Time-dependent and partially reversible mitochondrial remodeling induced by coenzyme Q10 depletion in HepG2 cells

Aging and age related pathological conditions are long-term processes in which cellular states gradually change over extended periods. However, many experimental studies of oxidative stress in cultured cells rely on short-term exposure to exogenous oxidative agents, which may not adequately reflect chronic oxidative conditions. To address this limitation, we established a cellular model of long-term oxidative stress by reducing endogenous antioxidant capacity through inhibition of coenzyme Q10 (CoQ10) biosynthesis rather than applying acute oxidative insults. Using HepG2 cells treated with 4-nitrobenzoic acid, we compared mitochondrial responses to acute and chronic CoQ10 depletion. CoQ10 levels were reduced under both acute and chronic conditions and were restored by co- treatment with 4-hydroxybenzoic acid. Acute CoQ10 depletion resulted in a reduced number of mitochondria and mitochondrial enlargement, accompanied by an increase in mitochondrial DNA copy number (mtDNAcn). In contrast, long-term culture under continuous CoQ10 depletion restored mitochondrial number, size, and mtDNAcn to levels comparable to those of control cells, despite persistently reduced CoQ10 content. However, cell proliferation remained impaired, and mitochondrial ultrastructural properties differed from those of control cells, indicating incomplete recovery under chronic conditions. Furthermore, 4-hydroxybenzoic acid reversed the mitochondrial alterations observed under acute conditions. Together, these findings demonstrate that mitochondrial responses to CoQ10 deficiency are strongly time dependent and involve reversible yet incomplete adaptive remodeling, highlighting the importance of modeling chronic oxidative stress when interpreting mitochondrial phenotypes.