Thyroid hormone receptors in liver metabolism

Abstract

Thyroid hormone has a profound impact on mammalian metabolism; hypothyroidism causes decreased metabolic rate and increased serum cholesterol levels. One major target organ for the hormone is the liver, the most important metabolic organ in mammals, that balances anabolism of carbohydrates and lipids against catabolism. This organ is also the main regulator of cholesterol homeostasis. The active form of the hormone is T3, the ligand for thyroid hormone receptors (TRs) which are ligand dependent transcription factors belonging to the nuclear hormone receptor superfamily. TRs affect transcription by binding to the regulatory regions of target genes and subsequently either stimulate or repress transcription. The major ligand-binding isoforms of the receptor are TR alpha1 TR beta1 and TR beta2. In addition, alternative splicing of the TRalpha gene transcript generates TRalpha2, a variant isoform that cannot bind hormone. Gene targeting experiments with mice indicate that the different isoforms have both distinct and overlapping functions in vivo.

The aim of our studies was to investigate T3 action and TR isoform specificity on gene regulation and metabolism in liver. For this, we used different TR deficient mouse strains and determined responses to T3 in hepatic gene regulation as well as in cholesterol homeostasis.

To study overall hepatic gene regulation by T3, and TRbeta dependence in this, we used microarrays for investigating regulation by T3 of 4000 genes in hypothyroid TRbeta-/- and control mice. The study was designed to detect direct regulation of target genes by T3 as well as changes induced by sustained hyperthyroidism. We identified 250 genes that were regulated by T3, of which 100 had not previously been recognized as target genes. Sixty percent of the target genes were dependent on TRbeta. Approximately half of the affected genes were downregulated by the hormone. Interestingly, many of the rapidly and transiently regulated genes were involved in lipogenesis whereas e.g. genes in the mitochondrial respiratory chain responded only after prolonged hyperthyroidism. In addition, we identified genes that presumably are directly regulated by the P isoform.

T3 effectively decreases serum cholesterol levels. To determine the TR isoform responsible for this, the responses to T3 and dietary cholesterol were studied in different TR deficient mouse strains under hypo-and hyperthyroid conditions. We found that TRbeta is essential for normal T3 regulation of serum cholesterol levels and transcriptional regulation of cholesterol-7-alpha hydroxylase (CYP7A1), the ratelimiting enzyme in cholesterol degradation to bile acids. This dependence was specific for the TRbeta gene: overexpression of TRalpha 1, at levels that normalize the total hepatic T3 receptor content, failed to rescue the dysregulation. Furthermore, hypothyroid TRbeta-/- mice were resistant to development of hypercholesterolemia.

In the studies above, we found that many of the hepatic genes were dependent on TRbeta for transcriptional regulation by T3. Since TRbeta1 has a heterogeneous, zonal, distribution over the liver lobule, we hypothesized that the mechanism for the TRbeta dependence was governed by a co- localization of target gene and TR isoform. This was confirmed by our demonstration that TRalpha1 has a wider distribution over the lobule than TRbeta1. Secondly, we determined the dependence on TRbeta of three prototype hepatic target genes (malic enzyme, deiodinase 1 and spot 14) known to have differential lobular distributions. We found a good correlation between TRbeta dependence and lobular distribution of all three genes, suggesting that the zonal distribution of the respective receptors govern isoform specificity in gene regulation for at least these genes. Isoform dependence is thus dependent not only on the regulatory elements in the target genes, but also on the elements governing local expression of the receptors themselves.

In conclusion, we have identified TRbeta as the isoform responsible for T3 regulation of serum cholesterol levels as well as several important hepatic genes. We have also identified 100 novel hepatic T3 target genes and elucidated their dependence on TRbeta. Furthermore, our data indicate that receptor isoform dependence in hepatic gene regulation is governed by different zonal distribution of the respective TR isoform.