55) Hormone synthesis and action

Question 1

How does protein/peptide hormone synthesis take place?

Answer 1

• First we have transcription of DNA to RNA and then post-transcriptional processing occurs (removal of introns and modification of 3’ and 5’) which forms mature RNA from RNA
• The mature RNA is then translated into protein using tRNA to transfer amino acids ti the ribosomes
• Post-translational processing occurs where large pre-prohormones are cleaved off, proteins are folded and smaller groups are added (e.g. sugar and phosphates)

Question 2

Why is post-translational processing stage important for protein/peptide hormones?

Answer 2

• These hormones may be produced in the cell and stored as precursors in vesicles ready to be activated and released

Question 3

Describe the structure of the large precursor protein hormone?

Answer 3

• Preprohormones are made as a result of translation and contains a hormone sandwiched between a signalling sequence (at the start) and a peptide sequence (end terminal).
• The signalling sequence is recognised and causes the protein to be processed in the Golgi apparatus
• The peptide sequence directs the large proteins molecules to the right part of the cell

Question 4

How is the precursor protein converted to active hormone?

Answer 4

• First the peptide sequence guides the protein to its proper destination
• Then the cleavage of the signal sequence from the peptide occurs which leaves the hormone and the peptide sequence on the end terminal
• Finally the accessory protein/peptide sequence on the end terminal is also cleaved

Question 5

Describe the process of insulin secretion

Answer 5

• First the insulin gene is transcribed from the DNA through RNA splicing and then translation
• The translated polypeptide chain (pre-proinsulin) is long and has a signalling sequence at the end terminus
• The signal sequence directs the protein to the RER where the protein is produced
• The protein folds to form disulphide bonds between between different parts of the protein
• The signal sequence is then removed and the resulting molecule is proinsulin
• Finally the protein is transported to the Golgi apparatus where a large portion of the protein is cleaved (called the C-chain)
• This leaves behind an insulin molecule consisting of the Alpha and Beta chain held together by disulphide bonds
• The insulin molecule and the C-chain are held within secretory granules within pancreatic cells

Question 6

How does steroid hormone synthesis occur?

Answer 6

• All steroid hormones are produced from cholesterol (4 Carbon rings held together)
• Cholesterol is processed within steroidogenic cells (e.g. in the gonads and adrenal cortex) via the action of trophic hormones
• When the trophic hormones (e.g. LH and FSH) arrive at these cells they stimulate the target cells to switch on signalling molecules (c-AMP)
• The signalling molecule activates a protein (Protein Kinase A) which causes increased release of cholesterol from the lipid store and synthesis of StAR proteins
• StAR allows for the transport of cholesterol from the lipid store into the mitochondria
• In the mitochondria enzymes containing cytochrome P450 cleave the cholesterol into pregnenolone which is the steroidogenic precursor.
• This leaves the backbone (4 Carbon rings) of cholesterol which is used to make the other steroids
• Further cleavage and other hydroxylation reactions allows cholesterol to be converted into any steroid hormone

Question 7

How are thyroid hormones synthesised?

Answer 7

• Synthesis takes place in the follicular cells of the thyroid glands
• TSH released by the hypothalamus acts on follicular cells which allows the active uptake of I- (iodine ions) into the cell from the blood stream via the Na+/I- symporter
• This concentrates I- levels within the cell which is then moved towards the apical membrane
• Here it is oxidised into an iodinating intermediate through the enzyme Thyroid Peroxidase (which requires hydrogen peroxide)
• This allows for the iodination of tyrosine residues which are located inside thyroglobulin
• There can either be mono- or di- iodinating events which leads to a further coupling of the iodinated thyroxines together to produce T3 and T4
• These are stored in the colloid as droplets of thyroglobulin
• When the T3 and T4 are required to be released they are cleaved from the thyroglobulin and transported back out from the colloid into the follicular cell and from there into the blood stream

Question 8

Describe the hormone action of peptide and protein hormones

Answer 8

• They are water soluble that can only act on cell surface receptors of their target cells
• This is because they cannot pass through the cell membrane
• They require the activation of second messenger molecules to activate the enzyme and cytoplasmic machinery

Question 9

Describe the hormone action of steroid and thyroid hormones

Answer 9

• They lipophilic (lipid soluble) and can pass through the membrane
• Each steroid hormone has their own specific receptor within the cell which they can activate directly
• These receptors are called nuclear receptors as they have their effects in the nucleus although they are found in the cytoplasm initially

Question 10

Explain the adenyl cyclase/cAMP pathway for proteins and peptide hormones

Answer 10

• First the hormone binds to GPCRs causing the coupled G proteins to become activated and so they start to convert GTP to GDP
• This causes the alpha subunit of the G protein to dissociate and activate adenylate cyclase which converts ATP into cAMP
• cAMP activates intracellular signalling molecules such as Protein Kinase A
• Protein Kinase A activates cellular proteins which either have a direct effect (e.g. enzymes) or they can be transcription factors which can activate gene transcription responses when stimulated

Question 11

Explain the Phosphoinositide pathway (via DAG formation) for proteins and peptide hormones

Answer 11

• First the hormone binds to GPCRs causing the coupled G proteins to become activated and so they start to convert GTP to GDP
• This causes the alpha subunit of the G protein to dissociate and activate phospholipase C
• Phospholipase C converts PI3 to PIP3 and then activates Diacyl Glycerol (DAG) which activates downstream intracellular signalling
• The production of IP3 causes Ca2+ to be released from the SER which causes further downstream effects

