The amino acid glutamine is required for many biosynthetic pathways, including that of the antioxidant glutathione. Protein phosphatase 2A (PP2A) is crucial for glutamine sensing in yeast. PP2A is composed of a catalytic C subunit, a scaffolding A subunit, and one of 16 regulatory B subunits, and is regulated by α4. Reid et al. (see also Gwinn and Sweet-Cordero) investigated how cancer cells detect and respond to glutamine deprivation, which is experienced by growing tumors. Cell viability after glutamine deprivation was greater in human fibrosarcoma HT1080 cells and in mouse embryonic fibroblasts (MEFs) overexpressing α4 than in their vector-expressing counterparts. Glutamine deprivation of 3T3 MEFs increased the mRNA and protein abundance for B55α, which was ablated by supplementing the cells with the antioxidants N-acetyl-L-cystein (NAC) or cell-permeable glutathione, suggesting that the production of reactive oxygen species was responsible for the induction of B55α. The survival of 3T3 MEFs after glutathione deprivation was decreased by an shRNA directed against B55α, and this loss of viability occurred in cells overexpressing α4. In glutathione-deprived cells, the formation of the A-C-B55α complex was increased, whereas the association of α4 and C was decreased. p53 was phosphorylated and activated after glutamine deprivation, and the ability of α4 overexpression to confer resistance to glutamine deprivation was lost in p53-knockout MEFs. The induction of B55α in response to glutamine deprivation occurred in p53-knockout MEFs, indicating that B55α was upstream of p53. Mass spectrometry of B55α immunoprecipitates from 293T cells identified as a binding partner the ubiquitin ligase EDD (E3 identified by different display), which inhibits p53 by preventing its phosphorylation. In 3T3 MEFs, glutamine deprivation increased the interaction of endogenous EDD and B55α, and siRNA directed against EDD increased the viability of and rescued the induction of p53 in B55α-knockdown cells after glutamine deprivation. In a mouse xenograft model, tumors formed from HT1080 cells expressing B55α shRNA formed smaller tumors characterized by increased apoptosis, and tumors formed from cells expressing control shRNA showed an inverse correlation between glutamine and glutathione concentrations and the mRNA abundance for B55α. Thus, the depletion of glutathione and increased ROS concentrations caused by glutamine deprivation induces the PP2A subunit B55α, which promotes cell survival by activating p53.
M. A. Reid, W.-I Wang, K. R. Rosales, M. X. Welliver, M. Pan, M. Kong, The B55α subunit of PP2A drives a p53-dependent metabolic adaptation to glutamine deprivation. Mol. Cell 50, 200–211 (2013). [PubMed]
D. Gwinn, E. A. Sweet-Cordero, The phosphatase PP2A links glutamine to the tumor suppressor p53. Mol. Cell 50, 157–158 (2013). [Online Journal]