br Introduction Hematopoietic stem cells and

Introduction Hematopoietic stem Pyridoxal 5 phosphate and leukemic stem cells (HSCs and LSCs, respectively) both have a capacity of self-renewal. Whereas HSCs give rise to all blood lineages during lifetime hematopoiesis, LSCs are responsible for the initiation and propagation of leukemia, as well as drug resistance and disease relapse after treatment-induced remission (Visvader and Lindeman, 2012). Chronic myelogenous leukemia (CML) is a quintessential LSC-driven myeloproliferative disorder that results from transformation of HSCs by the BCR-ABL oncoprotein (Bhatia et al., 2003). BCR-ABL has constitutive tyrosine-kinase activity, and tyrosine-kinase inhibitors (TKIs), such as imatinib, induce remissions and improve survival in CML patients in the chronic phase (CP). CML LSCs do not, however, appear to depend on the BCR-ABL kinase activity for survival, and they are less sensitive to TKIs (Corbin et al., 2011). Failure to eliminate LSCs necessitates continuous TKI treatment to sustain remission (Mahon et al., 2010); when TKI resistance develops, CML relapses and/or progresses to an accelerated phase (AP) and/or blast crisis with features of aggressive acute leukemia of the myeloid or lymphoid phenotype. Treatment options for AP or blast crisis CML are limited, but CP represents a therapeutic window where eradication of LSCs may lead to a cure. β-catenin, activated by Wnt ligands or prostaglandins, is implicated in HSC regulation (Castellone et al., 2005, Goessling et al., 2009, Malhotra and Kincade, 2009), and levels of β-catenin activation determine the impact on HSC activities (Luis et al., 2011). On the other hand, β-catenin is involved in many aspects of leukemogenesis, including development of LSCs in pre-clinical models of CML and acute myeloid leukemia (AML) (Jamieson et al., 2004, Wang et al., 2010, Zhao et al., 2007). β-catenin is also necessary for maintaining CML LSCs (Heidel et al., 2012) and is a contributing factor to TKI resistance (Hu et al., 2009) and progression to blast crisis CML (Neviani et al., 2013, Scheller et al., 2013). Aberrant activation of β-catenin is a hallmark of tumor initiation, progression, and metastasis, making β-catenin a sought-after drug target in cancer therapy (Anastas and Moon, 2013). In a CML mouse model, blocking prostaglandin production diminishes β-catenin expression in CML LSCs and extends survival of CML mice in tertiary recipients (Heidel et al., 2012). Upon activation, β-catenin translocates into the nucleus, where it interacts with Tcf/Lef transcription factors to modulate gene expression (Staal et al., 2008, Xue and Zhao, 2012). Recently, we showed that two members of the Tcf/Lef family, Tcf1 and Lef1, are expressed in HSCs. Whereas HSCs require Tcf1/Lef1 for regenerative fitness, LSCs are more strongly dependent on both factors for self-renewal than HSCs (Yu et al., 2016). In the present study, we profiled Tcf1/Lef1 downstream genes in CML LSCs; and, in search of small molecules that simulate gene expression changes caused by Tcf1/Lef1 deficiency using the Connectivity Map, we identified prostaglandin E1 (PGE1). In both pre-clinical and xenograft models, PGE1 treatment greatly diminished the activity and persistence of CML LSCs. The action of PGE1 is mechanistically distinct from that of prostaglandin E2 (PGE2), despite their structural similarity. Whereas PGE2 stimulates β-catenin accumulation, PGE1 acts through E-prostanoid receptor 4 (EP4) and represses AP-1 factors in LSCs in a β-catenin-independent manner. Therefore, activating the “EP4-AP-1 repression” pathway represents a different approach from inhibiting the “PGE2-β-catenin activation” pathway to effectively subvert LSCs. PGE1 is a US Food and Drug Administration (FDA)-approved drug clinically known as alprostadil, and our study indicates that PGE1 can be repositioned in combination with TKIs for a more effective CML therapy, alleviating CML patients’ lifetime dependence on TKIs.