How Cells Communicate, Part 3: How Cells Respond to Signals.
Many of the chemicals that carry messages between cells never enter their target cell. Instead, they activate a receptor that passes through the cell membrane. The activated receptor then triggers a sequence of chemical signals within the cell. These intracellular signaling pathways play an important role in health and disease. For example, some oncogenes (genes that can cause cancer) are genes that code for a receptor protein (eg, the estrogen receptor) that is in a constitutively activated state (ie, it is in the "on" state all the time). A constitutively activated receptor activates its signaling pathway even if its ligand is absent. (1) For this reason, signal transduction pathways are becoming increasingly important as therapeutic targets, particularly in oncology but also in immunology. In oncology, the goal is to find some way to suppress the proliferation of cancer cells without causing undue harm to the body as a whole. In immunology, the goal is typically to find some way to suppress an inappropriate inflammatory response without causing dangerous immune suppression or other undesirable side effects. Scientists have discovered that some "old" drugs, such as theophylline and caffeine, work by inhibiting an enzyme involved in an intracellular signaling pathway. Scientists are now developing other agents that are specifically designed to act selectively on a particular target within a particular intracellular signaling pathway within a particular set of cells. To understand how those drugs work, you need to understand intracellular signal transduction pathways.
Proteins play crucial roles in intracellular signaling pathways, both as signaling chemicals and as receptors (sensors). As Part 2 explained, each protein tends to take on a predictable shape that depends on its genetically determined chemical structure. This precise shape allows each receptor to bind selectively to some ligands but not to others. Because this binding is selective and generally reversible, it is often described as a lock-and-key relationship. The bond between the ligand and its receptor can cause a temporary change in the conformation (shape) of the receptor protein. This temporary conformational change can then trigger other events. For example, a ligand that is outside the cell can bind to a receptor that crosses a cell membrane. The resulting change in the shape of the intracellular domains of the receptor protein can thus transmit a signal into the cell, even though the ligand remains outside of the cell. Some receptor proteins do not have a chemical ligand. Instead, they undergo a conformational change in response to other kinds of stimuli, such as light, temperature, mechanical stress, or osmotic pressure.
Some signaling chemicals directly trigger a simple response, such as the opening of a ligand-gated ion channel. Others trigger a long and complicated cascade of signals within the cell. The following are the basic elements of the signal transduction pathways:
* A ligand is a molecule that binds reversibly to a receptor. (See Part 2 for a discussion of ligand-receptor relationships.)
* First messengers are signaling compounds that come from outside the cell.
* Signal transduction typically begins when a receptor (the signal transducer) becomes occupied by a ligand or responds to some other stimulus. As a result, the receptor undergoes a conformational change (a change in the shape of the protein molecule). This conformational change then activates a primary effector.
* The primary effector may produce some direct change in the cell's behavior, such as opening an ion channel through the cell membrane. Or the primary effector may trigger the production of another signaling chemical, called the second messenger, within the cell.
* The second messenger may then go on to activate a secondary effector, which may then produce another second messenger, and so on.
* Each component within this signaling cascade is called a node.
* An efficient node can amplify a signal (a phenomenon known as signal gain). As a result, one signaling molecule can generate a response involving hundreds to millions of molecules.
* Conversely, an intracellular signal can also be suppressed by blocking an intracellular receptor or degrading a second messenger. For this reason, these nodes and the enzymes that regulate them are becoming important as therapeutic targets.
* Each cell has many intracellular signaling pathways, and these pathways can interact in complicated ways. To describe these interactions, cell biologists have borrowed radio terminology, such as networks, noise, and crosstalk.
TYPES OF RECEPTORS
Receptors can be sorted into 4 broad categories: ligand-gated ion channels, G-protein-coupled receptors, kinase-linked receptors, and nuclear receptors.
Ligand-Gated Ion Channels
Ligand-gated ion channels are also called ionotropic receptors because they can open or close a channel that would allow some sort of ion to travel into or out of the cell (Figure 1). They typically provide a signal that lasts for only a few milliseconds.
