Anterior Pituitary Flashcards
What is the mechanism that gives rise to the appearance of the 5 secretory cell types of the anterior pituitary?
Spatio-temporal expression of various transcription factors.
Which two transcription factors is the development of somatotrophs dependent on?
Prop-1.
Pit-1.
What does a progenitor of the Prop-1/Pit-1 family tree give rise to?
Mammosomatotrophs.
What is contained within the secretory granules of the somatotrophs?
Human growth hormone.
What is the difference between type I and type II lactotrophs?
Type I lactotrophs contain few large secretory granules, they are irregularly shaped and thought to be inactive in terms of growth hormone secretion.
Type II lactotrophs contains many small secretory granules and are thought to be active in terms of growth hormone seretion.
Which type of lactotroph is thought to be active in terms of growth hormone secretion?
Type II lactotrophs.
What is contained within the secretory granules of mammosomatotrophs?
Prolactin and growth hormone.
What is meant by the fact that the pituary is plastic?
The mammosomatotrophs are able to differentiate down either somatotroph or lactotroph lineage, they can be forced down either pathway.
What is the most abundant cell type in the anterior pituiary?
Somatotrophs
What % of all cell types in the anterior pituitary do somatotrophs represent?
50% - making growth hormone the most abundant hormone of the anterior pituitary.
What is somatotroph organisation really like in vivo?
It has become apparent the pituitary isn’t organised as bunches of islands of cells.
Use of GFP under the control of a growth hormone promoter allowed visualisation of all somatotrophs using confocal microscopy and from this, it was determined that all somatotrophs are physically ocnnected, NOT islands of cells but networks of cells.
*This is important because it means activating one somatotrophs causes a cascade because they’re all connected.
What is the locus of human growth hormone?
17q22-24.
What genes are present in the hGH-hPL gene cluster?
hGH-N = gene responsible for growth hormone expression and production in the pituitary.
hPL-1 and hPL-2 are placental lactogen genes.
hGH-V = variant form of growth hormone present in the placenta.
Describe the gene organisation of hGH-N?
5 exons, 4 introns, coding region starts at the end of exon 1 and goes to exon 5.
What is the size of human growth hormone?
22Kda.
Describe neuroedocrine regulation of somatotrophs - how is GH secretion controlled?
GH is under positive control from GHRH which binds to its receptor on the somatotroph surface and activates Gs-alpha which leads to the release of GH.
Somatotrophs are under negative control - somatostatin inhibits secretion of growth hormone.
Somatostatin binds to SSTR2 or SSTR5 causing release of a inhibitory G protein (Gi) which inhibits adenylyl cyclase.
What are the 3 functions of GHRH?
- Induces somatotrophs to proliferate, the network of somatotrophs expands in the post-natal pituitary due to the action of GHRH.
- GHRH promotes GH expression.
- GHRH stimulates GH secretion.
Why is the pituitary small in the absence of GHRH?
GHRH is reponsible for post-natal expansion of the somatotroph network as it promotes proliferation of somatotrophs.
Somatotrophs account for 50% of all cells in the anterior pituitary and it is for this reason that in the absence of GHRH the pituitary is small.
Describe the structure of human growth hormone?
191 amino acids.
4 alpha helices arranged in an up-up-down-down topology.
2 disulphide bridges.
2 non-identical biding domains.
What is the pattern of human growth hormone release?
human growth hormone is released in bursts.
Compare the release of growth hormone in male and female rats
In male rats there is a burst of GH every 3 hours, this is predictable and there is a 100-fold increase in GH in blood in a matter of minutes, GH goes from virtually undetectable in the troughs to off the charts in a minute.
Female rats produce more bursts of GH but those bursts are much more irregular and sporadic, GH is tonically higher in females and alway detectable, unlike males there are no obvious troughs
What is the mechanism underlying the pulsatile secretion of human growth hormone?
Bursts of GH are the result of integration of both GHRH and somatostatin
GHRH and somatostatin both have short half-lives, GHRH stimulates pulses of GH and somatostatin inhibits those pulses.
GHRH neurones originate in the arcuate nucleus and project to the median eminence releasing their product into the primary plexus.
Somatostatin neurones originate in the periventricular nucleus and project to the median eminance releasing their product into the pituitary stalk into the secondary plexus.
GHRH seems to be released hourly, but only results in a peak in GH when the GHRH pulse coincides with a trough in somatostatin expression.
The timing is controlled by GH itself, it feeds back to switch on somatostatin and switch off GHRH.
What effect does ghrelin have on GHRH neurones?
Ghrelin binds to a receptor on the GHRH neurones and promotes the release of GH when given as a single injection.
What factors stimulate the secretion of growth hormone?
- Deficiency in energy substrate/hypoglycaemia.
- Fasting.
- Vigorous exercise.
- Increasing circulating amino acids, particularly arginine.
- Leptin
- Oestrogens
- Sleep
What effect does leptin have on growth hormone secretion?
Injections of phosphate-buffered saline compared to injections of phosphate-buffered saline + leptin result in greater bursts of GH = leptin promotes GH secretion.
What factors inhibit the secretion of growth hormone?
- Glucose.
- Adrenal function.
- Sleep (REM).
- Growth hormone/IGF-1.
- Age.
- Obesity.
How does hypercortisolism and hypocortisolism affect GH secretion?
If cortisol secretion goes down (hypocortisolism) the sensitivity of the pituitary to GHRH goes down due to reduced number of receptors = less GH secretion, even though GHRH and somatostatin are normal.
