Fumarate hydratase deficiency and tumorigenesis: hypoxia-response is independent of fumarate levels Free

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Recessive defects in mitochondrial energy metabolism cause severe infantile multisystem diseases, leading to early death. Dominantly inherited fumarase (FH) defects underlie cancer predisposition in families with kidney cancer and leiomyomas, and mutations in succinate dehydrogenase cause paragangliomas and pheochromocytomas. The citric acid cycle, which integrates energy metabolism in mitochondrial matrix, seem thus to have essential roles in cell proliferation and/or differentiation control.

We used fumarase-deficient (FH-) and control primary fibroblasts to explore the role of energetic and redox metabolism in fumarase deficiency and their relation with cell proliferation. We also used primary fibroblasts with respiratory chain defects (RC-), to distinguish the general consequences of malfunctioning mitochondrial energy metabolism from those leading to tumour formation.

We analysed two FH- lines with homozygous defects, and two from compound heterozygous patients. The homozygous line had pronounced accumulation of fumarate, but neither of the compound heterozygous lines had fumarate levels differing from controls. Also in RC- cells, the fumarate levels within control range.

FH- and RC- cells had increased glycolytic energy production. This metabolic pattern is typical in addition to oxidative energy defect, also for hypoxia response, and we found nuclear accumulation of HIF-1a in both FH- and RC- cells. Furthermore, transcription of HIF-responsive genes was increased in FH- and RC- cells. Thus, both FH- and RC- activated hypoxia response, irrespective of fumarate accumulation.

To study the redox metabolism, we focused on H₂O₂ and the major cell redox buffer, glutathione (GSH). Steady-state amounts of H₂O₂ are similar in FH- and controls. Catalase and glutathione peroxidase (GPx) activities are lower in FH- cells, while induced in RC-. Mn-SOD expression is also decreased in FH-, suggesting a decreased generation of H₂O₂. GSH content is increased in FH- and unaffected in RC-. GSH synthesis is increased in FH- but not in RC-. In FH-, GSH redox cycle oxidizes GSH slower than controls and recycles it faster. In RC-, GSH is oxidized faster than in controls and recycled also faster. These results suggest that while RC- is associated with increased oxidant stimuli and the corresponding antioxidant response, FH- seem to have a more reduced redox potential, which is required to proliferative cell populations.

In conclusion, FH- and RC malfunction affect similarly the energy metabolism and hypoxia response. The reduced redox state of FH- cells may contribute to keep these cells in a more undifferentiated state, which may be a feature linking FH-deficiency to tumor predisposition.