Flt3 receptor inhibition reduces constitutive NFkappaB activation in high-risk myelodysplastic syndrome and acute myeloid leukemia.

High-risk myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML) are characterized by the activation of the anti-apoptotic transcription factor NFkappaB, via the IKK complex. Here, we show that constitutive activation of the receptor tyrosine kinase Flt3 is responsible for IKK activation. Chemical inhibition or knockdown of Flt3 with small interfering RNAs reduced NFkappaB activation in MDS and AML cell lines, as well as in primary CD34(+) bone marrow cells from patients, causing apoptosis. Epistatic analysis involving the simultaneous inhibition of Flt3 and IKK suggested that both kinases act in the same anti-apoptotic pathway. An IKK2 mutant with a constitutive kinase activity and a plasma membrane-tethered mutant of NEMO that activates IKK1/2 prevented the cytocidal action of Flt3 inhibition. Flt3 phosphorylates IKK2 in vitro, and Flt3 inhibition reduced the phosphorylation of IKK2 in MDS or AML cell lines. IKK2 and Flt3 physically associated in MDS and AML cells, and Flt3 inhibition disrupted this interaction. Flt3 inhibition only killed CD34(+) bone marrow cells from high-risk MDS and AML patients, in correlation with blast numbers and NFkappaB activity, yet had no lethal effect on healthy CD34(+) cells or cells from low-risk MDS. These results suggest that Flt3 inhibitors might exert an anti-neoplastic effect in high-risk MDS and AML through inhibition of NFkappaB.