Many postsynaptic receptors are ligand-gated ion channels. Binding to the neurotransmitter that serves as their ligand causes these receptors to undergo a conformational change that opens a channel that permits a flow of ions across the cell membrane. This flow of ions then produces either depolarization (in the case of excitatory receptors) or hyperpolarization (in the case of inhibitory receptors). Ligand-gated ion channels typically consist of an extracellular domain that includes the ligand-binding site and a transmembrane domain that includes the ion pore.
The largest family of cell-surface receptors are called G-protein-coupled receptors (GPCRs) (Figure 2) because they are associated with guanine nucleotide-binding proteins (G proteins). The time scale of a signal produced by activation of a GPCR is typically measured in seconds.
GPCRs are sometimes called metabotropic receptors because their action is mediated by metabolic functions (eg, enzyme activation). They do not have ion channels. However, binding of the ligand to some GPCRs can indirectly result in the opening of an ion channel elsewhere on the membrane. Many GPCRs are activated by binding with a ligand. Others are activated by other kinds of stimuli, such as light. For example, the light-sensing protein rhodopsin (also known as visual purple) is a GPCR. (2)
Because GPCRs are expressed on the outer surface of the cell, they can readily interact with water-soluble ligands, including drugs. Approximately 40% of all prescription drugs (including antihistamines and beta blockers) target a GPCR. (3) Analysis of the human genome suggests that there are hundreds of different GPCRs. (4) Many GPCRs have been found to respond to a particular ligand or other stimulus, whether it is an amine, an ion, a nucleoside, a lipid, a peptide, a protein, or (in the case of optical receptors) light. (3) There are also many "orphan" GPCRs, which have been identified through genetic analysis but whose ligands have not been identified.
Each GPCR consists of a protein that is anchored in the cell membrane by 7 transmembrane domains (ie, the receptor protein crosses the cell membrane 7 times). (See Part 2 for a discussion of protein structure.) When the receptor is in the unactivated state, its intracellular portion is linked to a G protein. G proteins are heterotrimeric, which means that they consist of 3 different subunits called G[alpha], G[beta], and G[gamma]. The G[alpha] subunit of the heterotrimeric G protein may be bound to guanosine diphosphate (GDP). When the receptor is activated, the receptor undergoes a conformational change that causes phosphorylation (addition of a high-energy phosphate group) of GDP, thus yielding guanosine triphosphate (GTP). As a result, the G[alpha]-GTP complex and the remaining G[beta][gamma] dimer break free from the GPCR and can interact with other receptors down-stream in the signal transduction cascade. Meanwhile, the intracellular portion of GPCR can bind to another heterotrimeric G protein to form a new complex that can initiate a new round of signal transduction. The signaling activity of the disassociated G protein ends when the GTP associated with G[alpha] is degraded to GDP (often by the G[alpha] subunit's own GTPase activity) and the G[alpha] subunit recombines with a G[beta][gamma] dimer to form a new heterotrimer, which can then bind to the intracellular domain of the transmembrane receptor. (5)
There are several different classes of G-proteins, which are defined by their different G[alpha] subunits. Some GPCRs can activate more than one G[alpha] subtype, but they usually have a preference for a particular subtype. The G[[alpha].sup.[sigma]] and G[[alpha].sup.i/o] subtypes activate the enzyme adenylyl cyclase, which catalyzes the formation of a second messenger called cyclic adenosine monophosphate (cAMP). When adenylyl cyclase is activated, it can generate many molecules of cAMP, thus amplifying the signal. Cyclic AMP often exerts its downstream effect by binding to and activating protein kinase A (PKA; also known as cAMP-dependent kinase), which then phosphorylates target proteins in the cell (Figure 3).
