Ubiquitination Of P21 Research Articles

Ubiquitination of p21 by E3 ligase RNF135 promotes the proliferation of human glioblastoma cells.

Glioblastoma multiforme (GBM) is a highly heterogeneous tumor of the central nervous system with a high mortality rate. The upregulation of RING finger protein 135 (RNF135), an E3 ligase, has been observed in GBM, but the associated mechanisms have not been fully elucidated. The aim of the present study was to identify the substrate of RNF135 and study its functions in GBM. Bioinformatics analyses were performed. In addition, RNF135 was overexpressed or knocked down in human U87 and U251 GBM cells, and the effect on cell proliferation was analyzed using Cell Counting Kit-8 and colony formation assays. Furthermore, the interaction of RNF135 with its potential substrate was analyzed using glutathione S-transferase, yeast two-hybrid, immunoprecipitation (IP), co-IP and immunoblotting assays. Bioinformatics analysis indicated that RNF135 serves as a marker of poor prognosis in GBM. The overexpression of RNF135 was demonstrated to promote the proliferation of GBM cells, while the knockdown of RNF135 inhibited GBM cell growth. In addition, the results of the interaction experiments indicate that RNF135 interacts with p21 and mediates the degradation of p21 by ubiquitination. The major site of RNF135-mediated p21 ubiquitination was identified as K163. Further experiments demonstrated that RNF135 promotes the proliferation of GBM cells mainly via p21. In summary, these findings suggest that RNF135 has potential as a therapeutic target for the treatment of GBM.

RIG-I promotes cell proliferation in esophageal squamous cell carcinoma by facilitating p21 degradation.

Retinoic acid-inducible gene-I (RIG-I) is considered a key sensor for host recognition of RNA virus infections. Recent studies have shown that RIG-I also regulates carcinogenesis. However, the role of RIG-I in esophageal squamous cell carcinoma (ESCC) remains unclear. We investigated the RIG-I expression in ESCC cells using a public database, immunohistochemistry, and Western blotting. We evaluated the proliferative activity of ESCC cells using CCK-8, colony formation, and EdU staining assays. Further, we determined the ESCC cell-cycle changes using flow cytometry and the ubiquitination of p21 in the cells using cycloheximide chase and ubiquitination assays. Finally, we verified the in vivo effects of RIG-I on ESCC cells by constructing xenograft models. RIG-I was highly expressed in ESCC cells and significantly promoted their proliferation and cell-cycle. Moreover, RIG-I knockdown inhibited xenograft growth in nude mice. Furthermore, RIG-I accelerated the cell-cycle by promoting the ubiquitination and degradation of p21. Overall, this study revealed that the increased expression of RIG-I due to ESCC accelerated the progression of esophageal cancer by promoting the ubiquitination and degradation of p21, which is related to the prognosis of ESCC. Thus, RIG-I may be a novel therapeutic target for ESCC treatment.

Sugt1 loss in skeletal muscle stem cells impairs muscle regeneration and causes premature muscle aging

Abstract Adult skeletal muscle stem cells (MuSCs) are essential for muscle homeostasis and regeneration. During aging, the number of MuSCs and their regenerative capacity gradually decline but the underlying mechanisms remain elusive. Here, we identify Sugt1 (suppressor of G2 allele of SKP1 homolog), which is a chaperone for kinetochore function during mitosis and is essential for muscle regeneration by regulating MuSC proliferation. Sugt1 expression level is low in quiescent MuSCs but highly induced when the cells become activated and expand as proliferating myoblasts. Inducible inactivation of Sugt1 in MuSCs causes impaired muscle regeneration upon acute injury by impairing MuSC proliferation. Furthermore, loss of Sugt1 leads to cell cycle arrest in the G2/M phase and cellular senescence. Moreover, long-term loss of Sugt1 in MuSCs results in precocious muscle aging by inhibiting MuSC cell proliferation and promoting cellular senescence. Mechanistically, we identify a cytosolic E3 ubiquitin-ligase, Trim21 as a protein interacting partner for Sugt1 in myoblast cells. We further demonstrate that Sugt1 promotes the ubiquitination of p21 via Trim21; and Sugt1 loss causes p21 accumulation to inhibit cell cycle progression and stimulates cellular senescence. Collectively, our findings uncover that Sugt1 is an essential regulator for MuSC regenerative function during muscle regeneration and aging.

