Abstract 4655: Iron chelation inhibits mTORC1 signaling in tumor cells

Abstract

Abstract Iron chelators are commonly used for treatment of iron overload diseases. Recent studies have shown that iron chelators possess potent anticancer activities. Iron chelators (ciclopirox and 3-AP) are in clinical trials for cancer treatment. However, the molecular mechanism underlying the anticancer effect of iron chelation is not well understood. Mammalian/mechanistic target of rapamycin (mTOR) is a master kinase, regulating cell growth/proliferation and survival through sensing environmental cues. mTOR functions as at least two complexes (mTORC1/2). Here we show that two iron chelators, ciclopirox olamine (CPX) and Dp44mT, inhibit tumor cell proliferation by suppressing mTORC1 signaling in human rhabdomyosarcoma (Rh30) and lung cancer (A549 and A427) cells. This is evidenced by the findings that treatment with CPX or Dp44mT inhibited the phosphorylation of 4E-BP1 and S6K1, two best known effector molecules of mTORC1, which could be blocked by addition of ferrous sulfate. Addition of ferrous sulfate prevented the growth inhibitory effect of CPX. Ectopic expression of constitutively active mTOR also conferred resistance to CPX-induced growth inhibition. Mechanistically, iron chelation inhibition of mTORC1 was partially linked to the activation of AMPK, a negative regulator of mTORC1. Overexpression of a dominant-negative AMPK (DN-AMPK) in Rh30 cells attenuated the inhibitory effect of CPX or Dp44mT on the phosphorylation of S6K1 and 4E-BP1. In addition, iron chelation also transiently stabilized HIF-1α and further upregulated the expression of its downstream effector Bnip3, another negative regulator of mTORC1. Knockdown of Bnip3 partially attenuated the inhibitory effect of iron chelation on mTORC1. Furthermore, we found that iron chelation increased the translocation of mTOR from the cytoplasm to the nucleus. The co-localization of mTOR with the third negative regulator of mTOR, PML (promyelocytic leukemia), increased in the nucleus upon iron chelation. Moreover, iron chelation did not obviously alter the phosphorylation of IGF-1 receptor, PI3K or PTEN. The effect of iron chelation on mTORC2-mediated phosphorylation of Akt was cell line dependent. Taken together, the results suggest that the anticancer effect of iron chelation is in part related to inhibition of mTORC1 signaling, and iron chelation inhibits mTORC1 signaling via multiple mechanisms. (Supported by the FWCC, LSU Health Sciences Center) Citation Format: Chaowei Shang, Hongyu Zhou, Shile Huang. Iron chelation inhibits mTORC1 signaling in tumor cells. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4655.