Glioblastoma (GBM) tumor microenvironment (TME) is a highly heterogeneous and complex system, which in addition to cancer cells, consists of various resident brain and immune cells as well as cells in transit through the tumor such as marrow-derived immune cells. The TME is a dynamic environment which is heavily influenced by alterations in cellular composition, cell-to-cell contact and cellular metabolic products as well as other chemical factors, such as pH and oxygen levels. Emerging evidence suggests that GBM cells appear to reprogram their the TME, and hijack microenvironmental elements to facilitate rapid proliferation, invasion, migration, and survival thus generating treatment resistance. GBM cells interact with their microenvironment directly through cell-to-cell by interaction mediated by cell-surface molecules, or indirectly through apocrine or paracrine signaling via cytokines, growth factors, and extracellular vehicles. The recent discovery of neuron–glioma interfaces and neurotransmitter-based interactions has uncovered novel mechanisms that favor tumor cell survival and growth. Here, we review the known and emerging evidence related to the communication between GBM cells and various components of its TME, discuss models for studying the TME and outline current studies targeting components of the TME for therapeutic purposes.

Glioblastoma (GBM) is the most common primary brain tumor. Despite aggressive treatment, GBM almost always recurs. The current standard-of-care for treatment of newly diagnosed GBM has remained relatively unchanged since 2005: maximal safe resection followed by concomitant chemoradiation (CRT) with temozolomide (TMZ), and subsequent adjuvant TMZ. In 2011, the first-generation tumor treating fields (TTF) device, known at the time as the NovoTTF-100A System (renamed Optune), was approved by the Food and Drug Administration (FDA) for treatment of recurrent GBM. The TTF device was subsequently approved as an adjuvant therapy for newly-diagnosed GBM in 2015. The following is a review of the TTF device, including evidence supporting its use and limitations.

BACKGROUND: The objective of this study was to estimate the incidence and describe distribution, clinical presentation, and prognosis of brain metastases in patients with prostrate carcinoma who were seen at The University of Texas M. D. Anderson Cancer Center (MDACC).METHODS: The authors reviewed the charts of 16,280 patients with prostate carcinoma in the MDACC patient data base. Of 131 patients with craniospinal metastases confirmed by neuroimaging (n = 53 patients) or autopsy (n = 78 patients), 103 of 16,280 patients (0.63%) had parenchymal metastases.RESULTS: The median patient age at diagnosis was 64 years (range, 16–85 years). The median interval from the diagnosis of prostate carcinoma to the detection of brain metastasis was 35 months for patients with adenocarcinoma and 48 months for patients with small cell carcinoma (SCC). Confusion, headache, and memory deficits were the most frequent initial symptoms. Eighty‐six percent of patients had single lesions, and 14% of patients had ≥ 2 lesions. Metastases were supratentorial in 81 of 103 patients (76%), infratentorial in 22 of 103 patients (21%), and both supratentorial and infratentorial in 3 of 103 patients (3%). SCC and cribriform subtypes were more likely than adenocarcinoma to metastasize to the brain (relative risk, 20.36; 95% confidence interval, 9.91–41.84). Regardless of histology, the median survival in untreated patients was 1 month compared with 3.5 months in patients who were treated with radiotherapy. Patients who underwent stereotactic radiosurgery (n = 5 patients) had a longer median survival (9 months). Survival was not affected by supratentorial or infratentorial location of metastases.CONCLUSIONS: Brain metastasis from prostate carcinoma is a rare, terminal event with death in < 1 year frequently due to advanced, systemic disease. The majority of metastases were single and supratentorial. The most common clinical presentation was nonfocal neurologic symptoms related to intracranial hypertension. A better understanding of the biology of prostate carcinoma will help clarify the basis for its metastasis to the brain. Cancer 2003;98:363–8. © 2003 American Cancer Society.DOI 10.1002/cncr.11522

Purpose: Fenretinide induces apoptosis in malignant gliomas in vitro. This two-stage phase II trial was conducted to determine the efficacy of fenretinide in adults with recurrent malignant gliomas.Patients and Methods: Twenty-two patients with anaplastic gliomas (AG) and 23 patients with glioblastoma (GBM) whose tumors had recurred after radiotherapy and no more than two chemotherapy regimens were enrolled. Fenretinide was given orally on days 1 to 7 and 22 to 28 in 6-week cycles in doses of 600 or 900 mg/m²bid.Results: Six of 21 (29%) patients in the AG arm and two of 23 (9%) patients in the GBM arm had stable disease at 6 months. One patient with AG treated at 900 mg/m²bid dosage had a partial radiologic response. Median progression-free survival (PFS) was 6 weeks for the AG arm and 6 weeks for the GBM arm. PFS at 6 months was 10% for the AG arm and 0% for the GBM arm. Grade 1 or 2 fatigue, dryness of skin, anemia, and hypoalbuminemia were the most frequent toxicities reported. The trial was closed after the first stage because of the inadequate activity at the fenretinide doses used. The first-administration mean plasma C_maxfor fenretinide was 832 ± 360 ng/mL at the 600 mg/m²bid dosage and 1,213 ± 261 ng/mL at the 900 mg/m²bid dosage.Conclusion: Fenretinide was inactive against recurrent malignant gliomas at the dosage used in this trial. However, additional studies using higher doses of the agent are warranted based on the tolerability of the agent and the potential for activity of a higher fenretinide dosage, as suggested in this trial.

Tumor stem cells, postulated to be the source cells for malignancies, have been identified in several cancers using cell-surface expression of markers including CD133, a pentaspan membrane protein. CD133+ve cells form neurospheres, exhibit self-renewal and differentiation, and are tumorigenic. However, despite its association with stem cells, a causal relationship of CD133 to tumorigenesis remains to be defined. Hypothesizing that specific epigenetic and transcription factors implicated in driving the stem cell state may concurrently regulate CD133 expression in stem cells, we analyzed the structure and regulation of CD133 promoter in glioma stem cells and glioma cell lines. Initially, a minimal promoter region was identified by analyzing the activity of CD133 promoter-driven luciferase-expressing 5'-and 3'-deletion-constructs upstream of the transcription start site. This region contained a CpG island that was hypermethylated in CD133−ve glioma stem cells (GSC) and glioma cells but unmethylated in CD133+ve ones. Of several predicted TF-binding sites in this region, the role of tandem Sp1 (−242 and −221) and two Myc (−541 and −25)-binding sites were examined. Overexpression of Sp1 or Myc increased CD133 minimal promoter-driven luciferase activity and CD133 levels in GSC and in glioma cell line. Mithramycin, a Sp1 inhibitor, decreased minimal promoter activity and downregulated CD133 levels in GSC. Gel-shift assays demonstrated direct binding of Sp1 to their predicted sites that was competitively inhibited by oligonucleotide-binding-site sequences and supershifted by anti-Sp1 confirming the interaction. Sp1 and Myc-antibody chromatin immunoprecipitation (ChIP) analysis in GSC showed enrichment of regions with Sp1 and Myc-binding sites. In CD133−ve cells, ChIP analysis showed binding of the methyl-DNA-binding proteins, MBD1, MBD2 and MeCP2 to the methylated CpG island and repression of transcription. These results demonstrate that Sp1 and Myc regulate CD133 transcription in GSC and that promoter methylation and methyl-DNA-binding proteins cause repression of CD133 by excluding transcription-factor binding.