Hydroxycinnamic acids (such as ferulic, caffeic, sinapic, andp-coumaric acids) are a group of compounds highly abundant in food that may account for about one-third of the phenolic compounds in our diet. Hydroxycinnamic acids have gained an increasing interest in health because they are known to be potent antioxidants. These compounds have been described as chain-breaking antioxidants acting through radical scavenging activity, that is related to their hydrogen or electron donating capacity and to the ability to delocalize/stabilize the resulting phenoxyl radical within their structure. The free radical scavenger ability of antioxidants can be predicted from standard one-electron potentials. Thus, voltammetric methods have often been applied to characterize a diversity of natural and synthetic antioxidants essentially to get an insight into their mechanism and also as an important tool for the rational design of new and potent antioxidants. The structure-property-activity relationships (SPARs) correlations already established for this type of compounds suggest that redox potentials could be considered a good measure of antioxidant activity and an accurate guideline on the drug discovery and development process. Due to its magnitude in the antioxidant field, the electrochemistry of hydroxycinnamic acid-based antioxidants is reviewed highlighting the structure-property-activity relationships (SPARs) obtained so far.

The extracellular pH (pHe) is a key determinant of the cellular (micro)environment and needs to be maintained within strict boundaries to allow normal cell function. Here we used HEK293 cells to study the effects of pHe acidification (24h), induced by mitochondrial inhibitors (rotenone, antimycin A) and/or extracellular HCl addition. Lowering pHe from 7.2 to 5.8 reduced cell viability by 70% and was paralleled by a decrease in cytosolic pH (pHc), hyperpolarization of the mitochondrial membrane potential (Δψ), increased levels of hydroethidine-oxidizing ROS and stimulation of protein carbonylation. Co-treatment with the antioxidant α-tocopherol, the mitochondrial permeability transition pore (mPTP) desensitizer cyclosporin A and Necrostatin-1, a combined inhibitor of Receptor-interacting serine/threonine-protein kinase 1 (RIPK1) and Indoleamine 2,3-dioxygenase (IDO), prevented acidification-induced cell death. In contrast, the caspase inhibitor zVAD.fmk and the ferroptosis inhibitor Ferrostatin-1 were ineffective. We conclude that extracellular acidification induces necroptotic cell death in HEK293 cells and that the latter involves intracellular acidification, mitochondrial functional impairment, increased ROS levels, mPTP opening and protein carbonylation. These findings suggest that acidosis of the extracellular environment (as observed in mitochondrial disorders, ischemia, acute inflammation and cancer) can induce cell death via a ROS- and mPTP opening-mediated pathogenic mechanism.

Inhibitor studies with isolated mitochondria demonstrated that complex I (CI) and III (CIII) of the electron transport chain (ETC) can act as relevant sources of mitochondrial reactive oxygen species (ROS). Here we studied ROS generation and oxidative stress induction during chronic (24h) inhibition of CI and CIII using rotenone (ROT) and antimycin A (AA), respectively, in intact HEK293 cells. Both inhibitors stimulated oxidation of the ROS sensor hydroethidine (HEt) and increased mitochondrial NAD(P)H levels without major effects on cell viability. Integrated analysis of cells stably expressing cytosolic- or mitochondria-targeted variants of the reporter molecules HyPer (H2O2-sensitive and pH-sensitive) and SypHer (H2O2-insensitive and pH-sensitive), revealed that CI- and CIII inhibition increased cytosolic but not mitochondrial H2O2 levels. Total and mitochondria-specific lipid peroxidation was not increased in the inhibited cells as reported by the C11-BODIPY(581/591) and MitoPerOx biosensors. Also expression of the superoxide-detoxifying enzymes CuZnSOD (cytosolic) and MnSOD (mitochondrial) was not affected. Oxyblot analysis revealed that protein carbonylation was not stimulated by CI and CIII inhibition. Our findings suggest that chronic inhibition of CI and CIII: (i) increases the levels of HEt-oxidizing ROS and (ii) specifically elevates cytosolic but not mitochondrial H2O2 levels, (iii) does not induce oxidative stress or substantial cell death. We conclude that the increased ROS levels are below the stress-inducing level and might play a role in redox signaling.

Mitochondria are cytoplasmic double-membraned organelles that are involved in a myriad of key cellular regulatory processes. The loss of mitochondrial function is related to the pathogenesis of several human diseases. Over the last decades, an increasing number of studies have shown that dietary polyphenols can regulate mitochondrial redox status, and in some cases, prevent or delay disease progression. This paper aims to review the role of four dietary polyphenols – resveratrol, curcumin, epigallocatechin-3-gallate nd quercetin – in molecular pathways regulated by mitochondria and their potential impact on human health. Cumulative evidence showed that the aforementioned polyphenols improve mitochondrial functions in different in vitro and in vivo experiments. The mechanisms underlying the polyphenols’ beneficial effects include, among others, the attenuation of oxidative stress, the regulation of mitochondrial metabolism and biogenesis and the modulation of cell-death signaling cascades, among other mitochondrial-independent effects. The understanding of the chemicalbiological interactions of dietary polyphenols, namely with mitochondria, may have a huge impact on the treatment of mitochondrial dysfunction-related disorders.