Effect of sequential docetaxel followed by mTOR inhibitor temsirolimus on suppression of PI3K overactivation resistance mechanism.

Abstract

88 Background: Mammalian target of rapamycin (mTOR) is a downstream regulatory protein of the PI3K/Akt signal transduction pathway. This is a common pathway for a several cell surface receptors including IGFR (Insulin-like Growth Factor Receptor) and EGFR (Epidermal Growth Factor Receptor). The activation of these receptors through PI3K/Akt pathway is essential in cell proliferation, angiogenesis, and anti-apoptosis process. The upregulation of PI3K could be a mechanism of resistance of mTOR inhibitors. Docetaxel (D) is commonly used in the treatment of lung cancer. We demonstrated previously that the sequence of D followed by mTOR inhibitor temsirolimus (T) in lung cancer cell lines (LCCL) had synergistic effect in suppressing cell proliferation compared with T→D. The exact mechanism of this effect is unknown. We studied the expression of mTOR and PI3K in these cell lines treated in different time points to investigate the activity of this pathway when using these sequences of drug treatment. Methods: Adenocarcinoma LCCL H2122 and H1437 were plated and exposed to temsirolimus 1000nM and docetaxel 100nM. The cell viability was measured by optical density (OD) at 24, 48, and 72h. We tested effect of drugs D and T alone as well as the sequence of D treated for 24h followed by addition of T and the reverse in both LCCL. We then prepared cell lysate at 24h, 48h, and 72h time points and studied the expression of phospho mTOR (pmTOR) and PI3K by western blot using antibody obtained from cell signaling. Results: The use of T alone increased the expression of PI3K in both H2122 and H1437 cell lines at 48h time point. The use of D had a variable response: absent in H1437 and present in H2122 at 48 H. The sequence of D→T suppressed the expression of pmTOR and PI3K at 48 and 72 h compared with the opposite sequence of T→D. Conclusions: The combination of D → T is synergistic in suppression of pmTOR and inhibited the overactivation of upstream PI3K in both LCCL compared with opposite sequence. Therefore, the sequential treatment of D followed by T is able to overcome the PI3K overactivation mechanism of resistance in lung cancer cell line when treated with T. This would have implications in the use of these agents in treatment of lung cancer.