Loss of SDHB Elevates Catecholamine Synthesis and Secretion Depending on ROS Production and HIF Stabilization

Germline mutations in genes encoding succinate dehydrogenase subunits are associated with the development of familial pheochromocytomas and paragangliomas [hereditary paraganglioma/pheochromocytoma syndrome (HPPS)]. In particular, a mutation in succinate dehydrogenase subunit B (SDHB) is highly associated with abdominal paraganglioma and subsequent distant metastasis (malignant paraganglioma), indicating the importance of SDHB genetic testing. The discovery of HPPS suggests an association among genetic mitochondrial defects, tumor development, and catecholamine oversecretion. To investigate this association, we transfected pheochromocytoma cells (PC12) with SDHB-specific siRNA. SDHB silencing virtually abolished complex II activity, demonstrating the utility of this in vitro model for investigating the pseudo-hypoxic drive hypothesis. Lack of complex II activity resulting from RNA interference of SDHB increased tyrosine hydroxylase (TH; the rate-limiting enzyme in catecholamine biosynthesis) activity and catecholamine secretion. Reduced apoptosis was observed accompanied by Bcl-2 accumulation in PC12 cells, consistent with the phenotypes of paragangliomas with SDHB mutations. In addition, SDHB silencing increased reactive oxygen species (ROS) production and nuclear HIF1α stabilization under normoxic conditions. Furthermore, phenotypes induced by complex II activity knockdown were abolished by pretreatment with N-acetyl cysteine (an ROS scavenger) and by prior HIF1α knockdown, indicating an ROS- and HIF1α-dependent mechanism. Our results indicate that increased ROS may act as signal transduction messengers that induce HIF1α stabilization and may be necessary for the pseudo-hypoxic states observed in our experimental model. To our knowledge, this is the first study demonstrating that pseudo-hypoxic states resulting from SDHB knockdown are associated with increased TH activity and catecholamine oversecretion.