Role of CXCR4/STAT3 pathway in mesenchymal stromal cell-mediated drug resistance of acute leukemia cells

Our aim is to explore the role of CXCR4/STAT3 in mesenchymalstromal cell (MSC)-mediated drug resistance of acute myeloid leukemia (AML) from the version of tumor mieroenvironment. AML cell lines U937 and KG1a and primary AML cells were co-cultured with MSC. The AML celllines cultured alone was used as controls. Apoptosis induced by mitoxantrone was measured by flow cytometry and Annexin V/PI double and 4’-6-diamidino-2-phenylindole (DAPI) staining. CXCR4 and STAT3 protein were detected by Western blot under both culturing conditions. Apoptosis of AML cells (U937and KG1a) significantly decreased during co-culturing with MSC (U937: 10.08%±1.53% vs 45.33%±1.03%, P=0.02; KG1a: 21.6%±1.82% vs 40.33%±3.29%, P=0.020) which suggested that drug resistance was induced after co-culture with MSC. The co-culturing of AML cells with MSC significantly induced the expression of phosphorylated STAT3 and CXCR4 protein. The inhibitor of STAT3 Cucurbitacin I could induce apoptosis of AML cells. After addition of Cucurbitacin I into the co-culture system, the apoptotic rates of primary AML cells significantly increased. Similarly, the apoptotic rates were also increased when AMD3100, the inhibitor of CXCR4, were added to overcome the stromal cell-mediated drug resistance. And AMD3100 induces an up-regulation of phosphorylated STAT3. Therefore targeting on STAT3 or CXCR4 protein could be a novel approach for the treatment of AML.