Cyclic AMP is broken down by cyclic nucleotide phosphodiesterase enzymes, which thus suppress and shorten the cAMP signal. Several drugs work by selectively inhibiting a particular cyclic nucleotide phosphodiesterase enzyme. For example, theophylline exerts its anti-inflammatory effect in the respiratory tract by inhibiting phosphodiesterase 4 (PDE4), which is found in a wide range of inflammatory cells. (6) In contrast, inhibitors of PDE5 are used for the treatment of erectile dysfunction. (6)
G[[alpha].sub.q/11] activates an enzyme called phospholipase C, which splits a membrane phospholipid called PIP2 into 2 second messengers: inositol triphosphate (IP3) and diacylglycerol (DAG). IP3 is water soluble and diffuses through the cytosol and binds to a ligand-gated C[a.sup.++] channel on the endoplasmic reticulum (or sarcoplasmic reticulum in muscle cells). The calcium ions that consequently enter the cytosol then bind to calcium-binding proteins such as calmodulin. In contrast, DAG is lipid-soluble and stays in the membrane. DAG activates protein kinase C, which then phosphorylates particular target proteins. There is also a G[[alpha].sub.12/13], which binds to RhoGEF proteins, which then activate the cytosolic small GTPase Rho, which is involved in regulating the cell's cytoskeleton.
Kinase-linked receptors produce a signal that can last for hours. A kinase is an enzyme that catalyzes the transfer of a phosphate group from a high-energy, phosphate-donating molecule to a specific substrate. Kinases play an important role in intracellular signaling cascades, and dysregulation of many kinases has been linked to the development of disease. Many kinase inhibitors are currently being investigated in clinical trials. (7) More than two dozen have been approved for human use, mainly in oncology but some for the treatment of rheumatoid arthritis or macular degeneration. Some protein kinases serve as receptors. The insulin receptor and other growth hormone receptors are receptor tyrosine kinases (RTKs). Other protein kinases serve as secondary effectors in the intracellular signaling cascade.
Receptor Tyrosine Kinases
The RTKs consist of an extracellular domain that binds to a ligand and an intracellular domain that functions as a kinase (Figure 4). An RTK consists of 2 sub units, which form a dimer anchored in the cell membrane. This dimer becomes stabilized when its extracellular portion binds to its ligand. This stabilization allows the tyrosine residues of the intracellular domains to become activated through phosphorylation. Once activated, they initiate phosphorylation signaling cascades that have effects on cell metabolism and differentiation.
Activated RTKs activate small G-proteins (from the Ras, Rho, and Raf families), which in turn activate guanine nucleotide exchange factors such as SOS1. When activated, these exchange factors can activate more small G proteins, thus amplifying the signal. Raf proteins are central components of the mitogen-activated protein kinase (MAPK) pathway that regulates cell proliferation. Mutations that increase the catalytic activity of Raf proteins have been identified in many human tumors. For this reason, the Raf kinase family has emerged as a promising target in the treatment of many cancers. (8) For example, sorafenib is used in the treatment of unresectable hepatocellular carcinoma, advanced renal cell carcinoma, and refractory thyroid carcinoma. (9)
Members of the type 1 and type 2 cytokine receptor families have no catalytic kinase activity of their own. Instead, they rely on Janus kinases (JAK) to phosphorylate and activate the downstream proteins in their signal transduction pathways. When activated, the JAKs phosphorylate and activate transcription factors called STATs (signal transducer and activator of transcription). JAK inhibitors (also known as Jakinibs) are being investigated for use in the treatment of patients with autoimmune diseases. One example is tofacitinib, which is approved for the treatment of patients with rheumatoid arthritis. (10)
Integrins are transmembrane receptors that facilitate the adhesion of cells to an extracellular matrix. (11) Epithelial cells normally have active integrins at their cell surface, which help to maintain their attachment to stromal cells. In contrast, circulating leukocytes normally have inactive integrins. However, their integrins can be activated by the inflammatory process. For example, the weak binding of a T lymphocyte to its specific antigen on the surface of an antigen-presenting cell triggers intracellular signaling pathways in the T cell that activate its [[beta].sub.2] integrins. The activated integrins then enable the T cell to adhere strongly to the antigen-presenting cell so that it remains in contact long enough to become stimulated fully. The integrins may then return to an inactive state, allowing the T cell to disengage. (11)
Integrins are heterodimers (ie, they consist of an [alpha] and [beta] subunit). (11) Integrins have no kinase activity. Instead, they transmit their signals through various intracellular protein kinases and adaptor molecules, especially integrin-linked kinase.
