Page 278 - Williams Hematology ( PDFDrive )

252 Part IV: Molecular and Cellular Hematology Chapter 17: Signal Transduction Pathways 253

95
receptors responsible for internalization are mapped, potentially recruitment of signaling adaptor molecules. Ubiquitin and SUMO
108
allowing intervention in this process. For example, activation of c-Mpl (small ubiquitin-like modifier) also play a vital role in SOCS- and PIAS-
108
by TPO leads to engagement of the adaptor protein-2 (AP2) complex, mediated repression of cytokine signaling. Like for the hematopoi-
which results in clathrin binding and receptor internalization. The etic phosphatases, dysfunction of SOCS proteins has been implicated
kinetics of this process is delayed, taking approximately 30 minutes for in malignancy. 109
near complete internalization, allowing the TPO signal to persist only
a short time. 96
INHIBITORY SIGNALS
Finally, some signals negatively impact signals derived from other
PHOSPHATASES receptors. One example is the interaction of the growth inhibitory sig-
As discussed earlier in “The Diversity of Downstream Signals,” phos- nals derived from TGF-β and the growth promoting signals triggered by
phorylation of numerous proteins and membrane lipids plays a vital several hematopoietic growth factors. TGF-β is constitutively expressed
role in signal transduction within the cell. Thus, elimination of these in the marrow stroma, and acts to reduce hematopoietic stem cell
modifications through the action of phosphatases would be expected to (HSC) cycling by driving the nuclear localization of a SMAD2–SMAD4
terminate such signals. Moreover, because some of the same signals are complex, which, in turn, is regulated by an inhibitable nuclear export
activated in malignant transformation, protein tyrosine phosphatases signal present on the complex. Stem cell factor (SCF), FMS-like tyrosine
(PTPs) might also be expected to play an important role in malignancy, kinase 3 (Flt-3) ligand, and TPO all induce promote HSC survival and
and possibly in autoimmunity. growth, in part through activation of the MAPKs: ERK1 and ERK2. In
Hematopoietic cell phosphatase (also termed SHP1) bears two turn, activated ERK1/2 then phosphorylates several sites on the linker
SH2 domains that interact with cytokine and inhibitory immune core- region of SMAD2, inhibiting the nuclear localization of the inhibitory
ceptors at ITIM (immunoreceptor tyrosine-based inhibitory motif) SMAD2/SMAD4 complex, reducing the suppressive effects of TGF-β on
sites that have been modified by Tyr phosphorylation. Once so engaged, the cell cycle. Another form of this type of crosstalk between cytok-
110
SHP1 becomes activated and dephosphorylates associated phosphoty- ines is illustrated by TPO and interferon (IFN)-α, the latter suppressing
rosine activation sites on receptors, adaptor molecules, and their asso- megakaryopoiesis driven by the former. By induction of SOCS-1, not
ciated kinases. One of the earliest clues that SHP1 plays an important usually induced by TPO, IFN-α inhibits TPO-mediated signaling. 111
97
role in hematopoietic signaling came from the discovery that the moth-
eaten mouse phenotype is a result of a genetic loss of function of SHP1.
98
These mice demonstrate a massive expansion and tissue accumulation SIGNAL COORDINATION AND
of monocytes and myeloid cells, resulting in chronic inflammation, mas-
sive immune defects, and premature death. Careful analysis of the mice CROSSTALK
revealed they manifest defective controls over the cellular activation In the foregoing discussion, several examples of the convergence of sig-
and proliferation response to exogenous stimuli, such as that induced naling pathways and receptor crosstalk were summarized. Over the past
by engagement of the B-cell antigen receptor (BCR) complex. At steady decade, two types of cell membrane-based supramolecular organizations
state SHP1 is thought to engage the BCR (through presently unclear have been identified: lipid rafts and tetraspanin webs. In their seminal
mechanisms) and maintains the antigen-binding subunits (immuno- fluid–mosaic model of the cell membrane, Singer and Nicolson posited
globulin [Ig]α and Igβ) in a dephosphorylated, quiescent state. The that integral membrane proteins float in a random array of membrane
phosphatase is displaced from the complex upon antigen engagement, lipids. This model was modified to account for local heterogeneity of
112
but is later re-recruited to the complex once ITIM containing inhibitory the lipid bilayer. Lipid rafts, local concentrations of specific membrane
coreceptors such as CD22, PIR-B, CD72, and FcγRIIb are phosphory- lipids and proteins, are defined by the methods to isolate them—the
lated and recruited to the activated complex. Once recruited to the insoluble components of a cold detergent extraction in which raft com-
99
BCR complex, SHP1 removes the activating Tyr phosphate sites on the ponents “float” to the top of a density gradient. Upon discovery that
113
ITAM (immunoreceptor tyrosine-based activation motif) sites of Igα/β, many of the proteins present in such rafts were involved in signal trans-
the coreceptor CD19, the adaptor BLNK and Lyn kinase, and the BCR duction, it became apparent that these membrane subdomains could
returns to its quiescent state. Similar roles for SHP1 have been identified represent a structural basis for communication between seemingly dis-
in T cells, NK cells, monocytes and macrophages, and erythroid parate components of the signal transduction apparatus. 37,114,115
100
102
101
cells. The latter is of particular interest, as mutation of the site on A second level of membrane-based structural organization of
103
EPOR to which SHP1 binds causes familial erythrocytosis, as a result of signaling molecules has been elucidated: the tetraspanin-enriched
prolongation of EPO signaling. Of interest, this mutation was identified microdomain or “web.” The tetraspanin family of membrane proteins
in a family containing a two-time Olympic gold medalist. 104 is characterized by four TM domains punctuating two extracellular
regions, a CCG motif, and several other conserved cysteine residues
SOCS Proteins in the extracellular domain. The tetraspanins now include more than
Another mechanism of growth factor signal termination is mediated by 30 members, most or all of which interact with other cell surface mol-
116
the suppressors of cytokine signaling (SOCS) proteins. The cloning of a ecules, and have been functionally linked to cell adhesion, migration,
STAT-inducible gene, CIS, and several additional genes that bear sub- differentiation, and signal transduction. Members of this family are
105
stantial sequence homology, 106,107 yielded a family of proteins that can thought to act as molecular facilitators of protein–protein interaction
directly suppress growth factor receptor-induced signals. The engage- by associating with “partners,” the bimolecular complexes then interact
ment of either HCRs or RTKs leads to STAT activation, as discussed with others in a slightly less avid manner, and the complexes loosely
earlier in “The Diversity of Downstream Signals.” One of the transcrip- associate in microdomains. CD9, CD63, and CD81 are the tetraspanins
tional targets of STATs are the SOCS and PIAS (protein inhibitor of most closely linked to hematopoietic cell function, are usually found in
117
activated STATs) genes (see Fig. 17–2), which, upon transcription and association with β and β integrins, affect many hematopoietic cell
3
1
translation, bind to phosphotyrosine residues and inhibit either JAK types, 118–120 and act in concert with multiple signaling receptors, kinases,
kinases, STATs, or the phosphorylated receptors themselves, blocking and phosphatases. 121,122

Kaushansky_chapter 17_p0247-0256.indd 253 9/17/15 5:45 PM