Nuclear Hormone Receptor is a ligand- dependent transcriptional factor 1.Classes of hormones 2.General properties of nuclear hormone receptors superfamily 3.Functional characterization of nuclear hormone receptors 4.Thyroid hormones and thyroid hormone receptors 5.Hormone response elements: palindromic (inverted repeat), directed repeat, everted repeat. 6.Mechanism of nuclear hormone action Cofactors in two steps dimerization pathway—Coactivators (CoA)-SRC, Corepressor (CoR)- SMRT
7.Mechanism of transcriptional repression competition for overlapping binding sites 8. Resistance to thyroid hormone (RTH) 9. Transgenic model to study the receptor functions. Dominant negative activity 10. Thyroid hormone coactivator, corepressor. 11. Mutant nuclear hormone has oncogenic potential
Binding of extracellular signal molecules Hydrophilic molecules bind to cell-surface receptors Hydrophobic molecules bind to intracellular receptors *
Small molecule coactivators of transcription Steroids
Small molecule coactivators of transcription Lipid-Soluble Vitamins & Thyroid Hormone Vitamin A Thyroid Hormone
Thyroid hormone receptors
Nuclear Receptor Family is Large but not ubiquitous: mammals have ~50-60 genes flies 21 worms 270 (!!!) plants 0 yeast 0 Only a handful of physiological ligands have been identified, (despite many genes, worms lack any known lipid based endocrine system)
For a steroid hormone to turn gene transcription on, its receptor must: bind to the hormone bind to a second copy of itself to form a homodimer be in the nucleus, moving from the cytosol if necessary bind to its response element activate other transcription factors to start transcription.
Steroid hormone receptors are part of the superfamily of nuclear receptors that contains over 30 members. All members have conserved regions of high homology Hormone binding domain 90% homologous 10% difference accounts for specificity DNA binding domain which contains zinc fingers Receptors are found complexed with heat shock proteins (HSP) Unoccupied receptor held in inactive conformation by HSP Ligand binding releases HSP and exposes DNA binding domain Hormone receptor complex then binds to response elements on gene and allows transcription to occur Nuclear receptor family (steroid)
Structure of Intracellular Receptors Steroid and thyroid hormone receptors are members of a large group ("superfamily") of transcription factors. All of these receptors are composed of a single polypeptide chain that has, in the simplest analysis, three distinct domains: A/B, C, E.
Thyroid hormone receptors
Thyroid just below the larynx. Follicular cells secrete T3, T4 (which are “assembled” in colloid). Parafollicular cells secrete calcitonin, which regulates blood calcium levels.
EFFECTS T3: - Stimulates protein synthesis. - Stimulates rate of cellular respiration. - Increases metabolic rate.
Diseases of the Thyroid Hypothyroid Decreased metabolic rate. Weight gain. Decreased ability to adapt to cold. Lethargy. Hyperthyroid: weight loss. Nervousness, irritability ( 易怒 ). Intolerance to heat. Bulging eyes.
CTE: JBC:277(28),25115–25124, 2002
Nuclear hormone receptor: domain structure Zinc finger DNA binding domain… inserts into major groove of DNA
Steroid hormone receptors family
Hormone response elements (HRE)
half-sites are arranged as direct repeats DNA Sequence
Interaction of Thyroid Hormone Receptors with DNA Thyroid hormone receptors bind to short, repeated sequences of DNA called thyroid or T3 response elements (TREs), a type of hormone response element. A TRE is composed of two AGGTCA "half sites" separated by four nucleotides. The half sites of a TRE can be arranged as direct repeats, palindromes or inverted repeats.
n n n Half site 5‘-AGGTCA-3’ IR: inverted repeat DR: directed repeat ER: everted repeat
In the absence of hormone, some intracellular receptors do bind their hormone response elements loosely and silence transcription, when complexed to hormone, become activated and strongly stimulate transcription.
Ligand-bound state: Binding of T3 to its receptor induces a conformational change in the receptor that makes it incompetent to bind the corepressor complex, but competent to bind a group of coactivator proteins. The coactivator complex contains histone transacetylase (HAT) activity, which imposes an open configuration on adjacent chromatin. The coactivator complex associated with the T3-bound receptor functions to activate transcription from linked genes.
