Abstract 303: Compensatory up-regulation of NEK9 contributes to resistance to PLK1 inhibition in triple-negative breast cancer cells

Patients with triple-negative breast cancers (TNBCs) have very poor prognosis in terms of relapse and overall survival. There is currently no targeted therapy for this heterogeneous group of patients and it presents a pressing unmet medical need. Recent data suggest that Polo-like kinase 1 (PLK1) is a plausible target for treating TNBCs, and many small-molecule inhibitors are currently under development for targeting this kinase. Since clinical data have shown that single-agent treatment often leads to resistance and relapse of the cancer, it is essential to identify potential resistance mechanisms of TNBCs to PLK1 inhibition such that combination therapy can be rationally designed to increase treatment efficacy. We have selected ten individual cell clones from a TNBC cell line, MDA-MB-231, through long-term incubation of the cells with increasing concentrations of a PLK1 inhibitor BI 2536. We hypothesized that increased expression and/or activities of other kinases can compensate for the PLK1 inhibition. To identify these kinases, we screened the expression levels of >500 human kinases in the resistant clones in comparison to the parental cells, using the Nanostring platform, and generated a list of candidate kinases. Among the candidates, NEK9, a downstream effector of PLK1, was confirmed to be over-expressed in all resistant cell clones through Western blotting. MDA-MB-231 parental cells were then infected with a lentiviral over-expression vector that contains NEK9 and GFP. When a mixed population of NEK9-over-expressing cells (GFP+) and parental cells (GFP-) were incubated in the presence of BI 2536, increased proportion of GFP+ cells and increased mean GFP fluorescence intensity was observed over time through flow cytometry. Increase of NEK9 expression was also confirmed with Western blotting in the mixed cell culture experiment, indicating the selection of the NEK9-over-expressing cells in the presence of BI 2536. In conclusion, we have constructed a list of over-expressed kinase candidates that may contribute to the resistance to PLK1 inhibition. We further identify that over-expression of NEK9 contributes to the resistance phenotype. Our data prompt the need to further study the biology around NEK9 and suggest the possibility of combination therapy through inhibition of PLK1 and NEK9 in treating TNBCs. Citation Format: Nelson K. Y. Wong, Devon Mitchell, Mohamed K. Khan. Compensatory up-regulation of NEK9 contributes to resistance to PLK1 inhibition in triple-negative breast cancer cells. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 303.