Combining mTOR Inhibition with Radiation Improves Antitumor Activity in Bladder Cancer Cells In Vitro and In Vivo: A Novel Strategy for Treatment

Abstract

PurposeRadiation therapy for invasive bladder cancer allows for organ preservation but toxicity and local control remain problematic. As such, improving efficacy of treatment requires radiosensitization of tumor cells. The aim of study is to investigate if the mammalian Target of Rapamycin (mTOR), a downstream kinase of the phosphatidylinositol 3-kinase (PI3K)/AKT survival pathway, may be a target for radiation sensitization.Experimental DesignClonogenic assays were performed using 6 bladder cancer cell lines (UM-UC3, UM-UC5, UM-UC6, KU7, 253J-BV, and 253-JP) in order to examine the effects of ionizing radiation (IR) alone and in combination with RAD001, an mTOR inhibitor. Cell cycle analysis was performed using flow cytometry. In vivo, athymic mice were subcutaneously injected with 2 bladder cancer cell lines. Treatment response with RAD001 (1.5 mg/kg, daily), fractionated IR (total 9Gy = 3Gy×3), and combination of RAD001 and IR was followed over 4 weeks. Tumor weight was measured at experimental endpoint.ResultsClonogenic assays revealed that in all bladder cell lines tested, an additive effect was observed in the combined treatment when compared to either treatment alone. Our data indicates that this effect is due to arrest in both G1 and G2 phases of cell cycle when treatments are combined. Furthermore, our data show that this arrest is primarily regulated by changes in levels of cyclin D1, p27 and p21 following treatments. In vivo, a significant decrease in tumor weight was observed in the combined treatment compared to either treatment alone or control.ConclusionsAltering cell cycle by inhibiting the mTOR signaling pathway in combination with radiation have favorable outcomes and is a promising therapeutic modality for bladder cancer.

Mammalian Target of Rapamycin Inhibition With Rapamycin Mitigates Radiation-Induced Pulmonary Fibrosis in a Murine Model

“…Although the use of mTOR inhibitors in this context is exciting due to its availability for clinical trials and evidence suggesting tumor radiosensitization with mTOR inhibition (41–46), this must be tempered by the previously observed toxicities of mTOR inhibitors (18). One adverse event in particular, reversible pneumonitis, is of obvious concern, but appears to be dosing sensitive and completely reversible with treatment cessation (47).…”

Improved antitumor effect of ionizing radiation in combination with rapamycin for treating nasopharyngeal carcinoma

“…Previous studies have shown that inhibition of these Akt-associated pro-survival signaling pathways enhances the radiosensitivity to RT of various cancers (12)(13)(14)(15). This improvement may be due to, firstly, the antifungal characteristics of rapamycin itself, and secondly, to the excessive autophagy induced by rapamycin (15). In addition, Liu et al (22) demonstrated that leukemia inhibitory factor (LIF) activated p70 S6 kinase (p70S6K) and its downstream molecules, GSK3α/β, thus leading to the proliferation of NPC cells.…”

“…Additionally, mTOR can decrease the kinase activity of GSK3β, responsible for the degradation of cyclin D1 (11). Previous studies have shown that mTOR inhibition improves the radiosensitivity of prostate, breast, bladder and brain cancer (12)(13)(14)(15).…”

“…Rapamycin, an important inhibitor of mTOR and a drug used for suppressing autoimmune responses in organ transplantation (16), has been reported to act as a radiosensitizer in the RT treatment of prostate, breast, bladder and brain cancer (12)(13)(14)(15). However, the anti-tumor effects of the combination of rapamycin and ionizing radiation (IR) for treating NPC are still unknown.…”

mTOR inhibitors as radiosensitizers in neuroendocrine neoplasms

“…In contrast, Nassim et al reported an increase of both G1 and G2/M when combining everolimus with radiation in bladder cancer cells (44). They suggest that the everolimusinduced G1 arrest confers an enhanced sensitivity for the following radiation as fewer cells were counted in the rather radioresistant S-phase (45) Equal amounts of protein were separated on SDS gels and the indicated molecules detected after western blotting.…”