Purpose: Mevalonate metabolites are vital for a variety of key cellular functions with the biosynthetic products including cholesterol and farnesyl and geranylgeranyl isoprenoids. Inhibition of this pathway using lovastatin induces a potent apoptotic response in a specific subset of human tumor-derived cell lines, including head and neck squamous cell carcinomas (HNSCC). In this study, we evaluated the potential of a number of chemotherapeutics that demonstrate activity in HNSCC, including an inhibitor of epidermal growth factor receptor (EGFR) to potentiate the cytotoxic effects of lovastatin.
Methods: We evaluated the cytotoxic effects of combining a variety of chemotherapeutics with lovastatin using the MTT assay and flow cytometry. The MCF-7 lovastatin-resistant breast adenocarcinoma cell line and the lovastatin-sensitive HNSCC cell lines SCC9 and SCC25 were tested. Expression levels of EGFR and ligand activated EGFR following lovastatin treatment were analyzed by Western blotting.
Results: Pretreatment or concomitant treatment of 10 microM lovastatin did not significantly augment the effects of a variety of chemotherapeutic agents tested in these cell lines. Co-administration with actinomycin D or cycloheximide, drugs that inhibit RNA and protein synthesis, respectively, inhibited lovastatin-induced apoptosis in these cell lines. This suggests a requirement for the cellular functions disrupted by these chemotherapeutic agents in lovastatin-induced apoptosis of HNSCC cells. In contrast to the chemotherapeutics analyzed, the AG1478 tyrosine kinase inhibitor of the EGFR demonstrated additive cytotoxic effects in combination with lovastatin in HNSCC cells. Mevalonate metabolites may regulate EGFR function, suggesting that lovastatin may inhibit the activity of this receptor. Indeed, lovastatin treatment inhibited EGF-induced autophosphorylation of the EGFR in the SCC9 and SCC25 cell lines. Pretreatment of SCC9 and SCC25 cell lines for 24 h with 10 microM lovastatin, conditions that demonstrated significant inhibition of EGF-induced EGFR autophosphorylation, induced significant additive effects in combination with AG1478.
Conclusion: These results demonstrated the ability of EGFR pathway inhibitors to potentiate lovastatin-induced apoptosis and suggested that lovastatin may target the EGFR pathway in HNSCC cells.