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Cooper GM. The Cell: A Molecular Approach. 2nd edition. Sunderland (MA): Sinauer Associates; 2000.

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The Cell: A Molecular Approach. 2nd edition.

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Pathways of Intracellular Signal Transduction

Most cell surface receptors stimulate intracellular target enzymes, which may be either directly linked or indirectly coupled to receptors by G proteins. These intracellular enzymes serve as downstream signaling elements that propagate and amplify the signal initiated by ligand binding. In most cases, a chain of reactions transmits signals from the cell surface to a variety of intracellular targets—a process called intracellular signal transduction. The targets of such signaling pathways frequently include transcription factors that function to regulate gene expression. Intracellular signaling pathways thus connect the cell surface to the nucleus, leading to changes in gene expression in response to extracellular stimuli.

The cAMP Pathway: Second Messengers and Protein Phosphorylation

Intracellular signaling was first elucidated by studies of the action of hormones such as epinephrine, which signals the breakdown of glycogen to glucose in anticipation of muscular activity. In 1958, Earl Sutherland discovered that the action of epinephrine was mediated by an increase in the intracellular concentration of cyclic AMP (cAMP), leading to the concept that cAMP is a second messenger in hormonal signaling (the first messenger being the hormone itself). Cyclic AMP is formed from ATP by the action of adenylyl cyclase and degraded to AMP by cAMP phosphodiesterase (Figure 13.18). As discussed earlier, the epinephrine receptor is coupled to adenylyl cyclase via a G protein that stimulates enzymatic activity, thereby increasing the intracellular concentration of cAMP (see Figure 13.11).

Figure 13.18. Synthesis and degradation of cAMP.

Figure 13.18

Synthesis and degradation of cAMP. Cyclic AMP is synthesized from ATP by adenylyl cyclase and degraded to AMP by cAMP phosphodiesterase.

How does cAMP then signal the breakdown of glycogen? This and most other effects of cAMP in animal cells are mediated by the action of cAMP-dependent protein kinase, or protein kinase A, an enzyme discovered by Donal Walsh and Ed Krebs in 1968. The inactive form of protein kinase A is a tetramer consisting of two catalytic and two regulatory subunits (Figure 13.19). Cyclic AMP binds to the regulatory subunits, leading to their dissociation from the catalytic subunits. The free catalytic subunits are then enzymatically active and able to phosphorylate serine residues on their target proteins.

Figure 13.19. Regulation of protein kinase A.

Figure 13.19

Regulation of protein kinase A. The inactive form of protein kinase A consists of two regulatory (R) and two catalytic (C) subunits. Binding of cAMP to the regulatory subunits induces a conformational change that leads to dissociation of the catalytic (more...)

In the regulation of glycogen metabolism, protein kinase A phosphorylates two key target enzymes (Figure 13.20). The first is another protein kinase, phosphorylase kinase, which is phosphorylated and activated by protein kinase A. Phosphorylase kinase in turn phosphorylates and activates glycogen phosphorylase, which catalyzes the breakdown of glycogen to glucose-1-phosphate. In addition, protein kinase A phosphorylates the enzyme glycogen synthase, which catalyzes glycogen synthesis. In this case, however, phosphorylation inhibits enzymatic activity. Elevation of cAMP and activation of protein kinase A thus blocks further glycogen synthesis at the same time as it stimulates glycogen breakdown.

Figure 13.20. Regulation of glycogen metabolism by protein kinase A.

Figure 13.20

Regulation of glycogen metabolism by protein kinase A. Protein kinase A phosphorylates both glycogen synthase and phosphorylase kinase. Glycogen synthase (which catalyzes glycogen synthesis) is inhibited by this phosphorylation, whereas phosphorylase (more...)

The chain of reactions leading from the epinephrine receptor to glycogen phosphorylase provides a good illustration of signal amplification during intracellular signal transduction. Each molecule of epinephrine activates only a single receptor. However, each receptor may activate up to a hundred molecules of Gs. Each molecule of Gs then stimulates the enzymatic activity of adenylyl cyclase, which can catalyze the synthesis of many molecules of cAMP. Signal amplification continues as each molecule of protein kinase A phosphorylates many molecules of phosphorylase kinase, which in turn phosphorylate many molecules of glycogen phosphorylase. Hormone binding to a small number of receptors thus leads to activation of a much larger number of intracellular target enzymes.

