Communication by extracellular signals usually involves six steps: (1) synthesis and (2) release of the signaling molecule by the signaling cell; (3) transport of the signal to the target cell; (4) detection of the signal by a specific receptor protein; (5) a change in cellular metabolism, function, or development triggered by the receptor-signal complex; and (6) removal of the signal, which often terminates the cellular response.
In many eukaryotic microorganisms (e.g., yeast, slime molds, and protozoans), secreted molecules coordinate the aggregation of free-living cells for sexual mating or differentiation under certain environmental conditions. Chemicals released by one organism that can alter the behavior or gene expression of other organisms of the same species are called pheromones. Yeast mating-type factors discussed later in this chapter are a well-understood example of pheromonemediated cell-to-cell signaling. Some algae and animals also release pheromones, usually dispersing them into the air or water, to attract members of the opposite sex. More important in plants and animals are extracellular signaling molecules that function within an organism to control metabolic processes within cells, the growth of tissues, the synthesis and secretion of proteins, and the composition of intracellular and extracellular fluids. This chapter focuses on such cell-to-cell signaling in single-celled eukaryotes and in a variety of higher eukaryotes, particularly mammals.
(a – c) Cell-to-cell signaling by extracellular chemicals occurs over distances from a few micrometers in autocrine and paracrine signaling to several meters in endocrine signaling. (d) Proteins attached to the plasma membrane of one cell can interact directly with receptors on an adjacent cell.
).
In endocrine signaling, signaling molecules, called hormones, act on target cells distant from their site of synthesis by cells of endocrine organs. In animals, an endocrine hormone usually is carried by the blood from its site of release to its target.
In paracrine signaling, the signaling molecules released by a cell only affect target cells in close proximity to it. The conduction of an electric impulse from one nerve cell to another or from a nerve cell to a muscle cell (inducing or inhibiting muscle contraction) occurs via paracrine signaling. The role of this type of signaling, mediated by neurotransmitters, in transmitting nerve impulses is discussed in Chapter 21. Many signaling molecules regulating development in multicellular organisms also act at short range. Some of these molecules are discussed in Chapter 23.
In autocrine signaling, cells respond to substances that they themselves release. Many growth factors act in this fashion, and cultured cells often secrete growth factors that stimulate their own growth and proliferation. This type of signaling is particularly common in tumor cells, many of which overproduce and release growth factors that stimulate inappropriate, unregulated proliferation of themselves as well as adjacent nontumor cells; this process may lead to formation of tumor mass.
Some compounds can act in two or even three types of cell-to-cell signaling. Certain small amino acid derivatives, such as epinephrine, function both as neurotransmitters (paracrine signaling) and as systemic hormones (endocrine signaling). Some protein hormones, such as epidermal growth factor (EGF), are synthesized as the exoplasmic part of a plasma- membrane protein; membrane-bound EGF can bind to and signal an adjacent cell by direct contact. Cleavage by a protease releases secreted EGF, which acts as an endocrine signal on distant cells.
As noted earlier, the cellular response to a particular extracellular signaling molecule depends on its binding to a specific receptor protein located on the surface of a target cell or in its nucleus or cytosol. The signaling molecule (a hormone, pheromone, or neurotransmitter) acts as a ligand, which binds to, or “fits,” a site on the receptor. Binding of a ligand to its receptor causes a conformational change in the receptor that initiates a sequence of reactions leading to a specific cellular response.
The response of a cell or tissue to specific hormones is dictated by the particular hormone receptors it possesses and by the intracellular reactions initiated by the binding of any one hormone to its receptor. Different cell types may have different sets of receptors for the same ligand, each of which induces a different response. Or the same receptor may occur on various cell types, and binding of the same ligand may trigger a different response in each type of cell. Clearly, different cells respond in a variety of ways to the same ligand. For instance, acetylcholine receptors are found on the surface of striated muscle cells, heart muscle cells, and pancreatic acinar cells. Release of acetylcholine from a neuron adjacent to a striated muscle cell triggers contraction, whereas release adjacent to a heart muscle slows the rate of contraction. Release adjacent to a pancreatic acinar cell triggers exocytosis of secretory granules that contain digestive enzymes. On the other hand, different receptor-ligand complexes can induce the same cellular response in some cell types. In liver cells, for example, the binding of either glucagon to its receptors or of epinephrine to its receptors can induce degradation of glycogen and release of glucose into the blood.
