Growth Hormones What is Growth? Growth Hormone (Somatotropin) Actions of GH Production of GH Regulation of GH Release GH Binding Protein GH Receptor Role of Somatomedins & IGF-Binding Proteins Other Factors Regulating Growth

hypertrophy hyperplasia What is Growth?  Growth is an increase in size of a tissue/organism due to - increase in cell size (hypertrophy) - increase in number of cells (hyperplasia) - increase in extracellular matrix around cells

Hormones and Growth  A number of hormones influence growth of specific target tissues: - FSH; ovary - ACTH; adrenal - estrogen; breast, uterus - TSH; thyroid - testosterone; prostate gland….

Growth Hormone (GH; Somatotropin )  The major hormone regulating growth in the body is growth hormone (GH; somatotropin).  Actions of Growth Hormone: - increases skeletal growth - increases muscular growth - increases amino acid uptake and protein synthesis - increased use of lipids for energy - decreased storage of carbohydrates

Pituitary Dwarfism  Due to lack of GH release from the pituitary, or lack of GH receptor expression (or other deficits….)  Results in delayed growth and short stature (below 5 ft) in adult.  Body development is proportional.

GH and Aging  Some data indicate that GH treatment may help counteract some effects associated with aging (loss of muscle tone, increased body fat).

Structure and Source of Growth Hormone  GH is a large peptide hormone, with 191 amino acids  GH is produced by somatotroph cells of the anterior pituitary

Regulation of GH Levels  GH is released from the pituitary in a pulsatile manner: hormone level time GH levels are low during the day, but increase during sleep. There is an overall increase in GH levels during puberty. Implications for wt loss (circ rhythm and food deprivation).

Control of GH Release  GH is under the control of two hypothalamic releasing factors, GHRH and somatostatin.  GHRH acts through a Gs protein-coupled receptor to increase cyclic AMP-dependent protein kinase A activity. -Increased PKA activity causes increased levels and activity of a transcription factor, PIT-1, resulting in increased GH synthesis and release.  Somatostatin acts through a Gi protein coupled receptor, decreasing cAMP levels and PKA activity, and decreasing PIT-1 activity.

Regulation of GH Expression in Somatotrophs GHRH AC Gs cAMP PKA PIT-1 GH synthesis Gi (-) somatostatin A txn factor in the Pit

Characteristics REM Sleep State NREM Sleep State Eye movements REM, closed lids Lacks REM Brain activity Active (dreaming) Resting phase Muscle activity Bursts of twitching Diminished Vital signsActive, irregular Decreased Phases of Sleep: REM versus NonREM

Control of GH Release (con’t)  GH release is stimulated by deep (nonREM) [complete relaxation] sleep, but is inhibited during REM sleep.  GH release is also stimulated by stress and exercise  nts (NE, epi)  incr glc from glycogen [activates phosphorylase in muscle and liver (Cori Cycle)].  Also, epi  lypolysis TGs  FFAs incr metabolism.  GH release is inhibited by elevated glc, and stimulated by high levels of certain amino acids (arg).

Control of GH Release (cont)  Perhaps the major regulation is achieved by negative feedback by GH and somatomedins (IGFs) at the pituitary and hypothalamic levels.  Increased GH or IGF levels result in inhibition of GHRH and stimulation of somatostatin release. Net result: inhibition of GH levels.

Growth Hormone Receptor  Growth hormone binds to a receptor that is closely associated with an intracellular tyrosine kinase (JAK-2). JAK-2 Cf., Fig. 12-1

 Binding of GH to the receptor results in phosphorylation of various substrates within the cell, resulting in a biological response.  This triggers a number of pathways: - Insulin responsive substrates, MAP kinase - phospholipase C/IP3/PKC - induction of fos/jun (AP-1) and myc expression Growth Hormone Receptor

Transport of GH in the Blood  About 50% of GH is found in the blood bound to a Growth Hormone-Binding Protein (GHBP).  GHBP increases the half-life of GH, but decreases biological activity (bound GH is not biologically available).  The GHBP is identical to the ligand binding domain of the GH receptor, and may be derived from alternative splicing of the GH receptor RNA.

GHBP binding site is identical to the GH receptor (GHR) binding site GH receptor mRNA AAAAAA GH binding domain (GHBD) Alternative splicing GHBD GHBP GHR

Relationship between GH Receptor and GH Binding Protein associated tyrosine kinase growth hormone receptor extracellular ligand-binding region growth hormone binding protein (JAK-2)

Direct Actions of GH  GH appears to act directly on cells to cause –Lipolysis (breakdown of stored fat into free fatty acids) –Glycogenolysis (breakdown of glycogen to form glucose)  Therefore, it makes sense that increased glucose levels will inhibit GH release.

Role of Somatomedins in GH Actions – the GH/IGF Axis  The effects of GH on skeletal and muscular growth appear to be due to the activity of somatomedins, or insulin-like growth factors (IGF-1 and IGF-2) – processed in the liver.  GH acts on the liver, and some other tissues, to increase the production of IGFs.  IGFs then enter the circulation and act on target tissues to enhance growth.

