The Evolving Role of Succinate in Tumor Metabolism: An 18F-FDG-Based Study

In recent years, inherited and acquired mutations in the tricarboxylic acid (TCA) cycle enzymes have been reported in diverse cancers. Pheochromocytomas and paragangliomas often exhibit dysregulation of glucose metabolism, which is also driven by mutations in genes encoding the TCA cycle enzymes or by activation of hypoxia signaling. Pheochromocytomas and paragangliomas associated with succinate dehydrogenase (SDH) deficiency are characterized by high F-18-FDG avidity. This association is currently only partially explained. Therefore, we hypothesized that accumulation of succinate due to the TCA cycle defect could be the major connecting hub between SDH-mutated tumors and the F-18-FDG uptake profile. Methods: To test whether succinate modifies the F-18-FDG metabolic profile of tumors, we performed in vitro and in vivo (smallanimal PET/CT imaging and autoradiography) experiments in the presence of succinate, fumarate, and phosphate-buffered saline (PBS) in different cell models. As a control, we also evaluated the impact of succinate on F-18-fluorocholine uptake and retention. Glucose transporter 1 (GLUT1) immunohistochemistry was performed to assess whether F-18-FDG uptake correlates with GLUT1 staining. Results: Intratumoral injection of succinate significantly increased F-18-FDG uptake at 24 h on small-animal PET/CT imaging and autoradiography. No effect of succinate was observed on cancer cells in vitro, but interestingly, we found that succinate caused increased F-18-FDG uptake by human umbilical vein endothelial cells in a concentration-dependent manner. No significant effect was observed after intratumoral injection of fumarate or PBS. Succinate, fumarate, and PBS have no effect on cell viability, regardless of cell lineage. Intramuscular injection of succinate also significantly increases F-18-FDG uptake by muscle when compared with either PBS or fumarate, highlighting the effect of succinate on connective tissues. No difference was observed between PBS and succinate on F-18-fluorocholine uptake in the tumor and muscle and on hind limb blood flow. GLUT1 expression quantification did not significantly differ between the study groups. Conclusion: The present study shows that succinate stimulates F-18-FDG uptake by endothelial cells, a finding that partially explains the F-18-FDG metabotype observed in tumors with SDH deficiency. Although this study is an F-18-FDG-based approach, it provides an impetus to better characterize the determinants of F-18-FDG uptake in various tumors and their surrounding microenvironment, with a special emphasis on the role of tumor-specific oncometabolites.