In many animal cells, increases in cAMP activate the transcription of specific target genes that contain a regulatory sequence called the cAMP response element, or CRE (Figure 13.21). In this case, the signal is carried from the cytoplasm to the nucleus by the catalytic subunit of protein kinase A, which is able to enter the nucleus following its release from the regulatory subunit. Within the nucleus, protein kinase A phosphorylates a transcription factor called CREB (for CRE-binding protein), leading to the activation of cAMP-inducible genes. Such regulation of gene expression by cAMP plays important roles in controlling the proliferation, survival, and differentiation of a wide variety of animal cells.

Figure 13.21. Cyclic AMP-inducible gene expression.

Figure 13.21

Cyclic AMP-inducible gene expression. The free catalytic subunit of protein kinase A translocates to the nucleus and phosphorylates the transcription factor CREB (CRE-binding protein), leading to expression of cAMP-inducible genes.

It is important to recognize that protein kinases, such as protein kinase A, do not function in isolation within the cell. To the contrary, protein phosphorylation is rapidly reversed by the action of protein phosphatases. Some protein phosphatases are transmembrane receptors, as discussed in the preceding section. A number of others are cytosolic enzymes that remove phosphate groups from either phosphorylated tyrosine or serine/threonine residues in their substrate proteins. These protein phosphatases serve to terminate the responses initiated by receptor activation of protein kinases. For example, the serine residues of proteins that are phosphorylated by protein kinase A are usually dephosphorylated by the action of a phosphatase called protein phosphatase 1 (Figure 13.22). The levels of phosphorylation of protein kinase A substrates (such as phosphorylase kinase and CREB) are thus determined by a balance between the intracellular activities of protein kinase A and protein phosphatases.

Figure 13.22. Regulation of protein phosphorylation by protein kinase A and protein phosphatase 1.

Figure 13.22

Regulation of protein phosphorylation by protein kinase A and protein phosphatase 1. The phosphorylation of target proteins by protein kinase A is reversed by the action of protein phosphatase 1.

Although most effects of cAMP are mediated by protein kinase A, cAMP can also directly regulate ion channels, independent of protein phosphorylation. Cyclic AMP functions in this way as a second messenger involved in sensing smells. Many of the odorant receptors in sensory neurons in the nose are G protein-coupled receptors that stimulate adenylyl cyclase, leading to an increase in intracellular cAMP. Rather than stimulating protein kinase A, cAMP in this system directly opens Na+ channels in the plasma membrane, leading to membrane depolarization and initiation of a nerve impulse.

Cyclic GMP

Cyclic GMP (cGMP) is also an important second messenger in animal cells, although its roles are not as clearly understood as those of cAMP. Cyclic GMP is formed from GTP by guanylyl cyclases and degraded to GMP by a phosphodiesterase. As discussed earlier in this chapter, different types of guanylyl cyclases are activated by both nitric oxide and peptide ligands. Stimulation of these guanylyl cyclases leads to elevated levels of cGMP, which then mediate biological responses, such as blood vessel dilation. The action of cGMP is frequently mediated by activation of a cGMP-dependent protein kinase, although cGMP can also act to regulate other targets, including ion channels.

The best-characterized role of cGMP is in the vertebrate eye, where it serves as the second messenger responsible for converting the visual signals received as light to nerve impulses. The photoreceptor in rod cells of the retina is a G protein-coupled receptor called rhodopsin (Figure 13.23). Rhodopsin is activated as a result of the absorption of light by the associated small molecule 11-cis-retinal, which then isomerizes to all-trans-retinal, inducing a conformational change in the rhodopsin protein. Rhodopsin then activates the G protein transducin, and the α subunit of transducin stimulates the activity of cGMP phosphodiesterase, leading to a decrease in the intracellular level of cGMP. This change in cGMP level in retinal rod cells is translated to a nerve impulse by a direct effect of cGMP on ion channels in the plasma membrane, similar to the action of cAMP in sensing smells.

Figure 13.23. Role of cGMP in photoreception.

