Growth Hormones

1
Growth Hormones

• Pituitary growth hormone is clearly associated
  with growth but most of its actions are mediated
  indirectly by somatomedins.
  
• GH plays an important part in growth regulation
  in addition to insulins actions on CHO and fat
  metabolism.
  
• Androgens play roles in growth processes and
  glucocorticoids and thyroid hormones are
  permissive to growth-promoting activities of
  growth hormones.

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GH Somatomedins

• Congential failure to synthesize and secrete GH
  leads to short stature, hypersecretion leads to
  gigantism or acromegaly.
  
• GH causes the growth of epiphyseal regions of the
  long bones. Growth of the bone can be monitored
  by measuring the incorporation of 35S into
  epiphyseal cartilage.
• When bones from young animals are incubated in a
  medium containing serum 35S is incorporated into
  the cartilage.

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GH Somatomedins

• When bones are incubated in serum from hypox
  animals, no incorporation takes place, even if GH
  is added to the serum.
  
• If hypox animals are injected with GH prior to
  blood collection, the serum supported
  incorporation of S into bone.
  
• GH works indirectly on bones by way of a
  sulfation factor.
  
• Sulfation factor is now known to consist of
  several peptides referred to as somatomedins.
  

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GH Somatomedins

• Injected radiolabeled GH localizes to the liver
  not the epiphyses of the bones.
  
• Somatomedins is used to refer to those growth
  factors found in the plasma that are under the
  control of GH, have insulin-like properties, and
  promote incorporation of S into cartilage.
• Two peptides that bear some structural
  relationship with proinsulin are IGF-I and IGF-II.

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IGFs

• IGFs are peptides that are structurally similar
  to proinsulin and exhibit some affinity for
  insulin receptors.
  
• Insulin, at high concentrations, will bind to IGF
  receptors. IGFs are secreted by the liver and by
  some other tissues in response to GH but they are
  not stored in the liver.
• The a.a. sequence of IGF-I and II are very
  similar to insulin. IGF-II is less abundant in
  the adult circulation than IGF-I and its origin
  and function is less well defined.

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IGFs

• Insulin is a more potent stimulator of metabolic
  effects in insulin target tissues than IGF-I or
  II, but a less potent stimulator of cell
  proliferation than these growth factors.
• Although classical actions of IGFs are though to
  be endocrine, they can function in an autocrine
  and paracrine manner at the tissue level.
  
• Functions of IGFs are mediated by two IGF
  receptors, the type I IGF receptor and the type
  II IGF receptor.

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IGFs

• IGF type I receptor exhibits ligand dependent
  tyrosine kinase activity and autophosphorylation
  and is similar structurally and functionally to
  the insulin receptor.
• IGF type I receptor binds IGF-I with a much
  higher affinity than IGF-II and even less for
  insulin.
  
• IGF type II receptor is homologous to the
  mannose-6-phosphate receptor.
  
• IGF type II receptor has higher affinity for
  IGF-II than IGF-I and does not bind insulin.
  
• IGF-I and II can bind to the insulin receptor
  with low potency.

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IGFs

• Local injection of GH into the epiphyseal growth
  plate of the proximal tibia of hypox rats
  stimulates unilateral longitudinal bone growth,
  and the number of IGF-I immunoreactive cells is
  increased.
• It was concluded that IGF-I is produced in
  proliferative chondrocytes in response to GH and
  the number of IGF-I containing cells is directly
  regulated by expansion of differentiated
  chondrocytes stimulated through autocrine and/or
  paracrine mechanisms.

9
IGFs

• T3 and IGF-I interact with epeiphyseal
  chondrocytes and both affect cell proliferation
  and maturation and therefore longitudinal bone
  growth.
• IGF-I is important for cell proliferation,
  whereas T3 may initiate terminal differentiation
  of chondrocytes.
  
• Dual effector theory of somatotropin action
  states that GH directly stimulates the
  differentiated state of certain cell types.
  IGF-I then acts to multiply the differentiated
  cell types.

