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Nuclear receptor

Mechanism nuclear receptor action. This figure depicts the mechanism of a class I nuclear receptor (NR) which, in the absence of ligand, is located in the cytosol. Hormone binding to the NR triggers dissociation of heat shock proteins (HSP), dimerization, and translocation to the nucleus where the NR binds to a specific sequence of DNA known as a hormone response element (HRE). The nuclear receptor DNA complex in turn recruits other proteins that are responsible for transcription of downstream DNA into mRNA which is eventually translated into protein which results in a change in cell function.
Structures of selected endogenous nuclear receptor ligands and the name of the receptor that each binds to.

In the field of molecular biology, nuclear receptors are a class of proteins found within the interior of cells that are responsible for sensing the presence of steroid and thyroid hormones and certain other molecules. In response, these receptors work in concert with other proteins to regulate the expression of specific genes thereby controlling the development, homeostasis, and metabolism of the organism.

Nuclear receptors have the ability to directly bind to DNA and regulate the expression of adjacent genes, hence these receptors are classified as transcription factors.[1][2] The regulation of gene expression by nuclear receptors only happens when a ligand—a molecule which affects the receptor's behavior—is present. More specifically, ligand binding to a nuclear receptor results in a conformational change in the receptor which in turn activates the receptor resulting in up-regulation of gene expression.

A unique property of nuclear receptors which differentiate them from other classes of receptors is their ability to directly interact with and control the expression of genomic DNA. Consequently nuclear receptors play key roles in both embryonic development and adult homeostasis. As discussed in more detail below, nuclear receptors may be classified either according to mechanism[3][4] or homology.[5][6]

Contents

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[edit] Species distribution

Nuclear receptors are specific to metazoans (animals) and are not found in protists, algae, fungi, or plants.[7] Among animal species, they are not found in sponges, but they are present in cnidarians and all other more advanced animals.[7] There are 270 nuclear receptors in C. elegans alone.[8] Humans, mice, and rats have respectively 48, 49, and 47 nuclear receptors each.[9]

[edit] Ligands

Ligands that bind to and activate nuclear receptors include lipophilic substances such as endogenous hormones, vitamins A and D, and xenobiotic endocrine disruptors. Because the expression of a large number of genes is regulated by nuclear receptors, ligands that activate these receptors can have profound effects on the organism. Many of these regulated genes are associated with various diseases which explains why the molecular targets of approximately 13% of U.S. Food and Drug Administration (FDA) approved drugs are nuclear receptors.[10]

A number of nuclear receptors, referred to as orphan receptors,[11] have no known (or at least generally agreed upon) endogenous ligands. Some of these receptors such as FXR, LXR, and PPAR bind a number of metabolic intermediates such as fatty acids, bile acids and/or sterols with relatively low affinity. These receptors hence may function as metabolic sensors. Other nuclear receptors, such as CAR and PXR appear to function as xenobiotic sensors up-regulating the expression of cytochrome P450 enzymes that metabolize these xenobiotics.[12]

[edit] Structure

Structural Organization of Nuclear Receptors
Top – Schematic 1D amino acid sequence of a nuclear receptor.
Bottom – 3D structures of the DBD (bound to DNA) and LBD (bound to hormone) regions of the nuclear receptor. The structures shown are of the estrogen receptor. Experimental structures of N-terminal domain (A/B), hinge region (D), and C-terminal domain (F) have not been determined therefore are represented by red, purple, and orange dashed lines respectively.

