Growth hormone (GH) is well recognized as an important regulator of body growth and metabolism. Human growth hormone (hGH) is a linear polypeptide with 191 amino acids and contains two intrachain disulfide bridges. The major biological effect of HGH is to promote growth. The organ systems affected include the skeleton, connective tissue, muscles, and viscera such as liver, intestine, and kidneys. Growth hormone (GH) stimulates the systemic release of insulin-like growth factor-1 (ICF-1) from the liver. Growth hormone exerts its action through interaction with specific receptors on cell membranes. Receptors are proteins found associated with cell surfaces that specifically bind proteins or small molecules in the extracellular environment (ligands) and which, upon binding such ligands, exert an effect on the metabolic or morphologic character of the cell. The specific signaling substance is often referred to as the ligand, a substance that binds to or fits in a site, the ligand binding site. When the signaling substance binds to a receptor, a receptor-ligand complex initiates a sequence or cascade of reactions that changes the function of the cell. A cell surface receptor polypeptide typically comprises an extracellular part which comprises a binding site where the ligand can interact, a transmembrane part that locates a receptor in the cell membrane, and an intracellular part that plays a role in transducing a signal further into the cell once a ligand has bound. A receptor polypeptide can span the cell membrane several times resulting in multiple extra- and intracellular domains.
Receptors can bind hormones and growth factors, i.e., ligands. Each type of receptor is specific for its ligand. The function of the receptor is to convey external signals, e.g., hormonal signals to the target cell. In addition, the soluble forms of the receptors may have an inherent targeting function which is useful for selective delivery of drugs, such as to injured areas. Receptors are composed of a DNA binding domain and a ligand binding domain. The DNA binding domain contains the receptor regulating sequence and binds DNA and the ligand binding domain binds the specific biological compound (ligand) to activate the receptor. The receptors include receptors for ligands heretofore identified as growth factors and growth hormones. Examples are the growth factor receptors for growth hormones such as human, bovine, porcine or bovine growth hormones and growth factors such as epidermal growth factor, insulin-line growth factor and transforming growth factors alpha and beta (EGF, IGF and TGF-alpha and TGF-beta). Membrane-bound receptors are characterized by a multi-domain structure comprising an extracellular ligand-binding domain and an intracellular effector domain that is typically involved in signal transduction. In certain membrane-bound receptors, the extracellular ligand-binding domain and the intracellular effector domain are located in separate polypeptides that comprise the complete functional receptor.
Hormones reacting with cell surface receptors are of a varied nature. Typical examples are amino acid derivatives such as epinephrine or histamine, prostaglandins, various peptide hormones such as glucagon, insulin, gastrin, secretin, ACTH, LH, FSH, TSH, TRH, LHRH, vasopressin, IGF-I or II, EGF, somatotropin (growth hormone), prolactin, erythropoietin, EGF, and others. The growth hormone receptor has been shown to be an integral membrane protein. Growth hormone receptors contain a carbohydrate component associated with the extracellular domain of the receptor in conjunction with the growth hormone-binding site. Steroid receptors are responsible for the regulation of complex cellular events, including transcription. The ovarian hormones, estrogen and progesterone, are responsible, in part, for the regulation of the complex cellular events associated with differentiation, growth and functioning of female reproductive tissues. These hormones play also important roles in development and progression of malignancies of the reproductive endocrine system. The crystal structure of growth hormone bound to its receptor revealed that GH has two receptor binding sites and binds two receptor molecules. The two receptor binding sites are referred to as site I and site II. Site I encompasses the Carboxy (C)-terminal end of helix D and parts of helix A and the A-B loop, whereas site II encompasses the Amino (N)-terminal region of helix A and a portion of helix C. Binding of growth hormone to its receptor occurs sequentially, with site I always binding first. Site II then engages a second GH receptor, resulting in receptor dimerization and activation of the intracellular signaling pathways that lead to cellular responses to GH. A growth hormone mutein in which site II has been mutated is able to bind a single growth hormone receptor, but is unable to dimerize GH receptors; this mutein acts as a GH antagonist in vitro, presumably by occupying GH receptor sites without activating intracellular signaling pathways. The growth hormone receptor is important for normal growth and development in animals and humans. Biological effects include linear growth, lactation, nitrogen retention, lipolysis, diabetogenic-like effects, macrophage activation, and others. Binding of GH induces dimerization of two receptor polypeptides. This double binding initiates signal transduction and at the same time the ubiquitin/proteasome system is activated and removes a considerable part of the cytosolic or intracellular tails of a receptor. This proteolytic event is an obligatory step in the cascade of reactions which lead to endocytosis and degradation of a receptor. It is generally known that growth factors and hormones, both in animals and in humans, stimulate important cellular processes concerning cell division, growth, maturation, differentiation, and the like. In addition, healing and regenerative processes are also regulated by these factors.
Human growth hormone (hGH) is a polypeptide chain of 190 amino acids and a molecular weight of 22 kDa. The 22 kDa polypeptide human growth hormone induces a variety or biological effects including linear growth, lactation, nitrogen retention, diabetogenic and insulin-like effects and macrophage activation. Each of these effects is initiated by hGH interaction with specific cell receptors. The hGH receptor (hGHR) belongs to a large cytokine receptor family which includes receptors for prolactin, erythropoietin, interleukins (IL)-2, 4, and -6, granulocyte macrophage-colony-stimulating factor (GM-CSF) and granulocyte-colony-stimulating factor (G-CSF). Growth hormone binds to a GHR, which consists of three domains: an extracellular hormone-binding domain, which is 28 kDa for the human GHR, a single pass transmembrane domain and an intracellular domain, which is 35 kDa for the human GHR. Growth hormone receptor (GHR) forms a complex with a tyrosine kinase. The kinase, termed JAK2, is a member of the Janus family of tyrosine kinases. In addition to having a kinase domain, these proteins are characterized by the presence of a second kinase-like domain and the absence of Src homology 2 (SH2), SH3, and membrane-spanning domains. Ligand binding to the cytokine receptor appears to activate the kinase, causing tyrosyl phosphorylation of both JAK2 and the receptor involved, such as GHR for GH mediated signaling. The steps beyond this point are largely unclear, although it has been presumed that other intracellular proteins are recruited to JAK2-receptor complexes. Specific binding of growth hormone has been correlated with metabolic responses in primary cultures of adipocytes, with developmental responses in a fibroblast cell line, and with growth responses in primary cultures of chondrocytes. The ability to control the activation of GHR is important in developing new therapies for certain diseases such as dwarfism and acromegaly. GH is not glycosylated and can be produced in a fully active form in bacteria. The protein has a short in vivo half-life and must be administered by daily subcutaneous injection for maximum effectiveness. Recombinant human growth hormone (rhGH) was approved recently for treating cachexia in AIDS patients and is under study for treating cachexia associated with other diseases. |
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