mTOR mediates survival signals in malignant mesothelioma grown as tumor fragment spheroids.
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- to 2-mm-diameter fragments, exposed to apoptotic agents for 48 hours 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 three 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 nontoxic adjunct to therapy directed against malignant mesothelioma, especially in those with high baseline expression of p-S6K.
Wilson
Shannon M
SM
Lung Biology Center, UCSF, San Francisco, CA 94143-0854, USA.
Barbone
Dario
D
Yang
Tsung-Ming
TM
Jablons
David M
DM
Bueno
Raphael
R
Sugarbaker
David J
DJ
Nishimura
Stephen L
SL
Gordon
Gavin J
GJ
Broaddus
V Courtney
VC
D045744:Cell Line, Tumor; D002470:Cell Survival / Q000187:drug effects; D002867:Chromones / Q000494:pharmacology; D003513:Cycloheximide / Q000494:pharmacology; D003841:Deoxycytidine / Q000031:analogs & derivatives / Q000494:pharmacology; D018432:Drug Resistance, Multiple / Q000187:drug effects; D019008:Drug Resistance, Neoplasm / Q000187:drug effects; D006801:Humans; D008654:Mesothelioma / Q000235:genetics / Q000378:metabolism* / Q000473:pathology*; D009025:Morpholines / Q000494:pharmacology; D010766:Phosphorylation / Q000187:drug effects; D011494:Protein Kinases / Q000378:metabolism*; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D034741:RNA, Small Interfering / Q000235:genetics; D019893:Ribosomal Protein S6 Kinases / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects; D020123:Sirolimus / Q000494:pharmacology; D018874:Spheroids, Cellular; D053221:TNF-Related Apoptosis-Inducing Ligand / Q000494:pharmacology; D058570:TOR Serine-Threonine Kinases; D000093542:Gemcitabine
D002867:Chromones; D009025:Morpholines; D034741:RNA, Small Interfering; D053221:TNF-Related Apoptosis-Inducing Ligand; D003841:Deoxycytidine; C085911:2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one; D003513:Cycloheximide; D011494:Protein Kinases; C546842:MTOR protein, human; D051057:Proto-Oncogene Proteins c-akt; D019893:Ribosomal Protein S6 Kinases; D058570:TOR Serine-Threonine Kinases; D020123:Sirolimus; D000093542:Gemcitabine