Nuclear receptors are modular in structure and contain the following domains:[13][14]

DNA binding domain (DBD)
Identifiers
Symbol zf-C4
Pfam PF00105
InterPro IPR001628
PROSITE PDOC00031
SCOP 1hra
Ligand-binding domain (LBD)
Identifiers
Symbol Hormone_recep
Pfam PF00104
InterPro IPR000536
SCOP 1lbd
  • A-B) N-terminal regulatory domain: Contains the activation function 1 (AF-1) whose action is independent of the presence of ligand.[15] The transcriptional activation of AF-1 is normally very weak, but it does synergize with AF-2 in the E-domain (see below) to produce a more robust upregulation of gene expression. The A-B domain is highly variable in sequence between various nuclear receptors.
  • C) DNA-binding domain (DBD): Highly conserved domain containing two zinc fingers which binds to specific sequences of DNA called hormone response elements (HRE).
  • D) Hinge region: Thought to be a flexible domain which connects the DBD with the LBD. Influences intracellular trafficking and subcellular distribution.
  • E) Ligand binding domain (LBD): Moderately conserved in sequence and highly conserved in structure between the various nuclear receptors. The structure of the LBD is referred to as an alpha helical sandwich fold in which three anti parallel alpha helices (the "sandwich filling") are flanked by two alpha helices on one side and three on the other (the "bread"). The ligand binding cavity is within the interior of the LBD and just below three anti parallel alpha helical sandwich "filling". Along with the DBD, the LBD contributes to the dimerization interface of the receptor and in addition, binds coactivator and corepressor proteins. The LBD also contains the activation function 2 (AF-2) whose action is dependent on the presence of bound ligand.[15]
  • F) C-terminal domain: Variable in sequence between various nuclear receptors.


[edit] Mechanism of action

Mechanism nuclear receptor action. This figure depicts the mechanism of a class II nuclear receptor (NR) which, regardless of ligand binding status is located in the nucleus bound to DNA. For the purpose of illustration, the nuclear receptor shown here is the thyroid hormone receptor (TR) heterodimerized to the RXR. In the absence of ligand, the TR is bound to corepressor protein. Ligand binding to TR causes a dissociation of corepressor and recruitment of coactivator protein which in turn recruit additional proteins such as RNA polymerase that are responsible for transcription of downstream DNA into RNA and eventually protein which results in a change in cell function.

Nuclear receptors (NRs) may be classified into two broad classes according to their mechanism of action and subcellular distribution in the absence of ligand.

Small lipophilic substances such as natural hormones diffuse past the cell membrane and bind to nuclear receptors located in the cytosol (type I NR) or nucleus (type II NR) of the cell. This causes a change in the conformation of the receptor which depending on the mechanistic class (type I or II), triggers a number of down stream events that eventually results in up or down regulation of gene expression.

Accordingly, nuclear receptors may be subdivided into the following four mechanistic classes:[3][4]

[edit] Type I

Ligand binding to type I nuclear receptors in the cytosol (includes members of the NR subfamily 3) results in the dissociation of heat shock proteins, homo-dimerization, translocation (i.e., active transport) from the cytoplasm into the cell nucleus, and binding to specific sequences of DNA known as hormone response elements (HRE's). Type I nuclear receptors bind to HREs consisting of two half sites separated by a variable length of DNA and the second half site has a sequence inverted from the first (inverted repeat).

The nuclear receptor/DNA complex then recruits other proteins which transcribe DNA downstream from the HRE into messenger RNA and eventually protein which causes a change in cell function.

[edit] Type II

Type II receptors (principally NR subfamily 1) in contrast are retained in the nucleus regardless of the ligand binding status and in addition bind as hetero-dimers (usually with RXR) to DNA. In the absence of ligand, type II nuclear receptors are often complexed with corepressor proteins. Ligand binding to the nuclear receptor causes dissociation of corepressor and recruitment of coactivator proteins. Additional proteins including RNA polymerase are then recruited to the NR/DNA complex which transcribe DNA into messenger RNA.

[edit] Type III

Type III nuclear receptors (principally NR subfamily 2) are similar to type I receptors in that both classes bind to DNA as homodimers. However, type III nuclear receptors, in contrast to type I, bind to direct repeat instead of inverted repeat HREs.

