? The?Journal?of?Clinical?Investigation http://www.jci.org Volume 115 Number 12 December 2005
The molecular mechanisms
that control thrombopoiesis
Department of Medicine, Division of Hematology/Oncology, University of California, San Diego, San Diego, California, USA.
Overview of platelets and thrombopoiesis
An adequate supply of platelets is essential to repair the minute
vascular damage that occurs with daily life, and to initiate throm-
bus formation in the event of overt vascular injury. Accumulating
evidence also indicates vital roles for platelets in wound repair, the
innate immune response, and metastatic tumor cell biology. The
average platelet count in humans ranges from 150 × 109 to 400 × 109
per liter, although the level for any individual is maintained within
fairly narrow limits from day to day. While 150 × 109 to 400 × 109
per liter is considered “normal,” the values derived from the
mean ± 2 SDs of a group of “healthy” individuals, epidemiological
evidence indicates that individuals who display platelet counts in
the highest quartile of the normal range have a 2-fold increased
risk of adverse cardiovascular events (1), and, in both experimen-
tal animal models of metastatic cancer and patients with tumors,
higher platelet levels carry an unfavorable prognosis (2).
With a lifespan of approximately 10 days, a blood volume of
5 liters, and one-third of platelets pooled in the spleen, the average
adult must produce each day approximately 1 × 1011 platelets to
maintain a normal platelet count under steady-state conditions,
a level of production that can increase more than 10-fold under
conditions of increased demand. The primary regulator of platelet
production is thrombopoietin, an acidic glycoprotein produced
primarily in the liver, kidney, and BM. The biochemistry and struc-
ture-activity relationships of thrombopoietin have been carefully
evaluated, as have the binding sites to its receptor, the product of
the cellular protooncogene c-Mpl (3, 4). This Review will focus on
the regulation of platelet production, how thrombopoietin stimu-
lates thrombopoiesis under normal and pathological conditions,
and the molecular mechanisms through which the hormone pro-
duces its biological effects. In addition to providing an understand-
ing of normal physiology, the discovery of the biological effects of
thrombopoietin and its receptor provides a platform on which to
understand a number of clinical disorders of hematopoiesis and
serves to suggest novel therapeutic approaches to several diseases.
Discovery of thrombopoietin and the c-Mpl receptor
Evidence for a humoral regulator of thrombopoiesis was first pro-
vided in the late 1950s when plasma from bleeding or thrombocy-
topenic rats was found to induce thrombocytosis when transfused
into secondary animals. Based on ongoing work in erythropoiesis,
the term “thrombopoietin” was first coined in 1958 to describe the
humoral substance responsible for increasing platelet production
(5). In the late 1960s, in vivo assays were developed to detect throm-
bopoietin (6) but were cumbersome, hampering its purification.
In the 1980s, in vitro megakaryocyte colony–forming assays were
developed, allowing the identification of megakaryocyte colony–
stimulating factors, but whether the identified substances, such
as IL-3, GM-CSF, IL-6, or IL-11, were the same as thrombopoietin
remained controversial (7).
Occasionally in science, findings from one discipline spur an
entirely distinct field of investigation. The description of the
murine myeloproliferative leukemia virus in 1986 (8) and its
corresponding oncogene (v-Mpl) and cellular protooncogene
(c-Mpl) in the early 1990s (9, 10) had such an effect on the search
for thrombopoietin. Based on a number of shared primary and
secondary structural features, c-Mpl was identified as an orphan
member of the hematopoietic cytokine receptor family of pro-
teins, leading to the cloning of its ligand, thrombopoietin, 2 years
later (reviewed in ref. 11). Initial experiments with the recombi-
nant protein established thrombopoietin as the primary physi-
ological regulator of thrombopoiesis, as its levels were inversely
related to platelet or megakaryocyte mass and its infusion mas-
sively increased platelet production (12).
