Submitted March 1, 2006
Accepted November 19, 2006
Spi-1/PU.1 participates in erythroleukemogenesis by inhibiting apoptosis in cooperation with Epo signaling and by blocking erythroid differentiation
Pauline Rimmele, Olivier Kosmider, Patrick Mayeux, Francoise Moreau-Gachelin, and Christel Guillouf*
Institut Curie, Institut National de la Sante Et de la Recherche Medicale (Inserm) U528, Paris, France
Institut Cochin, Paris, France
* Corresponding author; email: guillouf{at}curie.fr.
Overexpression of the transcription factor Spi-1/PU.1 in mice leads to acute erythroleukemia characterized by a differentiation block at the proerythroblastic stage. In this study, we made use of a new cellular system allowing to reach graded expression of Spi-1 in preleukemic cells to dissect mechanisms of Spi-1/PU-1 in erythroleukemogenesis. This system is based on conditional production of one or two spi-1 interfering RNAs stably inserted into spi-1 transgenic proerythroblasts. We show that Spi-1 knock-down was sufficient to reinstate the erythroid differentiation program. This differentiation process was associated with an exit from the cell cycle. Evidence is provided that in the presence of Epo, Spi-1 displays an anti-apoptotic role that is independent of its function in blocking erythroid differentiation. Apoptosis inhibited by Spi-1 did not involve activation of the Fas/FasL signaling pathway nor a failure to activate EpoR. Furthermore, we found that reducing the Spi-1 level yields to ERK dephosphorylation and increased phosphorylation of AKT and STAT5 suggesting that Spi-1 may affect major signaling pathways downstream the EpoR in erythroid cells. These findings reveal two distinct roles for Spi-1 during erythroleukemogenesis: Spi-1 blocks the erythroid differentiation program and acts to impair apoptotic death in cooperation with an Epo signaling.
