Thrombopoietin in thrombocytopenic mice: evidence against regulation at the mRNA level and for a direct regulatory role of platelets.
Thrombopoietin (TPO), originally described as an activity in the serum of thrombocytopenic animals that leads to increased production of platelets, has recently been isolated and cloned. Its closest relative in the cytokine superfamily, erythropoietin (EPO), is transcriptionally regulated during anemia, and it was expected that TPO would similarly be regulated during thrombocytopenia. We induced thrombocytopenia in mice and confirmed that TPO activity was upregulated, as determined by a bioassay. Liver and kidney were found to be the major sources of TPO mRNA. Surprisingly, TPO mRNA in these tissues was not upregulated in thrombocytopenic mice. Using a sensitive RNase protection assay that can distinguish between TPO isoforms, we found no change in the profile of mRNA for these isoforms. A semiquantitative reverse transcription-polymerase chain reaction assay also did not demonstrate upregulation of TPO mRNA in the spleen. Thus, the increase of TPO activity during thrombocytopenia is not caused by regulation at the level of TPO mRNA. Furthermore, isolated mouse platelets absorbed high amounts of bioactive TPO out of TPO-conditioned medium in a dose-dependent fashion. Our results are consistent with TPO protein being regulated at a posttranscriptional level and/or directly through absorption and metabolism by platelets.
Stoffel
R
R
Department of Pharmacology, University of Basel, Switzerland.
Wiestner
A
A
Skoda
R C
RC
D000818:Animals; D001483:Base Sequence; D001792:Blood Platelets / Q000276:immunology / Q000502:physiology*; D001921:Brain / Q000378:metabolism; D002460:Cell Line; D005786:Gene Expression Regulation; D007106:Immune Sera / Q000633:toxicity; D007668:Kidney / Q000378:metabolism; D008099:Liver / Q000378:metabolism; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D008969:Molecular Sequence Data; D009363:Neoplasm Proteins; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D012333:RNA, Messenger / Q000096:biosynthesis / Q000235:genetics; D011817:Rabbits; D011833:Radiation Injuries, Experimental / Q000503:physiopathology; D018121:Receptors, Cytokine; D053628:Receptors, Thrombopoietin; D011994:Recombinant Proteins / Q000378:metabolism; D013737:Testis / Q000378:metabolism; D013921:Thrombocytopenia / Q000209:etiology / Q000235:genetics / Q000503:physiopathology*; D013926:Thrombopoietin / Q000096:biosynthesis / Q000235:genetics / Q000502:physiology*; D014916:Whole-Body Irradiation / Q000009:adverse effects
D007106:Immune Sera; D009363:Neoplasm Proteins; D011518:Proto-Oncogene Proteins; D012333:RNA, Messenger; D018121:Receptors, Cytokine; D053628:Receptors, Thrombopoietin; D011994:Recombinant Proteins; C082218:MPL protein, human; D013926:Thrombopoietin