Targeting the 5-Lipoxygenase as a Novel Principle of Stem Cell Therapy In Acute Myeloid Leukemia

Abstract 1846

Chromosomal translocations such as t(15;17), t(8,21) or t(6;9) lead to the formation of chimeric genes encoding the PML/RAR, AML-1/ETO or DEK/CAN fusion proteins (FP). These FP are able to induce and to maintain acute myeloid leukemia (AML) by both blocking terminal differentiation of early hematopoietic progenitors and increasing the self renewal potential of the leukemic stem cells (LSC). LSCs are potential therapeutic targets and it is of great importance to elucidate which signaling pathways control their development and maintenance. Recently it has been shown that the presence of the 5-Lipoxygenase activity (5-LO) is indispensable for the induction and the maintenance of the BCR/ABL induced CML-like disease in mice. Its depletion or inhibition impairs the LSCs in the CML-like disease.

5-LO is the key enzyme in the biosynthesis pathway of leukotrienes, a group of proinflammatory lipid mediators derived from arachidonic acid. Furthermore we have shown that Sulindac sulfide, a dual Cycloxygenase/5 –LO inhibitor, was able, at 5-LO inhibitory concentrations, to interfere with the stem cell capacity of PML/RAR-positive LSC. It also overcame the differentiation block in PML/RAR-positive HSC.

To disclose whether a “leukemic stem cell therapy” in AML is feasible if based on selectively targeting the 5-LO, we used two different selective 5-LO inhibitors, Zileuton and CJ-13,610, in a PML/RAR- and DEK/CAN-positive leukemia model. Zileuton, an anti-asthmatic drug, is a reversible inhibitor of 5-LO activity which leads to the inhibition of leukotrienes (LTB4, LTC4, LTD4, and LTE4) formation. CJ-13,610 is novel non redox, non iron chelating 5-LO inhibitor. As stem cell models we used Sca-1+/lin-murine HSC retrovirally transduced either with PML/RAR or DEK/CAN. Here we report that both Zileuton and CJ -13,610 at clinically feasible concentrations of 0.3 – 3μM interfered with the aberrant replating efficiency of PML/RAR and DEK/CAN expressing HSCs; ii.) inhibited the short-term stem cell (ST-HSC) capacity of PMR/RAR- and DEK/CAN-positive HSCs as assessed by colony forming unit-spleen day 12 assays in lethally irradiated recipient mice; and iii.) reduced the frequency of long-term HSC in a long term competitive repopulation stem cell assays. The effects of both compounds were not due related to the induction of apoptosis. Interestingly, on normal control HSC both Zileuton and CJ-13,610 exhibited a “paradox” effect by increasing ST-HSC as well as LT-HSC capacity.

Our here presented data establish the inhibition of 5-LO by selective inhibitors as a feasible approach of molecular stem cell therapy in AML. Furthermore it strongly suggest an important role of leukotrienes for the maintenance of leukemic stem cells. The exact mechanisms by which the inhibition of 5-LO interferes with the LSC have still to be disclosed.