These examples show that a receptor protein is characterized by binding specificity for a particular ligand, and the resulting hormone-ligand complex exhibits effector specificity (i.e., mediates a specific cellular response). For instance, activation of either epinephrine or glucagon receptors on liver cells by binding of their respective ligands induces synthesis of cyclic AMP (cAMP), one of several intracellular signaling molecules, termed second messengers, which regulate various metabolic functions; as a result, the effects of both receptors on liver-cell metabolism are the same. Thus, the binding specificity of epinephrine and glucagon receptors differ, but their effector specificity is identical.
In most receptor-ligand systems, the ligand appears to have no function except to bind to the receptor. The ligand is not metabolized to useful products, is not an intermediate in any cellular activity, and has no enzymatic properties. The only function of the ligand appears to be to change the properties of the receptor, which then signals to the cell that a specific product is present in the environment. Target cells often modify or degrade the ligand and, in so doing, can modify or terminate their response or the response of neighboring cells to the signal.
(a) Steroid hormones, thyroxine, and retinoids, being lipophilic, are transported by carrier proteins in the blood. After dissociation from these carriers, such hormones diffuse across the cell membrane and bind to specific receptors in the cytosol or nucleus. The receptor-hormone complex then acts on nuclear DNA to alter transcription of specific genes. (b) Polypeptide hormones and catecholamines (e.g., epinephrine), which are water soluble, and prostaglandins, which are lipophilic, all bind to cell-surface receptors. This binding triggers an increase or decrease in the cytosolic concentration of second messengers (e.g., cAMP, Ca2+), activation of a protein kinase, or a change in the membrane potential.
). Recently, nitric oxide, a
gas, has been shown to be a key regulator controlling many cellular responses.
We discuss this important regulator later in this chapter. Here we briefly
describe the three main types of hormones; later we discuss the mechanisms that
regulate their synthesis, release, and degradation.
). Hormones of this type include the steroids (e.g.,
cortisol, progesterone, estradiol, and testosterone), thyroxine, and
retinoic acid (see Figure
10-65).
All steroids are synthesized from cholesterol and have similar chemical skeletons. After crossing the plasma membrane, steroid hormones interact with intracellular receptors, forming complexes that can increase or decrease transcription of specific genes (see Figure 10-68). These receptor-steroid complexes also may affect the stability of specific mRNAs. Steroids are effective for hours or days and often influence the growth and differentiation of specific tissues. For example, estrogen and progesterone, the female sex hormones, stimulate the production of egg-white hormones in chickens and cell proliferation in the hen oviduct. In mammals, estrogens stimulate growth of the uterine wall in preparation for embryo implantation. In insects and crustaceans, α-ecdysone (which is chemically related to steroids) triggers the differentiation and maturation of larvae; like estrogens, it induces the expression of specific gene products.
These two thyroid hormones stimulate increased expression of many cytosolic enzymes (e.g., liver hexokinase) that cata-lyze the catabolism of glucose, fats, and proteins and of mitochondrial enzymes that catalyze oxidative phosphorylation.
Retinoids are polyisoprenoid lipids derived from retinol (vitamin A). They perform multiple regulatory functions in diverse cellular processes. Retinoids regulate cellular proliferation, differentiation, and death, and they have numerous clinical applications. Their diverse effects reflect, at least in part, the multiplicity of retinoid derivatives, the existence of two different classes of receptors that form heterodimers, and differences in their cis-acting regulatory sites on DNA. During development retinoids act as local mediators of cell-cell interaction. For instance, during the formation of motor neurons in the chick, one class of motor neurons generates a retinoid signal which regulates the number and type of neighboring motoneurons.
