A retinoid X receptor (RXR1) homolog from Schistosoma japonicum: its ligand-binding domain may bind to 9-cis-retinoic acid.

Retinoid X receptor (RXR) is an important member of the nuclear receptor superfamily of ligand-activated transcription factors that are present in all major groups of metazoans. A full-length cDNA encoding RXR, an orthologue of SmRXR1 in platyhelminth Schistosoma japonicum (SjRXR1) was identified and characterized. The SjRXR1 cDNA is 2806 bp long, and contains an open reading frame encoding a 745 amino acid protein. The deduced SjRXR1 protein sequence which was aligned with RXR proteins from other species revealed a highly conserved DNA binding domain (DBD) and moderately conserved ligand binding domain (LBD). The gene structure of SjRXR1 was analyzed and showed that it consists of seven exons spanning 18.4 kbp. The relative mRNA expression of SjRXR1 was evaluated in six different S. japonicum developmental stages in the final host (days 7-42 post-infection) and showed higher expression at days 21 and 35. In an in vitro study the transcription of SjRXR1 mRNA was shown to increase almost 3-fold and the SjRXR1 protein expression was also upregulated at the 48 h time point by treating the S. japonicum with 5.0 μM 9-cis-retinoic acid (RA). Flow cytometry analysis demonstrated that the percentage of HeLa cells expressing SjRXR1LBD-Myc fusion protein is approximately 11%. Over-expression of SjRXR1LBD-Myc in HeLa cells may result in the inhibition of innate apoptosis of this cancer cell line induced by 9-cis-RA. Our studies suggested that the retinoid signaling pathways may be conserved in the platyhelminth. The full cDNA sequence of SjRXR1 reported here has been submitted to the GenBank with accession no. JX111997.