mTOR Mediates Survival Signals in Malignant Mesothelioma Grown as Tumor Fragment Spheroids

American Journal of Respiratory Cell and Molecular Biology. 2008 May 29. [Epub ahead of print] [Link]

Wilson SM, Barbone D, Yang TM, Jablons DM, Bueno R, Sugarbaker DJ, Nishimura S, Gordon GJ, Broaddus VC.

University of California San Francisco, Lung Biology Center, San Francisco, CA, United States.


Solid tumors such as mesothelioma exhibit a stubborn resistance to apoptosis that may derive from survival pathways, such as PI3K/Akt/mTOR, that are activated in many tumors including mesothelioma. To address the role of PI3K/Akt/mTOR, we used a novel approach to study mesothelioma ex vivo as tumor fragment spheroids. Freshly resected mesothelioma tissue from 15 different patients was grown in vitro as 1-2 mm diameter fragments, exposed to apoptotic agents for 48 h with or without PI3K/Akt/mTOR inhibitors and doubly stained for cytokeratin and cleaved caspase 3 to identify apoptotic mesothelioma cells. Mesothelioma cells within the tumor spheroids exhibited striking resistance to apoptotic agents such as TRAIL plus gemcitabine that were highly effective against monolayers. In a majority of tumors (67%; 10 of 15), apoptotic resistance could be reduced by more than 50% by rapamycin, an mTOR inhibitor, but not by LY294002, a PI3K inhibitor. Responsiveness to rapamycin correlated with staining for the mTOR target, p-S6K, in the original tumor, but not for p-Akt. As confirmation of the role of mTOR, siRNA knockdown of S6K reproduced the effect of rapamycin in 3 rapamycin-responsive tumors. Finally, in 37 mesotheliomas on tissue microarray, p-S6K correlated only weakly with p-Akt suggesting the existence of Akt-independent regulation of mTOR. We propose that mTOR mediates survival signals in many mesothelioma tumors. Inhibition of mTOR may provide a non-toxic adjunct to therapy directed against malignant mesothelioma, especially in those with high baseline expression of p-S6K.