Midkine Inhibits Cholesterol Efflux by Decreasing ATP-Binding Membrane Cassette Transport Protein A1 via Adenosine Monophosphate-Activated Protein Kinase/Mammalian Target of Rapamycin Signaling in Macrophages.

Background: Midkine (MK), a heparin-binding protein, participates in multiple cellular processes, such as immunity, cellular growth and apoptosis. Overwhelming evidence indicates that MK plays an important role in various pathological processes, including chronic inflammation, autoimmunity, cancer, and infection. Recent studies demonstrated that MK may be involved in the development of atherosclerosis, yet the mechanism has not been fully explored. Therefore, this study aims to investigate the effect and mechanism of MK on macrophage cholesterol efflux.

Methods and results: Using Oil Red O staining, NBD-cholesterol fluorescence labeling and enzymatic methods, it observed that MK markedly promoted macrophage lipid accumulation. Liquid scintillation counting (LSC) showed that MK decreased cholesterol efflux. Moreover, cell immunofluorescence, western blotting and quantitative real-time polymerase chain reaction (qRT-PCR) showed that MK downregulated ATP-binding membrane cassette transport protein A1 (ABCA1) expression. Functional promotion of ABCA1 expression attenuated the inhibitory effects of MK on cholesterol efflux, which reduced lipid accumulation. Additionally, intervention of adenosine monophosphate activated protein (AMPK)-mammalian target of rapamycin (mTOR) signaling molecule by the AMPK activator, AICAR, increased p-AMPK and ABCA1 expression, decreased p-mTOR expression and promoted cholesterol efflux, resulting in an obvious reduction in intracellular lipid content.

Conclusions: These data suggest that MK reduces the expression of ABCA1, inhibits the efflux of cholesterol and promotes the accumulation of lipids in RAW264.7 macrophages, and AMPK-mTOR signaling is involved in MK-mediated regulation of cholesterol metabolism in RAW264.7 macrophages.