K63 Ubiquitination of P21 Can Facilitate Pellino-1 in the Context of Chronic Obstructive Pulmonary Disease and Lung Cellular Senescence.

Chronic obstructive pulmonary diseases (COPD) is a kind of age-related, airflow-obstruction disease mostly caused by cigarette smoke. However, the relationship between COPD and lung cellular senescence is still not fully understood. Here, we found silencing Pellino-1 could inhibit the protein level of P21. Then, through constructing cell lines expressed ubiquitin-HA, we found that the E3 ubiquitin ligase Pellino-1 could bind to senescence marker p21 and modify p21 by K63-site ubiquitination by co-IP assays. Furthermore, we found that p21-mediated lung cellular senescence could be inhibited by silencing Pellino-1 in a D-galactose senescence mice model. Moreover, by constructing a COPD mouse model with shPellino-1 adenovirus, we found that silencing Pellino-1 could inhibit COPD and inflammation via reduction of SASPs regulated by p21. Taken together, our study findings elucidated that silencing E3 ligase Pellino-1 exhibits therapeutic potential for treatment to attenuate the progression of lung cellular senescence and COPD.

Abstract 5373: Intact regulation of G1/S transition renders esophageal squamous cell carcinomas sensitive to PI3Kα inhibitors by blocking SKP2-mediated p21 degradation

Abstract Esophageal cancer is the seventh most common cancer and the sixth leading cause of cancer-related mortality in the world, of which about 80% are esophageal squamous cell carcinomas (ESCC). Mutation or amplification of PIK3CA frequently occurs in ESCC patients, indicating PI3Kα is a promising target for ESCC treatment. CYH33 is a novel PI3Kα-selective inhibitor, which is currently in clinical trials for the treatment of solid tumors including advanced ESCC. PI3Kα inhibitors including CYH33 and the marketed alpelisib displayed highly variable activity against the proliferation of a panel of ESCC cells, reflecting the heterogeneity of ESCC. We sought to discover predictive biomarkers for the efficacy of PI3Kα inhibitors in ESCC by evaluating its activity to against the growth of a panel of 16 clinically relevant patient-derived xenografts (PDXs). We found that PDXs with CCND1 amplification were more sensitive to CYH33, which was consistent with the positive correlation between CYH33 activity and CyclinD1 expression in PDXs detected by tissue microarray. As Cyclin D1 plays a critical role in G1/S transition in cell cycle progression, proteins regulating G1 phase were profiled in ESCC cells. Elevated expression of Cyclin D1, p21 and Rb was found in CYH33-sensitive cells compared to those in resistant cells, indicating intact G1/S regulation in sensitive cells. Accordingly, CYH33 significantly arrested sensitive cells at G1 phase, while it had little effect in resistant cells. The cellular level of p21 as well as p21 bound to the CDK4/6-Cyclin D1 or CDK2-Cyclin E complex increased upon CYH33 treatment in sensitive cells, which was accompanied with decreased phosphorylation of Rb. Though CYH33 had little effect on the mRNA level of p21, its treatment prolonged the half-life of p21 protein in sensitive cells. CYH33 was found to impede the interaction between the E3 ubiquitin ligase SKP2 and p21 in sensitive cells, resulting in decreased ubiquitination of p21. However, CYH33 failed to stabilize and accumulate p21 in resistant cells. In conclusion, we found that ESCC cells with intact G1 phase regulation were more sensitive to CYH33. CYH33 arrested sensitive ESCC cells at G1 phase via inhibiting the ubiquitination and degradation of p21 by SKP2, which further resulted in decreased activity of CDK4/6 and CDK2 as well as Rb phosphorylation. These findings revealed the insight mechanism of the differential activity CYH33 against ESCC and provided potential candidate biomarkers for the treatment of ESCC. Citation Format: Yuxiang Wang, Xu Zhang, Xi Zhang, Yanyan Shen, Qingyang Ma, Yuemei Qiao, Yi Wang, Biyu Yang, Lan Xu, Gaoxiang Ge, Landian Hu, Xiangyin Kong, Chunhao Yang, Jian Ding, Yi Chen, Linghua Meng. Intact regulation of G1/S transition renders esophageal squamous cell carcinomas sensitive to PI3Kα inhibitors by blocking SKP2-mediated p21 degradation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5373.

