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326 Part IV Disorders of Hematopoietic Cell Development

the disease is thought to be the dysregulation of Runx1-specific target activity of eIF2 or eIF4E results in an increase in expression of the
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genes that are directly dependent on the Runt domain of Runx1. shorter p30 isoform (reviewed by Calkhoven et al ).
Several groups have reported mutations in the C/EBPα gene
CCAAT Enhancer-Binding Protein Family of in a subset of patients (~10%) with AML presenting with normal
Transcription Factors karyotypes (reviewed by Muller and Pabst ). These mutations can
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CCAAT enhancer-binding proteins (C/EBPs) are a family of basic be broadly classified into two main categories. The first includes
region-leucine zipper (b-ZIP) transcription factors that recognize the in-frame mutations clustered in the highly conserved C-terminus
consensus DNA-binding sequence 5′TKN NGYAAK3′ (Y = C or T; of the C/EBPα protein. The second category involves frameshift
K = T or G) within the regulatory regions of target genes. C/EBP mutations at the N-terminus of C/EBPα resulting in the premature
family proteins bind DNA as either homo- or heterodimers. This termination of the full length C/EBPαp42 isoform while keeping the
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family of transcription factors, which plays a crucial role in hematopoi- truncated C/EBPα p30 protein intact. The remaining C/EBPαp42
esis, includes C/EBPα,-β,-γ, -δ,-ε, and -ζ (CHOP-GADD 153), all is thought to be rendered inactive by the dominant-negative activity
of which contain highly homologous carboxyl-terminal (C-terminal) of the p30 isoform by an unknown mechanism. In addition, mice
dimerization (leucine zipper) domains and DNA-binding (basic that express a vector inducing overexpression of p30 C/EBPα from
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region) motifs but differ in their amino-terminal (N-terminal) the C/EBPA locus, develop AML with complete penetrance. Thus
transactivation domains—with the exception of CHOP-GADD 153, changes in the expression ratio of the two C/EBPα isoforms play a
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which lacks this domain altogether. Of interest, CHOP-GADD role in cell fate (reviewed by Muller and Pabst and Kirstetter et al,
153 can dimerize with and inhibit transactivation by C/EBPα,-β, and references therein).
and -ε and is found at a breakpoint in liposarcomas resulting in the The expression of C/EBPα is associated with growth arrest and
TLS-CHOP fusion protein. differentiation of granulocyte precursor cells. This block in prolif-
With the exception of C/EBPε, which is expressed exclusively eration is thought to occur via the interaction of C/EBPα with
in the late stages of granulopoiesis and in T lymphocytes, the other the cyclin-dependent protein kinases cdk2 and cdk4, resulting in a
C/EBP members are expressed in a wide variety of cells including block in cell proliferation by inhibiting these cell cycle kinases. In
liver, adipose tissue, lung, intestine, adrenal gland, and peripheral addition, C/EBPα inhibits E2F-dependent transcription, which in
blood mononuclear cells and placenta. Both C/EBPβ and C/EBPδ turn contributes to inhibition of cell proliferation and induction
are expressed at high levels in late-stage granulocytes. The C/EBP of differentiation associated with C/EBPα-induced granulopoiesis.
