Lecture 11: GH and IGF Axis

Determinants of Human Growth

• Genetically determined

Fulfilment of Growth Potential

• Whether of not an individal reaches is this is determined by their nutrition, health and growth

Rate of Growth in:

a) Utero

b) Infancy

c) Adolescence

d) End of puberty

• a) Fastest - rate of 70cm per year

  • baby ~50 cm when born

• b) rapid period of growth

• c) rate accelerates to 10cm/year

• d) ceases

What Controls Post-Natal Growth

• GH and IGF-I hormone control

  • The axis is a key contributor to growth, with a contribution from thyroid hormones and gonadal steroids

Growth in Size Due to Tissue Accretion

• It is a growth in height due to an increase body tissue, growth in organs and soft tissue

What are Organ, Bone and Soft Tissue Growth Dependent on?

• Co-ordianted cellular function

Importance of Cellular Function

• Required to ensure:

  • optimal growth

  • Cell survival

  • Proliferation – cell divides – increase in N.o

  • Cell hypertrophy: cell increases in size

  • Differentiation

  • Metabolism – nutrients and metabolites required by cells to survive, grow and differentiate – signals important in how the cell uses and releases various nutrients

hypothalamic-pituitary-somatotropic axis.

• Hypothalamus release GHRH which acts as GHRH receptor at the APG Somatotropes

  •   GHRH receptor is a GCPR - causes an increase in cAMP      

• Somatotropes release GH into the circulation to act on its target organs

• Can have localised production of IGF-1 in response to GH e.g produced at the bone and acts within (auto/paracrine effects)

Organs With GH Receptor

• bone,

• adipose tissue (fat),

• muscle

• liver

Direct Actions of GH

• stimulates GH receptors on the target organs

Indirect Actions of GH

• Mediated through IGF-1 following its action in the liver

  • can act at IGF-1 receptors present on the muscle, adipose tissue and bone

Endocrine IGF-1

• Produced in the liver

• Circulates in the blood and travels to various organ

• Can have localised production in response to GH, produced at the bone and acts within via auto/paracrine effects

Growth Hormone Gene

• Part of a cluster composed of 5 closesly releated genes on chromosome 17

• GH-N expressed in the APG

  • normal gene

• GH-V expressed in placenta

  • related to hPL

• Arise through ancestral gene duplication 3.5 x 10⁸ years ago

  • Lots of sequence homology between species      

• human GH » 75% sequence homology with rat  & bovine GH

Growth Hormone (GH-N)

• Gives rise to:

  • 22 kDa protein (191 amino acids)

    •   Most abundant variant in plasma (90% of GH in plasma)

  • 20 kDa (deletion of residues 32-46)

    • Smaller variant – specific role is unknown

Growth Hormone Synthesis

• Produced as a precursor protein

  • N-terminal signal peptide is cleaved when secreted

• Secreted in pulses (more pronounced in males than females)

  • Predominantly released at night

• Pulsative release has implications for hormone measurements – must take multiple measures at given intervals

IGF-1

• gene on chromosome 12

• 7.5 kDa (70 amino acids) – significant homology with insulin

• Can bind to the insulin receptor at high concentrations

  • hormone is functionally similar to insulin

GH and IGF-1 Actions

• promote growth in long bones, soft tissues & organs

• effects on cellular:

  • proliferation – potent mitogens

  • survival – survival factors -prevent apoptosis

  • differentiation – stimulate pre-cursors

  • metabolism – affect storage and use of metabolites

5 Zones of Growth Plate

• Present at both ends of long bones and consist of

  • Reserve zone

  • Proliferation zone

  • Maturation zone

  • Hypertrophic Zone

  • Invasion Zone

Reserve Zone

• Small cluster of progenitor cells sat within the matrix of collagen and proteoglycans

  • Progenitor cells provide cells that can move into the growth plate - present here and proliferate to enter the proliferative zone

Proliferative Zone

• Progenitor cells undergo proliferation and enter this zone where they undergo further proliferation as chondroblasts

• Cells begin to orgnaise as columns

Maturation Zone

• Cells move in from the proliferative zone where they mature and become chondrocytes and move into the next

Hypertropic Zone

• Chondrocytes enter the zone and increase in size

• Cells begin to secrete a matrix composed of setpi, between columns of cells

  • This matrix is calcified and acts as a scaffold for new bone

Formation of The Bone

• Hypertrophic cells within the lacunae undergo apoptosis and leave holes in the matrix, which then calcify to form the basis of the bone

