Liver X receptor-α gene expression is positively regulated by thyroid hormone

The nuclear oxysterol receptors, liver X receptors (LXRs), and thyroid hormone receptors (TRs) cross talk mutually in many aspects of transcription, sharing the same DNA binding site (direct repeat-4) with identical geometry and polarity. In the current study, we demonstrated that thyroid hormone (T-3) up-regulated mouse LXR-alpha, but not LXR-beta, mRNA expression in the liver and that cholesterol administration did not affect the LXR-alpha mRNA levels. Recently, several groups have reported that human LXR-alpha autoregulates its own gene promoter through binding to the LXR response element. Therefore, we examined whether TRs regulate the mouse LXR-alpha gene promoter activity. Luciferase assays showed that TR-beta 1 positively regulated the mouse LXR-alpha gene transcription. Analysis of serial deletion mutants of the promoter demonstrated that the positive regulation by TR-beta 1 was not observed in the -1240/+ 30-bp construct. EMSA(s) demonstrated that TR-beta 1 or retinoid X receptor-alpha did not bind to the region from -1300 to -1240 bp (site A), whereas chromatin-immunoprecipitation assays revealed that TR-beta 1 and retinoid X receptor-alpha were recruited to the site A, indicating the presence of intermediating protein between the nuclear receptors and DNA site. We also showed that human LXR-alpha gene expression and promoter activities were up-regulated by thyroid hormone. These data suggest that LXR-beta mRNA expression is positively regulated by TR-beta 1 and thyroid hormone at the transcriptional level in mammals. This novel insight that thyroid hormone regulates LXR-alpha mRNA levels and promoter activity should shed light on a cross talk between LXR-alpha and TR-beta 1 as a new therapeutic target against dyslipidemia and atherosclerosis.