By Dr. Armaan Singh

NUCLEAR RECEPTORS FOR DEVELOPING NEW DRUG THERAPY

RECEPTOR :( Latin: means a receiver)

DEFINITION: A macromolecular component

of the organism that binds the drug

and initiates its effect.

To elicit the pharmacological

action , the functional group of the

drug have to interact with the

complementary chemical groupings

of the biologically important integral part of the organism is called receptor.

Functions of Receptors:

Receptors regulates the:-

1.Enzyme activity

2. Permeability/Transport

3. Structural features

4. Template function

5. Homoeostasis

6. Cellular regulatory signals etc

Historical background of receptor:

The concept of receptor is first established in 1878 by John N. Langley demonstrating the interaction of atropine and pilocarpine with cellular component to elicit the action.

Paul Ehrlich coined the cellular component as receptive substance or receptor in 1907.

John Newport Langley(1856-1926)An English Physiologist

Paul Ehrlich (1854-1915)A German Microbiologist

Types of RECEPTORS:

1. Cell surface receptors I. Ligand gated ion Channels)

eg: Nicotinic Cholinergic, GABAa II. G-protein coupled receptors.

eg: Muscarinic, β-adrenergic, DopamineD22. Enzyme linked receptors.

eg: Tyrosine Protein kinase receptor3. Nuclear receptors.

eg:GR receptor, AR receptor, PPAR

How does Cell surface Receptor elicits its action:

Cell Membrane

Unbound Endogenous Activator (Agonist) of Receptor

Inactive Cell Surface Receptor

Extracellular Compartment

Intracellular Compartment

N.B : Receptors are Present in equilibrium state of active and inactive conformation.

Agonists binds with the active conformations.

Extracellular Compartment

Intracellular Compartment

Cell Membrane

Bound Endogenous Activator (Agonist) of Receptor

Active Cell Surface Receptor

Cellular Response

Cell Membrane

Displaced Endogenous Activator (Agonist) of Receptor

Inactive Cell Surface Receptor

Extracellular Compartment

Intracellular Compartment

Bound Antagonist of Receptor (Drug)

N.B: Antagonist binds to the inactive conformations of the receptor.

How does Enzyme Receptor elicit its action:

Active Enzyme

Substrate Product

Cellular Function

Inactive Enzyme

Substrate

Bound Enzyme Inhibitor (Drug)

NUCLEAR RECEPTOR : Hormone receptors that bind ligand and act in the cell nucleus rather

than at the cell surface and regulate gene expression. Nuclear receptor is also called as true receptors which involved in

increase or decrease in the production of certain RNAs and mRNA along with corresponding enzyme and protein.

Recent data demonstrates that these are the prototypes of a large family of receptors for small lipophilic signaling molecules including steroid hormone, fat soluble vitamins fatty acid metabolites and cholesterol metabolites.

How does Nuclear Receptor elicit its action:

Intracellular Compartment

Nucleus

DNA

Modulation ofTranscription

Active Nuclear Receptor

Bound Endogenous Activator (Agonist) of Nuclear Receptor

Displaced Endogenous Activator (Agonist) of Nuclear Receptor

Intracellular Compartment

Nucleus

DNA

Bound Antagonist of Receptor (Drug)

Inactive Nuclear ReceptorIn Cytosolic Compartment

Inactive Nuclear ReceptorIn Nuclear Compartment

Classification of nuclear receptors:

The following is a list of the 48 known human nuclear receptors categorized into the following families:

Subfamily 1: Thyroid Hormone Receptor-like

Group A: Thyroid hormone receptor (Thyroid hormone)

1: Thyroid hormone receptor-α (TRα; NR1A1, THRA)

2: Thyroid hormone receptor-β (TRβ; NR1A2, THRB) Group B: Retinoic acid receptor (Vitamin A and related compounds)

1: Retinoic acid receptor-α (RARα; NR1B1, RARA)

2: Retinoic acid receptor-β (RARβ; NR1B2, RARB)

3: Retinoic acid receptor-γ (RARγ; NR1B3, RARG) Group C: Peroxisome proliferator-activated receptor(fatty acids,

prostaglandins)

1: Peroxisome proliferator-activated receptor-α (PPARα; NR1C1, PPARA)

