B7-H3 Silencing By RNAi Inhibits Tumor Progression and Enhances Chemosensitivity in U937 Cells

Background: Aberrant expression of the immunoregulatory protein B7-H3 in B7 family has been associated with more advanced disease and poor prognosis in a wide range of cancer. However, the role of B7-H3 in acute monocytic leukemia U937 cells has not been thoroughly investigated. In this study, we examined the gene expression and subcellular localization of B7-H3 in human peripheral blood cells and hematologic malignancy cell lines. Then, we determined the effects of down-regulating B7-H3 expression on U937 cells, which has the highest B7-H3 protein expression in the detected cell lines.

Methods: B7-H3 expression in 12 healthy volunteer peripheral blood cell samples and 13 human hematologic malignancy cell lines was determined by RT-PCR, western blot and flow cytometry. B7-H3 knockdown in the U937 cell line was performed using shRNA lentivirus transduction. The effects on cell proliferation, cycle, migration and invasion were investigated by CCK-8 assay, methyl cellulose colony forming assay, PI staining, and transwell assays in vitro. U937 xenograft models were used to assess the effects of B7-H3 on tumorigenicity and Ki-67 and PCNA was detected through immunohistochemical. Changes in cell growth inhibition and apoptosis, when combined with chemotherapy drugs, were determined using CCK-8, Annexin V-FITC/PI and Hoechst 33342 staining assays in vitro. The therapeutic effect of B7-H3 knockdown in combination with chemotherapy drugs were also studied by U937 xenograft models in vivo.

Results: B7-H3 mRNA was widely expressed in the 12 hematologic malignancy cell lines except for CZ1 and PB MNCs of volunteers. But the protein level of B7-H3 was only abnormally overexpressed in 12 hematologic malignancy cell lines except for CZ1, with subcellular localizations in nucleus and cytoplasm mostly determined. The down-regulation of B7-H3 in U937 cells significantly decreased cell growth and the rate of colony formation by 32.8% in 72 h and 70.3% in 14 d. Mean inhibition rate of tumor growth with B7-H3 knockdown was 59.4%, and expression of both Ki-67 and PCNA in xenografts was significantly reduced. After B7-H3 silencing, U937 cell cycle was arrested at G0/G1 phase, and the cell cycle-related proteins Cyclin D1 and CDK4 were lower. Cell migration rate of B7-H3 knockdown cells was reduced more than five-fold, and invasion capacity was decreased by 86.7%. The rates of distant metastasis in B7-H3 knockdown xenografts were significantly decreased. The invasion-related proteins MMP-2 and MMP-9 were lower in both B7-H3 knockdown cells and xenografts. B7-H3 RNAi profoundly increased the anti-tumor effect of chemotherapy and enhanced the activity of caspase-3 in vitro and in vivo. At the end of observation (on day 19 after inoculation), inhibition rates of tumor growth in B7-H3 shRNA combined with idarubicin, cytarabine, and idarubicin plus cytarabine groups were 70.5%, 80.0%, and 90.0%, respectively (P=0.006, 0.004 and 0.016). The TUNEL positive cells were significantly increased in the B7-H3 shRNA combined with chemotherapy drugs groups.

Conclusions: B7-H3 protein was abnormally overexpressed in 12 hematologic malignancy cell lines except for CZ1, with subcellular localizations in nucleus and cytoplasm mostly determined. B7-H3 may promote U937 cell progression, and shRNA targeting B7-H3 significantly enhances sensitivity to chemotherapeutic drugs. These results may provide new insight into the function of B7-H3 and a promising therapeutic approach targeting B7-H3 in acute monocytic leukemia.