p45SKP2 promotes p27Kip1 degradation and induces S phase in quiescent cells

Mechanoregulation of proliferation

The proliferation of all nontransformed adherent cells is dependent upon the development of mechanical tension within the cell; however, little is known about the mechanisms by which signals regulated by mechanical tension are integrated with those regulated by growth factors. We show here that Skp2, a component of a ubiquitin ligase complex that mediates the degradation of several proteins that inhibit proliferation, is upregulated when increased mechanical tension develops in intact smooth muscle and that its upregulation is critical for the smooth muscle proliferative response to increased mechanical tension. Notably, whereas growth factors regulate Skp2 at the level of protein stability, we found that mechanical tension regulates Skp2 at the transcriptional level. Importantly, we demonstrate that the calcium-regulated transcription factor NFATc1 is a critical mediator of the effect of increased mechanical tension on Skp2 transcription. These findings identify Skp2 as a node at which signals from mechanical tension and growth factors are integrated to regulate proliferation, and they define calcium-NFAT-Skp2 signaling as a critical pathway in the mechanoregulation of proliferation.

UBE3A/E6-AP regulates cell proliferation by promoting proteasomal degradation of p27

The UBE3A/E6-AP is known to function both as an E3 ubiquitin ligase of the ubiquitin proteasome system and as a transcriptional coactivator. E6-AP shows brain-specific imprinting and loss of function of maternally inherited E6-AP causes Angelman syndrome. However, how the loss of function of E6-AP causes disease pathogenesis is poorly understood. Here, we show that E6-AP interacts with and promotes proteasome-mediated degradation of cyclin-dependent kinase inhibitor p27. E6-AP also directly ubiquitinates p27 in an in vitro ubiquitination assay. Partial knockdown of E6-AP increases the level of p27 leading to cell cycle arrest. Interestingly, partial knockdown also increases the transcription of p27. Finally, we have demonstrated the increased levels of p27 in E6-AP-maternal-deficient and null mice brain. Our result suggests that E6-AP not only enhances the degradation but also regulates the expression of p27 and its loss of function in Angelman syndrome might cause cell cycle alteration leading to disease pathogenesis.

Phosphorylation of p27(KIP1) in the Mitotic Cells of the Corneal Epithelium

PURPOSE

The mechanism in regulation of the cell cycle and proliferation of corneal epithelium in the homeostatic ocular surface remains unclear. The aim of this study is to examine the expression of p27(KIP1) and its phosphorylation in corneal epithelium.

METHODS

The eyes of C57BL/6 mice (7 weeks old) were enucleated. Formalin-fixed and paraffin-embedded tissue sections were examined using immunohistochemistry with anti-p27(KIP1), threonine 187 phosphorylated p27(KIP1) (T187-phospho-p27), and phosphorylated Histon H3 (pHiston H3) antibodies. Anti-T187-phospho-p27 and anti-pHiston H3 polyclonal antibodies were used for parallel immunofluorescent staining.

RESULTS

pHiston H3-immunopositive cells were noted in basal cells of the corneal epithelium. At high magnification of DAPI nuclear staining, mitotic and non-mitotic cells were observed in corneal basal layer. p27(KIP1)-positive nuclei were detected in corneal basal cells, where non-mitotic basal cells were located. In contrast, mitotic cells showed under detectable level on p27(KIP1) immunoreactivity. Immunoreactivity for T187-phospho-p27 was detected in basal cells of the corneal epithelium. At high magnification, it was confirmed that the immunopositive cells were mitotic cells. Immunoreactivity of T187-phospho-p27 as well as pHiston H3 was localized in the same corneal basal cells using double-staining immunohistochemistry.

CONCLUSIONS

These results suggested that degradation of p27(KIP1) regulates progression into mitosis in corneal basal cells.