Toll-like receptors (TLRs) are named after their similarity to the toll protein identified in Drosophila. (12) The TLRs are single, membrane-spanning, noncatalytic receptors that are usually expressed on sentinel cells of the immune system (eg, macrophages and dendritic cells). The TLRs recognize molecules that are broadly shared by pathogens, such as bacteria and viruses. When activated, TLRs recruit adaptor proteins, which mediate specific protein-protein interactions that drive the formation of protein complexes. These protein complexes then activate other downstream proteins, including kinases, in a pathway that ultimately alters gene expression. The TLRs play an important role in the innate immune system and serve as an important link between innate and adaptive immunity. (13)
Intracellular (Nuclear) Receptors
Intracellular (nuclear) receptors produce signals that typically last for hours. Many signaling pathways end up activating an intracellular receptor that then goes on to alter the expression of a particular gene. Some intracellular receptors are activated by lipid-soluble first messengers, such as steroid hormones and vitamins A and D, that have diffused through the cell membrane. Other intracellular receptors are activated by second messengers that are produced by an intracellular signaling cascade.
Intracellular receptors can be found in the cytosol or in the nucleus (Figure 5). Before a cytosolic receptor can interact with a gene, the cytosolic receptor-ligand complex must pass into the nucleus. In the nucleus, the DNA-binding domain of the ligand-receptor complex binds to DNA. An activated steroid receptor will generally bind to a receptor-specific hormone-responsive element (HRE) sequence in the promoter region of a gene that is activated by that hormone-receptor complex. As a result, the receptor-ligand complex can alter the expression of that gene, for example by increasing or decreasing the transcription of the gene and thus influencing the production of the protein encoded by that gene.
The human body is made up of trillions of cells, each of which can be viewed as a separate organism. Yet these trillions of cells coordinate their activity in ways that allow the body to function as a single entity and to react to a wide range of environmental stressors. This coordination is accomplished largely through chemical signals, which are emitted by some cells and have effects on other cells. Part 1 of this 3-part presentation described how the endocrine system helps maintain homeostasis in the body. For example, hormones produced in the pancreas regulate blood sugar by causing the liver to store or release glucose, as needed. This third and final part explains what happens when one of these chemical signals reaches its target cell and how the cell's response to that signal can be modulated by medications that act inside the cell.
Inter- and intracellular signaling pathways play important roles in maintaining health, and disturbances in these pathways can result in disease. For example, disruptions in the pathways that regulate the activities of cells in the immune system can result in autoimmune or inflammatory diseases. Likewise, disruptions in the pathways that would ordinarily suppress a cell's division or command the cell to commit suicide (apoptosis, or programmed cell death) can lead to cancer. For this reason, research into intracellular signaling pathways has been yielding new medications for inflammatory and neoplastic disease.
By Laurie Endicott Thomas, MA, ELS / Author and freelance medical writer, Madison, NJ
Laurie Endicott Thomas is the author of Thin Diabetes, Fat Diabetes: Prevent Type 1, Cure Type 2 (www.thindiabetes.com). She can be reached at email@example.com.
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Conformational change--the change in shape (conformation) of a protein.
Cyclic AMP--cyclic adenosine monophosphate is an important second messenger in intracellular signaling cascades.
First messenger--a signaling compound that came from outside the cell.
Kinase--an enzyme that catalyzes phosphorylation, which is the transfer of a phosphate group from a high-energy phosphate-donating molecule (such as adenosine triphosphate) to a specific substrate.
Ligand--a molecule that binds reversibly to a receptor.
Phosphodiesterases--a class of enzymes that break down the phosphodiester bond in second-messenger molecules, such as cyclic AMP and cyclic guanosine monophosphate (cGMP).
Protein phosphorylation--The addition of a covalently bound phosphate group to an amino acid residue in a protein. This addition often changes the shape and function of the protein.
Second messenger--a signaling compound that is produced inside the cell as part of a signaling cascade.
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|Title Annotation:||SCIENCE SERIES|
|Author:||Thomas, Laurie Endicott|
|Publication:||American Medical Writers Association Journal|
|Date:||Dec 22, 2017|
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