4. Hormone Signaling Pathways

Question 1

Which subunit is the extracellular domain of tyrosine receptor kinase, and which is the intracellular domain?

Answer 1

The α subunit is extracellular, and of the β subunit is intracellular.

Question 2

What are the second messengers for Gs, Gi, Gq, and Gt?

Answer 2

Gq: PLC/IP3.

Gs: ↑ adenylate cyclase.

Gi: ↓ adenylate cyclase.

Gt: cGMP phosphodiesterase.

Question 3

What are the two phases of insulin secretion?

Why can’t both phases be released at the same time?

Answer 3

The “readily releasable pool (RRP).”

The “reserve pool.”

Granules in the reserve pool must undergo mobilization before they can gain release competence.

Question 4

What is bound to cytoplasmic lipophilic hormone receptors before the hormone binds?

Answer 4

Heat shock protein 90 (HSP90).

Question 5

What is the basic structure of insulin?

Answer 5

An A-chain connected to a B-chain by two disulfide bridges-with one disulfide bridge spanning a portion of the A-chain.

Question 6

G proteins have their own GTPase to turn themselves off, but what enzyme can accelerate the inactivation of these G proteins by up regulating their intrinsic GTPase?

Answer 6

GTPase-activating protein (GAP) -> ↑ intrinsic GTPase

Question 7

How does tamoxifen treat breast cancer?

Answer 7

Tamoxifen resembles estrogen, but after being metabolized by cytochrome P450 into 4-hydroxy-tamoxifen, attaches to the estrogen response element and recruits nuclear hormone receptor co-repressor, which promotes histone deacetylase activity (HDAC), which promotes tight binding of chromatin and inhibits transcription the cancer cells need to survive.

Question 8

What are the three units of the trimeric G proteins?

Answer 8

α β γ

Question 9

Where are estrogen receptor β’s (ERβ’s) most abundantly presented?

Answer 9

Ovaries.

Prostate.

Lower expression in lung, brain, bone, and vasculature.

Question 10

What happens to the three subunits of a G-protein receptor when it becomes activated?

Answer 10

The α subunit dissociates from the β/γ subunits.

Question 11

What is the effect of insulin on the number of insulin receptors?

Answer 11

Insulin down regulates its own receptor by decreasing rate of synthesis or increasing rate of degradation.

Question 12

What transporter will glucose use to enter the beta cells of the pancreas?

Answer 12

GLUT2

Question 13

What are the three domains of a receptor tyrosine kinase molecule?

Answer 13

An extracellular domain.

An α-helical transmembrane domain.

An intracellular domain with tyrosine kinase activity.

Question 14

What are the three major types of lipophilic hormones?

Answer 14

Steroid hormones -progesterone, estradiol, testosterone, cortisol, aldosterone, vitamin D [1,25-dihydroxycholecalciferol].

Thyroid hormones - (thyroxine).

Retinoids - (retinol, retinoic acid).

Question 15

How many times do G proteins cross the membrane?

Answer 15

G proteins are 7-transmembrane receptors!

(Apparently this is crazy high-yield)

Question 16

Explain RAS-independent insulin signaling to the best of your ability.

Answer 16

In the case of RAS-independent insulin signaling, insulin binds to RTK which recruits insulin receptor substrate-1. This time IRS-1 recruits phosphoinositide-3-kinase (PI3kinase). PI3 kinase then phosphorylates phosphoinositides to form phosphatidylinositol 3,4-bisphosphate (PIP2) and phosphatidylinositol 3,4,5 trisphosphate (PIP3). PIP2 and PIP3 then activate second messengers to stimulate the recruitment of protein kinase B to the membrane, and activate it via phosphorylation. Downstream, this causes an increase in GLUT4’s movement to the plasma membrane, as well as activation of glycogen synthase. Ultimately both result in an increase in glucose uptake and glycogen synthesis.

Question 17

Which estrogen receptor is thought to be the predominant form responsible for growth regulation of breast cancers?

Answer 17

Estrogen receptor α.

Question 18

How do sulfonylurea drugs work?

Answer 18

They inhibit the ATP-sensitive K+ channel in the beta cells of the pancreas, causing depolarization of the membrane and calcium influx into the cell, encouraging release of insulin granules.

Question 19

What five things, discussed in class, can lead to insulin resistance?

Answer 19

Down regulation of insulin receptor.

Defects in insulin signaling.

Defects in insulin receptor.

Defects in IRS 1 and IRS 2.

Serine/threonine kinase phosphorylating serine instead of tyrosine.

Question 20

What are the two main receptors involving hydrophilic hormone signaling?

Answer 20

G-protein coupled receptors (GPCRs).

Receptor tyrosine kinases (RTKs).

Question 21

What are some of the ways receptor tyrosine kinase signals are terminated?

Answer 21

Degradation of ligand by extracellular proteases.

Ligand induced endocytosis of receptor and its degradation.

RAS inactivation.

Dephosphorylation of protein targets by phosphatases.

Question 22

Describe the process of nuclear receptor binding from before the ligand attaches, to the binding of the receptor to the hormone response element.

Answer 22

Before the ligand binds, the nuclear receptor is surrounded by inhibitory proteins.

