Targeting the IGF-1/mTOR pathway
Authors: Peter Houghton
Journal: Clinical Cancer Research (2008)

PL05-03   The mammalian target of rapamycin (mTOR) is a conserved serine/threonine kinase that lies downstream of AKT in the PI3 kinase pathway. mTOR in the mTORC1 complex (raptor-mTOR) is regulated by cellular stress (growth factors, energy charge, nutrients, hypoxia), and controls transit from G1 phase to S phase of the cell cycle. The mTORC1 complex regulates many cellular processes including cap-dependent translation, transcription, proliferation and survival. The immunosuppressive agent rapamycin, a macrocyclic lactone antibiotic, is a highly selective inhibitor of the mTORC1 complex, and inhibits proliferation and in some cell lines induces apoptosis. While the activity of rapamycin as an antitumor agent was recognized over 30 years ago it was not developed due to its poor pharmaceutical properties (1). More recently analogs of rapamycin (rapalogs) with enhanced pharmaceutical properties have been developed. In preclinical models rapamycin and its analogs potently inhibit cancer cell proliferation, and have demonstrated broad-spectrum cytostatic activity against both rodent syngeneic tumors and human tumor xenografts (2). In vivo activity appears to be both directly on tumor cell proliferation and survival, and through its anti-angiogenic activities (3, 4). In clinical trials rapalogs have demonstrated wide-spectrum activity, with notable activity in mantle cell lymphoma, renal cell carcinoma, and endometrial carcinoma (5, 6).  In many preclinical models both in vitro and in vivo, rapamycin anecdotally activates Akt, leading to hyperphosphorylation of Ser473 required for full activation (7, 8). Several studies have shown that this is a result of diminished feedback inhibition on IRS-1 due to inhibition of S6K1 downstream of the mTORC1 complex (9). This leads to activation of IGF-1 receptor (IGF-1R)-mediated signaling and activation of mTORC2, the putative Ser473 kinase. However, recently, we have identified another mechanism by which AKT is activated; in the presence of rapamycin we found hyperphosphorylation of IGF-1R due to increased secretion of IGF’s (Kurmasheva and Houghton, unpublished data). IGF’s are potent survival factors, inhibiting the activity of pro-apoptotic proteins such as Bad through phosphorylation and sequestration with 14-3-3 proteins (10). Anti-apoptotic IGF-1R signaling may be either AKT-dependent or independent. Thus, there is reason to consider blocking IGF-1R signaling in the context of mTORC1 inhibition. We have previously shown that in the absence of exogenous IGF’s rapamycin induced apoptosis in cancer cells that are deficient or mutant for TP53, whereas cells with functional TP53 remained viable (11, 12). IGF’s rescued from rapamycin through a PI3K-dependent, AKT-independent pathway (13).  We have tested the hypothesis that dual blockade of IGF-1R with mTORC1 inhibition would be effective therapy for childhood sarcomas. In vitro combination of rapamycin with CP751871, a human monoclonal antibody that prevents ligand binding, was synergistic in inducing apoptosis in five Ewing sarcoma cell lines, and several rhabdomyosarcoma cell lines in vitro. The approach was further evaluated against rhabdomyosarcoma, Ewing sarcoma and osteosarcoma xenografts in scid-/- mice. The combination was synergistic in all osteosarcoma models, and several models of Ewing sarcoma or rhabdomyosarcoma, causing complete regressions of established xenografts.  Pharmacodynamic studies in these models are being undertaken to identify reliable biomarkers for predicting tumor response to CP751871 alone, rapamycin alone and the combination.  References  1.	Huang S, Bjornsti MA, Houghton PJ. Rapamycins: mechanism of action and cellular resistance. Cancer biology & therapy 2003;2(3):222-32.  2.	Easton JB, Houghton PJ. Therapeutic potential of target of rapamycin inhibitors. Expert Opin Ther Targets 2004;8(6):551-64.  3.	Bjornsti MA, Houghton PJ. The TOR pathway: a target for cancer therapy. Nat Rev Cancer 2004;4(5):335-48.  4.	Guba M, von Breitenbuch P, Steinbauer M, et al. Rapamycin inhibits primary and metastatic tumor growth by antiangiogenesis: involvement of vascular endothelial growth factor. Nat Med 2002;8(2):128-35.  5.	Witzig TE, Geyer SM, Ghobrial I, et al. Phase II trial of single-agent temsirolimus (CCI-779) for relapsed mantle cell lymphoma. J Clin Oncol 2005;23(23):5347-56.  6.	Atkins MB, Hidalgo M, Stadler WM, et al. Randomized phase II study of multiple dose levels of CCI-779, a novel mammalian target of rapamycin kinase inhibitor, in patients with advanced refractory renal cell carcinoma. J Clin Oncol 2004;22(5):909-18.  7.	Shi Y, Yan H, Frost P, Gera J, Lichtenstein A. Mammalian target of rapamycin inhibitors activate the AKT kinase in multiple myeloma cells by up-regulating the insulin-like growth factor receptor/insulin receptor substrate-1/phosphatidylinositol 3-kinase cascade. Mol Cancer Ther 2005;4(10):1533-40.  8.	O9Reilly KE, Rojo F, She QB, et al. mTOR inhibition induces upstream receptor tyrosine kinase signaling and activates Akt. Cancer research 2006;66(3):1500-8.  9.	Easton JB, Kurmasheva RT, Houghton PJ. IRS-1: auditing the effectiveness of mTOR inhibitors. Cancer Cell 2006;9(3):153-5.  10.	Kurmasheva RT, Houghton PJ. IGF-I mediated survival pathways in normal and malignant cells. Biochimica et biophysica acta 2006;1766(1):1-22.  11.	Huang S, Liu LN, Hosoi H, Dilling MB, Shikata T, Houghton PJ. p53/p21(CIP1) cooperate in enforcing rapamycin-induced G(1) arrest and determine the cellular response to rapamycin. Cancer research 2001;61(8):3373-81.  12.	Huang S, Shu L, Dilling MB, et al. Sustained activation of the JNK cascade and rapamycin-induced apoptosis are suppressed by p53/p21(Cip1). Molecular cell 2003;11(6):1491-501.  13.	Thimmaiah KN, Easton J, Huang S, et al. Insulin-like growth factor I-mediated protection from rapamycin-induced apoptosis is independent of Ras-Erk1-Erk2 and phosphatidylinositol 39-kinase-Akt signaling pathways. Cancer research 2003;63(2):364-74.