Cdt1 is differentially targeted for degradation by anticancer chemotherapeutic drugs

CDT1, transcriptionally regulated by E2F2, promotes lung adenocarcinoma progression

CDT1, a gene that shows excessive expression in various malignancies, functions as a pivotal regulator of replication licensing. In this study, we observed a positive correlation in expression between CDT1 and E2F2 among patients with lung adenocarcinoma (LUAD). Our findings substantiated that E2F2 directly interacted with the promoter region of CDT1, as confirmed by ChIP-qPCR assays, and depletion of E2F2 resulted in a downregulation of CDT1 expression in LUAD cell lines by gene interference technology. Furthermore, we identified an upregulation of CDT1 mRNA level in Chinese LUAD samples. Notably, in the loss-of-function assays, depletion of CDT1 in LUAD cell lines inhibited cell proliferation, migration, and invasion. Concurrently, it promoted cell apoptosis and induced G0/G1 phase arrest using MTT, flow cytometry, and Transwell assays, reinforcing its role as an oncogene.Furthermore, enhanced tumor ablation was determined in a CDT1-downregulated LUAD tumor-bearing nude mouse model. Collectively, our results strongly suggest that E2F2 positively regulates CDT1 expression and actively participates in the progression of lung adenocarcinoma, thereby providing valuable insights into identifying novel therapeutic targets for LUAD treatment.

CDT1 drives replication overlicensing and enhances tumorigenesis in the gut†

Colorectal carcinoma (CRC) is one of the most common forms of malignancy in the Western world. Recent decades have witnessed enormous progress in our understanding of the mechanisms that sustain CRC, even though the factors implicated in the initiation and progression of this neoplasia are not fully understood. A recent study published in The Journal of Pathology looked at the consequences of hyperactivity of chromatin licensing and DNA replication factor 1 (CDT1), a regulator of DNA replication that is produced in excess in CRC, on the course of intestinal tumorigenesis. Mice engineered to selectively overexpress CDT1 and/or lack Geminin, an inhibitor of CDT1, in the intestinal epithelium were more susceptible to experimental intestinal tumorigenesis compared to wild-type mice. This work supports the pro-tumorigenic role of CDT1 and suggests the potential prognostic value of this protein in CRC. © 2022 The Pathological Society of Great Britain and Ireland.

Intrinsic neural stem cell properties define brain hypersensitivity to genotoxic stress

Impaired replication has been previously linked to growth retardation and microcephaly; however, why the brain is critically affected compared with other organs remains elusive. Here, we report the differential response between early neural progenitors (neuroepithelial cells [NECs]) and fate-committed neural progenitors (NPs) to replication licensing defects. Our results show that, while NPs can tolerate altered expression of licensing factors, NECs undergo excessive replication stress, identified by impaired replication, increased DNA damage, and defective cell-cycle progression, leading eventually to NEC attrition and microcephaly. NECs that possess a short G1 phase license and activate more origins than NPs, by acquiring higher levels of DNA-bound MCMs. In vivo G1 shortening in NPs induces DNA damage upon impaired licensing, suggesting that G1 length correlates with replication stress hypersensitivity. Our findings propose that NECs possess distinct cell-cycle characteristics to ensure fast proliferation, although these inherent features render them susceptible to genotoxic stress.

Small Molecule Inhibitor Targeting CDT1/Geminin Protein Complex Promotes DNA Damage and Cell Death in Cancer Cells

DNA replication initiation requires the loading of MCM2-7 complexes at the origins of replication during G1. Replication licensing renders chromatin competent for DNA replication and its tight regulation is essential to prevent aberrant DNA replication and genomic instability. CDT1 is a critical factor of licensing and its activity is controlled by redundant mechanisms, including Geminin, a protein inhibitor of CDT1. Aberrant CDT1 and Geminin expression have been shown to promote tumorigenesis in vivo and are also evident in multiple human tumors. In this study, we developed an in vitro AlphaScreen™ high-throughput screening (HTS) assay for the identification of small-molecule inhibitors targeting the CDT1/Geminin protein complex. Biochemical characterization of the most potent compound, AF615, provided evidence of specific, dose-dependent inhibition of Geminin binding to CDT1 both in-vitro and in cells. Moreover, compound AF615 induces DNA damage, inhibits DNA synthesis and reduces viability selectively in cancer cell lines, and this effect is CDT1-dependent. Taken together, our data suggest that AF615 may serve as a useful compound to elucidate the role of CDT1/Geminin protein complex in replication licensing and origin firing as well as a scaffold for further medicinal chemistry optimisation.

