INTRODUCTION — Advances in molecular biology over the past 15 years have expanded our understanding of the processes of peptide hormone receptor binding and signal transduction that were previously impossible to study. The DNA sequences for hundreds of receptors and many signaling molecules involved in their regulation have been analyzed.
Signal transduction is a process in which a peptide hormone transfers specific information from the outside of the target cell to exert a cellular response. For this to occur, the hormone (eg, gastrin) exerts a signal through a specific receptor that transmits information from the extracellular compartment (blood) into the cell (acid-secreting cells of the stomach). This message is tightly controlled, especially in settings that are vital for cellular homeostasis.
The normal function of a cell depends upon an intact signal regulation/termination system. If this system malfunctions, the host may experience pathophysiological consequences such as abnormal secretion, motility, growth, or even the development of cancer [1,2].
The major physiological principles of cell signaling systems will be reviewed here. Discussions of individual peptide hormones are presented separately. (See appropriate topic reviews.)
RECEPTOR STIMULATION — Despite the vast array of information communicated to a cell, the basic components of the signaling system are relatively simple (show figure 1). A peptide hormone binds to a cell surface receptor and stimulates activation of an effector system. Cell surface receptors are capable of interacting with only certain chemical messages. The specificity of the hormone-receptor interaction is responsible for the unique cellular response.
The peptide hormone must initiate a change in the receptor such that the hormone-receptor complex activates an intracellular effector molecule such as a specific guanyl-nucleotide-binding protein (G-protein) (show figure 2). Most peptide hormone receptors act through G-proteins; as a result, these receptors are called G protein-coupled receptors (GPCRs).
G proteins — G-proteins are molecular intermediaries that initiate the intracellular communication process (show figure 2) [3,4]. After the hormone binds to its receptor, a G-protein is stimulated. Stimulation begins the intracellular process of signal transduction.
G-proteins are composed of three subunits (alpha, beta, and gamma) and are classified according to their alpha subunit. G-proteins that stimulate adenylyl cyclase are classified as the Gs type; those that inhibit adenylyl cyclase are called Gi. To date, 20 different G-protein alpha subunits have been identified [4].
Shortly after receptor stimulation, a series of events are initiated, which ultimately act to turn off signaling. The principle events in this process involve receptor desensitization and internalization, which reestablish cell responsiveness. (See "Desensitization" below and see "Internalization" below).
G protein-coupled receptors — G protein-coupled receptors are heptahelical proteins, with seven membrane spanning domains [5]. They contain an extracellular amino terminus and an intracellular carboxy terminus (show figure 3). When stimulated by the appropriate chemical messenger, the GPCR undergoes a conformational change that causes coupling to a specific G protein.
GPCRs are classified by their structure into three groups (show table 1). Group I, the largest group, contains the receptors for catecholamines, many peptide hormones, neuropeptides, and glycoproteins. Group II contains the secretin/glucagon/vasoactive intestinal peptide receptor family. Group III contains the metabotrophic receptors (eg, calcium-sensing and glutamate receptors).
Effector systems — Following receptor occupation, G-protein subunits cause activation of enzymes or other proteins, ultimately resulting in a variety of cellular responses (show figure 4). Enzymes, such as adenylyl cyclase or phospholipase C, generate specific second messengers; examples include cyclic adenosine monophosphate (cAMP) and inositol 1,4,5 triphosphate (IP3) and diacylglycerol. Some G-proteins couple directly with specific ion channels, such as potassium or calcium channels, and initiate changes in ion permeability (show figure 4). The effector systems are not understood for some receptors such as receptors involved with cell growth and differentiation (show table 2).
Adenylate cyclase — One of the most studied effector systems of receptor activation is the production of cAMP. As discussed above, Gs coupled G-protein-coupled receptors stimulate adenylate cyclase to produce cAMP. A conformational change occurs as the hormone binds to its receptor allowing the receptor to associate with Gs. Under basal (unstimulated) conditions Gs is bound to GDP. However, GDP is released during hormone binding and is replaced with GTP. The Gs-GTP complex then activates adenylyl cyclase, resulting in the formation of cAMP from ATP within the cytoplasm of the cell. cAMP is then capable of producing other effects within the cell, ultimately leading to responses such as secretion, motility, or growth.
The G alpha-GTP complex is gradually inactivated by GTPase, which converts GTP to GDP. This enzymatic conversion occurs spontaneously by the G-protein, which is itself a GTPase. The conversion of GTP to GDP no longer permits G-protein stimulation of adenylate cyclase and is one way by which the hormone signal is terminated and the basal condition is restored.
