Efficacy and Mechanisms of Apoptosis Induction by Simultaneous Inhibition of PI3K with GDC-0941 and Blockade of Bcl-2 (ABT-737) or FLT3 (Sorafenib) In AML Cells In the Hypoxic Bone Marrow Microenvironment

Field: Medicine (Health Sciences)

Abstract Abstract 777 Hypoxia and the interactions with bone marrow (BM) stromal cells have emerged as important mechanisms of leukemia cell survival and chemoresistance. The PI3K-Akt-mTOR-HIF1a signaling pathway has been shown to be activated in response to hypoxia in several cancer models including AML. Therefore, effective targeting of PI3K/Akt signaling pathway could suppress AML cell survival in the hypoxic BM microenvironment. Furthermore, concomitant intra-pathway blockade, or inhibition of the key pro-survival pathways may be more effective. In this study, we investigated the anti-leukemia effects and molecular mechanisms of apoptosis induction in the context of hypoxic bone marrow microenvironment by simultaneous use of a selective Class I PI3K inhibitor GDC-0941 (Genentech) with BH3 mimetic ABT-737 (Abbott) or FLT3 inhibitor sorafenib in AML cells. For hypoxia experiments, AML cells with FLT3-ITD mutation and wild-type FLT3 (FLT3-ITD: MOLM13, MV4;11, wt-FLT3: HL60) were cultured under 1.0% O2 for at least 14 days to assure their continuous proliferation and survival. Under hypoxia, more cells accumulated in G0/G1 phase, indicating that hypoxic conditions promote cell cycle quiescence in leukemic cells. GDC-0941 treatment in normoxia resulted in a reduction of cell proliferation with G0/G1 cell cycle arrest and minimal apoptosis induction, in a time and concentration-dependent manner (IC50 at 48 hrs; 0.7 μM for HL-60, 0.9 μM for MOLM13, 0.7 μM for MV4;11). In hypoxia, GDC-0941 further enhanced cell growth inhibition and G0/G1 cell cycle arrest. Importantly, co-culture with BM mesenchymal stroma cells (MSC), which protected AML cells from cytarabine induced apoptosis, did not affect cell growth inhibition by GDC-0941 both in normoxia and hypoxia. Combined treatment of GDC-0941 synergistically enhanced the ABT-737- or sorafenib induced apoptosis under both normoxic- and hypoxic conditions (p<0.05). Combination Index of GDC-0941/ABT-737 was 0.25 for HL-60 and 0.86 for MOLM13 cells; GDC-0941/sorafenib 0.58 for MV4;11 and 0.65 for MOLM13. In FLT3/ITD harboring MOLM13 and MV4;11 cells, ABT-737 and GDC-0941/ABT-737, or sorafenib and GDC-0941/sorafenib-induced apoptosis was partially reversed by MSC co-culture in normoxia, but not in hypoxia. We next examined the molecular mechanisms of apoptosis induction by simultaneous blockade of PI3K and mutant FLT3 in MOLM13 cells. GDC-0941 inhibited phospho-AktSer473, irrespective of oxygen tension. Hypoxia increased/induced phosphorylation of AktSer473, and of the pro-survival serine/threonine-protein kinase Pim-1, known to promote hypoxia-induced chemoresistance. Pim-1 is a downstream target of Akt and FLT-3 signaling that promotes cell survival and inhibits apoptosis through Bcl-2-dependent mechanisms. While GDC-0941 or ABT-737 alone did not affect hypoxia-induced Pim-1 levels, the GDC-0941/ABT-737 combination down-regulated Pim-1 by 86% (24 hrs). Treatment with sorafenib induced Pim-1 down-regulation by itself by 70%. Under hypoxia, expression levels of Bcl-2 family proteins Bcl-2 and BIM were not significantly changed by any treatment and cell culture conditions. In turn, Mcl-1 expression was upregulated by MSC co-cultures. Further, sorafenib alone but not GDC-0941 or ABT-737 decreased Mcl-1 protein levels, which was reversed by MSC co-culture in hypoxia; in turn, the GDC-0941/sorafenib or GDC-0941/ABT-737 combination potently suppressed Mcl-1 expression under MSC/hypoxia conditions. These changes were associated with the observed apoptotic responses. In summary, our findings indicate that hypoxic conditions of BM microenvironment result in stimulation of the pro-survival Akt and PIM kinase pathways and increase anti-apoptotic Mcl-1 protein in AML cells with mutated FLT3. In turn, simultaneous blockade of PI3K and FLT3 pathways, or of PI3K and Bcl-2 results in inhibition of these signaling modules and synergistic induction of apoptosis in AML. This data suggest that combined inhibition of Bcl-2 with PI3K or FLT3-ITD may constitute a targeted approach to eradicate chemoresistant AML cells sequestered in the hypoxic BM niches. Disclosures: No relevant conflicts of interest to declare.