| Stimulation of the insulin and IGF-1 receptor activates the PI3K/Akt/mTOR pathway causing pleiotropic cellular effects including an mTOR-dependent loss in IRS-1 expression leading to feedback down-regulation of signaling through the pathway. In model systems, tumors exhibiting mutational activation of PI3K/Akt kinase, a common event in cancers, are hypersensitive to mTOR inhibitors, including rapamycin. Despite the activity in model systems, in patients, mTOR inhibitors exhibit more modest antitumor activity. We now demonstrate that mTOR inhibition induces IRS-1 expression and abrogates feedback inhibition of the pathway, resulting in Akt activation both in cancer cell lines and in patient tumors treated with the rapamycin derivative RAD001. IGF-1R inhibition prevents rapamycin-induced Akt activation, enhances inhibition of cap-dependent translation, and sensitizes tumor cells to inhibition of mTOR. The data suggest that feedback down-regulation of receptor tyrosine kinase signaling is a frequent event in tumor cells with constitutive mTOR activation. Reversal of this feedback loop by rapamycin may attenuate its therapeutic effects, whereas combination therapy that ablates mTOR function and prevents Akt activation through IGF-IR may have improved antitumor activity.; We also find that IGF-1 reverses the antiproliferative effects of rapamycin in serum free media. We show that IGF-1 rescues mTOR inhibitor-treated cancer cells by overcoming rapamycin-induced translational blockade. In addition to reversing cell growth inhibition, cell cycle arrest, and cell death, IGF-1 rescues p-4E-BP1 (T37/46, S65, T70) and cyclin D3 levels in 50 nM rapamycin-treated cells. We show that by mediating 4E-BP1 phosphorylation in the setting of mTOR inhibition, IGF-1 prevents rapamycin-induced association of 4E-BP1 with the mRNA 7-methylguanosine cap, thus promoting cap-dependent translation, and rescuing cyclin D3 levels. The rescue of p-4E-BP1 despite dissociation of the mTORc1 and mTORc2 complexes and sustained inhibition of S6K (T389) suggests IGF-1-mediated, non-mTOR inputs to 4E-BP1. This data illustrates that growth factor receptor ligands like IGF-1 can render cancer cells resistant to single agent mTOR inhibition by disinhibiting cap-dependent translation. |
