Medical Physiology, 3rd Edition

Receptors That are Catalytic

A number of hormones and growth factors bind to cell-surface proteins that have—or are associated with—enzymatic activity on the cytoplasmic side of the membrane. Here we discuss five classes of such catalytic receptors (Fig. 3-12):

1. Receptor guanylyl cyclases catalyze the generation of cGMP from GTP.

2. Receptor serine/threonine kinases phosphorylate serine or threonine residues on cellular proteins

3. Receptor tyrosine kinases (RTKs) phosphorylate tyrosine residues on themselves and other proteins.

4. Tyrosine kinase–associated receptors interact with cytosolic (i.e., not membrane-bound) tyrosine kinases.

5. Receptor tyrosine phosphatases cleave phosphate groups from tyrosine groups of cellular proteins.

image

FIGURE 3-12 Catalytic receptors. A, Receptor guanylyl cyclases have an extracellular ligand-binding domain. B, Receptor serine/threonine kinases have two subunits. The ligand binds only to the type II subunit. C, RTKs similar to the NGF receptor dimerize on binding a ligand. D, Tyrosine kinase–associated receptors have no intrinsic enzyme activity but associate noncovalently with soluble nonreceptor tyrosine kinases. E, Receptor tyrosine phosphatases have intrinsic tyrosine phosphatase activity.

The receptor guanylyl cyclase transduces the activity of atrial natriuretic peptide, whereas a soluble guanylyl cyclase transduces the activity of nitric oxide

Receptor (Membrane-Bound) Guanylyl Cyclase

Some of the best-characterized examples of a transmembrane protein with guanylyl cyclase activity (see Fig. 3-12A) are the receptors for the natriuretic peptides. image N3-18 These ligands are a family of related small proteins (~28 amino acids) including atrial natriuretic peptide (ANP), B-type or brain natriuretic peptide (BNP), and C-type natriuretic peptide (CNP). For example, in response to atrial and ventricular stretch that occur with intravascular volume expansion, cardiac myocytes release ANP and BNP, which act through receptor guanylyl cyclases. Their action is to relax vascular smooth muscle and dilate blood vessels (see ANP in Table 20-7, and p. 553) as well as to enhance Na+ excretion into urine (natriuresis; see p. 843). Both activities contribute to lowering of effective circulating blood volume and thus blood pressure (see pp. 554–555).

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Atrial Natriuretic Peptide

Contributed by Emile Boulpaep

Granular inclusions in atrial myocytes, called Palade bodies, contain pro-ANP, the precursor of atrial natriuretic peptide (ANP; also called atrial natriuretic factor, ANF). Pro-ANP, comprising 126 amino acids, is derived from the precursor known as prepro-ANP (151 residues in the human). The converting enzyme corin—a cardiac transmembrane serine protease—cleaves the pro-ANP during or after release from the atria, which yields the inactive N-terminal fragment of 98 residues and the active C-terminal 28–amino-acid peptide called ANP. Release is primarily caused by stretch of the atrial myocytes. Hormones such as angiotensin, endothelins, arginine vasopressin, and glucocorticoid modulate ANP expression and release. It is noteworthy that expression of corin is reduced in heart failure, which blunts the release of ANP in the failing heart. This blunting might contribute to the inappropriate increase of extracellular fluid volume in heart failure.

ANP is a member of the NP (natriuretic peptide) family of peptides. The biological effects of ANP are potent vasodilation, diuresis, natriuresis, and kaliuresis, as well as inhibition of the renin-angiotensin-aldosterone system.

At least three types of natriuretic peptide receptors (NPRs) exist: NPRA (also called GC-A—GC for guanylyl cyclase), NPRB (also called GC-B), and NPR-C. NPRA and NPRB are receptors with a single transmembrane domain coupled to a cytosolic guanylyl cyclase (see p. 66). Activation of NPRA or NPRB leads to the intracellular generation of cGMP. In smooth muscle, intracellular cGMP activates the cGMP-dependent protein kinase that phosphorylates MLCK. Phosphorylation of MLCK inactivates MLCK; this leads to the dephosphorylation of myosin light chains, which allows muscle relaxation.

