Toxicity of phenolipids: Protocatechuic acid alkyl esters trigger disruption of mitochondrial membrane potential and caspase activation in macrophages.

Phenolipids are a class of phenolic compounds with a lipidic moiety that have been receiving increasing attention due to their promising biological activities; however data regarding their toxicity and mechanism of action are scarce. A series of 11 phenolipids consisting of alkyl esters derivatives of the natural molecule protocatechuic acid was synthesized and evaluated against a panel of cancer and non-cancer cell lines. The macrophage cell line RAW 264.7, widely used as a tool for screening anti-inflammatory drugs, was more susceptible to the toxicity of these molecules than human cancer cells, reason for which mechanist studies were conducted. The parent molecule was not toxic up to 100μM, however structural modifications by inclusion of carbon side chains resulted in increased toxicity, compounds bearing 8-14 carbons being the most toxic and displaying IC₅₀ in the nanomolar range. Mechanistic studies showed that phenolipids elicit chromatin condensation, loss of cell viability and disruption of mitochondrial membrane potential (ΔΨm), increased reactive oxygen species (ROS) and activation of caspase-9/3, thus pointing to the involvement of mitochondria in the programmed cell death process taking place. This is the first study addressing the toxicity and mechanism of action of protocatechuic acid derivatives, which is relevant in light of the recent interest in these molecules.