Receptors III – Intracellular receptors

What are intercellular receptors?

Ligand-activated transcription factors that regulate gene expression by binding specific DNA sequences called hormone response elements(HREs). They respond primarily to lipophilic molecules such as steroid hormones, thyroid hormones, retinoids, vitamin D, and fatty acids.

Why can intracellular receptor ligands enter cells easily?

Their ligands are lipophilic(fat-soluble) and can diffuse across plasma membranes

What is the function of the DNA-binding domain(DBD)

The DBD contains two zinc fingers that recognize and bind hormone response elements(HRE) on DNA

What is the function of the hinge region?

The hinge region provides flexibility between receptor domains

What are the functions of the ligand-binding domain(LBD)?

• ligand recognition

• dimerization

• coactivator/corepressor binding

What is AF-1?

The N-terminal activation function domain responsible for ligand-independent activation

What is AF-2?

The C-terminal activation function domain responsible for ligand-dependent activation

What are orphan receptors?

Receptors identified through sequence homology that had no known endogenous ligand when first discovered

What are adopted orphan receptors?

former orphan receptors whose natural ligands were later identified

Give examples of adopted orphan receptors and their ligands

• PPARs→ fatty acids and eicosanoids

• LXR→ oxysterols

• FXR→bile acids

• PXR→xenobiotics and steroids

What are the four DNA-binding classes of intracellular receptors?

1. Steroid Receptors(SR)

2. RXR Heterodimers

3. Dimeric orphan receptors(DOR)

4. Monomeric orphan receptors(MOR)

What DNA sequence do intracellular receptors bind?

Hormone response elements(HREs) located in target genome promoters

What happens after a ligand binds a Type 1 receptor?

• Ligand enters cell

• ligand binds to receptor

• conformational change occurs

• heat shock proteins(HSPs) dissociate

• receptor dimerizes

What happens after a Type 1 receptor dimerizes?

The homodimer translocates to the nucleus and binds palindromic HREs

How do Type 1 receptors regulate transcription?

they recruit coactivators(HATS) and either activate or repress transcription of target organs

What happens to Type II receptors in the absence of ligand?

They remain bound to DNA with corepressors(HDACs), causing transcription repression

What happens when ligand binds a Type II receptor?

• Corepressors are released

• Coactivators are recruited

• Chromatin remodeling occurs

• Gene transcription begins

How are intracellular receptor signals terminated?

• ligand dissociation

• receptor degradation/turnover

• negative feedback inhibition

Why do intracellular receptor responses have a slow onset?

Their effects require transcription and translation which take time

Why do intracellular receptor effects have a long duration?

They alter gene expression and protein synthesis, producing sustained cellular changes

Where are Type I receptors located before ligand binding?

in the cytoplasm, associated with heat shock proteins(HSPs)

Where are Type II receptors located before ligand binding?

in the nucleus, already bound to DNA

What type of dimers do Type 1 receptors form?

homodimers

What type of dimers do Type II receptors form?

Heterodimers with RXR(retinoid X receptor)

Give examples of Type 1 receptors

• glucocorticoid receptor

• estrogen receptor

• progesterone receptor

• androgen receptor

• mineralocorticoid receptor

Give examples of Type II receptors

• Thyroid hormone receptor

• retinoic acid receptor

• vitamin D receptor

• PPARs

What type of HREs do Type I receptors bind?

palindromic/inverted repeat HREs

What type of HREs do Type II receptors bind?

direct repeat(DR) response elements

What is the major transcriptional state of Type II receptors without ligand?

transcription is repressed due to corepressor binding

Compare Type I vs Type II intracellular receptors.

Feature
Location without ligand
Binding partners
Dimerization
DNA binding
Examples

Feature	Type I	Type II
Location without ligand	Cytoplasm	Nucleus
Binding partners	HSPs	DNA + corepressors
Dimerization	Homodimer	RXR heterodimer
DNA binding	After ligand binding	Before ligand binding
Examples	Steroid receptors	Thyroid receptor, VDR, RAR, PPARs

What is the primary physiological role of intracellular receptors?

Control gene expression involved in diverse physiological processes

Why is tissue specificity important for intracellular receptor signaling?

Responses depend on: receptor expression, cofactor availability, chromatin context

What is receptor signaling crosstalk?