Question 12

Explain the Raf/MEK/ERK1/2 pathway for proteins and peptide hormones

Answer 12

• Upon ligand binding the tyrosine residues on the intracellular part of the molecules become phosphorylated and activated
• This activates intracellular signalling accessory proteins (the first of which is called Ras)
• Activated Ras then activates Raf which then becomes phosphorylated which then activates MEK which also becomes phosphorylated
• Activated MEK then activates ERK 1/2 which becomes phosphorylated
• The phosphorylated ERK protein translocates into the nucleus where it activates downstream transcription factors leading to gene activated response

Question 13

Explain the Phosphatidylinositol kinase (PDK)/ATK pathway for proteins and peptide hormones

Answer 13

• Ligand binding causes tyrosine phosphorylation on the intracellular side of the receptor
• This activates PI 3-kinase which then activates AKT and downstream effectors such as mTOR
• mTOR is a major signalling kinase that is regulated in major pathways of the cell
• mTOR controls protein synthesis and nutrient metabolism

Question 14

Explain the JAK/STAT pathway for proteins and peptide hormones

Answer 14

• Upon ligand stimulation the receptor gets dimerised and the receptor (as well as the JAK proteins on the receptor) get phosphorylated
• This causes the STAT protein to be phosphorylated which will translocate into the nucleus
• Here it will bind to response elements in the promoter regions of genes within the DNA and activate transcription.

Question 15

What is the function of steroid hormones?

Answer 15

• Steroid hormones work systemically and so have effects on several tissues
• For example in females oestrogen controls menstrual cycle, breast tissue development, fertility, etc
• For example in males testosterone controls reproductive and supportive organs and development of sexual characteristics in men
• Corticosteroids have several physiological and immune pathways
• Mineralocorticoids control salt and water balance

Question 16

Describe the structure of steroid hormones

Answer 16

• Steroid hormone receptors are a family of transcription factors which exist in the cytoplasm or nucleus
• They have many structural units that activate transcription from DNA
• They have an N-terminal domain followed by at DNA binding domain (DBD). Then there is a hinge region along with a Ligand Binding Domain (LBD) and finally a C-terminal domain
• The LBD binds to specific steroid molecules with high affinity
• The DBD contains Zinc finger domains (Zn ion held by 4 cysteine residues leading to formation of a finger) which helps them binds to specific sequences on DNA
• There is an activation function domain in the C/end terminus which helps them to recruit gene activation machinery allowing them to switch on target genes

Question 17

Describe the process of how nuclear receptors work

Answer 17

• Nuclear receptors are found in the cell bound to Heat Shock Protein (HSP)
• The hormone (due to its lipid solubility) can pass through the plasma membrane and will bind to its own receptor
• This causes the receptors to dimerise resulting in the HSP dissociating
• The dimer that is bound to its hormone will translocate into the nucleus where it binds to specific regions in the DNA called Hormone Response Elements (HRE)
• HREs are located upstream on steroid responsive genes
• The nuclear receptor will then recruit gene activation machinery which will then cause transcription of those steroid responsive genes
• This will lead to the target protein being formed

Question 18

Why is it important to know about hormones, receptors and their signalling pathway?

Answer 18

• It allows us to understand endocrine disorders and provide targets for the development of new drugs

Question 19

What are the different types of diabetes and how are they caused?

Answer 19

• Diabetes is a disorder of protein/peptide hormone signalling
• Type 1: Due to the autoimmune destruction of the pancreatic islets (which secrete insulin into the system). This results in absolute insulin deficiency
• Type 2: (Also called insulin resistance) and is due to insulin resistance or partial loss of insulin production. It is often associated with obesity

Question 20

What is aromatase?

Answer 20

• A steriod hormone that converts androgens (e.g. testosterone) into estrogens

Question 21

What can aromatase deficiency cause?

Answer 21

• In men aromatase deficiency can cause elongated bones and a long stature as aromatase is needed for closure of the bones during puberty
• In women it means that children are born with male type characteristics, ambigious genitalia and enlarged clitoris

Question 22

What is androgen insensitivity?

Answer 22

• Lack of functional receptors causing a completel lack of response to testosterone.
• XY embryos develop as females even tho testosterone levels are normal to high
• Some females may not even realise they are XY until internal examinations are done as there can be some developmental anomalies and often infertility

Question 23

What is androgen deficiency?

Answer 23

• This can affect men of all ages
• In this case hormone levels are low
• If it occurs before puberty it can cause delayed puberty and developmental abnormalities and lack of sexual characteristics (e.g. no voice changed, reduced muscle growth).
• In older men it can cause loss of libido (sex drive), impotence (erectile dysfunction), loss of muscle mass

Question 24

What is goitre and how is it caused?

Answer 24

• Enlargement of thyroid glands
• Under normal circumstances the thyroxine hormone provides a negative feedback system to the anterior pituitary
• However if there is a lack of iodine and a lack of thyroxine production there will no longer be this negative feedback loop
• This causes an increased TSH secretion which will circulate in the body and cause the thyroid gland to grow
• If this continues it will cause an abnormal growth in the thyroid glands called goitre
• Graves disease can also cause goitre.
• This is where the receptor for the TSH is continually active cuasing overstimulation of the thyroid gland leading to goitre
• Thyroid cancers can also lead to goitre