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348
GROWTH FACTORS AND HEMATOPOIETIC CELL FATE
for inducing differentiation or clonal suppression in myeloid leukaemic
cell lines. EMBO J 16:451, 1997
16. Young JW, Steinman RM: The hematopoietic development of
dendritic cells: A distinct pathway for myeloid differentiation. Stem
Cells 14:376, 1996
17. Fukunaga R, Ishizaka-Ikeda E, Nagata S: Growth and differentiation signals mediated by different regions in the cytoplasmic domain of
granulocyte-stimulating factor receptor. Cell 74:1079, 1993
18. Dong F, Van Buitenen C, Pouwels K, Hoefsloot LH, Löwenberg
B, Touw IP: Distinct cytoplasmic regions of the human granulocyte
colony-stimulating factor receptor involved in induction of proliferation
and maturation. Mol Cell Biol 13:7774, 1993
19. Alexander WS, Maurer AB, Novak U, Harrison-Smith M:
Tyrosine-599 of the c-Mpl receptor is required for Shc phosphorylation
and the induction of cellular differentiation. EMBO J 15:6531, 1996
20. Dong F, Dale DC, Bonilla MA, Freedman M, Fasth A, Neijens
HJ, Palmblad J, Briars GL, Carlsson G, Veerman AJP, Welte K,
Löwenberg B, Touw IP: Mutations in the granulocyte colonystimulating factor receptor gene in patients with severe congenital
neutropenia. Leukemia 11:120, 1997
21. Kaushansky K, Broudy VC, Lin N, Jorgensen MJ, McCarty J,
Fox N, Zucker-Franklin D, Lofton-Day C: Thrombopoietin, the Mpl
ligand, is essential for full megakaryocyte development. Proc Natl Acad
Sci USA 92:3234, 1995
22. Metcalf D, Nicola NA: The clonal proliferation of normal mouse
hematopoietic cells: Enhancement and suppression by colonystimulating factor combinations. Blood 79:2861, 1992
23. Fairburn LJ, Cowling GJ, Reipert BM, Dexter TM: Suppression
of apoptosis allows differentiation and development of a multipotent
hemopoietic cell line in the absence of added growth factors. Cell
74:823, 1993
24. Metcalf D, Merchav S: Effects of GM-CSF deprivation on
precursors of granulocytes and macrophages. J Cell Physiol 112:411,
1982
25. Lieschke GJ, Grail D, Hodgson G, Metcalf D, Stanley E, Cheers
C, Fowler KJ, Basu S, Zhan YF, Dunn AR: Mice lacking granulocyte
colony-stimulating factor have chronic neutropenia, granulocyte and
macrophage progenitor cell deficiency, and impaired neutrophil mobilization. Blood 84:1737, 1994
26. Gurney AL, Carver-Moore K, de Sauvage FJ, Moore MW:
Thrombocytopenia in c-mpl-deficient mice. Science 265:1445, 1994
27. Alexander WS, Roberts AW, Nicola NA, Li R, Metcalf D:
Deficiencies in progenitor cells of multiple hematopoietic lineages and
defective megakaryocytopoiesis in mice lacking the thrombopoietin
receptor c-Mpl. Blood 87:2162, 1996
28. Roberts AW, Metcalf D: Granulocyte colony-stimulating factor
induces selective elevations of progenitor cells in the peripheral blood
of mice. Exp Hematol 22:1156, 1994
29. Kaushansky K, Lin N, Grossman A, Humes J, Sprugel KH,
Broudy VC: Thrombopoietin expands erythroid, granulocyte-macrophage, and megakaryocytic progenitor cells in normal and myelosuppressed mice. Exp Hematol 24:265, 1996
30. Metcalf D: The cellular basis for enhancement interactions
between stem cell factor and the colony stimulating factors. Stem Cells
11:1, 1993 (suppl 2)
Do Stem Cells Play Dice?
By T. Enver, C.M. Heyworth, and T.M. Dexter
T
HE DEBATE SURROUNDING the issue of whether or not
hematopoietic stem cell commitment and differentiation
are orchestrated in a cell-intrinsic or cell-extrinsic fashion is in
many ways akin to the nature versus nurture debate that typifies
discussions of human personal potentials. We have published
data that could be interpreted as supporting both hypotheses.1-3
Simply put, the question is this: is unilineage commitment the
result of a cell-autonomous or internally driven program or
rather is it the consequence of a cell responding to an external,
ie, environmentally imposed, agenda?