SIRT3 promotion reduces resistance to cisplatin in lung cancer by modulating the FOXO3/CDT1 axis

Background

Cisplatin is an extensively used chemotherapy agent for lung cancer, but its drug resistance serves as a huge obstacle for chemotherapy failure of lung cancer patients. Hence, researchers aimed to determine role of sirtuin 3 (SIRT3) considering its action in cisplatin resistance of lung cancer.

Methods

The expression patterns of SIRT3, FOXO3, and CDT1 were determined using RT‐qPCR and Immunoblotting in lung cancer. Immunofluorescence and Co‐IP were adopted to detect co‐localization and interaction of FOXO3 and CDT1. Loss‐ and gain‐function assays were conducted to determine roles of SIRT3, FOXO3, and CDT1 in resulting pathological changes, while biological behavior of cells was determined using a combination of CCK‐8, flow cytometry, colony formation, and Transwell assays. The effects of SIRT3 and CDT1 were determined in the nude mice xenografted with the tumor. The proliferation‐, angiogenesis‐, and apoptosis‐associated factors levels were determined using Immunoblotting.

Results

SIRT3, FOXO3, and CDT1 expression was suppressed in the lung cancer tissues and cells. FOXO3 positively regulates the CDT1 expression pattern and SIRT3 elevation inhibits FOXO3 at the acetylated level, thus, elevating FOXO3 expression. The elevation of SIRT3, FOXO3, or CDT1 inhibited cell cisplatin resistance of lung cancer cells as well as inhibited viability, proliferation, and invasion in vitro. In vivo experiments, SIRT3 depletion elevated Ki‐67 and VEGFA levels, but downregulated cleaved caspase 3 level.

Conclusion

Collectively, overexpressed SIRT3 elevates expression of FOXO3a/CDT1 axis, thus, contributing to enhanced sensitivity of lung cancer cells.

A Custom Ultra-Low-Cost 3D Bioprinter Supports Cell Growth and Differentiation

Advances in 3D bioprinting have allowed the use of stem cells along with biomaterials and growth factors toward novel tissue engineering approaches. However, the cost of these systems along with their consumables is currently extremely high, limiting their applicability. To address this, we converted a 3D printer into an open source 3D bioprinter and produced a customized bioink based on accessible alginate/gelatin precursors, leading to a cost-effective solution. The bioprinter's resolution, including line width, spreading ratio and extrusion uniformity measurements, along with the rheological properties of the bioinks were analyzed, revealing high bioprinting accuracy within the printability window. Following the bioprinting process, cell survival and proliferation were validated on HeLa Kyoto and HEK293T cell lines. In addition, we isolated and 3D bioprinted postnatal neural stem cell progenitors derived from the mouse subventricular zone as well as mesenchymal stem cells derived from mouse bone marrow. Our results suggest that our low-cost 3D bioprinter can support cell proliferation and differentiation of two different types of primary stem cell populations, indicating that it can be used as a reliable tool for developing efficient research models for stem cell research and tissue engineering.

CRL4 ubiquitin ligase stimulates Fanconi anemia pathway-induced single-stranded DNA-RPA signaling

Background

DNA-crosslinking agents like cisplatin and mitomycin C (MMC) are indispensible for the treatment of many solid malignancies. These anticancer drugs generate DNA interstrand crosslinks (ICLs) that cause cell death by blocking replication forks. Many factors counteracting ICL-induced DNA replication stress, including the Fanconi anemia (FA) pathway, are regulated by ubiquitination and, therefore, ubiquitin ligases are potential targets for the sensitization of cancer cells to crosslinking agents. In this study, we investigated the function of the CRL4 ubiquitin ligase in modulating the response of cancer cells to ICL induction.