CMTM6 inhibits tumor growth and reverses chemoresistance by preventing ubiquitination of p21 in hepatocellular carcinoma

Hepatocellular carcinoma is one of the most common malignancies and has a poor prognosis. The ubiquitin-proteasome pathway is required for the degradation of most short-lived proteins. CMTM6 has been implicated in the progression of various tumors, but its biological function and the underlying molecular mechanisms in HCC are still unknown. In this study, we found that the expression of CMTM6 was significantly reduced in HCC and predicted better prognosis of HCC patients. Through in vitro and in vivo experiments, CMTM6 was shown to inhibit the proliferation of HCC cells by blocking the G1/S phase transition. Mechanistically, CMTM6 interacted with p21 and prevented its ubiquitination mediated by SCFSKP2, CRL4CDT2 and APC/CCDC20 in a cell-cycle–independent manner. As a result, CMTM6 stabilized p21 protein, leading to the inactivation of pRB/E2F pathway. Additionally, CMTM6 sensitized HCC cells to doxorubicin and cisplatin, positively correlated with better clinical outcomes of the transarterial chemoembolization (TACE) treatment for postoperative recurrence. Taken together, our study reports a novel mechanism by which p21 can be stabilized by CMTM6 and pinpoints a crucial role of the CMTM6-p21 axis in suppressing the progression of HCC and sensitizing patients with postoperative recurrence to TACE treatment.

HERC3 directly targets RPL23A for ubiquitination degradation and further regulates Colorectal Cancer proliferation and the cell cycle.

Aims: Colorectal cancer (CRC) has high mortality and morbidity rates; however, the mechanism of CRC cells uncontrolled proliferation is unclarified, E3 ligases are widely reported to have crucial functions in cancers. HERC3 was once recognized as an important role in CRC, however its effects on CRC cell proliferation and cell cycle are blank.Methods: Correlation between HERC3 and clinical characteristics was analyzed. Coimmunoprecipitation, mass spectrometry analysis and GST-pull down were performed to identify interacting-proteins of HERC3. Expression pattern of RPL23A and its correlation between HERC3 was researched via qRT-PCR, western blot and immunohistochemistry. In vivo and vitro gain-and loss-of-function assays and rescue experiments concentrating HERC3-RPL23A axis in terms of cell proliferation and cell cycle were conducted. The ubiquitination regulatory mechanism between HERC3 and RPL23A were identified via vivo ubiquitylation assays, cycloheximide analysis and mass spectrometry analysis. GSEA aided to research the potential functional mechanism of RPL23A and validated by western blot and in vivo ubiquitylation assays.Results: HERC3 expression decreased gradually from colorectal tissues in healthy individuals to adjacent-tumors normal tissue in CRC patients, and to tumor tissues and HERC3 could inhibit CRC cell proliferation and arrest cells in the G0-G1 phase. RPL23A which was recognized as one potential target of HERC3 was identified to be overexpressed in CRC and could serve as a prognostic biomarker in CRC. RPL23A could also independently regulate the cell cycle and cell proliferation and attenuate the influence of HERC3 on CRC. In addition, HERC3 directly interacted with RPL23A and served as an E3 ligase to ubiquitination degrade RPL23A via K48-dependant manner through the HECT domain. Furthermore, HERC3 could regulate the ubiquitination of p21 and further modulate protein expression of c-Myc and p21 via regulating RPL23A.Conclusion: HERC3 controlled CRC proliferation, the cell cycle and regulated the c-Myc/p21 axis via directly targeting RPL23A for ubiquitination degradation.