Regulation of thrombopoietin production
Investigators have found that blood and marrow levels of thrombo-
poietin are inversely related to platelet count. Patients with aplas-
tic anemia or thrombocytopenia secondary to myelosuppressive
therapy display high levels of the hormone (13, 14). However, there
are some notable exceptions to this relationship. The first is seen in
states of platelet destruction, where levels of the hormone are not
as high as would be anticipated from the degree of thrombocytope-
nia (15, 16). Such instances, seen in most patients with idiopathic
thrombocytopenic purpura, are characterized by megakaryocyte
hypertrophy, which likely contributes to thrombopoietin regulation
(17). A second instance in which thrombopoietin levels are not accu-
rately predicted by blood platelet count is in patients with inflam-
Nonstandard?abbreviations?used: ET, essential thrombocythemia; GSK3β, glycogen
synthase kinase-3β; IMF, idiopathic myelofibrosis; ORF, open reading frame; PV, poly-
cythemia vera; SDF-1, stromal cell–derived factor-1.
Conflict?of?interest: The author has declared that no conflict of interest exists.
Citation?for?this?article: J. Clin. Invest. 115:3339–3347 (2005).
3340? The?Journal?of?Clinical?Investigation http://www.jci.org Volume 115 Number 12 December 2005
matory, reactive thrombocytosis, where levels of the hormone are
higher than expected (18–21). However, an important unanswered
question is whether alterations in thrombopoietin production
explain the thrombocytosis associated with iron deficiency.
A major component of thrombopoietin regulation is achieved by
receptor-mediated uptake and destruction (Figure 1), a mechanism
of hematopoietic growth factor regulation first established for
M-CSF (22). Platelets bear high-affinity thrombopoietin receptors
that remove the hormone from solution (23), thereby establishing
an autoregulatory loop; as platelet counts rise, they remove more of
the hormone from the circulation, driving levels down, whereas in
thrombocytopenic states there are less platelets to adsorb throm-
bopoietin, allowing levels to rise and drive increased thrombopoi-
esis. However, not all thrombopoietin receptors contribute to this
effect; while endothelial cells display c-Mpl receptors (24) and some
endothelial cell types proliferate or migrate in response to the hor-
mone (25), transplantation studies have shown that endothelial cell
c-Mpl does not materially affect thrombopoietin levels despite a
100-fold more expansive cell surface (and predicted greater c-Mpl
mass) than that displayed by the totality of megakaryocytes and
platelets (26). Therefore, the mere presence of c-Mpl does not guar-
antee that it is involved in regulating thrombopoietin blood levels.
A second exception to the relatively simple platelet-adsorption-
and-destruction model of platelet homeostasis is illustrated by the
physiological response to severe thrombocytopenia; studies in both
mice and humans show that while marrow stromal cells display very
little thrombopoietin mRNA under normal conditions, transcripts
for the cytokine greatly increase in the presence of thrombocyto-
penia (Figure 1) (27). The precise humoral or cellular mediators of
this effect are under intense study. For example, in one report, the
platelet α-granule proteins PDGF and FGF-2 increased, but platelet
factor 4, thrombospondin, and TGF-β decreased thrombopoietin
production from primary human BM stromal cells (28). However,
others have reported that HGF is responsible for thrombopoietin
production from hepatocytes (29).
