1. ¹Division of Hematology, Institute of Clinical Medicine, University of Tsukuba, Tsukuba, Ibaraki, Japan
2. ²Division of Medical Sciences, Institute of Clinical Medicine, University of Tsukuba, Tsukuba, Ibaraki, Japan

Correspondence: Dr H Kojima, Department of Clinical Oncology, Ibaraki Prefectural Central Hospital and Cancer Center, 6528 Koibuchi, Kasama, Ibaraki 309-1793, Japan. E-mail: h-kojima@chubyoin.pref.ibaraki.jp

³Current address: Department of Hematology, Tsukuba Memorial Hospital, Tsukuba, Ibaraki, Japan.

⁴Current address: Department of Clinical Oncology, Ibaraki Prefectural Central Hospital and Cancer Center, Kasama, Ibaraki, Japan.

Received 24 April 2008; Revised 23 November 2008; Accepted 29 December 2008; Published online 12 February 2009.

Abstract

To elucidate whether caspase activation is involved in megakaryopoiesis, we characterized megakaryocytes (MKs) in vav-bcl-2 transgenic (Tg) mice, in which Bcl-2 is overexpressed in hematopoietic cells. To exclude the effect of splenomegaly in Tg mice on megakaryopoiesis, splenectomy was performed. After splenectomy, basal platelet counts in peripheral blood were not significantly different between Tg and wild-type (WT) mice. However, when experimental thrombocytopenia was induced by injecting 5-fluorouracil into splenectomized mice, overshoot of platelet counts during the recovery phase was hardly observed in Tg mice. Analyses of MK ploidy during the recovery phase showed that MKs less than 16 N ploidy were significantly decreased in Tg mice, suggesting that MK supply from progenitors is impaired. Supporting this, differentiation of CD34-/c-kit+/Sca-1+/Lineage- stem cells into MKs was significantly hampered in Tg mice, whereas megakaryocyte-erythroid progenitors (MEPs) normally differentiated into MKs. It suggests that differentiation into MKs is impaired in Tg mice before the stage of MEP. Furthermore, MK colony formation in WT cells was dose-dependently inhibited in the presence of a caspase inhibitor. Contrary, Bcl-2-overexpressing MKs showed normal ability for in vitro platelet production. We thus believe that caspase activation is involved in the differentiation of progenitors into megakaryocytic lineage but not in platelet production.