Proteomics indicates modulation of tubulin polymerization by L-menthol inhibiting human epithelial colorectal adenocarcinoma cell proliferation

Menthol is a naturally occurring cyclic monoterpene used in oral hygiene products, confectionary, pharmaceuticals, cosmetics, pesticides, and as a flavoring agent. In the present study, we analyzed the differentially expressing proteome in L-menthol-treated Caco-2 cell line as it was found to inhibit cell proliferation. Interestingly, free tubulin proteins were observed to be limited after menthol treatment. Semiquantitative RT-PCR with α-tubulin primers showed no change in the level of RNA expression in menthol-treated cell line. However, tubulin polymerization assay with menthol indicated a trend similar to taxol in promoting microtubule assembly. Further, physical counting of apoptotic nuclei and active caspase-3 assays confirmed onset of apoptosis though the rate was slower as compared with that of taxol treatment. This study is the first report of a monoterpene L-menthol modulating tubulin polymerization and apoptosis to inhibit cancer cell proliferation.