Nuclear Cunts Flashcards
What are some hormones that act via nuclear receptors?
Derivatives of cholesterol: cortisol, aldosterone, estradiol, testosterone, vitamin D3.
Derivatives of vitamin A: all trans retinoic acid, 9-cis retinoic acid.
Amino acid derivatives: triiodothyronine (T3) (derivative of tyrosine).
Metabolic intermediates: fatty acids and bile acids.
What are nuclear receptors?
Intracellular receptors for small lipophilic ligands which include various steroids and lipophilic vitamin metabolites. 48 human genes encode nuclear receptors, and most serve as transcriptional regulators that act in a ligand-dependent manner through their DNA-binding.
What are the two classes of nuclear receptors?
Class I: present in the cytoplasm. Operate as homodimers. Mainly endocrine ligands. High affinity for HSP90. Since they’re symmetric, they bind to palindromic repeats on DNA. Bind to DNA head to head.
Class II: present in the nucleus. Operate as heterodimers (Except when it’s an RXR receptor). Mainly lipid or endocrine ligands, along with metabolic molecules. Low affinity for HSP90. Bind to repressors until activated. Larger binding pocket and bind at lower affinity, more “promiscuously”. Bind head to tail rather than head to head, so they bind DNA in a different way.
What are orphan receptors?
Cognate ligand has not been identified.
What makes nuclear receptor ligands different than ligands for transmembrane receptors such as epinephrine?
Compounds such as epinephrine can easily be carried in the blood but cannot easily pass membranes, so their receptor is on the surface of the targets and allows for them to bind and transmit their signal inside the cell. Nuclear receptor ligands are hydrophobic, and as such cannot be easily transported through the blood, so they need binding protein which helps transport them.
If a nuclear receptor ligand is bound to a binding protein, how does it get into the cell and bind to it’s receptor.
They are in equilibrium between bound and unbound states. So there’s a fraction that is unbound and able to easily permeate that membrane to it’s receptor in the cytoplasm.
How can binding of the receptor cause transcriptional regulation?
The ligand binds to the receptor in the cytoplasm and causes it to dimerize, which then allows it to be transported to the nucleus through nuclear pores. Once in it can repress or activate transcription.
What are the roles of chaperone proteins such as HSP90 that are bound to the receptors in the absence of ligand?
Helps in the folding of the receptor and in doing so increases the affinity for ligand. Also blocks the nuclear localization signal on the receptor so it won’t get transported to the nucleus in the absence of ligand. Also blocks the DNA binding domain so it can’t affect transcription without the ligand. Ligand binding allows for dissociation of the chaperone.
Nuclear localization signal; why is it needed?
Small molecules and ions can get through the nuclear pores, but proteins and larger molecules cannot. So they need a nuclear localization signal which binds to the nuclear pore and allows entrance. If you put a nuclear localization signal on other proteins it’ll transport them as well.
What does the receptor do once in the nucleus?
Binds to specific sites on DNA called response elements (cis-elements). The receptor itself is a trans-acting factor since it’s not part of the DNA.
How is gene expression regulated?
There are transcriptional activators, coactivators, general transcription factors, promoter, etc. general transcription factors bind near the promoter and enable RNA pol to be loaded on. Response elements can be great distances away from the start site and still enhance transcription because DNA is flexible and can bend. They can interact with transcriptional factors as well as RNA pol either directly or through a mediator protein. Some of them can even be palindromic and able to be reversed but still functional.
What are some features of transcriptional regulation?
Binding site for activator/repress or proteins is a distance from the promoter. Multiple inputs are integrated together (can work together or in opposition). DNA binding proteins act synergistically to regulate giving a greater effect than just a simple additive effect. So effect can be greatly magnified as opposed to them acting on their own.
Give an example of response elements for non-nuclear receptors.
Some genes contain CRE (cAMP response element) regulatory site that is recognized by the CRE-binding (CREB) protein. Activated PKA can enter the nucleus and phosphorylate CREB which can subsequently recruit a coactivator (CBP) to regulate gene transcription by binding to a response element. PKC can go through a similar pathway that results in binding to Phorbol ester sites.
How can nuclear repressors both activate and inhibit gene transcription?
They can bind to coactivators to increase transcription in the presence of a ligand, and can bind to corepressors in the absence of ligand or presence of antagonist. Some nuclear receptors are found in the nucleus so this explains why they can bind DNA without ligand present.
Coactivators and corepressors
Cells have multiple coactivators and corepressors. Each is expressed at different levels in different tissues. Each coregulator has unique structural preferences for interaction with nuclear receptor ligand complexes, so nuclear receptors are thought to have different conformational states that allow for them to bind to different coregulators and have different effects on DNA. Coregulators can bind to one state when agonist is bound, while different coregulators bind to the state in which antagonist or no agonist is bound.
