Ligand Activation of FLT3 Induces Cell Cycle Arrest in Acute Lymphoblastic Leukemia with 11q23 Translocation.

A fms-like kinase (FLT3) is widely known to be involved in proliferation of normal hematopoietic stem cells and precursors. Acute lymphoblastic leukemia (ALL) with 11q23 translocation, often found in infantile leukemia with very poor prognosis, is thought to be derived from a population of cells at the developmental stage very close to hematopoietic stem cells, and was recently shown that they express FLT3 at high levels compared with other types of leukemia. In the present study, we examined the effects of FLT3 ligand (FL) on leukemia cells with or without 11q23 translocation to evaluate a biological implication of the FLT3/FL interaction in ALL. Three of 8 leukemic cell lines without 11q23 translocation showed a proliferative response to FL in the 3H-thymidine uptake assays. However, five of 7 B-precursor leukemic cell lines with 11q23 translocation, unexpectedly, showed an inhibitory response (23-69% inhibition) to FL in a dose-dependent manner (1–20ng/ml), although the special cell line with D835 mutation in FLT3 (KOCL-33) was not affected by the addition of FL. This inhibitory effect was almost abrogated in the presence of a FLT-3 kinase inhibitor PKC412. Inhibition of 3H-thymidine uptakes were not due to induction of apoptosis but due to induction of the Go/G1 arrest. This cell cycle arrest was mediated, at least in part, by a marked up-regulation of p27 due to suppression of its degradation, and promoted resistance of cell lines to radiation-induced apoptosis. Of interest, the addition of FL induced a complete disappearance of constitutive phosphorylation of STAT5 but upregulated phosphorylation of MAPK and Akt. These results suggest that the FLT3/FL interaction in ALL with 11q23 translocation transmits the inhibitory signal specifically to the JAK/STAT pathway via the kinase activity of FLT3, in the process of which the JAK/STAT-specific inhibitory molecules such as SOCS-2 and CIS-1 may be implicated.