P057 CRL4DCAF2 promotes cell proliferation and limits the development of colitis

Abstract Background Ulcerative colitis (UC) is an idiopathic intestinal inflammatory disease, which leads to chronic intestinal mucosal barrier damage. More and more evidences show that ubiquitination of proteins regulates the occurrence and development of intestinal inflammation. DCAF family proteins could form E3 ubiquitin ligase with CRL4-DDB1 to regulate cell growth, differentiation, apoptosis and other life activities. CRL4DCAF2 is a crucial regulator in cell cycle regulation, but there are few studies on its application in intestinal epithelium. This study aims to explore the specific mechanism of CRL4DCAF2 in regulating the proliferation and repairment of intestinal epithelial cells. Methods DSS - induced colitis in mice was used as the experimental model in vivo. HCT116 and SW480 cell lines were used as experimental models in vitro studies.The Cre-loxP system was used to construct a mouse model of intestinal epithelium-specific DCAF2 knockout. The intestinal mucosa biopsy specimens of 11 normal patients and 11 UC patients were collected. In addition, qRT-PCR, Western blot, RNA-seq and immunofluorescent staining were used to detect the expression levels of target genes in human colon biopsy specimens, mouse colon tissues, HCT116 or SW480 cells Results DCAF2 gene was highly expressed in the colon of mice. The occurrence and development of DSS-induced experimental colitis was accompanied by a significant down-regulation of DCAF2 protein expression in colon. DCAF2 mRNA level was significantly decreased in UC patients. Mouse with intestinal epithelial-specific knockout of DCAF2(i.e. DCAF2IEC-KO) suffered from embryonic death. Compared with wild-type adult C57BL/6J mice, DCAF2IEC-KD mouse showed more severe intestinal inflammation in DSS-induced colitis model. CCK-8 test, PI staining and EDU staining flow cytometry experiments showed that the proliferation of intestinal epithelial cells with DCAF2 overexpression was faster than that of the control (P < 0.05) in HCT116 and SW480 cell lines, while in knockdown of DCAF2 models, the opposite results were obtained. Its effect may be related to the ubiquitination of p21. At the same time, MLN4924 in vivo and in vitro experiments further verified our experimental results. Combined with RNA-seq and Western blot, we also found that DCAF2 may reduce the symptoms of colitis by maintaining the stability of autophagy. Conclusion DCAF2 is low expressed in patients with ulcerative colitis, which may promote the activation and proliferation of intestinal epithelial cells. It could maintain autophagy stability, and restore intestinal barrier, thus alleviate the development of ulcerative colitis

Suppression of TCAB1 expression induced cellular senescence by lessening proteasomal degradation of p21 in cancer cells

BackgroundTCAB1, a.k.a. WRAP53β or WDR79, is an important molecule for the maintenance of Cajal bodies and critically involved in telomere elongation and DNA repair. Upregulation of TCAB1 were discovered in a variety types of cancers. However, the function of TCAB1 in tumor cell senescence remains absent.MethodsThe TCAB1 knockdown cell lines were constructed. The expression levels of TCAB1, p21, p16 and p53 were detected by qRT-PCR and western blotting. Staining of senescence-associated β-galactosidase was used to detect senescent cells. The ubiquitination of the p21 was analysed by immunoprecipitation and in vivo ubiquitination assay. TCGA databases were employed to perform in silico analyses for the mRNA expression of TCAB1, p21, p16 and p53.ResultsHere, we discovered that knockdown of TCAB1 induced rapid progression of cellular senescence in A549, H1299 and HeLa cells. In exploiting the mechanism underlining the role of TCAB1 on senescence, we found a significant increase of p21 at the protein levels upon TCAB1 depletion, whereas the p21 mRNA expression was not altered. We verified that TCAB1 knockdown was able to shunt p21 from proteasomal degradation by regulating the ubiquitination of p21. In rescue assays, it was demonstrated that decreasing the expression of p21 or increasing the expression of TCAB1 were able to attenuate the cellular senescence process induced by TCAB1 silencing.ConclusionsThis study revealed the importance of TCAB1 for its biological functions in the regulation of cell senescence. Our results will be helpful to understand the mechanisms of senescence in cancer cells, which could provide clues for designing novel strategies for developing effective treatment regimens.