Because water-soluble signaling molecules cannot diffuse across the plasma membrane, they all bind to cell-surface receptors. This large class of compounds is composed of two groups: (1) peptide hormones, such as insulin, growth factors, and glucagon, which range in size from a few amino acids to protein-size compounds, and (2) small charged molecules, such as epinephrine and histamine (see Figure 21-28), that are derived from amino acids and function as hormones and neurotransmitters.
Many water-soluble hormones induce a modification in the activity of one or more enzymes already present in the target cell. In this case, the effects of the surface-bound hormone usually are nearly immediate, but persist for a short period only. These signals also can give rise to changes in gene expression that may persist for hours or days. In yet other cases water-soluble signals may lead to irreversible changes, such as cellular differentiation.
The primary lipid-soluble hormones that bind to cell-surface receptors are the prostaglandins. There are at least 16 different prostaglandins in nine different chemical classes, designated PGA – PGI. Prostaglandins are part of an even larger family of 20 carbon–containing hormones called eicosanoid hormones. In addition to prostaglandins, they include prostacyclins, thromboxanes, and leukotrienes. Eicosonoid hormones are synthesized from a common precursor, arachidonic acid. Arachidonic acid is generated from phospholipids and diacylglycerol.
Many prostaglandins act as local
mediators during paracrine and autocrine signaling and are de-stroyed near
the site of their synthesis. They mod-ulate the responses of other hormones
and can have profound effects on many cellular processes. Certain
prostaglandins cause blood platelets to aggregate and adhere to the walls of
blood vessels. Because platelets play a key role in clotting blood and
plugging leaks in blood vessels, these prostaglandins can affect the course
of vascular disease and wound healing; aspirin inhibits their synthesis by
acetylating (and thereby irreversibly inhibiting) prostaglandin
H2 synthase. Other prostaglandins initiate the contraction of
smooth muscle cells; they accumulate in the uterus at the time of childbirth
and appear to be important in inducing uterine contraction.
Recent studies have shown that a family of plant steroids, called brassinosteroids, regulates many aspects of development. These lipophilic compounds, like prostaglandins, act through cell-surface receptors.
Common ligands for each receptor type are listed in parentheses. (a) G protein – linked receptors. Binding of ligand (maroon) triggers activation of a G protein, which then binds to and activates an enzyme that catalyzes synthesis of a specific second messenger. (b) Ion-channel receptors. A conformational change triggered by ligand binding opens the channel for ion flow. (c) Tyrosine kinase – linked receptors. Ligand binding causes formation of a homodimer or heterodimer, triggering the binding and activation of a cytosolic protein-tyrosine kinase. The activated kinase phosphorylates tyrosines in the receptor; substrate proteins then bind to these phosphotyrosine residues and are phosphorylated. (d) Receptors with intrinsic ligand-triggered enzymatic activity in the cytosolic domain. Some activated receptors are monomers with guanine cyclase activity and can generate the second messenger cGMP (left). The receptors for many growth factors have intrinsic protein-tyrosine kinase activity (right). Ligand binding to most such receptor tyrosine kinase (RTKs) causes formation of an activated homodimer, which phosphorylates several residues in its own cytosolic domain as well as certain substrate proteins. [Part (c) see J. E. Darnell et al., 1994, Science 264:1415. Part (d) see S. Schulz et al., 1989, FASEB J. 3:2026; D. Garbers, 1989, J. Biol. Chem. 264:9103; and W. J. Fantl et al., 1993, Annu. Rev. Biochem. 62:453.]
. Binding of ligand to some of these receptors induces
second-messenger formation, whereas ligand binding to others does not. For
convenience, we can sort these receptors into four classes:
): Ligand binding activates a G protein, which in turn
activates or inhibits an enzyme that generates a specific second
messenger or modulates an ion channel, causing a change in membrane
potential. The receptors for epinephrine, serotonin, and glucagon
are examples.