In the former case, stochastic processes are often invoked as
instigators of lineage decisions. In instructive, or so-called
deterministic schemes, cell-cell interactions or diffusible signals
dictate cell fate decisions. Much attention has revolved around
the role played by hematopoietic growth factors in this process:
do they play an instructive, ie, a deterministic role, or are they
From the Section of Gene Function and Regulation, Institute of
Cancer Research, Chester Beatty Laboratories, London, UK; and the
CRC Department of Haemopoietic Cell and Gene Therapeutics,
Paterson Institute for Cancer Research, Christie Hospital NHS Trust,
Manchester, UK.
Supported by the Cancer Research Campaign and the Leukaemia
Research Fund. T.M.D. is a Gibb Research Fellow.
Address reprint requests to T. Enver, PhD, Section of Gene Function
and Regulation, Institute of Cancer Research, Chester Beatty Laboratories, London, SW3 6JB, UK; e-mail: [email protected].
r 1998 by The American Society of Hematology.
0006-4971/98/9202-0045$3.00/0
simply permissive or selective, ie, allow the survival and
proliferation of independently committed cells? Certainly, experiments using multipotent cell lines in vitro suggest that the
addition of exogenous growth factors is essential for the
survival and proliferation of the cells but not for the lineage
commitment or subsequent differentiation and development
into mature cells.1,3 However, it has been argued that cultured
cell lines, albeit with a normal karyotype, growth factor
dependence, and ability to undergo multilineage differentiation,
may not adequately reflect what happens with freshly isolated
normal cells. To address this criticism, the intuitive experiment
of adding cytokines, alone or in combination, to a population of
multipotent cells in vitro and recording lineage output would,
on the face of it, be a simple way of distinguishing between
stochastic versus deterministic control of differentiation. Unfortunately, this experiment is flawed. For example, the elicitation
of granulocytes from a pool of multipotent cells by the addition
of G-CSF would seem to argue for an instructive role for
G-CSF. However, the result could equally well be explained by
suggesting that G-CSF is selecting for the survival and proliferation of a subpopulation of cells that have been already
programmed for neutrophil development (by cell-intrinsic/
stochastic means), with the other cells, with different lineage
potential, either remaining undeveloped/unexpanded or simply
dying. In this respect, in vivo experiments with all the currently
assessable knock-out mice lacking growth factors and/or their
receptors have strongly suggested that growth factors are not
essential for lineage determination. For example, mice lacking
From www.bloodjournal.org by guest on June 15, 2017. For personal use only.
GROWTH FACTORS AND HEMATOPOIETIC CELL FATE
GM-CSF show no obvious deficiencies in the production of
myeloid progenitor cells in the bone marrow4; mice lacking
G-CSF show a reduced but still substantial production of
neutrophils5; and the erythropoietin (Epo) and Epo receptor
knock-out mice still produce near-normal levels of lineagecommitted erythroid precursor cells.6 Although these data do
not rule out the possibility that these growth factors can be
involved in lineage commitment decisions, the data do show
that lineage commitment decisions can be made in the absence
of these growth factors.
Further information has come from molecular studies of the
cytokine receptors themselves. While maintaining specific
private ligand binding domains, many receptors share common
or public signaling domains. Delivering a deterministic signal
through common or shared subunits poses something of a
mechanistic conundrum and raises the familiar specter of
redundancy. Receptors that deliver signals through homodimerization perhaps offer more potential for instructive signaling,
but even these appear to activate, if not identical, than very
similar or highly overlapping, downstream signaling pathways.