Akt finds its new path to regulate cell cycle through modulating Skp2 activity and its destruction by APC/Cdh1

Skp2 over-expression has been observed in many human cancers. However, the mechanisms underlying elevated Skp2 expression have remained elusive. We recently reported that Akt1, but not Akt2, directly controls Skp2 stability by interfering with its association with APC/Cdh1. As a result, Skp2 degradation is protected in cancer cells with elevated Akt activity. This finding expands our knowledge of how specific kinase cascades influence proteolysis governed by APC/Cdh1 complexes. However, it awaits further investigation to elucidate whether the PI3K/Akt circuit affects other APC/Cdh1 substrates. Our results further strengthen the argument that different Akt isoforms might have distinct, even opposing functions in the regulation of cell growth or migration. In addition, we noticed that Ser72 is localized in a putative Nuclear Localization Sequence (NLS), and that phosphorylation of Ser72 disrupts the NLS and thus promotes Skp2 cytoplasmic translocation. This finding links elevated Akt activity with the observed cytoplasmic Skp2 staining in aggressive breast and prostate cancer patients. Furthermore, it provides the rationale for the development of specific Akt1 inhibitors as efficient anti-cancer therapeutic agents.

Phosphorylation of p27Kip1 by Epstein-Barr virus protein kinase induces its degradation through SCFSkp2 ubiquitin ligase actions during viral lytic replication

Epstein-Barr virus (EBV) productive replication occurs in an S-phase-like cellular environment with high cyclin-dependent kinase (CDK) activity. The EBV protein kinase (PK), encoded by the viral BGLF4 gene, is a Ser/Thr protein kinase, which phosphorylates both viral and cellular proteins, modifying the cellular environment for efficient viral productive replication. We here provide evidence that the EBV PK phosphorylates the CDK inhibitor p27(Kip1), resulting in ubiquitination and degradation in a proteasome-dependent manner during EBV productive replication. Experiments with BGLF4 knockdown by small interfering RNA and BGLF4 knock-out viruses clarified that EBV PK is involved in p27(Kip1) degradation upon lytic replication. Transfection of the BGLF4 expression vector revealed that EBV PK alone could phosphorylate the Thr-187 residue of p27(Kip1) and that the ubiquitination and degradation of p27(Kip1) occurred in an SCF(Skp2) ubiquitin ligase-dependent manner. In vitro, EBV PK proved capable of phosphorylating p27(Kip1) at Thr-187. Unlike cyclin E-CDK2 activity, the EBV PK activity was not inhibited by p27(Kip1). Overall, EBV PK enhances p27(Kip1) degradation effectively upon EBV productive replication, contributing to establishment of an S-phase-like cellular environment with high CDK activity.

The impact of Skp2 overexpression on recurrence-free survival following radical prostatectomy

BACKGROUND

In several human cancers, overexpression of Skp2 (S-phase kinase associated protein 2), which targets p27 for degradation, portends a poorer prognosis. We examined whether Skp2 overexpression is associated with recurrence following radical prostatectomy (RP) for prostate cancer.

METHODS

Immunohistochemical staining for Skp2, p27, and MIB-1 was performed on 109 men with node-negative prostate cancer surgically managed from 1985-1996. Associations between the stains were tested and Cox regression was used to determine the association between Skp2 expression and time to biochemical recurrence following RP.

RESULTS

The 12 tumors (11%) with Skp2 overexpression all had correspondingly low p27 expression (P=0.006), and a similar inverse Skp2/p27 relationship was seen in vitro in LNCap cells. Skp2 overexpression in tissue was associated with higher Gleason score (P=0.002), more advanced pathological stage (P=0.01), and higher MIB-1 index (P=0.03), but a more favorable PSA profile (P=0.04). Five men received a TURP. Among 104 who received RP, median follow-up was 67 months (range: 0.2-218). After adjusting for PSA, pathologic stage, and Gleason score, Skp2 overexpression remained significantly associated with a shorter time to biochemical recurrence (adjusted hazard ratio 4.8 (95% C.I. 1.6-14, P=0.004)). The median time to recurrence with high vs. low Skp2 was 4 vs. 54 months.

CONCLUSIONS

Skp2 overexpression was seen in a significant minority of surgically-managed men and was independently associated with a higher risk of recurrence, raising the possibility that Skp2 could be useful as a prognostic biomarker and as a potential molecular target for novel systemic agents in prostate cancer.