The ANP C-type receptor NPRC is not coupled to a messenger system but serves mainly to clear the natriuretic peptides from the circulation.

The heart, brain, pituitary, and lung synthesize an ANP-like compound termed BNP, originally known as brain natriuretic peptide (32 residues in the human). The biological actions of BNP are similar to those of ANP.

The hypothalamus, pituitary, and kidney synthesize C-type natriuretic peptide or CNP, which is highly homologous to ANP and BNP. CNP binds only to NPRB and is only a weak natriuretic but a strong vasodilator.

The kidney also synthesizes an ANP-like natriuretic compound known as urodilatin or URO. URO has four additional amino acids compared to ANP and also binds to the ANP A-type receptor. Its biological effect in the target tissue is also transduced by cGMP.

Natriuretic peptide receptors NPRA and NPRB are membrane proteins with an extracellular ligand-binding domain and a single membrane-spanning segment (see Fig. 3-12A). The intracellular domain has two consensus catalytic domains for guanylyl cyclase activity. Binding of a natriuretic peptide induces a conformational change in the receptor that causes receptor dimerization and activation. Thus, binding of ANP to its receptor causes the conversion of GTP to cGMP and raises intracellular levels of cGMP. In turn, cGMP activates a cGMP-dependent kinase (PKG or cGK) that phosphorylates proteins at certain serine and threonine residues. In the renal medullary collecting duct, the cGMP generated in response to ANP may act not only via PKG but also by directly modulating ion channels (see p. 768).

Soluble Guanylyl Cyclase

In contrast to the receptor guanylyl cyclase, which is activated by ANP, the cytosolic soluble guanylyl cyclase (sGC) is activated by nitric oxide (NO). This sGC is unrelated to the receptor guanylyl cyclase and contains a heme moiety that binds NO.

NO is a highly reactive, short-lived free radical. This dissolved gas arises from a family of NO synthase (NOS) enzymes that catalyze the reaction

image

Here, NADPH and NADP+ are the reduced and oxidized forms of nicotinamide adenine dinucleotide phosphate, respectively. Tetrahydrobiopterin is a cofactor. The NOS family includes neuronal or nNOS (NOS1), inducible or iNOS (NOS2), and endothelial or eNOS (NOS3). nNOS and iNOS are soluble enzymes, whereas eNOS is linked to the plasma membrane. The activation of NOS begins as an extracellular agonist (e.g., ACh) binds to a plasma-membrane receptor, triggering the entry of Ca2+, which binds to cytosolic CaM and then stimulates NOS. In smooth muscle, NO stimulates the sGC, which then converts GTP to cGMP, activating PKG, which leads to smooth-muscle relaxation.

Why NO is so ubiquitous and when its release is important are not known. However, abnormalities of the NO system are involved in the pathophysiological processes of adult respiratory distress syndrome, high-altitude pulmonary edema, stroke, and other diseases. For example, the importance of NO in the control of blood flow had long been exploited unwittingly to treat angina pectoris. Angina is the classic chest pain that accompanies inadequate blood flow to the heart muscle, usually as a result of coronary artery atherosclerosis. Nitroglycerin relieves this pain by spontaneously breaking down and releasing NO, which relaxes the smooth muscles of peripheral arterioles, thereby reducing the work of the heart and relieving the associated pain. Understanding the physiological and pathophysiological roles of NO has led to the introduction of clinical treatments that modulate the NO system. In addition to the use of NO generators for treatment of angina, examples include the use of gaseous NO for treatment of pulmonary edema and inhibitors of cGMP phosphodiesterase (see p. 53) such as sildenafil (Viagra) for treatment of erectile dysfunction.

In addition to acting as a chemical signal in blood vessels, NO generated by iNOS appears to play an important role in the destruction of invading organisms by macrophages and neutrophils. NO generated by nNOS also serves as a neurotransmitter (see pp. 315–317) and may play a role in learning and memory.

The importance of the NO signaling pathway was recognized by the awarding of the 1998 Nobel Prize for Physiology or Medicine to R.F. Furchgott, L.J. Ignarro, and F. Murad for their discoveries concerning NO as a signaling molecule in the cardiovascular system. image N3-18A

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Robert F. Furchgott, Louis J. Ignarro, and Ferid Murad

For more information about Robert F. Furchgott, Louis J. Ignarro, and Ferid Murad and the work that led to their Nobel Prize, visit http://www.nobelprize.org/nobel_prizes/medicine/laureates/1998/ (Accessed March 2015).

