Final report of the first funding period 2013 - 2015
Prof. Dr. Lamszus (Hamburg):"Relevance of tumor metabolism for the glioblastoma‐host interaction"
In previous work on glioblastoma cells we discovered that enzymes of the PPP are highly expressed under normoxic conditions, whereas hypoxia causes downregulation of the PPP in conjunction with increased expression of glycolysis enzymes and a glucose flux shift towards glycolysis. This metabolic switch is associated with increased cell migraton under hypoxia as opposed to increased cell proliferation under normoxia. Oxygenation of hypoxic cells led to an inverse metabolic switch. In our project funded by the Anni Hofmann Stiftung, we deepened our analysis of the associations between glycolysis/PPP and cell migration/proliferation. We identified a mechanistic relationship between metabolism and glioblastoma cell behavior which is independent of hypoxia/oxygenation. The overall goal of our studies is to identify molecular targets to disrupt key metabolic processes which are linked with the highly invasive as well as highly proliferative, aggressive growth of glioblastoma cells. The main findings of our work are as follows:
1. The associations between PPP and proliferation as opposed to glycolysis and migration exist also independently of changes in oxygen tension. Highly proliferative glioblastoma cells exhibit strikingly increased levels of PPP enzymes but decreased expression of glycolysis enzymes in comparison with slowly proliferating cells. Conversely, migrating cells display elevated expression of glycolysis enzymes but reduced levels of PPP enzymes. These findings indicate that glycolysis and the PPP are linked intrinsically with the dichotomous activation of pro-migratory ("go") versus pro-proliferative ("grow") functional programs, respectively.
2. The PPP and glycolysis are directly mechanistically important for the preferential activation of "go versus grow" cellular programs. We used enzyme inhibitors and shRNA-mediated downregulation of key enzymes of both pathways, namely aldolase C (ALDOC), which is part of the preparatory phase of glycolysis, and glucose-6-phosphate dehydrogenase (G6PD), which catalyzes the first and key regulatory step of the PPP. Interference with the PPP led to reduced proliferation, partly in conjunction with enhanced migration, whereas interference with glycolysis caused reduced migration, partly associated with increased proliferation. These findings indicate that disruption of glycolysis versus the PPP has opposite functional effects in vitro.
3. Downregulation of G6PD in a highly aggressive and highly proliferative glioblastoma cell line retarded the growth of xenograft tumors in vivo and prolonged the survival of mice bearing intracerebral glioblastomas. In contrast, downregulation of ALDOC enhanced tumor growth and led to shorter survival. In a different xenograft model that uses a glioblastoma cell line which grows diffusely invasive but proliferates only slowly, downregulation of either enzyme had no effect on the overall tumor extension in vivo. We conclude that inhibition of G6PD and the PPP can be useful to reduce the rapid growth of the main tumor mass that is typical of human glioblastomas.
4. The hypoxia-induced downregulation of PPP enzymes concomitant with upregulation of glycolysis enzymes is a universal phenomenon which is consistently associated with increased migration and decreased proliferation in a broad spectrum of different cell types, including tumor cells and normal cells present in adult human brain. This indicates that enhanced migration of normal brain cells (e.g. astrocytes, mesenchymal stem cells, endothelial cells) along hypoxic gradients in glioblastoma-bearing brain may foster the mutual interaction between the tumor and stromal cells and thereby contribute to tumor growth.
In summary, our findings show that metabolic and functional cellular programs are linked intrinsically with each other, and that glycolysis and the PPP are mechanistically relevant for regulating cell migration versus proliferation.