Thrombin induces tumor cell cycle activation and spontaneous growth by down-regulation of p27Kip1, in association with the up-regulation of Skp2 and MiR-222

The effect of thrombin on tumor cell cycle activation and spontaneous growth was examined in synchronized serum-starved tumor cell lines and a model of spontaneous prostate cancer development in TRAMP mice. BrdUrd incorporation and propidium iodide staining of prostate LNCaP cells arrested in G(0) and treated with thrombin or serum revealed a 48- and 29-fold increase in S phase cells, respectively, at 8 hours. Similar results were obtained with TRAMP cells and a glioblastoma cell line, T98G. Cell cycle kinases and inhibitors in synchronized tumor cells revealed high levels of p27(Kip1) and low levels of Skp2 and cyclins D1 and A. Addition of thrombin, TFLLRN, or serum down-regulated p27(Kip1) with concomitant induction of Skp2, Cyclin D1, and Cyclin A with similar kinetics. LNCaP p27(Kip1)-transfected cells or Skp2 knockdown cells were refractory to thrombin-induced cell cycle activation. MicroRNA 222, an inhibitor of p27(Kip1), was robustly up-regulated by thrombin. The in vitro observations were tested in vivo with transgenic TRAMP mice. Repetitive thrombin injection enhanced prostate tumor volume 6- to 8-fold (P < 0.04). Repetitive hirudin, a specific potent antithrombin, decreased tumor volume 13- to 24-fold (P < 0.04). Thus, thrombin stimulates tumor cell growth in vivo by down-regulation of p27(Kip1).

Transcriptional activation of Skp2 by BCR-ABL in K562 chronic myeloid leukemia cells

We addressed how BCR-ABL oncoprotein increased Skp2 expression. Treatment of Imatinib or LY294002 reduced Skp2 mRNA in BCR-ABL-positive K562 cells. Knockdown of AKT by small hairpin RNA also reduced Skp2 expression. We found that BCR-ABL up-regulated Skp2 via Sp1 because (1) the Sp1 site located at the -386/-380 promoter region was important for BCR-ABL-induced Skp2 promoter activity, (2) chromatin immunoprecipitation assay demonstrated that Imatinib inhibited the recruitment of p300 to the Sp1 site of Skp2 promoter and (3) knockdown of Sp1 reduced Skp2 expression in K562 cells. These results suggest that BCR-ABL controls Skp2 gene transcription via the PI3K/AKT/Sp1 pathway.

RBX1/ROC1 disruption results in early embryonic lethality due to proliferation failure, partially rescued by simultaneous loss of p27

RBX1 (RING box protein-1) or ROC1 (regulator of cullins-1) is the RING component of SCF (Skp1, Cullins, F-box proteins) E3 ubiquitin ligases, which regulate diverse cellular processes by targeting various substrates for degradation. However, the in vivo physiological function of RBX1 remains uncharacterized. Here, we show that a gene trap disruption of mouse Rbx1 causes embryonic lethality at embryonic day (E)7.5, mainly due to a failure in proliferation; p27, a cyclin dependent kinase inhibitor, normally undetectable in the early embryos, accumulates at high levels in the absence of Rbx1. Although mice heterozygous for the Rbx1 gene trap appear viable and fertile without obvious abnormalities, the Rbx1(+/Gt) MEFs do show retarded growth with G1 arrest and p27 accumulation. Simultaneous loss of p27 extended the life span of Rbx1(Gt/Gt) embryos from E6.5 to E9.5, indicating that p27-mediated cell cycle inhibition contributes to the early embryonic lethality in the Rbx1-deficient embryos. Our study demonstrates that the in vivo physiological function of RBX1 is to ensure cell proliferation by preventing p27 accumulation during the early stage of embryonic development.

Overexpression of UbcH10 alternates the cell cycle profile and accelerate the tumor proliferation in colon cancer

Background

UbcH10 participates in proper metaphase to anaphase transition, and abrogation of UbcH10 results in the premature separation of sister chromatids. To assess the potential role of UbcH10 in colon cancer progression, we analyzed the clinicopathological relevance of UbcH10 in colon cancer.

Methods

We firstly screened the expression profile of UbcH10 in various types of cancer tissues as well as cell lines. Thereafter, using the colon cancer cells line, we manipulated the expression of UbcH10 and evaluated the cell cycle profile and cellular proliferations. Furthermore, the clinicopathological significance of UbcH10 was immunohistologically evaluated in patients with colon cancer. Statistical analysis was performed using the student's t-test and Chi-square test.

