title: Silencing of ribosomal protein S9 elicits a multitude of cellular responses inhibiting the growth of cancer cells subsequent to p53 activation.
abstract: BACKGROUND
  Disruption of the nucleolus often leads to activation of the p53 tumor suppressor pathway through inhibition of MDM2 that is mediated by a limited set of ribosomal proteins including RPL11 and RPL5. The effects of ribosomal protein loss in cultured mammalian cells have not been thoroughly investigated. Here we characterize the cellular stress response caused by depletion of ribosomal protein S9 (RPS9).
  
  
  RESULTS
  Depletion of RPS9 impaired production of 18S ribosomal RNA and induced p53 activity. It promoted p53-dependent morphological differentiation of U343MGa Cl2:6 glioma cells as evidenced by intensified expression of glial fibrillary acidic protein and profound changes in cell shape. U2OS osteosarcoma cells displayed a limited senescence response with increased expression of DNA damage response markers, whereas HeLa cervical carcinoma cells underwent cell death by apoptosis. Knockdown of RPL11 impaired p53-dependent phenotypes in the different RPS9 depleted cell cultures. Importantly, knockdown of RPS9 or RPL11 also markedly inhibited cell proliferation through p53-independent mechanisms. RPL11 binding to MDM2 was retained despite decreased levels of RPL11 protein following nucleolar stress. In these settings, RPL11 was critical for maintaining p53 protein stability but was not strictly required for p53 protein synthesis.
  
  
  CONCLUSIONS
  p53 plays an important role in the initial restriction of cell proliferation that occurs in response to decreased level of RPS9. Our results do not exclude the possibility that other nucleolar stress sensing molecules act upstream or in parallel to RPL11 to activate p53. Inhibiting the expression of certain ribosomal proteins, such as RPS9, could be one efficient way to reinitiate differentiation processes or to induce senescence or apoptosis in rapidly proliferating tumor cells.
authors.affiliation: Department of Oncology-Pathology, Karolinska Institutet, Stockholm, Sweden. mikael.lindstrom@ki.se
authors.forename: Mikael S
authors.initials: MS
authors.lastname: Lindström
authors.forename: Monica
authors.initials: M
authors.lastname: Nistér
mesh_terms: D001932:Brain Neoplasms / Q000378:metabolism; D045744:Cell Line, Tumor; D002466:Cell Nucleolus / Q000378:metabolism; D002467:Cell Nucleus / Q000378:metabolism; D049109:Cell Proliferation; D003593:Cytoplasm / Q000378:metabolism; D020868:Gene Silencing; D005910:Glioma / Q000378:metabolism; D006367:HeLa Cells; D006801:Humans; D034741:RNA, Small Interfering / Q000378:metabolism; D000091243:Ribosomal Protein S9; D012269:Ribosomal Proteins / Q000235:genetics* / Q000378:metabolism; D014162:Transfection; D016159:Tumor Suppressor Protein p53 / Q000235:genetics*
chemical_list: D034741:RNA, Small Interfering; D000091243:Ribosomal Protein S9; D012269:Ribosomal Proteins; C495901:TP53 protein, human; D016159:Tumor Suppressor Protein p53; C027624:ribosomal protein L11