Disruption of Multiple Overlapping Functions Following Step-Wise Inactivation of the Extended Myc Network

Myc, a member of the Myc Network, supervises proliferation, metabolism and ribosomal function. The Mlx Network cross-talks with the Myc Network and regulates overlapping functions. We describe here the consequences of conditional Myc and/or Mlx gene knockouts (KOs) in primary and immortalized murine embryonic fibroblasts (MEFs). MycKO and MycKOxMlxKO “double KO” (DKO) primary MEFs, but not MlxKO MEFs, rapidly growth-arrested and displayed features of aging and senescence. In DKO MEFs, these were transient, indicating that Mlx was necessary to maintain them. KO MEFs deregulated transcripts pertaining to mitochondrial and ribosomal structure and function, cell cycle, aging, senescence and DNA damage. The expression of DNA damage-related proteins was also abnormal. Immortalized KO MEFs remained proliferation-competent but demonstrated differential sensitivities to genotoxic agents. Immortalized MycKO MEFs spontaneously developed tetraploidy that was Mlx-dependent. Different aspects of MEF aging, senescence and DNA damage responses are therefore differentially regulated by the Myc and Mlx Networks. Overall design: Comparative gene expression profiling analysis of RNA-seq data for MycKO, MlxKO, DKO(MycKO + MlxO) and WT in primary and immortalized MEF cells.