Toxic Effects of Penetrating Cations

As mitochondria are negatively charged organelles, penetrating cations are used as parts of chimeric molecules to deliver specific compounds into mitochondria. In other words, they are used as electrophilic carriers for such chemical moieties as antioxidants, dyes, etc., to transfer them inside mitochondria. However, unmodified penetrating cations affect different aspects of cellular physiology as well. In this review, we have attempted to summarise the data about the side effects of commonly used natural (e.g., berberine) and artificial (e.g., tetraphenylphosphonium, rhodamine, methylene blue) penetrating cations on cellular physiology. For instance, it was shown that such types of molecules can (1) facilitate proton transport across membranes; (2) react with redox groups of the respiratory chain; (3) induce DNA damage; (4) interfere with pleiotropic drug resistance; (5) disturb membrane integrity; and (6) inhibit enzymes. Also, the products of the biodegradation of penetrating cations can be toxic. As penetrating cations accumulate in mitochondria, their toxicity is mostly due to mitochondrial damage. Mitochondria from certain types of cancer cells appear to be especially sensitive to penetrating cations. Here, we discuss the molecular mechanisms of the toxic effects and the anti-cancer activity of penetrating cations.

Size-dependent effects of polystyrene microplastics on cytotoxicity and efflux pump inhibition in human Caco-2 cells

Microplastics in the environment may gain entry the human gastrointestinal tract through the food chain. However, information on different adverse effects of microplastics at nanometer or micrometer scales in human intestine cells is limited. This study compared the cytotoxicity and efflux pump inhibition ability of 0.1 μm and 5 μm polystyrene microplastics (PS-MPs) in the human colon adenocarcinoma Caco-2 cells. Both PS-MP sizes exhibited low toxicity on cell viability, oxidative stress, and membrane integrity and fluidity. However, the mitochondrial membrane potential was disrupted by both sizes of PS-MPs, and the 5 μm PS-MPs induced higher effects than 0.1 μm PS-MPs. Furthermore, 0.1 μm (≥20 μg/mL) or 5 μm (≥80 μg/mL) PS-MPs inhibited plasma membrane ATP-binding cassette (ABC) transporter activity and increased arsenic (one substrate of ABC transporter) toxicity. The 0.1 μm PS-MPs might act as substrates of ABC transporter to reduce the transport capacity of other substrates. However, high concentrations of 5 μm PS-MPs might reduce ABC transporter activity through induction of mitochondrial depolarization and potential depletion of ATP. This study provides basic information on the toxicity of 0.1 μm and 5 μm PS-MPs in human intestine cells, which are useful for assessing the risk of PS-MPs in humans.

Penetrating cations induce pleiotropic drug resistance in yeast

Substrates of pleiotropic drug resistance (PDR) transporters can induce the expression of corresponding transporter genes by binding to their transcription factors. Penetrating cations are substrates of PDR transporters and theoretically may also activate the expression of transporter genes. However, the accumulation of penetrating cations inside mitochondria may prevent the sensing of these molecules. Thus, whether penetrating cations induce PDR is unclear. Using Saccharomyces cerevisiae as a model, we studied the effects of penetrating cations on the activation of PDR. We found that the lipophilic cation dodecyltriphenylphosphonium (C₁₂TPP) induced the expression of the plasma membrane PDR transporter genes PDR5, SNQ2 and YOR1. Moreover, a 1-hour incubation with C₁₂TPP increased the concentration of Pdr5p and Snq2p and prevented the accumulation of the PDR transporter substrate Nile red. The transcription factor PDR1 was required to mediate these effects, while PDR3 was dispensable. The deletion of the YAP1 or RTG2 genes encoding components of the mitochondria-to-nucleus signalling pathway did not prevent the C₁₂TPP-induced increase in Pdr5-GFP. Taken together, our data suggest (i) that the sequestration of lipophilic cations inside mitochondria does not significantly inhibit sensing by PDR activators and (ii) that the activation mechanisms do not require mitochondria as a signalling module.