The magnitude of CXCR4 signaling regulates resistance to quizartinib in FLT3/ITD cells via RUNX1.

CXCR4 antagonists sensitize FLT3/ITD AML cells to FLT3 inhibitors; however, CXCR4 signaling can induce apoptosis in AML cells, raising the question of whether CXCR4 signaling exerts divergent effects on FLT3/ITD cells. The present study investigated the paradoxical function of CXCR4 in resistance to FLT3 inhibitors. The FLT3 inhibitor quizartinib significantly decreased the number of FLT3/ITD Ba/F3 cells, whereas 1 ng/ml CXCL12 showed a significant protective effect against quizartinib. In contrast, CXCL12 over 100 ng/ml significantly decreased FLT3/ITD cell viability with concomitant downregulation of Runx1. Moreover, the survival of FLT3/ITD Ba/F3 or MOLM13 cells with low surface CXCR4 expression incubated with quizartinib was significantly enhanced by 100 ng/ml CXCL12; however, this protective effect of CXCL12 against quizartinib was barely detected in cells with high surface CXCR4 expression. Although silencing Runx1 downregulated CXCR4 expression, RUNX1 expression levels were significantly higher in CXCR4 FLT3/ITD Ba/F3 cells incubated with 100 ng/ml CXCL12 than in CXCR4 cells, coincident with an increase in FLT3 phosphorylation. Silencing RUNX1 partially abrogated resistance to quizartinib in CXCR4 cells incubated with CXCL12, whereas ectopic RUNX1 significantly restored resistance in CXCR4 cells. These results indicate that CXCR4 signaling of different magnitudes paradoxically regulates resistance to quizartinib in FLT3/ITD cells via RUNX1.