Induction of Vascular Endothelial Growth Factor Secretion by Childhood Acute Lymphoblastic Leukemia Cells via the FLT-3 Signaling Pathway

Abstract

Human leukemia cells secrete VEGF, which can act in a paracrine manner within the bone marrow microenvironment to promote leukemia cell survival and proliferation. The FLT-3 receptor tyrosine kinase plays an essential role in regulating normal hematopoiesis, but its constitutive activation via mutation in acute leukemias is generally associated with poor outcome. The aim of this study was to investigate interactions between the FLT-3 and VEGF signaling pathways in acute leukemia using cell lines and ex vivo cultures of pediatric acute lymphoblastic leukemia cells following expansion of direct patient explants in immunodeficient mice. Different xenograft lines exhibited variable cell surface FLT-3 expression, as well as basal and FLT-3 ligand-induced VEGF secretion, whereas the MV4;11 cell line, which expresses constitutively active FLT-3, secreted high levels of VEGF. The FLT-3 inhibitor, SU11657, significantly reduced VEGF secretion in three of six xenograft lines and MV4;11 cells, in conjunction with inhibition of FLT-3 tyrosine phosphorylation. Moreover, exposure of xenograft cells to the FLT-3-blocking antibody, D43, also reduced VEGF secretion to basal levels and decreased FLT-3 tyrosine phosphorylation. In terms of downstream signaling, SU11657 and D43 both caused dephosphorylation of extracellular signal-regulated kinase 1/2, with no changes in AKT or STAT5 phosphorylation. Finally, partial knockdown of FLT-3 expression by short interfering RNA also resulted in inhibition of VEGF secretion. These results indicate that FLT-3 signaling plays a central role in the regulation of VEGF secretion and that inhibition of the FLT-3/VEGF pathway may disrupt paracrine signaling between leukemia cells and the bone marrow microenvironment.

Flt3 Regulation in the Mononuclear Phagocyte System Promotes Ocular Neovascularization

“…These findings support the notion that inhibition of Flt3 by AC220 affects retinal new vessel formation in the retina and detachment induced by VEGF in these models. Markovic et al showed that the Flt3-blocking antibody D43 decreased Flt3 tyrosine phosphorylation and reduced Flt3-induced VEGF secretion, indicating that Flt3 signaling is highly involved in VEGF regulation [ 60 ].…”

Transcriptomic responses provide a new mechanistic basis for the chemopreventive effects of folic acid and tributyrin in rat liver carcinogenesis

“…Several mechanisms may contribute to the observed suppression of angiogenesis in the livers of rats treated with folic acid and tributyrin. First, treatment of rats with folic acid and tributyrin alone or in combination caused down‐regulation of several key genes encoding activators of the angiogenesis signaling pathway, including Itgb6 , Itgad , Ftl3 , Map3k6 , Mgp and Src . It has been demonstrated that these proteins stimulate a VEGFA secretion through activation of the MAPK pathway (integrins, FLT3, MAP3K6 and SRC) or induction of the TGF‐β1 pathway (MGP).…”

“…First, treatment of rats with folic acid and tributyrin alone or in combination caused down-regulation of several key genes encoding activators of the angiogenesis signaling pathway, including Itgb6, Itgad, Ftl3, Map3k6, Mgp and Src. [32][33][34][35][36][37] It has been demonstrated that these proteins stimulate a VEGFA secretion through activation of the MAPK pathway (integrins, FLT3, MAP3K6 and SRC) or induction of the TGF-b1 pathway (MGP). Second, treatment of rats with folic acid and tributyrin alone, or a combination of folic acid and tributyrin resulted in marked down-regulation of the Edn1 gene.…”

Breaking and entering into the CNS: clues from solid tumor and nonmalignant models with relevance to hematopoietic malignancies

“…Such varying microenvironmental milieus may impose considerably different survival stresses. Many hematologic malignancies elaborate significant amounts of VEGF with mounting evidence of a role in CNS breach [57, 58, 65, 75–78]. Angiogenesis in this context may impose microenvironmental effects—increased microvessel density can correlate with worse prognosis/outcome in both lymphomatous mass/nodule and leukemic contexts [75, 77].…”