Silencing carboxylesterase 1 in human THP-1 macrophages perturbs genes regulated by PPARγ/RXR and RAR/RXR: down-regulation of CYP27A1-LXRα signaling

Abstract

Macrophage foam cells store excess cholesterol as cholesteryl esters, which need to be hydrolyzed for cholesterol efflux. We recently reported that silencing expression of carboxylesterase 1 (CES1) in human THP-1 macrophages [CES1KD (THP-1 cells with CES1 expression knocked down) macrophages] reduced cholesterol uptake and decreased expression of CD36 and scavenger receptor-A in cells loaded with acetylated low-density lipoprotein (acLDL). Here, we report that CES1KD macrophages exhibit reduced transcription of cytochrome P45027A1 (CYP27A1) in nonloaded and acLDL-loaded cells. Moreover, levels of CYP27A1 protein and its enzymatic product, 27-hydroxycholesterol, were markedly reduced in CES1KD macrophages. Transcription of LXRα (liver X receptor α) and ABCA1 (ATP-binding cassette transporter A1) was also decreased in acLDL-loaded CES1KD macrophages, suggesting reduced signaling through PPARγ-CYP27A1-LXRα. Consistent with this, treatment of CES1KD macrophages with agonists for PPARγ, RAR, and/or RAR/RXR partially restored transcription of CYP27A1 and LXRα, and repaired cholesterol influx. Conversely, treatment of control macrophages with antagonists for PPARγ and/or RXR decreased transcription of CYP27A1 and LXRα Pharmacologic inhibition of CES1 in both wild-type THP-1 cells and primary human macrophages also decreased CYP27A1 transcription. CES1 silencing did not affect transcript levels of PPARγ and RXR in acLDL-loaded macrophages, whereas it did reduce the catabolism of the endocannabinoid 2-arachidonoylglycerol. Finally, the gene expression profile of CES1KD macrophages was similar to that of PPARγ knockdown cells following acLDL exposures, further suggesting a mechanistic link between CES1 and PPARγ. These results are consistent with a model in which abrogation of CES1 function attenuates the CYP27A1-LXRα-ABCA1 signaling axis by depleting endogenous ligands for the nuclear receptors PPARγ, RAR, and/or RXR that regulate cholesterol homeostasis.

Keywords: 2-arachidonoylglycerol; PPARγ; RXR; carboxylesterase; cholesterol; macrophage.