Figure 13.23

Role of cGMP in photoreception. Absorption of light by retinal activates the G protein-coupled receptor rhodopsin. The α subunit of transducin then stimulates cGMP phosphodiesterase, leading to a decrease in intracellular levels of cGMP.

Phospholipids and Ca2+

One of the most widespread pathways of intracellular signaling is based on the use of second messengers derived from the membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2). PIP2 is a minor component of the plasma membrane, localized to the inner leaflet of the phospholipid bilayer (see Figure 12.2). A variety of hormones and growth factors stimulate the hydrolysis of PIP2 by phospholipase C—a reaction that produces two distinct second messengers, diacylglycerol and inositol 1,4,5-trisphosphate (IP3) (Figure 13.24). Diacylglycerol and IP3 stimulate distinct downstream signaling pathways (protein kinase C and Ca2+ mobilization, respectively), so PIP2 hydrolysis triggers a two-armed cascade of intracellular signaling.

Figure 13.24. Hydrolysis of PIP2.

Figure 13.24

Hydrolysis of PIP2. Phospholipase C (PLC) catalyzes the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) to yield diacylglycerol (DAG) and inositol trisphosphate (IP3). Diacylglycerol activates members of the protein kinase C family, and IP (more...)

It is noteworthy that the hydrolysis of PIP2 is activated downstream of both G protein-coupled receptors and protein-tyrosine kinases. This occurs because one form of phospholipase C (PLC-β) is stimulated by G proteins, whereas a second (PLC-γ) contains SH2 domains that mediate its association with activated receptor protein-tyrosine kinases (Figure 13.25). This interaction localizes PLC-γ to the plasma membrane as well as leading to its tyrosine phosphorylation, which increases its catalytic activity.

Figure 13.25. Activation of phospholipase C by protein-tyrosine kinases.

Figure 13.25

Activation of phospholipase C by protein-tyrosine kinases. Phospholipase C-γ (PLC-γ) binds to activated receptor protein-tyrosine kinases via its SH2 domains. Tyrosine phosphorylation increases PLC-γ activity, stimulating the hydrolysis (more...)

The diacylglycerol produced by hydrolysis of PIP2 activates protein-serine/threonine kinases belonging to the protein kinase C family, many of which play important roles in the control of cell growth and differentiation. A good illustration of this role of protein kinase C is provided by the action of phorbol esters (Figure 13.26), which have been studied extensively because they promote the growth of tumors in animals. This tumor-promoting activity of the phorbol esters is based on their ability to stimulate protein kinase C by acting as analogs of diacylglycerol. Protein kinase C then activates other intracellular targets, including a cascade of protein kinases known as the MAP kinase pathway (discussed in detail in the next section), leading to transcription factor phosphorylation, changes in gene expression, and stimulation of cell proliferation.

Figure 13.26. Structure of a phorbol ester.

Figure 13.26

Structure of a phorbol ester. Phorbol esters stimulate protein kinase C by acting as analogs of diacylglycerol.

Whereas diacylglycerol remains associated with the plasma membrane, the other second messenger produced by PIP2 cleavage, IP3, is a small polar molecule that is released into the cytosol, where it acts to signal the release of Ca2+ from intracellular stores (Figure 13.27). As noted in Chapter 12, the cytosolic concentration of Ca2+ is maintained at an extremely low level (about 0.1 μM) as a result of Ca2+ pumps that actively export Ca2+ from the cell. Ca2+ is pumped not only across the plasma membrane, but also into the endoplasmic reticulum, which therefore serves as an intracellular Ca2+ store. IP3 acts to release Ca2+ from the endoplasmic reticulum by binding to receptors that are ligand-gated Ca2+ channels. As a result, cytosolic Ca2+ levels increase to about 1 μM, which affects the activities of a variety of target proteins, including protein kinases and phosphatases. For example, some members of the protein kinase C family require Ca2+ as well as diacylglycerol for their activation, so these protein kinases are regulated jointly by both arms of the PIP2 signaling pathway.

Figure 13.27. Ca2+ mobilization by IP3.

