MYC and RAS are unable to cooperate in overcoming cellular senescence and apoptosis in normal human fibroblasts

The MYC and RAS oncogenes are sufficient for transformation of normal rodent cells. This cooperativity is at least in part based on suppression of RAS-induced cellular senescence by MYC and block of MYC-induced apoptosis by RAS – thereby canceling out two main barriers against tumor development. However, it remains unclear whether MYC and RAS cooperate in this way in human cells, where MYC and RAS are not sufficient for transformation. To address this question, we established a combined Tet-inducible H-RASV12 and hydroxytamoxifen-inducible MycER system in normal human BJ fibroblasts. We show here that activation of RAS alone induced senescence while activation of MYC alone or together with RAS triggered DNA damage, induction of p53 and massive apoptosis, suggesting that RAS cannot rescue MYC-induced apoptosis in this system. Although coexpression with MYC reduced certain RAS-induced senescence markers (histone H3 lysine 9 trimethylation and senescence-associated β-GAL activity), the induction of the senescence marker p16INK4A was further enhanced and the culture ceased to proliferate within a few days, revealing that MYC could not fully suppress RAS-induced senescence. Furthermore, depletion of p53, which enhanced proliferation and rescued the cells from RAS-induced senescence, did not abrogate MYC-induced apoptosis. We conclude that MYC and RAS are unable to cooperate in overcoming senescence and apoptosis in normal human fibroblasts even after depletion of p53, indicating that additional oncogenic events are required to abrogate these fail-safe mechanisms and pave the way for cellular transformation. These findings have implications for our understanding of the transformation process in human cells. Abbreviations and acronyms: CDK: Cyclin-dependent kinase; DDR: DNA damage response; DOX: Doxycycline; EdU: 5-ethynyl-2ʹ-deoxyuridine; FACS: Fluorescence Activated Cell Sorting; MycER: MYC-estrogen receptor; OHT: 4-hydroxytamoxifen; OIS: Oncogene-induced senescence; PP2A: Protein phosphatase 2A; ROS: Reactive oxygen species; SA-β-GAL: Senescence-associated β-galactosidase; SAHF: Senescence-associated heterochromatin foci; shRNA: Short hairpin RNA; YFP: Yellow fluorescent protein