Methods

The two cullin paralogs CUL4A and CUL4B, which form the CRL4 ligase scaffold, were depleted in cancer cells by small interfering RNA followed by analysis of the cellular and biochemical responses to ICLs elicited upon cisplatin or MMC treatment.

Results

We report that the combined depletion of CUL4A and CUL4B weakens an FA pathway-dependent S phase checkpoint response. CRL4 positively stimulates the monoubiquitination of FANCD2 required for the recruitment of XPF-ERCC1, a structure-specific endonuclease that, in turn, contributes to the display of single-stranded DNA (ssDNA) at ICLs. After CRL4 down regulation, the missing ssDNA results in reduced recruitment of RPA, thereby dampening activation of ATR and CHK1 checkpoint kinases and allowing for S phase progression despite ICL induction.

Conclusion

Our findings indicate that CRL4 promotes cell survival by potentiating an FA pathway-dependent ssDNA-RPA signaling platform installed at ICLs. The anticancer efficacy of crosslinking agents may, therefore, be enhanced by down regulating CRL4 activity.

A DNA-binding domain in the C-terminal region of Cdt2 enhances the DNA synthesis-coupled CRL4Cdt2 ubiquitin ligase activity for Cdt1

The Cullin-RING ubiquitin ligase CRL4Cdt2 maintains genome integrity by mediating the cell cycle- and DNA damage-dependent degradation of proteins such as Cdt1, p21 and Set8. Human Cdt2 has two regions, a conserved N-terminal seven WD40 repeat region and a less conserved C-terminal region. Here, we showed that the N-terminal region is sufficient for complex formation with CRL4, but the C-terminal region is required for the full ubiquitin ligase activity. UV irradiation-induced polyubiquitination and degradation of Cdt1 were impaired in Cdt2 (N-terminus only)-expressing cells. Deletion and mutation analysis identified a domain in the C-terminal region that increased ubiquitination activity and displayed DNA-binding activity. The identified domain mediated binding to double-stranded DNA and showed higher affinity binding to single-stranded DNA. As the ligase activity of CRL4Cdt2 depends on proliferating cell nuclear antigen (PCNA) loading onto DNA, the present results suggest that the DNA-binding domain facilitates the CRL4Cdt2-mediated recognition and ubiquitination of substrates bound to PCNA on chromatin.

Direct binding of Cdt2 to PCNA is important for targeting the CRL4Cdt2 E3 ligase activity to Cdt1

The C-terminal end of Cdt2 contains a PIP box for binding to PCNA to promote CRL4^Cdt2 function, creating a new paradigm where the substrate receptor and substrates bind to a common multivalent docking platform for ubiquitination.

The CRL4^Cdt2 ubiquitin ligase complex is an essential regulator of cell-cycle progression and genome stability, ubiquitinating substrates such as p21, Set8, and Cdt1, via a display of substrate degrons on proliferating cell nuclear antigens (PCNAs). Here, we examine the hierarchy of the ligase and substrate recruitment kinetics onto PCNA at sites of DNA replication. We demonstrate that the C-terminal end of Cdt2 bears a PCNA interaction protein motif (PIP box, Cdt2^PIP), which is necessary and sufficient for the binding of Cdt2 to PCNA. Cdt2^PIP binds PCNA directly with high affinity, two orders of magnitude tighter than the PIP box of Cdt1. X-ray crystallographic structures of PCNA bound to Cdt2^PIP and Cdt1^PIP show that the peptides occupy all three binding sites of the trimeric PCNA ring. Mutating Cdt2^PIP weakens the interaction with PCNA, rendering CRL4^Cdt2 less effective in Cdt1 ubiquitination and leading to defects in Cdt1 degradation. The molecular mechanism we present suggests a new paradigm for bringing substrates to the CRL4-type ligase, where the substrate receptor and substrates bind to a common multivalent docking platform to enable subsequent ubiquitination.

