Receptors Regulating Gene Expression (Nuclear Receptors)

Receptors Regulating Gene Expression [Transducer Mechanism]
Control of Gene Transcription.

1. Pharmacodynamics
Presented by: Megh Vithalkar
Roll No.: 07
M.PharmSemester-1 2020.
Department of Pharmacology.
Goa college of Pharmacy, Panaji-Goa.

2. SET-2
TRANSDUCER MECHANISMS
“Receptors regulating gene
expression (Transcription factors)”
Megh Vithalkar [M.Pharm Sem-1] (Goa College of Pharmacy)
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3. What areReceptors regulating gene
expression?
These are intracellular (cytoplasmic or nuclear) soluble
proteins which respond to lipid soluble chemical messengers
that penetrate the cells.
The receptor protein (specific for each hormone/ regulator) is
inherently capable of binding to specific genes, but is kept
inhibited till the hormone binds near its carboxy-terminus and
exposes the DNA binding regulatory segment located in the
middle of the molecule.
Megh Vithalkar [M.Pharm Sem-1] (Goa College of Pharmacy)
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4. History of Nuclear Receptors:
1958–Elwood Jensen –isolated the oestrogen
receptor.In 2004 he received the Albert LaskerAward
for Basic Medical Research for his research on these
oestrogen receptors.
1980s–cloning of the oestrogen, glucocorticoid, and
thyroid hormone receptorsby Pierre Chambon,
Ronald Evans, and BjörnVennströmrespectively
Megh Vithalkar [M.Pharm Sem-1] (Goa College of Pharmacy)
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5. Nuclear Receptors:
Nuclear receptors (NRs) belong to a large superfamily
that are ligand activated intracellular transcription
factors which up or down regulate the expression of
several genes.
Nuclear receptors are soluble proteins that can bind
to specific DNA regulatory elements(response
elements or REs) and act as cell type and promoter-
specific regulators of transcription.
Megh Vithalkar [M.Pharm Sem-1] (Goa College of Pharmacy)
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6. In contrast to other transcription factors, the activity
of nuclear receptors can be modulated by binding
to the corresponding ligands, small lipophilic
molecules that easily penetrate biological
membranes.
Nuclear receptors may be classified either according
to activation mechanism(type I or II), or sequence
homology(NR subfamilies 0-6).
Nowadays, there are more than 350 NR structures in
the Protein Data Bank.
Megh Vithalkar [M.Pharm Sem-1] (Goa College of Pharmacy)
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7. “Endogenous Ligands for Nuclear receptors”
Megh Vithalkar [M.Pharm Sem-1] (Goa College of Pharmacy)
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These are Lipophilic substances such as endogenous hormones.

8. Structure of Nuclear Receptors:
Megh Vithalkar [M.Pharm Sem-1] (Goa College of Pharmacy)
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Atypical nuclear receptor contains the following domains, with corresponding functions:

