Dual functions of the steroid hormone receptor coactivator 3 in modulating resistance to thyroid hormone

Mutations of the thyroid hormone receptor beta (TRbeta) gene cause resistance to thyroid hormone (RTH). RTH is characterized by increased serum thyroid hormone associated with nonsuppressible thyroid-stimulating hormone (TSH) and impaired growth. It is unclear how the actions of TRbeta mutants are modulated in vivo to affect the manifestation of RTH. Using a mouse model of RTH that harbors a knockin mutation of the TRbeta gene (TRbetaPV mouse), we investigated the effect of the steroid hormone receptor coactivator 3 (SRC-3) on RTH. In TRbetaPV mice deficient in SRC-3, dysfunction of the pituitary-thyroid axis and hypercholesterolemia was lessened, but growth impairment of RTH was worsened. The lessened dysfunction of the pituitary-thyroid axis was attributed to a significant decrease in growth of the thyroid and pituitary. Serum insulin-like growth factor 1 (IGF-1) was further reduced in TRbetaPV mice deficient in SRC-3. This effect led to reduced signaling of the IGF-1/phosphatidylinositol 3-kinase (PI3K)/AKT/mammalian target of rapamycin (mTOR) pathway that is known to mediate cell growth and proliferation. Thus, SRC-3 modulates RTH by at least two mechanisms, one via its role as a receptor coregulator and the other via its growth regulatory role through the IGF-1/PI3K/AKT/mTOR signaling.     

Identification of nuclear factors that enhance binding of the thyroid hormone receptor to a thyroid hormone response element

Using a gel shift assay, we analyzed the binding of in vitro translated alpha- and beta-thyroid hormone (T3) receptors to a T3-response element (TRE) derived from the rat GH gene. No receptor-TRE complexes were observed when translated receptor alone was incubated with the TRE. However, addition of a nuclear extract from liver to the translational products resulted in the formation of two receptor-DNA complexes for both the alpha- and beta-receptors. These complexes were shown to contain translated receptor by comigration of 32P-labeled TRE and 35S-labeled receptor in the gel shift assay. A competition experiment demonstrated that formation of the complexes was sequence specific. Preincubation of the liver nuclear extract at 60 C abolished formation of both complexes indicating that receptor-TRE binding required a heat-labile nuclear factor. Phosphocellulose chromatography of the nuclear extract resulted in separation of the activities required for formation of the two complexes. Analysis of nuclear extracts from different tissues revealed that one complex formed in the presence of all extracts, whereas the second complex appeared predominantly with a nuclear extract from liver. Addition of T3 to the binding reaction had no effect on receptor-TRE complex formation. We suggest that nuclear factors interact with the T3 receptor to enhance hormone-independent binding to a TRE.     

Capacity for cooperative binding of thyroid hormone (T3) receptor dimers defines wild type T3 response elements.

Thyroid hormone response elements (T3REs) have been identified in a variety of promoters including those directing expression of rat GH (rGH), alpha-myosin heavy chain (rMHC), and malic enzyme (rME). A detailed biochemical and genetic analysis of the rGH element has shown that it consists of three hexamers related to the consensus [(A/G)GGT(C/A)A]. We have extended this analysis to the rMHC and rME elements. Binding of highly purified thyroid hormone receptor (T3R) to T3REs was determined using the gel shift assay, and thyroid hormone (T3) induction was measured in transient tranfections. We show that the wild type version of each of the three elements binds T3R dimers cooperatively. Mutational analysis of the rMHC and rME elements identified domains important for binding T3R dimers and allowed a direct determination of the relationship between T3R binding and function. In each element two hexamers are required for dimer binding, and mutations that interfere with dimer formation significantly reduce T3 induction. Similar to the rGH element, the rMHC T3RE contains three hexameric domains arranged as a direct repeat followed by an inverted copy, although the third domain is weaker than in rGH. All three are required for full function and T3R binding. The rME T3RE is a two-hexamer direct repeat T3RE, which also binds T3R monomer and dimer. Across a series of mutant elements, there was a strong correlation between dimer binding in vitro and function in vivo for rMHC (r = 0.99, P less than 0.01) and rME (r = 0.67, P less than 0.05) T3REs. Our results demonstrate a similar pattern of T3R dimer binding to a diverse array of hexameric sequences and arrangements in three wild type T3REs. Addition of nuclear protein enhanced T3R binding but did not alter the specificity of binding to wild type or mutant elements. Binding of purified T3R to T3REs was highly correlated with function, both with and without the addition of nuclear protein. T3R dimer formation is the common feature which defines the capacity of these elements to confer T3 induction.     

The T3R alpha gene encoding a thyroid hormone receptor is essential for post-natal development and thyroid hormone production.