[edit] Type IV

Type IV nuclear receptors bind either as monomers or dimers, but only a single DNA binding domain of the receptor binds to a single half site HRE. Examples of type IV receptors are found in most of the NR subfamilies.

[edit] Coregulatory proteins

See also nuclear receptor coregulators

Nuclear receptors bound to hormone response elements recruit a significant number of other proteins (referred to as transcription coregulators) which facilitate or inhibit the transcription of the associated target gene into mRNA.[16][17] The function of these coregulators are varied and include chromatin remodeling (making the target gene either more or less accessible to transcription) or a bridging function to stabilize the binding of other coregulatory proteins.

[edit] Coactivators

Binding of agonist ligands (see section below) to nuclear receptors induces a conformation of the receptor that preferentially binds coactivator proteins. These proteins often have an intrinsic histone acetyltransferase (HAT) activity which weakens the association of histones to DNA, and therefore promotes gene transcription.

[edit] Corepressors

Binding of antagonist ligands to nuclear receptors in contrast induces a conformation of the receptor that preferentially binds corepressor proteins. These proteins in turn recruit histone deacetylases (HDACs) which strengthens the association of histones to DNA, and therefore represses gene transcription.

[edit] Agonism vs antagonism

Stuctural basis for the mechanism of nuclear receptor agonist and antagonist action.[18] The structures shown here are of the ligand binding domain (LBD) of the estrogen receptor (green cartoon diagram) complexed with either the agonist diethylstilbestrol (top, PDB 3ERD) or antagonist 4-hydroxytamoxifen (bottom, 3ERT). The ligands are depicted as space filling spheres (white = carbon, red = oxygen). When an agonist is bound to a nuclear receptor, the C-terminal alpha helix of the LDB (H12; light blue) is positioned such that a coactivator protein (red) can bind to the surface of the LBD. Shown here is just a small part of the coactivator protein, the so called NR box containing the LXXLL amino acid sequence motif.[19] Antagonists occupy the same ligand binding cavity of the nuclear receptor. However antagonist ligands in addition have a sidechain extension which sterically displaces H12 to occupy roughly the same position in space as coactivators bind. Hence coactivator binding to the LBD is blocked.

Depending on the receptor involved, the chemical structure of the ligand and the tissue that is being affected, nuclear receptor ligands may display dramatically diverse effects ranging in a spectrum from agonism to antagonism to inverse agonism.[20]

[edit] Agonists

The activity of endogenous ligands (such as the hormones estradiol and testosterone) when bound to their cognate nuclear receptors is normally to upregulate gene expression. This stimulation of gene expression by the ligand is referred to as an agonist response. The agonistic effects of endogenous hormones can also be mimicked by certain synthetic ligands, for example, the glucocorticoid receptor antiiflammatory drug dexamethasone. Agonist ligands work by inducing a conformation of the receptor which favors coactivator binding (see upper half of the figure to the right).

[edit] Antagonists

Other synthetic nuclear receptor ligands have no apparent effect on gene transcription in the absence of endogenous ligand. However they block the effect of agonist through competitive binding to the same binding site in the nuclear receptor. These ligands are referred to as antagonists. An example of antagonistic nuclear receptor drug is mifepristone which binds to the glucocorticoid and progesterone receptors and therefore blocks the activity of the endogenous hormones cortisol and progesterone respectively. Antagonist ligands work by inducing a conformation of the receptor which prevents coactivator and promotes corepressor binding (see lower half of the figure to the right).

