Relationship Between Self-Renewal and Differentiation Pathways in Stem Cells and Leukemia

• Published in: Blood 2014-12, Vol.124 (21), p.4789-4789
• Main Authors: Welner, Robert S, Tenen, Danielle E, Yang, Henry, Bararia, Deepak, Amabile, Giovanni, Tenen, Daniel G.
• Format: Article
• Language: English
• ISSN: 0006-4971; 1528-0020
• DOI: 10.1182/blood.V124.21.4789.4789

Hematopoietic stem cells are capable of perpetual self-renewal and multi-lineage differentiation, properties that are maintained throughout life by minimal cell cycle activity. Our work has focused on deciphering transcriptional driven differentiation versus self-renewal pathways in stem and progenitor cells. To this end, we have studied transcription factors that control the fate of hematopoietic stem cells by combining mouse models of activated self-renewal with models that can report transcription factor expression. We chose to study the Wnt pathway, activated in several types of leukemia, in combination with the ets family PU.1 transcription factor, vital to almost all myeloid and lymphoid lineages. PU.1 regulates a number of important myeloid specific genes that mediate differentiation to a specific cell fate. To understand the interaction of these pathways, we found that over-expression of Wnt signaling or beta-catenin, the downstream signaling component of the Wnt pathway, was able to inhibit PU.1-mediated differentiation in a PU.1-inducible cell line. There was little to no up-regulation of the myeloid markers Mac1 or Gr1 with activation of Wnt signaling upon induction with 4-hydroxy-tamoxifen (4-OHT). Additionally, many genes related to myeloid differentiation were not increased as compared to control-induced cultures. To understand how these interactions might function in vitro, we crossed a Cre-responsive activated beta-catenin (floxed allele Exon3) mouse to a PU.1-GFP knock-in mouse. From this model, we are able to see changes in PU.1 (GFP) expression in specific populations of hematopoietic progenitors upon activation of beta-catenin. Most importantly, in the LT-HSCs (defined by Lin- cKitHi Sca1+ CD150+ CD48-), we observed a significant increase in GFP (PU.1) intensity upon activation of active beta-catenin. Additionally, there was an increase in the total number of LT-HSCs, as defined by surface markers. LT-HSCs with active beta-catenin and GFP (PU.1) were found to be more in cycle and they express lower levels of transcription factors related to differentiation. These results demonstrate that when beta-catenin is activated, PU.1’s role is modified and the self-renewal program is enhanced at the expense of differentiation. Furthermore, activation of beta-catenin in the hematopoietic cells of mice has been shown to lead to impaired differentiation and eventual death. Even though active beta-catenin has been shown to be essential in several sub