• Osteoblasts then invade from the bone marrow to the trabachea and form the new bone of cartilage matrix

  • if reserve cells are present - they feed the process and allow the growth and lengthening of bone

Open Growth Plate

• Bones are growing

• Cells are able to ‘feed’ the process

Importance of GH and IGF-1

• Stimulate:

• the progenitor cells,

• the proliferation of chondroblasts in the proliferation zone,

• maturation into chondrocytes

• Hypertrophy to allow the deposition of the matrix to be calcified

Actions of GH in Metabolism

• An anabolic hormone that stimulates:

  • Lipolysis - breakdown of stored fats - increase in fatty acids

  • Muscle uptake of amino acids to stimulate protein synthesis

Inapropirate Use of GH

• Performance enhancer by athletes

  • Difficult to detect exogenous vs endogenous

How Does GH Promote Hyperglycaemia

• Increases breakdown of glycogen in the liver to increase glucose output

• Stimulation of gluconeogenesis in the liver – synthesis of glucose from non-carbohydrate precursors

• Prevents the uptake of glucose into peripheral tissues (muscle and fat)

  • hormone is not the primary regulator of carbohydrate metabolism (insulin and glucagon), but is activated to prevent hypoglycaemia in fasting

IGF-1 Actions in Metabolism

• Acts at adipose tissue to stimulate lipogenesis

• In muscle, it promotes amino acid uptake and protein synthesis 

• It is ‘insulin-like’ and decreases glucose in the circulation, by stimulating its uptake into peripheral tissue

What type of Receptors Are GH and IGF-1 Receptors

• Cell surface

• Hormones are protein hormones- hydrophilic

  • Can’t cross the membrane

Enzyme-Couple Receptor: GH Receptor

• Once hormone is bound, receptor must recruit an enzyme

Enzyme- Coupled Receptor: IGF Receptor

• Receptor already has an enzyme built into the cytoplasmic portion of the receptor

  • Once the hormone is bound to the receptor, the intrinsic enzyme activated to relay the signal further down the cascade

Rapid Actions of IGF and GH

• Occurs within seconds

  • e.g.glucose/ amino acid uptake

Slow Actions of IGF and GH

• Alter gene expression and protein synthesis

  • Effects cellular function

GH and IGF-1 Action on Kinase Cascade

• Activate signalling cascade to mediate a response

• A kinase enzyme is activated by the addition of a phosphate to Ser, Thr or Tyr residues, which in turn can activate another molecule, with the addition of a phosphate

• Phosphatases turn signals off by removing phosphate group – molecule inactivated

  • These hormones have phosphocascades

GH Receptor

• A Homodimer in the plasma membrane

• Binding of hormone induces a conformational change

Consequence of GH Binding to GH Receptor

• Hormone binds to its receptor, inducing a conformational change

  • One subunit rotates, revealing an intracellular binding site for the JAK2 enzyme that must be recruited

• JAK2, activated via its photophosphorylation at the cytoplasmic portion of the GH receptor, allows the recruitment of a transcription factor stat5

• Stat 5 is phosphorylated and activated to move into the nucleus, where it binds to the promoter regions of genes that have the appropriate response element

GH Binding Protein (GHBP)

• Formed from the cleavage of the extracellular region of the GH receptor via Metalloproteases and TNF-converting enzymes

• Protein is able to bind the hormone as the binding sites are present

  • Physiological significance is poorly understood

• It may prolong GH half-life, preventing its degradation

How Can GHBP Regulate the Action of GH

• Competes with GH receptors for GH

IGF-1 Receptor

• Heterotetramer of 4 subunuts

  • 2a (extracellular) and 2B (membrane-spanning and intracellular) subunits

IGF-1- PI3 Kinase Pathway

• When IGF binds to the receptor, the B-subunits are activated and phosphorylated, where they then bind to the next signalling molecule in the chain (IRS1), which can then activate the next molecule in the chain (PI-3K), which can then activate AKt

  • activated receptor can also phosphorylate Ras which can bind to and activate MEK which can then phosphorylate MPAK

• Pathway important in the proliferative and differentiation action of IGF-1

Type IGF-II Receptor

• Unknown if this is able to signal when IGF is bound

• May act as a clearance receptor

Type I IGF Receptor

• Largely mediates the actions of IGF-1 via the PI-3K or MAPK pathway

Insulin-Like Growth Factor Binding Protein

• 6 types of proteins

  • evolutionary homology

    Some structural similarity at the N- and C-terminus - sites where the BP binds to IGF

