Transcriptional Regulation of Human CYP27 Integrates Retinoid, Peroxisome Proliferator-Activated Receptor, and Liver X Receptor Signaling in Macrophages

Abstract

Cholesterol uptake and efflux are key metabolic processes associated with macrophage physiology and atherosclerosis. Peroxisome proliferator-activated receptor gamma (PPAR␥) and liver X receptor alpha (LXR␣) have been linked to the regulation of these processes. It remains to be identified how activation of these receptors is connected and regulated by endogenous lipid molecules. We identified CYP27, a p450 enzyme, as a link between retinoid, PPAR␥, and LXR signaling. We show that the human CYP27 gene is under coupled regulation by retinoids and ligands of PPARs via a PPAR-retinoic acid receptor response element in its promoter. Induction of the enzyme's expression results in an increased level of 27-hydroxycholesterol and upregulation of LXR-mediated processes. Upregulated CYP27 activity also leads to LXR-independent elimination of CYP27 metabolites as an alternative means of cholesterol efflux. Moreover, human macrophage-rich atherosclerotic lesions have an increased level of retinoid-, PPAR␥-, and LXR-regulated gene expression and also enhanced CYP27 levels. Our findings suggest that nuclear receptor-regulated CYP27 expression is likely to be a key integrator of retinoic acid receptor-PPAR␥-LXR signaling, relying on natural ligands and contributing to lipid metabolism in macrophages.Handling of lipids by macrophages is an important metabolic process in the context of hypercholesterolemia and the development of atherosclerotic lesions (20,32,44). For this reason it is critical to understand the regulatory processes associated with cholesterol and fatty acid uptake and release (efflux) in this cell type. A regulatory network has been associated with macrophage lipid metabolism in recent years. First, it has been shown that peroxisome proliferator-activated receptor gamma (PPAR␥), a member of the nuclear receptor superfamily, can be linked to macrophage maturation and uptake of modified (oxidized) low-density lipoprotein (LDL) (35,45). Later, the oxysterol receptor liver X receptor (LXR) was linked to macrophage lipid metabolism by showing that LXR␣ is a direct transcriptional target of PPAR␥ and could induce lipid transporters such as ABCA1 (9, 40) and ABCG1 (26). A coordinated lipid transport is likely to be regulated by these receptors. Linking of the two receptor systems (PPAR␥ and LXR␣) provides an attractive but not well understood pathway to explain lipid and cholesterol uptake and efflux from macrophages.

9-cis-Retinoic Acid (9cRA), a Retinoid X Receptor (RXR) Ligand, Exerts Immunosuppressive Effects on Dendritic Cells by RXR-Dependent Activation: Inhibition of Peroxisome Proliferator-Activated Receptor γ Blocks Some of the 9cRA Activities, and Precludes Them to Mature Phenotype Development

“…These facts are in agreement with a widespread point of view that there is a close relation among all these receptors, suggesting a complex and tight regulation dependent on secondary messengers and cofactors. This regulation has already been described in macrophages and Wnt signaling in other cell types (61,62) but, to our knowledge, not until now in DCs. In this sense, there would be a nuclear receptor modulation of the DC response depending on the external signaling or the extracellular medium composition, together with an intrinsic stage of the cells.…”

Nuclear receptors in macrophages: A link between metabolism and inflammation

“…The regulation of lipid influx and efflux by nuclear receptors is also different. CYP27 can be induced by retinoids and PPAR-c only in human macrophages [50]. Deletion of CYP27 does not induce atherosclerosis in mice [53][54][55] only in men [45,46].…”

“…Mutation of the enzyme leads to a human disease cerebrotendinous xanthomatosis (CTX), a rare sterol-storage disease characterized by xanthomas in tendons and also in the central nervous system leading to ataxia, spinal cord paresis, neurological dysfunctions, normolipidemic xanthomatosis and accelerated atherosclerosis [47][48][49]. We found that retinoid receptors and PPAR-c induce the expression of CYP27 resulting in increased production 27-hydroxycholesterol in macrophages which activates LXR and enhances cholesterol efflux from macrophages through ABC transporters [50].…”

NLRC5 Limits the Activation of Inflammatory Pathways

“…7. The comparative C T method was used to quantify transcripts, and the expression level was normalized to that of the human cyclophilin or mouse RPL19 (17). All PCR reactions were performed in triplicate in 10-ml volumes, with one control reaction that contained cDNA but no RT enzyme.…”