MBC 3320 Pituitary hormones IMBC 3320 Pituitary hormones I| Growth hormone releasing hormone | Growth hormone | Somatomedins (IGF) | Prolactin |
While somatostatin inhibits release of growth hormone by the anterior pituitary, growth hormone releasing hormone (GHRH) stimulates the release of growth hormone. So the hypothalamus exerts both positive and negative control over pituitary growth hormone levels. GHRH is a 44 amino acid peptide. That is all on GHRH!
Growth Hormone Synthesis: GH is derived from a prohormone which has an additional 26 AA attached to the N-terminal end. This tail is cleaved by an endopeptidase to generate active hormone.
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Growth Hormone Release: GH is normally released in several discrete bursts at a time (pulsatile fashion) during the day as opposed to continuous release. Bursts also occur during sleep and after a meal with high protein content. The timing of GH release is somewhat logical, since energy and materials needed for growth are absent during fasting and subject to competition during excercise.
Continuous release of growth hormone is undesirable. In young people whose growth is not complete, GH leads to gigantism. The tallest person on record, whose height resulted from this condition, died at age 22 in 1940 with a height of 8'11'. In persons who have finished growing, continued GH release causes a condition known as acromegaly. In acromegaly, the bones of the hands, feet, and face continue to grow, but other bones do not. Underproduction of growth hormone often results in adults 3 to 4 feet tall. Stunted growth from lack of GH can be averted by timely administration of human recombinant hormone.
The half-life of GH is about 25 minutes (relatively long compared to other peptide hormones). How is this longer half-life achieved. Growth hormone travels through the circulatory system adsorbed onto plasma proteins. Adsorbed GH has a lower rate of inactivation by proteolytic enzymes than would free GH. Later, we will see that neurophysins and the posterior pituitary hormons ADH and oxytocin use the same strategy. The Principle: the half-life (T1/2) of a peptide is increased by binding to plasma proteins which hinder proteolysis and other metabolic mechanisms.
Two strategies to store and transport peptide hormones. One: mask the peptide in a portion of a larger protein which serves as prodrug, as seen done by GH. MSH and proopiomelanocorticotropin will be later examples. Two: bind with a larger protein such as a plasma protein for enhanced stability, as also used by GH.
Growth Hormone (GH) is a single polypeptide chain which is made of 191 amino acids. Unlike TRH and GnRH, neither terminal is blocked (free acid/base).
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The figure of GH above shows a simplified depiction of important disulfide bonds and their relationship to proteolytic fragments. This depiction is not meant to imply three dimensional structure. Trypsin is an enzyme which can cleave a protein into fragments. Each protein fragment can be tested for biological activity. Trypsin specifically cleaves, or digests, a protein on the C-terminal side of Lysine or Arginine occuring within certain amino acid sequences. Depending on how long a digestion of GH by trypsin is allowed to proceed, either amino acids 135-140 are removed, which leaves a protein of similar activity to that of GH, or amino acids 135-145 (135-140 + 141-145) are removed, which results in a protein with greater activity than undigested GH. The two disulfides (53-165 and 182-189) in the digested proteins can be chemically or naturally reduced to thiols, resulting in unlinked peptide products. The dissociated protein fragments have been tested for GH activity. The results indicate that only the C-terminal region of GH, somewhere between amino acids 146 to 191, binds the GH receptor.
Nutropin (somatropin, Genentech). In early 1997, Genentech recieved approval from the FDA to market somatropin. Somatotropin, which is a second generation (methionine-free) human growth hormone is produced by recombinant technology. Nutropin (somatropin) is indicated for girls suffering from Turner's syndrome, and will be administered only during growth years. Turner's syndrome occurs when one or a part of one X chromosome is lost from either the ovum or sperm during incorrect cell division. The incidence is 1 in 2,500 to 7,000 births, with up to 95% of affected fetuses spontaneously aborted. Human growth hormone, androgens, and some estrogens are needed to minimize the disabilities associated with the syndrome. Nutropin is also indicated for growth hormone deficiency and for stunted growth caused by kidney problems. Nutropin may be expected to require long term administration to children short in stature as caused by a lack of endogenous growth hormone secretion. The treatment of growth failure in children caused by renal insufficiency is a stop-gap measure, since most of these children are awaiting kidney transplantation. Prior to somatropin, the first biosynthetic growth hormone was somatrem. Somatrem contains a methionine residue which shows a greater antibody response, heightened allergic reaction, and lessened growth response profile compared to the methionine free Somatotropin.
