title: Elevated DDX21 regulates c-Jun activity and rRNA processing in human breast cancers.
abstract: BACKGROUND
  The DDX21 RNA helicase has been shown to be a nucleolar and nuclear protein involved in ribosome RNA processing and AP-1 transcription. DDX21 is highly expressed in colon cancer, lymphomas, and some breast cancers, but little is known about how DDX21 might promote tumorigenesis.
  
  
  METHODS
  Immunohistochemistry was performed on a breast cancer tissue array of 187 patients. In order to study the subcellular localization of DDX21 in both tumor tissue and tumor cell lines, indirect immunofluorescence was applied. The effect of DDX21 knockdown was measured by cellular apoptosis, rRNA processing assays, soft agar growth and mouse xenograft imaging. AP-1 transcriptional activity was analyzed with a luciferase reporter and bioluminescence imaging, as well as qRT-PCR analysis of downstream target, cyclin D1, to determine the mechanism of action for DDX21 in breast tumorigenesis.
  
  
  RESULTS
  Herein, we show that DDX21 is highly expressed in breast cancer tissues and established cell lines. A significant number of mammary tumor tissues and established breast cancer cell lines exhibit nuclear but not nucleolar localization of DDX21. The protein expression level of DDX21 correlates with cell proliferation rate and is markedly induced by EGF signaling. Mechanistically, DDX21 is required for the phosphorylation of c-Jun on Ser73 and DDX21 deficiency markedly reduces the transcriptional activity of AP-1. Additionally, DDX21 promotes rRNA processing in multiple breast cancer cell lines. Tumor cells expressing high levels of endogenous DDX21 undergo apoptosis after acute DDX21 knockdown, resulting in significant reduction of tumorigenicity in vitro and in vivo.
  
  
  CONCLUSIONS
  Our findings indicate that DDX21 expression in breast cancer cells can promote AP-1 activity and rRNA processing, and thus, promote tumorigenesis by two independent mechanisms. DDX21 could serve as a marker for a subset of breast cancer patients with higher proliferation potential and may be used as a therapeutic target for a subset of breast cancer patients.
authors.forename: Yandong
authors.initials: Y
authors.lastname: Zhang
authors.forename: Kathleen C
authors.initials: KC
authors.lastname: Baysac
authors.forename: Lian-Fai
authors.initials: LF
authors.lastname: Yee
authors.forename: Anthony J
authors.initials: AJ
authors.lastname: Saporita
authors.forename: Jason D
authors.initials: JD
authors.lastname: Weber
mesh_terms: D000818:Animals; D001943:Breast Neoplasms / Q000201:enzymology* / Q000235:genetics; D059447:Cell Cycle Checkpoints; D045744:Cell Line, Tumor; D002466:Cell Nucleolus / Q000378:metabolism; D002467:Cell Nucleus / Q000378:metabolism; D049109:Cell Proliferation; D002470:Cell Survival; D002471:Cell Transformation, Neoplastic / Q000378:metabolism; D053487:DEAD-box RNA Helicases / Q000502:physiology*; D005260:Female; D006801:Humans; D016688:Mice, Inbred NOD; D008819:Mice, Nude; D016513:Mice, SCID; D009368:Neoplasm Transplantation; D016755:Proto-Oncogene Proteins c-jun / Q000378:metabolism*; D012323:RNA Processing, Post-Transcriptional; D012335:RNA, Ribosomal / Q000378:metabolism*; D018808:Transcription Factor AP-1 / Q000378:metabolism
chemical_list: D016755:Proto-Oncogene Proteins c-jun; D012335:RNA, Ribosomal; D018808:Transcription Factor AP-1; C501726:DDX21 protein, human; D053487:DEAD-box RNA Helicases