After the ligand binds, the inhibitory proteins dissociate, the activation function domain (AF 1 / “transcription activating domain”) changes confirmation, and co-activator proteins bind.

The nuclear receptor complex then binds to the receptor binding element of the hormone response element and alters gene expression.

Question 23

What is the basic pathway for the stimulation of insulin release in beta cells of the pancreas by glucose?

Answer 23

Glucose can enter the beta cells of the pancreas via GLUT2 (facilitated diffusion), glucokinase and become glucose 6-phosphate.

This glucose six phosphate travels through the TCA cycle and creates ↑ ATP.

This ATP causes the closing of an ATP-sensitive K+ channel, which causes an overall depolarization of the cell membrane.

This depolarization opens calcium channels, and the influx of calcium causes the release of insulin granules.

Question 24

What happens when a receptor tyrosine kinase (RTK) receives a signal?

Answer 24

The intracellular tyrosine kinase domain becomes phosphorylated (and usually dimerizes), causing those phospho-tyrosines to be recognized by the adapter and docking proteins which activate downstream signaling pathways via RAS-dependent and RAS-independent pathways.

Either pathway results in phosphorylation of specific protein targets in the cytoplasm or nucleus.

Question 25

Which have longer half-lives, hydrophilic medications or lipophilic medications?

What is the approximate half-life for each?

Answer 25

Hydrophilic medications last seconds to minutes.

Lipophilic medications have longer half-lives from hours to days.

Question 26

How do all lipophilic hormones bring about a change in their target cell?

Answer 26

They can either combined with the cytosolic receptor and translocates to the nucleus where they act as transcription factors, or go to the nucleus and bind with a nuclear receptor already present in the nucleus bound to the DNA.

Question 27

How is insulin signaling terminated?

Answer 27

The insulin receptor is internalized via endocytosis and either degraded by proteases or recycled later into the plasma membrane to be re-utilized.

Question 28

To what do the DNA-binding domains on nuclear receptors bind?

Answer 28

Hormone response elements (HREs) on the DNA upstream of the target gene.

Question 29

What is the function of guanine nucleotide exchange factor (GEF)?

Answer 29

Exchanges GDP for GTP in order to activate G proteins.

Question 30

What are the three major types of hydrophilic hormones?

Answer 30

Amino acid-derived - (histamine, serotonin, melatonin, dopamine, norepinephrine, epinephrine.)

Lipid metabolism-derived (acetylcholine).

Polypeptides – (insulin, glucagon, cytokines, and thyroid stimulating hormone).

Question 31

Where are estrogen receptor α’s (ERα’s) most abundantly presented?

Answer 31

Female reproductive tract: (uterus, vagina, ovaries).

Mammary gland.

Hypothalamus.

Endothelial cells.

Vascular smooth muscle.

Question 32

Explain the RAS-dependent insulin signaling pathway to the best of your ability.

Answer 32

In the case of the RAS-dependent signaling pathway, the Insulin Receptor Substrate (IRS-1) binds to the phosphorylated receptor tyrosine kinase after insulin has bound to it. The insulin receptor substrate then recruits an adapter protein called GRB-2, which activates the RAS and MAP kinase pathway. Multiple proteins are phosphorylated which leads to alterations in gene transcription – specifically ↑ transcription of glucokinase. This results in increased glucose uptake and glycogen synthesis.

Question 33

Describe the pathway of insulin production to the best of your ability.

Answer 33

Preproinsulin is created by the ribosomes on the endoplasmic reticulum. It then travels into the endoplasmic reticulum, where it is cleaved by a protease to form proinsulin. That is then folded and transported to the goal G to be packaged into immature granules. At this point the C-peptide (C-chain/connecting peptide) is cleaved by proteases to form insulin and a C-peptide remnant. This causes the immature granules to become mature granules, which now contain hexameric crystallized insulin (three dimers). The insulin and the C peptide are released together.

Question 34

What is the hormone response element (HRE)?

Answer 34

It is a specific DNA sequence in the promoter region of specific genes which binds to a lipophilic hormone-cytosolic receptor complex.

Question 35

Is cortisol water-soluble or lipid soluble?

Answer 35

Lipid soluble.

Question 36

Describe the basic mechanism of action of the estrogen receptor.

Answer 36

The estrogen receptor exists as a monomer in the nucleus.

When estrogen binds, it dimerizes, allowing it to bind to the estrogen response element on the DNA and recruit cofactors.

This binding and present cofactors promotes histone acetyltransferase (HAT), which loosens chromatin and favors transcription.

Question 37

How does serine phosphorylation prevent insulin signaling?

Answer 37

Serine phosphorylation appears to inactivate IRS 1 and IRS 2, leading to degradation of the insulin receptor.

Question 38

What five things, discussed in class, activates serine/threonine kinase?

Answer 38

Cytokines.

Free fatty acids.

Diacylglycerol.

Ceramide.

Inflammatory molecules.

Question 39

What is the relationship between the primary and secondary response to a steroid hormone?

Answer 39

The receptor complex induces synthesis of the primary response proteins.

The primary response proteins then shut off the primary response genes and activate the secondary response genes.