Page 275 - Williams Hematology ( PDFDrive )

250 Part IV: Molecular and Cellular Hematology Chapter 17: Signal Transduction Pathways 251

G-PROTEIN–COUPLED RECEPTOR FAMILY gene transcription. 50–52 Although sex, glucocorticoid, and thyroid hor-
Several molecules that play essential roles in blood cell development mones may play subtle roles in blood cell biology, retinoid receptors,
or function signal by engaging G-protein–coupled receptors (GPCRs), which most commonly bind as heterodimers with the RXR receptor to
the largest family of cell surface receptors in organisms as diverse as retinoid response elements of the form PuGTTCA(N)2,5PuGTTCA,
yeast and humans, estimated to comprise approximately 1000 distinct play vital developmental roles in a myriad of cell systems, and play
gene products, or approximately 3 percent of the human genome. Also similar roles in hematopoiesis. Amongst the hematopoietic targets of
53
termed serpentine or heptahelical receptors (for their seven transmem- retinoid receptors are c-myc, C/EBPε, and p21. However, because this
brane domains that form four extracellular and three intracellular class of receptors represents a nearly direct pathway from stimulus to
loops; see Fig. 17–1), GPCRs are so named because they use three small response, without intervening signaling, they are not discussed further
guanine nucleotide (or G)-binding proteins (Gα, Gβ, and Gγ) for sig- in this chapter.
nal transduction. In the unstimulated state, all three G proteins bind to
the intracellular loops of the receptor. Individual ligands engage GPCR THE DIVERSITY OF DOWNSTREAM
in one of many different ways. For example, small lipophilic molecules
(e.g., epinephrine) bind to transmembrane (TM) domains of the recep- SIGNALS
tor, disrupting the interactions between TM3 and TM6, leading to con- PROTEIN PHOSPHORYLATION
formational changes that alter G-protein binding. Other GPCRs use
38
additional extracellular domains (e.g., the “Venus flytrap” domain) Protein phosphorylation is the critical first and vital response to engage-
39
to bind and dimerize receptors. Still others, which are engaged by pro- ment of signaling molecules of nearly all classes of cell surface receptors,
teases, are activated by protease cleavage of the receptor amino termi- including those that affect blood cell production and function. Numer-
nus, leading to the “unmasking” of a hexapeptide at the new amino ous studies reveal that protein tyrosine phosphorylation is detectible
terminus, which then interacts with one of the receptor extracellular within a minute of the addition of a wide variety of hematopoietic
or TM domains. By each of these and other mechanisms a confor- cytokines to blood cells and their progenitors. Evidence from nearly
40
mational change occurs in the GPCR, allowing monomeric Gα and all studies employing chemical inhibitors of kinase function or various
dimeric Gβγ complexes to dissociate from the intracellular loops and knockout and knockin strategies shows that JAK activation is critical
each to engage secondary signaling pathways. Examples of critical for hematopoietic cell survival, growth, and differentiation, and mature
41
molecules that employ GPCRs and display hematologic activity are cell response to a wide range of stimuli (Fig. 17–2). 54,55 Several studies
thrombin, adrenergic hormones, and chemokines. The outcomes of have elucidated an important mechanism of regulation of JAK kinases,
such engagement include cellular growth and survival, functional acti- one that is altered in the myeloproliferative diseases polycythemia vera,
vation, and migration. essential thrombocytosis, and idiopathic myelofibrosis (Chaps. 84 to 86).
Based on homologies to a number of other proteins JAK kinases
INTEGRINS AND OTHER ADHESION display 7 domains. These include (1) the domains that tether the kinase
to the cytoplasmic domain of the cytokine receptor (JH3-JH7), (2) the
MOLECULES kinase domain (JH1), and (3) a pseudokinase domain (JH2), so termed
Although adhesion molecules were named for the vital structural role because of its homology to other tyrosine kinases but lack of kinase
they play in tissue cohesion, physically bridging cells in the marrow with activity. Nevertheless, despite its lack of kinase activity, the pseudoki-
each other and with extracellular matrix macromolecules, and at sites at nase domain inhibits the kinase activity of the kinase domain, as shown
56
which mature blood cells interact with the endothelium, engagement of by single- and double-domain expression studies. Based on the known
blood and progenitor cell integrins and other adhesion molecules also structures of other kinases, the kinase and pseudokinase domains
generates vital cellular signals that affect their survival, proliferation, of JAK2 have been modeled. Both structural and functional studies
57
and functional activation. 42–44 In fibroblasts, cell adhesion is most clearly revealed that unexpectedly, the “pseudokinase” domain is, in fact, a
manifest at contact sites termed focal adhesions, and the signaling com- kinase that phosphorylates two negative regulatory tyrosine residues,
plexes that form on cytoplasmic domains of the integrins that support and suggests that the Val Phe mutation present in virtually all patients
617
them are termed focal adhesion complexes. Within such complexes with polycythemia vera and approximately half with essential throm-
45
are components of the actin cytoskeleton, kinases both specific for focal bocythemia and idiopathic myelofibrosis (Chaps. 84 to 86) 58–61 acts to
adhesions and several others found in other cytoplasmic sites, 46–48 and stiffen the pseudokinase domain, altering its function. 57
a number of scaffolding molecules upon which adhesion strengthening Among the phosphorylation targets of JAKs and other immedi-
and signaling take place. Moreover, growth factor receptors functionally ately responsive kinases in both normal and neoplastic hematopoiesis
interact with integrins, adding to signaling complexity. Thus, adhesion are the signaling receptor itself, adaptor molecules (Shc, Grb2, IRS, Gab,
molecules must also be considered as signaling receptors. Tensin2) that once modified recruit additional signaling substrates, reg-
ulatory subunits of secondary kinases (p85 phosphoinositol 3′-kinase
[PI3K]), latent transcription factors (signal transducers and activators
NUCLEAR RECEPTORS of transcription [STATs]), and several phosphatases (SHP2, SHIP). By
Nuclear receptors (NRs) are nascent transcription factors that play a wide catalyzing the phosphorylation of Tyr residues present in certain recep-
variety of roles in cellular physiology by binding small lipophilic hor- tor motifs, RTKs and JAK2 modify many signaling proteins to acquire
mones. Some NRs, such as glucocorticoid hormone receptors, remain the capacity to bind Src homology (SH)-2 domain-containing proteins.
sequestered in the cytoplasm in the absence of their cognate ligand, and Perhaps equally important are ser/thr phosphorylation sites induced by
upon ligand engagement translocate to the nucleus and bind and acti- activated TGF-β receptors. However, most of the downstream signal-
vate palindromic, direct repeat, or inverted palindromic sequences that ing components of all of the receptors that use JAK2 and other kinases
comprise nucleotide hormone response elements. Other NRs, such to transduce growth and differentiation cues have been derived from
49
as receptors for vitamin A metabolites (retinoids), remain bound to candidate gene approaches; the availability of antibodies to specific sig-
nuclear DNA and repress transcription, until engaged by ligand upon naling mediators has dictated the molecules that have been studied. An
which nuclear coactivators are recruited leading to enhancement of unbiased approach to identifying the entire “signaling space” used by all

Kaushansky_chapter 17_p0247-0256.indd 250 9/17/15 5:45 PM