Lecture 22 Flashcards
What is common to all ligands of the nuclear receptor subfamily
Aromatic groups that are hydrophobic
What are the distinguishing features of cNOS
Although cNOS is constitutively expressed, its enzymatic activity is induced by Ca2+/Calmodulin. This activity is short-lived and leads to the generation of pmol amounts of NO
What is the major difference between nuclear receptor transcription factors that bind to DNA as homodimers and heterodimers in terms of the sequence with which they interact
Homodimers bind to palindromic sequences whereas heterodimers bind tandem repeat sequences
Heterodimeric intracellular receptors are always bound to DNA, even in the absence of ligand binding, T or F
T
Explain what happens to nuclear receptors following ligand binding
Inhibitory proteins hold the receptors in an inactive state. Binding to the ligand removes the inhibitors leading to a conformational change. The receptors can now interact with coactivators to direct transcription
How does sildenafil citrate act to treat erectile dysfunction
ViagraTM acts downstream of NO(g) to inhibit phosphodiesterase 5. Phosphodiesterase-5 converts cGMP back to GMP and counteracts guanylyl cyclase. Increases in cGMP by the inhibition of PD-5 leads to the relaxation of vascular smooth muscles increasing blood flow to the penis.
Explain the mechanism by which heterodimeric receptor transcription factors act
Heterodimeric receptors are located exclusively in the nucleus. They act as transcriptional repressors in the absence of ligand by recruitment of histone deacetylases and as activators in the presence of ligand by recruitment of histone acetylases
Overproduction of NO in the lungs and intestines can cause what diseases
Rheumatoid arthritis, Crohn’s disease and asthma
Give an example of a small molecule that binds to an intracellular receptor
NO
What is the name of the enzyme that catalyses NO formation
Nitric oxide synthase (NOS)
In the CNS, nNOS is tethered close to NMDA-type glutamate receptors. Explain the role of NO in the brain
nNOS in the brain can respond to the Ca2+ increases near the open NMDA channel. This has implications in synaptic plasticity. Glutamate is produced by a presynaptic terminal and this activates glutamate receptors, especially NMDA receptors, which in turn stimulate the NO pathway in the postsynaptic terminal. NO produced in this way travels to the presynaptic terminal and increases the production of glutamate, thus creating the state of long-term potentiation.
Explain the mechanism by which homodimeric receptor transcription factors act
Upon ligand/hormone binding, homodimeric transcription factors translocate to the nucleus following the release of inhibitor proteins that are bound to its inactivated form and anchor it to the membrane. Once allowed to translocate to the nucleus these homodimers can influence gene transcription.
iNOS plays a crucial role in the immune response, T or F
T
What neurotransmitter can cause the activation of NOS enzymes
Autonomic nervous system release of acetylcholine activates NOS enzymes in endothelial cells
What is the name of the cofactor that interacts with heterodimeric intracellular receptors
RXR cofactor
Nuclear receptors are essentially transcription factors activated by hydrophobic ligands, T or F
T
Give an example of an inhibitor protein that anchors homodimeric receptors to the membrane of the cell
Hsp90
Give an example of a lipid-soluble that binds to an intracellular receptor
Retinoic acid
Explain the role of nitrous oxide in the immune system
NO acts in the immune system by killing bacteria and parasites and inducing programmed cell death. iNOS in inflammatory cells uses NO as a cytostatic and cytotoxic agent. This involves much higher amounts of NO and is due to its effects as a free radical causing cell death
What kinds of stimuli induce iNOS expression
LPS, IFN-? and IL-1
How is NO naturally produces
Produced by two successive oxidations of L-arginine to L-citrulline
What are the two main categories of NOS enzymes
Constitutive NOS and Inducible NOS
Give examples of homodimeric nuclear receptors
Glucocorticoid and oestrogen receptor
There are two subgroups of constitutive NOS enzymes, eNOS and nNOS, what are the main differences between these enzymes
Endothelial or eNOS is a membrane tethered isoform of the cNOS enzyme. It is present in endothelium, cardiac myocytes renal mesangial cells amongst others. nNOS or neuronal NOS is a cytosolic NO receptor enzyme. It is present in the central nervous system, NANC neurons and enteric nervous system.
Explain how NO leads to the relaxation of vascular smooth muscle
Acetylcholine released activates NOS enzymes which convert arginine to NO.NO diffuses across membranes and binds to guanylyl cyclase and activates it. Activation guanylyl cyclase converts GTP to cGMP. Increases in cGMP activate the cGMP-dependant protein kinase-G which acts to increase myosin light chain phosphatase (MLCP) levels ultimately leasing to the relaxation of smooth muscle.
Nitrous oxide is a gas that binds to an intracellular receptor, it is known as a free radical. What is meant by this term
NO is a free radical meaning that it is a highly reactive species that contains an unpaired electron in its outer shell. It is very unstable with a half-life of roughly 5-10secs
Explain the role of nitroglycerine in the treatment of angina
Nitroglycerine breaks down in vivo to generate NO. NO then acts to relax the coronary blood vessels thus decreasing the load on the heart and increasing heart blood supply
The effects of the ligands are modified in different tissues by the expression of different coactivators, T or F
T
Give examples of heterodimeric nuclear receptors
Vitamin D, thyroid hormone and retinoic acid receptors
Describe the difference between the early primary and delayed secondary response to intracellular receptor activation
The early primary response occurs within 1-6 hours. These effects are relatively slow because they are transcription factors that regulate gene transcription. The delayed secondary response however takes 6-48hours. This is the case because the genes upregulated in primary response include these transcription factors that then mediate the secondary response
What distinguishes iNOS from cNOS
Inducible NOS (iNOS) is an enzyme whose expression is required to be induced. Once, induced iNOS enzymatic activity is constitutive and leads to the production of larger amounts of NO, in the nmol range. iNOS has longer lasting effects and is usually transcriptionally induced by pathological stimuli
What are the two main types of ligands that bind to intracellular receptors
Hydrophobic molecules or ones small enough to diffuse across the plasma membrane
Homodimeric receptors are exclusively located in the nucleus whereas, heterodimeric receptors are found in the cytoplasm in the absence of ligand binding, T or F
F – vice versa
Give some examples of ligands for intracellular receptors
Cortisol, estradiol, testosterone, thyroxine
What are the effects of nitrous oxide on the vasculature
Endothelial cells release NO which causes the surrounding smooth muscle to relax (vasodilation). NO also inhibits platelet aggregation and vascular smooth muscle proliferation and thus is anti-atherosclerotic