If cortisol secretion increases, somatostatin secretion goes up = less GH secretion.
Describe the structure of the GH-R gene?
Exon 2 = signal sequence.
Exon 3-7 = extracellular binding domain.
Exon 9-10.5 = intracellular binding domain + 3’UTR.
What regualtes the circulating levels of GH binding protein?
GH itself.
In mice, there is lower GHBP, this makes sense because they have much lower GH tonically.
What effect does the presence of GHBP have on plasma GH levels?
The presence of GHBP and the formation of a GHBP-GH complex delays removal/clearance of GH from the system and therefore lengthens the peaks in GH.
Describe the process of GH-receptor signal transduction
GH binds the GH-R - binding domain 1 interacts with the first receptor and the receptor flips presenting binding domain 2 of GH to the other receptor.
There is GH-induced receptor dimerisation.
This results in phosphorylation of JAK2.
This results in phosphorylation of STAT5.
This activates the Ras-Raf-MEK-MAPK pathway.
There is also increased activation of the PLC-DAG-PKC pathway.
There is increased mRNA expression e.g. of the Sox famiily of transcription factors.
How many receptors bind 1 GH?
2 - they dimerise.
Binding domain 1 of hGH-N binds to the first receptor which flips and presents binding domain 2 of hGH-N to the second receptor.
What are the key tissues with GH receptors?
Adipose tissue Bone Chondrocytes Cartilage Liver
What tissues have GH receptors?
Adipose tissue Bone Chondrocytes Osteoblasts Osteoclasts Brain Cartilage Corpus luteum Heart Kidney Intestine Liver Lung Lymphatic/immune cells Pancreas Skeletal muscle Testis
What are the actions of GH?
- Skeletal growth promotion.
- Metabolic effects - increased protein synthesis, reduces plasma glucose/utilisation, mobilisation of fatty acids from adipose tissue (lipolysis).
- Anabolic effects - DNA synthesis.
- Other effects - activation of PRL receptors - at very high levels, GH can bind to the prolactin receptor and activate.
- Inhibits preadipocyte differentiation.
Describe the dual effector theory
GH released from the anterior pituitary binds to receptors in the growth plate of long bone promoting proliferation and differentiation of chondrocytes.
GH binds to receptors on the liver to induce secretion of IGF-1 which enters circulation and binds receptors in the growth plate promoting proliferation of chodrocytes.
In response to IGF-1 binding, the chondrocytes produce more IGF-1 = more proliferation = positve feedback.
At the top of the growth plate, there is the germinal/resting zone which expresses GH receptors, GH binds and induces differentiation of these cells and they enter the proliferative zone and IGF-1 expression is induced.
Cells in the proliferative zone are responsive to IGF-1.
Under the control of IGF-1 cells undergo clonal expansion and expand and enter the hypertrophic zone controlled by T3 and corticosterone.
Chondrocytes become calcified and form trabecular bone.
What are the two drivers of skeletal growth in the dual effector theory?
IGF-1 and GH.
Cells in which zone of the growth plate express GH receptors and are responsive to GH?
Germinal /resting zone.
Cells in which zone of the growth plate express IGF-1 receptors?
Proliferative zone.
What do happens to chondrocytes in response to IGF-1?
Clonal expansion.
Describe a piece of evidence that confirms that GH causes skeletal growth?
Using a bone-specific promoter to drive GH expression, we can see that femoral length in trangenic mice increases = GH definitely is acting in the bone.
Describe a piece of evidence that confirms that the dual effector theory may not be correct
According to the dual effector theory, GH binding to the liver GH receptors induces production of IGF-1 and IGF-1 goes to the proliferative zone of the growth plate promoting clonal expansion of chondrocytes.
A liver-specific IGF-1 knockout revealed no different in grwtoh, the mice growth normal and have normal bone length suggesting perhaps IGF-1 from the liver acting on the growth plate is incorrect.
Fat cells located where are exquistely sensitive to GH?
Bone marrow.
How do we know the lipolytic effect of GH in humans is pattern dependent?
Daily injections of GH = high plasma levels of fatty acids, much higher than intravenous infusion of GH.
This suggests that bursts of GH are again more effective.
What is the result of adult-onset excess GH?
Acromegaly
What are the symptoms of acromegaly?
Coarseing of the facial features. Enlarged hands and feets. Thickening of palms and soles. Carpel Tunnel syndrome. Sweating and oily skin. Headaches. Visual disturbance. Lethargy. Reduced fertility. Lactation in the absence of pregnancy and in men.
What is the most common cause of acromegaly?
GH-secreting tumour.
What is the result of excess GH in childhood?
Gigantism.
What are the symtpoms of gigantism?
Features of acromegaly.
Excess skeletal growth.
What can be done to treat someone with GH excess?
Treat with a somatostatin analogue e.g. octreotide.
Mutate the 2nd binding site of growth hormone so it cannot initiate intracellular signal transduction e.g. pegvisomant prevents dimerisation of human growth hormone receptors.
What is the result of chilhood onset GH-deficiency?
Dwarfism.
What are the symtpoms of dwarfism/childhood onset GH-deficiency?
Increased adiposity Reduced muscle mass Increased fracture rate Lethargy Anxiety/depression Social isolation
What are the causes of GH-deficiency in children?
Craniopharyngiomas
Chromophobe tumours
Thrombosis of pituitary vessels
What is the treatment for GH-deficiency/dwarfism?
Recombinant human growth hormone.
