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J Biol Chem. 2002 Feb 15;277(7):4713-21. Epub 2001 Dec 10.

Oxysterol activators of liver X receptor and 9-cis-retinoic acid promote sequential steps in the synthesis and secretion of tumor necrosis factor-alpha from human monocytes.

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Department of Biochemistry, Faculty of Health Sciences, McMaster University, Hamilton, Ontario L8N 3Z5, Canada.


Liver X receptor alpha (LXRalpha), is a nuclear hormone receptor that is activated by oxysterols and plays a crucial role in regulating cholesterol and lipid metabolism in liver and cholesterol efflux from lipid-loaded macrophages. Here we show that treatment of human peripheral blood monocytes or monocytic THP-1 cells with the LXR ligand 22(R)-hydroxycholesterol (22(R)-HC), in combination with 9-cis-retinoic acid (9cRA), a ligand for the LXR heterodimerization partner retinoid X receptor (RXR), results in the specific induction of the potent pro-apoptotic and pro-inflammatory cytokine tumor necrosis factor-alpha (TNF-alpha). Promoter analysis, inhibitor studies, and order-of-addition experiments demonstrated that TNF-alpha induction by 22(R)-HC and 9cRA occurs by a novel two-step process. The initial step involves 22(R)-HC-dependent induction of TNF-alpha mRNA, and intracellular accumulation of TNF-alpha protein, mediated by binding of LXRalpha/RXRalpha to an LXR response element at position -879 of the TNF-alpha promoter. Subsequent cell release of TNF-alpha protein occurs via a separable 9cRA-dependent, LXRalpha-independent step that requires de novo transcription and protein synthesis. Our findings reveal a potentially new dimension of the physiological role of LXRalpha and identify a unique multistep pathway of TNF-alpha production that may be of consequence to the normal function of LXR in monocyte/macrophages and in disease conditions such as atherosclerosis.

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