10
IGFs

• IGF-I stimulates hypothalamic SST release and
  inhibits stimulated pituitary GH release.
  
• ICV injection of IGFs causes a marked decrease in
  GH secretion.
  
• Replacement therapy in GH-deficient adults alters
  body composition and energy metabolism through
  its lipolytic, protein anabolic and
  antinatriuretic actions, resulting in decreased
  fat mass, increased fat-free mass sodium
  retention and increased energy expenditure.

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GH Actions

• GH regulates growth indirectly through hepatic
  IGF-I production. IGF-I then affects growth
  through its actions on skeletal tissues and many
  other tissues and organs.
• Several hormones such as Prl, placental lactogen,
  and insulin may exert part of their
  growth-promoting activity through effects on
  hepatic IGF-I production, because their
  structures are very similar to that of GH.
• IGF-I levels remain normal in hypox pregnant rats
  even in the absence of Prl and GH, then decline
  postpartum.
  

13
GH Actions

• GH affects other tissues in addition to hepatic
  IGF production, and these actions can be
  diabetogenic in nature.
  
• GH stimulates lipolysis which provides substrates
  for glucose formation and thus has a direct
  sparing effect on glucose utilization.
  

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GH and IGF-I

• Individuals with hypopituitary short stature have
  very minimal plasma levels of IGF-I and II.
  Levels of IGF-I are greatly increased in
  acromegalics but IGF-II levels are not.
• IGF-II is also GH dependent but this only becomes
  apparent when GH falls below the normal level.
  
• Dwarfism in German shepards may be genetically
  transmitted by a simple recessive mode of
  inheritance and is caused by GH deficiency
  resulting in low levels of IGF-I.

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GH and IGF-I

• In the dwarf mouse strain decreased growth is
  associated with a partial deficiency of GH
  production, but the pituitary is insensitive to
  GHRH. However, GH secretion can be stimulated by
  dcAMP. This suggests a defect in the GHRH
  receptor or receptor signalling.
• Individuals with Laron dwarfism have elevated
  levels of GH but reduced circulating levels of
  IGF-I due to a defect in the GH receptor.
  

17
GH Receptor

• GH receptor is activated on binding of GH to
  stimulate growth and metabolism of muscle, bone,
  and cartilage cells.
  
• This receptor is a member of a group of receptors
  involved in cell growth and differentiation
  called the hematopoietic superfamily.
  
• These receptors have a three-domain organization
  comprising an extracellular ligand binding
  domain, a single transmembrane segment, and an
  intracellular domain.
• Like tyrosine kinase receptors the mechanism to
  evoke a cellular response is not known.

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19
GH Binding Proteins

• About 40 50 of the circulating GH is bound to
  to the GH BP.
  
• GH BP complex represents a reservoir of hormone
  and modifies the amount of that can have access
  to the receptors.
  
• GH BP is though to originate from proteolytic
  cleavage of the the liver membrane receptor.
  
• IN the rat, alternate splicing of a single
  primary DNA transcript may give rise to distinct
  mRNAs, one that encodes the full length membrane
  receptor, the other a truncated receptor
  (extracellular binding domain.

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IGF Binding Proteins

• Binding proteins can be important modulators of
  ligand-receptor interaction on target cells.
  
• IGFs circulate in the serum as high molecular
  weight complexes, the majority of which are
  complexed as a 150 kDa GH dependent binding
  protein complex (IGF-I-acid labile
  subunit-IGFBP-3).
• Lower molecular weight BPs occur within
  extracellular fluids and tissues.
  
• At least six different IGFBPs have been isolated
  and sequenced. These are identified as IGFBP-1
  through-6.

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IGF Binding Proteins

• IGFBPs differ in molecular weight, tissue
  distribution and the whether they inhibit or
  potentiate the effects of IGFs.
  
• IGFBPs have a greater affinity for the ligand
  than the IGF receptor.
  
