Distinct modulatory roles for thyroid hormone receptors TRalpha and TRbeta in SREBP1-activated ABCD2 expression

Adrenoleukodystrophy-related protein, a peroxisomal ABC transporter encoded by ABCD2, displays functional redundancy with the disease-associated X-linked adrenoleukodystrophy protein, making pharmacological induction of ABCD2 a potentially attractive therapeutic approach. Sterol regulatory element (SRE)-binding proteins (SREBPs) induce ABCD2 through an SRE overlapping with a direct repeat (DR-4) element. Here we show that thyroid hormone (T(3)) receptor (TR)alpha and TRbeta bind this motif thereby modulating SREBP1-dependent activation of ABCD2. Unliganded TRbeta, but not TRalpha, represses ABCD2 induction independently of DNA binding. However, activation by TRalpha and derepression of TRbeta are T(3)-dependent and require intact SRE/DR-4 motifs. Electrophoretic mobility shift assays with nuclear extracts support a direct interaction of TR and SREBP1 at the SRE/DR-4. In the liver, Abcd2 expression is high in young mice (with high T(3) and TRalpha levels) but downregulated in adults (with low T(3) and TRalpha but elevated TRbeta levels). This temporal repression of Abcd2 is blunted in TRbeta-deficient mice, and the response to manipulated T(3) states is abrogated in TRalpha-deficient mice. These findings show that TRalpha and TRbeta differentially modulate SREBP1-activated ABCD2 expression at overlapping SRE/DR-4 elements, suggesting a novel mode of cross-talk between TR and SREBP in gene regulation.     

An unliganded thyroid hormone beta receptor activates the cyclin D1/cyclin-dependent kinase/retinoblastoma/E2F pathway and induces pituitary tumorigenesis

Thyroid-stimulating hormone (TSH)-secreting tumors (TSH-omas) are pituitary tumors that constitutively secrete TSH. The molecular genetics underlying this abnormality are not known. We discovered that a knock-in mouse harboring a mutated thyroid hormone receptor (TR) beta (PV; TRbeta(PV/PV) mouse) spontaneously developed TSH-omas. TRbeta(PV/PV) mice lost the negative feedback regulation with highly elevated TSH levels associated with increased thyroid hormone levels (3,3',5-triiodo-l-thyronine [T3]). Remarkably, we found that mice deficient in all TRs (TRalpha1(-/-) TRbeta(-/-)) had similarly increased T3 and TSH levels, but no discernible TSH-omas, indicating that the dysregulation of the pituitary-thyroid axis alone is not sufficient to induce TSH-omas. Comparison of gene expression profiles by cDNA microarrays identified overexpression of cyclin D1 mRNA in TRbeta(PV/PV) but not in TRalpha1(-/-) TRbeta(-/-) mice. Overexpression of cyclin D1 protein led to activation of the cyclin D1/cyclin-dependent kinase/retinoblastoma protein/E2F pathway only in TRbeta(PV/PV) mice. The liganded TRbeta repressed cyclin D1 expression via tethering to the cyclin D1 promoter through binding to the cyclic AMP response element-binding protein. That repression effect was lost in mutant PV, thereby resulting in constitutive activation of cyclin D1 in TRbeta(PV/PV) mice. The present study revealed a novel molecular mechanism by which an unliganded TRbeta mutant acts to contribute to pituitary tumorigenesis in vivo and provided mechanistic insights into the understanding of pathogenesis of TSH-omas in patients.     

Novel mode of deoxyribonucleic acid recognition by thyroid hormone receptors: thyroid hormone receptor beta-isoforms can bind as trimers to natural response elements comprised of reiterated half-sites

Thyroid hormone receptors (TRs) regulate gene expression by binding to specific DNA sequences, denoted thyroid hormone response elements (TREs). The accepted paradigm for TRs proposes that they bind as homo- or heterodimers to TREs comprised of two AGGTCA half-site sequences. In the prototypic TRE, these half-sites are arranged as direct repeats separated by a four-base spacer. This dimeric model of TR binding, derived from analysis of artificial DNA sequences, fails to explain why many natural TREs contain more than two half-sites. Therefore, we investigated the ability of different TR isoforms to bind to TREs possessing three or more half-sites. We report that the TRbeta isoforms (TRbeta0, TRbeta1, TRbeta2), but not TRalpha1, can bind to reiterated DNA elements, such as the rat GH-TRE, as complexes trimeric or greater in size. The TRbeta0 isoform, in particular, formed homo- and heterotrimers (with the retinoid X receptor) with high efficiency and cooperativity, and TRbeta0 preferentially used reporters containing these reiterated elements to drive gene expression in vivo. Our data demonstrate that TRbeta isoforms can form multimeric receptor complexes on appropriately reiterated DNA response elements, providing a functional distinction between the TR isoforms and an explanation for TREs possessing three or more half-sites.     

