Variable RXR requirements for thyroid hormone responsiveness of endogenous genes.

Thyroid hormone receptors heterodimerize with retinoid X receptors in vitro and it is widely assumed that these heterodimers mediate the T3 induction of target genes. However, the importance of RXR for the T3 induction of endogenous genes has not been assessed. We used cDNA microarrays to identify 54 genes induced by T3 in Neuro2a cells that express thyroid hormone receptor beta. RNA interference-mediated knock down of endogenous RXRs showed that these genes vary from being highly dependent on RXR for T3 induction to being independent of RXR. Thus, the availability of RXR may differentially regulate the T3 induction of subsets of genes within a cell. Furthermore, coregulatory proteins that preferentially interact with TR homodimers or RXR-TR heterodimers may further expand the range of T3 response for genes within the same cell.

A method for efficient production of recombinant thyroid hormone receptors reveals that receptor homodimer-DNA binding is enhanced by the coactivator TIF2.

Thyroid hormone receptors (TRs) are ligand-activated transcription factors that mediate the biological effects of thyroid hormone (T3) by binding to thyroid hormone response elements (TREs), typically located in the promoter regions of T3-responsive genes. It is generally held that T3-induced gene activation is mediated by retinoid X receptor (RXR)-TR heterodimers. Although TR homodimers can bind to some TREs, T3 destabilizes this interaction, calling into question the ability of TR to activate transcription in the absence of RXR. TR-DNA binding has been difficult to study in vitro because mammalian TRs are notoriously difficult to produce in Escherichia coli. We considered that this may be due to codon bias. Therefore, we produced TRbeta1 in E. coli Rosetta 2(DE3) which contains a plasmid that overexpresses the tRNAs corresponding to the seven rarest E. coli codons. This resulted in an improved yield of full-length TRbeta1, which we then used in electrophoretic mobility shift assays. We found the coactivator TIF2 greatly enhances binding of T3-occupied TRs to a subset of TREs, suggesting TRs may activate transcription from these TREs in an RXR-independent manner.