Abstract LB-29: Disruption of annexin II tetramer/receptor axis suppresses leukemia cell homing and engraftment

Acute lymphoblastic leukemia (ALL) is the most frequent cancer in children and adolescents. Although multi-agent chemotherapy regimens have helped achieve cure rates of over 80%, the patients with refractory disease or those who suffer from a relapse of the disease are faced with a poor prognosis. The bone marrow microenvironment plays an important role in ALL cell proliferation, maintenance, and even resistance to chemotherapy. Consequently, it is interesting to decipher the molecular interactions that mediate ALL cell binding and retention within the bone marrow. One protein abundantly expressed on cells within the bone marrow is a phospholipid-binding protein called annexin II (ANX2) in complex with p11 to form ANX2/p11 tetramer (ANX2T). ANX2T has been reported to play an important role in the maintenance of normal hematopoietic stem cells within the bone marrow (Jung et al, Blood, 2007). ANX2T serves as a ligand for the Annexin II receptor (ANX2R), which is a 25-KDa transmembrane protein. We present evidence that ANX2R is upregulated in patients suffering relapse as well as in refractory ALL cell lines. Exogenous application of purified ANX2T protected ALL cells from serum-starvation induced apoptosis. This increased cell survival was mediated by ANX2T-mediated induction of signaling pathways such as Erk1/2. Furthermore, small molecule inhibitors that disrupt interactions between ANX2T and ANX2R (as described in Reddy et al., J Med Chem, 2011) induced apoptosis in ALL cells confirming the significance of ANX2T/ANX2R interactions in ALL cell survival. In addition, these inhibitors and anti-ANX2 antibody which also disrupts this complex, abrogated cell adhesion between ALL cells and bone marrow osteoblasts, suggesting that ANX2T/ANX2R interactions facilitate binding and retention of ALL cells in the bone marrow. Finally, we utilized an orthotopic leukemia xenograft mouse model in which human ALL cells when injected via the tail-vein in NOD/SCID mice home to the bone marrow, are retained there, and proliferate to establish human disease in mice. In a short-term homing assay, we injected patient-derived ALL cells in the presence or absence of ANX2T inhibitor and assayed the number of leukemic cells in the bone marrow, spleen, liver and kidneys 16 hours post cell injection. We observed that the number of ALL cells homing to the bone marrow was reduced by 50% in mice injected with the inhibitor. The number of ALL cells in the liver was equivalent in both groups indicating that equal numbers of ALL cells were injected in all mice. In a long-term engraftment assay, we observed that the number of circulating ALL cells in mouse blood was reduced by 90% in mice treated with inhibitor. Taken together, we show that disruption of ANX2T/ANX2R interactions results in ALL apoptosis, reduced cell adhesion to osteoblasts, and suppression of ALL cell homing and engraftment. Citation Format: Gopalakrishnapillai Anilkumar, Priyanka Dhanan, Edward A. Kolb, Andrew Napper, Robert Mason, Sonali P. Barwe. Disruption of annexin II tetramer/receptor axis suppresses leukemia cell homing and engraftment. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr LB-29. doi:10.1158/1538-7445.AM2014-LB-29