Lecture 22

Question 1

What is common to all ligands of the nuclear receptor subfamily

Answer 1

Aromatic groups that are hydrophobic

Question 2

What are the distinguishing features of cNOS

Answer 2

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

Question 3

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

Answer 3

Homodimers bind to palindromic sequences whereas heterodimers bind tandem repeat sequences

Question 4

Heterodimeric intracellular receptors are always bound to DNA, even in the absence of ligand binding, T or F

Answer 4

T

Question 5

Explain what happens to nuclear receptors following ligand binding

Answer 5

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

Question 6

How does sildenafil citrate act to treat erectile dysfunction

Answer 6

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.

Question 7

Explain the mechanism by which heterodimeric receptor transcription factors act

Answer 7

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

Question 8

Overproduction of NO in the lungs and intestines can cause what diseases

Answer 8

Rheumatoid arthritis, Crohn’s disease and asthma

Question 9

Give an example of a small molecule that binds to an intracellular receptor

Answer 9

NO

Question 10

What is the name of the enzyme that catalyses NO formation

Answer 10

Nitric oxide synthase (NOS)

Question 11

In the CNS, nNOS is tethered close to NMDA-type glutamate receptors. Explain the role of NO in the brain

Answer 11

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.

Question 12

Explain the mechanism by which homodimeric receptor transcription factors act

Answer 12

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.

Question 13

iNOS plays a crucial role in the immune response, T or F

Answer 13

T

Question 14

What neurotransmitter can cause the activation of NOS enzymes

Answer 14

Autonomic nervous system release of acetylcholine activates NOS enzymes in endothelial cells

Question 15

What is the name of the cofactor that interacts with heterodimeric intracellular receptors

Answer 15

RXR cofactor

Question 16

Nuclear receptors are essentially transcription factors activated by hydrophobic ligands, T or F

Answer 16

T

Question 17

Give an example of an inhibitor protein that anchors homodimeric receptors to the membrane of the cell

Answer 17

Hsp90

Question 18

Give an example of a lipid-soluble that binds to an intracellular receptor

Answer 18

Retinoic acid

Question 19

Explain the role of nitrous oxide in the immune system

Answer 19

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

Question 20

What kinds of stimuli induce iNOS expression

Answer 20

LPS, IFN-? and IL-1

Question 21

How is NO naturally produces

Answer 21

Produced by two successive oxidations of L-arginine to L-citrulline

Question 22

What are the two main categories of NOS enzymes

Answer 22

Constitutive NOS and Inducible NOS

Question 23

Give examples of homodimeric nuclear receptors

Answer 23

Glucocorticoid and oestrogen receptor

Question 24

There are two subgroups of constitutive NOS enzymes, eNOS and nNOS, what are the main differences between these enzymes

Answer 24

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.

Question 25

Explain how NO leads to the relaxation of vascular smooth muscle

Answer 25

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.

Question 26

Nitrous oxide is a gas that binds to an intracellular receptor, it is known as a free radical. What is meant by this term

Answer 26

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

Question 27

Explain the role of nitroglycerine in the treatment of angina

Answer 27

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

Question 28

The effects of the ligands are modified in different tissues by the expression of different coactivators, T or F

Answer 28

T

Question 29

Give examples of heterodimeric nuclear receptors

Answer 29

Vitamin D, thyroid hormone and retinoic acid receptors

Question 30

Describe the difference between the early primary and delayed secondary response to intracellular receptor activation

Answer 30

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

Question 31

What distinguishes iNOS from cNOS

Answer 31

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

Question 32

What are the two main types of ligands that bind to intracellular receptors

Answer 32

Hydrophobic molecules or ones small enough to diffuse across the plasma membrane

Question 33

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

Answer 33

F – vice versa

Question 34

Give some examples of ligands for intracellular receptors

Answer 34

Cortisol, estradiol, testosterone, thyroxine

Question 35

What are the effects of nitrous oxide on the vasculature

Answer 35

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