Interaction of intracellular receptor pathways with other signaling pathways such as growth factors and kinases.

Why are intracellular receptors major pharmacologic targets?

They regulate endocrine, metabolic, inflammatory, and neoplastic diseases through direct gene regulation

What endocrine disorders involve intracellular receptors?

• Addison's disease

• Cushing's syndrome

• Hypothyroidism

What metabolic disorders involve intracellular receptors?

• Metabolic syndrome

• Diabetes

• Dyslipidemia

Which hormone-dependent cancers involve intracellular receptors?

• Breast cancer

• Prostate cancer

Which inflammatory diseases involve intracellular receptor pathways?

Various inflammatory and autoimmune diseases

What are glucocorticoids used for?

Anti-inflammatory and immunosuppressive therapy.

What are examples of SERMs?

Tamoxifen and raloxifene

What receptor is targeted by thiazolidinediones?

PPARγ (peroxisome proliferator-activated receptor gamma)

What are examples of drugs acting on Type II intracellular receptors?

Vitamin D analogs and retinoids.

What are the two most tested structural domains of intracellular receptors?

• DBD = DNA binding (zinc fingers)

• LBD = ligand binding, dimerization, cofactor interactions

Which intracellular receptor type is cytoplasmic before activation?

Type I receptors.

Which intracellular receptor type is already nuclear and DNA-bound before activation?

Type II receptors.

Do intracellular receptors use second messengers?

No. They cause direct gene regulation without second messengers.

What is the single best summary of intracellular receptors?

Intracellular receptors are ligand-activated transcription factors that directly regulate gene expression and serve as major drug targets in endocrine, metabolic, inflammatory, and cancer diseases

What are the five major receptor classes discussed in pharmacology?Enzymes

• Ion Channels

• Membrane Transporters

• G-Protein Coupled Receptors (GPCRs)

• Tyrosine Kinase Receptors

• Intracellular Receptors

What types of molecules typically activate intracellular receptors?

• Steroid hormones

• Thyroid hormones

• Retinoids

• Vitamin D

• Fatty acids

What is an HRE?

A Hormone Response Element (HRE) is a specific DNA sequence that intracellular receptors bind to regulate gene transcription

What are the major structural domains of intracellular receptors?

• N-terminal domain (AF-1)

• DNA-binding domain (DBD)

• Hinge region

• Ligand-binding domain (LBD)

• C-terminal domain (AF-2)

Which receptor domain contains zinc fingers?

The DNA-binding domain (DBD).

What is the function of zinc fingers?

They recognize and bind hormone response elements on DNA

What proteins are associated with inactive Type I receptors?

Heat Shock Proteins (HSPs).

What happens when ligand binds a Type I receptor?

A conformational change occurs and HSPs dissociate.

What happens after Type I receptor homodimerization?

The receptor complex translocates into the nucleus

What DNA sequence do Type I receptors bind?

Palindromic hormone response elements.

How do Type I receptors activate transcription?

By recruiting coactivators such as histone acetyltransferases (HATs)

What receptor does every Type II receptor heterodimerize with?

Retinoid X Receptor (RXR)

What happens when Type II receptors are unliganded?

They remain bound to DNA with corepressors and repress transcription.

Name an important corepressor enzyme associated with Type II receptors.

Histone deacetylases (HDACs)

What happens after ligand binding to a Type II receptor?

Corepressors dissociate and coactivators are recruited

What cellular events occur after Type II receptor activation?

chromatin remodeling and target gene transcription

what ligand activates LXR?

oxysterols

what ligand activates FXR?

bile acids

What ligands activate PXR?

xenobiotics and steroids

What is Class I (SR) receptor?

Steroid receptors that form homodimers and bind inverted repeat HREs.

What is Class II receptor?

RXR heterodimers that bind direct repeat response elements.

What receptors belong to Class II?

• Thyroid hormone receptor

• Retinoic acid receptor

• Vitamin D receptor

• PPARs

What are Class III receptors?

Dimeric orphan receptors that form homodimers and bind direct repeats

Examples of Class III receptors?

ERRs and RORs.

What are Class IV receptors?

Monomeric orphan receptors that bind DNA as monomers.

Examples of Class IV receptors?

NGFI-B, Nurr1, and NOR1