Some catalytic receptors are serine/threonine kinases

We have already discussed how activation of various G protein–linked receptors can initiate a cascade that eventually activates kinases (e.g., PKA, PKC) that phosphorylate proteins at serine and threonine residues. In addition, some receptors are themselves serine/threonine kinases—such as the one for transforming growth factor-β (TGF-β)—and are thus catalytic receptors.

The TGF-β superfamily includes a large group of cytokines, including five TGF-βs, antimüllerian hormone (see p. 1080), as well as the inhibins and activins (see p. 1113), bone morphogenic proteins, and other glycoproteins, all of which control cell growth and differentiation. Members of this family participate in embryogenesis, suppress epithelial-cell growth, promote wound repair, and influence immune and endocrine functions. Unchecked TGF-β signaling is important in progressive fibrotic disorders (e.g., liver cirrhosis, idiopathic pulmonary fibrosis) that result in replacement of normal organ tissue by deposits of collagen and other matrix components.

The receptors for TGF-β and related factors are glycoproteins with a single membrane-spanning segment and intrinsic serine/threonine-kinase activity. Receptor types I and II (see Fig. 3-12B) are required for ligand binding and catalytic activity. The type II receptor first binds the ligand, and this binding is followed by the formation of a stable ternary complex of ligand, type II receptor, and type I receptor. After recruitment of the type I receptor into the complex, the type II receptor phosphorylates the type I receptor, thereby activating the serine/threonine kinase activity of the type I receptor. The principal targets of this kinase activity are SMAD proteins, which fall into three groups. imageN3-19 The largest group is the receptor-activated SMADs (SMADs 1, 2, 3, 5, and 8), which—after phosphorylation by activated type I receptors—association with SMAD4, the only member of the second group. This heterodimeric complex translocates to the nucleus, where it regulates transcription of target genes. The third group (SMAD6, SMAD7) is the inhibitory SMADs, which can bind to type I receptors and prevent the phosphorylation of the receptor-activated SMADs.

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SMADs

Contributed by Ed Moczydlowski

The largest group of SMAD proteins is the receptor-activated SMADs (SMADs 1, 2, 3, 5, and 8), which have a type I receptor–interacting domain that is phosphorylated by the activated type I receptor; this phosphorylation results in their disassociation from the receptor and subsequent association with the regulatory SMAD, SMAD4. This heterodimeric complex translocates to the nucleus where it can regulate transcription of target genes by both direct and indirect mechanisms.

The signaling specificity of this system comes from two mechanisms. First, distinct members of the receptor-activated SMAD group interact with specific type I receptors. For example SMAD2 and SMAD3 associate with the TGF-β type I receptor ALK-5, whereas SMAD1 associates with bone morphogenic protein (BMP) type I receptors such as ALK-2 and ALK-3. Second, the receptor-activated SMAD/SMAD4 heterodimer regulates not only downstream effector gene expression but also the expression of a third group of SMADs, the inhibitory SMADs. These proteins (SMAD6, SMAD7), once expressed, can bind to type I receptors and prevent the association and activation of receptor-activated SMADs.

RTKs produce phosphotyrosine motifs recognized by SH2 and phosphotyrosine-binding domains of downstream effectors

In addition to the class of receptors with intrinsic serine/threonine kinase activity, other plasma-membrane receptors have intrinsic tyrosine kinase activity. All RTKs discovered to date phosphorylate themselves in addition to other cellular proteins. Epidermal growth factor (EGF), platelet-derived growth factor (PDGF), vascular endothelial growth factor (VEGF), insulin and insulin-like growth factor type 1 (IGF-1), fibroblast growth factor (FGF), and nerve growth factor (NGF) can all bind to receptors that possess intrinsic tyrosine kinase activity.