Noteworthy here, however, and potentially supportive of an
instructive role is the molecular delineation within the cytoplasmic signaling domain of the G-CSF receptor of separable
proliferation and differentiation-associated regions.7,8
Attention has also focused on the expression pattern of the
cytokine receptors, particularly within multipotent cells: a
cytokine-mediated instructive signal cannot be delivered to a
multipotent cell that lacks a receptor for it! Most recently,
McKinstry et al9 have examined receptor distribution on
purified stem and progenitor cells using radiolabeled cytokine
ligands as probes. Heterogeneity was observed both in terms of
the percentage of the population expressing receptors and the
numbers of receptors expressed by labeled cells. Expression
levels were in many cases low (#100 receptors/cell). Strikingly,
the expression of receptors for GM-CSF (GM-CSFR) and
M-CSF (M-CSFR) on stem cells was below the level of
detection, and M-CSFR could not be detected on progenitor
cells despite the fact that these cell populations could functionally respond to added GM-CSF or M-CSF.9,10 Similar results
have been documented by reverse transcription-polymerase
chain reaction (RT-PCR) analysis of single multipotent FDCPmix A4 and sorted CD341 lin2 mouse bone marrow cells.11
Whereas heterogeneity was observed in these studies (although
more marked in A4 than in CD341 lin2 cells), many individual
cells coexpressed multiple different receptors and, despite the
low levels of expression seen, could functionally respond to the
appropriate ligands. These data may be argued both ways:
heterogeneity favoring perhaps a stochastic view and being
consistent with the random assortment of blood cell lineages
reported many years ago by Ogawa12 and receptor coexpression
on the other hand setting up the possibility of receiving an
instructive cue from one or more of an assortment of cytokines.
Manipulation of cytokine and cytokine receptor expression
both in vitro and in vivo has been performed in many studies.
Ectopic expression of receptors in multipotent cells, largely
through retroviral transduction, has not led to a marked bias in
lineage output but rather elicited expansion of the multipotent
compartment in response to the cognate cytokine. For example,
M-CSFR–transduced multipotent cells proliferate in response
to M-CSF but do not preferentially commit to an M-CSF–
349
affiliated pathway.13 One striking exception to this pattern was
reported by Borzillo et al,14 who ectopically expressed the
M-CSFR in an IL-7–dependent pre-B–cell line. The addition of
M-CSF resulted in a lymphoid to macrophage lineage switch.
Although this has been frequently interpreted as a deterministic
event, an alternative explanation is that there may be a default
pathway for differentiation that results in cells with macrophagelike characteristics whose survival is enhanced by M-CSF.
These results should be contrasted with natural and engineered
cytokine and cytokine receptor loss of function mutants in
mice.15 Despite the multiplicity of these knock-outs, no evidence for any defects in unilineage commitment has been
observed. As with all knock-out studies, the action of compensatory/parallel and/or redundant pathways, obscuring an authentic
in vivo role, cannot be ruled out, but the results currently being
obtained from the cross-breeding of different knockouts makes
this less likely.16 However, it should be emphasized that,
although unilineage commitment itself seems unimpaired in
these animals, the production of mature, functional, terminally
differentiated cells is, in many cases, severely affected, once
again arguing for a role for growth factors in the postcommitment amplification and maturation phase. Similar data have
been obtained using hybrid receptors in which the signaling
domains of given receptors have been fused to the ligand
binding domains of others or to binding domains for synthetic
ligands. A most recent and elegant example of this has been
provided by Stoffel et al17 at ASH 1997. These investigators
have used a knock-in approach to express a hybrid receptor
consisting of the extracellular domain of c-mpl and the cytoplasmic domain of the G-CSFR under the control of the c-mpl
regulatory elements. Once again, a nonspecific survival or
proliferation response was observed, but no changes in commitment bias were detected, arguing against a deterministic role for
growth factors in lineage commitment.
Because the signals themselves often activate similar if not
identical pathways, different outcomes could presumably be
dependent on different cellular perceptions or interpretations, ie,
the notion of context-dependent interpretation or cellular history. An example comes from studies of the G-CSFR in which
ectopic expression and activation in hematopoietic cell lines can
lead a proliferative response if expressed in BAF3 cells, but
induce differentiation if transfected into L-GM cells.7
What then might likely constitute context at the molecular
level? The status of cross-regulatory or cross-talking signaling
pathways undoubtedly factor into the mix, but a more significant component is likely provided by the transcription factor
profiles of individual multipotent cells. Many lineage-restricted
or lineage-affiliated transcription factors have been described
and their role in lineage specification addressed, primarily
through gene targeting in mice.18 Strikingly, there is not a yet a
single example of a resulting lineage-specific ablation at the
level of commitment. That is not to say that the hematopoietic
systems of these animals are not in many instances grossly
affected with major defects in mature blood cell development.
