Page 405 - Hematology_ Basic Principles and Practice ( PDFDrive )

Chapter 27 Granulocytopoiesis and Monocytopoiesis 327

Transcriptional Networks in Emergency Granulopoiesis function in BM). A combination of differential splicing and alternate
The role of transcription factors in emergency granulopoiesis has promoter usage results in four messenger RNA (mRNA) isoforms 2.6
only recently begun to be addressed. In an elegant study, expression kilobases and 1.3 to 1.5 kilobases in size, from which three proteins of
−/−
levels of C/EBP family members were measured in early granulocytic 32.2 kDa, 27.8 kDa, and 14.3 kDa have been described. C/EBPε
precursor cells following induction of emergency granulopoiesis with mice produce hyposegmented granulocytes that are functionally
cytokines or Candida albicans infection. While expression levels of C/ defective. Late in life, these mice develop myelodysplasia. Absence of
EBPα, C/EBPδ and C/EBPε were downregulated in the granulocytic C/EBPε is thought to block the later steps in terminal differentiation
precursor cells, C/EBPβ levels remained elevated, suggesting that of mature segmented granulocytes. Mutant mice usually survive 2
3
C/EBPβ may play a role in emergency granulopoiesis. While C/ to 5 months and eventually succumb to low-pathogenicity bacterial
EBPα-null hematopoietic precursor cells are capable of generating infections. C/EBPε thus plays a crucial role in terminal granulocytic
3–5
neutrophils in substantial numbers upon cytokine stimulation or differentiation.
−/−
3
upon infection with C. albicans, C/EBPβ-null mice were unable to C/EBPε mice have wild-type levels of the G-CSF receptor, and
3,6
support emergency granulopoiesis in response to cytokines, even the defects manifested in these mice are confined to late-stage gene
though steady-state granulopoiesis remained unaffected. Based on expression associated with the function of the mature neutrophil. It
these observations, it has been suggested that C/EBPα and C/EBPβ has been demonstrated that the ability of G-CSF to regulate myeloid
function specifically but antagonistically towards one another during differentiation is dependent on the induction of C/EBPε. Verbeek
3
steady-state and emergency granulopoiesis. On the one hand, C/ and colleagues demonstrated that the mRNA of several genes includ-
EBPα functions as a master regulator of steady-state granulopoi- ing p47 phox (a component of the neutrophil–NADPH oxidase
esis by limiting proliferation via inhibition of expression of the complex), as well as the secondary granule protein (SGP) genes,
−/−
7
8
cyclin-dependent kinases, cdk2 and cdk4 and of c-Myc and by are either absent or abnormal in the BM of C/EBPε mice. These
promoting granulocytic differentiation. On the other hand, C/EBPβ investigators further suggest that C/EBPε plays a critical role in the
drives emergency granulopoiesis largely because it does not block the regulation of host antimicrobial defense.
−/−
expression of cdk2, cdk4, or c-Myc, thereby allowing proliferation of Neutrophils from C/EBPε mice have morphologic and bio-
granulocytic progenitors and increasing neutrophil numbers during chemical features very similar to those observed in patients with
emergency granulopoiesis. neutrophil-specific SGD. SGD is an extremely rare congenital disor-
According to the current model, the switch from steady-state to der that is characterized by frequent and severe bacterial infections.
emergency granulopoiesis is thought to be regulated by the increased Patients with SGD have defects in neutrophil function including
levels of cytokines such as G-CSF and to a lesser extent GM-CSF atypical nuclear morphology, impaired bactericidal activity, and
and IL-6. These cytokines are known to be upregulated in response to abnormalities in neutrophil migration; they also lack both neutro-
pathogen invasion. G-CSF signaling involves activation of the JAK- phil and eosinophil secondary granule proteins. Sequence analysis
signal transducer and activator of transcription (STAT) pathway. In of genomic DNA from two patients with SGD revealed mutations
9
particular, STAT3, a known regulator of granulopoiesis, is activated within the C/EBPε gene, resulting in the identification of a mutant
in response to G-CSF, and together with C/EBPβ binds to the protein lacking the dimerization and DNA-binding domains and
proximal promoter of c-Myc, thereby activating gene expression. hence transcriptional activity. Lack of functional C/EBPε activity has
This in turn suppresses binding of C/EBPα to the c-Myc promoter, been postulated to underlie the observed pathology in these patients.
