Cytochrome-c-oxidase (COX) subunit 4 (COX4) plays important roles in the function, assembly and regulation of COX (mitochondrial respiratory complex 4), the terminal electron acceptor of the oxidative phosphorylation (OXPHOS) system. The principal COX4 isoform, COX4-1, is expressed in all tissues, whereas COX4-2 is mainly expressed in the lungs, or under hypoxia and other stress conditions. We have previously described a patient with a COX4-1 defect with a relatively mild presentation compared to other primary COX deficiencies, and hypothesized that this could be the result of compensatory upregulation of COX4-2. To this end, COX4-1 was downregulated by shRNAs in human foreskin fibroblasts (HFF), and compared to patient's cells. COX4-1, COX4-2 and HIF-1α were detected by immunocytochemistry. The mRNA transcripts of both COX4 isoforms and HIF-1 target genes were carried out by RT-qPCR. COX activity and OXPHOS function were measured by enzymatic and oxygen consumption assays, respectively. Pathways were analyzed by CEL-Seq2 and by RT-qPCR. We demonstrate elevated COX4-2 levels in the COX4-1-deficient cells with a concomitant HIF-1α stabilization, nuclear localization and upregulated hypoxia and glycolysis pathways. We suggest that COX4-2 and HIF-1α has the are upregulated, also in normoxia as a compensatory mechanism in COX4-1 deficiency.

Ethoxyquin (EQ) is used as a synthetic antioxidant mainly in animal food, but it has also been reported to have neuroprotective properties by diminishing the peripheral neuropa-thy resulting from anti-cancer chemotherapy in animal models. However, EQ has not been thoroughly evaluated in humans and its use is under debate. The objective of this work was to examine the effect of EQ on primary fibroblasts derived from controls and patients with confirmed mitochondrial dysfunction. To this end, we incubated primary fibroblasts from patients with mitochondrial respiratory chain complex 1 (C1) deficiency, cyto-chrome-c-oxidase (COX) deficiency and genetic form of Parkinson’s disease (PD). Alt-hough EQ did not alter ATP production it significantly increased oxygen consumption and improved energy status in C1 and COX cells while decreasing ROS production in COX, PD and control cells and maintaining mitochondrial content. These effects could possibly be attributed to the antioxidant and chaperone-modulating properties of EQ and to induced SOD2 expression in some cells. Our results indicate that EQ has the potential to improve some mitochondrial functions. Still, caution should be exerted since overdose of this com-pound affected cell growth.