Transcription Factor Dosage: Maximal STAT5-Induced Proliferation and Stem Cell Self-Renewal at Intermediate STAT5 Activity Levels.

The level of transcription factor activity critically regulates cell fate decisions such as hematopoietic stem cell self-renewal and differentiation. Here, we describe a model in which STAT5 activity is introduced into human hematopoietic stem/progenitor cells in a dose-dependent manner by overexpression of tamoxifen-inducible STAT5A-ER fusion proteins that can be activated upon treatment with 4-OHT. For this study, we tested STAT5A(wt), STAT5A(1*6) and STAT5A(S711F)-ER fusion proteins. Introduction of STAT5A activity in cord blood-derived CD34+ cells resulted in impaired myelopoiesis and induction of erythropoiesis which was most pronounced at the highest STAT5A transactivation levels. In contrast, intermediate STAT5A activity levels resulted in the most pronounced proliferative advantage of CD34+ cells. This coincided with increased CAFC and LTC-IC frequencies that were predominantly elevated at intermediate STAT5A activity levels, but not at high STAT5A activity levels. Self-renewal of progenitors was addressed by serial replating of CFUs grown in methylcellulose, and only progenitors containing intermediate STAT5A activity levels contained self-renewal capacity. Extensive gene expression profiling in CB CD34+ cells over a range of STAT5A activation levels identified subsets of genes that were induced by low, intermediate and high STAT5A transcriptional activity. Thus, we could cluster gene expression levels of STAT5 target genes into a ‘differentiation signature’ versus a ‘proliferation/self-renewal signature’. In conclusion, our data suggest that intermediate rather than high STAT5 activity levels might be required to induce leukemic transformation.