Hypoxia Inducible Factor (HIF)-2α Enhances Proliferation Of Malignant Hematopoietic Cells In The Hypoxic Malignant Bone Marrow

Hypoxia and hypoxia-inducible factors (HIFs) are implicated in the regulation of normal and malignant hematopoiesis. HIF-1α stabilization makes leukemia stem cells and normal HSC dormant and is necessary to maintain their self-renewal potential. In sharp contrast, HIF-2α, which shares 60% homology with HIF-1a, promotes proliferation of renal clear carcinoma and embryonic stem cells by enhancing expression of oct-4, sox2 and activating c-myc. In this study, we investigated the role of hypoxia and HIF-2α in leukemia. In normal mouse and human bone marrow (BM), HIF-2α mRNA expression was observed predominantly in non-hematopoietic stromal cells, while hematopoietic cells displayed low to undetectable levels. In contrast, HIF-2α mRNA and protein were detected in the BM of moribund NOD/SCID mice engrafted with 3 different human ALL, and in cultured human ALL and AML cell lines, suggesting that HIF-2α is abnormally expressed in leukemic cells. To investigate the potential roles of HIF-2α in leukemic cells, we cloned human HIF-2α cDNA into the MXIE retroviral vector. In a 1st model the GM-CSF-dependent mouse pre-leukemic cell line FDCP1, which does not express HIF-2α, was retrovirally transduced with HIF-2α. HIF-2α provided a significant proliferative advantage to FDCP1 cells in hypoxic or normoxic cultures and reduced GM-CSF dependency. We next transplanted retrovirally transduced FDCP1 cells into non-irradiated syngeneic DBA/2 mice. All recipients of FDCP1 transduced with HIF-2α-MXIE vector succumbed to leukemia by week 28 post-transplantation. In sharp contrast, mice receiving FDCP1 transduced with empty MXIE vector, displayed a leukemia penetrance of only 15% by week 45 (Fig. 1a; p=0.0001 log rank, hazard ratio = 12.28).Fig. 1Percent survival of recipients of (a) FDCP1 cells retrovirally transduced with HIF-2α-MXIE vector or MXIE control empty vector, (b) vavBcl2 HSC transduced with HIF-2α-MXIE vector or MXIE empty vector, and (c) HL60 cells transduced with HIF-2α knocked-down or scrambled control lentiviral vectors.Fig. 1. Percent survival of recipients of (a) FDCP1 cells retrovirally transduced with HIF-2α-MXIE vector or MXIE control empty vector, (b) vavBcl2 HSC transduced with HIF-2α-MXIE vector or MXIE empty vector, and (c) HL60 cells transduced with HIF-2α knocked-down or scrambled control lentiviral vectors. In a 2nd model, HSC from vavBcl2 transgenic mice were transduced with human HIF-2α-containing or empty MXIE retroviral vectors and subsequently transplanted into lethally irradiated wild-type recipients. Transduction of vavBcl2 HSC with HIF-2α resulted in the outgrowth of HIF-2α-expressing B cells which was not observed in recipients of vavBcl2 HSC transduced with empty vector. Consequently recipients of HIF-2α transduced vavBcl2 HSC succumbed more rapidly to spontaneous lymphoma compared to controls (Fig. 1b; p=0.036 log rank, hazard ratio = 2.971, MXIE median survival = 56 weeks, HIF2α median survival = 41 weeks). Finally, HIF-2α was knocked-down in human leukemia cell lines U937 and HL60 using a shRNA lentiviral vector. HIF-2α knock-down resulted in a 2-fold decrease in proliferation in vitro. We next transplanted HL60-HIF-2a shRNA and HL60-scrambled shRNA cells into NOD/SCID/ IL2Rγ-/- (NSG) mice for each group. Notably, all recipients of HL60-HIF-2a shRNA cells succumbed to leukemia significantly later than recipients of HL60-scrambled shRNA cells (Fig. 1c; p=<0.027 Log-rank, hazard ratio = 0.1918, Scrambled median survival = 5 weeks, HIF-2α knock down median survival = 6 weeks). Together these data suggest that expression of HIF-2α in malignant hematopoietic cells provides a proliferative advantage in the hypoxic malignant BM enabling them to proliferate in the hypoxic leukemic BM while the proliferation of normal HSC, which do not express HIF-2α, is blocked by hypoxia-stabilized HIF-1α. Disclosures: No relevant conflicts of interest to declare.