of the Max - MDC

C/EBPα and C/EBPβ were found to recruit the ATP-dependent
SWI/SNF chromatin remodeling complex to activate
genes. Moreover, interaction with SWI/SNF is also required
for C/EBP mediated proliferation arrest. The transcriptional
function of C/EBPβ is unleashed by ras / MAP-kinase signaling
and phosphorylation of the RD that causes a conformational
change and releases the transactivation potential.
This involves alteration of interaction with the MED23 subunit
of “Mediator”, a complex that contacts the basic transcription
machinery, including RNA Polymerase II.
A chromatin function of the Myb (proto-)
oncoprotein
The classical retroviral Myb oncogene (v-myb) causes
myeloblastosis (yet the name: Myb) and represents a
mutated derivative of its cellular (c-Myb) counterpart which
is essential for hematopoietic progenitor proliferation.
The Myb protein harbors a DNA binding SANT-domain.
Three distinct amino acid substitutions in the leukemic
v-myb DNA binding SANT-domain are essential for the
leukemic potential of the oncoprotein that arrests progenitors
on the myeloblast stage and maintains their selfrenewal.
The Myb-SANT domain was since long supposed to harbor
epigenetic functions. Yeast-two-hybrid screening revealed
that the c-Myb SANT domain interacts specifically with the
N-terminal histone tails of H3 and H3.3 whereas v-Myb fails
to do so. Moreover, the oncogenic mutations abrogate
acetylation of H3, a requirement for collaborative activation
of differentiation genes together with C/EBP. This suggests
that loss-of-function mutations in Myb converts it into a
dominant-negative mutant, which causes leukemia.
Importantly, pharmacologic enhancement of H3 acetylation
(by an inhibitor of histone deacetylases) restored activation
of differentiation genes and induced cell differentiation,
implying that pharmacologic adjustment of an epigenetic
mark can overcome the leukemic function of v-myb.
Translational regulation of hematopoietic
transcription factors
Translational control has emerged as a major mechanism of
regulated gene expression. Both, C/EBPα and C/EBPβ transcripts
harbor small upstream open reading frames (uORF)
in their mRNAs that serve as relays for alternative initiation
at downstream start sites. As a result, N-terminally truncated
proteins with distinct biological functions are generated.
In the case of C/EBPα and C/EBPβ, truncated forms sustain
proliferation, whereas full length forms are strong
inhibitors of cell division. Anaplastic large cell lymphoma
and Hodgkin Lymphoma express large amounts of the truncated
form of C/EBPβ. Rapamycin, an antibiotic that
inhibits mTOR signaling, shuts down the truncated C/EBPβ
isoform and concomitantly inhibits growth in both types of
lymphomas. Ectopic expression of truncated C/EBPβ
restored proliferation, suggesting truncated C/EBPβ as a
translationally controlled oncogene in lymphoma.
The stem cell and leukemia gene SCL/Tal1 also contains
uORFs and is expressed as several isoforms. SCL is important
for HSC biology and the SCL locus is found frequently
translocated in childhood T-cell leukemia. Deletion of the
SCL uORF alters the expression of protein isoforms and modulats
progenitor biology in colony assays, underscoring the
importance of uORF regulation in SCL biology.
For both, C/EBPβ and SCL, we have now generated targeted
mutations in the mouse that favor expression of distinct
isoforms. Preliminary results show profound biological and
pathological alterations in these animal strains.
Canonical Wnt signaling in HSC
Hematopoietic stem cells (HSC) respond to Wnt signaling
but the functional role of β-catenin in self-renewal of HSCs
remains controversial. The role of activated β-catenin in the
hematopoietic system was explored by a conditional gainof-function
β-catenin gene in the mouse: Through a series
of stem cell transplantation and functional assays, we
demonstrated that excess of canonical Wnt signaling
impairs HSCs and multi-lineage progenitor functions
(Figure 2). Constitutive activation of β-catenin blocks
differentiation and induces anemia, neutropenia, cytopenia,
and death. Blood cell depletion was accompanied by
complete failure of HSCs to differentiate into mature cells.
Activation of β-catenin enforced cell cycle entry of HSCs,
thus leading to exhaustion of long-term stem cells, yet
preservation of short-term transit amplifying cells. Our data
suggest that fine-tuned Wnt stimulation is essential for
hematopoiesis and will be critical for any therapeutic use.
90 Cancer Research