Thyroid hormone action T4 T3 Deiodinase
Thyroid hormone action T3 TR Thyroid responsive element (TRE) Transcription machinery Pol II Transcriptional start site RXR
A two-step model for TR/RXR heterodimer receptor activation
Coactivator Annu. Rev. Physiol :315–60
Corepressor Annu. Rev. Physiol :315–60
Nuclear receptor function -Ligand +Ligand
Transcriptional Repression Mechanisms
Mechanisms of Transcriptional Repression
Histone acetylation is reversible HDACs (histone deacetylase complexes) remove acetyl groups from N-terminal lysines, restoring their positive charge Histone deacetylation correlates with transcriptional inactivation.
Key repression step: Deacetylation of N-terminal lysines Acetyl group no positive charge transcriptional inactivation
Resistance to thyroid hormone (RTH) TR 1 mutation Autosomal dominant disorder Elevated free thyroid hormones in the serum TSH elevated or inappropriated normal
Mutation sites for TR 1
Resistance to thyroid hormone Childhood: Failure to thrive, growth retardation and attention-deficit hyperactivity disorder. Adults: goitre and tachycardia.
Approximately 60 different mutations in TR have been identified in RTH patients from over 100 families. 2. The clinical phenotype can vary among these families that harbor the same mutation and also vary within a family. This suggests that there maybe other genetic modifiers that determine the phenotype. 3. The mutant TRs had defective dissociation from this corepressor. In general, the release of SMRT correlated with the T3 -binding affinity of the mutant receptors.
Mechanism of Dominant Negative Activity T3 T3T3T3RXR TRE TRE MRXR M
Mutant Thyroid Hormone Receptor 1. Mutation within 3 regions of hormone binding domain (Reduced T3/T4 binding activity). 2. Retain DNA binding. 3. Retain dimerization activity. 4. Are transcriptional inactive. 5. Block normal receptor function (dominant negative activity).
Knockout and transgenic mice
TR 1, 2 deleted Transgenic mice in which both TR -1 and c-erbA -2 have been deleted (TR -/-) have a more severe phenotype with hypothyroidism, intestinal malformation, growth retardation, and early death shortly after weaning.
TR null (-/-) mice 1. Abnormal heart rate (20% lower ) 2. Abnormal body temperature (0.5 degrees C lower ). 3. A mild hypothyroidism. EMBO Journal. 17(2):455-61, 1998
TR null (-/-) mice 1. Abnormal intestinal morphology, assessed by a decrease in the number of epithelial cells along the crypt-villus axis and a decrease in proliferating crypt cells. 2. No changes occurred in T3Rbeta (-/-) mice. Gastroenterology. 116: , 1999
TR -null (-/-) Elevated thyroid hormone levels Abnormal central regulation of thyrotropin Develop profound hearing loss J Clinical Investigation. 104: , 1999
TR 2-null (-/-) mice 1. have preserved expression of the TR 1 and TR 1 isoforms. 2. Develop a central resistance to thyroid hormone 3. Growth hormone gene expression is marginally reduced. 4.TR beta 2-null mice exhibit no evidence of hearing impairment. 5. TR 2 may be involved in retinal development. J Clinical Investigation. 104: , 1999
A targeted dominant negative mutation of the thyroid hormone alpha 1 receptor causes increased mortality, infertility, and dwarfism in mice. Proc Natl Acad Sci U S A Dec
SRC-1: Steroid receptor coactivator Kda; 1440 aa. 2.In a reporter assay system, enhanced transactivation (5-10X) by the receptors. 3.Contain 3 transferable activation domains. 4.Antagonist RU486 abrogates the ability of SRC-1 to interact with and coactivate PR. (Science 270:1354,1995). 5. Also call NcoA LXXLL structure to interact with NRs (AF-2 domain). 7. SRC-3 (AIB-1) is amplified in many cases of breast cancer. Recruit CBP and PCAF.
Several common features among these putative coactivators. 1.there are multiple putative nuclear hormone receptor interaction sites that seem to bear a signature LXXLL sequence motif in which X represents any amino acid. This sequence has been shown to be important for coactivator binding to nuclear hormone receptors. 2.In the amino-terminal region, there is a basic helix- loop-helix (bHLH) motif, suggesting that these coactivators may bind to DNA.
N-CoR/SMRT (Nuclear receptor corepressor, Silencing Mediator for Retnoid and Thyroid receptor) kD. Structural and functional similarity. 2. Binding to unliganded TR and RARs 3. Hormone binding results in dissociation of the corepressors. 4. CoR box in LBD of NR is essential for corepressor binding. 5. Play an important role in the syndrome of resistance to thyroid hormone.
