<p>Claudin family proteins play an important role in the formation of tight junctions in several tissues. Individual claudins display organ- and tissue-specific expression. Claudin domain containing 1 (CLDND1), also known as claudin 25 (CLDN25), is a homolog of the claudin family, and its expression was reported to be downregulated in a mouse model of cerebellar hemorrhage. We have also reported that the retinoic acid receptor-related orphan receptor α (RORα) is involved in the transcriptional activation of CLDND1 by binding to the RORα responsive element (RORE) in the CLDND1 promoter region. Cholesterol and its derivative oxysterol reportedly serve as ligands for the nuclear receptor RORα. However, the effect of cholesterols on CLDND1 expression is unclear. The present study aimed to evaluate the effect of inhibiting steroid synthesis via lovastatin on RORα-mediated CLDND1 transcriptional regulation. Chromatin immunoprecipitation and luciferase reporter assays revealed that RORα-mediated transcriptional regulation of CLDND1 was suppressed upon lovastatin treatment of HepG2 cells; however, this inhibitory effect was attenuated by supplementation with cholesterol. Furthermore, quantitative reverse transcription-PCR and immunoblotting analyses revealed the downregulated expression of CLDND1 mRNA and protein in HepG2 cells upon lovastatin treatment with no parallel changes in RORα mRNA and protein levels. These results confirm that cholesterol serve as ligands for RORα and are, therefore, involved in the activation of CLDND1 transcriptional regulation by RORα.</p>