Nuclear Receptors

lipid soluble/hydrophobic ligands

• small

• can cross plasma membrane (come via passive diffusion, thyroid hormones via active transport)

• interact with intracellular nuclear receptors (large proteins)

what receptors are included in the type I steroid receptor family

• glucocorticoid receptor

• mineralocorticoid receptor

• androgen receptor

• estrogen receptors

• progesterone receptor

what receptors are included in the type II thyroid receptor family

• Tra and TrB = thyroid hormone receptors

• RAR = retinoic acid receptor (vitamin A metabolite)

• RXR = retinoic X receptor (vitamin A metabolite)

• VDR = vitamin D receptor

• PPARs = peroxisome proliferator activated receptors (fatty acids, prostagalndins)

• "orphan nuclear receptors” (endogenous ligands: not yet identified or aren’t needed)

nuclear receptors generalized signaling pathway

• lipid soluble ligand passively diffuses or is actively transported across cell membrane

• for cytoplasmic, heat shock proteins anchored nuclear receptors (NRs): ligand binding causes dissociation from HSPs & exposes nuclear translocation signal in receptor

• dimerization of receptors & receptor ligand dimer complex enters nucleus

• association of receptors with specific response element (“HRE”) sequence in DNA; transcription factor activities

can some nuclear receptors be membrane associated receptors

yes, they enact more rapid responses through interactions with intracellular signaling pathways

unliganded glucocorticoid receptor and mineralocorticoid receptor

reside in the cytosol associated with chaperone proteins such as heat shock protein (HSP) 90

other unliganded steroid receptors

some proportion might reside in the cytosol associated with chaperone proteins such as HSP90, but a high proportion is nuclear (especially estrogen receptors)

unliganded Type II nuclear receptors

• thyroid hormone receptors

• typically in the nucleus, bound to DNA but associated with complex containing copressor proteins

• do not interact with HSP90 in cytosol

copressor proteins

• are associated with a complex of proteins that have histone deactylase (HDAC) activity-repress gene expression by maintaining chromatin in condensed conformation

• ligand binding to these nuclear receptors “releases” corepressors and allows for change in DNA expression

in promotor region…

preinitiation complex form RNA polymerase II, general transcription factors and other regulators

what does RNA polymerase II do

enzyme that transcribes genes to mRNA

hormone receptor complex

bound to specific hormone response element of DNA, recruits general transcription factors or intermediary co-activators, promote assembly and stabilization of transcription preinitiation complex

transcriptional regulators

their presence & their interaction with DNA regulatory elements affects rate of gene transcription

chromatin looping

• distal regulatory elements located far from the transcriptional start site can impact gene expression

• can come into closer proximity due to changes in 3D structure of DNA & chromatin

DNA binding domain nuclear receptors

• targets receptor to specific DNA sequence comprising half of a response element (HRE)

• usually the response element is direct or inverted repeats separated by 3 nucleotides

• dna binding domain is highly conserved

• basic, 2 zinc fingers, 2 alpha helices which physically interacts with DNA

hinge region nuclear receptors

has sequences for receptor dimerization & nuclear localization

N terminus nuclear receptors

contains ligand independent activation function (AF-1) domain that facilitates receptor coregulator interactions

ligand binding domain

direct specificity of biology response; often contains ligand dependent activation function (AF-2) domain that facilitates receptor co-activator interactions

post translational modifications that affect activity

• acetylation, ubiquitination and phosphorylation

• modifications are common in the amino terminal AF-1 domain where they can affect the affinity for certain coactivator proteins (thereby modulate receptor activity)

how can nuclear receptors act indirectly

by tethering to other transcription factors

interacting with coregulators

• coregulators are subunits of multiprotein complexes with several functions

• chromatin remodeling, enzymatic modification of histone tails, modulation of preinitiation complex via interactions with RNA polymerase II & general transcription factors

interactions of specific coregulators with nuclear receptors depends on

• whether the coregulator has been post translationally modified

• phosphorylation can affect its recruitment, its interaction with other coregulators, or the stability of the activation complex

tamoxifen vs estradiol interactions with ERa

• tamoxifen inhibits the activation factor 2 (AF-2) domain of the estrogen receptor by blocking coactivator binding

• ~2/3 of breast cancers express ERa

• allows for AF1 domain to be open and AF2 to be blocked

“nuclear” receptors- membrane initiated signaling

• most steroid/thyroid hormone effects are via gene transcription but certain rapid effects are incompatible with this mechanism

• nuclear receptors in plasma membrane associated complexes can activate/interact with members of intracellular signaling pathways (e.g., MAPK, PI3K, JNK)

metabolite ligands

• some nuclear receptors have metabolites (fatty acids and bile acids) as ligands

• have expansive effects on metabolic & inflammatory processes by regulating levels of key enzymes, signaling factors, etc