Correspondence

Signal Transduction in Platelets from Patients with Polycythemia Vera

N Engl J Med 1998; 339:127-128July 9, 1998

Article

To the Editor:

In the February 26 issue, Moliterno et al.1 report on the reduced expression of the thrombopoietin receptor Mpl as a new characteristic of polycythemia vera and idiopathic myelofibrosis. On the basis of this reduced expression, they suggest a subsequent failure of activation of the Janus kinase signaling system due to an unknown upstream defect, with subsequent loss of phosphorylation of JAK2 and STAT5. After stimulation of platelets from normal subjects with thrombopoietin or thrombin, the authors observed tyrosine phosphorylation of proteins of approximately 125, 95, and 85 kd. Interestingly, this phosphorylation pattern could be observed in the patients with polycythemia vera only after stimulation with thrombin but not with thrombopoietin. However, the identity of these proteins is not discussed.

Recently, it was shown that thrombopoietin and thrombin induce tyrosine phosphorylation of Vav, a 95-kd protein found mainly in hematopoietic cells, in the platelets of healthy subjects.2 The same authors also observed a constitutive association of the 28-kd adaptor protein Grb2 with Vav. Another recent observation is the increased tyrosine phosphorylation of the 85-kd regulatory subunit of phosphatidylinositol 3-kinase (PI3K) after thrombopoietin stimulation of the Mpl receptor.3 Since Vav induces tyrosine phosphorylation of PI3K in human B cells,4 it seems likely that it is also the activating kinase of PI3K in platelets. An Mpl receptor that had completely lost its capacity to activate JAK and STAT, as a result of a deletion mutation, was shown to activate Vav and other proteins through tyrosine phosphorylation after thrombopoietin stimulation.3

Taken together, these observations suggest that it is very likely that the tyrosine-phosphorylated proteins observed after thrombopoietin stimulation of normal platelets are p85, PI3K, and Vav. A candidate substrate for the increased tyrosine phosphorylation of a protein of about 125 kd might be a complex composed of Vav and Grb2, since the authors also report increased phosphorylation of a protein of about 28 kd after both thrombin and thrombopoietin stimulation; another candidate substrate could be p120 Cbl, which was also tyrosine-phosphorylated after stimulation of platelets with thrombopoietin.5

The most likely signaling pathway downstream of the Mpl receptor is therefore first mediated by Vav and second, or alternatively, by JAK2. Lack of expression of the Mpl receptor will result in impaired signal transduction due to nonphosphorylation of signaling proteins. The reported lack of phosphorylation of proteins with apparent sizes of 125, 95, and 85 kd, as reported by Moliterno et al., fits very well in this model.

Winand Lange, M.D.
Uwe M. Martens, M.D.
Cornelius F. Waller, M.D.
Medizinische Universitätsklinik Freiburg, D-79106 Freiburg, Germany

5 References

1. 1

   Moliterno AR, Hankins WD, Spivak JL. Impaired expression of the thrombopoietin receptor by platelets from patients with polycythemia vera. N Engl J Med 1998;338:572-580
   Full Text | Web of Science | Medline

2. 2

   Miyakawa Y, Oda A, Druker BJ, et al. Thrombopoietin and thrombin induce tyrosine phosphorylation of Vav in human blood platelets. Blood 1997;89:2789-2798
   Web of Science | Medline

3. 3

   Dorsch M, Fan PD, Danial NN, Rothman PB, Goff SP. The thrombopoietin receptor can mediate proliferation without activation of the Jak-STAT pathway. J Exp Med 1997;186:1947-1955
   CrossRef | Web of Science | Medline

4. 4

   Weng WK, Jarvis L, LeBien TW. Signaling through CD19 activates Vav/mitogen-activated protein kinase pathway and induces formation of a CD19/Vav/phosphatidylinositol 3-kinase complex in human B cell precursors. J Biol Chem 1994;325:14-21
   

5. 5

   Oda A, Ozaki K, Druker BJ, et al. p120c-cbl Is present in human blood platelets and is differentially involved in signaling by thrombopoietin and thrombin. Blood 1996;88:1330-1338
   Web of Science | Medline

Author/Editor Response

The authors reply:

To the Editor: We thank Lange et al. for their comments. As they indicate, the signal-transduction pathways initiated by Mpl after interaction with its cognate ligand, thrombopoietin, are both multivalent and redundant. Furthermore, thrombin can induce tyrosine phosphorylation of many of the same platelet proteins involved in the signaling cascade induced by thrombopoietin–Mpl, but presumably through a different mechanism.1,2

It was not our intention to imply that only the JAK–STAT pathway was involved in thrombopoietin-mediated signal transduction. Indeed, we were unable to identify the activation of TYK2 or the tyrosine phosphorylation of Shc after exposure of platelets from patients with polycythemia vera to thrombopoietin, implying that there was a generalized abnormality in signaling through Mpl that would include the pathways involving Ras, Vav, and p120 Cbl.3,4 Since our experiments with thrombin indicated that these pathways and the mechanisms to activate them were intact in platelets from patients with polycythemia vera, and since surface expression of Mpl was reduced both functionally and immunologically, we can conclude that thrombopoietin-mediated signal transduction is globally impaired in platelets of patients with polycythemia vera.

Alison R. Moliterno, M.D.
W. David Hankins, Ph.D.
Jerry L. Spivak, M.D.
Johns Hopkins University School of Medicine, Baltimore, MD 21205-2196

4 References

1. 1

   Rodriguez-Linares B, Watson SP. Phosphorylation of JAK2 in thrombin-stimulated human platelets. FEBS Lett 1994;352:335-338
   CrossRef | Web of Science | Medline

2. 2

   Cichowski K, Brugge JS, Brass LF. Thrombin receptor activation and integrin engagement stimulate tyrosine phosphorylation of the proto-oncogene product, p95^vav, in platelets. J Biol Chem 1996;271:7544-7550
   CrossRef | Web of Science | Medline

3. 3

   Sasaki K, Odai H, Hanazono Y, et al. TPO/c-mpl ligand induces tyrosine phosphorylation of multiple cellular proteins including proto-oncogene products, Vav and c-Cbl, and Ras signaling molecules. Biochem Biophys Res Commun 1995;216:338-347
   CrossRef | Web of Science | Medline

4. 4

   Moliterno AR, Siebel KE, Sun AY, Hankins WD, Spivak JL. A novel thrombopoietin signaling defect in polycythemia vera platelets. Stem Cells (in press).