Finally, despite the fact that the VgEcR-RXR system is not expected to transactivate host genes by itself, changes in endogenous gene levels have been previously observed in mammalian cells treated with ecdysone receptor ligands (Oehme et al. 2006; Panguluri et al. 2007). In the experiments described here, activation of the transcriptional machinery was shown to be sufficient for changes in expression of at least one host gene, SEPP1. Due to the complex nature of selenoprotein translation (Tujebajeva et al. 2000; Small-Howard et al. 2006; Howard et al. 2007), many cell lines that are commonly used express selenoproteins poorly; however, HEK-293 cells have been successfully used in other studies for the expression of selenoenzymes (Madeja et al. 2005; Squires et al. 2007). Therefore, this 293-EcR system may function as a particularly effective system for the study of SelP transcription and translation process. While serving as a beneficial tool in the studies presented herein, the potential for this system to transactivate host genes may be considered as a possible limitation to the use of this inducible gene expression system in other studies.
