The Role of the Stress Response Gene Gadd45b in Senescence

Abstract

The Gadd45 family of proteins (Gadd45a, Gadd45b, and Gadd45g) has been shown to act as stress sensors in response to various physiological and environmental stressors, including oncogenic stress. However, the role of Gadd45b in senescence remained unclear. Here, we show for the first time that primary mouse embryo fibroblasts (MEFs) from Gadd45b null mice proliferate slowly; accumulate increased levels of DNA damage, and senesce prematurely. Notably, this is in contrast with Gadd45a null MEFs that show enhanced growth rate and escape senescence. This difference in growth rate increases with increasing passage number, suggesting that senescence results from exposure to environmental stressors. The impaired proliferation and increased senescence in Gadd45b null MEFs can be partially reversed by culturing cells at physiological oxygen levels, indicating that in the absence of Gadd45b, primary MEFs are less able to cope with elevated levels of oxidative stress. Interestingly, in contrast to other senescent MEFs, which arrest at G1 phase of cell cycle, Gadd45b null MEFs arrest at the G2/M phase of cell cycle. Furthermore, FACS analysis of Gadd45b null MEFs showed less phospho-histone H3-positive cells compared to wild type MEFs indicating that Gadd45b null MEFs are arrested in G2 phase rather than M phase. Interestingly, other stressors such as sub-lethal H2O2 and UV irradiation, that are known to increase oxidative stress, triggered increased premature senescence in Gadd45b null MEFs compared to wild type MEFs. By staining embryos for SA-β-gal gal, we also show that embryos from Gadd45b null mice exhibit increased SA-β-gal gal staining compared to wild type embryos, thus providing in vivo evidence for increased senescence in Gadd45b null mice. Finally, investigating the effect of loss of Gadd45b on senescence related diseases, we show that loss of Gadd45b promotes senescence and aging phenotypes in the skin as well as increased senescence and attenuated fibrotic response to CCl4 induced liver fibrosis. Together, these results highlight a novel and significant role for Gadd45b in the senescence response of cells to stress.