Dysregulation of mTORC1 Signaling in Childhood Solid Tumors
The mTORC1 complex regulates cap-dependent translation, and is essential for cell proliferation. Nutrient deficiency, and hypoxia, conditions characteristic of solid tumors, suppress mTORC1 signaling, yet tumor cells maintain proliferation. We have examined the pathway by which hypoxia suppresses mTORC1 signaling and examined the consequences of maintained signaling under hypoxic conditions.
Regulation of mTORC1 signaling was examined by phosphorylation of downstream substrates (4E-BP1 and S6 protein) in cell lines with or without ATM, as well as mutants defective in DNA damage repair (NBS1-/-), p53-/-, and REDD1. Expression of ATM (Affymetrix U133+2, and protein levels) was determined in the xenograft models developed by the Pediatric Preclinical Testing Program (PPTP). Immunohistochemical staining for ATM, pS6 on normal tissues and PPTP solid tumor and acute lymphoblastic leukemia xenografts were undertaken.
Hypoxia (0.2% O2) rapidly suppresses mTORC1 signaling in ATM+/+ cells but not in ATM-/- cells, placing ATM in the hypoxia-regulated mTORC1 signaling pathway. ATM activation is independent of DNA damage and does not require ATM phosphorylation (S1981), or NBS1. ATM phosphorylates HIF1α(S696), increasing its stability, nuclear localization and transcription of REDD1. Maintained mTORC1 signaling under hypoxia results in p53-dependent, CD95-dependent apoptosis. IHC studies shows mTORC1 signaling in hypoxic tumor zones and the expression of ATM is 10- to 20-fold lower in solid tumor xenografts than in acute lymphoblastic leukemia xenografts.
Hypoxia regulation of mTORC1 signaling is ATM-dependent, and ATM rapidly phosphorylates and stabilizes HIF1α. Maintained mTORC1 signaling under hypoxia leads to apoptosis. These results suggest that ATM expression is suppressed early in development of childhood solid tumors, and subsequently either p53 signaling or components of the extrinsic death pathway are abrogated.