Abstract
Geranylgeranoic acid (GGA) was initially recognized as an acyclic retinoid due to its ligand activity on retinoid receptors; however, its chemical structure suggested a possibility that it could be biosynthesized via the mevalonate pathway in animal cells. Therefore, we started to study the biosynthesis of GGA in mammalian cells and investigated the mechanism of its biological activity by focusing on the effect of GGA on the induction of cell death in human hepatoma cells. As a result, we found that GGA was detected as an endogenous lipid in the most organs of rats including liver, brain, and testis, that GAA was biosynthesized via the mevalonate pathway in several human hepatoma-derived cell lines, and that monoamine oxidase B (MAOB) was primarily involved in this metabolic pathway. As for the biological activity of GGA, we found that the lipid-induced unfolded protein response (UPR), which is an upstream cellular process of the incomplete autophagic response, and Toll-like receptor 4 (TLR4)-mediated pyroptosis were involved in GGA-induced cell death of human hepatoma cell lines. GGA-induced UPR immediately activated caspase-4 (CASP4) and gasdermin D (GSDMD) was translocated to the cell membrane after producing an N-terminal fragment of GSDMD. A second gradual up-regulation of intracellular Ca2+ concentration was followed by activation of CASP1, indicating that GGA activated the inflammasome. The increases in CASP1 activity and cell death by GGA were inhibited by co-treatment with oleic acid, VIPER (an inhibitory peptide of TLR4), MCC950 (a selective inhibitor of the NLRP3 inflammasome), or a CASP4 inhibitor peptide. In summary, our results suggest that GGA is de novo synthesized via MAOB and the mevalonate pathway in human hepatoma cells and that GGA, when its concentrations reach micromolar range, causes pyroptotic cell death via TLR4 signaling in hepatoma cells.
