Experimental hypoxia as a potent stimulus for radiotracer uptake in vitro: comparison of different tumor cells and primary endothelial cells

Introduction:

Hypoxia is a common feature of solid malignant tumors. The vascular endothelial growth factor (VEGF) is upregulated by hypoxia inducible factors (HIFs) as a result of central hypoxia and plays an outstanding role for the formation of new tumoral blood vessels by promoting proliferation and migration of endothelial cells. Beside [^18F]fluorodeoxyglucose (¹⁸F-FDG) as standard radiotracer for tumor imaging with PET, oxygenation status in experimental and human tumors can be visualized by using [¹⁸F]fluoromisonidazole (¹⁸F-FMISO).

Experimental:

The aim of our study was the evaluation of the uptake of the radiotracers ¹⁸F-FDG and ¹⁸F-FMISO under standardized hypoxic conditions in various primary endothelial cells compared with three tumor cell lines in vitro. Experimental hypoxia was characterized by measurement of expression of HIF-1α, VEGF and various VEGF receptors by quantitative real-time PCR.

Results and Discussion:

Experimental hypoxia was confirmed by significant upregulation of characteristic hypoxia-related genes like VEGF in all analyzed primary endothelial cells and the tumor cell lines. VEGF receptors 1, 2 and 3 were almost absent in tumor cells and only expressed in endothelial cells. In contrast, the VEGF co-receptor Neuropilin-1 showed an increased expression in tumor cells under experimental hypoxia. In comparison to normoxic conditions, cellular uptake of ¹⁸F-FDG was significantly increased under experimental hypoxia in both tumor and endothelial cells. Otherwise, uptake of ¹⁸F-FMISO was always higher in tumor cells than in endothelial cells under hypoxic conditions.

Conclusion:

Our data showed a strong dependence of ¹⁸F-FDG uptake on the availability of oxygen in vitro. The study demonstrates relevance of endothelial cells as one important part of the tumor microenvironment under hypoxic conditions. This may have implications when performing PET studies aiming at the visualization and characterization of highly vascularized tumors, in particular, when using ¹⁸F-FDG.