Antiapoptotic protein Bcl-x(L) is up-regulated during megakaryocytic differentiation of CD34(+) progenitors but is absent from senescent megakaryocytes

Objective. The expression of Bcl-x(L) has been shown to be regulated during the maturation process of different hematopoietic cell Lineages (i.e., ery throid cells, neutrophils, monocytes/macrophages). In the present study, we examined the expression of Bcl-x(L) in megakaryocytes derived from CD34(+) progenitors and in the megakaryoblastic cell line UT7. Materials and Methods. Expression of Bcl-x(L) was analyzed in CD41(+) cells cultured in the presence of thrombopoietin and in UT7 cells treated with p horbol diester by Western blot, flow cytometry, and immunocytochemistry ana lysis. Apoptosis was determined at different culture times by terminal deox ynucleotidyl transferase-mediated dUTP-biotin nick end labeling and propidi um iodide uptake. Results. Bcl-x(L) but not Bcl-2 was up-regulated in the megakaryocytic popu lation (CD41(+)) during the first 15 days of culture, which was consistent with the pattern of Bcl-x(L) expression in UT7 cells differentiated to mega karyocytes by incubation with phorbol diester, However, by day 20 of cultur e, the levels of Bcl-x(L) in CD41(+) cells were greatly reduced, and this e xpression pattern was accompanied by an increase in the number of apoptotic cells. At this culture time, we detected the presence of cytoplasmic fragm ents resembling proplatelets with prominent Bcl-x immunostaining, most like ly due to the Bcl-x(L) isoform, in close proximity to Ecl-x(L) senescent me gakaryocytes. The presence of Bcl-x(L) but not of Bcl-2 in platelets was co nfirmed by Western blot analysis. Conclusion. Although little is known regarding the functional significance of survival proteins T within the megakaryocytic compartment, the changes i n the Bcl-x(L) expression pattern observed in UT7 and CD41(+) cells may pla y a role in the survival of developing megakaryocytes and the lifespan of m ature platelets. (C) 2001 International Society for Experimental Hematology , Published by Elsevier Science Inc.