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Inhibition of the phosphatidylinositol-3-kinase pathway is selectively cytotoxic to primitive and mature acute myeloid leukemia progenitor cells while sparing their normal counterparts

University of British Columbia

Acute myeloid leukemia (AML) is a disorder of the hematopoietic system in which there is an overproduction of immature myeloblast (blast) cells in the bone marrow and suppression of normal hematopoiesis. Like normal hematopoiesis, AML is structured hierarchically and is maintained by rare progenitor cells capable of self-renewal and production of large numbers of leukemic blasts. These rare cells, leukemia-initiating cells (L-IC), are capable of engraftment and proliferation in immunocompromised mice. Dysregulation of signal transduction pathways are frequent occurrences in AML, leading to enhanced cell proliferation and resistance to apoptosis. Pathways which are frequently constitutively active in AML blasts include the phosphatidylinosityl-3-kinase (PI3K) and FMS-like tyrosine kinase 3 (FLT3) receptor signalling pathways. Constitutive PI3K activation, as measured by Akt phosphorylation, was detected in the majority of the 36 AML patient samples tested. The PI3K signalling intermediate integrin linked kinase (ILK) was ubiquitously expressed, and FLT3 abundantly expressed although at variable levels. Furthermore, in the five samples tested, ILK and FLT3, as well as PI3K pathway activation, were also detected in CD34+CD38- sorted and quiescent AML cells, populations enriched for L-IC. siRNA inhibition of ILK caused a decrease in AML colony forming cell (CFC) production. To further test the effects of ILK inhibition on AML, the small molecule inhibitor QLT0267 was used. In vitro kinase assays showed that QLT0267 inhibits both ILK and FLT3 kinases to a similar degree. Treatment of AML blasts with QLT0267 caused dose-dependent killing of AML-CFC, with AML samples containing FLT3 mutations being more sensitive to the inhibitor, although some samples with wildtype FLT3 were also effectively killed. Treatment of AML cells with QLT0267 also decreased survival of both primitive leukemic progenitor cells and L-IC in a dose-dependent fashion. In contrast, QLT0267 showed little toxicity toward normal bone marrow progenitors detected in the same assays. When combined with cytarabine or daunorubicin, QLT0267 worked synergistically to kill AML-CFC from a proportion of AML samples tested. Thus, FLT3, ILK, and constitutive PI3K activation are present in L-ICs, and these candidate leukemic stem cells can be eliminated by inhibition of these pathways, while their normal hematopoietic counterparts are spared.

Text

2009-12-10

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/16327

Genetics

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/