2: Peroxisome proliferator-activated receptor-β/δ (PPARβ/δ; NR1C2, PPARD)

3: Peroxisome proliferator-activated receptor-γ (PPARγ; NR1C3, PPARG)

Group D: Rev-ErbA (heme)

1: Rev-ErbAα (Rev-ErbAα; NR1D1)

2: Rev-ErbAβ (Rev-ErbAβ; NR1D2) Group F: RAR-related orphan receptor(cholesterol, ATRA)

1: RAR-related orphan receptor-α (RORα; NR1F1, RORA)

2: RAR-related orphan receptor-β (RORβ; NR1F2, RORB)

3: RAR-related orphan receptor-γ (RORγ; NR1F3, RORC) Group H: Liver X receptor-like (oxysterol)

3: Liver X receptor-α (LXRα; NR1H3)

2: Liver X receptor-β (LXRβ; NR1H2)

4: Farnesoid X receptor (FXR; NR1H4) Group I: Vitamin D receptor-like

1: Vitamin D receptor (VDR; NR1I1, VDR) (vitamin D)

2: Pregnane X receptor (PXR; NR1I2) (xenobiotics)

3: Constitutive androstane receptor (CAR; NR1I3) (androstane)

Subfamily 2: Retinoid X Receptor-like Group A: Hepatocyte nuclear factor-4(HNF4) (fatty acids)

1: Hepatocyte nuclear factor-4-α (HNF4α; NR2A1, HNF4A) 2: Hepatocyte nuclear factor-4-γ (HNF4γ; NR2A2, HNF4G)

Group B: Retinoid X receptor(RXRα) (retinoids) 1: Retinoid X receptor-α (RXRα; NR2B1, RXRA) 2: Retinoid X receptor-β (RXRβ; NR2B2, RXRB) 3: Retinoid X receptor-γ (RXRγ; NR2B3, RXRG)

Group C: Testicular receptor1: Testicular receptor 2 (TR2; NR2C1) 2: Testicular receptor 4 (TR4; NR2C2)

Group E: TLX/PNR 1: Human homologue of the Drosophila tailless gene (TLX; NR2E1) 3: Photoreceptor cell-specific nuclear receptor (PNR; NR2E3)

Group F: COUP/EAR 1: Chicken ovalbumin upstream promoter-transcription factor I (COUP-TFI; NR2F1) 2: Chicken ovalbumin upstream promoter-transcription factor II (COUP-TFII; NR2F2) 6: V-erbA-related gene|V-erbA-related (EAR-2; NR2F6)

Subfamily 3: Estrogen Receptor-like: Group A: Estrogen receptor (Sex hormones: Estrogen)

1: Estrogen receptor-α (ERα; NR3A1, ESR1)

2: Estrogen receptor-β (ERβ; NR3A2, ESR2) Group B: Estrogen related receptor

1: Estrogen-related receptor-α (ERRα; NR3B1, ESRRA)

2: Estrogen-related receptor-β (ERRβ; NR3B2, ESRRB)

3: Estrogen-related receptor-γ (ERRγ; NR3B3, ESRRG) Group C: 3-Ketosteroid receptors

1: Glucocorticoid receptor (GR; NR3C1) (Cortisol)

2: Mineralocorticoid receptor (MR; NR3C2) (Aldosterone)

3: Progesterone receptor (PR; NR3C3, PGR) (Sex hormones: Progesterone)

4: Androgen receptor (AR; NR3C4, AR) (Sex hormones: Testosterone)

Subfamily 4: Nerve Growth Factor IB-like Group A: NGFIB/NURR1/NOR1

1: Nerve Growth factor IB (NGFIB; NR4A1) 2: Nuclear receptor related 1 (NURR1; NR4A2) 3: Neuron-derived orphan receptor 1 (NOR1; NR4A3)

Subfamily 5: Steroidogenic Factor-like Group A: SF1/LRH1

1: Steroidogenic factor 1 (SF1; NR5A1) (phospholipids) 2: Liver receptor homolog-1 (LRH-1; NR5A2)

Subfamily 6: Germ Cell Nuclear Factor-like Group A: GCNF

1: Germ cell nuclear factor (GCNF; NR6A1) 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)

Why Nuclear Receptor is preferred:

Nuclear receptors have the ability to directly bind to DNA and regulate the expression of adjacent genes.