Creation of Phosphotyrosine Motifs

Most RTKs are single-pass transmembrane proteins that have an extracellular ligand-binding domain and a single intracellular kinase domain (see Fig. 3-12C). Binding of a ligand, such as NGF, facilitates the formation of receptor dimers that in turn promote the direct association and trans-phosphorylation of the adjacent kinase domains; the result is activation of the receptor complex. The activated receptors then catalyze the addition of phosphate to tyrosine (Y) residues on the receptor itself as well as specific membrane-associated and cytoplasmic proteins. The resulting phosphotyrosine (pY) motifs serve as high-affinity binding sites for the recruitment of a number of intracellular signaling molecules, discussed in the next paragraph. These interactions lead to the formation of a signaling complex and the activation of downstream effectors. Some RTKs, such as the insulin and IGF-1 receptors, image N3-20 exist as dimers even before ligand binding but undergo a conformational change that promotes autophosphorylation and activation of the kinase domains (see pp. 1041–1042).

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Insulin and IGF-1 Receptors

Contributed by Emile Boulpaep, Walter Boron

The insulin receptor (see Fig. 51-5) and the IGF-1 receptor are activated by somewhat different mechanisms, as we discuss on pages 1041–1042 for the insulin receptor and on page 996 for the IGF-1 receptor. In brief, these receptors are tetrameric; they are composed of two α and two β subunits. The α subunit contains a cysteine-rich region and functions in ligand binding. The β subunit is a single-pass transmembrane protein with a cytoplasmic tyrosine kinase domain. The α and β subunits are held together by disulfide bonds (as are the two α subunits), forming a heterotetramer. Ligand binding produces conformational changes that appear to cause allosteric interactions between the two α and β pairs, as opposed to the dimerization characteristic of the first class of RTKs (see Fig. 3-12C). Thus, insulin binding results in the autophosphorylation of tyrosine residues in the catalytic domains of the β subunits. The activated insulin receptor also phosphorylates cytoplasmic substrates such as IRS-1 (insulin-receptor substrate 1; see Fig. 51-6), which, once phosphorylated, serves as a docking site for additional signaling proteins.

Recognition of pY Motifs by SH2 and Phosphotyrosine-Binding Domains

The pY motifs created by tyrosine kinases serve as high-affinity binding sites for the recruitment of cytoplasmic or membrane-associated proteins that contain either an SH2 domain or PTB (phosphotyrosine-binding) domain. SH2 domains are ~100 amino acids in length. They are composed of relatively well conserved residues that form the binding pocket for pY motifs as well as more variable residues that are implicated in binding specificity. These residues that confer binding specificity primarily recognize the three amino acids located on the C-terminal side of the phosphotyrosine. For example, the activated PDGF receptor has five such pY motifs (Table 3-5), each of which interacts with a specific SH2-containing protein.

TABLE 3-5

Tyrosine Phosphopeptides of the PDGF Receptor That Are Recognized by SH2 Domains on Various Proteins

TYROSINE (Y) THAT IS PHOSPHORYLATED IN PDGF RECEPTOR

pY MOTIF RECOGNIZED BY SH2-CONTAINING PROTEIN

SH2-CONTAINING PROTEIN

Y579

pYIYVD

Src family kinases

Y708

pYMDMS

p85

Y719

pYVPML

p85

Y739

pYNAPY

GTPase-activating protein

Y1021

pYIIPY

PLCγ

In contrast to SH2 and PTB domains, which interact with highly regulated pY motifs, Src homology 3 (SH3) domains interact constitutively with proline-rich regions in other proteins in a manner that does not require phosphorylation of the motif. However, phosphorylation at distant sites can change the conformation near the proline-rich region and thereby regulate the interaction. Like SH2 interactions, SH3 interactions appear to be responsible for targeting of signaling molecules to specific subcellular locations. SH2- or SH3-containing proteins include growth factor receptor–bound protein 2 (GRB2), PLCγ, and the p85 subunit of the phosphatidylinositol-3-kinase.

The MAPK Pathway

A common pathway by which activated RTKs transduce their signal to cytosol and even to the nucleus is a cascade of events that increase the activity of the small GTP-binding protein Ras. This Ras-dependent signaling pathway involves the following steps (Fig. 3-13):

Step 1: A ligand binds to the extracellular domain of a specific RTK, thus causing receptor dimerization.

