title: Adenosine triphosphatase pontin is overexpressed in hepatocellular carcinoma and coregulated with reptin through a new posttranslational mechanism.
abstract: Reptin and Pontin are related ATPases associated with stoichiometric amounts in several complexes involved in chromatin remodeling, transcriptional regulation, and telomerase activity. We found that Reptin was up-regulated in hepatocellular carcinoma (HCC) and that down-regulation of Reptin led to growth arrest. We show here that Pontin messenger RNA (mRNA) is also up-regulated in human HCC 3.9-fold as compared to nontumor liver (P = 0.0004). Pontin expression was a strong independent factor of poor prognosis in a multivariate analysis. As for Reptin, depletion of Pontin in HuH7 cells with small interfering RNAs (siRNAs) led to growth arrest. Remarkably, Pontin depletion led to down-regulation of Reptin as shown with western blot, and vice versa. Whereas siRNAs induced a decrease of their cognate mRNA targets, they did not affect the transcripts of the partner protein. Translation of Pontin or Reptin was not altered when the partner protein was silenced. However, pulse-chase experiments demonstrated that newly synthesized Pontin or Reptin stability was reduced in Reptin- or Pontin-depleted cells, respectively. This phenomenon was reversed upon inhibition of proteasome or ubiquitin-activating enzyme (E1). In addition, proteasome inhibition could partly restore Pontin steady-state levels in Reptin-depleted cells, as shown by western blot. This restoration was not observed when cells were also treated with cycloheximide, thus confirming that proteasomal degradation in this setting was restricted to newly synthesized Pontin.
  
  
  CONCLUSIONS
  Reptin and Pontin protein levels are strictly controlled by a posttranslational mechanism involving proteasomal degradation of newly synthesized proteins. These data demonstrate a tight regulatory and reciprocal interaction between Reptin and Pontin, which may in turn lead to the maintenance of their 1:1 stoichiometry.
authors.affiliation: Institut National de la Santé et de la Recherche Médicale , U889 Bordeaux, France.
authors.forename: Valérie
authors.initials: V
authors.lastname: Haurie
authors.forename: Ludovic
authors.initials: L
authors.lastname: Ménard
authors.forename: Alexandra
authors.initials: A
authors.lastname: Nicou
authors.forename: Christian
authors.initials: C
authors.lastname: Touriol
authors.forename: Philippe
authors.initials: P
authors.lastname: Metzler
authors.forename: Jérémy
authors.initials: J
authors.lastname: Fernandez
authors.forename: Danièle
authors.initials: D
authors.lastname: Taras
authors.forename: Patrick
authors.initials: P
authors.lastname: Lestienne
authors.forename: Charles
authors.initials: C
authors.lastname: Balabaud
authors.forename: Paulette
authors.initials: P
authors.lastname: Bioulac-Sage
authors.forename: Hervé
authors.initials: H
authors.lastname: Prats
authors.forename: Jessica
authors.initials: J
authors.lastname: Zucman-Rossi
authors.forename: Jean
authors.initials: J
authors.lastname: Rosenbaum
mesh_terms: D000074183:ATPases Associated with Diverse Cellular Activities; D017209:Apoptosis; D006528:Carcinoma, Hepatocellular / Q000473:pathology*; D002352:Carrier Proteins / Q000235:genetics / Q000502:physiology*; D049109:Cell Proliferation; D004265:DNA Helicases / Q000235:genetics / Q000502:physiology*; D006801:Humans; D008113:Liver Neoplasms / Q000473:pathology*; D061988:Proteasome Inhibitors; D014176:Protein Biosynthesis; D012333:RNA, Messenger / Q000032:analysis
chemical_list: D002352:Carrier Proteins; D061988:Proteasome Inhibitors; D012333:RNA, Messenger; D000074183:ATPases Associated with Diverse Cellular Activities; D004265:DNA Helicases; C486549:RUVBL1 protein, human; C122079:RUVBL2 protein, human