Topic B

Question 1

What are the essential functions of cell to cell communication?

Answer 1

1. Regulation of development and organisation of cells into tissues
2. Control of death, growth and division of cells
3. Coordination of a diverse range of cellular activities

Question 2

What are the 3 types of hormones and examples:

Answer 2

1. Polypeptide/protein hormones:
   - stored in secretory vesicles for up to 1 day as pro-hormones
   - secretion is regulated by other hormones
   - circulate free in the blood
   - bind to cell-surface receptors
   - relatively short lifespan= minutes
   e. g. insulin, glucagon, leptin, growth hormone
2. Peptide-amine hormones:
   - derived from the amino acid tyrosine
   e. g. epinephrine and norepinephrine:
   - synthesised in adrenal medulla and CNS
   - stored in vesicles for several days
   - secreted in response to signals from CNS
   - free in blood
   - bind to cell surface receptors
   - very short lifespan (seconds)
   e. g. thyroid hormones- T3 and T4:
   - synthesised in thyroid gland
   - lipophilic- bind intracellular receptors
3. Lipophilic hormones:
   e. g. steroid hormones: testosterone, estradiol and cortisol
   e. g. vitamin D
   - bind intracellular receptors
   - transported in blood attached to plasma proteins
   - longer lifespan

Question 3

What are nuclear receptors?

Answer 3

• intracellular receptors that mediate signals from lipophilic ligand (e.g. steroid hormones, thyroid hormones) and transmit the signal to the nucleus of the cell to alter gene expression and physiology
• many nuclear receptors function as ligand-dependent transcription factors

Question 4

Describe a nuclear receptor structure:

Answer 4

• DNA binding domain: e.g. zinc fingers; that will bind with the major groove of DNA
• Transcription activating domains: that bind other molecules that help regulate gene expression such as coactivator proteins
• Ligand binding domain: binding of the ligand to this domain leads to a confirmational change allowing DNA binding and transcription activating domains to bind their targets; binding of ligand may remove inhibitory protein complexes

Question 5

What is the basic function of a nuclear receptor?

Answer 5

1. Bind ligand within the cytoplasm or nucleus
2. Ligand binding to the ligand binding domain causes the receptor to translocate to the nucleus (if not already there)
3. The ligand binding the ligand binding site causes a confirmational change in the receptor- inhibitory proteins are removed, coactivator proteins attach to the transcription-activating domain and the DNA binding domain binds to the target DNA

Question 6

What is a SERM/SARM?

Answer 6

SERMs= selective estrogen receptor modulators 
SARMs= selective androgen receptor modulators

Question 7

How do SERMs/SARMs work?

Answer 7

• These molecules bind to estrogen receptors/androgen receptors an recruit different co-activators/co-repressors (usually co-repressors) and thus alter the effect the activation of the ER/AR has on genes. They can be agonists (upregulate the receptor activity) or antagonists (downregulate receptor activity) depending on cell type or tissue

Question 8

Why is the drug Tamoxifen perhaps not a good choice for treating advanced breast cancer?

Answer 8

• Tamoxifen is a ER antagonist in breast tissue that prevents estrogen signalling and thus prevents proliferation of some breast cancers- however it is a ER agonist in endometrium and it may promote tumour growth.

Question 9

For treating breast/prostate cancer what other treatments other than SERMs/SARMs are used?

Answer 9

• Targeting the production of the ER/AR ligand e.g. estrogen/testosterone via inhibiting enzymes

Question 10

What is a G-coupled protein receptor (GCPR)?

Answer 10

• A large cell surface receptor with 7 membrane spanning segments that interacts directly with a trimeric GTP-binding protein (G-protein)
• When the ligand binds to the extracellular receptor domain it activates the G-protein and the G-protein goes onto active a cascade of second messengers
  e. g. beta-adrenergic receptors

Question 11

How does GPCR activation occur?

Answer 11

1. Ligand binds with GPCR on the extracellular domain
2. Receptor changes confirmation and interacts with the inactive GDP bound G-protein
3. Causes the G-protein to eject GDP for GTP which induces a confirmational change in the G-protein causing it to release the receptor
4. The activated G-protein activates a range of second messengers including adenylyl cyclase which causes the conversion of ATP -> cAMP
5. cAMP activates protein kinase which phosphorylates many other proteins which induces a change in the cell

Question 12

What are 5 ways to down-regulate GPCRs?

Answer 12

1. Receptor sequestration (receptor and ligand ingested into endosome- ligand removed- receptor returned to cell surface)
2. Down-regulation of receptor (receptor and ligand ingested into endosome and broken down by lysosome)
3. Receptor inactivation
4. Inactivation of signalling proteins
5. Introduction of inhibitory proteins

Question 13

What is an enzyme coupled receptor?

Answer 13

• A cell-surface receptor where the extracellular domain of the enzyme binds a ligand and becomes a dimer (thus activated) which in turn activates a sequence of second messengers that can alter cytosolic proteins or act on DNA to induce/repress gene expression

Question 14

What are the 3 main families of enzyme coupled receptors?

Answer 14

1. Receptor tyrosine kinases
2. Tyrosine-kinase associated receptors
3. Receptor serine/threonine kinases

Question 15

How do receptor tyrosine kinases work?

