Pseudoalteromonas haloplanktis TAC125 produces 4-hydroxybenzoic acid that induces pyroptosis in human A459 lung adenocarcinoma cells

In order to exploit the rich reservoir of marine cold-adapted bacteria as a source of bioactive metabolites, ethyl acetate crude extracts of thirteen polar marine bacteria were tested for their antiproliferative activity on A549 lung epithelial cancer cells. The crude extract from Pseudoalteromonas haloplanktis TAC125 was the most active in inhibiting cell proliferation. Extensive bioguided purification and mass spectrometric characterization allowed the identification of 4-hydroxybenzoic acid (4-HBA) as the molecule responsible for this bioactivity. We further demonstrate that 4-HBA inhibits A549 cancer cell proliferation with an IC50 value ≤1μg ml-1, and that the effect is specific, since the other two HBA isomers (i.e. 2-HBA and 3-HBA) were unable to inhibit cell proliferation. The effect of 4-HBA is also selective since treatment of normal lung epithelial cells (WI-38) with 4-HBA did not affect cell viability. Finally, we show that 4-HBA is able to activate, at the gene and protein levels, a specific cell death signaling pathway named pyroptosis. Accordingly, the treatment of A549 cells with 4-HBA induces the transcription of (amongst others) caspase-1, IL1beta, and IL18 encoding genes. Studies needed for the elucidation of mode of action of 4-HBA will be instrumental in depicting novel details of pyroptosis.