The Role of Dihydroorotate Dehydrogenase in Apoptosis Induction in Response to Inhibition of the Mitochondrial Respiratory Chain Complex III

A mechanism for the induction of programmed cell death (apoptosis) upon dysfunction of the mi-tochondrial respiratory chain has been studied. Previously, we had found that inhibition of mitochondrial cy-tochrome bc1, a component of the electron transport chain complex III, leads to activation of tumor suppressor p53, followed by apoptosis induction. The mitochondrial respiratory chain is coupled to the de novo pyrimidine biosynthesis pathway via the mitochondrial enzyme dihydroorotate dehydrogenase (DHODH). The p53 activa-tion induced in response to the inhibition of the electron transport chain complex III has been shown to be trig-gered by the impairment of the de novo pyrimidine biosynthesis due to the suppression of DHODH. However, it remained unclear whether the suppression of the DHODH function is the main cause of the observed apoptotic cell death. Here, we show that apoptosis in human colon carcinoma cells induced by the mitochondrial respira-tory chain complex III inhibition can be prevented by supplementation with uridine or orotate (products of the reaction catalyzed by DHODH) rather than with dihydroorotate (a DHODH substrate). We conclude that apop-tosis is induced in response to the impairment of the de novo pyrimidine biosynthesis caused by the inhibition of DHODH. The conclusion is supported by the experiment showing that downregulation of DHODH by RNA interference leads to accumulation of the p53 tumor suppressor and to apoptotic cell death.