Abstract
The mTOR pathway is aberrantly stimulated in many cancer cells, including pancreatic ductal adenocarcinoma (PDAC), and thus it is a potential target for therapy. However, the mTORC1/S6K axis also mediates negative feedback loops that attenuate signaling via insulin/IGF receptor and other tyrosine kinase receptors. Suppression of these feed-back loops unleashes over-activation of upstream pathways that potentially counterbalance the antiproliferative effects of mTOR inhibitors. Here, we demonstrate that treatment of PANC-1 or MiaPaCa-2 pancreatic cancer cells with either rapamycin or active-site mTOR inhibitors suppressed S6K and S6 phosphorylation induced by insulin and the GPCR agonist neurotensin. Rapamycin caused a striking increase in Akt phosphorylation at Ser(473) while the active-site inhibitors of mTOR (KU63794 and PP242) completely abrogated Akt phosphorylation at this site. Conversely, active-site inhibitors of mTOR cause a marked increase in ERK activation whereas rapamycin did not have any stimulatory effect on ERK activation. The results imply that first and second generation of mTOR inhibitors promote over-activation of different pro-oncogenic pathways in PDAC cells, suggesting that suppression of feed-back loops should be a major consideration in the use of these inhibitors for PDAC therapy. In contrast, metformin abolished mTORC1 activation without over-stimulating Akt phosphorylation on Ser(473) and prevented mitogen-stimulated ERK activation in PDAC cells. Metformin induced a more pronounced inhibition of proliferation than either KU63794 or rapamycin while, the active-site mTOR inhibitor was more effective than rapamycin. Thus, the effects of metformin on Akt and ERK activation are strikingly different from allosteric or active-site mTOR inhibitors in PDAC cells, though all these agents potently inhibited the mTORC1/S6K axis.
Publication types
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Research Support, N.I.H., Extramural
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Research Support, Non-U.S. Gov't
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Research Support, U.S. Gov't, Non-P.H.S.
MeSH terms
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Antibiotics, Antineoplastic / pharmacology
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Carcinoma, Pancreatic Ductal / drug therapy
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Carcinoma, Pancreatic Ductal / genetics*
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Carcinoma, Pancreatic Ductal / metabolism
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Carcinoma, Pancreatic Ductal / pathology
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Cell Line, Tumor
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Cell Proliferation / drug effects
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Extracellular Signal-Regulated MAP Kinases / antagonists & inhibitors
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Extracellular Signal-Regulated MAP Kinases / genetics*
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Extracellular Signal-Regulated MAP Kinases / metabolism
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Feedback, Physiological / drug effects
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Gene Expression Regulation, Neoplastic / drug effects*
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Humans
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Hypoglycemic Agents / pharmacology
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Indoles / pharmacology
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Insulin / pharmacology
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Metformin / pharmacology*
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Morpholines / pharmacology
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Neurotensin / pharmacology
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Pancreatic Neoplasms / drug therapy
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Pancreatic Neoplasms / genetics*
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Pancreatic Neoplasms / metabolism
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Pancreatic Neoplasms / pathology
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Phosphorylation / drug effects
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Proto-Oncogene Proteins c-akt / antagonists & inhibitors
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Proto-Oncogene Proteins c-akt / genetics*
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Proto-Oncogene Proteins c-akt / metabolism
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Purines / pharmacology
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Pyrimidines / pharmacology
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Ribosomal Protein S6 Kinases, 70-kDa / antagonists & inhibitors
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Ribosomal Protein S6 Kinases, 70-kDa / genetics
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Ribosomal Protein S6 Kinases, 70-kDa / metabolism
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Signal Transduction / drug effects
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Sirolimus / pharmacology*
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TOR Serine-Threonine Kinases / antagonists & inhibitors
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TOR Serine-Threonine Kinases / genetics*
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TOR Serine-Threonine Kinases / metabolism
Substances
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Antibiotics, Antineoplastic
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Hypoglycemic Agents
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Indoles
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Insulin
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Morpholines
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Purines
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Pyrimidines
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Neurotensin
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Ku 0063794
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Metformin
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MTOR protein, human
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Proto-Oncogene Proteins c-akt
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Ribosomal Protein S6 Kinases, 70-kDa
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TOR Serine-Threonine Kinases
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Extracellular Signal-Regulated MAP Kinases
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PP242
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Sirolimus