For example, GATA-1 knockout mice are severely anemic as a
result of a failure in erythroid maturation, although erythroid
commitment (ie, progenitor numbers) is largely unimpaired. As
with growth factor receptors, the issue of overlapping functions
and expression patterns as well as compensating pathways may
well as yet be obscuring authentic roles for some of these
From www.bloodjournal.org by guest on June 15, 2017. For personal use only.
350
transcription factors in commitment. The next generation of
multiple and conditional knockout mice will undoubtedly help
to resolve this.
Ectopic master gene experiments based on the myoD paradigm in muscle have been conducted with some success,
although primarily in chicken cells. Kulessa et al19 have shown
that ectopic expression of individual transcription factors can
reprogram the lineage output of myb-ets–transformed chicken
progenitor cells. GATA-1 is a case in point, and here quantitatively different levels of GATA-1 are associated with different
lineage outcomes suggesting that threshold effects may be in
play.19 Importantly, their work, as well as our own, also
emphasizes the role of negative regulation in lineage specification.20-22 Rejection of other lineage options is in some sense a
corollary of unilineage commitment, and the observation that
transcription factors can promote one lineage program while
simultaneously and actively repressing another squares well
with this. The recent demonstration that Ikaros may function to
sequester loci into transcriptionally repressive regions of the
nucleus may also be important in this regard.23
Because lineage determination ultimately results from the
collective assembly of stable transcriptional complexes at
lineage affiliated loci such as b-globin, myeloperoxidase (mpo),
Ig, etc, it is relatively easy to see how transcription factors must
play a crucial role in differentiation through activation of a host
of lineage-affiliated or lineage-specific target genes. It is
perhaps less obvious to imagine how the ball starts rolling, and
indeed rolling in any one particular direction.
Some intriguing evidence has recently come to light from the
detailed molecular analysis of multipotent stem and progenitor
cells. Studies of chromatin structure of multipotent cells have
shown that a number of lineage-affiliated genes (globin, mpo,
GROWTH FACTORS AND HEMATOPOIETIC CELL FATE
IgH, CD3d) have accessible control regions (enhancers/LCRs)
before unilineage commitment.24,25 This accessibility is reflected in the fact that low-level, possibly spontaneous, transcription can be detected from a number of these genes and that
RT-PCR analysis of a variety of differently purified multipotent
cells has demonstrated expression of a variety of different
lineage-affiliated transcription factors and cytokine receptors.11
Furthermore, analysis of single cells has shown that many
different lineage-affiliated components can be expressed in the
same cell, although heterogeneity in specific patterns or profiles
of expression was observed. These data are consistent with a
model of lineage specification of type shown in Fig 1, in which
low level multilineage gene activity establishes a ground state
or level of noise from which regulatory networks can start to
build and be amplified or diminished either through quasirandom or stochastic changes in the components (ie, spontaneous transcription of an accessible activator or repressor) or
through positive/negative reinforcement through extracellular
signalling via stochastically expressed receptor molecules. Such
a model incorporates both cell-extrinsic and cell-intrinsic
components and may critically not be dependent on any one
specific component to instigate a commitment decision.
Finally, it should be noted that what is true for multipotent or
stem cells, which must service a lifetime’s supply of blood cells,
may not hold for more developmentally restricted progenitor
cells. If differentiation of stem cells was simply instructive, it is
difficult to imagine how stem cell homeostasis can be maintained in the face of competing demands for one of the stem cell
lineages in response to physiological insults. Notably, cell
proliferation in the bone marrow must be able to respond
rapidly and efficiently to bleeding, infection, etc, and here
perhaps it is more reasonable to propose that growth factors
Fig 1. In this model, lineagespecific genes (a, b, and c) are in
a primed state in the multipotent progenitor cells, characterized by open chromatin and
some low-level of sporadic expression. Lineage-affiliated regulators (colored shapes) are initially coexpressed at levels that
fluctuate within thresholds. This
low-level multilineage gene activity establishes a ground state
from which regulatory networks
can develop through negative
and positive feedback loops.
From www.bloodjournal.org by guest on June 15, 2017. For personal use only.