Answer 15

1. The signal molecule e.g. a growth factor will bind the exracellular domain of an inactive monomer RTK causing it to become dimerised
2. The dimerization brings the two tyrosine kinase domains of the RTKs together causing them to autophosphorylate and activate
3. The activated tyrosine kinase domains cause the C terminus tails of the dimer tyrosine residues to become autophosphorylated
4. This creates new binding sites- the specificity of which are determined by the amino acids residues surrounding the phosphorylated tyrosine residues- for signalling proteins to bind to and become activated via phosphorylation e.g. RAS, PI-3 kinase etc.

Question 16

What is the Ras Family of GTPases? What do they relate to?

Answer 16

• A common family of downstream signalling proteins activated by the signalling cascade of RTKs
• A monomeric GTP protein that oscillates between inactivate (GDP bound form) and active (GTP bound form)
• Provides a crucial link in the intracellular signalling cascade of RTKs

Question 17

Give an example of an RTK signalling cascade involving Ras:

Answer 17

1. RTK binds signalling molecule and becomes dimerised activating the tyrosine kinase domain causing an auto phosphorylation of the C terminal tail tyrosine residues
2. A secondary adaptor protein binds via its SH2 domain to the phosphorylated tyrosine residue(s) on the activated RTK
3. The SH3 domain of this secondary protein that is also bound to the activated RTL will bind to a Ras-GEF protein (guanine exchange protein) activating it
4. The Ras-GEF protein stimulates the exchange of the GDP on an inactive Ras protein for a GTP which in turn activates the Ras protein causing a conformational change and allows for a series of downstream signals including the MAP kinase pathway

Question 18

What is the MAP kinase pathway?

Answer 18

• A signalling pathway activated by the activation of Ras protein as part of a RTK signalling cascade
• It is a serine/threonine signalling cascade:
  1. Activated Ras protein will activate the MAP kinase kinase kinase (Raf) which is a serine/threonine kinase that phosphorylates MAP kinase kinase (Mek)
  2. Phosphorylation activates Mek and causes it to phosphorylate the serine/threonine residues of the MAP kinase (Erk)
  3. Erk goes onto phosphorylate a number of cellular proteins resulting in changes in protein activity and changes in gene expression
• This Ras-Raf-Mek-Erk signalling pathway can be inhibited- used to treat cancer and disease

Question 19

What are receptor serine/threonine kinases?

Answer 19

• These receptors bind signal molecules such as TGF-B and act via the following mechanism:
  1. Binding of the signal e.g. a growth factor, causes the dimerization of two receptors which results in activation of the serine/threonine kinase domains
  2. This triggers the binding of the secondary protein e.g. Smads which bind to the tail of the receptor and become phosphorylated and detach forming a trimer complex
  3. This Smad protein complex moves into the nucleus and acts as a transcription regulatory complex

Question 20

What is the difference between type I and type II diabetes Mellitus?

Answer 20

Type I diabetes:
- Also known as juvenile diabetes and insulin dependent diabetes mellitus (IDDM)
- An autoimmune disease in which there is a destruction of insulin producing beta cells in the pancreas
Type II diabetes:
- Much more prevalent in Australia (90% of diabetes cases)
- Also known as non-insulin dependent diabetes mellitus (NIDDM)
- The cells of the body are resistant to the effects of insulin and ultimately there will be impaired insulin production

Question 21

How insulin synthesised and stored?

Answer 21

• Insulin is produced by B cells of the islets of Langerhans in the pancreas
• Syntheised as a preprohormone is is cleaved to produce proinsulin
• It is stored as proinsulin in vesicles until a singal (high levels of circulating blood glucose) causes them to be secreted into the blood stream

Question 22

How does the secretion of insulin occur?

Answer 22

• When blood glucose is high e.g. after a meal GLUT 2 receptors (which have a high Km) will transport glucose into the pancreatic β cells
• When glucose is transported into the cell if it is in high enough concentrations it will be converted to glucose 6 phosphate by hexokinase IV and then it will move down the glycolytic pathway, CAC and the ETC leading to the production of ATP
• An increase in ATP levels in the pancreatic β cells close ATP gated K+ channels in the plasma channels in the cell membrane, leading to a depolarisation of the cell membrane
• The depolarisation triggers the opening of a voltage dependent Ca2+ channel causing an influx of calcium into the cell and a increase in intracellular calcium levels
• The increase in cytosolic calcium levels also causes the release of calcium from the ER of the cell leading to a further increase in cytosolic calcium
• The high calcium levels triggers the secretion of insulin from the cell

Question 23

How does insulin affect fatty acid synthesis?

Answer 23

• Insulin stimulates the entry of glucose into the glycolytic pathway which results in the production of pyruvate which can be converted into acetyl-CoA which in turn is converted into fatty acids which can be used made into TAGs for efficient storage
• A lack of insulin leads to an increase in lipolysis and a failure to synthesise FAs
• This also elevates ketone levels

Question 24

Describe the effect of Type I diabetes on lipid metabolism:

Answer 24

• No stimulation of the uptake of TAGs by adipocytes
• No stimulation for conversion of FAs to TAGs
• No inhibition for the breakdown of TAGs to fatty acids and glycerol
• There is an elevation of lipids in the blood stream