Lecture 2: Signaling by Steroid Hormone Receptors

How do small lipid-soluble hormones enter their target cells?

by diffusing through plasma and nuclear membranes

List some lipid-soluble hormones.

• steroid hormones (estrogen, progesterone, testosterone, glucocorticoid, mineralocorticoid)

• retinoids

• thyroid hormones

• vitamin D

• fatty acid metabolites

What is this hormone?

cortisol

What is this hormone?

retinoic acid

What is this hormone?

thyroxine

Steroids act via which type of receptors?

intracellular receptors

What did Jensen and Gorski discover?

estrogen acts by binding to estrogen receptors in the cytoplasm; the complex then moves to the nucleus

What did O’Malley demonstrate?

estrogen and progesterone receptors act in the nucleus to induce specific messenger RNAs

What is the signal transduction pathway of nuclear hormone receptors?

1. hormone enters the cell

2. hormone binds to intracellular receptor

3. initiation of a series of events

4. altered gene expression and cellular function

True or False: A ligand will enter the cytoplasm again if there is low receptor concentration.

true

True or False: There is conservation in the multiple functional domains in the following receptors: estrogen receptor, progesterone receptor, glucocorticoid receptor, thyroid hormone receptor, and retinoic acid receptor. The receptor structure consists of amino acid sequences and modular functional domains: DNA-binding domain (DBD), ligand-binding domain (LBD), and two transactivation domains 1 and 2.

true

In the absence of hormones, steroid receptors exist where and with what?

exist in the cytosol in association with chaperone proteins (Hsp90)

True or False: In the absence of hormones, receptors cannot interact with target gene, which leads to transcriptional inactivation.

true

True or False: The binding of hormones to receptors releases chaperone proteins → receptors can interact with target genes → transcriptional activation.

true

Certain nuclear receptors, such as TR, RAR, and VDR, can bind to target genes in the absence of the cognate hormone. These receptors typically function as what in the unoccupied form?

transcriptional repressors

Hormonal binding induces what in the receptor?

a conformational change in the ligand binding domain (LBD) of the receptor

Describe the LBD domain in the presence and absence of a ligand.

• in the absence of a ligand, the LBD has a fairly open conformation

• upon ligand binding, the LBD assumes a compact conformation

The DNA-binding domain mediates recognition of what?

specific target genes

What is the area called where the receptor recognizes specific DNA sequences in the promoter region of target genes?

hormone response elements (HRE)

The DNA-binding domain (DBD) of the receptor makes contact with atoms in the major groove of target via which bonds?

via hydrogen bonds

Describe the response elements for the glucocorticoid and estrogen receptors.

responses elements are 6 base pair inverted repeats separated by any 3 base pairs

Describe the response elements of vitamin D, thyroid hormone, and retinoic acid receptors.

response elements are direct repeats of the same sequence recognized by the estrogen receptor, separated by 3-5 base pairs

Steroid hormone receptors bind to their inverted repeat response elements as which type of dimers?

homodimers

TR, RAR, and VDR bind to their direct repeat response elements as which type of dimers?

heterodimers with a common nuclear receptor partner called RXR

Describe the DNA-binding domain of each nuclear receptor.

• 2 signature C4 zinc fingers

• each zinc finger motif comprises of a group of 4 conserved cysteines, which bind a zinc ion

Describe the binding of the glucocorticoid receptor homodimer to its response element.

• 2 alpha-helices (one in each monomer interacts with the DNA)

• zinc ions

• binds as a homodimer—allowing 2-fold rotational symmetry

True or False: Nuclear hormone receptors display both positive and negative modes of transcriptional regulation.

true

Cellular coregulatory complexes regulate transcription by which type of receptors?

nuclear hormone receptors

Coactivator and corepressor complexes contain which type of activities?

chromatin-modifying activities

True or False: Negatively charged DNA binds tightly to positively charged lysine residues near the N termini of histone protein subunits—leading to transcriptional silencing.

true

True or False: Acetylation neutralizes the positive charge on histones and loosens the interaction between histones and DNA. This allows transcription factors access to DNA—leading to transcriptional activation.

true

Corepressors recruited by nuclear receptors promote what?

deacetylation at the target promoter

Why do many corepressors influence chromatin structure?

corepressors have an association with histone deacetylase activity

True or False: When recruited to the promoter, a corepressor induces deacetylation of histone N-terminal tails in the nearby nucleosomes → local compaction of chromatin → inhibition of the assembly of a transcription initiation complex.

true

Coactivators recruited by nuclear receptors induce what?

histone acetylation at the target promoter

True or False: Many coactivators possess histone acetylation activity.

true

True or False: When these molecules are recruited to the target promoter, the resulting local hyperacetylation of histone N-terminal tails relaxes the chromatin → facilitates access of general transcription factors required for initiation.

true

Multiple chromatin remodeling/modifying activities act in concert with what?

nuclear receptors

Which activities lead to transcriptional activation and repression?

activation

• HATs

• HMTs

• histone kinases

repression

• HDACs

• HMTs

What is Mifepristone?

anti-progesterone

What is Tamoxifen?

anti-estradiol

True or False: Steroid receptor agonists and antagonists induce distinct conformational changes in the receptor.

true

True or False: Estrogen receptor assumes different conformations in the presence of agonists estradiol and DES or antagonist tamoxifen. The conformation dictates transcriptional activity of the receptor.

true

Designer steroids modulate function of steroid receptors in such areas as what?

• mammary gland

• reproductive tract

• immune response

• bone metabolism

• cardiovascular health

• brain function and behavior

What does the antiprogestin RU486 do?

inhibits function of the progesterone receptor in the uterus and thus blocks embryo implantation

What does the antiestrogen tamoxifen do?

used in breast cancer therapy to inhibit the action of estrogen receptors to block tumor cell proliferation