell-cell communication and reduced resting membrane potential in CSCs. Our data demonstrate a pro-tumorigenic role for gap junctions that is dependent on connexin expression.

SUMMARY Tumors contain hostile inflammatory signals generated by aberrant proliferation, necrosis, and hypoxia. These signals are sensed and acted upon acutely by the Toll-like receptors (TLRs) to halt proliferation and activate an immune response. Despite the presence of TLR ligands within the microenvironment, tumors progress, and the mechanisms that permit this growth remain largely unknown. We report that self-renewing cancer stem cells (CSCs) in glioblastoma have low TLR4 expression that allows them to survive by disregarding inflammatory signals. Non-CSCs express high levels of TLR4 and respond to ligands. TLR4 signaling suppresses CSC properties by reducing retinoblastoma binding protein 5 (RBBP5), which is elevated in CSCs. RBBP5 activates core stem cell transcription factors, is necessary and sufficient for self-renewal, and is suppressed by TLR4 overexpression in CSCs. Our findings provide a mechanism through which CSCs persist in hostile environments because of an inability to respond to inflammatory signals.

Objective Glioblastomas (GBMs) are lethal cancers that display cellular hierarchies parallel to normal brain. At the apex are GBM stem cells (GSCs), which are relatively resistant to conventional therapy. An important driver of malignancy and self-renewal in GSCs are interactions with the adjacent perivascular niche. Extracellular matrix (ECM) cues instruct neural stem/progenitor cell-niche interactions and the objective of our study was to elucidate its composition and contribution to GSC maintenance in the perivascular niche. Methods We interrogated human tumor tissue for immunofluorescence analysis and derived GSC from tumor tissues for functional studies. Bioinformatics analyses were conducted by mining publicly available databases. Results We find that laminin ECM proteins are localized to the perivascular GBM niche and inform negative patient prognosis. To identify the source of laminins, we characterized cellular elements within the niche and found that laminin α chains were expressed by non-stem tumor cells and tumor associated endothelial cells (ECs). RNA interference targeting laminin α2 inhibited GSC growth and self-renewal. In co-culture studies of GSCs and ECs, laminin α2 knockdown in ECs resulted in decreased tumor growth. Interpretation Our studies highlight the contribution of non-stem tumor cell-derived laminin juxtracrine signaling. As laminin α2 has recently been identified as a molecular marker of aggressive ependymoma, we propose that the brain vascular ECM promotes tumor malignancy through maintenance of the GSC compartment providing not only a molecular fingerprint but also a possible therapeutic target.

CD55 is a membrane complement regulatory protein that attenuates complement-mediated cytotoxicity. Saygin et al. elucidate a new role for CD55 as a signaling hub for cancer stem cell self-renewal and cisplatin resistance pathways in endometrioid tumors and open a new line of research into chemotherapeutic-refractory cancers.