Figure 13.27

Ca2+ mobilization by IP3. Ca2+ is pumped from the cytosol into the endoplasmic reticulum, which therefore serves as an intracellular Ca2+ store. IP3 binds to receptors that are ligand-gated Ca2+ channels in the endoplasmic reticulum membrane, thereby (more...)

Many of the effects of Ca2+ are mediated by the Ca2+-binding protein calmodulin, which is activated by Ca2+ binding when the concentration of cytosolic Ca2+ increases to about 0.5 μM (Figure 13.28). Ca2+/calmodulin then binds to a variety of target proteins, including protein kinases. One example of such a Ca2+/calmodulin-dependent protein kinase is myosin light-chain kinase, which signals actin-myosin contraction by phosphorylating one of the myosin light chains (see Figure 11.28). Other protein kinases that are activated by Ca2+/calmodulin include members of the CaM kinase family, which phosphorylate a number of different proteins, including metabolic enzymes, ion channels, and transcription factors. One form of CaM kinase is particularly abundant in the nervous system, where it regulates the synthesis and release of neurotransmitters. In addition, CaM kinases can regulate gene expression by phosphorylating transcription factors. Interestingly, one of the transcription factors phosphorylated by CaM kinase is CREB, which (as discussed earlier) is phosphorylated at the same site by protein kinase A. This phosphorylation of CREB illustrates one of many intersections between the Ca2+ and cAMP signaling pathways. Other examples include the regulation of adenylyl cyclases and phosphodiesterases by Ca2+/calmodulin, the regulation of Ca2+ channels by cAMP, and the phosphorylation of a number of target proteins by both protein kinase A and Ca2+/calmodulin-dependent protein kinases. The cAMP and Ca2+ signaling pathways thus function coordinately to regulate many cellular responses.

Figure 13.28. Function of calmodulin.

Figure 13.28

Function of calmodulin. Calmodulin is a dumbbell-shaped protein with four Ca2+-binding sites. The active Ca2+/calmodulin complex binds to a variety of target proteins, including Ca2+/calmodulin-dependent protein kinases.

Ca2+ is an extremely common second messenger, and it is important to note that IP3-mediated release of Ca2+ from the endoplasmic reticulum is not the only mechanism by which the intracellular concentration of Ca2+ can be increased. One alternative pathway involves the entry of extracellular Ca2+ through Ca2+ channels in the plasma membrane. In many cells, the transient increase in intracellular Ca2+ resulting from production of IP3 is followed by a more sustained increase resulting from extracellular Ca2+ entry. The entry of extracellular Ca2+ is particularly important in the electrically excitable cells of nerve and muscle, in which voltage-gated Ca2+ channels in the plasma membrane are opened by membrane depolarization (Figure 13.29). The resulting increases in intracellular Ca2+ then trigger the further release of Ca2+ from intracellular stores by activating distinct Ca2+ channels known as ryanodine receptors. One effect of increases in intracellular Ca2+ in neurons is to trigger the release of neurotransmitters, so Ca2+ plays a critical role in converting electric to chemical signals in the nervous system. In muscle cells, Ca2+ is stored in the sarcoplasmic reticulum, from which it is released by the opening of ryanodine receptors in response to changes in membrane potential. This release of stored Ca2+ leads to large increases in cytosolic Ca2+, which trigger muscle contraction (see Chapter 11). Cells thus utilize a variety of mechanisms to regulate intracellular Ca2+ levels, making Ca2+ a versatile second messenger that controls a wide range of cellular processes.

Figure 13.29. Regulation of intracellular Ca2+ in electrically excitable cells.

Figure 13.29

Regulation of intracellular Ca2+ in electrically excitable cells. Membrane depolarization leads to the opening of voltage-gated Ca2+ channels in the plasma membrane, causing the influx of Ca2+ from extracellular fluids. The resulting increase in intracellular (more...)

