Phosphorylation and degradation of MdmX is inhibited by Wip1 phosphatase in the DNA damage response

Dephosphorylation of γ-H2AX by WIP1: an important homeostatic regulatory event in DNA repair and cell cycle control

DNA double strand breaks are a particularly toxic form of DNA damage and the mammalian cell has evolved an intricate set of responses to repair this type of DNA lesion. A key early event in the DNA damage response (DDR) is ATM phosphorylation of the histone variant H2AX at serine 139 at the site of the DNA break. Phosphorylated S139 H2AX, or γ-H2AX, forms a docking site for binding of MDC1, leading to sustained recruitment of other DNA repair factors that mediate the repair of the DNA double strand break.  Moreover, recruitment of MDC1 to the break site activates cell cycle checkpoints, protecting the cell from replication of damaged DNA templates. While the molecular events leading to DNA double strand break repair have been well described, the deactivating or homeostatic mechanisms following completion of repair remain largely unexplored. Recent publications by our laboratories and the Medema laboratory shed new light on this issue. Both publications showed that the Wild-type p53-Induced Phosphatase 1 (WIP1) directly dephosphorylates γ-H2AX. WIP1 migrates to the sites of irradiation-induced foci (IRIF), though at a delayed rate relative to MDC1 and mediates γ-H2AX dephosphorylation, presumably after DNA repair is complete. This prevents recruitment of other repair factors such as MDC1 and 53BP1 to the DNA damage sites and promotes the dissolution of IRIF.  In addition, overexpression of WIP1 has a suppressive effect on DNA double strand break repair. Taken together, these reports further implicate WIP1 as a critical homeostatic regulator of the DDR.

The oncogenic phosphatase WIP1 negatively regulates nucleotide excision repair

Nucleotide excision repair (NER) is the only mechanism in humans to repair UV-induced DNA lesions such as pyrimidine (6-4) pyrimidone photoproducts and cyclobutane pyrimidine dimers (CPDs). In response to UV damage, the ataxia telangiectasia mutated and Rad3-related (ATR) kinase phosphorylates and activates several downstream effector proteins, such as p53 and XPA, to arrest cell cycle progression, stimulate DNA repair, or initiate apoptosis. However, following the completion of DNA repair, there must be active mechanisms that restore the cell to a prestressed homeostatic state. An important part of this recovery must include a process to reduce p53 and NER activity as well as to remove repair protein complexes from the DNA damage sites. Since activation of the damage response occurs in part through phosphorylation, phosphatases are obvious candidates as homeostatic regulators of the DNA damage and repair responses. Therefore, we investigated whether the serine/threonine wild-type p53-induced phosphatase 1 (WIP1/PPM1D) might regulate NER. WIP1 overexpression inhibits the kinetics of NER and CPD repair, whereas WIP1 depletion enhances NER kinetics and CPD repair. This NER suppression is dependent on WIP1 phosphatase activity, as phosphatase-dead WIP1 mutants failed to inhibit NER. Moreover, WIP1 suppresses the kinetics of UV-induced damage repair largely through effects on NER, as XPD-deficient cells are not further suppressed in repairing UV damage by overexpressed WIP1. Wip1 null mice quickly repair their CPD and undergo less UV-induced apoptosis than their wild-type counterparts. In vitro phosphatase assays identify XPA and XPC as two potential WIP1 targets in the NER pathway. Thus WIP1 may suppress NER kinetics by dephosphorylating and inactivating XPA and XPC and other NER proteins and regulators after UV-induced DNA damage is repaired.

The p53 orchestra: Mdm2 and Mdmx set the tone

The activities of p53 cover diverse aspects of cell biology, including cell cycle control, apoptosis, metabolism, fertility, differentiation and cellular reprogramming. Although loss of p53 function engenders tumor susceptibility, hyperactivation of p53 is lethal. Therefore, p53 activity must be strictly regulated to maintain normal tissue homeostasis. Critical for the control of p53 function are its two main negative regulators: Mdm2 and Mdmx. Recent reports have provided insight into the complex mechanisms that regulate these two proteins and have revealed novel functions for each. Here, we review and evaluate models of Mdm2- and Mdmx-dependent regulation of p53 activity. Both Mdm2 and Mdmx receive input from numerous signaling pathways and interact with many proteins in addition to p53. Therefore, we also consider roles for Mdm2 and Mdmx in additional cancer-related networks, including Notch signaling and the epithelial-to-mesenchymal transition.