9. Structure of Nuclear Receptors:
The N-terminal domain displays the most heterogeneity.
It harbours the AF1 (activation function 1) site that binds to
other cell specific transcription factors in a ligand-
independent way and modifies the binding or activity of the
receptor itself.
Activation Function-1, placed in the N-terminus region is an
important domain for the transcriptional activationof nuclear
receptors is the ligand-independent activation function (AF-1),
which generally resides in the N-terminal region of nuclear
receptors.
AF-1 functions in a promoter-context and/or cell-type specific
manner and cooperates with AF-2 in the regulation of gene
transcription.
Megh Vithalkar [M.Pharm Sem-1] (Goa College of Pharmacy)
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10. Alternative splicing of genes may yield several receptor
isoforms each with slightly different N-terminal regions.
The core domain of the receptor is highly conserved and
consists of the structure responsible for DNA recognition and
binding (DBD).
At the molecular level, this comprises two zinc fingers-cysteine-
(or cysteine-/histidine-)rich loops in the amino acid chain that
are held in a particular conformation by zinc ions.
The main function of this portion of the molecule is to recognise
and bind to the hormone response elements located in genes
that are regulated by this family of receptors, but it also plays a
part in regulating receptor dimerization as well.
Megh Vithalkar [M.Pharm Sem-1] (Goa College of Pharmacy)
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11. It is the highly flexible hinge region in the molecule that allows
it to dimerise with other NRs and also to exhibit DNA binding in
a variety of configurations.
Finally,the C-terminal domain contains the ligand binding
module and is specific to each class of receptor.
A highly conserved AF2 region is important in ligand-
dependent activation.
The ligand-dependent activation function (AF-2) is the key
region for NR ligand interaction. AF-2 makes the function of
agonist/antagonist ligand response, changing the LBD
conformation and regulating the gene transcription,
according to each case and with participation of other
coregulatory elements.
Megh Vithalkar [M.Pharm Sem-1] (Goa College of Pharmacy)
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12. CLASSIFICATION OF NUCLEAR RECEPTORS:
The NR superfamily consists of two main classes (I and II),
together with a third that shares some of the
characteristics of both.
Class I NRsconsists largely of receptors for the steroid
hormones, including the glucocorticoid and
mineralocorticoid receptors (GR and MR), as well as the
oestrogen, progesterone and androgen receptors (ER, PR
and AR, respectively).
Megh Vithalkar [M.Pharm Sem-1] (Goa College of Pharmacy)
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13. These ligandedreceptors generally form homodimers
and translocate to the nucleus, where they can trans-
activate or trans-repress genes by binding to ‘positive’ or
‘negative’ hormone response elements.
Large numbers of genes can be regulated in this way by
a single ligand. For example, it is estimated that the
activated GR itself can regulate transcription of ~1% of
the genome either directly or indirectly.
Megh Vithalkar [M.Pharm Sem-1] (Goa College of Pharmacy)
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14. Class II NRs function in a slightly different way. Their
ligands are generally lipids already present to some
extent within the cell.
This group includes the peroxisome proliferator-activated
receptor(PPAR) that recognises fatty acids; the liver
oxysterol receptor (LXR) that recognises and acts as a
cholesterol sensor, the farnesoid(bile acid) receptor
(FXR), a xenobiotic receptor (SXR; in rodents the PXR)
that recognises a great many foreign substances,
including therapeutic drugs, and the constitutive
androstanereceptor(CAR), which not only recognises
the steroid androstanebut also some drugs such as
phenobarbital.
Megh Vithalkar [M.Pharm Sem-1] (Goa College of Pharmacy)
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15. Unlike the receptors in class I, these NRs almost always
operate as heterodimers together with the retinoid
receptor (RXR).
They tend to mediate positive feedback effects (e.g.
occupation of the receptor amplifies rather than inhibits a
particular biological event).
When class II monomeric receptors bind to RXR, two types
of heterodimer may be formed: a non-permissive
heterodimer, which can be activated only by the RXR
ligand itself, and the permissive heterodimer, which can
be activated either by retinoic acid itself or by its
partner’s ligand.
Megh Vithalkar [M.Pharm Sem-1] (Goa College of Pharmacy)
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16. A third group of NRs is really a subgroup of class II in the
sense that they form obligate heterodimers with RXR, but
rather than sensing lipids, they play a part in endocrine
signalling.
The group includes the thyroid hormone receptor(TR),
the vitamin D receptor(VDR) and the retinoic acid
receptor (RAR).
Megh Vithalkar [M.Pharm Sem-1] (Goa College of Pharmacy)
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17. Revision
Megh Vithalkar [M.Pharm Sem-1] (Goa College of Pharmacy)
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18. CONTROL OF GENE TRANSCRIPTION:
Hormone response elements are the short (four or five base
pairs) sequences of DNA to which the NRs bind to modify
gene transcription.
They are usually present symmetrically in pairs or half sites,
although these may be arranged together in different ways
(e.g. simple repeats or inverted repeats).
Each NR exhibits a preference for a particular consensus
sequence but because of the family homology, there is a
close similarity between these sequences.
Once in the nucleus, the ligand-bound receptor recruits
further proteins including co-activators or co-repressors to
modify gene expression through its AF1 and AF2 domains.
Megh Vithalkar [M.Pharm Sem-1] (Goa College of Pharmacy)
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19. Some of these co-activatorsare enzymes involved in chromatin
remodelling such as histone acetylase/deacetylase which,
together with other enzymes, regulate the unravelling of the
DNA to facilitate access by polymerase enzymes and hence
gene transcription.
Co-repressorcomplexes are recruited by some receptors and
comprise histone deacetylase and other factors that cause the
chromatin to become tightly packed, preventing further
transcriptional activation.
Some un-ligandedclass II receptors such as TR and VDR are
constitutively bound to these repressor complexes in the
nucleus, thus ‘silencing’ the gene. The complex dissociates on
ligand binding, permitting an activator complex to bind.
Megh Vithalkar [M.Pharm Sem-1] (Goa College of Pharmacy)
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20. “Nuclear receptors are key elements for
control of gene expression”
Megh Vithalkar [M.Pharm Sem-1] (Goa College of Pharmacy)
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21. Megh Vithalkar [M.Pharm Sem-1] (Goa College of Pharmacy)
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The glucocorticoid(G)
penetrates the cell
membrane and binds to
the glucocorticoid
receptor (GR) protein that
normally resides in the
cytoplasm in association
with heat shock protein 90
(HSP90) + other proteins.
The GR has a steroid
binding domain near the
carboxyterminus and a
mid region DNA binding
domainjoined by a ‘hinge
region’.