PIP2 not only serves as the source of diacylglycerol and IP3, but is also the starting point of a distinct second messenger pathway that plays a key role in regulating cell survival. In this pathway, PIP2 is phosphorylated on the 3 position of inositol by the enzyme phosphatidylinositide (PI) 3-kinase (Figure 13.30). Like phospholipase C, one form of PI 3-kinase is activated by G proteins, while a second has SH2 domains and is activated by association with receptor protein-tyrosine kinases. Phosphorylation of PIP2 yields phosphatidylinositol 3,4,5-trisphosphate (PIP3), which functions as a distinct second messenger. A key target of PIP3, which is critical for signaling cell survival, is a protein-serine/threonine kinase called Akt. PIP3 binds to a domain of Akt known as the pleckstrin homology domain (Figure 13.31). This interaction recruits Akt to the inner face of the plasma membrane, where it is phosphorylated and activated by other protein kinases (called PDKs) that also contain pleckstrin homology domains and bind PIP3. The formation of PIP3 thus results in the association of both Akt and PDKs with the plasma membrane, leading to phosphorylation and activation of Akt. Once activated, Akt phosphorylates a number of target proteins, including proteins that are direct regulators of cell survival, transcription factors, and other protein kinases that regulate cell metabolism and protein synthesis.

Figure 13.30. Activity of PI 3-kinase.

Figure 13.30

Activity of PI 3-kinase. PI 3-kinase phosphorylates the 3 position of inositol, converting PIP2 to PIP3.

Figure 13.31. Activation of the Akt protein kinase.

Figure 13.31

Activation of the Akt protein kinase. Akt is recruited to the plasma membrane by binding to PIP3 via its pleckstrin homology (PH) domain. It is then activated as a result of phosphorylation by another protein kinase (PDK) that also binds PIP3.

Second messengers can also be derived from other phospholipids. The hydrolysis of phosphatidylcholine is stimulated by a variety of growth factors, providing a second source of diacylglycerol, in addition to that derived from PIP2. While PIP2 hydrolysis is a transient response to growth factor stimulation, the hydrolysis of phosphatidylcholine typically persists for several hours, providing a sustained source of diacylglycerol that may be important in signaling long-term responses, such as cell proliferation. Sphingomyelin is also cleaved in response to a variety of extracellular stimuli, resulting in the formation of ceramide. Although its targets remain to be fully elucidated, ceramide regulates a number of protein kinases and phosphatases that can affect cell proliferation and survival.

Ras, Raf, and the MAP Kinase Pathway

The MAP kinase pathway refers to a cascade of protein kinases that are highly conserved in evolution and play central roles in signal transduction in all eukaryotic cells, ranging from yeasts to humans. The central elements in the pathway are a family of protein-serine/threonine kinases called the MAP kinases (for mitogen-activated protein kinases) that are activated in response to a variety of growth factors and other signaling molecules. In yeasts, MAP kinase pathways control a variety of cellular responses, including mating, cell shape, and sporulation. In higher eukaryotes (including C. elegans, Drosophila, frogs, and mammals), MAP kinases are ubiquitous regulators of cell growth and differentiation.

The best-characterized forms of MAP kinase in mammalian cells belong to the ERK (extracellular signal-regulated kinase) family. ERK activation plays a central role in signaling cell proliferation induced by growth factors that act through either protein-tyrosine kinase or G protein-coupled receptors. As already noted, protein kinase C can also activate the ERK pathway, which appears to be responsible for the stimulation of cell proliferation induced by phorbol ester tumor promoters. In addition, both the Ca2+ and cAMP pathways intersect with ERK signaling, either stimulating or inhibiting the ERK pathway in different types of cells.

Activation of ERK is mediated by two upstream protein kinases, which are coupled to growth factor receptors by a GTP-binding protein called Ras (Figure 13.32). Activation of Ras leads to activation of the Raf protein-serine/threonine kinase, which phosphorylates and activates a second protein kinase called MEK (for MAP kinase/ERK kinase). MEK is a dual-specificity protein kinase that activates members of the ERK family by phosphorylation of both threonine and tyrosine residues separated by one amino acid (e.g., threonine-183 and tyrosine-185 of ERK2). Once activated, ERK phosphorylates a variety of targets, including other protein kinases and transcription factors.

Figure 13.32. Activation of the ERK MAP kinases.

Figure 13.32

Activation of the ERK MAP kinases. Stimulation of growth factor receptors leads to activation of the small GTP-binding protein Ras, which interacts with the Raf protein kinase. Raf phosphorylates and activates MEK, a dual-specificity protein kinase that (more...)