The diverse functions of thyroid hormones are thought to be mediated by two nuclear receptors, T3R alpha1 and T3R beta, encoded by the genes T3R alpha and T3R beta respectively. The T3R alpha gene also produces a non-ligand-binding protein T3R alpha2. The in vivo functions of these receptors are still unclear. We describe here the homozygous inactivation of the T3R alpha gene which abrogates the production of both T3R alpha1 and T3R alpha2 isoforms and that leads to death in mice within 5 weeks after birth. After 2 weeks of life, the homozygous mice become progressively hypothyroidic and exhibit a growth arrest. Small intestine and bones showed a strongly delayed maturation. In contrast to the negative regulatory function of the T3R beta gene on thyroid hormone production, our data show that the T3R alpha gene products are involved in up-regulation of thyroid hormone production at weaning time. Thus, thyroid hormone production might be balanced through a positive T3R alpha and a negative T3R beta pathway. The abnormal phenotypes observed on the homozygous mutant mice strongly suggest that the T3R alpha gene is essential for the transformation of a mother-dependent pup to an 'adult' mouse. These data define crucial in vivo functions for thyroid hormones through a T3R alpha pathway during post-natal development.     

Mutations of the rat growth hormone promoter which increase and decrease response to thyroid hormone define a consensus thyroid hormone response element

We have previously identified sequences required for thyroid hormone (T3) induction of the rat GH (rGH) promoter, which lie in a region from -188 to -164 upstream of the mRNA start site. Within this region, Domains A, -189 to -184 and B, -179 to -174, are imperfect direct repeats, and domain C, -172 to -167, is a divergent inverted copy that matches the A domain at 4/6 positions. A series of synthetic mutant versions of this sequence were inserted upstream of a truncated rGH promoter, or as a replacement for wild-type sequences in a synthetic 237 base pair rGH promoter or upstream of the heterologous thymidine kinase promoter. Mutations changing the B domain to a perfect copy of the A domain significantly increased T3 induction (21.3-fold) relative to the wild type (3.6-fold). A single point mutation making the C domain a better match to the A domain also increased T3 induction to 16.2-fold. Combining this up-mutation with any of three down-mutations in the A, B, or C domains strongly decreased response, showing that all three domains contribute to the amplified T3 response. Binding affinity of the various mutant oligonucleotides was assessed using in vitro translated receptor and affinity paralleled the functional responses for most binding site mutations. Requirements for in vitro binding were, however, less rigorous than those for functional T3 induction. Based on these results, we propose a consensus T3 receptor binding half-site, AGGT(C/A)A, at least two copies of which are required for a T3 response.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Thyroid hormone receptors form distinct nuclear protein-dependent and independent complexes with a thyroid hormone response element

We have examined the binding of nuclear proteins and recombinant thyroid hormone receptors (TRs) to the palindromic thyroid hormone responsive element AGGTCATGACCT (TREp) using a gel electrophoretic mobility shift assay. Four specific protein-DNA complexes were detected after incubation of nuclear extracts (NE) from T3-responsive pituitary (GH3) cells with a TREp-containing DNA fragment. This was compared with the TREp binding of reticulocyte lysate-synthesized TRs. TR alpha 1 and TR beta 2 each formed a single major TR:TREp complex which comigrated with the least retarded complex formed by GH3 NE, while TR beta 1 formed multiple complexes suggesting that it can bind to TREp as an oligomer. Interestingly, coincubation of 35S-TR alpha 1, GH3 NE, and unlabeled TREp resulted in not only the 35S-TR:TREp complex, but in two additional more greatly retarded complexes containing 35S-TR alpha 1 and comigrating with those formed by GH3 extract alone. Incubation of each of the TRs with NE from COS-7 cells, which do not possess sufficient endogenous TRs to mediate T3-responses, resulted in formation of a new, more greatly shifted complex. A similar, heat labile activity which altered mobility of the TR:TRE complex was also present in NE from T3-unresponsive JEG-3 cells. At high concentration of NE, all of the TR bound to TREp was more greatly retarded than in the absence of NE. Truncation of TR alpha 1 at amino acid 210 prevented additional complex formation in the presence of NE without affecting DNA binding, suggesting that the carboxyl-terminus of the TRs is essential for interaction with nuclear proteins.(ABSTRACT TRUNCATED AT 250 WORDS)     

Sodium functions as a negative allosteric modulator of the oxytocin receptor

The oxytocin receptor, a class A G protein coupled receptor (GPCR), is essentially involved in the physiology of reproduction. Two parameters are crucially important to support high-affinity agonist binding of the receptor: Mg²⁺ and cholesterol, both acting as positive modulators. Using displacement assays with a high-affinity fluorescent antagonist (OTAN-A647), we now show that sodium functions as a negative allosteric modulator of the oxytocin receptor. In membranes from HEK293 cells stably expressing the oxytocin receptor, oxytocin binding occurred with about 15-fold lower affinity when sodium chloride was increased from 0 to 300 mM, whereas antagonist binding remained largely unchanged. The effect was concentration-dependent, sodium-specific, and it was also observed for oxytocin receptors endogenously expressed in Hs578T breast cancer cells. A conserved Asp (Asp 85) is known to stabilize the sodium binding site in other GCPRs. Mutations of this residue into Ala or Asn are known to yield non-functional oxytocin receptors. When Asp 85 was exchanged for Glu, most of the oxytocin receptors were localized in intracellular structures, but a faint plasma membrane labeling with OTAN-A647 and the appearance of oxytocin-induced calcium responses indicated that these receptors were functional. However, a sodium effect was not detectable for the mutant D85E oxytocin receptors. Thus, the oxytocin receptor is allosterically controlled by sodium similar to other GPCRs, but it behaves differently concerning the involvement of the conserved Asp 85. In case of the oxytocin receptor, Asp 85 is obviously essential for proper localization in the plasma membrane.