IGF Receptors  IGFs bind to specific receptors (type-I IGF receptor and the insulin receptor) to stimulate growth.  The type-I IGF receptor is similar to the insulin receptor, with intrinsic tyrosine kinase activity.  Binding of IGFs to their receptors results in phosphorylation of insulin-responsive substrates (IRSs), which stimulate tissue growth and differentiation. phosphorylation of IRSs Growth and differentiation plasma membrane extracellular domains (ligand binding) tyrosine kinase domains

Some Observations in Knockout Mice: IGF-II  Gene knockout experiments can create mice which lack expression of certain genes.  Knockout of IGF-II gene results in slower fetal development, with low birth weights. However, after birth these mice grow at normal rates.  This finding suggests that IGF-II is an important fetal growth factor, with unclear role in growth after birth.

Some Observations in Knockout Mice: IGF-I  Knockout of the IGF-I gene also results in slow fetal development and low birth weights.  However, these mice also display marked lack of growth after birth as well.  Thus, IGF-I is important for growth at all stages of development (before and after birth).

Some Observations in Knockout Mice: GH  Knockout of the GH gene results in normal fetal growth, and normal birthweight.  However, after birth, growth is impaired.  Together these results indicate that IGF-I and IGF-II are NOT regulated by GH during fetal development.

Roles of IGF Binding Proteins  IGFs bind to several (at least ten) IGF binding proteins (IGFBPs).  Several possible actions of IGFBPs have been proposed.  Some IGFBPs are believed to inhibit the action of IGFs by binding them and making them less biologically available: IGF IGFBP IGF Receptor

Roles of IGF Binding Proteins  Some IGFBPs may also enhance IGF action (possibly by delivering IGFs to the cell, or increasing half-life), resulting in increased stimulation of the IGF receptor.  Also, IGFBPs may act independently of IGFs. Specific IGFBP receptors have been observed on cell membranes.  Thus, regulating the expression of IGFBPs influences IGF activity.

IGFBP binds IGF IGF receptor binding OR binds IGFBP Receptor [In liver cell membrane or cytoplasm] Txn of proapoptotic genes E.g., IGFBP gene is induced by hypoxia  incr expression of IGFBP and  developmental delay and retardation of embryonic Growth.

IGFBP Proteases  The activity of IGFBPs is also regulated by proteases which degrade IGFBPs.  By regulating IGFBPs, these proteases may be important regulators of IGF bioactivity and bioavailability.  Specific proteases have been identified for most IGFBPs. IGFBP IGF IGFBP protease

Example: Prostate-Specific Antigen  Prostate cancer: hyperplasia of prostate cells  Normally, the action of IGF-I on prostate cells is inhibited by binding to IGFBP3.  Patients with prostate cancer have elevated levels of prostate-specific antigen (PSA), which is a protease for IFGBP3.  Increased protease levels may result in increased mitogenic effect of IGF on prostate cells.

IGFBP3 IGF PSA A protease cleaves IGF (mitogen) may now bind its receptor on prostate cells and incr growth of prostate gland

Other Factors Regulating Growth  Recall that estrogen and androgens stimulate skeletal and muscular growth. -estrogens (and androgens) act on and maintain bone, inhibit osteoclast activity -androgens act on muscle, increasing size (anabolic steroids) -estrogens and androgens also stimulate GH release

Other Factors Regulating Growth  Last lecture we saw how vitamin D and PTH were involved in regulation of bone growth.  Also recall that thyroid hormone is required for GH synthesis and action: - TRE on GH gene (effects synthesis of GH) - T3 induces GH receptor expression

Genetic Factors: Inheritance of Height  The genetic factors responsible for the inheritance of height are largely unknown.  Identical twins show a high correlation for height (0.9), but not for weight (0.2)

Effects of Nutrition  There is also an interaction between genetic factors and nutrition.  Adequate intake of nutrients (ie, vitamins and minerals) and calories is required to reach full growth potential, especially during childhood.  Recall that arginine (an amino acid) stimulates GH release. vitamins, minerals, calories from lipids, carbohydrates, and proteins food growth GH

Other Growth Factors: Fibroblast Growth Factor (FGF)  Several forms of FGFs exist (acidic, basic, etc.)  FGFs act as local regulators of growth and differentiation (autocrine/paracrine function).  A number of FGF receptors have been identified.  FGFs stimulate the development of organs and of bone.

Epidermal Growth Factor (EGF)  EGF is named after its stimulatory effect on skin cell proliferation.  EGF appears to play a role in development and growth.  Associated with eyelid development (in species born with closed eyelids), and eruption of teeth.  EGF acts through the intrinsic tyrosine kinase activity of its receptor.  Interestingly, knockout of EGF has little effect on growth. Redundant mechanisms must exist.

Nerve Growth Factor (NGF)  NGF appears to be important in survival of neurons, and in the innervation of target tissues.  Acts through a tyrosine kinase activity.  Also, NGF acts as a IGFBP-3 protease, increasing the influence of IGFs on nerve cells.  May be important in recovery from damage to nervous tissue.

Inhibitory Growth Factors  Transforming growth factor alpha (TGF  ): local factor which can act to both stimulate cell growth as well as inhibiting cell growth, depending upon the situation.  Tumor Necrosis Factor (TNF): Inhibits growth of tumor and other cells by initiating programmed cell death.

Next Time Second Midterm Exam Includes material starting at the lecture after the first midterm (end of steroid receptor lecture) and through today’s lecture.