Hydroxytyrosol [2-(3,4-dihydroxyphenyl)-ethanol], a natural phenolic compound found in the olive, alters Ca2+ signaling and viability in human HepG2 hepatoma cells

Abstract

Hepatotoma is the leading type of primary liver cancer in adults and third cause of death in the world. Hydroxytyrosol is a natural phenol existing in olive (Olea europaea L.). Hydroxytyrosol is the chief ingredient of olive oil, which was early deemed to be the most robust antioxidant in olive oil. Hydroxytyrosol is known to inhibit various types of cancer by different methods. This study was aimed to delineate the action of hydroxytyrosol on viability and [Ca2+]i in HepG2 hepatoma cells. Fura-2 was used to detect [Ca2+]i, and WST-1 assays were applied to explore cell cytotoxicity. Hydroxytyrosol elicited [Ca2+]i raises. Eliminating external Ca2+ diminished the Ca2+ signal by 30%. Hydroxytyrosol-evoked Ca2+ influx was diminished by 20% by three inhibitors of store-operated Ca2+ channels and by a protein kinase C activator and an inhibitor. In the absence of Ca2+, thapsigargin eradicated hydroxytyrosol-provoked [Ca2+]i raises. Suppression of phospholipase C (PLC) with U73122, a PLC inhibitor, did not inhibit hydroxytyrosol-elicited [Ca2+]i raises. Hydroxytyrosol reduced cell viability. This cytotoxic action was not reversed by preincubation with BAPTA/AM, a cytosolic Ca2+ binder. In sum, in HepG2 hepatoma cells, hydroxytyrosol elicited [Ca2+]i raises by provoking PLC-unrelated discharge of Ca2+ from ER and Ca2+ influx through PKC-sensitive store-operated Ca2+ entry. In addition, hydroxytyrosol elicited Ca2+-dissociated cytotoxicity.