The SCL/TAL1 Transcription Factor Represses the Stress Protein DDiT4/REDD1 in Human Hematopoietic Stem/Progenitor Cells.

Hematopoietic stem/progenitor cells (HSPCs) are regulated through numerous molecular mechanisms that have not been interconnected. The transcription factor stem cell leukemia/T-cell acute leukemia 1 (TAL1) controls human HSPC but its mechanism of action is not clarified. In this study, we show that knockdown (KD) or short-term conditional over-expression (OE) of TAL1 in human HSPC ex vivo, respectively, blocks and maintains hematopoietic potentials, affecting proliferation of human HSPC. Comparative gene expression analyses of TAL1/KD and TAL1/OE human HSPC revealed modifications of cell cycle regulators as well as previously described TAL1 target genes. Interestingly an inverse correlation between TAL1 and DNA damage-induced transcript 4 (DDiT4/REDD1), an inhibitor of the mammalian target of rapamycin (mTOR) pathway, is uncovered. Low phosphorylation levels of mTOR target proteins in TAL1/KD HSPC confirmed an interplay between mTOR pathway and TAL1 in correlation with TAL1-mediated effects of HSPC proliferation. Finally chromatin immunoprecipitation experiments performed in human HSPC showed that DDiT4 is a direct TAL1 target gene. Functional analyses showed that TAL1 represses DDiT4 expression in HSPCs. These results pinpoint DDiT4/REDD1 as a novel target gene regulated by TAL1 in human HSPC and establish for the first time a link between TAL1 and the mTOR pathway in human early hematopoietic cells. Stem Cells 2015;33:2268-2279.