A Novel Cytokine Response Modulatory Function of MEK Inhibitors Mediates Therapeutic Efficacy

Despite the recent success of immune-checkpoint blockade therapy for late-stage non-small cell lung cancer (NSCLC), lung cancer is still the leading cause of cancer deaths worldwide. One of the most important characteristics of lung cancer in therapeutic decision-making are the targetable molecules, including EGFR, ALK, BRAF, and MEK. The excitement of immune-checkpoint blockade therapy has triggered concerted efforts that focus on exploring combinations of immune checkpoint therapy with other approved therapeutic regimens aiming at further augmenting positive outcomes and survival. However, the lack of understanding of underlying mechanisms and evidence-based clinical testing has hindered the progress to a cure. Hence, the goal of the study presented here is to elucidate the cross-talk between targeted agents and cytokine response pathways in cancer cells. The goal of the first part of this study was to investigate the molecular mechanisms of cancer cells in response to MEK inhibitors (MEKi), through which lung cancer cells are rendered sensitive to TNFα and IFNγ. We found that BRAF inhibitor and MEKi treatment of melanoma patients led to the activation of tumor NF-κB activity. In vitro studies revealed that MEKi potentiate the response to TNFα, a potent activator of NF-κB. In both melanoma and lung cancer cells, MEKi induced a strong increase in the cell surface expression of TNFα receptor 1 (TNFR1), subsequently enhancing NF-κB activation and augmenting the expression of genes regulated by TNFα and IFNγ. Screening of 289 targeted agents for the ability to increase TNFα + IFNγ target gene expression demonstrated that this was a general activity of inhibitors of MEK and ERK kinases. Treatment with MEKi led to the acquisition of a novel vulnerability to TNFα + IFNγ-induced apoptosis in lung cancer cells that were refractory to MEKi killing as well as the augmentation of cell cycle arrest. Lung cancer cell knock-out of TNFR1 impaired the anti-tumor efficacy of MEKi, whereas TNFα + IFNγ administration in MEKi-treated mice enhanced the anti-tumor response. Finally, immunotherapeutics known to induce the expression of these cytokines synergized with MEKi in eradicating tumors. These findings define a novel cytokine response modulatory function of MEKi that can be therapeutically exploited. The second part of this study explored the above-described concept in a more translationally relevant setting. In immune therapies, TNFα and IFNγ are primarily produced by tumor-infiltration lymphocytes (TILs) that are adoptively transferred or reactivated by immune-checkpoint blockade. Whereas activated T cells recognize and eliminate specific antigen-expressing cancer cells, the cytokines secreted by those T cells impact broader targets, including those that are able to escape from direct T cell antigen recognition. This non-specific anti-tumor effect is important for long-term therapeutic response but is currently overlooked and less exploited. To understand the relative contribution of the cytokine mediated killing, an in vitro co-culture system that measures target cell survival via a firefly bioluminescence assay was used. We demonstrated that cytokines secreted by activated T cells contribute significantly to an anti-tumor effect and can be further enhanced by MEKi, depletion of TNFα, and IFNγ signaling, all of which dampened the overall anti-tumor effect and resulted in a loss of synergy with MEKi. These findings provided further evidence for the development of a pre-clinical model and clinical approach to combine immune therapies with MEKi for lung cancer treatment. Overall, we show that lung cancer cells are rendered sensitive to MEKi by TNFα and IFNγ, providing a strong mechanistic rationale for combining MEKi with immunotherapeutics that enhance the expression of these cytokines