Step 2: The now-activated RTK phosphorylates itself on tyrosine residues of the cytoplasmic domain (autophosphorylation).

Step 3: GRB2, an SH2-containing protein, recognizes pY residues on the activated receptor.

Step 4: Because GRB2 constitutively associates with the guanine nucleotide exchange factor SOS (son of sevenless), via an SH3-proline interaction, the recruitment of GRB2 automatically results in the recruitment of SOS as well.

Step 5: SOS activates the small G protein Ras by catalyzing the replacement of GDP with GTP.

Step 6: The activated GTP-Ras complex activates other proteins by physically recruiting them to the plasma membrane. In particular, active GTP-Ras interacts with the N-terminal portion of the cytosolic serine/threonine kinase Raf-1 (also known as MAP kinase kinase kinase or MAPKKK or MAP3K), which is the first in a series of sequentially activated protein kinases that ultimately transmits the activation signal.

Step 7: Raf-1 phosphorylates and activates a protein kinase called MEK (also known as MAP kinase kinase or MAPKK). MEK is a multifunctional protein kinase that phosphorylates substrates on both tyrosine and serine/threonine residues.

Step 8: MEK phosphorylates MAPKs, cytosolic serine/threonine kinases also called extracellular signal–regulated kinases (ERK1, ERK2). Activation of MAPK requires dual phosphorylation on neighboring serine and tyrosine residues. Raf, MEK, and MAPK typically assemble on a scaffolding protein at the inner side of the cell membrane to facilitate interaction/phosphorylation during the activation process.

Step 9: MAPK is an important effector molecule in Ras-dependent signaling to the cytoskeleton. MAPK phosphorylates multiple proteins involved in actin cytoskeletal assembly and cell-matrix interactions; this phosphorylation leads to Ras-dependent changes in cell morphology and cell migration.

Step 10: Once activated, MAPK disassociates from the scaffold and translocates primarily to the nucleus, where it phosphorylates a number of nuclear proteins that are transcription factors. The result is either enhancement or repression of the DNA binding and transcriptional activity of these nuclear proteins. image N3-21

image

FIGURE 3-13 Regulation of transcription by the Ras pathway. A ligand, such as a growth factor, binds to a specific RTK, and this leads to an increase in gene transcription in a 10-step process.

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Transcription Factors Phosphorylated by MAPK

Contributed by Peter Igarashi

The following table summarizes some transcription factors that MAPK phosphorylates, together with the site of phosphorylation on the transcription factor and the effect.

Transcription Factor

Site

Effect

c-Myc

Ser-62

Stabilizes protein

c-Jun

Ser-243 (Ser-63/Ser-73 in the activation domain are phosphorylated by a distinct Ras-dependent kinase)

Inhibits DNA binding

c-Fos

Ser-374 (direct)
Ser-362 (indirect via ribosomal S6 kinase, which is activated by MAPK)

Stimulates transrepression

p62TCF = Elk-1

Multiple

Stimulates transactivation and possibly also DNA binding

C/EBPβ = LAP or NF–IL-6

Thr-235

Stimulates transactivation

ATF-2

Thr-69 and Thr-71 (via p38 and JNK MAPK)

Stimulates transactivation

ATF-2, activating transcription factor 2; C/EBPβ, CCAAT/enhancer–binding protein β; JNK, c-Jun N-terminal kinases.

Two other signal-transduction pathways (cAMP and Ca2+) can modulate the activity of some of the protein intermediates in this MAPK cascade, which suggests multiple points of integration for the various signaling systems.

The Phosphatidylinositol-3-Kinase Pathway

The phosphatidylinositol-3-kinase (PI3K) is an SH2 domain–containing protein that commonly signals downstream of RTKs. PI3K is a heterodimer consisting of a p85 regulatory subunit and p110 catalytic subunit. p85 has an SH2 domain for targeting the complex to activated receptors and an SH3 domain that mediates constitutive association with p110. p110 is a lipid kinase that phosphorylates PIP2 (see p. 58) on the 3 position of the inositol ring to form PIP3. PIP2 is a relatively common lipid in the inner leaflet of the cell membrane, whereas PIP3 constitutes <1% of membrane lipids in a quiescent cell.

