Molecular mechanism for nobiletin to enhance ABCA1/G1 expression in mouse macrophages

Background and aims: Nobiletin (NOB), a functional ingredient found in citrus peel, is said to act against diabetes, obesity, and atherosclerosis. It has been reported to activate AMPK pathway, as well as increase SREBP1c, PPAR alpha and PPAR gamma expression. However, no molecular mechanism has been elucidated to be able to integrate these sporadic findings with some controversies to lead to concrete outcomes. In this study, regulation of HDL biogenesis by NOB was investigated modulating ABCA1 and ABCG1 expression. Methods and results: Regulation of ABCA1/G1 by NOB was investigated in mouse macrophages J774.1. NOB increased mRNA and protein levels of ABCA1/G1, and cell cholesterol release by these factors. It also increased mRNA of PPAR gamma and LXR alpha but not PPAR alpha. The increase in ABCA1/G1 mRNA levels by NOB was suppressed by antagonists of PPAR gamma and LXR alpha. The increase in PPAR. mRNA levels by NOB was suppressed by an LXR alpha antagonist, and the increase in LXR alpha mRNA levels was suppressed by a PPAR gamma antagonist. NOB increased CD36 mRNA and this was suppressed by an LXR alpha antagonist. The increase in ABCA1 mRNA by a PPAR alpha agonist was also suppressed by an LXR alpha antagonist. NOB did not influence LPL1 mRNA expression levels. NOB stimulated AMPK phosphorylation, and the increase in ABCA1/G1, LXR alpha and PPAR gamma mRNA levels and ABCA1/G1 protein levels by NOB was reversed by an AMPK inhibitor. AMPK siRNA suppressed ABCA1 expression. Conclusions: NOB activates AMPK and subsequently LXR alpha to promote the expression of ABCA1 and ABCG1, and an LXR alpha - PPAR gamma loop pathway amplifies these signals.