Prevention of oral carcinogenesis in rats by Dracaena cinnabari resin extracts

OBJECTIVES

In vivo study was performed to determine the chemopreventive efficacy of the DC resin methanol extract on a 4-nitroquinoline-1-oxide (4NQO) oral cancer animal model.

MATERIALS AND METHODS

This study involves administration of 4NQO solution for 8 weeks alone (cancer induction) or with Dracaena cinnabari (DC) extract at 100, 500, and 1000 mg/kg. DC extract administration started 1 week before exposure until 1 week after the carcinogen exposure was stopped. All rats were sacrificed after 22 weeks, and histological analysis was performed to assess any incidence of pathological changes. Immunohistochemical expressions of selected tumor marker antibodies were analyzed using an image analyzer computer system, and the expression of selected genes involved in apoptosis and proliferative mechanism related to oral cancer were evaluated using RT²-PCR.

RESULTS

The incidence of OSCC decreased with the administration of DC extract at 100, 500, and 1000 mg/kg compared to the induced cancer group. The developed tumor was also observed to be smaller when compared to the induced cancer group. The DC 1000 mg/kg group inhibits the expression of Cyclin D1, Ki-67, Bcl-2, and p53 proteins. It was observed that DC 1000 mg/kg induced apoptosis by upregulation of Bax and Casp3 genes and downregulation of Tp53, Bcl-2, Cox-2, Cyclin D1, and EGFR genes when compared to the induced cancer group.

CONCLUSIONS

The data indicated that systemic administration of the DC resin methanol extract has anticarcinogenic potency on oral carcinogenesis.

CLINICAL RELEVANCE

Chemoprevention with DC resin methanol extract may significantly reduce morbidity and possibly mortality from OSCC.

Geminin ablation in vivo enhances tumorigenesis through increased genomic instability

Geminin, a DNA replication licensing inhibitor, ensures faithful DNA replication in vertebrates. Several studies have shown that geminin depletion in vitro results in rereplication and DNA damage, whereas increased expression of geminin has been observed in human cancers. However, conditional inactivation of geminin during embryogenesis has not revealed any detectable DNA replication defects. In order to examine its role in vivo, we conditionally inactivated geminin in the murine colon and lung, and assessed chemically induced carcinogenesis. We show here that mice lacking geminin develop a significantly higher number of tumors and bear a larger tumor burden than sham-treated controls in urethane-induced lung and azoxymethane/dextran sodium sulfate-induced colon carcinogenesis. Survival is also significantly reduced in mice lacking geminin during lung carcinogenesis. A significant increase in the total number and grade of lesions (hyperplasias, adenomas, and carcinomas) was also confirmed by hematoxylin and eosin staining. Moreover, increased genomic aberrations, identified by increased ATR and γH2AX expression, was detected with immunohistochemistry analysis. In addition, we analyzed geminin expression in human colon cancer, and found increased expression, as well as a positive correlation with ATM/ATR levels and a non-monotonic association with γH2AX. Taken together, our data demonstrate that geminin acts as a tumor suppressor by safeguarding genome stability, whereas its overexpression is also associated with genomic instability. Copyright © 2018 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Mutations at multiple CDK phosphorylation consensus sites on Cdt2 increase the affinity of CRL4Cdt2 for PCNA and its ubiquitination activity in S phase