TRs have common and isoform-specific functions in regulation of the cardiac myosin heavy chain genes.

TRalpha1 and TRbeta mediate the regulatory effects of T3 and have profound effects on the cardiovascular system. We have analyzed the expression of the cardiac myosin heavy chain (MyHC) genes alpha and beta in mouse strains deficient for one or several TR genes to identify specific regulatory functions of TRalpha1 and TRbeta. The results show that TRalpha1 deficiency, which slows the heart rate, causes chronic overexpression of MyHCbeta. However, MyHCbeta was still suppressible by T3 in both TRalpha1- and TRbeta-deficient mice, indicating that either receptor can mediate repression of MyHCbeta. T3-dependent induction of the positively regulated MyHCalpha gene was similar in both TRalpha1- and TRbeta-deficient mice. The data identify a specific role for TRalpha1 in the negative regulation of MyHCbeta, whereas TRalpha1 and TRbeta appear interchangeable for hormone-dependent induction of MyHCalpha. This suggests that TR isoforms exhibit distinct specificities in the genes that they regulate within a given tissue type. Thus, dysregulation of MyHCbeta is likely to contribute to the critical role of TRalpha1 in cardiac function.     

Differential expression of thyroid hormone receptor isoforms dictates the dominant negative activity of mutant Beta receptor

Mutations in the thyroid hormone receptor beta gene (TRbeta) cause resistance to thyroid hormone (RTH). Genetic analyses indicate that phenotypic manifestation of RTH is due to the dominant negative action of mutant TRbeta. However, the molecular mechanisms underlying the dominant negative action of mutants and how the same mutation results in marked variability of resistance in different tissues in vivo are not clear. Here we used a knock-in mouse (TRbetaPV mouse) that faithfully reproduces human RTH to address these questions. We demonstrated directly that TRbeta1 protein was approximately 3-fold higher than TRalpha1 in the liver of TRbeta(+/+) mice but was not detectable in the heart of wild-type and TRbetaPV mice. The abundance of PV in the liver of TRbeta(PV/PV) was more than TRbeta(PV/+) mice but not detectable in the heart. TRalpha1 in the liver was approximately 6-fold higher than that in the heart of wild-type and TRbetaPV mice. Using TR isoforms and PV-specific antibodies in gel shift assays, we found that in vivo, PV competed not only with TR isoforms for binding to thyroid hormone response elements (TRE) but also competed with TR for the retinoid X receptors in binding to TRE. These competitions led to the inhibition of the thyroid hormone (T(3))-positive regulated genes in the liver. In the heart, however, PV was significantly lower and thus could not effectively compete with TRalpha1 for binding to TRE, resulting in activation of the T(3)-target genes by higher levels of circulating thyroid hormones. These results indicate that in vivo, differential expression of TR isoforms in tissues dictates the dominant negative activity of mutant beta receptor, thereby resulting in variable phenotypic expression in RTH.     

Requirement for thyroid hormone receptor beta in T3 regulation of cholesterol metabolism in mice

T3 potently influences cholesterol metabolism through the nuclear thyroid hormone receptor beta (TRbeta), the most abundant TR isoform in rodent liver. Here, we have tested if TRalpha1, when expressed at increased levels from its normal locus, can replace TRbeta in regulation of cholesterol metabolism. By the use of TRalpha2-/-beta-/- animals that overexpress hepatic TRalpha1 6-fold, a near normalization of the total amount of T3 binding receptors was achieved. These mice are similar to TRbeta-/- and TRalpha1-/-beta-/- mice in that they fail to regulate cholesterol 7alpha-hydroxylase expression properly, and that their serum cholesterol levels are unaffected by T3. Thus, hepatic overexpression of TRalpha1 cannot substitute for absence of TRbeta, suggesting that the TRbeta gene has a unique role in T3 regulation of cholesterol metabolism in mice. However, examination of T3 regulation of hepatic target genes revealed that dependence on TRbeta is not general: T3 regulation of type I iodothyronine deiodinase and the low density lipoprotein receptor were partially rescued by TRalpha1 overexpression. These in vivo data show that TRbeta is necessary for the effects of T3 on cholesterol metabolism. That TRalpha1 only in some instances can substitute for TRbeta indicates that T3 regulation of physiological and molecular processes in the liver occurs in an isoform-specific fashion.