A third mechanism of thrombopoietin regulation occurs in
states of reactive thrombocytosis, where hormone concentrations
are higher than that predicted by the degree of thrombocytosis. For
example, inflammatory stimuli affect thrombopoietin production,
with the acute-phase response mediator IL-6 increasing thrombo-
poietin transcription from the liver (30). These in vitro effects are
also seen in vivo; administration of IL-6 to mice or cancer patients
increases thrombopoietin-specific mRNA in the liver and levels of
the hormone in the blood. Since an anti-thrombopoietin antibody
neutralizes the thrombopoietic effects of administered IL-6 (30), it
is now clear that thrombopoietin is the final mediator of inflam-
Additional modes of thrombopoietic regulation
In addition to thrombopoietin, additional factors likely influence
thrombopoiesis, as the genetic elimination of thrombopoietin or
its receptor leads to profound but not absolute thrombocytopenia
(the platelet counts in these settings are about 10% of a normal
level). In order to determine whether any of the known hema-
topoietic cytokines contributes to the residual thrombopoiesis
in the c-Mpl–null state, such mice have been crossed with other
cytokine- or cytokine receptor–deficient animals; from these stud-
ies it is clear that IL-3 (31, 32), IL-6, IL-11, and LIF (33) are not
basal, physiological mediators of thrombopoiesis. However, the
chemokine stromal cell–derived factor-1 (SDF-1) exerts numerous
influences on megakaryopoiesis, and studies indicate that it may
be responsible for thrombopoiesis not related to thrombopoietin.
The regulation of thrombopoietin levels. A steady-state amount of
hepatic thrombopoietin (TPO) is regulated by platelet c-Mpl receptor–
mediated uptake and destruction of the hormone. Hepatic production
of the hormone is depicted. Upon binding to platelet c-Mpl receptors,
the hormone is removed from the circulation and destroyed, which
reduces blood levels. In the presence of inflammation, IL-6 is released
from macrophages and, through TNF-α stimulation, from fibroblasts
and circulates to the liver to enhance thrombopoietin production.
Thrombocytopenia also leads to enhanced marrow stromal cell pro-
duction of thrombopoietin, although the molecular mediator(s) of this
effect is not yet completely understood.
? The?Journal?of?Clinical?Investigation http://www.jci.org Volume 115 Number 12 December 2005
Fibbe, W.E. 1993. Continuous infusion of inter-
leukin-6 in sublethally irradiated mice accelerates
platelet reconstitution and the recovery of myeloid
but not of megakaryocytic progenitor cells in bone
marrow. Exp. Hematol. 21:1621–1627.
80. Sosman, J.A., et al. 1997. Concurrent phase I tri-
als of intravenous interleukin 6 in solid tumor
patients: reversible dose-limiting neurological tox-
icity. Clin. Cancer Res. 3:39–46.
81. Kozak, M. 2001. Constraints on reinitiation of trans-
lation in mammals. Nucleic Acids Res. 29:5226–5232.
82. Cazzola, M., and Skoda, R.C. 2000. Translational
pathophysiology: a novel molecular mechanism of
human disease. Blood. 95:3280–3288.
83. Ihara, K., et al. 1999. Identification of mutations
in the c-mpl gene in congenital amegakaryocytic
thrombocytopenia. Proc. Natl. Acad. Sci. U. S. A.
84. van den Oudenrijn, S., et al. 2000. Mutations in the
thrombopoietin receptor, Mpl, in children with
congenital amegakaryocytic thrombocytopenia.
Br. J. Haematol. 110:441–448.
85. Ballmaier, M., Germeshausen, M., Krukemeier, S.,
and Welte, K. 2003. Thrombopoietin is essential
for the maintenance of normal hematopoiesis
in humans: development of aplastic anemia in
patients with congenital amegakaryocytic throm-
bocytopenia. Ann. N. Y. Acad. Sci. 996:17–25.
86. Sabath, D.F., Kaushansky, K., and Broudy, V.C.
1999. Deletion of the membrane-distal cytokine
receptor homology domain of MPL results in con-
stitutive cell growth and loss of thrombopoietin
binding. Blood. 94:365–367.
87. Onishi, M., et al. 1996. Identification of an oncogen-
ic form of the thrombopoietin receptor using retro-
virus-mediated gene transfer. Blood. 88:1399–1406.
88. Ding, J., et al. 2004. Familial essential thrombo-
cythemia associated with a dominant-positive
activating mutation of the c-MPL gene, which
encodes for the receptor for thrombopoietin. Blood.
89. Moliterno, A.R., et al. 2004. Mpl Baltimore: a
thrombopoietin receptor polymorphism associat-
ed with thrombocytosis. Proc. Natl. Acad. Sci. U. S. A.