[edit] Inverse agonists

Finally, some nuclear receptors promote a low level of gene transcription in the absence of agonists (also referred to as basal or constitutive activity). Synthetic ligands which reduce this basal level of activity in nuclear receptors are known as inverse agonists.[21]

[edit] Selective receptor modulators

A number of drugs that work through nuclear receptors display an agonist response in some tissues and an antagonistic response in other tissues. This behavior may have substantial benefits since it may allow retaining the desired beneficial therapeutic effects of a drug while minimizing undesirable side effects. Drugs with this mixed agonist/antagonist profile of action are referred to as selective receptor modulators (SRMs). Examples include Selective Estrogen Receptor Modulators (SERMs) and Selective Progesterone Receptor Modulators (SPRMs). The mechanism of action of SRMs may vary depending on the chemical structure of the ligand and the receptor involved, however it is thought that many SRMs work by promoting a conformation of the receptor that is closely balanced between agonism and antagonism. In tissues where the concentration of coactivator proteins is higher than corepressors, the equilibrium is shifted in the agonist direction. Conversely in tissues where corepressors dominate, the ligand behaves as an antagonist.[22]

[edit] Alternative mechanisms

[edit] Transrepression

The most common mechanism of nuclear receptor action involves direct binding of the nuclear receptor to a DNA hormone response element. This mechanism is referred to as transactivation. However some nuclear receptors not only have the ability to directly bind to DNA, but also to other transcription factors. This binding often results in deactivation of the second transcription factor in a process known as transrepression.[23]

[edit] Non-genomic

The classical direct effects of nuclear receptors on gene regulation normally takes hours before a functional effect is seen in cells because of the large number of intermediate steps between nuclear receptor activation and changes in protein expression levels. However it has been observed that some effects from the application of hormones such as estrogen occur within minutes which is inconsistent with the classical mechanism of nuclear receptor action. While the molecular target for these non-genomic effects of nuclear receptors has not been conclusively demonstrated, it has been hypothesized that there are variants of nuclear receptors which are membrane associated instead of being localized in the cytosol or nucleus. Furthermore these membrane associated receptors function through alternative signal transduction mechanisms not involving gene regulation.[24][25]

[edit] Family members

The following is a list of the 48 known human nuclear receptors[26] categorized according to sequence homology.[5][6] The list is organized as follows:

Subfamily: name

Group: name (endogenous ligand if common to entire group)
Member: name (abbreviation; NRNC Symbol[5], gene) (endogenous ligand)
Phylogenetic tree of human nuclear receptors

[edit] Subfamily 1: Thyroid Hormone Receptor-like

[edit] Subfamily 2: Retinoid X Receptor-like

[edit] Subfamily 3: Estrogen Receptor-like

See also steroid and sex hormone receptors

[edit] Subfamily 4: Nerve Growth Factor IB-like

  • Group A: NGFIB/NURR1/NOR1

[edit] Subfamily 5: Steroidogenic Factor-like

[edit] Subfamily 6: Germ Cell Nuclear Factor-like

  • Group A: GCNF

[edit] Subfamily 0: Miscellaneous

  • Group B: DAX/SHP
    • 1: Dosage-sensitive sex reversal, adrenal hypoplasia critical region, on chromosome X, gene 1 (DAX1, NR0B1)
    • 2: Small heterodimer partner (SHP; NR0B2)
  • Group C: Nuclear receptors with two DNA binding domains (2DBD-NR) (A novel subfamily)[27][28]

[edit] History

Below is a brief selection of key events in the history of nuclear receptor research.[29]

