Rapamycin induces feedback activation of Akt signaling through an IGF-1R-dependent mechanism

Abstract

Rapamycin and several analogs, such as CCI-779 and RAD001, are currently undergoing clinical evaluation as anticancer agents. In this study, we show that inhibition of mammalian target of rapamycin (mTOR) signaling by rapamycin leads to an increase of Akt phosphorylation in Rh30 and RD human rhabdomyosarcoma cell lines and xenografts, and insulin-like growth factor (IGF)-II-treated C2C12 mouse myoblasts and IGF-IIoverexpressing Chinese hamster ovary cells. RNA interference-mediated knockdown of S6K1 also results in an increase of Akt phosphorylation. These data suggest that mTOR/S6K1 inhibition either by rapamycin or small interfering RNA (siRNA) triggers a negative feedback loop, resulting in the activation of Akt signaling. We next sought to investigate the mechanism of this negative feedback regulation from mTOR to Akt. Suppression of insulin receptor substrate (IRS)-1 and tuberous sclerosis complex-1 by siRNAs failed to abrogate rapamycininduced upregulation of Akt phosphorylation in both Rh30 and RD cells. However, pretreatment with h7C10 antibody directed against insulin-like growth factor-1 receptor (IGF-1R) led to a blockade of rapamycin-induced Akt activation. Combined mTOR and IGF-1R inhibition with rapamycin and h7C10 antibody, respectively, resulted in additive inhibition of cell growth and survival. These data suggest that rapamycin mediates Akt activation through an IGF-1R-dependent mechanism. Thus, combining an mTOR inhibitor and an IGF-1R antibody/inhibitor may be an appropriate strategy to enhance mTOR-targeted anticancer therapy.

Validation of the type 1 insulin-like growth factor receptor as a therapeutic target in renal cancer

“…4A), suggesting that rapamycin had released mTOR-induced feedback inhibition on IGF1R signaling. Similar enhancement of Akt phosphorylation has been observed following rapamycin treatment in myeloblasts, myeloma, prostate and breast cancer cells in vitro, and in RCC biopsies following clinical mTOR inhibitor treatment (44)(45)(46), suggesting that this phenomenon is clinically relevant.…”

14-3-3 Proteins regulate Akt Thr308 phosphorylation in intestinal epithelial cells

“…Consistent with our findings, Akt Thr308 phosphorylation has been observed in other models where raptor has been downregulated or inactivated (rapamycin). [44][45][46] Thus, based on this premise, we suggest that probably association of raptor with 14-3-3 proteins could serve two major purposes: (1) inhibit mTORC1 function to trigger autophagy and therefore 14-3-3η degradation and PDK1 activation; and (2) release the TOR components to 12 μM) or IFNγ/Akt Inhibitor VIII. Treatment was performed for 36 h. Actin was used as a loading control.…”

Identification of dual mTORC1 and mTORC2 inhibitors in melanoma cells: Prodigiosin vs. obatoclax

“…inhibition, which leads to AKT activation through IGF-1/IRS-1 signaling (19). The effectiveness of prodiginines relies on their ability to inhibit mTOR activity.…”

“…These pathways are critical to melanoma progression and both are deregulated in melanoma, but not in normal cells (19,20). Thus, compounds that counteract these feedback loops are considered in cancer therapy.…”

“…Inhibition of mTORC1 induces activation of S6K-1/IRS-1 negative feedback (19). Dual inhibition of mTORC1 and mTORC2 might counteract this mechanism.…”