ReviewThe ARF/p53 pathway
Introduction
The retinoblastoma protein (RB) and p53 are canonical tumor suppressors, each negatively regulating different steps in cell-cycle progression 1, 2. In turn, their activities are modulated by two products of the INK4a/ARF locus, p16INK4a and p19ARF, both of which act in tumor surveillance 3, 4, 5. Together, the loci encoding these four proteins are among those most commonly inactivated in cancer. RB phosphorylation by cyclin-dependent kinases (CDKs) during G1 phase disrupts its association with histone deacetylase and E2F transcription factors, allowing the transcription of genes whose activities promote DNA synthesis (Figure 1). By inhibiting the activity of cyclin-D-dependent kinases, p16INK4a can prevent E2F activation and block cells from exiting G1 phase. p53 is a homotetrameric transcription factor induced by DNA damage or by inappropriate mitogenic signaling, and its activation and accumulation, largely through protein stabilization, can trigger cell-cycle arrest or apoptosis. Among the p53-responsive gene products are p21CIP1, a potent inhibitor of cyclin-dependent kinases, and Mdm2, a negative feedback regulator that helps terminate the p53 response. By inhibiting Mdm2, p19ARF — a 19 kDa polypeptide in the mouse, but p14ARF in humans — stabilizes and activates p53 6, 7, 8, 9, braking the cell cycle in response to inappropriate mitogenic signals.
p16INK4a and p19ARF are encoded in part by unique first exons (designated 1α in INK4a and 1β in ARF). RNAs from exons 1 are spliced to sequences specified by a common second exon translated in two different reading frames, underscoring an unprecedented economy of genome organization in higher eukaryotes [10]. Because ARF is induced by E2F-1 11, 12, it biochemically connects the RB and p53 pathways (Figure 1). Moreover, the induction of ARF by oncoproteins such as Myc, adenovirus E1A, Ras, and v-Abl 12, 13, 14, 15 highlights its role in sensing hyperproliferative signals in incipient cancer cells. ARF’s ability to act upstream of p53 in tumor surveillance has been reviewed recently 4, 5, and here we summarize progress gained over the past year in understanding ARF function.
Section snippets
The ARF–Mdm2 interaction
Mdm2 is a multifunctional protein that negatively regulates p53 in several ways. First, its binding interferes with p53’s ability to transactivate target genes 16, 17. Second, Mdm2 has an intrinsic ubiquitin ligase activity that most likely contributes to p53 degradation [18]. At least in vitro, ARF can interfere with this reaction [19] but whether this is central to ARF’s actions in vivo is unknown. Third, Mdm2 relocalizes p53 from the cell nucleus to the cytoplasm where it undergoes
From cultured primary cells to mouse models
Cultured mouse embryo fibroblasts (MEFs) lacking INK4a/ARF [30] or ARF alone [31], as with those that sustain p53 mutations, do not senesce but continue to proliferate as established cell lines. In human fibroblasts, senescence can be bypassed temporarily by disrupting both RB and p53 function but it also depends on a mitotic counting mechanism that senses telomere length [32]. Yet, wild-type MEFs stop growing after only 20–30 doublings, a time well before any significant erosion of their
Conclusions and the road ahead
Part of the continuing confusion about ARF function relates to p16INK4a and the manner by which it may contribute to the phenotypes observed in INK4a/ARF-null mice and in those interbred with other strains. The Bmi-1 knockout mouse provides an interesting case in point [43••]. It would be of obvious benefit to know the phenotype of a mouse lacking only INK4a, and to understand which Bmi-1−/− phenotypes are rescued by either ARF or INK4a inactivation. Although the extent to which p16INK4a
Acknowledgements
The authors thank Martine Roussel, James Roberts, Wade Harper, Galit Rotman, Tom Curran, and the other members of their laboratory for helpful criticisms of the manuscript. CJ Sherr is an Investigator and JD Weber is an Associate of the Howard Hughes Medical Institute. Both authors are also supported by American Lebanese Syrian Associated Charities (ALSAC) of St Jude Children’s Research Hospital.
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest
•• of outstanding interest
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