15 Cell Signaling and Signal Transduction：Communication Between Cells

605 15.1 The Basic Elements of Cell Signaling Systems 15.2 A Survey of Extracellular Messengers and Their Receptors 15.3 G Protein-Coupled Receptors and Their Second Messengers 15.4 Protein-Tyrosine Phosphorylation as a Mechanism for Signal Transduction 15.5 The Role of Calcium as an Intracellular Messenger 15.6 Convergence, Divergence, and Cross-Talk Among Different Signaling Pathways 15.7 The Role of NO as an Intercellular Messenger 15.8 Apoptosis (Programmed Cell Death) The Human Perspective: Disorders Associated with G Protein-Coupled Receptors 15 Cell Signaling and Signal Transduction: Communication Between Cells he English poet John Donne expressed his belief in the interdependence of humans in the phrase “No man is an island.” The same can be said of the cells that make up a complex multicellular organism. Most cells in a plant or animal are specialized to carry out one or more specific functions. Many biological processes require various cells to work together and to coordinate their activities. To make this possible, cells have to communicate with each other, which is accomplished by a process called cell signaling. Cell signaling makes it possible for cells to respond in an appropriate manner to a specific environmental stimulus. T Three-dimensional, X-ray crystallographic structure of a �2-adrenergic receptor (�2-AR), which is a representative member of the G protein-coupled receptor (GPCR) superfamily. These inte- gral membrane proteins are characterized as containing seven transmembrane helices. As a group, these proteins bind an astonishing array of biological messengers, which constitutes the first step in eliciting many of the body’s most basic responses. The �2-AR is a resident of the plasma membrane of a variety of cells, where it normally binds the ligand epinephrine and mediates such responses as increased heart rate and relaxation of smooth muscle cells. Beta-adrenergic receptors are the targets of a number of important drugs, including �-blockers, which are widely prescribed for the treatment of high blood pressure and heart arrhythmias. GPCRs have been very difficult to crystallize so that high-resolution structures of these important proteins have been lacking. This situation is now changing as the result of recent advances in crystalliza- tion technology, and it is hoped that these new high-resolution structures will lead to the devel- opment of new classes of structure-designed drugs. The image shown here depicts two �2-ARs, which were crystallized in the presence of cholesterol and palmitic acid (yellow) and a receptor- binding ligand (green). (FROM VADIM CHEREZOV ET AL., COURTESY OF RAYMOND C. STEVENS, SCIENCE 318:1258, 2007; © COPYRIGHT 2007, AMERICAN ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE.) JWCL151_ch15_605-649.qxd 8/9/09 12:05 AM Page 605 606 Chapter 15 CELL SIGNALING AND SIGNAL TRANSDUCTION: COMMUNICATION BETWEEN CELLS Cell signaling affects virtually every aspect of cell struc- ture and function, which is one of the primary reasons that this chapter appears near the end of the book. On one hand, an understanding of cell signaling requires knowledge about other types of cellular activity. On the other hand, insights into cell signaling can tie together a variety of seemingly in- dependent cellular processes. Cell signaling is also intimately involved in the regulation of cell growth and division. This makes the study of cell signaling crucially important for un- derstanding how a cell can lose the ability to control cell di- vision and develop into a malignant tumor. ■ 15.1 THE BASIC ELEMENTS OF CELL SIGNALING SYSTEMS It may be helpful to begin the discussion of this complex sub- ject by describing a few of the general features that are shared by most signaling pathways. Cells usually communicate with each other through extracellular messenger molecules. Extracellular messengers can travel a short distance and stim- ulate cells that are in close proximity to the origin of the mes- sage, or they can travel throughout the body, potentially stimulating cells that are far away from the source. In the case of autocrine signaling, the cell that is producing the messenger expresses receptors on its surface that can respond to that messenger (Figure 15.1a). Consequently, cells releasing the message will stimulate (or inhibit) themselves. During paracrine stimulation (Figure 15.1b), messenger molecules travel only short distances through the extracellular space to cells that are in close proximity to the cell that is generating the message. Paracrine messenger molecules are usually lim- ited in their ability to travel around the body because they are inherently unstable, or they are degraded by enzymes, or they bind to the extracellular matrix. Finally, during endocrine signaling, messenger molecules reach their target cells via passage through the bloodstream (Figure 15.1c). Endocrine