Hydroxytyrosol [2-(3,4-dihydroxyphenyl)-ethanol], a natural phenolic compound found in the olive, alters Ca 2+ 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 [Ca 2+] i in HepG2 hepatoma cells. Fura-2 was used to detect [Ca 2+] i, and WST-1 assays were applied to explore cell cytotoxicity. Hydroxytyrosol elicited [Ca 2+] i raises. Eliminating external Ca 2+ diminished the Ca 2+ signal by 30%. Hydroxytyrosol-evoked Ca 2+ influx was diminished by 20% by three inhibitors of store-operated Ca 2+ channels and by a protein kinase C activator and an inhibitor. In the absence of Ca 2+, thapsigargin eradicated hydroxytyrosol-provoked [Ca 2+] i raises. Suppression of phospholipase C (PLC) with U73122, a PLC inhibitor, did not inhibit hydroxytyrosol-elicited [Ca 2+] i raises. Hydroxytyrosol reduced cell viability. This cytotoxic action was not reversed by preincubation with BAPTA/AM, a cytosolic Ca 2+ binder. In sum, in HepG2 hepatoma cells, hydroxytyrosol elicited [Ca 2+] i raises by provoking PLC-unrelated discharge of Ca 2+ from ER and Ca 2+ influx through PKC-sensitive store-operated Ca 2+ entry. In addition, hydroxytyrosol elicited Ca 2+-dissociated cytotoxicity.