Thyroid hormone affects the gene expression of retinoid X receptors in the adult rat.

Gene expression of three nuclear retinoid X receptors (RXR alpha, beta and gamma) was examined by Northern blot analysis in various rat tissues. The RXR alpha mRNA (5.5 kb) was detected in most tissues and particularly expressed at a high level in the liver. The RXR beta transcripts (2.4 and 3.0 kb) were expressed ubiquitously, and particularly at high levels in the brain and testis. In the liver, heart, kidney and lung, the RXR gamma mRNA (2.0 kb) was specifically detected. Furthermore, we examined the effect of retinoid, vitamin D and thyroid hormone status on the gene expression of RXR alpha, beta and gamma. Though retinoid and vitamin D did not affect the mRNA levels of three RXRs, the mRNA levels of two genes were controlled by thyroid hormone. Namely, positive (RXR beta) and negative (RXR gamma) regulations by thyroid hormone were observed with no effect on the gene expression of RXR alpha. These results suggest that thyroid hormone might affect the signal transduction of retinoid, vitamin D and thyroid hormone by changing RXR levels.