Portal de Periódicos da CAPES

Transcription-independent ARF regulation in oncogenic stress-mediated p53 responses

2010; Nature Portfolio; Volume: 464; Issue: 7288 Linguagem: Inglês

10.1038/nature08820

1476-4687

Delin Chen, Jing Shan, Wei‐Guo Zhu, Jun Qin, Wei Gu,

Retinoids in leukemia and cellular processes

Of the known tumour suppressors, ARF and p53 are among the most relevant clinically, with p53 loss or inactivation found in more than half of human cancers and ARF known to activate p53 during oncogenic stress. A study of the mechanism of ARF-mediated p53 activation now shows that ARF is unstable in normal human cells but stable in cancer cells. In normal cells ARF ubiquitination and degradation is promoted by a ligase termed ULF (ubiquitin ligase for ARF), and this activity is abrogated in cancer cells. In response to oncogenic stress, the tumour suppressor ARF activates the p53 protein. ARF protein is highly stable in most human cell lines, so it has been thought that ARF activation occurs mainly at the level of transcription. Here, however, ARF is shown to be unstable in normal human cells but stable in cancer cells, through a transcription-independent mechanism. A ubiquitin ligase for ARF is identified and shown to promote ARF degradation in normal cells. This activity is prevented in cancer cells, stabilizing ARF. The tumour suppressor ARF is specifically required for p53 activation under oncogenic stress1,2,3,4,5,6. Recent studies showed that p53 activation mediated by ARF, but not that induced by DNA damage, acts as a major protection against tumorigenesis in vivo under certain biological settings7,8, suggesting that the ARF–p53 axis has more fundamental functions in tumour suppression than originally thought. Because ARF is a very stable protein in most human cell lines, it has been widely assumed that ARF induction is mediated mainly at the transcriptional level and that activation of the ARF–p53 pathway by oncogenes is a much slower and largely irreversible process by comparison with p53 activation after DNA damage. Here we report that ARF is very unstable in normal human cells but that its degradation is inhibited in cancerous cells. Through biochemical purification, we identified a specific ubiquitin ligase for ARF and named it ULF. ULF interacts with ARF both in vitro and in vivo and promotes the lysine-independent ubiquitylation and degradation of ARF. ULF knockdown stabilizes ARF in normal human cells, triggering ARF-dependent, p53-mediated growth arrest. Moreover, nucleophosmin (NPM) and c-Myc, both of which are commonly overexpressed in cancer cells, are capable of abrogating ULF-mediated ARF ubiquitylation through distinct mechanisms, and thereby promote ARF stabilization in cancer cells. These findings reveal the dynamic feature of the ARF–p53 pathway and suggest that transcription-independent mechanisms are critically involved in ARF regulation during responses to oncogenic stress.