👤 Tetsuya Hisada

ATP-binding cassette transporters (ABC) A1 and G1 are key molecules in cholesterol efflux from macrophages, which is an initial step of reverse cholesterol transport (RCT), a major anti-atherogenic property of high-density lipoprotein (HDL). Astaxanthin is one of the naturally occurring carotenoids responsible for the pink-red pigmentation in a variety of living organisms. Although astaxanthin is known to be a strong antioxidant, it remains unclear through what mechanism of action it affects cholesterol homeostasis in macrophages. We therefore investigated the effects of astaxanthin on cholesterol efflux and ABCA1/G1 expressions in macrophages. Astaxanthin enhanced both apolipoprotein (apo) A-I- and HDL-mediated cholesterol efflux from RAW264.7 cells. In supporting these enhanced cholesterol efflux mechanisms, astaxanthin promoted ABCA1/G1 expression in various macrophages. In contrast, peroxisome proliferator-activated receptor γ, liver X receptor (LXR) α and LXRβ levels remained unchanged by astaxanthin. An experiment using actinomycin D demonstrated that astaxanthin transcriptionally induced ABCA1/G1 expression, and oxysterol depletion caused by overexpression of cholesterol sulfotransferase further revealed that these inductions in ABCA1/G1 were independent of LXR-mediated pathways. Finally, we performed luciferase assays using human ABCA1/G1 promoter-reporter constructs to reveal that astaxanthin activated both promoters irrespective of the presence or absence of LXR-responsive elements, indicating LXR-independence of these activations. In conclusion, astaxanthin increased ABCA1/G1 expression, thereby enhancing apoA-I/HDL-mediated cholesterol efflux from the macrophages in an LXR-independent manner. In addition to the anti-oxidative properties, the potential cardioprotective properties of astaxanthin might therefore be associated with an enhanced anti-atherogenic function of HDL. Show less

Pioglitazone, a peroxisome proliferator-activated receptor γ (PPARγ) agonist, reportedly reduces cardiovascular events in diabetic patients. ATP cassette binding transporters (ABC) A1 and G1 are pivotal molecules for cholesterol efflux (ChE) from macrophages and high density-lipoprotein biogenesis, and the A1 transporter is regulated by a PPARγ-liver receptor X (LXR) pathway. Also, pioglitazone induces ABCG1 expression, though the exact mechanism remains unclear. We therefore investigated the effects of pioglitazone on ABCA1/G1 expression in vitro and ex vivo. The effects of pioglitazone on ChE and ABCA1/G1 expressions in macrophages were assessed. Then, mRNA was quantified in macrophages when PPARγ/LXR inhibition by siRNA or overexpression of oxysterol sulfotransferase was performed. ABCA1/G1 promoter activity with mutated LXR-responsive elements was also measured. As an ex vivo study, 15 type 2 diabetic patients were administered pioglitazone or placebo, and ChE assays and protein expressions were determined using macrophages cultured with the corresponding sera. Pioglitazone increased LXRα/ABCA1/G1 expressions, which enhanced ChE from macrophages. Inhibition of PPARγ/LXR pathways revealed that LXR was primarily involved in pioglitazone's transactivation of ABCA1 but only partially involved for ABCG1. Promoter assays showed that ABCG1 was regulated more by the promoter in intron 4 than that upstream of exon 1 but both promoters were responsive to LXR activation. Sera obtained after pioglitazone treatment promoted ChE and ABCA1/G1 expressions in macrophages. Pioglitazone enhanced ChE from macrophages by increasing ABCA1/G1 in LXR-dependent and -independent manners. Our comparable in vitro and ex vivo results shed new light on pioglitazone's novel anti-atherogenic property. Show less