Serostim (somatropin, Serono). In fall 1996, Serono obtained accelerated approval from the FDA to treat AIDS wasting (cachexia) with somatropin. In wasting, the body burns lean body mass inappropriately. Wasting is the major cause of death in AIDS patients.Somatropin must be given with concomitant antiviral therapy. After up to twelve weeks of somatropin therapy in clinical trials, patients gained an average of 1.6 kg of lean body mass and a decrease in body fat when compared to placebo. While no difference in survival was detected in that trial, other trials indicate increased weight improves survival. In late 1996, Human Growth Hormone received an indication for treatment of cardiac insufficiency. The reasons for this may include the following. The interaction of Growth Hormone on IGF-1 (Insulin-like growth factor I) helps control heart morphology and function in adults (Nature Medicine 1996, 2, 1, 29-31 January). GH, or agonists and antagonists, may improve deteriorated cardiac performance caused by ventricular wall thinning or cardiac hypertrophy (nonproductive myocardial growth, a major risk factor for heart attacks) by encouraging useful muscle tissue growth.
Caution: Growth hormone does increase skeletal mass and strength. However, athletes tempted to use GH to attain greater growth, especially muscle, should be aware of the bizarre growth patterns that can result from GH use. Persons with closed epiphyses (ossified cartilage at ends of long bones fused with bone ends), usually over age 18, who take GH can experience abnormal bone growth patterns, such as acromegaly, with a lack of long bone elongation.
IGF or somatomedins are a family of peptide hormones structurally similar to insulin with mitogenic properties and insulin-like effects that mediate the action of growth hormone (somatotropin) on skeletal tissue. They are synthesized by tissue in the liver, pituitary, brain, and various fetal organs. Important somatomedins include somatomedin A (SM-A), somatomedin C (SM-C), insulin-like growth factor I (IGF-I), insulin-like growth factor II (IGF-II), and multiplication stimulating activity (MSA). IGF-I and IGF-II are also known as NSILAS (-I, -II) for non-suppressible insulin-like acting substance (-I, -II). In the literature, NSILAS and NSILA are equivalent.
Somatomedins increase cartilige growth, stimulate protein synthesis, stimulate DNA thymidine incorporation, and stimulate mitosis.
Somatomedins are primarily made in the liver and either reside there or are secreted.
Somatomedins have a very long half-life (T1/2) of about 20 hours. Somatomedins are carried in the blood bound to plasma proteins, as seen for GH. The protein carriers act to ensure a continous supply of somatomedins to target cells (nature's own slow release formulation).
IGFs / Somatomedins cannot replace insulin since they have a higher affinity for carrier proteins in the plasma than they do for insulin receptors.
IGFs / Somatomedins: the Non-suppressible Insulin-like Growth Factors (NSILA) from human plasma.
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NSILA-I and NSILA-II show homology with positions 1-29 of the insulin b-chain and with positions 42-62 of the insulin a-chain. Also, conserved dissulfide bridge positions suggest conserved simularities in their 3-dimensional conformations.
IGFs or somatomedins, MW 6500-8500, are slightly larger than insulin (MW 5800). Somatomedin is old but convenient nomenclature. In early experiments, anti-insulin antibodies were used to precipitate insulin by from the blood. After removing the immunoprecipitated insulin, researchers found that the insulin-free blood still showed some insulin-like activity. The proteins eventually isolated from this insulin-free blood were thus called "nonsuppressible insulin-like activity", or NSILA for short. The NSILA stimulates growth as does insulin. Instead of somatomedins, they are now referred to as IGFs for "insulin-like growth factors" There are a number of IGFs designated IGF-1 and IGF-2, etc.
Both growth hormone and/or somatomedins can stimulate growth in abnormally short children (0.3 standard deviations from normal, adjusted to parents' heights).
Both are used when the Anterior Pituitary does not respond to GHRH, or when GH production by the anterior pituitary would be normal, but no GHRH is being made by the hypothalamus.
In 1995, Somatomedin-1 made in the USA by Biogen was introduced in Sweden to treat growth hormone insensitivity. It has been indicated for the treatment of children with growth disorders caused by hormone insensitivity and for the treatment of a hereditary condition known as Laron-type dwarfism which is otherwise untreatable. Somatomedin-1 induces a rapid stimulation of linear growth of both body and extremeties, and has been shown to be important in early brain development. It also is a potent hypoglycemic agent and may be used for the treatment of diabetic patients, muscle atrophy, osteoporosis, and catabolism in critically ill patients. Not yet available in the U.S.A., FDA approval is being sought.
