Myc, a member of the “Myc Network” of bHLH-ZIP transcription factors, supervises proliferation, metabolism, and translation. It also engages in crosstalk with the related “Mlx Network” to co-regulate overlapping genes and functions. We investigated the consequences of stepwise conditional inactivation of Myc and Mlx in primary and SV40 T-antigen-immortalized murine embryonic fibroblasts (MEFs). Myc-knockout (MycKO) and Myc × Mlx “double KO” (DKO)—but not MlxKO—primary MEFs showed rapid growth arrest and displayed features of accelerated aging and senescence. However, DKO MEFs soon resumed proliferating, indicating that durable growth arrest requires an intact Mlx network. All three KO MEF groups deregulated multiple genes and functions pertaining to aging, senescence, and DNA damage recognition/repair. Immortalized KO MEFs proliferated in Myc’s absence while demonstrating variable degrees of widespread genomic instability and sensitivity to genotoxic agents. Finally, compared to primary MycKO MEFs, DKO MEFs selectively downregulated numerous gene sets associated with the p53 and retinoblastoma (Rb) pathways and G2/M arrest. Thus, the reversal of primary MycKO MEF growth arrest by either Mlx loss or SV40 T-antigen immortalization appears to involve inactivation of the p53 and/or Rb pathways.

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MYC proto-oncogene dysregulation alters metabolic, translational, cell cycle and other functions in ways that support tumor induction and maintenance. Myc+/- mice are healthier and longer-lived than control mice but the long-term ramifications of more complete Myc loss remain unknown. We now describe the life-long consequences of body-wide Myc inactivation initiated post-natally. ″MycKO″ mice rapidly acquire numerous features of premature aging including altered body composition and habitus, metabolic dysfunction, hepatic steatosis and the dysregulation of numerous gene sets involved in functions that normally deteriorate with aging. Yet, MycKO mice have an extended life span that correlates with a 4-5-fold lower lifetime cancer incidence. Aging tissues from normal mice and humans deregulate many of the same gene sets as do young MycKO mice while also down-regulating Myc and many of its target genes. Normal aging and its associated cancer predisposition are thus highly linked via Myc and its target genes and can be genetically separated.