[edit] See also

[edit] References

  1. ^ Evans RM (1988). "The steroid and thyroid hormone receptor superfamily". Science 240 (4854): 889–95. doi:10.1126/science.3283939. PMID 3283939. 
  2. ^ Olefsky JM (2001). "Nuclear receptor minireview series". J. Biol. Chem. 276 (40): 36863–4. doi:10.1074/jbc.R100047200. PMID 11459855. 
  3. ^ a b Mangelsdorf DJ, Thummel C, Beato M, Herrlich P, Schutz G, Umesono K, Blumberg B, Kastner P, Mark M, Chambon P, Evans RM (1995). "The nuclear receptor superfamily: the second decade". Cell 83 (6): 835–9. doi:10.1016/0092-8674(95)90199-X. PMID 8521507. 
  4. ^ a b Novac N, Heinzel T (2004). "Nuclear receptors: overview and classification". Curr Drug Targets Inflamm Allergy 3 (4): 335–46. doi:10.2174/1568010042634541. PMID 15584884. http://www.ingentaconnect.com/content/ben/cdtia/2004/00000003/00000004/art00002. 
  5. ^ a b c Nuclear Receptors Nomenclature Committee (1999). "A unified nomenclature system for the nuclear receptor superfamily". Cell 97 (2): 161–3. doi:10.1016/S0092-8674(00)80726-6. PMID 10219237. 
  6. ^ a b Laudet V (1997). "Evolution of the nuclear receptor superfamily: early diversification from an ancestral orphan receptor". J. Mol. Endocrinol. 19 (3): 207–26. doi:10.1677/jme.0.0190207. PMID 9460643. 
  7. ^ a b Escriva H, Langlois MC, Mendonça RL, Pierce R, Laudet V (May 1998). "Evolution and diversification of the nuclear receptor superfamily". Annals of the New York Academy of Sciences 839: 143–6. doi:10.1111/j.1749-6632.1998.tb10747.x. PMID 9629140. 
  8. ^ Sluder AE, Maina CV (April 2001). "Nuclear receptors in nematodes: themes and variations". Trends in Genetics : TIG 17 (4): 206–13. doi:10.1016/S0168-9525(01)02242-9. PMID 11275326. 
  9. ^ Zhang Z, Burch PE, Cooney AJ, Lanz RB, Pereira FA, Wu J, Gibbs RA, Weinstock G, Wheeler DA (April 2004). "Genomic analysis of the nuclear receptor family: new insights into structure, regulation, and evolution from the rat genome". Genome Research 14 (4): 580–90. doi:10.1101/gr.2160004. PMID 15059999. 
  10. ^ Overington JP, Al-Lazikani B, Hopkins AL (2006). "How many drug targets are there?". Nature reviews. Drug discovery 5 (12): 993–6. doi:10.1038/nrd2199. PMID 17139284. 
  11. ^ Benoit G, Cooney A, Giguere V, Ingraham H, Lazar M, Muscat G, Perlmann T, Renaud JP, Schwabe J, Sladek F, Tsai MJ, Laudet V (2006). "International Union of Pharmacology. LXVI. Orphan nuclear receptors". Pharmacol. Rev. 58 (4): 798–836. doi:10.1124/pr.58.4.10. PMID 17132856. 
  12. ^ Mohan R, Heyman RA (2003). "Orphan nuclear receptor modulators". Curr Top Med Chem 3 (14): 1637–47. doi:10.2174/1568026033451709. PMID 14683519. 
  13. ^ Kumar R, Thompson EB (1999). "The structure of the nuclear hormone receptors". Steroids 64 (5): 310–9. doi:10.1016/S0039-128X(99)00014-8. PMID 10406480. 
  14. ^ Klinge CM (2000). "Estrogen receptor interaction with co-activators and co-repressors". Steroids 65 (5): 227–51. doi:10.1016/S0039-128X(99)00107-5. PMID 10751636. 
  15. ^ a b Wärnmark A, Treuter E, Wright AP, Gustafsson J-Å (2003). "Activation functions 1 and 2 of nuclear receptors: molecular strategies for transcriptional activation". Mol. Endocrinol. 17 (10): 1901–9. doi:10.1210/me.2002-0384. PMID 12893880. 
  16. ^ Glass CK, Rosenfeld MG (2000). "The coregulator exchange in transcriptional functions of nuclear receptors". Genes Dev 14 (2): 121–41. doi:10.1101/gad.14.2.121 (inactive 2008-12-06). PMID 10652267. 
