Simvastatin enhances radiation sensitivity of colorectal cancer cells

Introduction: Neoadjuvant chemoradiation (CRT) for rectal cancer induces variable responses, and better response has been associated with improved oncologic outcomes. Our group has previously shown that the administration of HMG-CoA reductase inhibitors, commonly known as statins, is associated with improved response to neoadjuvant CRT in rectal cancer patients. The purpose of this study was to study the effects of simvastatin on colorectal cancer cells and explore its potential as a radiation-sensitizer in vitro. Methods: Four colorectal cancer cell lines (SW480, DLD1, SW837, and HRT18) were used to test the effects of simvastatin alone, radiation alone, and combination therapy. Outcome measures included ATP-based cell viability, colony formation, and protein (immunoblot) assays. Results: The combination of radiation and simvastatin inhibited colony formation and cell viability of all four CRC lines, to a greater degree than either treatment alone (p < 0.01). In addition, the effects of simvastatin in this combination therapy were dose dependent, with increased concentrations resulting in more potentiated inhibitory effects. The radiosensitizing effects of simvastatin on cell viability were negated by the presence of exogenous GGPP in the media. On protein analyses of irradiated cells, simvastatin treatment inhibited phosphorylation of ERK1/2, in a dose-dependent manner, while the total levels of ERK1/2 remained stable. In addition, the combined treatment resulted in increased levels of cleaved caspase 3, indicating greater apoptotic activity in the cells treated with radiation and simvastatin together. Conclusions: Treatment with simvastatin hindered CRC cell viability and enhanced radiation sensitivity in vitro. These effects were tied to the depletion of GGPP and the decreased phosphorylation of ERK1/2, suggesting a prominent role for the EGFR-RAS-ERK1/2 pathway, through which statin enhances radiation sensitivity.