Ribosomal protein S27-like regulates autophagy via the β-TrCP-DEPTOR-mTORC1 axis

RPS27L (Ribosomal protein S27-like), an evolutionarily conserved ribosomal protein, is a p53 target and a physiological p53 regulator. We previously reported that Rps27l disruption enhanced lymphomagenesis in Trp53^+/- mice by triggering genome instability and sensitized Trp53^+/- mice to radiation by blocking DNA damage response. Whether and how RPS27L modulates autophagy is totally unknown. Here we report that RPS27L silencing significantly induced autophagy in breast cancer MB231 and SK-BR3 cells harboring mutant p53. Mechanistically, RPS27L silencing remarkably inactivated mTORC1, a major negative autophagy regulator, but not mTORC2. Autophagy induction and mTORC1 inactivation was also observed in MEFs with Rps27l deletion. More specifically, RPS27L silencing shortened the protein half-life of β-TrCP, a substrate receptor of Skp1-Cullin 1-F-box (SCF) ubiquitin ligase, which is responsible for DEPTOR degradation, leading to DEPTOR accumulation to inhibit mTORC1 activity. Furthermore, RPS27L silencing-induced autophagy and mTORC1 inactivation can be partially rescued by simultaneous DEPTOR silencing, suggesting a causal role of DEPTOR. Biologically, autophagy inhibitor, chloroquine (CQ), or Bafilomycin A1 (BAF A1), significantly induced apoptosis in RPS27L silenced cells, indicating that autophagy is a cellular survival mechanism in response to RPS27L loss. Finally, RPS27L levels were reduced in human breast cancers, as compared to adjacent normal tissues. Collectively, our study suggests that RPS27L reduction might play a promoting role during breast tumorigenesis by autophagy induction via the β-TrCP-DEPTOR-mTORC1 axis.