Blood, Vol. 92 No. 6 (September 15), 1998:
pp. 2189-2191
CORRESPONDENCE
Primary Role of the Liver in Thrombopoietin Production
Shown by Tissue-Specific Knockout
 |
LETTER |
To the Editor:
Maintenance of an appropriate number of circulating platelets is
required for hemostasis and blood coagulation. The control of platelet
production is mediated by thrombopoietin (Tpo) through its effect on
the proliferation of megakaryocyte progenitors and on megakaryocyte
ploidy.1,2 The levels of Tpo available to stimulate
platelet production are thought to be directly regulated by the
platelet mass itself.3 In this model, Tpois constitutively produced and released in the circulation, where it is being cleared by
platelets through c-mpl-mediated binding.4 But, to date, the site(s) of Tpo production remains to be defined. Using Northern blot analysis, the liver and kidneys appear to be the major site of Tpo
mRNA expression.5 However, using more sensitive methods, such as reverse transcription-polymerase chain reaction
(RT-PCR), Tpo transcripts can be detected in most tissues,
including the bone marrow stroma, which could produce Tpo directly in
the megakaryocyte microenvironment. To evaluate the contribution of the
liver to the Tpo production required to maintain a normal platelet
count, we have generated tissue-specific knockout mice by transplanting the liver of Tpo-deficient mice6 into wild-type
recipients.7 As shown in Fig
1A, RT-PCR analysis of wild-type mice shows
the presence of Tpo transcripts in all tissue analyzed, whereas there is no detectable transcript in any tissue from Tpo-KO mice. In wild-type mice transplanted with a Tpo-KO liver, the Tpo transcripts were absent from the liver only. Circulating Tpo levels were assessed by a c-Mpl-dependent proliferation assay5 and were below
the detection limit in both wild-type and transplanted animals (data not shown). Blood cell counts analysis up to 12 weeks after the operation indicated that the circulating platelet levels of the transplanted mice were intermediate between those of normal and Tpo-KO
mice, suggesting that the liver contributes approximately 60% of the
Tpo required for maintenance of a normal platelet count (Fig 1B). All
other blood parameters analyzed, including red blood cells and total
white blood cells, were normal in animals transplanted with a wild-type
or Tpo-KO liver.
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| Fig 1.
RT-PCR analysis and platelet counts in liver specific
Tpo-KO mice. Liver from Tpo-KO mice or wild-type littermates were
transplanted in normal recipients, as previously described. (A) Tpo
mRNA expression was analyzed in the kidney (K), liver (L), muscle (M),
and spleen (S) by RT-PCR. (B) Platelet counts were measured from
retroorbital bleeds 12 weeks after transplant; n = 6 mice per
group.
|
|
Thrombocytopenia associated with liver cirrhosis has historically been
attributed to hypersplenism; however, lower than normal platelet counts
seem to persist in many patients, even after splenectomy or portal
decompression.8,9 The present results suggest that thrombocytopenia associated with liver disease such as cirrhosis may be
attributable to reduced Tpo production. This is consistent with the
finding that Tpo levels in these patients are not elevated even with
their reduced platelet count, suggesting a potential defect in Tpo
production.10 We have previously shown using quantitative PCR that Tpo mRNA levels were reduced in cirrhotic liver.10 The reduction in Tpo production may not be restricted to decrease of
mRNA but may be combined with a posttranscriptional decrease in active
protein, as has been documented for other liver-derived proteins.11 Our data demonstrate that the liver is the
major site of Tpo production and that altered hepatic Tpo production will lead to a significant reduction in platelet levels. Secondary sites of Tpo production are likely to include the kidneys, which express significant amount of Tpo mRNA and are also responsible for
erythropoietin production. The bone marrow stroma could provide part of
the remaining Tpo, because it would be directly delivered in the
megakaryocyte microenvironment.
Shinguang Qian
Fumin Fu
Wei Li
Thomas E. Starzl Transplantation
Institute
Department of Surgery
University of Pittsburgh Medical
Center
Pittsburgh, PA
Qi Chen
Frederic J. de
Sauvage
Department of Molecular Oncology
Genentech
Inc
South San Francisco, CA
| |
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