Apelin-13Increases Expression Of ATP-binding Cassette Transporter A1via Activating Protein Kinase C α Signaling In THP-1Macrophage-derived Foam Cells

Background and objective: Apelin is a recently discovered adipocytokine. Apelinfamily of peptides (include apelin-13) are ligands for the cognate G-protein coupledreceptor APJ, showing cardiovascular protective and anti-inflammatory activity.Apelin was shown to initiate a series of cellular signaling pathways by activatingprotein kinase C (PKC). ATP-binding cassette A1(ABCA1), as a pivotal transporterof cholesterol, can mediate the efflux of cellular cholesterol, which inhibits theformation of foam cells and the development of atherosclerosis. However, thepotential effect of apelin on ABCA1expression is unknown. We used THP-1macrophage-derived foam cells to explore the influence of apelin-13on cellularABCA1expression and possible mechanism involved.Methods: We induced THP-1macrophages to be foam cells by ox-LDL. Thenexamined ABCA1expression after treated apelin-13with different concentrations (0,1,10,100,1000nM) for24h or incubated with apelin-13(100nM) for increasingperiods of time (0,6,12,24,48h). High performance liquid chromatography and Oilred O staining were used to evaluate the cellular lipid accumulation. Cholesterolefflux was determined by liquid scintillator. The level of ABCA1mRNA wasdetermined by real time quantitative RT-PCR. Western bolt were used to determinethe protein levels of ABCA1, PKC and so on. Calpain activity assay was performedusing a kit. Our experiment also used PTX or PKC inhibitor or PKC siRNA toincubate together with apelin-13for24h. The levels of ABCA1, p-ABCA1calpainactivity and cholesterol efflux were examined. Results: Apelin-13increased ABCA1protein levels and cellular cholesterol efflux ina dose and time-dependent manner, while had no effects on ABCA1mRNA. Furtherstudies showed that apelin-13phosphorylated ABCA1serine and inhibited calpainactivity, protected ABCA1from degradation mediated by calpain, thereby promotedcellular cholesterol efflux. However, pretreatment with PTX or PKC inhibitors, ortransferred with PKCα siRNA could abolish the effects of apelin-13on ABCA1expression, phosphorylation of ABCA1and calpain activity.Conclusions: Apelin-13phosphorylated ABCA1and inhibited calpain activitythrough activating PKCα, thereby preventing ABCA1from calpain-mediatedproteolysis, leading to the increase of expression of ABCA1and cellular cholesterolefflux.