• Have different regulation and tissues sources → centre of the proteins differs

• Most IGF present in circulation is bound

  • <5% is free

IGFBP3 and IGFBP5

• Tertiary Insulin-Like Growth Factor Binding Protein

  • (150kDa)

• Composed of:

  • BP

  • IGF-1

  • Acids Labile Subunit

BP1, BP2, BP4, BP6

• Binary insulin-like binding proteins

  • 40-50kDa

• Composed of

  • BP

  • IGF-1

IGFBP-3

• Main IGFBP in circulation

  • Levels don’t fluctuate as much in circulation - one-off blood samples can be taken

• prolongs IGF half-life – protected from proteolysis (lasts hrs)

Need for Binary Complexes

• Large IGFBP such as IGFBP-3 is retained in the circulation as it is too large

• IGF must therefore be transported to tissues via the smaller 40-50kDa=

  • modify actions of IGF at the tissue

Regulation of IGF Activity

• Mediated by the binding proteins

• When hormone is bound to the protein, its activity is blocked from acting at the receptor.

• It must be released from the binding protien to have an effect

  • BP must be cleaved and fragmented by proteases to remove the hormone binding site

Role of Proteolysis and Post Translational Modification of Binding Proteins

• Regulates it activity and interaction with IGF

• Phosphorylation and Glycosylation can affect its affinity for IGF

  • carefully regulated at the extracellular membrane to release IGF

How is IGF activity regulated?

• IGF activity is regulated by its own levels and by GH.

• IGF can also regulate its own production through negative feedback at the APG and hypothalamus

What factors affect GH production?

• GH production is affected by psychological stress, physical stress, sleep, and ghrelin.

• Prolonged psychological stress can have a negative impact on GH production, while acute physical stress and deep sleep can increase GH production.

• REM sleep and dreaming can decrease GH production.

• Ghrelin, a hormone produced by the stomach, is a potent stimulator of GH.

Ghrelin

• A hormone produced by the stomach that is a potent stimulator of GH

  • Represents the link between nutrition and growth, with its secretion regulated by glucose, amino acids and fatty acids

Defects in the Hypothalamus-Pituitary-Growth Axis

• Problems with GHRH hormone

• Problem with GHRH receptor

• Problem with GH production

  • problems at any point can result in GH deficency

Normal GH Profile

• Increase in adolescence

• Peaks at 20 year olds

• Gradually declines

GH Deficiency Profile

• No peak in GH seen, resulting in

  • short stature

  • Adiposity - more fat deposition

What May Cause Problems In IGF-1 Action

• Mutation in GH receptor

• Mutation in IGF-1 gene

• Mutation in IGF-1 receptor

  • Same phenotype seen

Hormone Deficiency Treatment: Defect at GHRH, GHRH receptor or GH

• GH replacement injections daily

Hormone Deficiency Treatment: Defect at IGF gene, or GH receptor

• IGF replacemnt treatment given

Laron Syndrome

• Mutation in the IGF receptor

  • Receptor insensitive to GH - IGF and GH hormone replacement not appropriate

• Treatment: recombinant insulin-like growth factor 1 (rhIGF-1), bypasses the need for growth hormone by directly stimulating growth through the IGF-1 pathway e.g. Mecasermin

Excess GH

• Effects are dependent on the point at which the excess occurs and can result in one of 2 conditoins

  • gigantism

  • acromegaly

• Often occurs in response to an APG tumour secreting excess GH, producing excess IGF-1

Gigantism

• Excess GH present before growth plate closure, resulting in an increase in height

Acromegaly

• Excess GH present after growth plate closure - rare

• No increase in height is seen

• Phenotype seen: thickening of bone and changes to soft tissue and organ growth

Treatments For Excess Hormone

• Surgery to remove tumor      

• Somatostatin analogues – stops secretion from APG

  • E.g. octreotide

• GH receptor antagonists – blocks action at the receptor

  • E.g. pegvisumon

When Can An Individual Growth

• When the growth plate is open and cells are present in the reserve zone to feed proliferative zone

Growth Plate Closure

• Occurs in response to oestrogen, which increases in puberty

• Leads to apoptosis of cells in the reserve zone

  • Once they have apoptoses, nothing is left to feed into the reserve zone

Role of Thyroid Hormones in Growth

• Contributes to long bone growth

  • Acts at the hypertrophic zone and stimulates hypertrophy of the chondrocytes