Prolactin (PRL): The release of prolactin is controlled by dopamine (Prolactin Release Inhibitory Hormone) produced by the hypothalamus. Prolactin is a polypeptide (small protein) of 198 amino acids (MW ~ 23,000), with 6 cysteines, 3 dissulfide bonds and a half-life (T1/2) of about 10 minutes. Neither end of the protein is blocked. It is synthesized as a 227 amino acid precursor protein which has 29 residues cleaved off to generate the active hormone. Comparing prolactin with growth hormone, GH has 191 amino acids and is also synthesized as a prohormone with an extra 26 AA attached to its N-terminus. Prolactin also shares regions of homology with GH (growth hormone).
Comparison of human growth hormone (hGH) and human prolactin (hPRL) raises the concept of meaningful protein homology . In general, when two protein sequences are analyzed for regions of identity, one segment of one protein is moved in relation to the other until the maximum number of residues are able to make a one-to-one match. The order of amino acids occuring in the proteins is maintained (one does not take a C-terminal region of one protein or peptide and match it to the N-terminal region of the other). If one trial alignment produces 10% matches, another 20%, then the protein homology is said to be 20%. The overall homology between hPRL and hGH is found to be only 16%. However, when one considers just the first 50 amino acids of each, the homology is 24%. Replacement of 8AA from hGH into hPRL makes a hPRL able to bind well to the hGH receptor. This 8AA change increases the overall homology from 16% to 23%. Homology is important, but homology between active site amino acids is more important. We could have randomly changed 8 amino acids of hPRL to make it 23% homologous to hGH, but we probably would not have improved its binding to the hGH receptor.
Ten to 15% of prolactin is glycosylated at an asparagine (Asn) residue [Asn side chain -CH2-C(O)-NH2 --> -CH2C(O)-NH-sugar chain. Proteins can be glycosylated in three ways, on the Asn amide nitrogen, the oxygen of Ser/Thr, and on an oxidized form of Lys called Hylys. Glycosylated prolactin is only 20 to 25% as active as non-glycosylated prolactin, but is more stable. More stable translates to longer duration of action. Glycosylation is often found on extracellular proteins and is thought to help target such proteins for excretion from the cell. Thyroid stimulating hormone (TSH) is another glycosylated anterior pituitary hormone that we will study later (lecture 5).
Prolactin has a synergistic action with estrogen to promote mammary gland proliferation. It also brings about the release of progesterone from lutein cells which renders the uterine mucosa suitable for the imbedding of the ovum, should fertilization occur. Growth hormone also binds the lactogenic receptor (the receptor to with prolactin binds) but is not a significant source of lactation stimulation at normal physiological levels. Excessive levels of growth hormone, however, may lead to galactorrhea. This is analogous to high levels of prolactin binding to growth hormone receptor and inducing acromegaly.
Take home message: at high levels, prolactin and growth hormone can bind to one anothers receptors!
The synergistic relationship between prolactin and progesterone release for preparation of the uterus for implantation is important. As previously noted (p22), the main characteristic of high levels of prolactin is infertility! Since prolactin is neededto get pregnant, one explanation is that high prolactin levels cause a decrease in the number of prolactin receptors in the uterine lining, decreasing the sensitivity of the tissue to the actions of prolactin. The mechanism of receptor down regulation has already been encountered with GnRH agonists, such as Leuprolide (p19). Leuprolide increases testosterone production to levels that eventually down regualate testosterone receptors. Prolactin may act similarly with its receptors.
New nursing mothers have high levels of prolactin and are less able to become pregnant. This makes some physiological sense and could be nature's way of spacing out child-rearing responsibilities. In fall of 1995, the World Health Organization recommended that only women who can no longer nurse be given baby formula in Third World countries. This recommendation was to encourage mothers to nurse their children for as long as possible. Multiple benefits were expected. First, contaminated water mixed with the powdered baby formulas often kills or makes ill the infants. Secondly, increased breast feeding increases infant-mother bonding. Thirdly, nursing by increasing prolactin levels acts as a natural form of birth control in overpopulated regions!
No products or derivatives of prolactin have been developed. However, as mention previously, bromocriptine and pergotril, dopamine Type 2 receptor agonists, are used to inhibit prolactin excretion by the pituitary.