Consequently nuclear receptors play key roles in both embryonic development and adult homeostasis.

Differences of between Nuclear Receptors and other types of receptors:

Other Types of Receptors Nuclear Receptors

1.They present in different sites of tissues and cells and bind and elicit without involvement of nucleus.

2. Elicit pharmacological response due to release/inhibition of endogenous mediators. ( transmitter, hormones, autacoids).

3.Drugs acts by modulating the receptor to produce/inhibit the release of mediators.

4. They shows changes in physiological parameters,

1.They present with association of nucleus and elicit the response with involvement of nucleus.

2. Elicit the response on the basis of transcription of gene, regulates protein synthesis.

3. Drugs acts which cause the transcription of gene from its genetics based.

4. They shows changes on structural basis of target cells.

Functions of Nuclear receptor:

To propagate the regulatory signals from outside to within effectors cell.

To amplify the signal. To integrate various extracellular and intracellular regulatory signals. To adapt changes and maintain homeostasis. To facilitates gene expression so that specific mRNA is synthesized ,

which moves to ribosome and directs synthesis of specific proteins which regulates the activity of target cells.

Nuclear Receptors in Therapy:

Nuclear Receptors are one of the major target area of modern therapy and of research.

A large number of receptors are identified and so screening of their Ligands are going in clinical trials as well as few of them becoming patent.

PPAR (Peroxisome proliferators activated receptor)

Types of PPAR Receptors:

PPARα expressed in Liver, Kidney, Heart, Muscle, Adipose Tissue and others.

PPAR β expressed mainly in brain, adipose tissue and skin.

PPAR γ almost all the tissue.

PPARs heterodimerize

with Liver X Receptors ( LXR),

or Retinoid X Receptors( RXR),

or Vitamin D receptors.

Functions: Control of Cellular differentiation and Development. Control of Metabolism ( Carbohydrate, Lipid, protein).Genetics:

PPARα - chromosome22q12-13.1 (OMIM 170998) PPARβ/δ - chromosome 6p21.2-21.1 (OMIM 600409) PPARγ- chromosome 3p25 (OMIM 601487).

Disease involved : Diabetes Type 2, Atherosclerosis, Obesity and Hyperlipidemia.

PPAR Agonists for treatment:

PPARα : Fibrates ( Clofibrate, Gemfibrozil, Fenofibrate).

PPARγ : Thiazolidinediones ( Roziglitazone, Pioglitazone, Perflurooctanoic acid).

Pharmacology:

Agonist Receptor Enhance Transcription of responsive genes

Glucose entry to muscle

and suppressed Reverse insulin resistance

Gluconeogenesis.

Gene transcription cartoon showing interactions of ligand/PPAR/RXR heterodimer with cofactors and basal transcription machinery(RNA Polymerase II).

GR ( Glucocorticoid Receptors):Physiological function: GR express and regulates the genes controlling for

development , metabolism and immmune response.GR and LRH-1(Liver Receptor Homolog-1) upregulates the

expression anti-inflamatory proteins in nucleas.

Disease involved: Inflammatory bowel syndrome, Auto Immune disorder, rheumatoid arthritis, gout etc.

Agonists : Cortisol, Prednisolone, Dexamethasone.

Genetics: NR3C1, NR5A2 , NR3C2 GenePharmacology: ( Anti- inflammatory activity)

Agonist Receptor Enhance Transcription of responsive genes

.Inhibition of PhospolipaseA2 Production of lipocortin TNF, PAFand related proteins.

Immunosuppressive activity of GRAgonist Receptor Transcription of

Responsive Genes

Decrease proliferation of Inhibition of IL synthesis.

B-Lymphocytes.

Inhibition of T-lymphocyte production Decrease antibody

production

Xeroreceptors targets for Liver Disease

Xeroreceptors:CAR ( Constitutive Androstane Receptor)

PXR (Pregnane X Receptors).

Physiological function: Activation and consequences on Lipid metabolism

Glucose Homeostasis Inflammatory Responses

Genetics: Targeted Genes CYP3A4, CYP2B6, CYP2C9 MDR1, GT1A1, MRP2, UGT1A1

Disease Involved: Non-alchoholic fatty liver, Hepatic Steatosis,Obesity, Inflammation

Agonists : Rifampin, Nifidipine, Hyperferlin, RU486, HNF4

Nuclear Receptor Targets of Cancer Therapy:

Nuclear Receptors targeted

mainly for Cancer therapy are.

