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Abstract

Glioblastoma (GBM) is a highly aggressive tumor that leads to the patient’s deaths within approximately one year. Despite recent considerable advances in our understanding of the pathogenetic alterations present in the tumor cells and the nature of the glioblastoma microenvironment, it has not been yet possible to develop effective therapies. The role of tumor-derived metabolites in the interaction of the tumor cells with cells of the tumor stroma, e.g. immune cells, are a promising aspect of glioblastoma biology for defining new therapeutic targets. This thesis explores different approaches to characterize the branched-chain amino acid (BCAA) metabolism in the tumor-stroma interaction. In the first part I investigated whether and how branched-chain ketoacids (BCKAs), generated in the first step of the BCAA catabolism, are excreted from glioblastoma cells. I found that while the monocarboxylate transporters MCT1 and MCT4 are both capable and sufficient to transport BCKAs across the membranes of living cells, excretion of BCKAs from glioblastoma cells appears to be mediated mostly by MCT1. Additionally, I could show that MCT1 locates in close proximity to the BCKA-generating branched-chain transaminase 1 (BCAT1), suggesting possible functional interaction of the proteins. In the second part I investigated the fate and function of tumor-secreted BCKAs in the tumor-stroma interaction using macrophages as a model of the stroma compartment. Using in vitro isotope tracing analysis, it was demonstrated that BCKAs are taken up by macrophages and catabolized to BCAAs. Additionally, exposure to BCKAs reduced the phagocytic activity of macrophages suggesting that in glioblastoma, tumor cell-secreted BCKAs might be able to modulate the tumor-associated macrophages/microglia, contributing to their role in tumor immune suppression and supporting glioblastoma tumor growth and progression. Furthermore, I was able to provide evidence, that the manipulation of BCAT1 expression in the tumor compartment can impact the immune phenotype of tumor-associated macrophages of the stroma compartment via BCKAs and likely other diffusible factors. Ultimately, this study provides further evidence for the eminent role of BCAA catabolism in glioblastoma by demonstrating that tumor-excreted BCKAs might have a direct role in tumor immune suppression.