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
- Research Support, N.I.H., Extramural
- Research Support, Non-U.S. Gov't
- Research Support, U.S. Gov't, Non-P.H.S.
MeSH terms
- Antibiotics, Antineoplastic / pharmacology
- Carcinoma, Pancreatic Ductal / drug therapy
- Carcinoma, Pancreatic Ductal / genetics*
- Carcinoma, Pancreatic Ductal / metabolism
- Carcinoma, Pancreatic Ductal / pathology
- Cell Line, Tumor
- Cell Proliferation / drug effects
- Extracellular Signal-Regulated MAP Kinases / antagonists & inhibitors
- Extracellular Signal-Regulated MAP Kinases / genetics*
- Extracellular Signal-Regulated MAP Kinases / metabolism
- Feedback, Physiological / drug effects
- Gene Expression Regulation, Neoplastic / drug effects*
- Humans
- Hypoglycemic Agents / pharmacology
- Indoles / pharmacology
- Insulin / pharmacology
- Metformin / pharmacology*
- Morpholines / pharmacology
- Neurotensin / pharmacology
- Pancreatic Neoplasms / drug therapy
- Pancreatic Neoplasms / genetics*
- Pancreatic Neoplasms / metabolism
- Pancreatic Neoplasms / pathology
- Phosphorylation / drug effects
- Proto-Oncogene Proteins c-akt / antagonists & inhibitors
- Proto-Oncogene Proteins c-akt / genetics*
- Proto-Oncogene Proteins c-akt / metabolism
- Purines / pharmacology
- Pyrimidines / pharmacology
- Ribosomal Protein S6 Kinases, 70-kDa / antagonists & inhibitors
- Ribosomal Protein S6 Kinases, 70-kDa / genetics
- Ribosomal Protein S6 Kinases, 70-kDa / metabolism
- Signal Transduction / drug effects
- Sirolimus / pharmacology*
- TOR Serine-Threonine Kinases / antagonists & inhibitors
- TOR Serine-Threonine Kinases / genetics*
- TOR Serine-Threonine Kinases / metabolism
Substances
- Antibiotics, Antineoplastic
- Hypoglycemic Agents
- Indoles
- Insulin
- Morpholines
- Purines
- Pyrimidines
- Neurotensin
- Ku 0063794
- Metformin
- MTOR protein, human
- Proto-Oncogene Proteins c-akt
- Ribosomal Protein S6 Kinases, 70-kDa
- TOR Serine-Threonine Kinases
- Extracellular Signal-Regulated MAP Kinases
- PP242
- Sirolimus