Following RTK activation, the newly recruited PI3K produces PIP3 locally in the membrane, where it serves as a binding site for proteins such as 3-phosphoinositide–dependent kinase 1 (PDK1), PKC (see pp. 60–61), and the guanine nucleotide exchange factor Vav. image N3-22 The events downstream of the insulin receptor are a good example of PDK1 signaling (see Fig. 51-6). As PDK1 binds to PIP3, its kinase activity leads to phosphorylation and activation of the downstream target Akt (also known as protein kinase B or PKB). Akt is itself a serine/threonine kinase that regulates multiple downstream targets, inhibiting glycogen synthase kinase 3-β (GSK-3β), activating the mTOR pathway via phosphorylation and inhibition of tuberin image N3-23 and inhibiting the proapoptotic proteins BAD and caspase-9 (see Fig. 62-6).

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PIP3-Binding Domains

Contributed by Ed Moczydlowski

Proteins that bind PIP3 contain either a pleckstrin homology domain, PTB domain, or FYVE domain that binds to the membrane-associated PIP3.

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Tuberous Sclerosis

Contributed by Ed Moczydlowski

Tuberous sclerosis is a result of the mutation of the gene TSC1 or TSC2.

Tyrosine kinase–associated receptors activate cytosolic tyrosine kinases such as Src and JAK

Some of the receptors for cytokines and growth factors that regulate cell proliferation and differentiation do not themselves have intrinsic tyrosine kinase activity but can associate with nonreceptor tyrosine kinases (see Fig. 3-12D). Receptors in this class include those for several cytokines, including IL-2, IL-3, IL-4, IL-5, IL-6, leukemia inhibitory factor (LIF), granulocyte-macrophage colony-stimulating factor (GM-CSF), and erythropoietin (EPO). The family also includes receptors for growth hormone (GH), prolactin (PRL), leptin, ciliary neurotrophin factor (CNTF), oncostatin M, and IFN-α, IFN-β, and IFN-γ.

The tyrosine kinase–associated receptors typically comprise multiple subunits that form homodimers (αα), heterodimers (αβ), or heterotrimers (αβγ). For example, the IL-3 and the GM-CSF receptors are heterodimers (αβ) that share common β subunits with transducing activity. image N3-24 However, none of the cytoplasmic portions of the receptor subunits contains kinase domains or other sequences with recognized catalytic function. Instead, tyrosine kinases of the Src family and Janus family (JAK or Janus kinases) associate noncovalently with the cytoplasmic domains of these receptors. Thus, these are receptor-associated tyrosine kinases. Ligand binding to these receptors results in receptor dimerization and activation of the associated tyrosine kinase. The activated kinase then phosphorylates tyrosines on both itself and the receptor. Thus, tyrosine kinase–associated receptors, together with their tyrosine kinases, function much like the RTKs discussed in the previous section. A key difference is that for the tyrosine kinase–associated receptors, the receptors and kinases are encoded by separate genes and the proteins are only loosely associated with one another.

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Multimeric Composition of Tyrosine Kinase–Associated Receptors

Contributed by Laurie Roman

In the text, we pointed out that the IL-3 and GM-CSF receptors are heterodimers (αβ) that share a common β subunit that has transducing activity. A similar example is the group of three receptors for IL-6, oncostatin M (OncM), and IL-11. These three receptors use a common transducer subunit called gp130 as well as a unique subunit for ligand binding.

The Src family of receptor-associated tyrosine kinases includes at least nine members. Alternative initiation codons and tissue-specific splicing result in at least 14 related gene products.

The conserved regions of Src-related proteins can be divided into five domains: (1) an N-terminal myristylation site, through which the kinase is tethered to the membrane; (2) an SH3 domain, which binds to proline-rich regions of the kinase itself or to other cytosolic proteins; (3) an SH2 domain, which binds phosphorylated tyrosines; (4) the catalytic domain, which has tyrosine kinase activity; and (5) a noncatalytic C terminus. Members of this family are kept in the inactive state by tyrosine phosphorylation at a conserved residue in the C terminus that causes the pY to bind to the N-terminal SH2 domain of the same molecule. The result is a masking of the intervening kinase domain. Dephosphorylation of the pY residue after the activation of such phosphatases as receptor protein tyrosine phosphatase α (RPTPα) or the cytosolic SHP2 releases this inhibition, and the kinase domain can then phosphorylate its intracellular substrates.