): Ligand binding changes the
conformation of the receptor so that specific ions flow through it;
the resultant ion movements alter the electric potential across the
cell membrane. The acetylcholine receptor at the nerve-muscle
junction is an example.
): These receptors lack intrinsic catalytic
activity, but ligand binding stimulates formation of a dimeric
receptor, which then interacts with and activates one or more
cytosolic protein-tyrosine kinases. The receptors for many
cytokines, the interferons, and human growth factor are of this
type. These tyrosine kinase – linked
receptors sometimes are referred to as the cytokine-receptor
superfamily.
): Several
types of receptors have intrinsic catalytic activity, which is
activated by binding of ligand. For instance, some activated
receptors catalyze conversion of GTP to cGMP; others act as protein
phosphatases, removing phosphate groups from phosphotyrosine
residues in substrate proteins, thereby modifying their activity.
The receptors for insulin and many growth factors are
ligand-triggered protein kinases; in most cases, the ligand binds as
a dimer, leading to dimerization of the receptor and activation of
its kinase activity. These
receptors — often referred to as
receptor serine/threonine kinases or receptor tyrosine
kinases — autophosphorylate
residues in their own cytosolic domain and also can phosphorylate
various substrate proteins.The discussion in this chapter focuses primarily on signaling pathways initiated by G protein – coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs). The general structure and mechanism of action of the intracellular receptors for steroid hormones are discussed in Chapter 10; ion channels are covered in detail in Chapters 15 and 21; and certain receptor serine/threonine kinases as well as other developmentally relevant cell-surface receptors are described in Chapter 23.
Their abbreviations are indicated. The calcium ion (Ca2+) and several membrane-bound inositol phospholipids (phosphoinositides) also act as second messengers but are not shown (see Figure 20-38).
).
The elevated intracellular concentration of one or more second messengers following hormone binding triggers a rapid alteration in the activity of one or more enzymes or nonenzymatic proteins. The metabolic functions controlled by hormone-induced second messengers include uptake and utilization of glucose, storage and mobilization of fat, and secretion of cellular products. These intracellular molecules also control proliferation, differentiation, and survival of cells, in part by regulating the transcription of specific genes. The mode of action of cAMP and other second messengers is discussed in a later section. Removal (or degradation) of a ligand or second messenger, or inactivation of the ligand-binding receptor, can terminate the cellular response to an extracellular signal.
In addition to cell-surface receptors and second messengers, several types of conserved proteins function in signaltransduction pathways stimulated by extracellular signals. Here we introduce the three main classes of these intracellular signaling proteins; their structures and functions are described in detail in later sections.
(a) GTP-binding proteins with GTPase activity function as molecular switches. When bound to GTP they are active; when bound to GDP, they are inactive. They fall into two categories, trimeric G proteins and Ras-like proteins. (b) Protein kinases modulate the activity or the binding properties of substrate proteins by phosphorylating serine, threonine, or tyrosine residues. The phosphorylated form of some proteins is active, whereas the dephosphorylated form of other proteins is active. The combined action of kinases and phosphatases, which dephosphorylate specific substrates, can cycle proteins between active and inactive states. (c) Adapter proteins contain various protein-binding motifs that promote the formation of multiprotein signaling complexes.
). In the absence of a signal, the
protein is bound to GDP. Signals activate the release of GDP, and the
subsequent binding to GTP over GDP is favored by the higher concentrations
of GTP in the cell. The intrinsic GTPase activity of these GTP-binding
proteins hydrolyzes the bound GTP to GDP and Pi, thus converting
the active form back to the inactive form. The kinetics of hydrolysis
regulates the length of time the switch is “on.”
There are two classes of GTPase switch proteins: trimeric G proteins, which as noted already are directly coupled to certain receptors, and monomeric Ras and Ras-like proteins. Both classes contain regions that promote the activity of specific effector proteins by direct protein-protein interactions. These regions are in their active conformation only when the switch protein is bound to GTP. G proteins are coupled directly to activated receptors, whereas Ras is linked only indirectly via other proteins. The two classes of GTPbinding proteins also are regulated in very different ways.
