Page 1848 - Williams Hematology ( PDFDrive )

1822 Part XII: Hemostasis and Thrombosis Chapter 111: Megakaryopoiesis and Thrombopoiesis 1823

of experimental animals and patients with thrombocytopenia or throm- CXCL12 and other stimuli on megakaryocyte growth extends to cell
bocytosis support this model. 144,145 Moreover, thrombopoietin knock- surface adhesion. 38
out mice display a gene dosage effect. Platelet levels in heterozygous
146
mice are intermediate between that seen in wild-type and nullizygous Transforming Growth Factor-β
animals, suggesting active regulation of the remaining thrombopoietin In addition to the many positive regulators of megakaryopoiesis, several
allele cannot compensate for the mild (60 percent of normal) thrombo- substances down-modulate their development. Five isoforms of TGF-
cytopenia induced by the loss of one allele. β have been identified, all disulfide-linked homodimers each contain-
162
A second model suggests thrombopoietin expression is a regu- ing 112 residues. TGF-β is the predominant type of TGF found in
1
lated event. Very-low platelet levels can induce thrombopoietin-specific hematopoietic tissues. Platelet α granules are a particularly rich source
mRNA production. Several studies show that thrombopoietin mRNA of the cytokine. In general, transforming growth factors are inhibitors
levels are modulated in response to moderate to severe thrombocytope- of hematopoiesis, 163,164 particularly of megakaryocyte development. 165,166
nia, at least in the marrow. 145,147 The signal(s) responsible for this form The best understood TGF-β growth inhibitory effects are exerted on
of thrombopoietin regulation is being uncovered, but is, at least in part, cell-cycle progression. After binding to one of five receptors, two path-
mediated by transcriptional enhancement. CD40 ligand, platelet-de- ways that block cell-cycle progression are activated. pRb is hypophos-
148
167
rived growth factor, FGF, TGF-β, platelet factor-4, and thrombospondin phorylated, antagonizing the effects of G -phase cyclin-dependent
1
modulate thrombopoietin production from marrow stromal cells. 149,150 kinases, and cell-cycle inhibitors, including p27 and p15 INK , are upregu-
The human thrombopoietin gene 5′ flanking region lacks a TATA lated, affecting cell-cycle progression. 168,169 In contrast to these negative
box or CAAT motif and directs transcription initiation at multiple sites effects of TGF-β on cell proliferation, the cytokine enhances megakary-
over a 50-nucleotide region. Reporter gene analysis in a hepatocyte ocyte differentiation.
151
cell line identified an Ets2 transcription factor-binding motif respon-
sible for high-level expression of the gene. The 5′ flanking region also Interferon-α
152
includes SP-1, AP-2, and nuclear factor-κB binding sites, although A second class of cytokines that negatively impact thrombopoiesis are
the contribution of these transcription factors to thrombopoietin gene the interferons (IFNs), proteins first defined by their ability to induce
expression, either under steady-state or inflammatory conditions, has an antiviral state in mammalian cells. Biochemical fractionation has
170
not been studied. revealed three classes of IFNs: IFN-α, a family of 17 distinct but highly
homologous molecules; IFN-β, a single molecule more distantly related
CXCL12 (Stromal Cell-Derived Factor-1) to the various isoforms of IFN-α; and IFN-γ, a unique molecule that
Chemokines are members of a rapidly growing class of molecules shares functional properties but not structure with the others. IFNs
153
that play multiple roles in blood cell physiology. Initially defined exert profound inhibitory effects on hematopoiesis. 171
as substances that induce leukocyte chemotaxis, four classes of the The genes for the IFN-α/β subfamily cluster on the short arm of
8- to 12-kDa polypeptides have been recognized, based on the spac- chromosome 9 and encode 165- to 172-residue polypeptides, of which
ing of cysteine residues close to the aminoterminus of the proteins. An 35 percent are invariant across the family of IFN-α molecules. IFNs of
equally rapidly growing family of chemokine receptors also has been the α/β type are produced by transcriptional upregulation in fibroblasts
discovered, classified by the subfamily of chemokines they serve. All and leukocytes in response to viruses and other infectious agents and
chemokine receptors are members of the seven-transmembrane family to inflammatory cytokines. Once bound to the IFN receptors, a cascade
of receptors that signal through heterotrimeric G proteins. of kinases and intracellular mediators are triggered, initiated by JAKs
Most work has been conducted with the CC and CXC subfamilies (Janus family kinases), STAT (signal transducer and activator of tran-
of chemokines, molecules that display modest inhibitory effects on cell scription) factors, and p38 MAPK (Chap. 17), resulting in changes in
proliferation when used alone and potent effects when used in com- gene transcription.
bination on hematopoietic progenitors at all levels of development. IFN-α inhibits megakaryopoiesis, the clinical use of which is
154
On many levels, the CXC chemokine CXCL12 (previously termed SDF- responsible for modest to severe thrombocytopenia in a significant
1) and its receptor CXCR4 are notable exceptions to the many features number of patients undergoing therapy for chronic viral hepatitis. 172,173
shared by most members of the chemokine and chemokine receptor The mechanisms responsible for the inhibitory effect of IFN-α are
families. For example, although all the other genes for the known CXC multifactorial. Some studies suggest a direct inhibitory effect of IFN-
chemokines reside on the long arm of human chromosome 14, CXCL12 α on growth factor-induced proliferation pathways. For example, the
localizes to the long arm of chromosome 10. Moreover, most chemok- cytokine augments double-stranded RNA-activated protein kinase
155
ine receptors can be activated by multiple ligands. For example, the activity, inhibiting translation initiation factor-2, implicating reduction
chemokine CCL3 (macrophage inflammatory protein [MIP]-1α) can of the growth factor-induced protein synthesis necessary for growth
bind and activate CCR1 and CCR5, and IL-8 can bind both CXCR1 factor response. IFN-β induces expression of the cell-cycle inhibi-
174
and CXCR2. In contrast, as the phenotype of genetic elimination of tor p27 Kip1 , arresting cells in G /G 175 Other studies have demonstrated
156
1.
0
both CXCR4 and CXCL12 are almost identical, 157,158 CXCR4 appears to IFN-α induces a SOCS (suppressor of cytokine signaling)-1–based
be the only receptor for CXCL12, and CXCL12 is the only ligand for feedback mechanism that cross-reacts and depresses thrombopoie-
CXCR4. tin signaling. Thus, in addition to the multiple positive mediators of
176
The marrow stroma is the primary source of CXCL12, and most megakaryopoiesis, several cytokines block the process and can lead to
of the cell types known to express CXCR4 are hematopoietic in ori- thrombocytopenia.
gin. One of the major phenotypes in CXCL12- or CXCR4-deficient
neonatal mice is marrow aplasia, thought to be secondary to failure
of perinatal hematopoietic stem cell homing (Chap. 16). In addi- MEGAKARYOCYTE MICROENVIRONMENT
159
tion, megakaryocytes display CXCR4 and migrate in response to an Chapter 5 details the role of the marrow microenvironment in hemato-
160
CXCL12 concentration gradient. Several groups have shown that poiesis. This chapter discusses only aspects particularly vital for
161
CXCL12 augments thrombopoietin-induced megakaryocyte growth in megakaryocyte growth. The cellular concentration within the marrow
suspension culture. 37,160 Later studies have shown the synergy between is estimated to be 10 /mL. Consequently, cell–cell and cell–matrix
9

Kaushansky_chapter 111_p1813-1828.indd 1823 9/21/15 4:11 PM