TRIM27 Functions as a Novel Oncogene in Non-Triple-Negative Breast Cancer by Blocking Cellular Senescence through p21 Ubiquitination

In the current study, we aimed to explore the correlation between TRIM27 and breast cancer prognosis, as well as the functions of TRIM27 in breast cancer and their underlying mechanisms. Bioinformatics analyses were used to examine the correlation between TRIM27 and breast cancer prognosis. Moreover, TRIM27 knockdown and overexpression in breast cancer cells were performed to investigate its functions in breast cancer. Tamoxifen (TAM) was applied to evaluate the influence of TRIM27 on chemoresistance of breast cancer cells, while co-immunoprecipitation (coIP) was performed to identify the E3 ubiquitin ligase capability of TRIM27. High expression of TRIM27 was found in non-triple-negative breast cancer (non-TNBC) tumor tissues and was positively correlated with the mortality of non-TNBC patients. Moreover, TRIM27 could suppress non-TNBC cell apoptosis and senescence, promote cell viability and tumor growth, counteract the anti-cancer effects of TAM, and mediate ubiquitination of p21. In addition, EP300 could enhance the expression of TRIM27 and its transcription promoter H3K27ac. TRIM27, through ubiquitination of p21, might serve as a prognostic biomarker for non-TNBC prognosis. TRIM27 functions as a novel oncogene in non-TNBC cellular processes, especially suppressing cell senescence and interfering with non-TNBC chemoresistance.

Melanoma antigen A12 regulates cell cycle via tumor suppressor p21 expression.

Melanoma-associated antigen family A (MAGE-A) is a family of cancer/testis antigens that are expressed in malignant tumors but not in normal tissues other than the testes. MAGE-A12 is a MAGE-A family gene whose tumorigenic function in cancer cells remains unclear. Searches of the Oncomine and NextBio databases revealed that malignant tumors show up-regulation of MAGE-A12 mRNA relative to corresponding normal tissue. In PPC1 primary prostatic carcinoma cells and in HCT116 colorectal cancer cells (wild type and p53-depleted), MAGE-A12 gene knockdown using siRNA or shRNA diminishes cancer cell proliferation as assessed by cellular ATP levels, cell counting, and clonogenic assays. FACS analyses of annexin V-PI staining and DNA content show that MAGE-A12 knockdown causes G2/M arrest and apoptosis. In tumor xenografts of HCT116 cells, conditional knockdown of MAGE-A12 suppresses tumor growth. The depletion of MAGE-A12 leads to the accumulation of tumor suppressor p21 in PPC1, HCT116, and p53-depleted HCT116 cells. Conversely, CDKN1A knockdown partially rescues the viability of PPC1 cells transfected with siRNA targeting MAGE-A12, while p21 overexpression leads to proliferation arrest in PPC-1 cells. Furthermore, exogenous MAGE-A12 expression promotes the ubiquitination of p21. Our findings reveal that MAGE-A12 plays crucial roles in p21 stability and tumor growth, suggesting that MAGE-A12 could provide a novel target for cancer treatment.

Abstract 42: The NF-κB/MDM2 pathway is engaged in activin/ PI3 kinase induced degradation of p21 in colon cancer cells