CRL4^Cdt2 ubiquitin ligase plays an important role maintaining genome integrity during the cell cycle. A recent report suggested that Cdk1 negatively regulates CRL4^Cdt2 activity through phosphorylation of its receptor, Cdt2, but the involvement of phosphorylation remains unclear. To address this, we mutated all CDK consensus phosphorylation sites located in the C-terminal half region of Cdt2 (Cdt2-18A) and examined the effect on substrate degradation. We show that both cyclinA/Cdk2 and cyclinB/Cdk1 phosphorylated Cdt2 in vitro and that phosphorylation was reduced by the 18A mutation both in vitro and in vivo. The 18A mutation increased the affinity of Cdt2 to PCNA, and a high amount of Cdt2-18A was colocalized with PCNA foci during S phase in comparison with Cdt2-WT. Poly-ubiquitination activity to Cdt1 was concomitantly enhanced in cells expressing Cdt2-18A. Other CRL4^Cdt2 substrates, Set8 and thymine DNA glycosylase, begin to accumulate around late S phase to G2 phase, but the accumulation was prevented in Cdt2-18A cells. Furthermore, mitotic degradation of Cdt1 after UV irradiation was induced in these cells. Our results suggest that CDK-mediated phosphorylation of Cdt2 inactivates its ubiquitin ligase activity by reducing its affinity to PCNA, an important strategy for regulating the levels of key proteins in the cell cycle.

Mismatch repair proteins recruited to ultraviolet light-damaged sites lead to degradation of licensing factor Cdt1 in the G1 phase

Cdt1 is rapidly degraded by CRL4^Cdt2 E3 ubiquitin ligase after UV (UV) irradiation. Previous reports revealed that the nucleotide excision repair (NER) pathway is responsible for the rapid Cdt1-proteolysis. Here, we show that mismatch repair (MMR) proteins are also involved in the degradation of Cdt1 after UV irradiation in the G1 phase. First, compared with the rapid (within ∼15 min) degradation of Cdt1 in normal fibroblasts, Cdt1 remained stable for ∼30 min in NER-deficient XP-A cells, but was degraded within ∼60 min. The delayed degradation was also dependent on PCNA and CRL4^Cdt2. The MMR proteins Msh2 and Msh6 were recruited to the UV-damaged sites of XP-A cells in the G1 phase. Depletion of these factors with small interfering RNAs prevented Cdt1 degradation in XP-A cells. Similar to the findings in XP-A cells, depletion of XPA delayed Cdt1 degradation in normal fibroblasts and U2OS cells, and co-depletion of Msh6 further prevented Cdt1 degradation. Furthermore, depletion of Msh6 alone delayed Cdt1 degradation in both cell types. When Cdt1 degradation was attenuated by high Cdt1 expression, repair synthesis at the damaged sites was inhibited. Our findings demonstrate that UV irradiation induces multiple repair pathways that activate CRL4^Cdt2 to degrade its target proteins in the G1 phase of the cell cycle, leading to efficient repair of DNA damage.

Systematic Protein Level Regulation via Degradation Machinery Induced by Genotoxic Drugs

In this study we monitored protein dynamics in response to cisplatin, 5-fluorouracil, and irinotecan with different concentrations and administration modes using "reverse-phase" protein arrays (RPPAs) in order to gain comprehensive insight into the protein dynamics induced by genotoxic drugs. Among 666 protein time-courses, 38% exhibited an increasing trend, 32% exhibited a steady decrease, and 30% fluctuated within 24 h after drug exposure. We analyzed almost 12,000 time-course pairs of protein levels based on the geometrical similarity by correlation distance (dCor). Twenty-two percent of the pairs showed dCor > 0.8, which indicates that each protein of the pair had similar dynamics. These trends were disrupted by a proteasome inhibitor, MG132, suggesting that the protein degradation system was activated in response to the drugs. Among the pairs with high dCor, the average dCor of pairs with apoptosis-related protein was significantly higher than those without, indicating that regulation of protein levels was induced by the drugs. These results suggest that the levels of numerous functionally distinct proteins may be regulated by common degradation machinery induced by genotoxic drugs.