messengers are also called hormones, and they typically act on target cells located at distant sites in the body. An overview of cellular signaling pathways is depicted in Figure 15.2. Cell signaling is initiated with the release of a messenger molecule by a cell that is engaged in sending messages to other cells in the body (step 1, Figure 15.2). Cells can only respond to an extracellular message if they express receptors that specifically recognize and bind that particular messenger molecule (step 2). In most cases, the messenger molecule (or ligand) binds to a receptor at the extracellular surface of the responding cell. This interaction causes a signal to be relayed across the membrane to the receptor’s cytoplas- mic domain (step 3). Once it has reached the inner surface of the plasma membrane, there are two major routes by which the signal is transmitted into the cell interior, where it elicits the appropriate response. The particular route taken depends on the type of receptor that is activated. In the following discussion, we will focus on these two major routes of signal transduction, but keep in mind there are other ways that ex- tracellular signals can have an impact on a cell. For example, we saw on page 164 how neurotransmitters act by opening plasma membrane ion channels and on page 514 how steroid hormones diffuse through the plasma membrane and bind to intracellular receptors. In the two major routes discussed in this chapter: ■ One type of receptor (Section 15.3) transmits a signal from its cytoplasmic domain to a nearby enzyme (step 4), which generates a second messenger (step 5). Because it brings about (effects) the cellular response by generating a second messenger, the enzyme responsible is referred to as an effector. Second messengers are small substances that typically activate (or inactivate) specific proteins. Depend- ing on its chemical structure, a second messenger may dif- fuse through the cytosol or remain embedded in the lipid bilayer of a membrane. ■ Another type of receptor (Section 15.4) transmits a signal by transforming its cytoplasmic domain into a recruiting station for cellular signaling proteins (step 4a). Proteins in- teract with one another, or with components of a cellular membrane, by means of specific types of interaction do- mains, such as the SH3 domain discussed on page 60. Whether the signal is transmitted by a second messenger or by protein recruitment, the outcome is similar; a protein that is positioned at the top of an intracellular signaling path- way is activated (step 6, Figure 15.2). Signaling pathways are the information superhighways of the cell. Each signaling FIGURE 15.1 Autocrine (a), paracrine (b), and endocrine (c) types of intercellular signaling. (a) (b) (c) JWCL151_ch15_605-649.qxd 8/9/09 12:05 AM Page 606 15.1 THE BASIC ELEMENTS OF CELL SIGNALING SYSTEMS 607 pathway consists of a series of distinct proteins that operate in sequence (step 7). Each protein in the pathway typically acts by altering the conformation of the subsequent (or down- stream) protein in the series, an event that activates or inhibits that protein (Figure 15.3). It should come as no surprise, af- ter reading about other topics in cell biology, that alterations in the conformation of signaling proteins are often accom- plished by protein kinases and protein phosphatases that, re- spectively, add or remove phosphate groups from other proteins (Figure 15.3). The human genome encodes more than 500 different protein kinases and approximately 150 dif- ferent protein phosphatases. Most protein kinases transfer phosphate groups to serine or threonine residues of their protein substrates, but a very important group of kinases phosphory- lates tyrosine residues. Some protein kinases and phos- phatases are soluble cytoplasmic proteins; others are integral membrane proteins. It is remarkable that, even though thou- sands of proteins in a cell contain amino acid residues with the potential of being phosphorylated, each protein kinase or phosphatase is able to recognize only its specific substrates and ignore all of the others. Some protein kinases and phos- phatases have numerous proteins as their substrates, whereas others phosphorylate or dephosphorylate only a single amino acid residue of a single protein substrate. Many of the protein substrates of these enzymes are enzymes themselves—most often other kinases and phosphatases—but the substrates also include ion channels, transcription factors, and various types of regulatory proteins. It is thought that at least 50 percent of transmembrane and cytoplasmic proteins are phosphory- lated at one or more sites. Protein phosphorylation can change protein behavior in several different ways. Phosphorylation can activate or inactivate an enzyme, it can increase or de- crease protein–protein interactions, it can induce a protein to move from one subcellular compartment to another, or it can FIGURE 15.2 An overview of the major