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Blood, 1 September 2005, Vol. 106, No. 5, pp. 1857-1863.
Prepublished online as a Blood First Edition Paper on May 17, 2005; DOI 10.1182/blood-2004-11-4344.
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Submitted November 15, 2004
Accepted May 10, 2005
Bcl-xL prevents apoptosis of late-stage erythroblasts but does not mediate the anti-apoptotic effect of erythropoietin
Melissa M Rhodes, Prapaporn Kopsombut, Maurice C Bondurant, James O Price, and Mark J Koury*
Department of Pediatrics, Vanderbilt University Medical Center, Nashville, TN, USA
Department of Medicine, Vanderbilt University Medical Center, Nashville, TN, USA
Tennessee Valley Healthcare System VA Medical Center, Nashville, TN, USA; Department of Medicine, Vanderbilt University Medical Center, Nashville, TN, USA
Tennessee Valley Healthcare System VA Medical Center, Nashville, TN, USA; Department of Pathology, Vanderbilt University Medical Center, Nashville, TN, USA
* Corresponding author; email: mark.koury{at}vanderbilt.edu.
Bcl-xL, an outer mitochondrial membrane protein, has been proposed to mediate erythropoietin's anti-apoptotic action on erythroid progenitor cells and to be necessary for heme synthesis in erythroblasts. Mice with conditional knockout of Bcl-xL (conditional bcl-x -/- mice) develop severe anemia that has been attributed to hemolysis and that is accompanied by splenomegaly. We characterized further the anemia of conditional bcl-x -/- mice and investigated the role of Bcl-xL in erythropoietin's action and in heme synthesis. We analyzed peripheral blood cells and cultured splenic erythroblasts of conditional bcl-x -/- mice and littermates that were rendered anemic by bleeding. Although they had massive splenic erythroblastosis, conditional bcl-x -/- mice had decreased circulating reticulocytes compared to littermates even prior to bleeding the littermates. Compared to erythroblasts of bled littermates, bcl-x -/- erythroblasts cultured with erythropoietin underwent apoptosis during the later, hemoglobin-synthesizing stages of differentiation. bcl-x -/- erythroblasts synthesized heme, but at reduced rates compared to bled littermate erythroblasts. When cultured without erythropoietin, bcl-x -/- erythroblasts underwent apoptosis at early stages of differentiation, prior to hemoglobin synthesis. Bcl-xL is not required for heme synthesis and does not mediate erythropoietin's anti-apoptotic effects, but it prevents ineffective erythropoiesis due to apoptosis in late-stage, hemoglobin-synthesizing erythroblasts.

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