Sensitization of resistant cells with a BET bromodomain inhibitor in a cell culture model of deep intrinsic resistance in breast cancer

Abstract

Background: Cell culture models of cancer typically favor proliferative but therapy-sensitive cells because body-like selection pressures are absent. To address this limitation of cell culture, we previously described a function-based selection strategy to model deep intrinsic resistance in cultures of triple-negative breast cancer cells. To determine the validity of this approach in identifying noncytotoxic drugs that could inhibit the relapse of poor-prognosis minimal residual disease in breast cancer, we used our novel cell culture model to evaluate a well-known BET bromodomain inhibitor, JQ1, which modulates cancer epigenome. Methods: We treated highly metabolically adaptable (SUM149-MA) cells and their control parental SUM149-Luc cell line with JQ1 for long periods to determine its efficacy at inhibiting resistant cells. To measure JQ1-mediated sensitization of resistant cancer cells, we first eradicated approximately 99% of relatively chemotherapy-sensitive cancer cells in culture dishes by long treatments with doxorubicin or paclitaxel, and then analyzed the remaining resistant cells for survival and growth into colonies. Our methods were designed to reveal resistant cancer cells while minimizing the contribution of the vast majority of nonresistant cells that simply proliferate in cell culture. We also performed Western blotting to determine whether JQ1 treatment affected PD-L1 protein levels in cancer cells. Results: After 20 days of treatment with 1-2 µM JQ1, which killed a majority of cells in the parental cell line, a large number of SUM149-MA cells survived, consistent with their pan-resistant nature. Interestingly, though, the JQ1 treatment sensitized resistant cancer cells in both the SUM149-MA and SUM149-Luc cell lines to subsequent treatment with doxorubicin and paclitaxel. In addition, combination, rather than sequential, treatment with JQ1 and doxorubicin was also effective in overcoming resistance. Notably, JQ1-treated cancer cells also had significantly lower levels of PD-L1 protein than did untreated cells. Conclusions: Our results suggest that the noncytotoxic drug JQ1 could inhibit the growth of resistant breast cancer cells at the minimal residual disease stage, prior to relapse. Because it can also reduce PD-L1 protein levels, JQ1 may reduce tumor-mediated immune suppression and improve response to immune therapy targeting PD-L1. In all, our results support the validity of a cell culture-based approach for modeling a cancer adaptability phenotype, namely the opportunistic switching of cancer cells between quiescence and proliferation.