Effects of the tumour microenvironment on protoporphyrin IX accumulation in glioblastoma
Abstract Aims Glioblastoma is the most common primary brain tumour and has a poor prognosis. Standard clinical intervention involves the resection of the tumour volume, chemotherapy and radiotherapy. However, achieving gross-total resection is challenging due to poorly defined boundaries as a result of tumour infiltration. Fluorescence-guided surgery (FGS) utilises an apparently selective accumulation of protoporphyrin IX (PPIX) that occurs in areas of glioblastoma after systemic administration of the metabolite 5-aminolevulinic acid (5-ALA). We have investigated the metabolic basis for the heterogeneity of the PPIX fluorescent signal, and its implications for glioma biology. Method Using glioblastoma cell lines and patient-derived primary cells, we have monitored the uptake of 5-ALA and conversion to the fluorescent molecule PPIX. Stable isotope tracing coupled with GCMS and LCMS was used to analyse intra- and extracellular metabolite levels arising from exogenous 5-ALA administration under both normoxic (21% O2) and hypoxic (1% O2) conditions. Results Uptake of exogenous 5-ALA from culture media and conversion to PPIX is observed in a time and dose-dependent manner in both normoxia and hypoxia. High levels of PPIX accumulation are associated with reduced cell proliferation despite the majority of the PPIX synthesised not being retained within the tumour cell, but exported into the medium. Under hypoxic conditions, reduced fluorescence is observed as a result of the decrease in oxygen availability likely affecting the oxygen-dependent enzymes. Stable isotope tracing experiments indicate an increase in the glutamine-derived succinate pool in response to exogenous 5-ALA, which is dependent on flux through the heme pathway. Conclusion Our data suggest that different microenvironments within the tumour alter the activity of the heme biosynthetic pathway, resulting in differential fluorescence in glioblastoma. It paves the way by which we could work to alter the glioblastoma microenvironment in order to further improve the use of FGS in guiding surgery across these devastating tumours.
