Is there a negative TRE in the luciferase reporter cDNA?

Transient expression assays using a luciferase (LUC) reporter gene are often used in studies of positive and negative thyroid hormone response elements (TREs) or of wild-type and mutated thyroid hormone receptors (TRs). However, unliganded TR (the beta isoform > than the alpha) increases LUC expression from 2 different TK-LUC vectors in several cell types, especially JEG-3 cells, and 3,5,3'-triiodothyronine (T3) reduces that expression by as much as 80% in a TR-dependent manner. The TR effects require an intact TR DNA-binding domain and the results suggest that there may be a negative TRE in the LUC cDNA. We conclude that great care must be used in the interpretation of studies of thyroid hormone action using LUC expression plasmids especially when these are performed in JEG-3 cells.