A new PICTure of nucleolar stress

Abstract

Cell growth demands new protein synthesis, which requires nucleolar ribosomal functions. Ribosome biogenesis consumes a large proportion of the cell's resources and energy, and so is tightly regulated through an intricate signaling network to guarantee fidelity. Thus, events that impair ribosome biogenesis cause nucleolar stress. In response to this stress, several nucleolar ribosomal proteins (RPs) translocate to the nucleoplasm and bind to MDM2. MDM2-mediated ubiquitination and degradation of the tumor suppressor p53 is then blocked, resulting in p53 accumulation and the induction of p53-dependent cell cycle arrest and apoptosis. Nucleolar stress is therefore a quality control surveillance mechanism that monitors the synthesis and assembly of the rRNA and protein components of ribosomes. Although nucleolar stress signaling pathways have been extensively analyzed, critical questions remain about their regulatory mechanisms. For example, how do RPs translocate from the nucleolus to the nucleoplasm to exert their functions, and do these p53-regulating RPs influence the prognosis of human cancer patients? Our laboratory recently identified the nucleolar protein PICT1 as a novel regulator of nucleolar stress. PICT1 sequesters the ribosomal protein RPL11 in the nucleolus, preventing it from binding to MDM2. MDM2 is then free to degrade p53, favoring tumor cell growth. Accordingly, the level of PICT1 in a tumor is becoming a useful prognostic marker for human cancers. This review summarizes the evidence that links nucleolar stress to tumorigenesis, and casts PICT1 as an oncogenic player in human cancer biology.

Figure 1

Stress pathways leading to MDM2 inactivation and p53 accumulation. In a cell without stress, ribosomal proteins (RPs) and ARF are confined to the nucleolus, and MDM2 is active in the nucleus and facilitates the eventual degradation of p53 protein. In response to oncogenic stress, ARF binds to MDM2 and inhibits its E3 ligase activity, blocking p53 ubiquitination and eventual degradation. Similarly, in response to DNA stress, the checkpoint kinases ATM and ATR phosphorylate p53 and MDM2, blocking their interaction and promoting p53 accumulation. In response to stimuli causing nucleolar stress, nucleolar RPs such as RPL11, L5, L23, and S7 are released from nucleolus. These RPs translocate into the nucleoplasm and bind to MDM2, preventing p53 degradation. All three stress pathways result in p53 accumulation and activation that induces cell cycle arrest/apoptosis.

Figure 2

PICT1 regulates p53 protein level. Immunoblot detecting p53 and Pict1 proteins in embryonic stem (ES) cells (PICT1tetTg⁺/PICT1^Δ/−), which were left untreated (PICT1^+/+) or treated with Dox (PICT1^−/−) for 2 days.

Figure 3

PICT1 regulates nucleolar stress by sequestering RPL11 in the nucleolus. Embryonic stem (ES) cells (PICT1tetTg⁺/PICT11^Δ/−) were transfected with RPL11‐DsRed. At 16 h post‐transfection, cells were left untreated (PICT1^+/+) or treated with Dox (PICT1^−/−), and RPL11 protein localization was determined by confocal microscopy. 4′6′‐diamidino‐2‐phenylindole dihydrochloride (DAPI) was used to counterstain nuclei.

Figure 4

Regulation of the nucleolar stress pathway via PICT1‐RPL11 binding. Left: When PICT1 is present in the nucleolus, RPL11 is retained in the nucleolus and MDM2 is free to ubquitinate p53, promoting its degradation. Right: When PICT1 is absent from the nucleolus or its expression is reduced, nucleolar RPL11 escapes into the nucleoplasm and binds to MDM2, blocking p53 ubiquitination. p53 accumulation in PICT1‐deficient or PICT1^low tumor cells then slows tumor progression.