6. binding to antagonist occupied steroid receptors (RU 486/PR, tamoxifen/ER), suggesting that active repression is an important part of hormone antagonist action. 7. Interact with Sin3A, which in turns interacts with histone deacetylases.
Mutant receptor has oncogenic potential Thyroid abnormalities and hepatocellular carcinoma in mice transgenic for v-erbA C Barlow, B Meister, M Lardelli, U Lendahl and B Vennstrom Department of Cell and Molecular Biology, Karolinska Institute, Stockholm, Sweden. The EMBO Journal Vol 13, pp , 1994
Thyroid hormone receptor (c-erbA) in cancer 1.Retroviral v-erbA oncogene that contributes to the appearance of erythroleukemia and sarcomas in bird. (Sap. et al., 1986 ) 4. v-erbA encodes a highly mutanted chicken TRα1 protein that does not bonding T3 and constitutive repressor of TR genes. (Yen et al.,1994) ｌ 2. v-erbA transgenic mice develop hepatocellular carcinomas. (Barlow et al., 1994) 3.A knockin mutant mouse by targeting a mutation (PV) into TRβ gene, spontaneously develop follicular thyroid carcinoma. (Kato et al., 2004) (c-erbA)
4. The evidence of alterations that affect TR genes, their expression and the activity of their protein products in human cancer. These alterations include: ● Deletion ● Gene rearrangement ● Hypermethylation ● Aberrant splicing ● Point mutation (dominant negative) ● Overexpression of inhibitory variant (Jose et al., 2003)
Aim To determine the role of thyroid hormone receptor (TR) during hepato- carcinogenesis
Pituitary tumor-transforming Gene 1 1. PTTG1 is characterized proto-oncogene that was originally cloned from a rat pituitary tumor. (Dominguez et al.1998) 2. hPTTG1 cDNA encodes a protein of 202 amino acids. 3. PTTG1 is highly expressed in various human tumors, including pituitary, adrenal, kidney, liver, and ovarian tumors. ( Molec. Endocr. 13: , 1999)
Reduce expression or complete loss of normal TR function (Lin et al, 1999) Out of control in T3-Regulated Gene Hepatocellular carcinoma Overexpression of PTTG1/Furin Hypothesis
Effect of T3 on PTTG1 protein expression in HepG2 cell lines
To clarify whether Sp family does indeed activated the PTTG1 promoter
Sp1 activates PTTG1 transcription by directly binding to the Sp1 sites located between -620 and -512 bp
T3 mediates repressed Sp1 by specifically decreasing the PTTG1 expression in HepG2-TRα1 cells
The PTTG1 was regulated by TR through Sp1 in human HCC specimens 29/107 (27.1%) Linear regression analysis (T/N ratios)
Summary 1. PTTG1 gene was mediated by Sp1 and indirectly down-regulated by T3. 2. T3/TR plays a suppressor role in hepatocarcinogenesis. Cancer Research :68(6), , 2008
Furin Furin is a proprotein convertases that process latent precursor proteins into their biologically active products. Furin substrates are: proparathyroid hormone, transforming growth factor beta 1 precursor, proalbumin, pro-beta-secretase, membrane type-1 matrix metalloproteinase, PDGF, IGF1, beta subunit of pro-nerve growth factor. This gene is thought to play a role in tumor progression.
Activation of furin by T3 in HepG2 cell lines mRNA protein
The experimental protocol
Furin over-expression induced by T3 o r TGF-β enhances HepG2-TRα1 cell invasion in SCID mice
Co-operative model for T3-and TGFβ-activated furin gene expression
Conclusion T3 directly regulates furin gene expression, thereby enhancing its invasiveness. Endocrinology, 149, 3817, 2008.
References and Reviews Brent GA: Mechanisms of disease: The molecular basis of thyroid hormone action. New Eng J Med 331: , Tsai M, O'Malley BW: Molecular mechanisms of action of steroid/thyroid receptor superfamily members. Annu Rev Biochem 63: , Zhang J, Lazer MA: The mechanism of action of thyroid hormones. Annu Rev Physiol 62: , Yen,PM Physiology and molecular basis of thyroid hormone action Physiological Review 81, Annu. Rev. Physiol :315–60 Annu. Rev. Physiol :309–33