22. Megh Vithalkar [M.Pharm Sem-1] (Goa College of Pharmacy)
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The DNA binding domain
has two ‘zinc fingers’, each
made up of a loop of
amino acids with chelated
zinc ion.
Binding of the steroid to GR
dissociates the complexed
proteins (HSP90, etc)
removing their inhibitory
influence on it.
A dimerization region that
overlaps the steroid binding
domain is exposed,
promoting dimerization of
the occupied receptor.

23. Megh Vithalkar [M.Pharm Sem-1] (Goa College of Pharmacy)
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The steroid bound receptor
dimer translocatesto the
nucleus, binds
coactivator/corepressor
proteins and interacts with
specific DNA sequences
called ‘glucocorticoid
responsive elements’
(GREs) within the regulatory
region of appropriate
genes.
The expression of these
genes is consequently
altered resulting in
promotion (or suppression)
of their transcription.

24. Megh Vithalkar [M.Pharm Sem-1] (Goa College of Pharmacy)
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The specific mRNAthus
produced is directed to
the ribosome where the
message is translated
into a specific pattern
of protein synthesis,
which in turn modifies
cell function.

25. Scientific Interests:
Nuclear receptors are
transcription factors involved in
such important physiological
functions as control of embryonic
development, organ physiology,
cell differentiation and
homeostasis.
Due to the role of nuclear
receptors in gene expression
control, members of this family
are suitable targets for new drug
development.
Megh Vithalkar [M.Pharm Sem-1] (Goa College of Pharmacy)
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26. Megh Vithalkar [M.Pharm Sem-1] (Goa College of Pharmacy)
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27. References:
RANG AND DALE’S Pharmacology, 7th Edition,
pp:40-43.
Essentials of Medical Pharmacology, K.D Tripathi,
7th Edition, pp:50-52.
https://www.google.com/url?sa=t&source=web&rc
t=j&url=http://sbi.imim.es/web/files/projects/maste
r/2006/nuclear_receptors.pdf
Megh Vithalkar [M.Pharm Sem-1] (Goa College of Pharmacy)
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28. End of Set-2
Megh Vithalkar [M.Pharm Sem-1] (Goa College of Pharmacy)
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29. THANK YOU
Megh Vithalkar [M.Pharm Sem-1] (Goa College of Pharmacy)
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