Hormones are present in the plasma and interstitial tissue at concentrations ranging from 10-7 M to 10-10 M. Due to these very low physiological concentrations, sensitive protein receptors have evolved in target tissues to perceive the presence of very weak signals. In addition, systemic feedback mechanisms have evolved to regulate the production of endocrine hormones. Once the hormone is secreted by the endocrine tissue, it usually binds to a specific plasma protein carrier and the complex is transmitted to distant tissues. Plasma carrier proteins are present in all classes of endocrine hormones. The carrier proteins for peptide hormones prevent hormone destruction by plasma proteases. Carriers of steroids and thyroid hormones allow these very hydrophobic materials to be present in plasma at concentrations several hundred times higher than their solubility in water. The carrier of small, hydrophilic amino acid-derived hormones prevents them from filtering through the glomerulus, greatly extending their circulating half-life. Tissues that are capable of responding to endocrine have two common characteristics: they have receptors that have a very high affinity for hormones, and that receptors are coupled to processes that regulate the metabolism of target cells. Most amino acid-derived hormones and receptors for all peptide hormones are located on the plasma membrane. The activation of these receptors by hormones (first messengers) results in the production of a second messenger in the cell, such as cAMP, which is responsible for initiating intracellular biological responses. Steroids and thyroid hormones are hydrophobic and diffuse from binding proteins in plasma, through the plasma membrane to receptors localized within the cell. The resulting steroid and receptor complex binds to the reactive elements of nuclear DNA and regulates mRNA production by specific proteins.
Receptors for Peptide Hormones
With the exception of the thyroid hormone receptor, the receptors for amino acid-derived and peptide hormones are located in the plasma membrane. Receptor structure is varied: some receptors consist of a single polypeptide chain with a domain on either side of the membrane, connected by a membrane-spanning domain. Some receptors are comprised of a single polypeptide chain that is passed back and forth in serpentine fashion across the membrane, giving multiple intracellular, transmembrane, and extracellular domains. Other receptors are composed of multiple polypeptides. For example, the insulin receptor is a disulfide-linked tetramer with the β-subunits spanning the membrane and the α-subunits located on the exterior surface.
Subsequent to hormone binding, a signal is transduced to the interior of the cell, where second messengers and phosphorylated proteins generate appropriate metabolic responses. The main second messengers are cAMP, Ca2+, inositol-1,4,5-triphosphate (IP3), and diacylglycerol (DAG). The generation of cAMP occurs via activation of G-protein coupled receptors (GPCRs) whose associated G-proteins activated adenylate cyclase. GPCRs also couple to G-protein activation of phospholipase C-β (PLCβ). Activated PLCβ hydrolyzes membrane phospholipids (as described below) resulting in increased levels of IP3 and DAG. Downstream signaling proteins are phosphorylated on serine and threonine by PKA and DAG-activated protein kinase C (PKC) leading to alterations in their activities. Additionally, a series of membrane-associated and intracellular tyrosine kinases phosphorylate specific tyrosine residues on target enzymes and other regulatory proteins.
The hormone-binding signal of most, but not all, plasma membrane receptors is transduced to the interior of cells by the binding of receptor-ligand complexes to a series of membrane-localized GDP/GTP binding proteins known as G-proteins. The classic interactions between receptors, G-protein transducer, and membrane-localized adenylate cyclase are illustrated below using the pancreatic hormone glucagon as an example. When G-proteins bind to receptors, GTP exchanges with GDP bound to the α subunit of the G-protein. The Gα-GTP complex binds adenylate cyclase, activating the enzyme. The activation of adenylate cyclase leads to cAMP production in the cytosol and to the activation of PKA, followed by regulatory phosphorylation of numerous enzymes. Stimulatory G-proteins are designated Gs, inhibitory G-proteins are designated Gi.
1. Kuntal PAL.; et al. Structure and mechanism for recognition of peptide hormones by Class B G-protein-coupled receptors. Acta Pharmacologica Sinica. 2012,33:300-311.