Objective-To determine whether arsenic inhibits transcriptional activation of the liver X receptor (LXR)/retinoid X receptor (RXR) heterodimers, thereby impairing cholesterol efflux from macrophages and potentially contributing to a proatherogenic phenotype. Methods and Results-Arsenic is an important environmental contaminant and has been linked to an increased incidence of atherosclerosis. Previous findings showed that arsenic inhibits transcriptional activation of type 2 nuclear receptors, known to heterodimerize with RXR. Environmentally relevant arsenic doses decrease the LXR/RXR ligand-induced expression of the LXR target genes . Arsenic failed to decrease cAMP-induced ABCA1 expression, suggesting a selective LXR/RXR effect. This selectivity correlated with the ability of arsenic to decrease LXR/RXR ligand-induced, but not cAMP-induced, cholesterol efflux. By using chromatin immunoprecipitation assays, we found that arsenic inhibits the ability of LXR/RXR ligands to induce activation markers on the ABCA1 and SREBP-1c promoters and blocks ligand-induced release of the nuclear receptor coexpressor (NCoR) from the promoter. Arsenic did not alter the ability of LXR to transrepress inflammatory gene transcription, further supporting our hypothesis that RXR is the target for arsenic inhibition. Conclusion-Exposure to arsenic enhances the risk of atherosclerosis. We present data that arsenic inhibits the transcriptional activity of the liver X receptor, resulting in decreased cholesterol-induced gene expression and efflux from macrophages. Therefore, arsenic may promote an athersclerotic environment by decreasing the ability of macrophages to efflux excess cholesterol, thereby favoring increased plaque formation. Key Words: atherosclerosis Ⅲ macrophages Ⅲ receptors Ⅲ arsenic Ⅲ liver X receptor A rsenic has been widely known as a carcinogen; however, recent evidence suggests that those exposed to arsenic also have an increased risk for diabetes mellitus and cardiovascular diseases. In humans, a consequence of arsenic exposure is Blackfoot disease, which is characterized by severe peripheral arteriosclerosis. [1][2][3] Copper smelt workers exposed to arsenic were reported to have increased mortality from ischemic heart disease. 4,5 A significant correlation was found in an area of Taiwan between length of exposure to arsenic in the ground water and the risk of ischemic heart disease. 6 In the United States, higher levels in the drinking water correlated with an increased risk of death from vascular disease. 7 More recently, arsenic accelerated carotid atherosclerosis, with a multivariate-adjusted odds ratio of 3.1. 8 In vivo and in vitro data support and correlate with these epidemiological data, although the mechanism by which arsenic increases atherosclerosis is unclear. Inflammation is a key component of the generation of atherosclerotic lesions, and increased inflammatory molecule expression after arsenic exposure has been reported. 9 Cells treated in vitro with arsenic have increased levels of 5-lipoxygenase, ...

We have previously published that 2 proven treatments for acute promyelocytic leukemia, As 2 O 3 and retinoic acid, can be antagonistic in vitro. We now report that As 2 O 3 inhibits ligand-induced transcription of the retinoic acid receptor, as well as other nuclear receptors that heterodimerize with the retinoid X receptor α (RXRα). As 2 O 3 did not inhibit transactivation of the estrogen receptor or the glucocorticoid receptor, which do not heterodimerize with RXRα. We further show that As 2 O 3 inhibits expression of several target genes of RXRα partners. Phosphorylation of RXRα has been reported to inhibit nuclear receptor signaling, and we show by in vivo labeling and phosphoamino acid detection that As 2 O 3 phosphorylated RXRα in the N-terminal ABC region exclusively on serine residues. Consistent with our previous data implying a role for JNK in As 2 O 3 -induced apoptosis, we show that pharmacologic or genetic inhibition of JNK activation decreased As 2 O 3 -induced RXRα phosphorylation and blocked the effects of As 2 O 3 on RXRα-mediated transcription. A mutational analysis indicated that phosphorylation of a specific serine residue, S32, was primarily responsible for inhibition of RXRα-mediated transcription. These data may provide some insight into the rational development of chemotherapeutic combinations involving As 2 O 3 as well as into molecular mechanisms of arsenicinduced carcinogenesis resulting from environmental exposure.

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