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 Show more
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, PPARĪ± and PPARĪ³ 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. 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 PPARĪ³ and LXRĪ± but not PPARĪ±. The increase in ABCA1/G1 mRNA levels by NOB was suppressed by antagonists of PPARĪ³ and LXRĪ±. The increase in PPARĪ³ mRNA levels by NOB was suppressed by an LXRĪ± antagonist, and the increase in LXRĪ± mRNA levels was suppressed by a PPARĪ³ antagonist. NOB increased CD36 mRNA and this was suppressed by an LXRĪ± antagonist. The increase in ABCA1 mRNA by a PPARĪ³ agonist was also suppressed by an LXRĪ± antagonist. NOB did not influence LPL1 mRNA expression levels. NOB stimulated AMPK phosphorylation, and the increase in ABCA1/G1, LXRĪ± and PPARĪ³ mRNA levels and ABCA1/G1 protein levels by NOB was reversed by an AMPK inhibitor. AMPK siRNA suppressed ABCA1 expression. NOB activates AMPK and subsequently LXRĪ± to promote the expression of ABCA1 and ABCG1, and an LXRĪ± - PPARĪ³ loop pathway amplifies these signals. Show less
Release of cellular cholesterol by ATP-binding cassette transporter (ABC)A1 and apolipoproteins is a major source of plasma high-density lipoprotein (HDL). Expression of ABC transporter A1 (ABCA1) is Show more
Release of cellular cholesterol by ATP-binding cassette transporter (ABC)A1 and apolipoproteins is a major source of plasma high-density lipoprotein (HDL). Expression of ABC transporter A1 (ABCA1) is directly stimulated by liver X receptor (LXR)/retinoid X receptor (RXR) activation. We evaluated the abilities of two RXR agonists, PA024 and HX630, to increase ABCA1 expression. In differentiated THP-1 cells, the two agonists efficiently enhanced ABCA1 mRNA expression and apoA-I-dependent cellular cholesterol release. However, in RAW264 cells and undifferentiated THP-1 cells, PA024 was highly effective while HX630 was inactive in increasing ABCA1 mRNA. In parallel, the two agonists had different abilities to activate ABCA1 promoter in an LXR-responsive-element (LXRE)-dependent manner and to directly stimulate LXRalpha/RXR transactivation. The ability of HX630 to enhance ABCA1 expression was correlated closely with the cellular PPARgamma mRNA level. Moreover, HX630 was able to activate PPARgamma/RXR. Transfection of PPARgamma in RAW264 cells induced HX630-mediated activation of LXRE-dependent transcription and ABCA1 promoter, suggesting the ability of HX630 to activate PPARgamma-LXR-ABCA1 pathway. We conclude that RXR agonist PA024 and HX630 have different abilities to activate LXR/RXR, and that the cell-type-dependent effect of HX630 on ABCA1 expression and HDL generation is closely associated with this defect. Show less