resulting in C/EBPβ-mediated gene expression prevailing over that of
C/EBPα, leading to increased cell cycle progression and c-Myc acti- C/EBPζ
10
vation during emergency granulopoiesis. In summary, emergency C/EBPζ-C/EBP homologous protein (CHOP) is a C/EBP family
granulopoiesis is triggered by the very high levels of granulocytic member that was originally isolated as the product of a gene
cytokines (G-CSF in particular) in response to microbial infections, induced in response to DNA-damaging agents. It has subsequently
leading to the activation of the JAK-STAT pathway, which together been shown to be induced by various extracellular or endoplasmic
with C/EBPβ stimulate granulocyte precursor proliferation and reticulum stresses. The basic region of CHOP is less well conserved
differentiation. than that of the other C/EBP family members, and CHOP does
not seem to bind to canonical C/EBP cis elements. CHOP has been
C/EBPγ shown to interfere with the transcriptional activity of C/EBPβ in a
C/EBPγ is a ubiquitously expressed C/EBP family member that was manner dependent on its leucine zipper.
first identified by its affinity for cis-regulatory sites in the Ig heavy
chain promoter and enhancer. C/EBPγ contains a C/EBP-like b-Zip PU.1
domain but lacks an N-terminal transactivation domain and can PU.1 is a member of the Ets family of transcription factors and is
inhibit transcriptional activation of other C/EBP members in some expressed abundantly in B cells and macrophages. Expression of
cell types. Impairment of natural killer (NK) cytotoxic activity and of PU.1 has also been reported in granulocytes and eosinophils as well
−/−
+
interferon-γ production has been reported in C/EBPγ mice. as in CD34 hematopoietic progenitor cells. High levels of PU.1
expression in fetal livers of mice preferentially direct macrophage
C/EBPδ development, whereas low levels of PU.1 result in B-cell development.
C/EBPδ is expressed at low or undetectable levels in several tissues of C-Jun, another member of the b-Zip family of transcription factors,
adult mice and humans. Expression has been shown to dramatically serves as a coactivator of PU.1 during macrophage development.
increase on induction with bacterial lipopolysaccharide and inflam- It has been demonstrated that overexpression of c-Jun in myeloid
matory cytokines, suggesting a role for C/EBPβ in acute-phase and progenitor cells results in macrophage development. Furthermore,
inflammatory response. Double-knock-out experiments using C/ downregulation of c-Jun by C/EBPα is necessary for granulocytic
EBPβ and C/EBPβ suggest a synergistic role for these two C/EBP maturation and appears to be the mechanism through which C/
family members in controlling terminal adipocyte differentiation. EBPα blocks macrophage development. C/EBPα not only binds to
Of interest, both C/EBPβ and C/EBPβ are expressed during late the promoter of the c-jun gene and decreases its expression but also
neutrophil development and have been postulated to play roles in binds PU.1, thereby inhibiting its activity.
late neutrophil gene expression. PU.1-binding sites have been reported in almost all myeloid-
specific promoters reported to date, including those for M-CSF,
C/EBPε GM-CSF, and G-CSF receptors, all of which play critical roles in
CCAAT enhancer-binding protein ε (C/EBPε) is the most recently myeloid cell development. PU.1 activity is modulated both by cova-
described C/EBP protein. The human C/EBPε gene resides on lent modifications and by protein–protein interactions. For example,
chromosome 14 and is transcribed by two alternative promoters, phosphorylation of PU.1 by casein kinase II or by JNK kinase leads
Pα (thought to function in mature neutrophils) and Pβ (thought to to increased transcriptional activity.