Synergistic actions of atorvastatin with γ‐tocotrienol and celecoxib against human colon cancer HT29 and HCT116 cells

The synergistic actions of atorvastatin (ATST) with γ‐tocotrienol (γ‐TT) and celecoxib (CXIB) were studied in human colon cancer cell lines HT29 and HCT116. The synergistic inhibition of cell growth by ATST and γ‐TT was demonstrated by 3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide (MTT) assay and isobologram analysis. δ‐TT exhibited a similar inhibitory action when combined with ATST. Mevalonate and geranylgeranyl pyrophosphate eliminated most of the growth inhibitory effect of ATST, but only marginally decreased that of γ‐TT; whereas farnesyl pyrophosphate and squalene exhibited little effect on the inhibitory action of ATST and γ‐TT, indicating protein geranylgeranylation, but not farnesylation are involved in the inhibition of colon cancer cell growth. Both mevalonate and squalene restored the cellular cholesterol level that was reduced by ATST treatment, but only mevalonate eliminated the cell growth inhibitory effect, suggesting that the cholesterol level in cells does not play an essential role in inhibiting cancer cell growth. Protein level of HMG‐CoA reductase increased after ATST treatment, and the presence of γ‐TT attenuated the elevated level of HMG‐CoA reductase. ATST also decreased membrane‐bound RhoA, possibly due to a reduced level of protein geranylgeranylation; addition of γ‐TT enhanced this effect. The mediation of HMG‐CoA reductase and RhoA provides a possible mechanism for the synergistic action of ATST and γ‐TT. The triple combination of ATST, γ‐TT and CXIB showed a synergistic inhibition of cancer cell growth in MTT assays. The synergistic action of these three compounds was also illustrated by their induction of G₀/G₁ phase cell cycle arrest and apoptosis.