The E3 ubiquitin ligase Mule acts through the ATM-p53 axis to maintain B lymphocyte homeostasis.

The Journal of experimental medicine

PubMedID: 22213803

Hao Z, Duncan GS, Su YW, Li WY, Silvester J, Hong C, You H, Brenner D, Gorrini C, Haight J, Wakeham A, You-Ten A, McCracken S, Elia A, Li Q, Detmar J, Jurisicova A, Hobeika E, Reth M, Sheng Y, Lang PA, Ohashi PS, Zhong Q, Wang X, Mak TW. The E3 ubiquitin ligase Mule acts through the ATM-p53 axis to maintain B lymphocyte homeostasis. J Exp Med. 2012;209(1):173-86.
Cellular homeostasis is controlled by pathways that balance cell death with survival. Mcl-1 ubiquitin ligase E3 (Mule) is an E3 ubiquitin ligase that targets the proapoptotic molecule p53 for polyubiquitination and degradation. To elucidate the role of Mule in B lymphocyte homeostasis, B cell-specific Mule knockout (BMKO) mice were generated using the Cre-LoxP recombination system. Analysis of BMKO mice showed that Mule was essential for B cell development, proliferation, homeostasis, and humoral immune responses. p53 transactivation was increased by two- to fourfold in Mule-deficient B cells at steady state. Genetic ablation of p53 in BMKO mice restored B cell development, proliferation, and homeostasis. p53 protein was increased in resting Mule-deficient mouse embryonic fibroblasts (MEFs) and embryonic stem (ES) cells. Loss of Mule in both MEFs and B cells at steady state resulted in increased levels of phospho-ataxia telangiectasia mutated (ATM) and the ATM substrate p53. Under genotoxic stress, BMKO B cells were resistant to apoptosis, and control MEFs exhibited evidence of a physical interaction between Mule and phospho-ATM. Phospho-ATM, phospho-p53, and Brca1 levels were reduced in Mule-deficient B cells and MEFs subjected to genotoxic stress. Thus, Mule regulates the ATM-p53 axis to maintain B cell homeostasis under both steady-state and stress conditions.