Abstract Introduction: The signaling pathways for activin and TGFβ are frequently disrupted in colorectal cancers (CRC) and these mutations are associated with metastatic disease. Understanding the cellular consequences wrought by mutations to these signaling pathways may provide opportunities for the development of new therapeutic approaches. Our lab has previously shown opposing effects on a downstream target of TGFβ and activin, namely the cyclin-dependent kinase (CDK) inhibitor p21. TGFβ induces SMAD4-dependent upregulation of p21, while activin downregulates p21 in a SMAD4-independent mechanism via ubiquitination and degradation allowing for enhanced cell migration. However, the mode of regulation leading to p21 degradation is not completely known. We hypothesize that regulation of the E3 ubiquitin ligase MDM2 by NF-κB is involved in p21 degradation. Here we explore the role of MDM2 in the regulation of p21 in response to activin and TGFβ treatment in FET CRC cells. Methods: Colon cancer cell lines were assessed for ligand-induced PI3K/NF-κB/MDM2/p21 pathway engagement. FET cells were transfected with increasing concentrations of either wild type p65 or a dominant-negative mutant of p65 and treated with activin or TGFβ prior to immunoblot analysis for IκB-alpha, MDM2 and p21. DNA-binding activity of NF-κB was assessed by electrophoretic mobility shift assay (EMSA). The requirement for NF-κB activation was determined through use of the selective inhibitor peptide NBD (NEMO-binding domain), and PI3 kinase activity was confirmed by inhibition with LY294002. Results: Because activin induces ubiquitination of p21, we investigated the potential involvement of MDM2 and further the regulation of MDM2 by NF-κB. NF-κB has been reported to regulate MDM2 through direct binding to an NF-κB binding site in the promoter of MDM2. Activin induced activation of NF-κB as measured by increased DNA binding activity to an NF-κB consensus-binding site as well as by a reduction in IκB-alpha protein. Furthermore, we found this regulation to be dependent on activation of PI3 kinase. Overexpression of wild type p65 plasmid but not dominant negative p65 was found to increase MDM2 expression while the NF-κB specific inhibitor NBD blocked the activin-induced increase in MDM2. By contrast, TGFβ treatment did not contribute to an increase in MDM2 levels via NF-κB pathway. Conclusions: Here, we provide evidence to indicate that in colon cancer cells, activin may be working through NF-κB to promote MDM2 activity leading to the degradation of p21 in a PI3 kinase dependent mechanism. Future experiments will explore if pharmaceutical intervention in this pathway can inhibit the metastatic activity of activin as a therapeutic approach for advanced colon cancer. Citation Format: Arundhati Jana, Seung Hyua Baik, Timothy Carroll, Ozkan Ozden, Nancy L. Krett, Barbara Jung. The NF-κB/MDM2 pathway is engaged in activin/ PI3 kinase induced degradation of p21 in colon cancer cells. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 42. doi:10.1158/1538-7445.AM2015-42

The ability of TRIM3 to induce growth arrest depends on RING-dependent E3 ligase activity

Mutation of the TRIM (tripartite motif)-NHL family members brat and mei-P26 perturb the differentiation of transit-amplifying progenitor cells resulting in tumour-like phenotypes. The NHL (named after the NCL1, HT2A and LIN41 repeat) domain is essential for their growth suppressive activity, and they can induce cell-cycle exit in a RING-independent manner. TRIM3 is the only bona fide tumour suppressor in the mammalian TRIM-NHL subfamily and similar to the other members of this family, its ability to inhibit cell proliferation depends on the NHL domain. However, whether the RING domain was required for TRIM3-dependent cell-cycle exit had not been investigated. In the present study, we establish that the RING domain is required for TRIM3-induced growth suppression. Furthermore, we show that this domain is necessary to promote ubiquitination of p21in a reconstituted invitro system where UbcH5a is the preferred E2. Thus the ability of TRIM3 to suppress growth is associated with its ability to ubiquitinate proteins.

Nucleophosmin/B23 interacts with p21WAF1/CIP1 and contributes to its stability

The cyclin-dependent kinase inhibitor p21WAF1/CIP1 is a critical regulator of cell cycle, and it is easily degraded by proteasome through ubiquitin-dependent and –independent pathway. The mechanism of the post-translational regulation of p21 stability remains to be further clarified. In the present study, we have identified nucleophosmin (NPM)/B23, a multifunctional protein that bound p21 and contributed to its stability. The direct interac-tion between p21 and NPM was confirmed by reciprocal co-immunoprecipitation and GST pull-down assay. Confocal microscopy showed that NPM partially co-localized with p21 in nucleoplasm and their co-localization increased treated with Act D which induces the nu-cleoplasmic translocation of NPM. We observed the half life of p21 was prolonged with over-expression of NPM or Act D treatment. Knockdown of NPM by siRNA resulted in down-regulation of p21 and impaired up-regulation of p21 treated with Act D. Further, we examined the effect of NPM expression on the ubiquitination of p21. Over-expression of NPM inhibited the ubiquitination of p21, and depletion of NPM remarkably improved the ubiquitination of p21. Altogether, we provide evidence for a direct binding between NPM and p21, and assign NPM as a positive post-translational regulator of p21.

