Cytoplasmic E3 ubiquitin ligase CUL9 controls cell proliferation, senescence, apoptosis and genome integrity through p53.

CUL9 is a member of the cullin family of E3 ubiquitin ligases, and it localizes predominantly in the cytoplasm. Deletion of Cul9 in mice results in increased DNA damage, widespread aneuploidy, spontaneous tumor development, accelerated Eμ-Myc-induced lymphomagenesis and susceptibility to carcinogenesis. CUL9 binds to p53 and causes cell apoptosis when ectopically expressed. Whether the function of CUL9 in maintaining genomic integrity and suppressing tumorigenesis is linked to p53 has not been genetically tested. Here we report that deletion of CUL9 in human cells results in attenuated p21 induction and impaired cellular response to DNA damage. We show that disruption of Cul9-p53 binding in mouse embryo fibroblasts (MEFs) by a knock-in mutation in Cul9 (Δp53) increases S-phase cell population, accumulates DNA damage during DNA replication and decreases apoptosis to both endogenous and exogenous DNA-damaging agents. The extent of these alterations in Cul9 MEFs is indistinguishable to those seen in Cul9 MEFs and comparable to those seen in p53 MEFs. Deletion of CUL9 in p53 null cells does not lead to further increase of DNA damages. Both Cul9 and Cul9 MEFs proliferate faster and undergo spontaneous immortalization while retaining both Arf and p53. These results demonstrate that the functions of CUL9 in regulating cell proliferation and maintaining genomic integrity are mainly mediated by p53, and that CUL9 is a critical p53 activator.