Immune check point inhibitor therapy has shown efficacy in Triple-negative breast cancer (TNBC), highlighting the immune-responsive microenvironment of TNBC. Recently, anti-cancer therapeutics have been shown to induce immunogenic cell death (ICD). MEK/ERK cascade mediates T cell receptor signaling and involves in immune response. MEK inhibitor cobimetinib is approved for advanced melanoma and is studied in several clinical trials including breast cancer. Here, we aimed to explore the anti-tumor and immunomodulatory effects of cobimetinib in TNBC.
Damage-associated molecular patterns (DAMPs), cell-surface translocation of calreticulin (CRT), extracellular release of ATP, and increase in high-mobility group box protein B1 (HMGB1) release from dying tumor cells, were examined by immunoblotting and flow cytometric analysis. The pan-caspase inhibitor z-VAD-FMK was used to validate cobimetinib-induced signaling. The immunocompetent and immunodeficient mice were conducted to investigate the efficacy of cobimetinib.
Cobimetinib significantly induced cell apoptosis and DAMPs in TNBC cells (human MDA-MB-231, MDA-MB-468, and murine 4T1 cells). Cobimetinib downregulated p-ERK and activated caspase-8, in association with increased DAMPs such as HMGB1 and CRT. Cobimetinib-induced CRT was attenuated by ectopic ERK expression. Moreover, cobimetinib-induced cell apoptosis and increased CRT expressions were restored by the pan-caspase inhibitor. The anti-tumor activity of cobimetinib was superior in immunocompetent than in immunodeficient mice. DAMPs and caspase-8 cascade were upregulated in cobimetinib-treated 4T1 xenografts. Interestingly, the immunomodulation of cobimetinib in 4T1-bearing mice were different from that in non-tumor bearing immunocompetent mice. Importantly, cobimetinib-treated 4T1-bearing immunocompetent mice exhibited increased total number of CD8+ T cells, effector CD4+ T cells, regulatory T cells and reduced the number of myeloid-derived suppressor cells, compared to vehicle-treated mice.
Our findings suggest that the MEK inhibitor cobimetinib acts as an ICD-inducer and exerts immunomodulatory effects in TNBC.
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All authors have declared no conflicts of interest.