  17. ^ Aranda A, Pascual A (2001). "Nuclear hormone receptors and gene expression" (abstract). Physiol. Rev. 81 (3): 1269–304. PMID 11427696. http://physrev.physiology.org/cgi/content/abstract/81/3/1269. 
  18. ^ Brzozowski AM, Pike AC, Dauter Z, Hubbard RE, Bonn T, Engström O, Öhman L, Greene GL, Gustafsson J-Å, Carlquist M (1997). "Molecular basis of agonism and antagonism in the oestrogen receptor". Nature 389 (6652): 753–8. doi:10.1038/39645. PMID 9338790. 
  19. ^ Shiau AK, Barstad D, Loria PM, Cheng L, Kushner PJ, Agard DA, Greene GL (1998). "The structural basis of estrogen receptor/coactivator recognition and the antagonism of this interaction by tamoxifen". Cell 95 (7): 927–37. doi:10.1016/S0092-8674(00)81717-1. PMID 9875847. 
  20. ^ Gronemeyer H, Gustafsson JA, Laudet V (2004). "Principles for modulation of the nuclear receptor superfamily". Nature reviews. Drug discovery 3 (11): 950–64. doi:10.1038/nrd1551. PMID 15520817. 
  21. ^ Busch BB, Stevens WC, Martin R, et al. (2004). "Identification of a selective inverse agonist for the orphan nuclear receptor estrogen-related receptor alpha". J. Med. Chem. 47 (23): 5593–6. doi:10.1021/jm049334f. PMID 15509154. 
  22. ^ Smith CL, O'Malley BW (2004). "Coregulator function: a key to understanding tissue specificity of selective receptor modulators". Endocr Rev 25 (1): 45–71. doi:10.1210/er.2003-0023. PMID 14769827. 
  23. ^ Pascual G, Glass CK (2006). "Nuclear receptors versus inflammation: mechanisms of transrepression". Trends Endocrinol Metab 17 (8): 321–7. doi:10.1016/j.tem.2006.08.005. PMID 16942889. 
  24. ^ Björnström L, Sjöberg M (2004). "Estrogen receptor-dependent activation of AP-1 via non-genomic signalling". Nucl Recept 2 (1): 3. doi:10.1186/1478-1336-2-3. PMID 15196329. 
  25. ^ Zivadinovic D, Gametchu B, Watson CS (2005). "Membrane estrogen receptor-alpha levels in MCF-7 breast cancer cells predict cAMP and proliferation responses". Breast Cancer Res. 7 (1): R101–12. doi:10.1186/bcr958. PMID 15642158. 
  26. ^ Zhang Z, Burch PE, Cooney AJ, Lanz RB, Pereira FA, Wu J, Gibbs RA, Weinstock G, Wheeler DA (2004). "Genomic analysis of the nuclear receptor family: new insights into structure, regulation, and evolution from the rat genome". Genome Res 14 (4): 580–90. doi:10.1101/gr.2160004. PMID 15059999. 
  27. ^ Wu W, Niles EG, El-Sayed N, Berriman M, LoVerde PT (2006). "Schistosoma mansoni (Platyhelminthes, Trematoda) nuclear receptors: sixteen new members and a novel subfamily". Gene 366 (2): 303–15. doi:10.1016/j.gene.2005.09.013. PMID 16406405. 
  28. ^ Wu W, Niles EG, Hirai H, LoVerde PT (2004). "Evolution of a novel subfamily of nuclear receptors with members that each contain two DNA binding domains". BMC Evol Biol 7 (Feb 23): 27. doi:10.1186/1471-2148-7-27. PMID 17319953. 
  29. ^ Tata JR (2005). "One hundred years of hormones". EMBO Rep. 6 (6): 490–6. doi:10.1038/sj.embor.7400444. PMID 15940278. 

[edit] External links


Retrieved from "http://en.wikipedia.org/wiki/Nuclear_receptor"

 

The content of this section is licensed under the GNU Free Documentation License (local copy). It uses material from the Wikipedia article "Nuclear receptor" modified November 23, 2009 with previous authors listed in its history.

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