Up-Regulation of Heme Oxygenase-1 Is Induced by the Leukemia Stroma-Cell Interaction and Rescues Acute Myeloid Leukaemia (AML) Cells From Cytarabin Induced Cell Death

Abstract 533

Although the majority of patients achieve a complete remission after induction chemotherapy with cytarabine and antracyclines relapse is still the major cause of death in acute myeloid leukemia (AML). In order to address the question of resistance mechanisms in residual leukemic cells we employed a murine leukemia model system. We transplanted Rag2^−/−g_c^−/− mice with human KG1a leukemia cells and treated them with four doses of cytarabine at the onset of AML. Residual human leukemic cells were collected from murine marrow and spleen and RNA was further subjected to whole genome microarray analysis. Heme Oxygenase-1 (HO-1) was found to be highly up-regulated in the persistent cell fraction that had survived cytarabine chemotherapy in vivo. The clinical relevance of HO-1 up-regulation was confirmed with two AML data sets. In a small set of 31 unselected AML patients treated in one institution, overall survival (OS) was significantly worse in patients with high HO-1 expression as determined by qPCR analysis (log rank: p < 0.0001). A second patient cohort of 286 patients was therefore retrospectively analyzed for HO-1 expression based on whole genome expression profiling. Again, OS was inferior in patients expressing HO-1 at high levels (p<0.005).

HO-1 is a well-known stress-response protein also termed heat-shock protein 32 (hsp32) that catalyzes the enzymatic cleavage of heme into biliverdin, carbon monoxide and iron, but also decreases cellular levels of reactive oxygen species (ROS). Employing an in vitro co-culture system we show that mesenchymal stem cells (MSC) increase HO-1 expression in leukemic cells causing a decrease in ROS levels and protection from apoptosis particularly in the adherent fraction of leukemic cells. The cytoprotective effect induced by leukemia stromal cell interaction can be overcome by concomitant treatment of cytarabin together with zinc protophoryrin (ZnPP), a specific competitive inhibitor of HO-1. Taken together we demonstrate that expression of HO-1 may play a critical role for chemotherapy resistance in AML in particular with regard to the leukemia stromal cell interaction. ZnPP is a specific HO-1 inhibitor that has already proven anti-tumor activity in vivo and might therefore be a promising candidate for further targeted drug therapy strategies that intend to eradicate persistent leukemic cells.