The stress phenotype makes cancer cells addicted to CDT2, a substrate receptor of the CRL4 ubiquitin ligase

CDT2/L2DTL/RAMP is one of the substrate receptors of the Cullin Ring Ubiquitin Ligase 4 that targets for ubiquitin mediated degradation a number of substrates, such as CDT1, p21 and CHK1, involved in the regulation of cell cycle and survival. Here we show that CDT2 depletion was alone able to induce the apoptotic death in 12/12 human cancer cell lines from different tissues, regardless of the mutation profile and CDT2 expression level. Cell death was associated to rereplication and to loss of CDT1 degradation. Conversely, CDT2 depletion did not affect non-transformed human cells, such as immortalized kidney, lung and breast cell lines, and primary cultures of endothelial cells and osteoblasts. The ectopic over-expression of an activated oncogene, such as the mutation-activated RAS or the amplified MET in non-transformed immortalized breast cell lines and primary human osteoblasts, respectively, made cells transformed in vitro, tumorigenic in vivo, and susceptible to CDT2 loss. The widespread effect of CDT2 depletion in different cancer cells suggests that CDT2 is not in a synthetic lethal interaction to a single specific pathway. CDT2 likely is a non-oncogene to which transformed cells become addicted because of their enhanced cellular stress, such as replicative stress and DNA damage.

Mitotic UV irradiation induces a DNA replication-licensing defect that potentiates G1 arrest response

Cdt1 begins to accumulate in M phase and has a key role in establishing replication licensing at the end of mitosis or in early G1 phase. Treatments that damage the DNA of cells, such as UV irradiation, induce Cdt1 degradation through PCNA-dependent CRL4-Cdt2 ubiquitin ligase. How Cdt1 degradation is linked to cell cycle progression, however, remains unclear. In G1 phase, when licensing is established, UV irradiation leads to Cdt1 degradation, but has little effect on the licensing state. In M phase, however, UV irradiation does not induce Cdt1 degradation. When mitotic UV-irradiated cells were released into G1 phase, Cdt1 was degraded before licensing was established. Thus, these cells exhibited both defective licensing and G1 cell cycle arrest. The frequency of G1 arrest increased in cells expressing extra copies of Cdt2, and thus in cells in which Cdt1 degradation was enhanced, whereas the frequency of G1 arrest was reduced in cell expressing an extra copy of Cdt1. The G1 arrest response of cells irradiated in mitosis was important for cell survival by preventing the induction of apoptosis. Based on these observations, we propose that mammalian cells have a DNA replication-licensing checkpoint response to DNA damage induced during mitosis.

Cell cycle staging of individual cells by fluorescence microscopy

Progression through the cell cycle is one of the most fundamental features of cells. Studies of the cell cycle have traditionally relied on the analysis of populations, and they often require specific markers or the use of genetically modified systems, making it difficult to determine the cell cycle stage of individual, unperturbed cells. We describe a protocol, suitable for use in high-resolution imaging approaches, for determining cell cycle staging of individual cells by measuring their DNA content by fluorescence microscopy. The approach is based on the accurate quantification by image analysis of the integrated nuclear intensity of cells stained with a DNA dye, and it can be used in combination with several histochemical methods. We describe and provide the algorithms for two automated image analysis pipelines and the derivation of cell cycle profiles with both commercial and open-source software. This 1-2-d protocol is applicable to adherent cells, and it is adaptable for use with several DNA dyes.

Generation of cell-based systems to visualize chromosome damage and translocations in living cells

Traditional methods for the generation of DNA damage are not well suited for the observation of spatiotemporal aspects of damaged chromosomal loci. We describe a protocol for the derivation of a cellular system to induce and to visualize chromosome damage at specific sites of the mammalian genome in living cells. The system is based on the stable integration of endonuclease I-SceI recognition sites flanked by bacterial LacO/TetO operator arrays, coupled with retroviral-mediated integration of their fluorescent repressors (LacR/TetR) to visualize the LacO/TetO sites. Expression of the I-SceI endonuclease induces double-strand breaks (DSBs) specifically at the sites of integration, and it permits the dynamics of damaged chromatin to be followed by time-lapse microscopy. Sequential LacO-I-SceI/TetO-I-SceI integrations in multiple chromosomes permit the generation of a system to visualize the formation of chromosome translocations in living cells. This protocol requires intermediate cell culture and molecular biology skills, and it is adaptable to the efficient derivation of any integrated clonal reporter system of interest in ≈ 3-5 months.

