Statin Treatment Inhibits FLT3 Signaling by Preventing Glycosylation of the Mature Receptor and Leads to Cell Death In Mutant FLT3 Expressing Cells

Abstract 779

FLT3 is a tyrosine kinase receptor expressed on the surface of hematopoietic stem cells/progenitors and contributes to leukemogenesis as evidenced by its frequent mutation in acute myeloid leukemia (AML). FLT3 normally undergoes N-linked glycosylation to produce an immature 130 Kd protein in the endoplasmic reticulum followed by further processing in the Golgi to form the final 160 Kd mature glycoprotein that translocates to the surface. At the surface, FLT3 binds to FLT3 ligand (FL) resulting in phosphorylation and activation of multiple signaling cascades. FLT3 activating mutations primarily occur as internal tandem duplications (ITD) within the juxtamembrane domain or as point mutations in the kinase domain and have become the target of numerous small molecule tyrosine kinase inhibitors (TKI), several of which have entered clinical trials for AML. However, clinical trials to date have shown limited success due to a variety of reasons including the emergence of resistance mediated by alternative pathway activation, the acquisition of point mutations, expression of increased FLT3 ligand levels and poor inhibition of FLT3 kinase activity for a number of reasons. Statins are a well-defined class of drugs that have been safely used for treatment of hypercholesterolemia. Here, we show for the first time that statins can inhibit constitutive activation of both kinase domain and ITD mutants. Western blotting demonstrates that fluvastatin inhibits FLT3 phosphorylation by preventing receptor glycosylation, initially leading to loss of the mature receptor followed by instability of the receptor and subsequent degradation of FLT3 protein and ultimately, cell death. MTT analysis indicates that BaF3 cells transfected with FLT3/ITD are killed in a dose-dependent fashion by fluvastatin with an IC₅₀ of <1 uM, a level which has been achieved in clinical trials. Annexin V binding confirms that BaF3/ITD cells undergo apoptosis when treated with Fluvastatin. Fluvastatin treatment causes a reduction in surface FLT3 expression as measured by flow cytometry that parallels induction of apoptosis. This finding is consistent with results obtained by immunofluoresence microsopy showing increased intracellular retention of FLT3/ITD. Additionally, fluvastatin eliminates the 2–3 fold shift in the dose-response to TKI seen when FLT3/ITD expressing cells are stimulated by FL. Also, a number of mutations that confer high level resistance to TKI are overcome by fluvastatin. These results demonstrate that statins, a class of drugs already clinically approved, might be useful in the management of AML cases in which FLT3 is over expressed, mutated or resistant to other clinically available inhibitors, either alone or more likely in combination with FLT3 TKI.