Mevalonate suppression mediates the impact of geranylgeraniol on cell cycle arrest, apoptosis and differentiation in murine B16 melanoma cells
The diterpene geranylgeraniol suppresses the growth of human liver, lung, ovary, pancreas, colon, stomach, and blood tumors with undefined mechanisms. We evaluated the growth-suppressive activity of geranylgeraniol in murine B16 melanoma cells. Geranylgeraniol induced dose-dependent suppression of B16 cell proliferation following a 48-h incubation in 96-well plates. Cell cycle arrest at the G1 and G2/M phases, measured by flow cytometry, and apoptosis, detected by Guava Nexin™ assay and fluorescence microscopy following acridine orange and ethidium bromide dual staining, contributed to the growth suppression. Geranylgeraniol also induced the activity of alkaline phosphatase activity, a biomarker for cell differentiation, in B16 cells. Mouse 3T3-L1 fibroblasts were more resistant than B16 cells to geranylgeraniol. The impact of geranylgeraniol on B16 cell proliferation, cell cycle arrest and apoptosis was attenuated by supplemental mevalonate, the product of 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase and an essential intermediate required for growth. Geranylgeraniol and d-d-tocotrienol, also a down-regulator of HMG CoA reductase activity, synergistically suppressed the proliferation of B16 cells. The anti-tumor activity of geranylgeraniol may be mediated by mevalonate starvation. Geranylgeraniol individually and in combination with other mevalonate suppressors may have potential in melanoma prevention and/or therapy.