Autophagy Induction with RAD001 Enhances Chemosensitivity and Radiosensitivity through Met Inhibition in Papillary Thyroid Cancer

Abstract

Although autophagy is generally considered a prosurvival mechanism that preserves viability, there is evidence that it could drive an alternative programmed cell death pathway in cells with defects in apoptosis. Because the inhibition of autophagic activity promotes resistance to both chemotherapy and external beam radiation in papillary thyroid cancer (PTC), we determined if RAD001, a potent activator of autophagy, improves the efficacy of either therapy. We found that RAD001 increased the expression level of light chain 3-II, a marker for autophagy, as well as autophagosome formation in cell lines and in human PTC ex vivo. RAD001 sensitized PTC to doxorubicin and external beam radiation in a synergistic fashion, suggesting that combination therapy could improve therapeutic response at less toxic concentrations. The effects of RAD001 were abrogated by RNAi knockdown of the autophagy-related gene 5, suggesting that RAD001 acts, in part, by enhancing autophagy. Because the synergistic activity of RAD001 with doxorubicin and external radiation suggests distinct and complementary mechanisms of action, we characterized how autophagy modulates signaling pathways in PTC. To do so, we performed kinome profiling and discovered that autophagic activation resulted in Src phosphorylation and Met dephosphorylation. Src inhibition did not reverse the effects of RAD001, whereas Met inhibition reversed the effects of autophagy blockade on chemosensitivity. These results suggest that the anticancer effects of autophagic activation are mediated largely through Met. We conclude that RAD001 induces autophagy, which enhances the therapeutic response to cytotoxic chemotherapy and external beam radiation in PTC.

Autophagy and human diseases

“…Many drugs and compounds that modulate autophagy are currently receiving considerable attention [11,89,108]. These include, for example, autophagy inducers such as the mTORC1 inhibitor rapamycin [109] and its analogues (e.g., CCI-779 [109], RAD001 [110,111], and AP23573 [112]), mTOR kinase inhibitors (e.g., Torin 1 [113], and PP242 [114]), trehalose [115,116], carbamazepine [117], and the newly identified autophagy-inducing peptide Tatbeclin 1 [118]; autophagy inhibitors such as chloroquine [119,120] and hydroxychloroquine [121], Lys05 [122], 3-methyladenine [123] and its derivatives [124], PIK3C3 inhibitors [125], ATG4B inhibitors [126,127], and ATG7 inhibitors [128,129]. Autophagy-modulating drugs that are currently used in clinical trials are summarized in Table 2.…”

Autophagy as a mediator of chemotherapy-induced cell death in cancer

“…Such a death-promoting effect has been described for a large variety of drugs in different cancer cell types. The first examples are for DNA-damaging agents in cell models in which autophagy is inhibited either by pharmacological inhibitors like 3-methyladenine or by the use of siRNA targeting Beclin-1, Atg5 or Atg7: hepatoma cells [42] or cervical carcinoma cells [43] exposed to etoposide, papillary thyroid cancer cells [44] or different sarcoma cell lines [45] incubated with doxorubin, cervical cancer SiHa cells exposed to carboplatin [46] or pancreatic cancer cells treated with gemcitabine [47]. Autophagy also contributes to cell death induced by microtubule targeting agents like paclitaxel [48,49] as well as by the new "smart" drugs.…”

Pathological processes and therapeutic advances in radioiodide refractory thyroid cancer

“…In addition to effects on survival and proliferation, autophagy is also involved in cellular differentiation (Mizushima & Levine 2010, Helgason et al 2013. In TC specifically, autophagy has been shown to intertwine with oncogenic signaling and in vitro activation of autophagy is demonstrated to increase sensitivity of TC to treatment with chemotherapy, radiotherapy, TRAIL and mTOR kinase inhibitors (Lin et al 2010, Jin et al 2014, Morani et al 2014, Yi et al 2014, Plews et al 2015 and to promote resistance to BRAF inhibitors (Wang et al 2017). Furthermore, germline-genetic variants in autophagy genes, specifically ATG5 (rs2245214) and ATG16L1 p.Thr300Ala (rs2241880), known to functionally impair the autophagy machinery, are associated with genetic susceptibility to TC development and/or resistance to RAI treatment (Huijbers et al 2012, Plantinga et al 2014b.…”