family members are known to exert pleiotropic effects in the tissues
in which they are expressed. This may be because of their tissue- and C/EBPβ
stage-specific expression, their ability to dimerize with members of Expression of C/EBPβ increases during myeloid maturation and is
their own family and of the Fos/Jun and ATF/CREB families of important for monocyte/macrophage gene expression and develop-
transcription factors, and their ability to interact with other tran- ment. Mice lacking the C/EBPβ gene demonstrate reduced B-cell
scription factors such as nuclear factor-κB (NFκB) and specificity numbers and defects in macrophage activation and function, and
protein-1 (Sp-1). they are more prone to microbial infections. The C/EBPβ knock-out
The C/EBP factors have been implicated in regulating the dif- studies reveal that this transcription factor is not essential for myeloid
ferentiation of a variety of tissues. C/EBPα plays a role in adipocyte development per se, but knock-in of C/EBPβ into the C/EBPα
−/−
differentiation: Inhibition of C/EBPα blocks adipocyte differentia- locus of C/EBPα mice rescues granulopoiesis. Several monocyte/
tion, and overexpression of C/EBPα induces adipocyte differentia- macrophage-specific genes are activated by C/EBPβ, including the
tion. Regulation of constitutive hepatic genes as well as acute-phase G-CSF receptor, lysozyme, CD11c, monocyte chemoattractant
response genes in the liver involves several C/EBP family members, in protein-1 (MCP-1), IL-6, IL-8, and nitric oxide synthase, among
particular, C/EBPα. Modulation of myelomonocytic differentiation others. Similar to C/EBPα, multiple isoforms of C/EBPβ are gener-
is also attributed to the activity of C/EBP family members. The ated from a single transcript through the use of three translation
importance of this family of transcription factors in myeloid dif- initiation sites and a leaky ribosome scanning mechanism. The
ferentiation has been demonstrated by the study of hematopoietic shortest of these isoforms, initiated at the most 3′ AUG, results in
abnormalities observed in mice with targeted disruption of C/EBPα, the formation of LIP (liver-enriched inhibitory protein), which lacks
-β, and -ε. the N-terminal activation domain present in full-length C/EBPβ and
has been implicated as a negative regulator of C/EBPβ function. It
C/EBPα has been suggested that the ratio of C/EBPβ to LIP may affect cellular
C/EBPα has been postulated to be a master regulator of the proliferation and differentiation. The activity of C/EBPβ is regulated
granulopoietic developmental program. It is expressed at high levels posttranscriptionally through protein–protein interactions and cova-
throughout myeloid differentiation and has been shown to bind to lent modifications. For example, in early myeloid progenitor cells,
the promoters of multiple myeloid-specific gene promoters regulating C/EBPβ is found in an unphosphorylated state in the cytoplasm.
gene expression at many different stages of myeloid maturation. However, on differentiation, C/EBPβ becomes phosphorylated and
−/−
Although C/EBPα mice die perinatally because of defects in gluco- translocates to the nucleus. 19
neogenesis that result in fatal hypoglycemia, they also have a selective
early block in the differentiation of granulocytes without affecting C/EBPβ in Emergency Granulopoiesis
either monocyte/macrophage maturation or the differentiation of Since they are very short-lived cells, neutrophils must be continuously
−/−
other hematopoietic lineages. Myeloid cells of C/EBPα mice lack produced in the BM under normal steady-state conditions. The large
G-CSFR, and it has been postulated that lack of mature neutrophils storage pool of neutrophils is sufficient to provide an immediate
in these mice may be because of the lack of G-CSFR. However, the increase in circulating neutrophils in response to acute infection.
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myeloid defect in C/EBPα mice is more severe than that seen in However, if there is a severe or persistent demand for neutrophils
−/−
G-CSFR mice, suggesting that C/EBPα has additional functions related to a prolonged or particularly severe infection, a switch from
vital to granulocytic maturation. steady-state to emergency granulopoiesis occurs to meet the increased
C/EBPα is a single exon gene, but it is expressed as two isoforms demand for neutrophils. This marked de novo increase in neutrophil
that arise from alternate translation start sites that give rise to a full- production is defined as “emergency granulopoiesis,” and is critical
length C/EBPαp42 and a truncated dominant negative C/EBPαp30 for survival of the host. As a rule, the emergency granulopoiesis
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isoform. Translational control of C/EBPα isoform expression is pathway follows three distinct steps, pathogen sensing and alerting
orchestrated by a conserved upstream open reading frame (uORF) the innate immune system of infection, triggering the molecular
in the 5′ untranslated region (UTR). This region is thought to be events that lead to increased neutrophil production in the BM and
responsive to the activities of the translation initiation factors eIF4E finally, restoration of steady-state conditions following clearance of
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1
and eIF2 (reviewed by Khanna-Gupta ) such that an increase in the the pathogen from the system (reviewed by Dao et al ).