). In some cases
kinases are part of the receptor itself, and in others they are found in the
cytosol or associated with the plasma membrane. Animal cells contain two
types of protein kinases: those directed toward tyrosine and those directed
toward either serine or threonine. The structures of the catalytic core of
both types are very similar. In general, protein kinases become active in
response to the stimulation of signaling pathways. The catalytic activities
of kinases are modulated by phosphorylation, by direct binding to other
proteins, and by changes in the levels of various second messengers. The
activity of protein kinases is opposed by the activity of protein
phosphatases, which remove phosphate groups from specific
substrate proteins. The activities of kinases and phosphatases during cell
cycle control are described in some detail in Chapter 13.
). Adapter proteins do not have catalytic activity, nor do they
directly activate effector proteins. Rather, they contain different
combinations of domains, which function as docking sites for other proteins.
For instance, different domains bind to phosphotyrosine residues (SH2 and
PTB domains), proline-rich sequences (SH3 and WW domains), phosphoinositides
(PH domains), and unique C-terminal sequences with a C-terminal hydrophobic
residue (PDZ domains). In some cases adapter proteins contain arrays of a
single binding domain or different combinations of domains. In addition,
these binding domains can be found alone or in various combinations in
proteins containing catalytic domains. These combinations provide enormous
potential for complex interplay and cross-talk between different signaling
pathways.
Hormone binding to the receptor initiates a series of events leading to phosphorylation of specific substrate proteins, which mediate the cellular responses such as changes in the activity of metabolic enzymes, gene expression, and cytoskeletal structures. The kinase cascade entails sequential activation of specific protein kinases induced by a signal from activated Ras protein. Second messengers (SM) play a role in some RTK signaling pathways, although not in the pathway depicted here. Likewise, some GPCR pathways do not involve second messengers; these lead to activation of MAP kinase. See text for discussion.
illustrates the main
components of the key signaling pathways downstream from G
protein – coupled receptors (GPCRs) and receptor
tyrosine kinases (RTKs), the two receptor classes that we consider in detail in
this chapter. Although a GTPase switch protein occurs in both types of pathways,
its position in the pathway relative to the receptor differs. Second messengers
are critical components of most GPCR pathways and some RTK pathways. Adapter
proteins function in all RTK pathways but not in the main GPCR pathways. Protein
kinases, however, play a key role in all signaling pathways; ultimately an
activated protein kinase phosphorylates one or more substrate proteins. The
nature of the substrate proteins, which include enzymes, microtubules, histones,
and transcription factors, plays an important role in determining the cellular
response to a particular signal in a particular cell.
| Property | Steroids | Thyroxine | Peptides and Proteins | Catecholamines |
|---|---|---|---|---|
| Feedback regulation of synthesis | Yes | Yes | Yes | Yes |
| Storage of preformed hormone | Very little | Several weeks | One day | Several days, in adrenal medulla |
| Mechanism of secretion | Diffusion through plasma membrane | Proteolysis of thyroglobulin | Exocytosis of storage vesicles | Exocytosis of storage vesicles |
| Binding to plasma proteins | Yes | Yes | Rarely | No |
| Lifetime in blood plasma | Hours | Days | Minutes | Seconds |
| Time course of action | Hours to days | Days | Minutes to hours | Seconds or less |
| Receptors | Cytosolic or nuclear | Nuclear | Plasma membrane | Plasma membrane |
| Mechanism of action | Receptor-hormone complex controls transcription and stability of mRNAs | Receptor-hormone complex controls transcription and stability of mRNAs | Hormone binding triggers synthesis of cystolic second messengers or protein kinase activity | Hormone binding causes change in membrane potential or triggers synthesis of cystolic second messengers |
SOURCE: Adapted from E. L. Smith et al., 1983, Principles of Biochemistry: Mammalian Biochemistry, 6th ed., McGraw-Hill, p. 358. Reproduced by permission of McGraw-Hill.
