DDB1, a subunit of the damaged-DNA binding protein DDB, has been shown to function also as an adaptor for Cul4A, a member of the cullin family of E3 ubiquitin ligase. The Cul4A-DDB1 complex remains associated with the COP9 signalosome, and that interaction is conserved from fission yeast to human. Studies with fission yeast suggested a role of the Pcu4-Ddb1-signalosome complex in the proteolysis of the replication inhibitor Spd1. Here we provide evidence that the function of replication inhibitor proteolysis is conserved in the mammalian DDB1-Cul4A-signalosome complex. We show that small interfering RNA-mediated knockdown of DDB1, CSN1 (a subunit of the signalosome), and Cul4A in mammalian cells causes an accumulation of p27 Kip1 . Moreover, expression of DDB1 reduces the level of p27 Kip1 by increasing its decay rate. The DDB1-induced proteolysis of p27Kip1 requires signalosome and Cul4A, because DDB1 failed to increase the decay rate of p27Kip1 in cells deficient in CSN1 or Cul4A. Surprisingly, the DDB1-induced proteolysis of p27 Kip1 also involves Skp2, an F-box protein that allows targeting of p27Kip1 for ubiquitination by the Skp1-Cul1-F-box complex. Moreover, we provide evidence for a physical association between Cul4A, DDB1, and Skp2. We speculate that the F-box protein Skp2, in addition to utilizing Cul1-Skp1, utilizes Cul4A-DDB1 to induce proteolysis of p27 Kip1 .The Cul4A gene is amplified and overexpressed in breast and hepatocellular carcinomas (6, 42). Also, Cul4A is essential for mammalian development (18). It encodes a protein of the cullin family. The cullins are central components of several E3 ubiquitin ligases (11). Cul4A associates with the damaged-DNA binding protein DDB (22,32). DDB consists of two subunits: DDB1 and DDB2. The DDB2 subunit is mutated in xeroderma pigmentosum (complementation group E) (reviewed in reference 35). Cul4A participates in the ubiquitination of the DDB2 subunit of DDB and induces its proteolysis through the ubiquitin-proteasome pathway (22). Recent studies indicated that the DDB1 subunit of DDB functions as an adaptor for substrate binding by Cul4A in a manner similar to how Skp1 functions in the Skp1-cullin1-F-box (SCF) complex (15). However, unlike the case for Skp1, there are instances where DDB1 directly targets a substrate without additional adaptor proteins. For example, Cul4A has been implicated in the proteolysis of the replication licensing protein Cdt1 following DNA damage (14, 44). It was shown that the interaction between Cul4A and Cdt1 is mediated by DDB1 (15). In other examples, Cul4A-DDB1 interacts with additional adaptors to target a specific protein. The DDB1-Cul4A complex associates with hDET1, an ortholog of Arabidopsis De-etiolated-1, and hCOP1, an ortholog of Arabidopsis constitutively photomorphogenic-1 (COP1), to induce proteolysis of the c-Jun protein through the ubiquitin-proteasome pathway (40). In that study, the authors proposed that the hDET1-hCOP1 functioned as the heteromeric substrate adaptor and, in keeping with the SCF E3 ligase,...

The xeroderma pigmentosum complementation group E (XP-E) gene product damaged-DNA binding protein 2 (DDB2) plays important roles in nucleotide excision repair (NER). Previously, we showed that DDB2 participates in NER by regulating the level of p21 Waf1/Cip1 . Here we show that the p21 Waf1/Cip1 -regulatory function of DDB2 plays a central role in defining the response (apoptosis or arrest) to DNA damage. The DDB2-deficient cells are resistant to apoptosis in response to a variety of DNA-damaging agents, despite activation of p53 and the pro-apoptotic genes. Instead, these cells undergo cell cycle arrest. Also, the DDB2-deficient cells are resistant to E2F1-induced apoptosis. The resistance to apoptosis of the DDB2-deficient cells is caused by an increased accumulation of p21 Waf1/Cip1 after DNA damage. We provide evidence that DDB2 targets p21 Waf1/Cip1 for proteolysis. The resistance to apoptosis in DDB2-deficient cells also involves Mdm2 in a manner that is distinct from the p53-regulatory activity of Mdm2. Our results provide evidence for a new regulatory loop involving the NER protein DDB2, Mdm2, and p21 Waf1/Cip1 that is critical in deciding cell fate (apoptosis or arrest) upon DNA damage.

Colon cancer is one of the deadliest cancers worldwide because of its metastasis to other essential organs. Metastasis of colon cancer involves a complex set of events, including epithelial to mesenchymal transition (EMT) that increases invasiveness of the tumor cells. Here we show that the xeroderma pigmentosum group E (XPE) gene product DDB2 is down-regulated in high-grade colon cancers, and it plays a dominant role in the suppression of EMT of the colon cancer cells. Depletion of DDB2 promotes mesenchymal phenotype, whereas expression of DDB2 promotes epithelial phenotype. DDB2 constitutively represses genes that are the key activators of EMT, indicating that DDB2 is a master regulator of EMT of the colon cancer cells. Moreover, we observed evidence that DDB2 functions as a barrier for EMT induced by hypoxia and TGF-β. Also, we provide evidence that DDB2 inhibits metastasis of colon cancer. The results presented here identify a transcriptional regulatory pathway of DDB2 that is directly linked to the mechanisms that suppress metastasis of colon cancer.

The mammalian Cul4 genes, Cul4A and Cul4B, encode the scaffold components of the cullin-based E3 ubiquitin ligases. The two Cul4 genes are functionally redundant. Recent study indicated that mice expressing a truncated CUL4A that fails to interact with its functional partner ROC1 exhibit no developmental phenotype. We generated a Cul4A-/- strain lacking exons 4-8 that does not express any detectable truncated protein. In this strain, the male mice are infertile and exhibit severe deficiencies in spermatogenesis. The primary spermatocytes are deficient in progression through late prophase I, a time point when expression of the X-linked Cul4B gene is silenced due to meiotic sex chromosome inactivation. Testes of the Cul4A-/- mice exhibit extensive apoptosis. Interestingly, the pachytene spermatocytes exhibit persistent double stranded breaks, suggesting a deficiency in homologous recombination. Also, we find that CUL4A localizes to the double stranded breaks generated in pre-pachytene spermatocytes. The observations identify a novel function of CUL4A in meiotic recombination and demonstrate an essential role of CUL4A in spermatogenesis.