The Xeroderma Pigmentosum Group E Gene Product DDB2 Activates Nucleotide Excision Repair by Regulating the Level of p21^Waf1/Cip1

Abstract

The xeroderma pigmentosum group E gene product DDB2, a protein involved in nucleotide excision repair (NER), associates with the E3 ubiquitin ligase complex Cul4A-DDB1. But the precise role of these interactions in the NER activity of DDB2 is unclear. Several models, including DDB2-mediated ubiquitination of histones in UV-irradiated cells, have been proposed. But those models lack clear genetic evidence. Here we show that DDB2 participates in NER by regulating the cellular levels of p21 Waf1/Cip1 . We show that DDB2 enhances nuclear accumulation of DDB1, which binds to a modified form of p53 containing phosphorylation at Ser18 (p53 S18P ) and targets it for degradation in low-dose-UV-irradiated cells. Waf1/Cip1 to the NER activity of DDB2.The UV rays in sunlight are considered to be the major cause of skin cancers. UV causes DNA damage by generating cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts, which are repaired mainly by the nucleotide excision repair (NER) pathway (reviewed in reference 41). NER involves excision of the strand containing CPDs or 6-4 photoproducts, followed by repair synthesis to fill the gap. Several genes involved in NER are mutated in xeroderma pigmentosum, a rare repair deficiency disease in which the patients are sun sensitive and develop skin cancer at a high frequency (see references 11 and 16 for reviews). Eight complementation groups, XPA through XPG and XPV, have been characterized in xeroderma pigmentosum. While XPV encodes an error-prone DNA polymerase, the other XP genes encode proteins that participate in the excision of the damaged DNA strand. For example, XPC participates in the recognition of damaged DNA. Subsequently, XPB and XPD participate in unwinding the strands at the damaged site; XPA is critical for positioning the endonucleases XPF and XPG with respect to the damaged site, leading to excision of the damaged strand. The gap generated following excision is filled by DNA polymerase delta that involves PCNA to carry out the repair synthesis (41). Thus, while all other XP genes have been functionally characterized, the mechanism by which the XPE gene participates in NER has remained controversial.The XPE gene encodes DDB2, a subunit of the damaged-DNA-binding protein DDB, which possesses a high affinity for CPDs and 6-4 photoproducts (reviewed in reference 48). Cells from XP-E patients exhibit a deficiency in NER. The NER deficiency in XP-E cells, along with several other in vivo studies, suggested a role for DDB2 in NER (25,36,48). Studies with DDB2Ϫ/Ϫ mice provided further evidence of a role for DDB2 in inhibiting UV-induced skin carcinogenesis (3,22,54), a characteristic of the XP gene products.Because of the high affinity of DDB, a complex of DDB1 and DDB2, for damaged DNA, several studies have implicated DDB2 and DDB in the early damaged-DNA recognition step of NER. However, a direct role for DDB2 or DDB in NER is a point of controversy. Initial studies reported a stimulatory activity of DDB in NER assays in vitro (51). However, two recent studies carri...

PCNA-dependent regulation of p21 ubiquitylation and degradation via the CRL4Cdt2 ubiquitin ligase complex

“…These results are consistent with the finding that an ATR-dependent pathway is essential for triggering the down-regulation of p21 after UV irradiation in mammalian cells (Bendjennat et al 2003). Another study suggested that the down-regulation of p21 in response to UV irradiation is facilitated by the ubiquitin ligase activity of the Cul4-RING ubiquitin ligase and its substrate recognition subunit DDB2 (CRL4 Ddb2 ) on p53 (Stoyanova et al 2008). Thus, the involvement of the ubiquitin system, in general, and Skp2 or other ubiquitin ligases in particular, in degrading p21 in UV-irradiated cells remains to be clarified.…”

UV Damage-Induced Phosphorylation of HBO1 Triggers CRL4DDB2-Mediated Degradation To Regulate Cell Proliferation

“…CRL4 DDB2 ubiquitylates histones (23,24) and XPC (25,26) in the nucleotide excision repair (NER) pathway after UV damage. CRL4 DDB2 also ubiquitylates p21 and targets it for ubiquitin-mediated protea-somal degradation (27), and it also ubiquitylates DDB2 itself (28,29).…”

Pirh2 E3 Ubiquitin Ligase Targets DNA Polymerase Eta for 20S Proteasomal Degradation

“…for 26S proteasomal degradation (1,29), and SCF Skp2 and Mdm2, which target p21 for 20S proteasomal degradation (13,18,32). Thus, whether p21 is directly regulated by Pirh2 is worth further investigation.…”