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The ARF tumor suppressor controls a well-described p53/ Mdm2-dependent oncogenic stress checkpoint. In addition, ARF has recently been shown to localize to mitochondria, and to induce autophagy; however, this has never before been demonstrated for endogenous ARF, and the molecular basis for this activity of ARF has not been elucidated. Using an unbiased mass spectrometry-based approach, we show that mitochondrial ARF interacts with the Bcl2 family member Bcl-xl, which normally protects cells from autophagy by inhibiting the Beclin-1/Vps34 complex, which is essential for autophagy. We find that increased expression of ARF decreases Beclin-1/Bcl-xl complexes in cells, thereby providing a basis for ARF-induced autophagy. Our data also indicate that silencing p53 leads to high levels of ARF and increased autophagy, thereby providing a possible basis for the finding by others that p53 inhibits autophagy. The combined data support the premise that ARF induces autophagy in a p53-independent manner in part by virtue of its interaction with Bcl-xl.The ARF tumor suppressor, p14 ARF in humans and p19 ARF in mouse, is a critical growth suppressor that is up-regulated by chronic mitogenic signals and localizes predominantly to the nucleolus. At the nucleolus and in the nucleoplasm, ARF can exert both p53-dependent and -independent growth suppressive function, by virtue of interaction with and inhibition of MDM2, nucleophosmin, E2F-1, CtBP, c-Myc, as well as others (see Ref. 1 for review). Recently, a small molecular weight variant of ARF, generated by translation from an internal methionine, has been discovered to localize primarily to mitochondria and to induce autophagy (2). More recently, another group has shown that full-length ARF, in addition to the small molecular weight variant, can likewise induce autophagy (3). However, neither of these studies revealed a mechanism whereby ARF induces autophagy.Autophagy is an evolutionarily conserved homeostatic process whereby cytosolic components are targeted for removal or turnover in membrane-bound compartments (autophagosomes) that fuse with the lysosome (for review see Ref. 4). This process regulates the turnover of damaged organelles and longlived proteins that are too large to be delivered to the proteasome. Autophagy occurs constitutively at low levels and is greatly induced during period of metabolic stress, where lysosome-mediated digestion of sequestered molecules serves to release free amino acids and ATP to fuel the continued survival of the cell.Several genes are implicated in the control of autophagy. Perhaps most notable of these is Beclin-1, which is an evolutionarily-conserved mediator of autophagy, with structural similarity to the yeast autophagy gene Apg6/Vps30. Beclin-1 is a component of the class III PI3 kinase complex that includes Vps34; this complex regulates the formation and nucleation of autophagosomes, and the regulation of the activity of this complex is tightly regulated. For example, Beclin-1 possesses a BH3 domain that interacts with the BH3 bind...

ARF (ARF) is a tumor suppressor gene that is frequently mutated in human cancer. ARF has multiple tumor suppressor functions, some of which are mediated by signaling to p53. Surprisingly, a significant fraction of human tumors retain persistently high levels of ARF, suggesting that ARF may possess a prosurvival function. We show that ARF protein is markedly up-regulated in cells exposed to nutrient starvation. Cells with silenced ARF show reduced autophagy and reduced viability when placed under conditions of starvation. We show for the first time that ARF silencing can limit the progression of some tumors, such as lymphoma, but not others, such as E1A/Ras-induced tumors. Specifically, mycdriven lymphomas with mutant p53 tend to overexpress ARF; we show that silencing ARF in these tumors greatly impedes their progression. These data are the first to show that ARF can act in a p53-independent manner to promote the progression of some tumors. [Cancer Res 2008;68(23):9608-13]

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A common polymorphism at codon 72 of exon 4 encoding either arginine or proline has been shown to confer a susceptibility to the development of skin tumor in renal transplant recipients. Moreover, this polymorphism may affect proteolytic degradation of p53 promoted by E6 protein from mucosal human papillomaviruses and represent a risk factor for human-papillomavirus-induced carcinogenesis. In this study, we analyzed the human papillomavirus presence and the TP53 allele distribution in cutaneous squamous cell carcinoma of renal transplant recipients and immunocompetent patients. Fifty-three squamous cell carcinomas from 40 renal transplant recipients, 50 benign epithelial skin lesions from 50 renal transplant recipients with no history of skin cancer, 51 squamous cell carcinomas from immunocompetent patients, and 29 blood samples from immunocompetent individuals without skin cancer were investigated. Human papillomavirus DNA was detected using polymerase chain reaction performed with two pairs of primers (MY09-MY11 and FAP59-FAP64). TP53 allele distribution was studied by denaturing gradient gel electrophoresis assay, followed by sequencing analysis. Human papillomavirus DNA was detected in 64% of squamous cell carcinoma and 79% of benign epithelial lesions from renal transplant recipients (NS) and only in 37% of squamous cell carcinoma from immunocompetent patients (p < 0.05). Mucosal oncogenic human papillomavirus types were predominant in squamous cell carcinoma from both renal transplant recipients and immunocompetent patients. Rate of arginine homozygosity in squamous cell carcinoma from renal transplant recipients was significantly higher (83%) than in immunocompetent patients with or without squamous cell carcinoma (60% and 59%, respectively). Our results suggest that TP53 arginine/arginine genotype could represent a potential risk factor for the development of squamous cell carcinoma in renal transplant recipients compared to immunocompetent patients. No association between TP53 arginine/arginine genotype and human papillomavirus status could be determined, however.

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Supplementary Figure 2 from The ARF Tumor Suppressor Can Promote the Progression of Some Tumors