Abstract
The type III receptor tyrosine kinase fms-like tyrosine kinase 3 (FLT3) is expressed on both normal hematopoietic stem cells and acute myeloid leukemia (AML) cells and regulates their proliferation. Internal tandem duplication (ITD) mutation of FLT3 is present in a third of AML cases, results in constitutive activation and aberrant signaling of FLT3, and is associated with adverse treatment outcomes. While wild-type (WT) FLT3 is predominantly a 150 kDa complex glycosylated cell surface protein, FLT3-ITD is partially retained in the endoplasmic reticulum as a 130 kDa underglycosylated species associated with the chaperones calnexin and heat shock protein (HSP) 90, and mediates aberrant STAT5 signaling, which upregulates the oncogenic serine/threonine kinase Pim-1. FLT3 contains a Pim-1 substrate consensus serine phosphorylation site, and we hypothesized that it might be a Pim-1 substrate. Pim-1 was indeed found to directly interact with and serine-phosphorylate FLT3. Pim-1 inhibition decreased the expression and half-life of 130 kDa FLT3, with partial abrogation by proteasome inhibition, in association with decreased FLT3 binding to calnexin and HSP90, and increased 150 kDa FLT3 expression and half-life, with abrogation by inhibition of glycosylation. These findings were consistent with Pim-1 stabilizing FLT3-ITD as a 130 kDa species associated with calnexin and HSP90 and inhibiting its glycosylation to form the 150 kDa species. Pim-1 knockdown effects were similar. Pim-1 inhibition also decreased phosphorylation of FLT3 at tyrosine 591 and of STAT5, and expression of Pim-1 itself, consistent with inhibition of the FLT3-ITD-STAT5 signaling pathway. Finally, Pim-1 inhibition synergized with FLT3 inhibition in inducing apoptosis of FLT3-ITD cells. This is, to our knowledge, the first demonstration of a role of Pim-1 in a positive feedback loop promoting aberrant signaling in malignant cells.
Highlights
The type III receptor tyrosine kinase fms-like tyrosine kinase 3 (FLT3) is expressed on both normal hematopoietic cells and acute myeloid leukemia (AML) cells and regulates their proliferation [1]
AML cells with FLT3-internal tandem duplication (ITD) are characterized by predominant expression of an underglycosylated, or high-mannose, 130 kDa FLT3 species that is retained in the endoplasmic reticulum (ER) and mediates aberrant signaling of STAT5, and the serine/threonine kinase Pim-1 is trancriptionally upregulated downstream of STAT5
We have demonstrated that Pim-1 in turn serine-phosphorylates and stabilizes 130 kDa FLT3 and thereby promotes aberrant signaling of STAT5, creating a positive feedback loop that sustains aberrant signaling in AML cells with FLT3-ITD
Summary
The type III receptor tyrosine kinase fms-like tyrosine kinase 3 (FLT3) is expressed on both normal hematopoietic cells and acute myeloid leukemia (AML) cells and regulates their proliferation [1]. FLT3 is mutated in leukemia cells of approximately a third of AML patients, most commonly by internal tandem duplication (ITD) within the juxtamembrane domain, resulting in constitutive activation and aberrant signaling [1,2]. FLT3 inhibitors have activity in AML with FLT3-ITD [3], but the single randomized clinical trial reported to date did not demonstrate improved treatment outcome [4]. STAT5 signaling in turn causes transcriptional activation of its downstream target Pim-1 [14,15,16], a serine/threonine kinase encoded by a proto-oncogene originally identified as the proviral insertion site in Moloney murine leukemia virus lymphomagenesis [17]. Pim-1 is a member of the pim kinase family, which includes Pim-2 and Pim-3 [20]
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