PIM Inhibitor SMI-4a Induces Cell Apoptosis and Cell Cycle Arrest in B-Cell Acute Lymphocytic Leukemia Cells Via the JAK2/STAT3 Pathway

The serine/threonine PIM protein kinases are critical regulators of turmorigenesis in mutiple hematologic malignancies and solid cancers. We used real-time PCR to detect the expression of PIM in B-cell acute lymphocytic leukemia (B-ALL) patients, and found the expression of PIM in B-ALL patients was significantly higher than that in normal controls. SMI-4a is a pan-PIM small molecule inhibitor, and this agent exhibits demonstrable preclinical antitumour activity in a wide range of hematologic malignant cell lines. To further explore the effect of SMI-4a on B-ALL cells, B-ALL cell lines CCRF-SB and Sup-B15 were treated with this small molecule inhibitor, and the results showed that SMI-4a inhibited B-ALL cell proliferation in a dose- and time-dependent manner. Moreover, SMI-4a significantly promoted B-ALL cell apoptosis and caused cell cycle arrest in the G0/G1 phase. The results of Western blot showed that SMI-4a increased the expression of Caspase-3, Caspase-9, Bax and P21, and decreased the expression of Bcl-2 and CDK4. Furthermore, we found that SMI-4a significantly inhibits the activation of the JAK2/STAT3 pathway and HO-1 interferes with the JAK2/STAT3 pathway to inhibit SMI-4a-induced ALL cell apoptosis. Finally, xenograft experiments in NOD/SCID mice were operated to investigate the potential role of SMI-4a in B-ALL tumorigenesis in vivo. To observe the effect of SMI-4a on tumor growth in vivo, NOD/SCID mice were transplanted with B-ALL devied cells, and the tumor-bearing mice were intraperitoneally injected with saline and SMI-4a, respectively. As a result, tretment with SMI-4a resulted in a significant inhibition on tumor growth. In addition, PIM inhibtor obviously reduced the volume and weight of B-ALL cell-derived tumors. TUNEL assay revealed the proportion of apoptotic cells was higher in the SMI-4a-treated group than in the control group. Taken together, our data showed PIM inhibitor (SMI-4a) significantly inhibits the growth of B-ALL cells in vitro and in vivo and promotes apoptosis and cell cycle arrest. This suppressive effect is mediated partly through inhibiting the JAK2/STAT3 pathway activation.