GROWTH FACTORS AND HEMATOPOIETIC CELL FATE
have a role to play in the differentiation of lineage-restricted
progenitor cells such as the bipotent granulocyte-macrophage
colony-forming cells (GM-CFC). Highly enriched GM-CFC
develop into granulocytes in the presence of SCF or G-CSF and
into macrophages if cultured in M-CSF. In a series of studies
aimed at investigating the molecular mechanisms underlying
lineage restriction in the presence of these growth factors, we
showed that macrophage development is associated with translocation of PKCa from the cytoplasm to the nucleus. Indeed, if
GM-CFC are grown in the presence of agents that translocate
PKCa to the nucleus (eg, TPA and IL-4) even in presence of
growth factors that usually promote granulocytic development,
macrophages develop.26,27 Similarly, using clonal analysis of
paired daughter cells of GM-CFC, Metcalf and Burgess28
showed that GM-CSF and M-CSF can apparently irreversibly
commit cells to different lineages. Thus, certain of the growth
factors may be able to play a role in lineage determination of
these bipotent GM-CFC via known regulatory mechanisms.
In conclusion, our current hypothesis is that primitive cell
differentiation involves mainly stochastic processes, but, as the
cell becomes more lineage restricted, deterministic processes
appear to be more relevant as the cell has to respond to
immediate changes in the environment. Ultimately, one cannot
help feeling that the all or none instructive versus selective
debate is no longer the question. It is clearly important now to
piece together the different molecular components, cell-intrinsic
and cell-extrinsic, and understand the circuitry of their interactions. The precise starting point of a lineage-determining loop is
probably not important. Indeed, it may vary between cells and
therefore be unknowable. It is the chicken and the egg scenario
all over again and who really cares which came first—apart
from the chicken!
REFERENCES
1. Just U, Stocking C, Spooncer, E, Dexter TM, Ostertag W:
Expression of the GM-CSF gene after retroviral transfer in hematopoietic stem cell lines induces synchronous granulocyte-macrophage
differentiation. Cell 64:1163, 1991
2. Heyworth CM, Dexter TM, Kan O, Whetton AD: The role of
hemopoietic growth factors in self-renewal and differentiation of
IL-3-dependent multipotential stem cells. Growth Factors 2:197, 1990
3. Fairbairn LJ, Cowling GJ, Reipert BM, Dexter TM: Suppression
of apoptosis allows differentiation and development of a multipotent
hemopoietic cell line in the absence of added growth factors. Cell
74:823, 1993
4. Stanley E, Lieschke GJ, Grail D, Metcalf D, Hodgson G, Gall JA,
Maher DW, Cebon J, Sinickas V, Dunn AR: Granulocyte/macrophage
colony-stimulating factor-deficient mice show no major perturbation of
hematopoiesis but develop a characteristic pulmonary pathology. Proc
Natl Acad Sci USA 91:5592, 1994
5. Lieschke GJ, Grail D, Hodgson G, Metcalf D, Stanley E, Cheers
C, Fowler KJ, Basu S, Zhan YF, Dunn AR: Mice lacking granulocyte
colony-stimulating factor have chronic neutropenia, granulocyte and
macrophage progenitor cell deficiency, and impaired neutrophil mobilization. Blood 84:1737, 1994
6. Wu H, Lui X, Jaenisch R, Lodish HF: Generation of committed
erythroid BFU-e and CFU-e does not require erythropoietin or the
erythropoietin receptor. Cell 80:613, 1995
7. Dong F, van Buitenen C, Pouwels K, Hoefsloot LH, Lowenberg B,
Touw IP: Distinct cytoplasmic regions of the human granulocyte
colony-stimulating factor receptor involved in induction of proliferation
and maturation. Mol Cell Biol 13:7774, 1993
351
8. Fukunaga R, Ishizaka Ikeda E, Nagata S: Growth and differentiation signals mediated by different regions in the cytoplasmic domain of
