Inhibition of cytochrome c oxidase subunit 4 precursor processing by the hypoxia mimic cobalt chloride

Cobalt is often used as a hypoxia mimic in cell culture, because it stabilizes the α subunits of the transcription factor, HIF (hypoxia-inducible factor). We have previously shown that HIF stabilization due to a deficiency of the von Hippel Lindau protein (pVHL) in clear cell renal carcinoma (CRCC) was correlated to a down-regulation of oxidative phosphorylation. To better understand this mechanism, we have used CoCl2 in CRCC expressing stably transfected vhl. We show that, in addition to its effect on HIF-α subunits, CoCl2 prevented the normal processing of the precursor of cytochrome c oxidase (COX) subunit 4 and induced COX degradation very likely by inhibiting the mitochondrial intermediate peptidase (MIP) that cleaves the COX4 precursor protein. This cobalt-induced MIP inhibition was however not observed in other human mitochondrial precursor sequences as previously predicted from comparison between human and yeast mitochondrial precursor sequences.