Lecture 28 - MIDTERM 3

Question 1

What are the 5 major classes of signaling molecules?

Answer 1

– neurotransmitters, hormones, cytokines, growth factors and pheromones

Question 2

What are the different types of hormones?

Answer 2

– Hormones can be 1) peptides (insulin or glucagon)

2) steroids (glucocorticoids and sex hormones)
3) amino acid derivatives (epinephrine, etc.)

Question 3

What are the different receptor classes?

Answer 3

– Nuclear Receptors: intracellular receptors

• • Cell Membrane:
    
    • —> G protein-couple receptors (ie. glucagon
  • —–> Receptor tyrosine kinases (ie insulin(: extracellular ligand binding causes auto-phosphorylation and activation of intracellular kinase domain
  • —–> Ligand-gated ion channels: ion transport and nerve transmission (acetylcholine receptor)

Question 4

What is the basic mechanism of signal transduction?

Answer 4

– Molecules are released from cells, travel to target cells and interact with receptors

– the interaction stimulates or inhibits events within the target cell

Question 5

What is the nature of ligand-receptor interaction?

Answer 5

– ligand-receptor interaction is very specific such that receptors don’t exert their downstream effects until corresponding ligand binds

Question 6

How do hormones work?

Answer 6

– hormones are secreted by endocrine glands into the bloodstream

– hormonal action works at low concentration and is short-lived, so that tissues can respond to changes quickly

– hormonal regulation involves a hierarchy of cell ty[es acting on each other to stimulate or modulate release or action of a hormone

Question 7

Describe G protein-coupled receptors (GCPR)

Answer 7

– interacts with a G-protein (guanine nucleotide-binding protein), a signals transducer

– signal transduction pathway occurs where a second messenger is synthesized

– sequence of human Beta2 - adrenergic receptor

– 7 transmembrane domains (rich in hydrophobic a.a.)

– Ligan binding is on extracellular domain, signaling domain is on intracellular domain

Question 8

T or F, adenylate cyclase forms cAMP from ATP

Answer 8

True

Question 9

What is the mechanism of GPCR activation of adenylate cyclase and synthesis of cAMP?

Answer 9

– activation by alpha subunit is limited by how quickly GTP is hydrolyzed

– GCPR can also activate different second messengers (ie. PI to DAG + IP3)

– Gs stimulates adenylate cyclase

– Gi inhibits adenylate cyclase

– alpha subunit is really important bc it activates nearby enzyme

– the alpha subunit then diffuses away and binds to adenylate cyclase

– adenylate cyclase takes ATP and makes cAMP

– after the alpha subunit activates adenylate cyclase it becomes in active

Question 10

Describe the action of epinephrine. (GPCR)

Answer 10

– Hormone (first messenger) binds to surface receptor

– stimulates cAMP formation (second messenger), which influences phosphorylation of target enzymes

– distribution of receptors on specific cells determines how hormones only effect certain tissues

Question 11

Describe the epinephrine pathway.

Answer 11

1. Hormone binding activates adenylate cyclase
2. cAMP binds to the R subunits of the PKA R2C2 tetramer, causing their dissociation from the C subunits –> once cAMP is activated it activates protein kinase A and then a signaling cascade occurs
3. The active C monomer of PKA activates phosphorylase b kinase
4. The active kinase converts the inactive phosphorylase b to the active phosphorylase a
5. Active phosphorylase a catalyzes glycogen breakdown

Question 12

What is gene expression activation by GPCR cascade?

Answer 12

– cAMP response element-binding protein (CREB)

– effector makes secondary messenger

– cAMP binds to regulatory subunits

– CREB binds to section of DNA which then promotes gene expression

Question 13

What is an overview of receptor tyrosine kinases (ie. insulin signaling)?

Answer 13

– they behave differently than G protein-coupled receptors

1. Cell surface receptors that are directly linked to intracellular enzymes (kinases)
2. Includes receptors for most growth factors (NGF, EGF, PDGF), insulin, and Src
3. Common structure: N terminal extracellular ligand-binding domain, single TM domain, cytosolic C-terminal domain with tyrosine kinase activity
4. Can be single polypeptide or dimer

Question 14

Describe how receptor TK responds to a ligand.

