Astrocytes in glioblastoma : enhancers of tumor growth, predictors of patient survival and potential therapeutic targets

Abstract

The tumor microenvironment plays an important role in glioblastoma, the most malignant primary brain tumor in adults. Astrocytes are a major component of the glioblastoma tumor microenvironment; therefore, their influence on glioblastoma biology needs to be clarified. In this thesis, the role of astrocytes was explored with regard to glioblastoma growth, patient survival and their potential as a therapeutic target through a set of in vitro and in vivo studies and analyses of clinical samples.

Co-culture experiments identified astrocytes as enhancers of glioblastoma cell growth in cell lines and in a patient-derived culture. Furthermore, orthotopic co-injection of astrocytes with glioblastoma cells reduced survival of NOD scid mice, compared to mice that received mono- injection of glioblastoma cells. A gene signature reflecting glioblastoma-activated astrocytes was associated with poor prognosis in two glioblastoma datasets. Through this set of experiments, astrocytes were thus shown to enhance glioblastoma growth. In a glioblastoma tissue collection, a subset of peritumoral astrocytes co-expressing PDGFRα and GFAP was examined for biomarker significance; experiments showed that such astrocytes did not carry tumor markers, supporting their non-malignant nature. Inter-case variability was observed, both with regard to the presence of such a subset and the general astrocyte density. High density in the peritumoral areas of the PDGFRα and GFAP co- expressing astrocytes, but not total astrocyte density, was identified as an independent poor prognostic factor. This observation suggests the presence of differentially functional astrocyte subsets in glioblastoma holding clinical relevance. A high-throughput screening assay was designed to screen a library of compounds in a novel glioblastoma/astrocyte co-culture system. The assay was implemented to identify compounds that specifically blocked the astrocyte-driven enhancement of glioblastoma growth. Three such compounds were identified and one of them was further validated in an additional cell line. Results from the high-throughput screen suggested the crosstalk between glioblastoma cells and astrocytes as a potential therapeutic target.

In conclusion, these studies suggest clinically and biologically relevant roles of astrocytes, as validated in patient datasets and peritumoral tissue. Co-culture specific drug response implies the crosstalk between malignant cells and astrocytes as a candidate target for novel therapies. Further studies will lead to better characterization of the mechanisms behind the glioblastoma-astrocyte crosstalk, while the clinical association of the novel PDGFRα+/GFAP+ peritumoral astrocyte subset should be further investigated and validated in larger cohorts.