Copper exposure induces mitochondrial dysfunction and hepatotoxicity via the induction of oxidative stress and PERK/ATF4 -mediated endoplasmic reticulum stress

Copper is an essential trace element, and excessive exposure could result in hepatoxicity, however, the underlying molecular mechanisms remain incompletely understood. The present study is aimed to investigate the molecular mechanisms of copper sulfate (CuSO4) exposure-induced hepatoxicity both in vivo and in vitro. In vitro, HepG2 and L02 cells were exposed to various doses of CuSO4 for 24 hours. Cell viability, ROS production, oxidative stress biomarkers, mitochondrial functions, ultrastructure, intracellular calcium (Ca2+) concentration, and the expression of proteins related to mitochondrial apoptosis and endoplasmic reticulum (ER) stress were assessed. In vivo, C57BL/6 mice were treated with CuSO4 at doses of 10 and 30 mg/kg BW/day and co-treated with 4-PBA at 100 mg/kg BW/day for 35 days. Subsequently, liver function, histopathological features, and protein expression were evaluated. Results found that exposure to CuSO4 at concentrations of 100-400 μM for 24 hours significantly decreased the viabilities of HepG2 and L02 cells and it was in a dose-dependent manner. Additionally, CuSO4 exposure induced significant oxidative stress and mitochondrial dysfunction in HepG2 cells, which were partially ameliorated by the antioxidant N-acetylcysteine (NAC). Furthermore, CuSO4 exposure prominently triggered ER stress, as evidenced by the upregulation of GRP94, GRP78, phosphorylated forms of PERK and eIF2α, and CHOP proteins in livers of mice and HepG2 cells. NAC treatment significantly inhibited CuSO4 exposure -induced ER stress in HepG2 cells. Pharmacological inhibition of ER stress through co-treatment with 4-PBA and the PERK inhibitor GSK2606414, as well as genetic knockdown of ATF4, partially mitigated CuSO4-induced cytotoxicity in HepG2 cells by reducing mitochondrial dysfunction and inhibiting the mitochondrial apoptotic pathway. Moreover, 4-PBA treatment significantly attenuated CuSO4-induced caspase activation and hepatoxicity in mice. In conclusion, these results reveal that CuSO4-induced hepatotoxicity involves mitochondrial dysfunction and ER stress by activating oxidative stress induction and PERK/ATF4 pathway.