signaling pathways by which extracellular messenger molecules can elicit intracellular responses. Two different types of signal transduction pathways are depicted, one in which a signaling pathway is activated by a diffusible second messenger and another in which a signaling pathway is activated by recruitment of proteins to the plasma membrane. Most signal transduction pathways involve a combination of these mechanisms. It should also be noted that signaling pathways are not typically linear tracks as depicted here, but are branched and interconnected to form a complex web. The steps are described in the text. Signaling cell Transmembrane receptor Second messenger Effector Activated target protein Extracellular signaling molecule (first messenger) 9 88 9 7 6 4a3 4 3 22 7 6 5 1 P Transcription Survival Protein synthesis Movement Cell death Metabolic change FIGURE 15.3 Signal transduction pathway consisting of protein ki- nases and protein phosphatases whose catalytic actions change the conformations, and thus the activities, of the proteins they modify. In the example depicted here, protein kinase 2 is activated by protein kinase 1. Once activated, protein kinase 2 phosphorylates protein kinase 3, activating the enzyme. Protein kinase 3 then phosphorylates a tran- scription factor, increasing its affinity for a site on the DNA. Binding of a transcription factor to the DNA affects the transcription of the gene in question. Each of these activation steps in the pathway is reversed by a phosphatase. Whereas protein kinases typically work as a single subunit, many protein phosphatases contain a key regulatory subunit that helps determine substrate specificity. ActiveInactive Protein kinase 2 P Protein kinase 2 Active Protein kinase 1 Active Protein kinase 3 Inactive Protein kinase 3 P Active Transcription factor Inactive Transcription factor P DNA mRNA P JWCL151_ch15_605-649.qxd 8/9/09 12:05 AM Page 607 608 Chapter 15 CELL SIGNALING AND SIGNAL TRANSDUCTION: COMMUNICATION BETWEEN CELLS 15.2 A SURVEY OF EXTRACELLULAR MESSENGERS AND THEIR RECEPTORS A large variety of molecules can function as extracellular car- riers of information. These include ■ Amino acids and amino acid derivatives. Examples include glutamate, glycine, acetylcholine, epinephrine, dopamine, and thyroid hormone. These molecules act as neurotrans- mitters and hormones. ■ Gases, such as NO and CO. ■ Steroids, which are derived from cholesterol. Steroid hormones regulate sexual differentiation, pregnancy, carbohydrate metabolism, and excretion of sodium and potassium ions. ■ Eicosanoids, which are nonpolar molecules containing 20 carbons that are derived from a fatty acid named arachidonic acid. Eicosanoids regulate a variety of processes including pain, inflammation, blood pressure, and blood clotting. Several over-the-counter drugs that are used to treat headaches and inflammation inhibit eicosanoid synthesis. ■ A wide variety of polypeptides and proteins. Some of these are present as transmembrane proteins on the sur- face of an interacting cell (page 247). Others are part of, or associate with, the extracellular matrix. Finally, a large number of proteins are excreted into the extracellular envi- ronment where they are involved in regulating processes such as cell division, differentiation, the immune response, or cell death and cell survival. Extracellular signaling molecules are usually, but not always, recognized by specific receptors that are present on the surface of the responding cell. As illustrated in Figure 15.2, receptors bind their signaling molecules with high affinity and translate this interaction at the outer surface of the cell into changes that take place on the inside of the cell. The receptors that have evolved to mediate signal transduction are indicated below. ■ G protein-coupled receptors (GPCRs) are a huge family of receptors that contain seven transmembrane � helices. These receptors translate the binding of extracellular sig- naling molecules into the activation of GTP-binding proteins. GTP-binding proteins (or G proteins) were discussed in connection with vesicle budding and fusion in Chapter 8, microtubule dynamics in Chapter 9, protein synthesis in Chapters 8 and 11, and nucleocytoplasmic transport in Chapter 12. In the present chapter, we will ex- plore their role in transmitting messages along “cellular in- formation circuits.” ■ Receptor protein-tyrosine kinases (RTKs) represent a second class of receptors that have evolved to translate the presence of extracellular messenger molecules into changes inside the cell. Binding of a specific extracellular ligand to an RTK usually results in receptor dimerization followed by activation of the receptor’s protein-kinase domain, which is present within its cytoplasmic region. Upon activation, these protein kinases phosphorylate spe- cific tyrosine residues of cytoplasmic substrate proteins, thereby altering their activity, their localization, or their ability