• The free hormone hypothesis as postulated for
  steroids and thyroid hormones would allow IGFBPs
  a passive role of physically transporting IGFs
  and inactivating IGFs by binding.
• However, studies indicate that IGFBPs interact
  with target cells and facilitate delivery to the
  receptors.
  
• IGFBPs may bind to cell surfaces to deliver IGF
  to adjacent receptors.

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23
Insulin

• Insulin, besides from playing a role in CHO
  metabolism, affects growth processes in animals.
  
• Children with diabetes fail to grow even though
  GH levels are normal, whereas infants of diabetic
  mothers with islet hyperplasia and
  hyper-insulinemia are of increased stature.
• Insulin is growth promoting and absence of
  insulin leads to accelerated protein catabolism.
  

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Insulin

• Insulin is required for the full anabolic effect
  of GH, perhaps through its action of glucose
  uptake by muscle, providing energy substrates for
  protein synthesis.
• Insulin also increases incorporation of a.a. into
  muscle by a mechanism independent of glucose
  metabolism.
  
• This may result form a direct action of insulin
  on transport of a.a. and activation of ribosomal
  translational capacity because protein synthesis
  is not dependent on glucose availability or RNA
  synthesis.

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Insulin

• Insulin at a high concentration can stimulate
  general body growth through low-affinity binding
  to IGF receptors.
  
• Even though there is a deficiency of insulin
  receptors in this case, the number of IGF
  receptors appears to be normal.
  
• High circulating levels of insulin cause
  overgrowth of extremities and enlargement of the
  kidney and adrenal glands by cross reacting with
  IGF receptors.

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Prolactin

• Because Prl is so similar to GH it is not
  surprising that many effects of Prl are
  growth-related, usually involved in reproductive
  processes.
• Prl, along with E2 and adrenal steroids is
  essential for ductile branching during
  prepubertal and postpubertal periods to form the
  fully developed system of the mammary glands.
• Prl also stimulates production of IGFs by the rat
  liver and could conceivably affect general body
  growth by a GH like action.

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Prolactin

• Prl also directly affects growth and function of
  the ovaries and testes by modulating actions of
  gonadotropins.
  
• There is a strong localized sequence identity
  between the receptors for GH and Prl in the
  extracellular and cytoplasmic domains.
  
• Signal transduction has not been identified for
  Prl, no clear effects on cAMP, cGMP, PIP2,
  phosphorylation calcium channels or ion channels.
  It does not appear to be a tyrosine kinase.

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Placental Lactogen

• Placental lactogen (PL) is similar in structure
  to GH but lacks the growth promoting activity but
  has lactogenic activity.
  
• It is found in the maternal plasma as early as 6
  wks of pregnancy and is synthesized by the
  synctiotrophoblastic cells of the chorionic villi
  of the placenta.

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Placental Lactogen

• PL increases sulfate uptake into cartilage in
  hypox rats and stimulates growth and induction of
  milk protein synthesis in mouse mammary
  explants.
• PL mimics the actions of GH on most tissues but
  is much less potent than GH and does not increase
  linear growth in hypopituitary short stature.
  
• Primary role of PL may be to stimulate the
  mammary gland without causing milk secretion.

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Placental Lactogen

• During the second half of pregnancy, PL may help
  to counter the effect of insulin in the maternal
  circulation, facilitating glucose and a.a.
  availability to the fetus.
• Human PL and GH are both single chain
  polypeptides of 191 a.a. with internal disulfide
  bridges. 162 residues of PL and GH are identical.

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Thymic Hormones

• Thymus is a lobular organ that lies in the upper
  thorax above the heart in front of the aorta.
  
• In humans and other mammals the thymus begins to
  atrophy shortly after puberty. Removal in the
  adult causes no harmful effects.
  
• It used to be thought it was a useless organ but
  now we know it is an endocrine organ that plays a
  role in development of the immune system.

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Thymic Hormones

• Thymectomy of mice immediately after birth
  results in a wasting disease characterized by
  sever depletion of white blood cells.
  Reimplantation of the thymus into young mice
  prevents the wasting disease.
• The lymphocytes produced in the animals with the
  reimplanted thymus were of host origin,
  suggesting the thymus was responsible for
  stimulating proliferation of host lymphocytes.