C-Jun NH2-terminal kinase decreases ubiquitination and promotes stabilization of p21 WAF1/CIP1 in K562 cell

Proteasome-dependent degradation of regulatory proteins is a known mechanism of cell cycle control. p21 WAF1/CIP1 (p21), a negative regulator of the cell division cycle, exhibits proteasome-sensitive turnover and ubiquitination. In the present study, we analyzed the regulatory effects of JNK1 on p21 protein accumulation in p53 null K562 cells. We found that JNK1 (wild type, WT) mediated H 2O 2-induced p21 protein up-regulation. Over-expression of JNK1 (WT) could elevate endogenous p21 protein level but did not affect p21 mRNA level and also prolong the p21 half-life as well as inhibited the p21 ubiquitination. These findings indicated that JNK1 could regulate cellular p21 level via inhibiting ubiquitination of p21, which provided a new insight for analyzing the regulatory effect of JNK after stress.

UV Induces p21 Rapid Turnover Independently of Ubiquitin and Skp2

It was previously reported that low doses, but not high doses, of UV trigger the Skp2-mediated proteasomal degradation of the cyclin-dependent kinase inhibitor p21 in mammalian cells. Here we show that both UV-C and UV-B lead to decrease of p21 protein, but not mRNA, level in a dose-dependent fashion in all of six human cell lines and five mouse cell lines tested. Also, high doses of UV reduce the half-life of p21. High doses, but not low doses, of UV induced p21 degradation in both skp2-proficient and -deficient murine embryonic fibroblast cells. UV-induced p21 reduction was rescued by proteasome inhibitors in all human and mouse cell lines tested. Neither a caspase inhibitor nor small interfering RNA against skp2 had an effect on the UV-induced p21 decrease, suggesting that this p21 degradation pathway may not involve caspases, or Skp2. Finally, UV did not induce p21 ubiquitination but still induced its degradation when the E1-activating enzyme was inactivated in an E1 temperature-sensitive mouse embryonic fibroblast cell line. Altogether, these results demonstrate that UV induces p21 degradation through an Skp2 and ubiquitin-independent pathway.

Ubiquitination of p21Cip1/WAF1 by SCFSkp2: Substrate Requirement and Ubiquitination Site Selection

Multiple proteolytic pathways are involved in the degradation of the cyclin-dependent kinase inhibitor p21(Cip1/WAF1). Timed destruction of p21(Cip1/WAF1) plays a critical role in cell-cycle progression and cellular response to DNA damage. The SCF(Skp2) complex (consisting of Rbx1, Cul1, Skp1, and Skp2) is one of the E3 ubiquitin ligases involved in ubiquitination of p21(Cip1/WAF1). Little is known about how SCF(Skp2) recruits its substrates and selects particular acceptor lysine residues for ubiquitination. In this study, we investigated the requirements for SCF(Skp2) recognition of p21(Cip1/WAF1) and lysine residues that are ubiquitinated in vitro and inside cells. We demonstrate that ubiquitination of p21(Cip1/WAF1) requires a functional interaction between p21(Cip1/WAF1) and the cyclin E-Cdk2 complex. Mutation of both the cyclin E recruitment motif (RXL) and the Cdk2-binding motif (FNF) at the N terminus of p21(Cip1/WAF1) abolishes its ubiquitination by SCF(Skp2), while mutation of either motif alone has minimal effects, suggesting either contact is sufficient for substrate recruitment. Thus, SCF(Skp2) appears to recognize a trimeric complex consisting of cyclin E-Cdk2-p21(Cip1/WAF1). Furthermore, we show that p21(Cip1/WAF1) can be ubiquitinated at four distinct lysine residues located in the carboxyl-terminal region but not two other lysine residues in the N-terminal region. Any one of these four lysine residues can be targeted for ubiquitination in the absence of the others in vitro, and three of these four lysine residues are also ubiquitinated in vivo, suggesting that there is limited specificity in the selection of ubiquitination sites. Interestingly, mutation of the carboxyl-terminal proline to lysine enables ubiquitin conjugation at the carboxyl terminus of the substrate both in vitro and in vivo. Thus, our results highlight a unique property of the ubiquitination enzymatic reaction in that substrate ubiquitination site selection can be remarkably diverse and occur in distinct spatial areas.