90. Spivak, J.L., et al. 2003. Chronic myeloproliferative
disorders. Hematology (Am. Soc. Hematol. Educ. Pro-
91. Cocault, L., et al. 1996. Ectopic expression of
murine TPO receptor (c-mpl) in mice is pathogen-
ic and induces erythroblastic proliferation. Blood.
92. Yan, X.-Q., et al. 1996. A model of myelofibrosis and
osteosclerosis in mice induced by overexpressing
thrombopoietin (mpl ligand): reversal of disease
by bone marrow transplant. Blood. 88:402–409.
93. Tsui, H.W., Siminovitch, K.A., de Souza, L., and
Tsui, F.W. 1993. Motheaten and viable motheaten
mice have mutations in the haematopoietic cell
phosphatase gene. Nat. Genet. 4:124–129.
94. Helgason, C.D., et al. 1998. Targeted disruption
of SHIP leads to hemopoietic perturbations, lung
pathology, and a shortened life span. Genes Dev.
95. Gitler, A.D., et al. 2004. Tie2-Cre-induced inactiva-
tion of a conditional mutant Nf1 allele in mouse
results in a myeloproliferative disorder that mod-
els juvenile myelomonocytic leukemia. Pediatr. Res.
96. Chan, I.T., et al. 2004. Conditional expression of
oncogenic K-ras from its endogenous promoter
induces a myeloproliferative disease. J. Clin. Invest.
97. Roder, S., Steimle, C., Meinhardt, G., and Pahl,
H.L. 2001. STAT3 is constitutively active in some
patients with Polycythemia rubra vera. Exp. Hema-
98. Silva, M., et al. 1998. Expression of Bcl-x in ery-
throid precursors from patients with polycythemia
vera. N. Engl. J. Med. 338:564–571.
99. Dai, C., Chung, I.J., and Krantz, S.B. 2005. Increased
erythropoiesis in polycythemia vera is associated
with increased erythroid progenitor proliferation
and increased phosphorylation of Akt/PKB. Exp.
100. Zanjani, E.D., Lutton, J.D., Hoffman, R., and Was-
serman, L.R. 1997. Erythroid colony formation by
polycythemia vera bone marrow in vitro. Depen-
dence on erythropoietin. J. Clin. Invest. 59:841–848.
101. Dai, C.H., et al. 1992. Polycythemia vera. II. Hyper-
sensitivity of bone marrow erythroid, granulocyte-
macrophage, and megakaryocyte progenitor cells
to interleukin-3 and granulocyte-macrophage
colony-stimulating factor. Blood. 80:891–899.
102. Axelrad, A.A., Eskinazi, D., Correa, P.N., and Amato,
D. 2000. Hypersensitivity of circulating progenitor
cells to megakaryocyte growth and development
factor (PEG-rHu MGDF) in essential thrombocy-
themia. Blood. 96:3310–3321.
103. Baxter, E.J., et al. 2005. Acquired mutation of the
tyrosine kinase JAK2 in human myeloproliferative
diseases. Lancet. 365:1054–1061.
104. Levine, R.L., et al. 2005. Activating mutation in the
tyrosine kinase JAK2 in polycythemia vera, essen-
tial thrombocythemia, and myeloid metaplasia
with myelofibrosis. Cancer Cell. 7:387–397.
105. James, C., et al. 2005. A unique clonal JAK2 muta-
tion leading to constitutive signalling causes poly-
cythaemia vera. Nature. 434:1144–1148.
106. Kralovics, R., et al. 2005. A gain-of-function muta-
tion of JAK2 in myeloproliferative disorders.
N. Engl. J. Med. 352:1779–1790.
107. Zhao, R., et al. 2005. Identification of an acquired
JAK2 mutation in polycythemia vera. J. Biol. Chem.
108. Wright, J.H. 1906. The origin and nature of the
blood plates. Boston Med. Surg. J. 23:643–645.