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Thymic Hormones

• When a thymus gland was placed into a small
  plastic capsule with pores so small that no
  thymus or other cells could get out or enter then
  implanted into an athymic mouse it was found to
  be capable of mounting an immune response.
• A partially purified thymus gland preparation
  called thymosin fraction five could correct some
  of the immunological deficiencies in these
  athymic models.
• Several thymosin fractions are active in
  immunological assays but only a few have been
  characterized.

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Roles of Thymosins

• White blood cells are produced in the bone marrow
  and stem cells in this hemopoietic tissue give
  rise to the white cell types granulocytes
  (neutrophils, eosinophils, basophils), monocytes
  (give rise to macrophages), and lymphocytes.
• Lymphocytes may reside within the thymus gland or
  pass on through to compose the circulating T
  lymphocytes.
  
• Other lymphocytes of bone cell origin (B
  lymphocytes) establish themselves in other
  lymphoid tissue and become transformed into
  plasma cells that secrete AB.

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Role of Thymosins

• T lymphocytes become competent to participate in
  the immune response either by passage through the
  thymus, where they contact thymic hormones, or
  they are stimulated to become competent in
  response to humoral factors (hormones) released
  by the thymus.
• Fully differentiated T cells function as killer
  cells to combat tumor cell development, release
  lymphokines to affect macrophage function, and
  function as helper cells with B cells in AB
  production.

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Role of Thymosins

• A thymosin polypeptide (thymosin a1) was isolated
  from calf thymus and is probably derived from a
  larger precursor molecule and appears to be
  identical to a similar peptide from humans, pig,
  sheep, chinchilla.
• Structure of thymosin b4 has been identified and
  appears to act on stem cells to form
  prothymocytes whereas thymosin a1 may act on
  prothymocytes to induce differentiation to mature
  T cells.

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Role of Thymosins

• Thymosins have been used in clinical tests with
  children who have primary immuno-deficiency
  diseases.
  
• Thymosins appear to trigger maturational
  progression of several early stages of T cell
  development and augment the capacity of T cells
  to respond to antigens.

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Platelet-Derived Growth Factor

• Many factors in serum are necessary for for
  viability and growth of cells in culture, serum
  prepared from cell-free plasma has no mitogenic
  activity. This activity can be restored by
  adding material released from platelets to sera.
• PDGF is released from platelets during platelet
  aggregation in clot formation
  
• At sites of injury, platelets release PDGF to
  induce proliferation of smooth muscle cells.

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PDGF

• Atherosclerosis is a disease process which
  results from endothelial disruption due to a
  number of causes and it involves smooth muscle
  proliferation, and formation of large amounts of
  collagen matrix. These athersclerotic plaques
  impede blood flow which may lead to stroke, heart
  attack or other circulatory complications.
• PDGF may play a part in atherosclerosis by
  increasing vasoreactivity due to its
  vasoconstrictor ability.
  

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Epidermal Growth Factor

• Mouse EGF is 53 a.a. long with 3 intrachain
  disulfide bonds.
  
• EGF is found in the salivary gland combined with
  an EGF binding protein that is an arginine
  esterase.
  
• Release of the peptide from the salivary glands
  may be under a adrenergic control because
  adrenergic agonists increase serum levels of
  EGF.
• EGF causes tooth eruption, eyelid opening, and
  skin proliferation in the neonate and stimulates
  growth in a number of cultured normal epithelial
  cells.

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EGF

• EGF is a mjor growth-promoting agent in brease
  milk and AB to human EGF neutralize human milks
  mitogenic activity.
  
• EGF is not entirely destroyed in the digestive
  tract and may act directly on tissues of the
  digestive tract.
  
• Several lines of evidence suggest salivary EGF
  plays a role in wound healing.
  
• Wound healing of skin is enhanced by licking it,
  transferring saliva to the wound.
  
• EGF rich excreta such as saliva, duodenal juice
  and tears may promote wound healing.

