Protein kinase C inhibitors sensitize GNAQ mutant uveal melanoma cells to ionizing radiation

Purpose. Uveal melanoma (UM) tumors require large doses of radiation therapy (RT) to achieve tumor ablation, which frequently results in damage to adjacent normal tissues, leading to vision-threatening complications. Approximately 50% of UM patients present with activating somatic mutations in the gene encoding for G protein α_q-subunit (GNAQ), which lead to constitutive activation of downstream pathways, including protein kinase C (PKC). In this study, we investigated the impact of small-molecule PKC inhibitors bisindolylmaleimide I (BIM) and sotrastaurin (AEB071), combined with ionizing radiation (IR), on survival in melanoma cell lines. Methods. Cellular radiosensitivity was determined by using a combination of proliferation, viability, and clonogenic assays. Cell-cycle effects were measured by flow cytometry. Transcriptomic and proteomic profiling were performed by quantitative real-time PCR, reverse-phase protein array analysis, and immunofluorescence. Results. We found that the PKC inhibitors combined with IR significantly decreased the viability, proliferation, and clonogenic potential of GNAQ^mt, but not GNAQ^wt/BRAF^mt cells, compared with IR alone. Combined treatment increased the antiproliferative and proapoptotic effects of IR in GNAQ^mt cells through delayed DNA-damage resolution and enhanced induction of proteins involved in cell-cycle arrest, cell-growth arrest, and apoptosis. Conclusions. Our preclinical results suggest that combined modality treatment may allow for reductions in the total RT dose and/or fraction size, which may lead to better functional organ preservation in the treatment of primary GNAQ^mt UM. These findings suggest future clinical trials combining PKC inhibitors with RT in GNAQ^mt UM warrant consideration.