Chemosensitizing effects and sustained G1-S cell cycle arrest by low-dose RAD001 (everolimus) for cisplatin and 5-fluorouracil in human gastric cancer cells

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RAD001 (Everolimus) is a rapamycin derivative that specifically inhibits the mammalian target of rapamycin (mTOR). Phosphoinositide-3-kinase (PI3K)/AKT/mTOR pathway is actively participating in cell proliferation and survival of human gastric cancers. The PI3K/AKT pathway is negatively regulated by a tumor suppressor gene, the phosphatase and tensin homologue gene (PTEN). Loss of heterozygosity of PTEN and activation of PI3K/AKT/mTOR pathway are significantly more (>60%) in poorly differentiated advanced gastric cancers. RAD001 is a potential targeted agent in the treatment of gastric cancers.

A panel of human gastric cancer cell lines (N87, AGS, SNU-1, and SNU-16 from ATCC) was used in this study. Using MTT colorimetric assay, we demonstrated that RAD001, although with only modest growth inhibitory effects as a single agent, has significant synergistic cytotoxicity with cisplatin or 5-FU in gastric cancer cells. The concentration of RAD001 needed for synergism with cisplatin and 5-FU is as low as 0.5 to 5 nM. It is reasonable to use RAD001 as a chemosensitizing agent in gastric cancer.

RAD001 significantly inhibits the downstream molecules, such as 4EBP1 and S6 kinase-1 (S6K1), in human gastric cancer cells. Further, we found that either 5-FU or cisplatin can inhibit p-AKT in gastric cancer, and the addition of RAD001, at the low concentration of 0.5 to 5 nM, further enhances phosphorylated AKT (p-AKT) suppression, and up-regulates p21 causing sustained G1-S cell cycle arrest by these two agents. Importantly, at higher concentration of RAD001 (10 to 25 nM), compensatory activation of p-AKT and abrogation of p21 up-regulation are noted in gastric cancer cells, and these events may offset the chemosensitizing activity of RAD001.

In summary, our data indicate that the optimal dose of RAD001 as a chemosensitizing agent may be lower than what we normally use; as the higher dose will trigger the compensatory activation of p-AKT, also abrogate p21 up-regulation and G1-S cell cycle arrest in gastric cancer.