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EGF

• Activation of the EGF receptor initiates
  intrinsic tyrosine kinase activity, followed by a
  rise in cytosolic Ca and receptor
  internalization.
• Specific gene transcription is stimulated within
  minutes, and within hours DNA synthesis and cell
  division occur.
  
• Tyrosine kinase activity of the receptor is
  necessary for all receptor actions.

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Angiogenic Factors

• Angiogenesis is the formation of blood vessels in
  situ and involves the orderly migration,
  proliferation, and differentiation of vascular
  beds.
• Angiogenins fall into two groups those that act
  directly on the vascular endothelial cells to
  stimulate locomotion or motility and those that
  act indirectly through mobilizing host cells to
  release endothelial growth factors.
• These angiogenic peptides are not restricted to
  tumors but are present in normal tissues and
  their release must be tightly regulated.

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Angiogenic Factors

• Besides FGF these polypeptides include angiogenin
  and TGF.
  
• FGF family consists of polypeptide growth factors
  characterized by a.a. sequence homology,
  heparin-binding, ability to promote angiogenesis,
  and mitogenic activity.
• FGF is a single chain peptide of 146 a.a. that
  can also exist in a NH2 terminally truncated form
  missing the frist 15 a.a., which is as potent as
  the long form.

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FGF

• bFGF has been purified from many mesoderm and
  neuroectoderm derived tissue which include the
  kidney, brain, retina, CL, adrenal gland,
  placenta, prostate, thymus. Depending on where
  it is purified from determines its length.
• It is not known if both are made in the same
  tissue or if the shorter one comes from cleavage
  of the longer.
  
• It has been shown that FGF can act as a morphogen
  at one of the earliest embryonic stages,
  including transformation of cells designed to be
  ectodermal into mesenchymal cells.

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TGF

• Two TGFs have been identified, TGFa, is composed
  of 50 a.a. and bears sequence homology to EGF and
  binds to the EGF receptor.
  
• TGFa may play a local role under normal
  conditions requiring autocrine or paracrine
  stimulation of growth as in embryogenesis.
  
• EGF on the other hand may fulfill and endocrine
  type of role.
  
• Application of TGFa to burns results in an
  increased rate of epidermal regeneration.

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TGFs

• TGFb is a homodimer of 112 a.a., and occurs as
  two related forms, TGFb1 and TGFb2.
  
• TGBb has been shown to be structurally similar to
  inhibin and MIF. Both inhibin and MIF, like
  TGFb, exhibit strong growth inhibitory
  activities.
• TGFb can stimulate or inhibit growth of certain
  nonendothelial cells depending on whether the
  cells are anchored or not and whether EGF are
  present.

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TGFs

• TGFb may be a misnomer since it is most often
  inhibitory to cell growth.
  
• It is possible that loss of responsiveness to
  TGFb may contribute to uncontrolled cell growth
  of cancer cells.
  
• Breast cancer patients who have estrogen
  dependent tumors may be treated with an
  antiestrogen that increase TGFb production.
  Estrogens themselves decrease TGFb production.
• TGFb inhibits proliferative actions of some
  hormones on cells, but it does not inhibit
  protein synthesis induced by these hormones.

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TGFb

• Normal skeletal growth is a balance of bone
  matrix synthesis and bone resorption.
  
• Bone matrix is enriched with TGFb activity which
  may regulate replication and differentiation of
  mesenchymal precursor cells, chondrocytes,
  osteoblasts and clasts.

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Cytokines

• A cytokine is defined as a soluble protein
  synthesized by immune or nonimmune cells that
  mediates intercellular communication.
  
• Cytokines transmit info to target cells through
  receptor-ligand interactions and regulate
  immunologic and physiologic events.
  
• Interleukins are the best know but least
  understood group, the eight or more interleukins
  have diverse biological activity.
  
• Interleukin described a leukocyte derived protein
  with activity for other leukocytes, but immune
  and nonimmune cells synthesize interleukins and
  other cytokines important in inflammation.