Sumoylation induced by the Arf tumor suppressor: A p53-independent function

摘要

The mouse p19Arf protein has both p53-dependent and p53-inde-pendent tumor-suppressive activities. Arf triggers sumoylation of many cellular proteins, including Mdm2 and nucleophosmin (NPM/ B23), with which p19Arf physically interacts in vivo, and this occurs equally well in cells expressing or lacking functional p53. In an Arf-null NIH 3T3 cell derivative (MT-Arf cells) engineered to reex-press an Arf transgene driven by a zinc-inducible metallothionein promoter, sumoylation of endogenous Mdm2 and NPM proteins was initiated as p19Arf was induced and was observed before p53-dependent cell cycle arrest. Predominately nucleoplasmic molecules visualized by immunofluorescence with antibodies to small ubiquitin-like modifier (SUMO) 1 localized to nucleoli as p19Arf accumulated there. Two Arf mutants, one of which binds to Mdm2 and NPM but is excluded from nucleoli and the other of which enters nucleoli but is handicapped in binding to Mdm2 and NPM, were defective in inducing sumoylation of these two target proteins and did not localize bulk sumoylated molecules to nucleoli. The CELO adenovirus protein, Gam1, which inhibits the SUMO activating enzyme (E1) and leads to down-regulation of the SUMO conjugating enzyme (E2/Ubc9), had no overt effect on the ability of p19Arf to activate p53 or the p53-responsive genes encoding Mdm2 and p21~(Cip1), despite the fact that Arf-induced sumoylation of Mdm2 was blocked. Reduction of Ubc9 levels with short hairpin RNAs rendered similar results. We suggest that Arf's p53-indepen-dent effects on gene expression and tumor suppression might depend on Arf-induced sumoylation.