Mitochondria-targeted phenolic antioxidants induce ROS-protective pathways in primary human skin fibroblasts.

Phytochemical antioxidants like gallic and caffeic acid are constituents of the normal human diet that display beneficial health effects, potentially via activating stress response pathways. Using primary human skin fibroblasts (PHSFs) as a model, we here investigated whether such pathways were induced by novel mitochondria-targeted variants of gallic acid (AntiOxBEN) and caffeic acid (AntiOxCIN). Both molecules reduced cell viability with similar kinetics and potency (72 h incubation, IC50 ~23 μM). At a relatively high but non-toxic concentration (12.5 μM), AntiOxBEN and AntiOxCIN increased ROS levels (at 24 h), followed by a decline (at 72 h). Further analysis at the 72 h timepoint demonstrated that AntiOxBEN and AntiOxCIN did not alter mitochondrial membrane potential (Δψ), but increased cellular glutathione (GSH) levels, mitochondrial NAD(P)H autofluorescence, and mitochondrial superoxide dismutase 2 (SOD2) protein levels. In contrast, cytosolic SOD1 protein levels were not affected. AntiOxBEN and AntiOxCIN both stimulated the gene expression of Nuclear factor erythroid 2-related factor 2 (NRF2; a master regulator of the cellular antioxidant response toward oxidative stress). AntiOxBEN2 and ANtiOxCIN4 differentially affected the gene expression of the antioxidants Heme oxygenase 1 (HMOX1) and NAD(P)H dehydrogenase (quinone) 1 (NQO1). Both antioxidants did not protect from cell death induced by GSH depletion and AntiOxBEN (but not AntiOxCIN) antagonized hydrogen peroxide-induced cell death. We conclude that AntiOxBEN and AntiOxCIN increase ROS levels, which stimulates NRF2 expression and, as a consequence, SOD2 and GSH levels. This highlights that AntiOxBEN and AntiOxCIN can act as prooxidants thereby activating endogenous ROS-protective pathways.