Abstract LB-109: BCR-ABL1 kinase activity but not its expression is dispensable for Ph+ quiescent stem cell survival which depends on the PP2A-controlled Jak2 activation and is sensitive to FTY720 treatment

Abstract

Background: The success of tyrosine kinase inhibitors (TKIs) depends on the addiction of Philadelphia-positive (Ph+) CML progenitors to BCR-ABL1 kinase activity. However, CML quiescent hematopoietic stem cells (HSC) are TKI-resistant and represent an active disease reservoir. We hypothesize that this innate drug-resistance depends on inhibition of the tumor suppressor protein phosphatase 2A (PP2A). PP2A can be reactivated by FTY720, a drug that targets CML but not normal progenitors. Here we investigated the mechanism controlling survival/self-renewal of quiescent leukemic HSCs and their sensitivity to PP2A-activating drugs. Methods: HSCs from CML (n=68) and healthy (n=12) donors were FACS-isolated, and the biologic importance of PP2A inhibition and pharmacologic PP2A activation on their survival/self-renewal was assessed by BM serial transplantation; CFSE and Annexin-V staining; LTC-IC and CFC/replating assays; lentiviral shRNA/cDNA-transduction; LEF/TCF and proximity-ligation assays; Western blot, confocal microscopy and FACS analyses. Results: We observed increased BCR-ABL1 expression with impaired kinase activity in quiescent CML HSCs, in which BCR-ABL1 per se is required for induction of JAK2 that subsequently activated β-catenin and inhibited PP2A. In fact, PP2A was suppressed in CML but not normal CD34+/CD38−/CD90+ HSCs. FTY720 and/or its non-immunosuppressive (S)-FTY720-OMe derivative markedly reduced survival and self-renewal of CML but not normal quiescent HSCs through BCR-ABL1 kinase-independent and PP2A-mediated JAK2 and β-catenin inhibition. Importantly, FTY720 also strongly diminished BCR-ABL1+ LT-HSC frequency in serial BM transplantation assays. Conclusions: The pharmacologic targeting of the newly-identified BCR-ABL1 kinase-independent JAK2/β-catenin interplay in quiescent HSCs with FTY720 and its derivatives, might lead to cessation of lifelong patient dependence on TKIs. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr LB-109. doi:10.1158/1538-7445.AM2011-LB-109

Protein phosphatase 2A: a target for anticancer therapy

“…LSCs in CML are characterized by aberrant activation of pathways that control the survival and self-renewal of normal stem cells, such as the wnt pathway(7). There is emerging evidence that PP2A is also inactivated in the LSC compartment in a BCR-ABL1-independent, Jak2-dependent manner and that treatment with PADs can impair their survival and self-renewal and weaken their ability to initiate and propagate CML in vivo without harming normal hematopoiesis(65, 66). …”

“…In fact, long-term FTY720 treatment (27 weeks) of leukemic animals extensively prolongs survival and restores normal myelopoiesis without exerting any toxic effects in hematopoietic and non-hematopoietic organs(53). Restoration of PP2A activity by FTY720 or its non-immunosuppressive derivatives ( S)-FTY720-OMe, (S)-FTY720-regioisomer and OSU-2S selectively suppresses the survival and self-renewal of CML but not normal quiescent HSCs and progenitors both in vitro and in animal models ( Neviani and Perrotti, manuscript submitted 2012)(66, 78–80). These FTY720 derivatives neither induce lymphopenia nor undergo phosphorylation or interact with the S1PR1 receptor but still activate PP2A(66, 80).…”

Minimal Residual Disease and Discontinuation of Therapy in Chronic Myeloid Leukemia: Can We Aim at a Cure?

“…FTY720 treatment of quiescent BCR-ABL ϩ stem cells degraded ␤-catenin and induced apoptosis, with no effect on normal stem cells. 45,46 One of the most important pathways for self-renewal is the Wnt-␤-catenin pathway. Inhibitors of Smo, a molecule in the Hedgehog pathway, have stem-cell-targeting activity, and the combination of LDE 225 and nilotinib in vitro resulted in a decrease in CML stem cells in a transgenic mouse model.…”

Chronic myeloid leukemia: the basis of treatment for tomorrow

“…6 Recent evidence suggests that FTY720 also targets other kinases through PP2A. 8 In primitive CML cells, the apoptotic effect of FTY720 might not require BCR-ABL1 activity 7 which, as reported, is not essential for their survival. 10 Indeed, it seems that alternative signaling pathways which require the expression of BCR-ABL1 for their activation and/or maintenance are necessary for the effect of FTY720 in the most primitive CML cells.…”

“…[6][7][8][9] The question arises whether BCR-ABL1 is necessary for this drug to work. In CML progenitors, FTY720 induces apoptosis because of the ability of active PP2A to simultaneously impair BCR-ABL1 activity/expression.…”