Abstract
B7 family proteins are important immune response regulators, and can mediate oncogenic signaling and cancer development. We have used human triple-negative breast cancer cell lines with different expression levels of B7-H3 to evaluate its effects on the sensitivity to 22 different anticancer compounds in a drug screen. API-2 (triciribidine) and everolimus (RAD-001), two inhibitors that target the PI3K/AKT/mTOR pathway, showed enhanced inhibition of cell viability and proliferation in B7-H3 knockdown tumor cells compared to their B7-H3 expressing counterparts. Similar inhibition was seen in control cells treated with an anti-B7-H3 monoclonal antibody. In B7-H3 overexpressing cells, the effects of the two drugs were reduced, supported also by in vivo experiments in which B7-H3 overexpressing xenografts were less sensitive to everolimus than control tumors. In API-2 and everolimus-treated B7-H3 overexpressing cells, phospho-mTOR levels were decreased. However, phosphorylation of p70S6K was differentially regulated in B7-H3 cells treated with API-2 or everolimus, suggesting a different B7-H3-mediated mechanism downstream of mTOR. Both API-2 and everolimus decreased the glycolysis of the cells, whereas knockdown of B7-H3 decreased and B7-H3 overexpression increased the glycolytic capacity. In conclusion, we have unveiled a previously unknown relationship between B7-H3 expression and glycolytic capacity in tumor cells, and found that B7-H3 confers resistance to API-2 and everolimus. The results provide novel insights into the function of B7-H3 in cancer, and suggest that targeting of B7-H3 may be a novel alternative to improve current anticancer therapies.