Many of the Src family members were first identified in transformed cells or tumors because of mutations that caused them to be constitutively active. When these mutations result in malignant transformation of the cell, the gene in question is designated an oncogene; the normal, unaltered physiological counterpart of an oncogene is called a proto-oncogene.

Box 3-5

Oncogenes

The ability of certain viral proteins (oncogenes) to transform a cell from a normal to a malignant phenotype was initially thought to occur because these viral proteins acted as transcriptional activators or repressors. However, during the last 20 years, only a few of these viral proteins have been found to work in this manner. The majority of oncogenes harbor mutations that transform them into constitutively active forms of normal cellular signaling proteins called proto-oncogenes. Most of these aberrant proteins (i.e., the oncogenes) encode proteins important in a key signal-transduction pathway. For example, expression of the viral protein v-erb B is involved in fibrosarcomas, and both v-erb A and v-erb B are associated with leukemias. v-erb B resembles a constitutively activated RTK (EGF receptor), and the retroviral v-erb A is derived from a cellular gene encoding a thyroid hormone receptor. Other receptors and signaling molecules implicated in cell transformation include Src, Ras, and PDGF receptor. A mutation in protein tyrosine phosphatase 1C results in abnormal hematopoiesis and an increased incidence of lymphoreticular tumors.

The Janus family of receptor-associated tyrosine kinases in mammals includes JAK1, JAK2, and Tyk2. JAK originally stood for “just another kinase.” Major downstream targets of the JAKs include one or more members of the STAT (signal transducers and activators of transcription) family. When phosphorylated, STATs interact with other STAT family members to form a complex that translocates to the nucleus (see p. 90). There, the complex facilitates the transcription of specific genes that are specialized for a rapid response, such as those that are characterized by the acute-phase response of inflammation (see p. 1202). For example, IL-6 binding to the heterodimeric IL6R/GP130 receptor on hepatocytes activates JAK/STAT, which enhances transcription of the acute-phase proteins. During inflammation, these acute-phase proteins function to limit tissue damage by inhibiting the proteases that attack healthy cells as well as diseased ones. The pattern of STAT activation provides a mechanism for cytokine individuality. For example, EPO activates STAT5a and STAT5b as part of the early events in erythropoiesis, whereas IL-4 or IL-12 activates STAT4 and STAT6.

Attenuation of the cytokine JAK-STAT signaling cascade involves the production of inhibitors that suppress tyrosine phosphorylation and activation of the STATs. For example, IL-6 and LIF both induce expression of the inhibitor SST-1, which contains an SH2 domain and prevents JAK2 or Tyk2 from activating STAT3 in M1 myeloid leukemia cells.

Receptor tyrosine phosphatases are required for lymphocyte activation

Tyrosine residues that are phosphorylated by the tyrosine kinases described in the preceding two sections are dephosphorylated by PTPs, which can be either cytosolic or membrane spanning (i.e., the receptor tyrosine phosphatases). We discussed the cytosolic PTPs above. Both classes of tyrosine phosphatases have structures very different from the ones that dephosphorylate serine and threonine residues. Because the tyrosine phosphatases are highly active, pY groups tend to have a short half-life and are relatively few in number in unstimulated cells.

The CD45 protein, found at the cell surface of T and B lymphocytes, is an example of a receptor tyrosine phosphatase. CD45 makes a single pass through the membrane. Its glycosylated extracellular domain functions as a receptor for antibodies, whereas its cytoplasmic domain has tyrosine phosphatase activity (see Fig. 3-12E). During their maturation, lymphocytes express several variants of CD45 characterized by different patterns of alternative splicing and glycosylation. CD45 plays a critical role in signal transduction in lymphocytes. For instance, CD45 dephosphorylates and thereby activates Lck and Fyn (two receptor-associated tyrosine kinases of the Src family) and triggers the phosphorylation of other proteins downstream in the signal-transduction cascade. This interaction between receptor tyrosine phosphatases and tyrosine kinase–associated receptors is another example of crosstalk between signaling pathways.