Answer 14

– EGF receptor but insulin receptor is very similar, except that it is already dimerized

– ligand binding stimulates dimerization of the monomeric receptor

– dimerization triggers activity

– tyrosine takes phosphate from ATP to phosphorylate other receptor –> cross phosphorylation

– Mechanism of activation of TKRs:

• ligand binding induces receptor dimerization (receptor crosslinking)
• dimerization leads to autophosphorylation of the receptor (cross-phosphorylation) –> signaling cascade that is activated
• phosphorylation increases kinase activity and also creates specific new binding sites
• proteins that bind to these new binding sites transmit intracellular signals

Question 15

T or F, insulin receptor can affect many signaling pathways

Answer 15

True; responds to “fed” state

Question 16

What are Insulin receptor effects on gene expression, glucose uptake and glycogen synthesis?

Answer 16

– remember insulin receptor is already dimerized

– ligand binds to receptor

– auto-phosphorylation (cross phosphorylation)

– proteins that bind this domain are recruited and phosphorylated

– These proteins can have many effects:

      - -> keeping glycogen synthase active 
      - -> promoting glucose uptake
      - -> transcribing genes that promote cell division and growth 

overall points: insulin, once it binds, can promote gene expression; once hormone binds there is a signaling cascade

Question 17

How do nuclear receptors act as transcription factors upon hormone binding?

Answer 17

*Note: only affect gene expression

– tend to be longer-term changes in growth and differentiation because receptor acts on DNA and affects gene transcription

– hormones can transverse the cell membrane (can go through membrane, no problem), bind receptor, and enter nucleus to bind to specific DNA elements and start transcription

Question 18

What is the importance of cysteine on conserved DNA domains?

Answer 18

– conserved DNA domains among nuclear receptors

– cysteine residues allow hormone receptors to bind to DNA

Question 19

Explain the mechanism of Estrogen/Progesterone receptors and cancer drugs.

Answer 19

– Nuclear receptors possess a zinc-containing N-terminal DNA binding domain and a C-terminal hormone-binding domain

• • Tamoxifen binds to estrogen receptor, keeps it form turning on estrogen-responsive genes (acts as an antagonist) that are important to growth of some types of breast cancer –> blocks breast cancer cell growth proliferation
    - - mimics effects of estrogen
• • RU486 is a progesterone receptor antagonist, and is also known as the “abortion pill.” It prevents a fertilized ovum from implanting in the uterus
    
    • –> mimics progesterone receptor; blocks effects of receptors

Question 20

Explain Signal Transduction, Growth Control and Cancer.

Answer 20

– Many tumor contain genetic mutations in proteins involved in signal transduction: protein kinases, G proteins, nuclear receptors, growth factor/receptors, etc.

– some cells produce an excess of a normal signal transduction protein

– genes that cause abnormal growth (cancer) from these changes are called Oncogenes (ex. Ras, Myc, p53) –> these are such important genes that mutations of these can cause cancer

– Genes that have the possibility of becoming Oncogenes are called Proto-Oncogenes

– Proto-oncogenes function to facilitate cell growth for human development while oncogenes often promotes proliferation of abnormal cells that lead to cancer

*Note: if it’s multiple mutations that are occurring at once then DNA repair can’t help; more than 2-3 mutations in oncogenes can cause cancer

Question 21

What is an examaple of how an oncogene works?

Answer 21

– 1. Mutations can cause the receptor to be consecutively active (always turned-on), even with an external signal –> whether you have a ligand or not

– 2. Alternatively, expressing MORE of the receptors, non-mutated, can amplify the signals and cause a similar effect

– cancer is continuous cell division

Question 22

What are tumor suppressor genes and how do they work?

Answer 22

– The normal form of this gene suppresses formation of tumors. Loss of function mutations lead to tumor formation

– most commonly known tumor suppressor gene is p53, a DNA-binding protein that regulates metabolic processes following DNA damage (any loss of p53 promotes cancer growth)

Example: p53

• • Tumor-suppressor genes: inhibit cell division
• • These genes regulate cell division so that it normally occurs only for growth/repair
• • Cancer cells lack restraints on cell division

– If DNA damage is moderate, cell cycle arrest occurs

– If extensive, cells will undergo apoptosis, programmed cell death

– Loss of the p53 checkpoint, would involve inappropriate cell growth