1.Estrogen Receptor ( ER)

2. Vitamin D Receptor (VDR)

3.PPAR γ

4. Androgen Receptor (AR)

5. Progesterone Receptor (PR)

Estrogen Receptor and Cancer

Estrogens are key regulator of growth, differentiation, and function in large number of tissue.

Biological effects of estrogens are mediated by ERα and ERß

Structure of estrogen

Esrogen Receptors Types: 1: Estrogen receptor-α (ERα; NR3A1, ESR1)

2: Estrogen receptor-β (ERβ; NR3A2, ESR2)

ERα is predominant and expressed in breast, uterus, cervix and Vagina.

ERß is predominantly localized in ovary, prostate, testis, spleen, Lung, hypothalamus and thymus.

SERM and Breast Cancer therapy:Pharmacology: Breast cancer dependent upon the presence of estrogen for

growth.

The objective of treatment is SERMs ( Selective estrogen receptor modulators) to inhibit tumor growth by interrupting the estrogen signaling process.

SERMs shows estrogenic effect in some tissue and anti estrogenic in others.

Tamoxifen ( Nolvadex®, Astra Zeneca ):-

It has antagonistic effect in mammary tissue, while agonist effect in bone, uterine and cardiovascular tissue.

An ideal agents foe breast cancer which retain the beneficial effects of estrogen in bone, brain and cardiovascular tissue.

It inhibit the mutagenic effect of estrogen in breast.

Role of VDR in Breast Cancer:

VDR binds with 1a, 25-dihydroxy cholecalciferol D3 and regulates the transcription of Vit-D responsive gene.

This regulates the calcium endocrinology and involved in homeostasis such as skin, muscle, pancreas and reproductive organs.

Activated VDR induces genes that suppress proliferation and stimulate differentiation of cells in the normal mammary gland.

Vitamin D receptor (VDR) and Postmenopauseal breast cancer:

VDR in Breast cancer therapy in post menopausesal women:

Pathogenesis:Aging reduces the production of Vitamin D by epidermis.

Estrogen deficiency decreases both metabolic activation of Vitamin D and expression of the VDR.

VDR agonists:

Nearly 400 structural analogs of Vit.D have been synthesized but few have advanced in clinical trials due to preliminary toxicity testing.

VDR and Colorectal Cancer:

N.B: Cancer of the large intestine (colon) and rectum, collectively termed colorectal cancer. It is the third most common form of cancer, behind lung and breast cancer.Vitamin D receptors is also considered as target of colon cancer along with Glucocorticoid receptor.

Pathogenesis: Diet with high fat

Bile acid ( Lithocholic acid) poorly reabsorbed A toxic metabolite in enterohepatic

circulation

High conc. In colon

Induce DNA strand breakage, inhibit DNA repair enzyme

Colon Cancer

Pharmacology of VDR in colon cancer

Vit-DVDR

LCA Catabolism of CYP3A, a target gene

Increased detoxification of LCA and elimination

Protection against colon cancer.

Examples: Ligands and target Genes of some nuclear receptors:

Receptor type Ligands / Drug Disease target

PPARα

PPARγ

PPARγ

GR

LXR

ER ( SERM)

ER β

Fibrates :ClofibrateGemfibrozilFenofibrate

Thiazolidinediones RoziglitazonePioglitazone Perflurooctanoic acid

RS544

DexamethasonePrednisolone

TO901317

TamoxifenRaloxifen

Mifepristone( Antagonist)

Diabetes Type2, Hyperlipidemia.

Diabetes Type2, HyperlipidemiaAtherosclerosis

Anaplastic cancer

Different inflammatory syndromes.

Alzheimer Disease

Breast cancer, Osteoporosis

Unwanted abortion

Conclusion:

Functioning as transcription factors and controlling cellular process at the level of gene expression.

Modulation of NR activity produces selective alterations in downstream gene expression.

Nuclear Receptors are an attractive and relatively unexploited target for drug development.

References:

Books: Essential of Medical Pharmacology, 6th Edition

by K. D. Tripathi, Page no. 40-53