PIP degron proteins, substrates of CRL4Cdt2, and not PIP boxes, interfere with DNA polymerase η and κ focus formation on UV damage

Proliferating cell nuclear antigen (PCNA) is a well-known scaffold for many DNA replication and repair proteins, but how the switch between partners is regulated is currently unclear. Interaction with PCNA occurs via a domain known as a PCNA-Interacting Protein motif (PIP box). More recently, an additional specialized PIP box has been described, the « PIP degron », that targets PCNA-interacting proteins for proteasomal degradation via the E3 ubiquitin ligase CRL4^Cdt2. Here we provide evidence that CRL4^Cdt2-dependent degradation of PIP degron proteins plays a role in the switch of PCNA partners during the DNA damage response by facilitating accumulation of translesion synthesis DNA polymerases into nuclear foci. We show that expression of a nondegradable PIP degron (Cdt1) impairs both Pol η and Pol κ focus formation on ultraviolet irradiation and reduces cell viability, while canonical PIP box-containing proteins have no effect. Furthermore, we identify PIP degron-containing peptides from several substrates of CRL4^Cdt2 as efficient inhibitors of Pol η foci formation. By site-directed mutagenesis we show that inhibition depends on a conserved threonine residue that confers high affinity for PCNA-binding. Altogether these findings reveal an important regulative role for the CRL4^Cdt2 pathway in the switch of PCNA partners on DNA damage.

PCNA-dependent ubiquitination of Cdt1 and p21 in mammalian cells

PCNA is a DNA clamp, acting on chromatin as a platform for various proteins involved in many aspects of DNA replication-linked processes. Most of these proteins have the PCNA-interaction protein motif (PIP box) that associates with PCNA. Recent works show that PCNA plays an important role as a matchmaker, connecting PCNA-interacting proteins to the ubiquitin ligase CRL4(Cdt2) for their degradation. Proteins degraded by CRL4(Cdt2) include Cdt1, p21, and Set8 in mammalian cells. These CRL4(Cdt2) substrates have a PIP degron that consists of the canonical PIP-box sequence and additional conserved amino acids required for ubiquitination. The degradation of these proteins is triggered when PCNA is loaded onto chromatin at the onset of S phase, and this process is important to prevent re-replication of DNA. These CRL4(Cdt2) substrates are also degraded through the same mechanism in response to DNA damage. In this chapter, we describe several approaches to investigate how PIP degron-containing proteins are degraded in a PCNA-dependent manner.

REGγ deficiency promotes premature aging via the casein kinase 1 pathway

Our recent studies suggest a role for the proteasome activator REG (11S regulatory particles, 28-kDa proteasome activator)γ in the regulation of tumor protein 53 (p53). However, the molecular details and in vivo biological significance of REGγ-p53 interplay remain elusive. Here, we demonstrate that REGγ-deficient mice develop premature aging phenotypes that are associated with abnormal accumulation of casein kinase (CK) 1δ and p53. Antibody array analysis led us to identify CK1δ as a direct target of REGγ. Silencing CK1δ or inhibition of CK1δ activity prevented decay of murine double minute (Mdm)2. Interestingly, a massive increase of p53 in REGγ(-/-) tissues is associated with reduced Mdm2 protein levels despite that Mdm2 transcription is enhanced. Allelic p53 haplodeficiency in REGγ-deficient mice attenuated premature aging features. Furthermore, introducing exogenous Mdm2 to REGγ(-/-) MEFs significantly rescues the phenotype of cellular senescence, thereby establishing a REGγ-CK1-Mdm2-p53 regulatory pathway. Given the conflicting evidence regarding the "antiaging" and "proaging" effects of p53, our results indicate a key role for CK1δ-Mdm2-p53 regulation in the cellular aging process. These findings reveal a unique model that mimics acquired aging in mammals and indicates that modulating the activity of the REGγ-proteasome may be an approach for intervention in aging-associated disorders.