LXR/RXR ligand activation enhances basolateral efflux of β-sitosterol in CaCo-2 cells

Abstract

To examine whether intestinal ABCA1 was responsible for the differences observed between cholesterol and β-sitosterol absorption, ABCA1-facilitated β-sitosterol efflux was investigated in CaCo-2 cells following liver X receptor/retinoid X receptor (LXR/RXR) activation. Both the LXR agonist T0901317 and the natural RXR/LXR agonists 22-hydroxycholesterol and 9-cis retinoic acid enhanced the basolateral efflux of β-sitosterol without altering apical efflux. LXR-mediated enhanced β-sitosterol efflux occurred between 6 h and 12 h after activation, suggesting that transcription, protein synthesis, and trafficking was likely necessary prior to facilitating efflux. The transcription inhibitor actinomycin D prevented the increase in β-sitosterol efflux by T0901317. Glybenclamide, an inhibitor of ABCA1 activity, and arachidonic acid, a fatty acid that interferes with LXR activation, also prevented β-sitosterol efflux in response to the LXR ligand activation. Influx of β-sitosterol mass did not alter the basolateral or apical efflux of the plant sterol, nor did it alter ABCA1, ABCG1, ABCG5, or ABCG8 gene expression or ABCA1 mass. Similar to results observed with intestinal ABCA1-facilitated cholesterol efflux, LXR/RXR ligand activation enhanced the basolateral efflux of β-sitosterol without affecting apical efflux.The results suggest that ABCA1 does not differentiate between cholesterol and β-sitosterol and thus is not responsible for the selectivity of sterol absorption by the intestine. ABCA1, however, may play a role in β-sitosterol absorption.

Keywords

• adenosine 5′-triphosphate binding cassette transporter A1
• absorption
• cholesterol
• intestine
• fatty acid
• liver X receptor