Download THE MAMMALIAN TARGET OF RAPAMYCIN (MTOR) AS A

THE MAMMALIAN TARGET OF RAPAMYCIN (MTOR) AS A TARGET
FOR CANCER CHEMOTHERAPY
James J. Gibbons, Jr.*, L. Toral-Barza, W-G Zhang, P. Frost, J. Skotnicki, C.
Gaydos, J. Lucas and K. Yu
Wyeth Research, 401 N. Middletown Rd., Pearl River, NY 10965
*[email protected]
The mammalian target of rapamycin (mTOR) was originally identified in yeast as
the protein responsible for mediating the anti-fungal activity of rapamycin1, a
natural product originally isolated and characterized at Wyeth in 19752.
Rapamycin has been a critical reagent in the elucidation of cell signaling
pathways regulated by mTOR. The work of many investigators has established
that mTOR regulates cell size and proliferation in part through control of protein
synthesis of a subset of cell cycle regulatory proteins3,4. In yeast, TOR activity is
regulated by nutrient availability and in mammalian cells mTOR appears to
integrate signals from both nutrient and growth factor pathways. Inasmuch as
human oncogenes and tumor suppressor genes are often components of growth
factor signaling pathways, mTOR is a potential target for anti-cancer
chemotherapy.
We have studied the effects of mTOR inhibition by the rapamycin ester CCI-779
(Temsirolimus) in preclinical models of breast, prostate, glioblastoma, and renal
cell carcinoma. In breast cancer models responsiveness to mTOR inhibition
correlated with activation of the PI3K pathway either through growth factor
dependence (estradiol), mutation of growth factor receptor (her2/neu), or loss of
the tumor suppressor gene PTEN.5 Cells resistant to the mTOR inhibitor were
rendered sensitive by stable transfection of activated Akt (myristoylated) or by
knockout of the PTEN gene using a cre-lox system. PTEN-/- lines across several
tissue types were invariably sensitive to CCI-7796,7 whereas PTEN+/+ lines were
either sensitive or insensitive. Sensitive lines tended to show evidence of Akt
activation.
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In a screening assay employing PTEN-/- cells, we identified non-rapamycin like
compounds that selectively inhibited proliferation of PTEN-/- cells. Preliminary
characterization of these compounds suggest they partially inhibit mTOR through
modulation of an Akt regulated target TSC2. Taken together, these data suggest
that persistent activation of Akt confers sensitivity to mTOR inhibitors.
The effect of mTOR inhibition on protein expression is not restricted to cell cycle
proteins8. In a human renal cell carcinoma model deficient for the Von Hippel
Lindau (VHL) tumor suppressor gene, CCI-779 inhibited production of the
hypoxia inducible transcription factor HIF-2α. Expression of HIF-2α regulated
genes such as vascular endothelial cell growth factor (VEGF) were also
suppressed by the mTOR inhibitor. In addition, proangiogenic cytokines such as
IL-8 and IL-6 were down regulated by CCI-779 by an as yet undefined
mechanism. These data prompted us to study the effect of CCI-779 in
combination with other anti-angiogenic agents. The combination of interferon-α
with CCI-779 in renal cell carcinoma xenografts synergistically inhibited tumor
growth despite showing little evidence of even additivity on cell proliferation in
vitro. These data suggest an anti-angiogenic effect of mTOR inhibition in this
renal cancer model.
mTOR inhibition by CCI-779 (Temsirolimus) in clinical trials has shown
objective responses in renal cell and breast carcinoma and mantle cell lymphoma.
Preclinical studies suggest the potential for direct anti-proliferative and antiangiogenic activity through mTOR inhibition. Inasmuch as the tendency in the
clinic of some tumor types to respond to mTOR inhibition with a stable disease
phenotype (renal, breast) and others with significant objective responses (mantle
cell lymphoma), the anti-tumor effector molecules downstream of mTOR may
depend on cell context and may differ amongst tumor types. Identification of
mTOR regulated anti-tumor targets will be essential to optimize the single agent
activity of mTOR inhibitors and to optimize combination therapies employing
mTOR inhibitors.
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REFERENCES
1. Heitman J, Movva NR, and Hall MN. Targets for cell cycle arrest by the
immuno-suppressant rapamycin in yeast. Science 53:905, 1991.
2. Vezina C, Kudelski A, and Sehgal SN. Rapamycin (AY-22, 989), a new
anti-fungal antibiotic. I. Taxanomy of the producing streptomycete and
isolation of the active principle. J Antibiot (Tokyo) 10:721, 1975.
3. Schmegle T and Hall MN. TOR, a central controller of cell growth. Cell
103:193, 2000.
4. Hay N and Sonenberg N. Upsteam and downstream of mTOR. Genes Dev
18:1926, 2004.
5. Yu K, Toral-Barza L, Discafani C, Zhang WG, Skotnicki J, Frost P, and
Gibbons JJ. mTOR, a novel target in breast cancer: the effect of CCI-779,
an mTOR inhibitor, in preclinical models of breast cancer. Endoci Relat
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Frost P, Gibbons JJ, Wu H, and Sawyers CL. Enhanced sensitivity of
PTEN-deficient tumors to inhibition of FRAP/mTOR. PNAS 98:103,
2001.
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Wang H, Yeng L, Gibbons JJ, Frost P, Dreisbeck V, Blenis J, Gaciong Z,
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of mTOR reduces neoplasia and normalizes P70/S6 kinase activity in
PTEN +/- mice. PNAS 98:10320, 2001.
8. Laughner E, Taghari P, Chiles K, Mahon PC, and Semenza GL. Her2
(neu) signaling increases the rate of hypoxia-inducible factor 1α (HIF-1α)
synthesis: a novel mechanism for HIF-1 mediated vascular endothelial
growth factor expression. Mol Cell Biol 21:3995, 2001.
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