Screening of citrus fruits on anti-atherogenic functionality of HDL

Background and objectives: ATP-binding cassette transporters (ABC) A1, G1and cavenger receptor class B type I (SR-BI) are key molecules in cholesterol efflux from macrophages, which is an initial step of reverse cholesterol transport, a major anti-atherogenic functionality of high-density lipoprotein (HDL). Citrus fruit intake protects against cardiovascular disease, it remains unclear through what mechanism of action it affects cholesterol homeostasis in macrophages. We therefore investigated the effects of citrus fruit on HDL-mediated cholesterol efflux and expressions of key molecules in macrophages. Methods: We investigated water-soluble and oil-soluble extracts of twenty-three types of citrus fruit. The differentiation of THP-1 monocytes into macrophages was induced in the presence of 320 nmol/L PMA for 72 hours. The macrophages were labeled with [3H]cholesterol (1.0 μCi/mL) in RPMI 1640 for 24 hours and incubated in the presence and absence of HDL (50 μg/mL) for 24 hours. The percentage cholesterol efflux was calculated by dividing the media-derived radioactivity by the sum of the radioactivity in the media and the cells. Results: Among citrus fruits, six types of oil-soluble extracts from citrus fruit peel (blood orange, minneola orange, kumquat, kabosu, navel and citron) enhanced cholesterol efflux from THP-1 macrophages mediated by HDL. Furthermore, we found that limonene, the major component in the oil of citrus fruit peels enhanced cholesterol efflux from THP-1 macrophages mediated by HDL. In supporting these enhanced cholesterol efflux mechanisms, limonene increased the protein levels of ABCA1 and ABCG1, but not SR-BI. Conclusions: The potential cardioprotective properties of citrus fruit peel might be associated with an enhanced anti-atherogenic functionality of HDL.