The central role of the ERK pathway in mammalian cells emerged from studies of the Ras proteins, which were first identified as the oncogenic proteins of tumor viruses that cause sarcomas in rats (hence the name Ras, from rat sarcoma virus). Interest in Ras intensified considerably in 1982, when mutations in ras genes were first implicated in the development of human cancers (discussed in Chapter 15). The importance of Ras in intracellular signaling was then indicated by experiments showing that microinjection of active Ras protein directly induces proliferation of normal mammalian cells. Conversely, interference with Ras function by either microinjection of anti-Ras antibody or expression of a dominant negative Ras mutant blocks growth factor-induced cell proliferation. Thus, Ras is not only capable of inducing the abnormal growth characteristic of cancer cells, but also appears to be required for the response of normal cells to growth factor stimulation.

The Ras proteins are guanine nucleotide-binding proteins that function analogously to the α subunits of G proteins, alternating between inactive GDP-bound and active GTP-bound forms (Figure 13.33). In contrast to the G protein α subunits, however, Ras functions as a monomer rather than in association with βγ subunits. Ras activation is mediated by guanine nucleotide exchange factors that stimulate the release of bound GDP and its exchange for GTP. Activity of the Ras-GTP complex is then terminated by GTP hydrolysis, which is stimulated by the interaction of Ras-GTP with GTPase-activating proteins. It is interesting to note that the mutations of ras genes in human cancers have the effect of inhibiting GTP hydrolysis by the Ras proteins. These mutated Ras proteins therefore remain continuously in the active GTP-bound form, driving the unregulated proliferation of cancer cells even in the absence of growth factor stimulation.

Figure 13.33. Regulation of Ras proteins.

Figure 13.33

Regulation of Ras proteins. Ras proteins alternate between inactive GDP-bound and active GTP-bound states.

The Ras proteins are prototypes of a large family of approximately 50 related proteins, frequently called small GTP-binding proteins because Ras and its relatives are about half the size of G protein α subunits. While the Ras proteins regulate cell growth and differentiation, other subfamilies of small GTP-binding proteins control distinct cellular activities. For example, the largest subfamily of small GTP-binding proteins (the Rab proteins) function to regulate vesicle trafficking, as discussed in Chapter 9. Other small GTP-binding proteins are involved in nuclear protein import (the Ran protein, discussed in Chapter 8) and organization of the cytoskeleton (the Rho subfamily, discussed later in this chapter).

The best understood mode of Ras activation is that mediated by receptor protein-tyrosine kinases (Figure 13.34). Autophosphorylation of these receptors results in their association with Ras guanine nucleotide exchange factors as a result of SH2-mediated protein interactions. One well-characterized example is provided by the guanine nucleotide exchange factor Sos, which is bound to the SH2-containing protein Grb2 in the cytosol of unstimulated cells. Tyrosine phosphorylation of receptors (or of other receptor-associated proteins) creates a binding site for the Grb2 SH2 domains. Association of Grb2 with activated receptors localizes Sos to the plasma membrane, where it is able to interact with Ras proteins, which are anchored to the inner leaflet of the plasma membrane by lipids attached to the Ras C terminus (see Figure 12.10). Sos then stimulates guanine nucleotide exchange, resulting in formation of the active Ras-GTP complex. In its active GTP-bound form, Ras interacts with a number of effector proteins, including the Raf protein-serine/threonine kinase. This interaction with Ras recruits Raf from the cytosol to the plasma membrane, where it is activated as a result of phosphorylation by both protein-tyrosine and protein-serine/threonine kinases.

Figure 13.34. Ras activation downstream of receptor protein-tyrosine kinases.

Figure 13.34

Ras activation downstream of receptor protein-tyrosine kinases. A complex of Grb2 and the guanine nucleotide exchange factor Sos binds to a phosphotyrosine-containing sequence in the activated receptor via the Grb2 SH2 domain. This interaction recruits (more...)

