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.