Isoketals form cytotoxic phosphatidylethanolamine adducts in cells

Levuglandins and their stereo- and regio-isomers (termed isolevuglandins or isoketals) are γ-ketoaldehydes (IsoK) that rapidly react with lysines to form stable protein adducts. IsoK protein adduct levels increase in several pathological conditions including cardiovascular disease. IsoKs can induce ion channel dysfunction and cell death, potentially by adducting to cellular proteins. However, IsoKs also adduct to phosphatidylethanolamine (PE) in vitro, and whether PE adducts form in cells or contribute to the effects of IsoKs is unknown. When radiolabeled IsoK was added to HEK293 cells, 40% of the radiolabel extracted into the chloroform lower phase suggesting the possible formation of PE adducts. We therefore developed methods to measure IsoK-PE adducts in cells. IsoK-PE was quantified by LC/MS/MS after hydrolysis to IsoK-ethanolamine by Streptomyces chromofuscus phospholipase D. In HEK293 and human umbilical vein endothelial cells (HUVEC), IsoK dose-dependently increased PE adduct concentrations to a greater extent than protein adduct. To test the biological significance of IsoK-PE formation, we treated HUVEC with IsoK-PE. IsoK-PE dose dependently induced cytotoxicity (LC50 2.2 μM). These results indicate that cellular PE is a significant target of IsoKs, and that formation of PE adducts may mediate some of the biological effects of IsoKs relevant to disease.