As already noted, activation of Raf initiates a protein kinase cascade leading to ERK activation. ERK then phosphorylates a variety of target proteins, including other protein kinases. Importantly, a fraction of activated ERK translocates to the nucleus, where it regulates transcription factors by phosphorylation (Figure 13.35). In this regard, it is notable that a primary response to growth factor stimulation is the rapid transcriptional induction of a family of approximately 100 genes called immediate-early genes. The induction of a number of immediate-early genes is mediated by a regulatory sequence called the serum response element (SRE), which is recognized by a complex of transcription factors including the serum response factor (SRF) and Elk-1. ERK phosphorylates and activates Elk-1, providing a direct link between the ERK family of MAP kinases and immediate-early gene induction. Many immediate-early genes themselves encode transcription factors, so their induction in response to growth factor stimulation leads to altered expression of a battery of other downstream genes, thereby establishing new programs of gene expression.

Figure 13.35. Induction of immediate-early genes by ERK.

Figure 13.35

Induction of immediate-early genes by ERK. Activated ERK translocates to the nucleus, where it phosphorylates the transcription factor Elk-1. Elk-1 binds to the serum response element (SRE) in a complex with serum response factor (SRF). Phosphorylation (more...)

Both yeasts and mammalian cells have multiple MAP kinase pathways that control distinct cellular responses. Each cascade consists of three protein kinases: a terminal MAP kinase and two upstream kinases (analogous to Raf and MEK) that regulate its activity. In the yeast S. cerevisiae, five different MAP kinase cascades regulate mating, sporulation, filamentation, cell wall remodeling, and response to high osmolarity. In mammalian cells, at least five MAP kinases have been identified. In addition to members of the ERK family, these include the JNK and p38 MAP kinases, which are preferentially activated in response to inflammatory cytokines and cellular stress (e.g., ultraviolet irradiation) (Figure 13.36). Whereas ERK signaling principally leads to cell proliferation, survival, and differentiation, the JNK and p38 MAP kinase pathways often lead to inflammation and cell death. Like ERK, the JNK and p38 MAP kinases can translocate to the nucleus and phosphorylate transcription factors that regulate gene expression. Multiple MAP kinase pathways thus function in all types of eukaryotic cells to control cellular responses to diverse environmental signals.

Figure 13.36. Pathways of MAP kinase activation in mammalian cells.

Figure 13.36

Pathways of MAP kinase activation in mammalian cells. In addition to ERK, mammalian cells contain JNK and p38 MAP kinases. Activation of JNK and p38 is mediated by protein kinase cascades parallel to that responsible for ERK activation. The protein kinase (more...)

The JAK/STAT Pathway

The MAP kinase pathway provides an indirect connection between the cell surface and the nucleus, in which a cascade of protein kinases ultimately leads to transcription factor phosphorylation. An alternative pathway, known as the JAK/STAT pathway, provides a much more immediate connection between protein-tyrosine kinases and transcription factors. In this pathway protein-tyrosine phosphorylation directly affects transcription factor localization and function (Figure 13.37).

Figure 13.37. The JAK/STAT pathway.

Figure 13.37

The JAK/STAT pathway. The STAT proteins are transcription factors that contain SH2 domains that mediate their binding to phosphotyrosine-containing sequences. In unstimulated cells, STAT proteins are inactive in the cytosol. Stimulation of cytokine receptors (more...)

The key elements in this pathway are the STAT proteins (signal transducers and activators of transcription), which were originally identified in studies of cytokine receptor signaling. The STAT proteins are a family of transcription factors that contain SH2 domains. They are inactive in unstimulated cells, where they are localized to the cytoplasm. Stimulation of cytokine receptors leads to recruitment of STAT proteins, which bind via their SH2 domains to phosphotyrosine-containing sequences in the cytoplasmic domains of receptor polypeptides. Following their association with activated receptors, the STAT proteins are phosphorylated by members of the JAK family of nonreceptor protein-tyrosine kinases, which are associated with cytokine receptors. Tyrosine phosphorylation promotes the dimerization of STAT proteins, which then translocate to the nucleus, where they stimulate transcription of their target genes.

Further studies have shown that STAT proteins are also activated downstream of receptor protein-tyrosine kinases, where their phosphorylation may be catalyzed either by the receptors themselves or by associated nonreceptor kinases. The STAT transcription factors thus serve as direct links between both cytokine and growth factor receptors on the cell surface and regulation of gene expression in the nucleus.

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By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2000, Geoffrey M Cooper.
Bookshelf ID: NBK9870


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