Modulation of Aldehyde Dehydrogenase and Retinoid Signaling Induces the Expansion of Human Hematopoietic Stem Cells.

Aldehyde dehydrogenases are cytosolic enzymes that convert aldehydes into carboxylic acids. Human aldehyde dehydrogenase 1 (ALDH1) is highly expressed in the liver and in hematopoietic stem cells (HSCs). Although ALDH1 is a selectable marker of HSCs, its HSC-specific function is unknown. We hypothesized that ALDH might play a critical role in HSC fate determinations since it is required for the production of retinoic acids, which are broadly implicated in tissue differentiation, tissue patterning and embryonic development in vertebrates. In this study, highly purified human CD34+CD38−lin− HSCs were cultivated with early acting cytokines, thrombopoietin, stem cell factor and Flt-3 ligand (TSF) in the presence or absence of the competitive ALDH inhibitor, diethylaminobenzaldehyde (DEAB). Treatment of human BM and CB HSCs with TSF x 7 days caused a loss of CD34+CD38− cells in culture, morphologic differentiation, amplification of committed colony forming cells (CFCs) and a loss of primitive cells capable of repopulating non-obese diabetic/severe combined immunedeficient mice (SCID-Repopulating Cells, SRCs). Conversely, 7 day culture of BM and CB HSCs with TSF plus 100 μM DEAB blocked the morphologic differentiation and lineage commitment of HSCs in culture, expanded the CD34+CD38− population, and amplified SRCs 2-fold compared to input, indicating a fundamental role for ALDH in HSC differentiation. The effects of DEAB could be reversed by the co-administration of the retinoic acid receptor (RAR) agonist, all-trans retinoic acid (ATRA), suggesting that the ability of ALDH to produce retinoic acids is important in determining HSC fate. Via screening studies of direct ligands of RAR and RXR, we also identified a selective RXR modulator which functioned similarly to DEAB by impeding HSC differentiation and causing the 4-fold expansion of SRCs. Interestingly, treatment with either DEAB or the RXR modulator reversed the down-modulation of HOXB4 transcription that was otherwise observed in CD34+CD38−lin- cells during culture with cytokines, suggesting that inhibition of ALDH or RXR modulation may promote HSC self-renewal via discrete interactions with other regulatory pathways. Modulation of ALDH activity and retinoid signaling is a novel and effective strategy to amplify human HSCs.