Cdk2-dependent Inhibition of p21 Stability via a C-terminal Cyclin-binding Motif

p21 is a member of the Cip/Kip family of cyclin-dependent kinase (CDK) inhibitors that includes p21, p27, and p57. Recent studies have suggested that Cdk2 activity may promote p21 degradation through a pathway similar to that for p27, although the mechanism by which this occurs has not been clarified. In the current report, co-expression with cyclin E and Cdk2 stabilized p21 in a manner that required the CDK-binding site of p21 and a cyclin-binding site (cy1) located in the p21 N terminus. Strikingly, however, a kinase-dead Cdk2 mutant stabilized p21 to a greater extent than did wild-type Cdk2, consistent with the notion that Cdk2 activity can destabilize p21. The ability of wild-type Cdk2 to destabilize p21 required a potential Cdk2 phosphorylation site in p21 at serine 130 and an intact cyclin-binding motif (cy2) in the p21 C terminus. Finally, p21 was phosphorylated by Cdk2 at Ser-130 in vitro, and this ability of Cdk2 to phosphorylate p21 was dependent, in large part, on the presence of cy2. These results support a model in which active Cdk2 destabilizes p21 via the cy2 cyclin-binding motif and p21 phosphorylation.

Hsp25 regulates the expression of p21(Waf1/Cip1/Sdi1) through multiple mechanisms.

Exposure of cells to external stresses leads to the induction or activation of certain proteins. Expression of heat shock proteins (Hsps) is induced in response to these stresses. Hsps are known to have molecular chaperone activities; but recent studies have shown that Hsps have a variety of functions such as the triggering of proliferation, differentiation, and apoptosis of cells. Previously, we found that overexpression of a 25 kDa Hsp (Hsp25) induced expression of cell cycle inhibitory protein p21 (Waf1/Cip1/Sdi1) in murine fibroblastoid L929 cells. However, the mechanisms underlying the induction of p21 by Hsp25 are unknown. In the present study, we investigated the mechanisms underlying the regulation of p21 expression by Hsp25 in these cells. The introduction of Hsp25 cDNA stimulated the accumulation of p21 transcripts through transcriptional but not posttranscriptional regulation in these cells. We also found that overexpression of Hsp25 markedly increased the translational rate of p21 and stabilized the protein. Studies involving proteasome inhibitors and Western blot analysis for ubiquitination of p21 demonstrated that the stabilization of p21 is regulated through a ubiquitin-independent pathway. However, no direct association of Hsp25 with p21 was observed. These findings suggest that Hsp25 induces p21 expression through multiple mechanisms, and that transcriptional, translational, and post-translational regulation are important in the regulation of p21.

Identification of the regulatory region required for ubiquitination of the cyclin kinase inhibitor, p21

The expression of cyclin kinase inhibitor p21 is regulated by the ubiquitin-proteasome protein degradation system, as well as by transcriptional regulation. Generally, ubiquitination is regulated by the phosphorylation of the substrate. In this study, we identified the region of p21 responsible for the regulation of ubiquitination. Since the phosphorylation sites of p21 are distributed in the C-terminal region, we constructed sequential C-terminal truncated fragments and examined their ubiquitination in eukaryotic cells. The ubiquitination was observed in the 1–164 (full length) and 1–157 fragments with the same efficiency, but not in the 1–147 fragment. The lack of ubiquitination in the 1–147 fragment was unlikely due to the removal of a Lys residue at position 154, since the p21 K154R mutant was ubiquitinated as efficiently as the full-length p21. Furthermore, the 148–157 deleted form of p21 was not ubiquitinated, just like the 1–147 fragment. Thus, the C-terminal 148–157 region, not a ubiquitination site by itself, should contain an essential regulatory region for the efficient ubiquitination of p21.