Actions of hormones - cell signalling

Question 1

how do hormones target specific cell types?

Answer 1

Only the target cells for a particular hormone express receptors for binding with this hormone

Question 2

what is signal transduction?:

Answer 2

Process where incoming signals are conveyed into the target cell where they are transformed into the dictated cellular response

Question 3

Describe how hydrophilic hormones alter pre-existing proteins via second messenger systems.

Answer 3

Most hydrophilic hormones bind to G-protein-coupled receptors (GPCRs)
Target cell effects of hormone binding are mediated via second-messenger systems (hormone is first messenger)

Question 4

what are the two major second-messenger pathways?

Answer 4

• Cyclic adenosine monophosphate (cAMP)

- Ca2+

Question 5

what do both pathways have in common?

Answer 5

Both pathways use a G protein (inner surface of PM) which acts as an intermediary between the receptor and an effector protein

Question 6

what is the active and inactive state of G proteins?

Answer 6

Guanosine triphosphate (GTP) when active, or
Guanosine diphosphate (GDP) when inactive

Question 7

what does an inactive G protein consist of?

Answer 7

3 subunits: alpha, beta and gamma

GDP is bound to the alpha subunit

Question 8

Describe the signal transduction with GPCRs.

Answer 8

• Hormone (1st messenger) binds to cell surface receptor
• Receptor attaches to G protein, resulting in release of GDP from the G protein complex.
• GTP then attaches to the alpha subunit, which activates the G protein.
• The alpha subunit dissociates from beta and gamma subunits and moves along the inner surface of the PM until it reaches an effector protein (enzyme or ion channel).
• The alpha subunit links up with the effector protein and alters its activity

Question 9

what are the two major G-protein coupled effectors?

Answer 9

1) Adenylyl cyclase (cAMP)

2) Phospholipase C (Ca2+)

Question 10

what does adenylyl cyclase do?

Answer 10

converts (intracellular) ATP to cAMP

Question 11

what does cAMP do?

Answer 11

activates a specific intracellular enzyme Protein Kinase A (PKA)

Question 12

what does PKA do?

Answer 12

phosphorylate a pre-existing intracellular protein (Thr residues e.g. a metabolic enzyme)

Question 13

why can cAMP induce differing responses in different cells?

Answer 13

Different cell types express different proteins that are phosphorylated by PKA

Question 14

what is the effect of glucagon ?

Answer 14

Increased glycogen
breakdown & reduced
glycogen synthesis

Question 15

Describe the actions of phospholipase C & Ca2+.

Answer 15

• Binding of the hormone, via G-protein, activates an enzyme Phospholipase C (PLC; inner side of PM)
• PLC breaks down PIP2 (phosphatidylinositol bisphosphate) to yield DAG (diacylglycerol) and IP3 (inositol trisphosphate)
• DAG remains in the lipid bilayer of PM, IP3 diffuses into the cytosol
• IP3 stimulates release of Ca2+ from the ER
• Ca2+ can influence the activity of ion channels, enzymes, secretion of other hormones etc. to produce a biological response.

-DAG regulates Protein Kinase C -(PKC) activity
PKC phosphorylates (Ser residues on) specific intracellular proteins.
Phosphorylated intracellular proteins accomplish the biological response to hormone binding

Question 16

Describe why amplification is important.

Answer 16

Low concentration of primary messenger- trigger more pronounced cellular response

Question 17

what peptide hormone does not act via a G-protein-coupled receptor?

Answer 17

insulin

Question 18

what does insulin bind to?

Answer 18

insulin binds to its cell surface receptor and activates ‘autophosphorylation

Question 19

what is an insulin receptor an example of?

Answer 19

tyrosine kinase

Question 20

Describe how lipophilic hormones promote synthesis of new proteins.

Answer 20

• Steroid hormones, calcitriol and thyroid hormone can all cross the cell membrane and bind to intracellular receptors (cytoplasmic or nuclear).
• Each receptor has a hormone binding region and a DNA binding region.
• The hormone-receptor complex binds with DNA at a specific attachment site: hormone response element (HRE)
• DNA transcription of specific genes can be induced (or suppressed)
• Result: changes in cellular expression of proteins encoded by these genes

Question 21

Summarise the overall actions of steroid hormones and hydrophilic peptide hormones.

Answer 21

• Steroid hormones-increase expression of proteins or turn off gene expression to reduce number of proteins
• Hydrophilic peptide hormones alter proteins that already exist