to interact with other proteins within the cell. ■ Ligand-gated channels represent a third class of cell- surface receptors that bind to extracellular ligands. The ability of these proteins to conduct a flow of ions across the plasma membrane is regulated directly by ligand bind- ing. A flow of ions across the membrane can result in a temporary change in membrane potential, which will af- fect the activity of other membrane proteins, for instance, voltage-gated channels. This sequence of events is the basis for formation of a nerve impulse (page 162). In addi- tion, the influx of certain ions, such as Ca2�, can change the activity of particular cytoplasmic enzymes. As dis- ?R EVIE W 1. What is meant by the term signal transduction? What are some of the steps by which signal transduction can occur? 2. What is a second messenger? Why do you suppose it is called this? act as a signal that initiates protein degradation. Large-scale proteomic approaches have been employed (page 68) to iden- tify the potential substrates of various protein kinases. The primary challenge is to understand the roles of these diverse posttranslational modifications in the activities of different cell types. Signals transmitted along such signaling pathways ulti- mately reach target proteins (step 8, Figure 15.2) involved in basic cellular processes (step 9). Depending on the type of cell and message, the response initiated by the target protein may involve a change in gene expression, an alteration of the activ- ity of metabolic enzymes, a reconfiguration of the cytoskele- ton, an increase or decrease in cell mobility, a change in ion permeability, activation of DNA synthesis, or even the death of the cell. Virtually every activity in which a cell is engaged is regulated by signals originating at the cell surface. This over- all process in which information carried by extracellular mes- senger molecules is translated into changes that occur inside a cell is referred to as signal transduction. Finally, signaling has to be terminated. This is important because cells have to be responsive to additional messages that they may receive. The first order of business is to eliminate the extracellular messenger molecule. To do this, certain cells pro- duce extracellular enzymes that destroy specific extracellular messengers. In other cases, activated receptors are internalized (page 612). Once inside the cell, the receptor may be degraded together with its ligand, which can leave the cell with de- creased sensitivity to subsequent stimuli. Alternatively, recep- tor and ligand may be separated within an endosome, after which the ligand is degraded and the receptor is returned to the cell surface. JWCL151_ch15_605-649.qxd 8/9/09 12:05 AM Page 608 15.3 G PROTEIN-COUPLED RECEPTORS AND THEIR SECOND MESSENGERS 609 cussed in Section 4.8, one large group of ligand-gated channels functions as receptors for neurotransmitters. ■ Steroid hormone receptors function as ligand-regulated tran- scription factors. Steroid hormones diffuse across the plasma membrane and bind to their receptors, which are present in the cytoplasm. Hormone binding results in a conformational change that causes the hormone–receptor complex to move into the nucleus and bind to elements present in the promot- ers or enhancers of hormone-responsive genes (see Figure 12.43). This interaction gives rise to an increase or decrease in the rate of gene transcription. ■ Finally, there are a number of other types of receptors that act by unique mechanisms. Some of these receptors, for example, the B- and T-cell receptors that are involved in the response to foreign antigens, associate with known signaling molecules such as cytoplasmic protein-tyrosine kinases. We will concentrate in this chapter on the GPCRs and RTKs. 15.3 G PROTEIN-COUPLED RECEPTORS AND THEIR SECOND MESSENGERS G protein-coupled receptors (GPCRs) are so named because they interact with G proteins, as discussed be- low. Members of the GPCR superfamily are also referred to as seven-transmembrane (7TM) receptors because they contain seven transmembrane helices (Figure 15.4). Thousands of dif- ferent GPCRs have been identified in organisms ranging from yeast to flowering plants and mammals that together regulate an extraordinary spectrum of cellular processes. In fact, GPCRs constitute the single largest superfamily of pro- teins encoded by animal genomes. Included among the natu- ral ligands that bind to GPCRs are a diverse array of hormones (both plant and animal), neurotransmitters, opium derivatives, chemoattractants (e.g., molecules that attract phagocytic cells of the immune system), odorants and tastants (molecules detected by olfactory and gustatory receptors elic- FIGURE 15.4 The membrane-bound machinery for transducing sig- nals by means of a seven transmembrane receptor and a heterotrimeric G protein. (a) Receptors of this type, including those that bind epi- nephrine and glucagon, contain seven membrane-spa