Organisms must be able to respond instantly to many changes in their internal or external environment. Such rapid responses are mediated primarily by peptide hormones and the catecholamines epinephrine, norepinephrine, and dopamine (see Figure 21-28). The cells that produce these signaling molecules store them in secretory vesicles just under the plasma membrane (see Figure 21-30). The supply of stored, preformed signaling molecules is sufficient for 1 day in the case of peptide hormones and for several days in the case of catecholamines. All peptide hormones, including insulin and adrenocorticotropic hormone (ACTH), are synthesized as part of a longer propolypeptide, which is cleaved by specific proteases to generate the active molecule just after it is transported to a secretory vesicle (see Figure 17-61).
Stimulation of signaling cells causes immediate exocytosis of the stored peptide hormone or catecholamine into the surrounding medium or the blood. Secreting cells also are stimulated to synthesize the signaling molecule and replenish the cell’s supply. Released peptide hormones persist in the blood for only seconds or minutes before being degraded by blood and tissue proteases. Released catecholamines are rapidly inactivated by different enzymes or taken up by specific cells (Section 21.4). The initial actions of these signaling molecules on target cells (the activation or inhibition of specific enzymes) also last only seconds or minutes. Thus the catecholamines and some peptide hormones can mediate short responses that are terminated by their own degradation.
The pathways for synthesizing steroid hormones from cholesterol involve 10 or more enzymes. Steroid-producing cells, like those in the adrenal cortex, store a small supply of hormone precursor but none of the mature, active hormone. When stimulated, the cells convert the precursor to the active hormone, which then diffuses across the plasma membrane into the blood. Likewise, thyroglobulin, the iodinated precursor of thyroxine is stored in thyroid follicles. When cells lining these follicles are exposed to thyroid-stimulating hormone (TSH), they take up thyroglobulin; controlled proteolysis of this glycoprotein by lysosomal enzymes yields thyroxine, which is released into the blood.
Retinol is stored in the liver and is found in high concentrations in blood in a complex with serum binding protein. Due to its lipophilic nature, retinol diffuses through the plasma membrane and forms a complex with a cytosolic retinol-binding protein called CRBP. Retinol is converted to retinal through the activity of retinol dehydrogenase, and retinal, in turn, is converted to retinoic acid by retinal dehydrogenase. Retinoic acid can act as a signal in the cell in which it is produced, or it can diffuse through the plasma membrane to influence the development of neighboring cells. Retinoic acid can also be further modified enzymatically to alter its signaling specificity.
The synthesis and/or release of many hormones are regulated by positive or negative feedback. This type of regulation is particularly important in coordinating the action of multiple hormones on various cell types during growth and differentiation. Often, the levels of several hormones are interconnected by feedback circuits, in which changes in the level of one hormone affect the levels of other hormones. One example is the regulation of estrogen and progesterone, steroid hormones that stimulate the growth and differentiation of cells in the endometrium, the tissue lining the interior of the uterus. Changes in the endometrium prepare the organ to receive and nourish an embryo. The levels of both hormones are regulated by complex feedback circuits involving several other hormones.
Extracellular signaling molecules regulate interaction between unicellular organisms and are critical regulators of physiology and development in multicellular organisms.
).
).Binding of extracellular signaling molecules to cell-surface receptors trigger intracellular pathways that ultimately modulate cellular metabolism, function, or development.
The level of second messengers, such as Ca2+, cAMP, and IP3 are modulated in response to binding of ligand to cell-surface receptors. These intracellular signaling molecules, in turn, regulate the activities of enzymes and nonenzymatic proteins.
). The
latter coordinate the formation of multicomponent signaling
complexes.Extracellular signals are often integrated into complex regulatory networks in which the synthesis, release, and degradation of hormones are precisely regulated.
