| The Ink4a-Arf1 locus encodes two functionally unrelated tumor suppressor proteins, p16Ink4a and p19 Arf. Mice deleted for both Ink4a and Arf are highly tumor prone, whilst primary embryonic fibroblasts derived from these animals fail to undergo senescence. Surprisingly, mice specifically lacking p19Arf exhibit an almost identical phenotype, implying that loss of Arf alone is sufficient to confer susceptibility to cancer in mice.; p19Arf antagonizes the function of the p53-negative regulator, Mdm2, thereby inducing p53-dependent cell cycle arrest. This pathway is presumed to operate in a linear manner, but on characterization of a cohort of Arf-null mice, we observed clear differences in the spectrum of tumors arising in these animals, as compared to mice lacking p53. We next generated mice lacking either Arf and, p53 , or Arf, p53 and Mdm2 to examine possible synergy between these genes in tumor suppression. Strikingly, the spectrum of tumors observed in these animals included novel tumor types, whilst a number of mice developed two or more independent tumors. Thus, the Arf-Mdm2-p53 pathway is not strictly linear and, alternative targets remain to be discovered. Transformation of primary murine embryonic fibroblasts (MEFs) requires the collaborative effects of Myc and oncogenic Ras. In contrast, immortal Arf-null MEFs can be transformed by oncogenic Ras alone. We therefore sought to determine whether Myc can regulate the expression of Arf. We show that Myc rapidly activates Arf and p53 gene expression in primary MEFs, leading to the induction of apoptosis. MEFs that survive Myc overexpression sustain p53 mutation or Arf loss during establishment and become immortal. Therefore, p19Arf is involved in a checkpoint that safeguards cells against hyperproliferative, oncogenic signals.; Finally, we asked whether additional cell types from Arf-null mice are immortal in culture. Whereas Arf-null pre-B cells were immortal just like Arf-null MEFs, both wild-type (WT) and Arf-null bone marrow-derived macrophages initially grew at a slow rate, but gave rise to rapidly and continuously growing variants that ceased to express p16Ink4a. In these cases, gene silencing was accompanied by methylation of the Ink4a promoter. Therefore, p19Arf and p16Ink4a differentially regulate the immortalization process, in a cell-type specific manner. |