granulocyte colony-stimulating factor receptor. Cell 74:1079, 1993
9. McKinstry WJ, Li CL, Rasko JE, Nicola NA, Johnson GR,
Metcalf D: Cytokine receptor expression on hematopoietic stem and
progenitor cells. Blood 89:65, 1997
10. Metcalf D, Johnson GR, Burgess AW: Direct stimulation by
purified GM-CSF of the proliferation of multipotential and erythroid
precursor cells. Blood 55:138, 1980
11. Hu M, Krause D, Greaves M, Sharkis S, Dexter M, Heyworth C,
Enver T: Multilineage gene expression precedes commitment in the
hemopoietic system. Genes Dev 11:774, 1997
12. Ogawa M: Differentiation and proliferation of hematopoietic
stem cells. Blood 81:2844, 1993
13. Pharr PN, Ogawa M, Hofbauer A, Longmore GD: Expression of
an activated erythropoietin or a colony-stimulating factor 1 receptor by
pluripotent progenitors enhances colony formation but does not induce
differentiation. Proc Natl Acad Sci USA 91:7482, 1994
14. Borzillo GV, Ashmun RA, Sherr CJ: Macrophage lineage
switching of murine early pre-B lymphoid cells expressing transduced
fms genes. Mol Cell Biol 10:2703, 1990
15. Lieschke GJ: CSF-deficient mice—What have they taught us?
Ciba Found Symp 204:60, 1997
16. Seymour JF, Lieschke GJ, Grail D, Quilici C, Hodgson G, Dunn
AR: Mice lacking both granulocyte colony-stimulating factor (CSF)
and granulocyte-macrophage CSF have impaired reproductive capacity,
perturbed neonatal granulopoiesis, lung disease, amyloidosis, and
reduced long-term survival. Blood 90:3037, 1997
17. Stoffel R, Ledermann B, de Sauvage FJ, Skoda RC: Evidence for
a selective-permissive role of cytokine receptors in hematopoietic cell
fate decisions. Blood 90:123a, 1997 (abstr, suppl 1)
18. Shivdasani RA, Orkin SH: The transcriptional control of hematopoiesis. Blood 87:4025, 1996
19. Kulessa H, Frampton J, Graf T: GATA-1 reprograms avian
myelomonocytic cell lines into eosinophils, thromboblasts, and erythroblasts. Genes Dev 9:1250, 1995
20. Sieweke MH, Tekotte H, Frampton J, Graf T: MafB is an
interaction partner and repressor of Ets-1 that inhibits erythroid
differentiation. Cell 85:49, 1996
21. Ford AM, Healy LE, Bennett CA, Navarro E, Spooncer E,
Greaves MF: Multilineage phenotypes of interleukin-3-dependent progenitor cells. Blood 79:1962, 1992
22. Cross MA, Heyworth CM, Dexter TM: How do stem cells decide
what to do? Ciba Found Symp 204:3, 1997
23. Brown KE, Guest SS, Smale ST, Hahm K, Merkenschlager M,
Fisher AG: Association of transcriptionally silent genes with Ikaros
complexes at centrometric heterochromatin. Cell 91:845, 1997
24. Jimenez G, Griffiths SD, Ford AM, Greaves MF, Enver T:
Activation of the beta-globin locus control region precedes commitment
to the erythroid lineage. Proc Natl Acad Sci USA 89:10618, 1992
25. Ford AM, Bennett CA, Healy LE, Navarro E, Spooncer E,
Greaves MF: Immunoglobulin heavy-chain and CD3 delta-chain gene
enhancers are DNase I-hypersensitive in hemopoietic progenitor cells.
Proc Natl Acad Sci USA 89:3424, 1992
26. Heyworth CM, Dexter TM, Nicholls SE, Whetton AD: Protein
kinase C activators can interact synergistically with granulocyte colonystimulating factor or interleukin-6 to stimulate colony formation from
enriched granulocyte-macrophage colony-forming cells. Blood 81:894,
1993
27. Whetton AD, Heyworth CM, Nicholls SE, Evans CA, Lord JM,
Dexter TM, Owen-Lynch PJ: Cytokine-mediated protein kinase C
activation is a signal for lineage determination in bipotential granulocyte macrophage colony-forming cells. J Cell Biol 125:651, 1994
28. Metcalf D, Burgess AW: Clonal analysis of progenitor cell
commitment of granulocyte or macrophage production. J Cell Physiol
111:275, 1982
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1998 92: 348-351
Do Stem Cells Play Dice?
T. Enver, C.M. Heyworth and T.M. Dexter
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