Phospholipase C — Other G-proteins, such as Go, activate the phosphoinositide system when bound to hormone. Phospholipase C (PLC) acts on inositol phospholipids found in the cell membrane. As an example, PLC can cause the hydrolysis of phosphatidylinositol 4, 5 bisphosphate (PIP2) to 1, 2 diacylglycerol and inositol 1,4,5 triphosphate (IP3). Diacylglycerol and IP3 can then act as regulators of cell metabolism. This pathway can alter cell function by increasing intracellular calcium levels.
SIGNAL REGULATION AND TERMINATION — Even while signal transduction is occurring, processes begin that will terminate receptor responsiveness.
Desensitization — For the cell to respond to future stimuli, signaling must be terminated completely and in a timely fashion; a process known as desensitization. Desensitization begins within seconds to minutes of hormone binding, and eventually results in signal termination [6].
Desensitization is the primary regulatory step that assures appropriate cell function. It involves the termination of receptor activation by receptor phosphorylation, which is initiated by specific G protein-coupled receptor kinases (GRKs) or second messenger-dependent kinases (eg, protein kinase A and protein kinase C).
Phosphorylation of receptors requires the recruitment of proteins to the hormone-receptor complex, which participate in regulating signaling. One of these is beta-arrestin, which is located in the cytoplasm of unstimulated cells [6]. Upon hormone receptor stimulation, beta-arrestin is translocated from the cytoplasm to the cell membrane and assists in signal termination and subsequent hormone-receptor internalization [6-8].
Internalization — Once the receptor is adequately phosphorylated, the hormone-receptor complex is moves from the cell membrane to the inside of the cell; a process known as "internalization." Internalization, which may also involve beta-arrestins [8], permits receptor processing to occur, which will most likely result in receptor dephosphorylation, removal/degradation of the peptide hormone, and receptor degradation or recycling. Regardless of the eventual fate of the hormone-receptor complex, the goal is to reestablish cell responsiveness, so the next hormone stimulus is capable of sending the necessary information into the cell.
Beta-arrestin — Arrestins are cytosolic proteins that are recruited to hormone bound receptors and bind to cytoplasmic regions of the receptor [9]. Once bound with beta-arrestin, the hormone-receptor complex is "targeted" to a specific endocytic pathway that turns off the signaling process. Endocytosis is the process by which the hormone-occupied receptor is brought from the plasma membrane into the cell. The eventual fate of the receptor depends in part upon the receptor type. Some receptors are rapidly internalized and recycled back to the cell membrane while others are destroyed and only newly produced receptors are expressed on the cell surface.
Non-G protein-coupled receptors
Receptor tyrosine kinases — Some peptides signal through receptors that are not linked to G proteins. One particular class of receptors possesses intrinsic protein tyrosine kinase activity. These receptors are comprised of an extracellular domain that is usually glycosylated, a single transmembrane domain, and a cytoplasmic domain that contains a protein tyrosine kinase region and a region that is a substrate for peptide ligand-activated phosphorylation.
With peptide binding, these receptors either phosphorylate themselves or are phosphorylated by other protein kinases [10]. After activation, these receptors initiate other intracellular signal transduction pathways including Ras that activates MAP kinase. MAP kinase, in turn, modulates other cellular proteins, particularly transcription factors. Specific phosphorylated tyrosine residues are also binding sites for Src homology regions 2 and 3 (SH2 and SH3 domains) that can activate various signaling pathways [11].
Examples of the receptor tyrosine kinase family include receptors for epidermal growth factor, insulin, insulin-like growth factor, fibroblast growth factor, vascular endothelial growth factor, platelet-derived growth factor, nerve growth factor, and macrophage colony stimulating factor [12].
Receptor serine/threonine kinases — Receptor serine/threonine kinases such as TGF-b receptors contain a single transmembrane domain. Stimulation of these receptors activates endogenous serine/ threonine kinase activity which modulates cellular protein function [13].
PATHOPHYSIOLOGIC RELEVANCE — Dysfunction of the control mechanisms of cellular signaling may lead to a number of pathophysiologic consequences [14]. Numerous receptor mutations have been identified that result in unregulated stimulation in the absence of hormone (constitutive activity). As examples, a constitutively active receptor has been found in thyroid adenomas producing clinical hyperthyroidism [15] and in precocious puberty secondary to a mutation in the luteinizing hormone receptor [16]. On the other hand, the McCune-Albright syndrome is due to postzygotic activating mutations in the gene encoding the G alpha s protein, resulting in activation of the signal-transduction pathway generating cyclic AMP [17-19]. The clinical manifestations include polyostotic fibrous dysplasia, cafe au lait spots, and hyperfunction of multiple glands that can lead to sexual precocity, Cushing's syndrome, acromegaly, hyperthyroidism, or hyperparathyroidism.
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