Results

Using the colon cancer cells, depletion of UbcH10 resulted in suppression of cellular growth whereas overexpression of UbcH10 promoted the cellular growth and oncogenic cellular growth. Mitotic population was markedly alternated by the manipulation of UbcH10 expression. Immunohistochemical analysis indicated that UbcH10 was significantly higher in colon cancer tissue compared with normal colon epithelia. Furthermore, the clinicopathological evaluation revealed that UbcH10 was associated with high-grade histological tumors.

Conclusion

The results show the clinicopathological significance of UbcH10 in the progression of colon cancer. Thus UbcH10 may act as a novel biomarker in patients with colon cancer.

Phosphorylation-dependent regulation of cytosolic localization and oncogenic function of Skp2 by Akt/PKB

Skp2 is an F-box protein that forms the SCF complex with Skp1 and Cullin-1 to constitute an E3 ligase for ubiquitylation. Ubiquitylation and degradation of the p27 are critical for Skp2-mediated entry to the cell cycle, and overexpression and cytosolic accumulation of Skp2 have been clearly associated with tumorigenesis, although the functional significance of the latter is still unknown. Here we show that Akt/protein kinase B (PKB) interacts with and directly phosphorylates Skp2. We find that Skp2 phosphorylation by Akt triggers SCF complex formation and E3 ligase activity. A phosphorylation-defective Skp2 mutant is drastically impaired in its ability to promote cell proliferation and tumorigenesis. Furthermore, we show that Akt-mediated phosphorylation triggers 14-3-3beta-dependent Skp2 relocalization to the cytosol, and we attribute a specific role to cytosolic Skp2 in the positive regulation of cell migration. Finally, we demonstrate that high levels of activation of Akt correlate with the cytosolic accumulation of Skp2 in human cancer specimens. Our results therefore define a novel proto-oncogenic Akt/PKB-dependent signalling pathway.

Array comparative genomic hybridization in retinoma and retinoblastoma tissues

In retinoblastoma, two RB1 mutations are necessary for tumor development. Recurrent genomic rearrangements may represent subsequent events required for retinoblastoma progression. Array-comparative genomic hybridization was carried out in 18 eye samples, 10 from bilateral and eight from unilateral retinoblastoma patients. Two unilateral cases also showed areas of retinoma. The most frequent imbalance in retinoblastomas was 6p gain (40%), followed by gains at 1q12-q25.3, 2p24.3-p24.2, 9q22.2, and 9q33.1 and losses at 11q24.3, 13q13.2-q22.3, and 16q12.1-q21. Bilateral cases showed a lower number of imbalances than unilateral cases (P = 0.002). Unilateral cases were divided into low-level (< or = 4) and high-level (> or = 7) chromosomal instability groups. The first group presented with younger age at diagnosis (mean 511 days) compared with the second group (mean 1606 days). In one retinoma case ophthalmoscopically diagnosed as a benign lesion no rearrangements were detected, whereas the adjacent retinoblastoma displayed seven aberrations. The other retinoma case identified by retrospective histopathological examination shared three rearrangements with the adjacent retinoblastoma. Two other gene-free rearrangements were retinoma specific. One rearrangement, dup5p, was retinoblastoma specific and included the SKP2 gene. Genomic profiling indicated that the first retinoma was a pretumoral lesion, whereas the other represents a subclone of cells bearing 'benign' rearrangements overwhelmed by another subclone presenting aberrations with higher 'oncogenic' potential. In summary, the present study shows that bilateral and unilateral retinoblastoma have different chromosomal instability that correlates with the age of tumor onset in unilateral cases. This is the first report of genomic profiling in retinoma tissue, shedding light on the different nature of lesions named 'retinoma'.

EB1089 induces Skp2-dependent p27 accumulation, leading to cell growth inhibition and cell cycle G1 phase arrest in human hepatoma cells

EB1089 exhibits a high level of antiproliferative activity against various tumors. However, it is not known whether the mechanism of EB1089 induced the growth inhibition in human hepatic-carcinoma. Here we found that EB1089 significantly reduced cell growth in human hepatoma cells (Hep-G2) and blocked Hep-G2 cell-associated tumor formation in nude mice. The growth inhibition was linked to cell cycle G1 phase arrest by the accumulation of p27 and a reduction of Skp2. Knockdown of Skp2 reversed the p27 induction and G1 arrest. Taken together, our data indicate that EB1089 inhibitory activity is associated with alteration of cell cycle checkpoints through Skp2-dependent p27 induction in Hep-G2 cells.

Clinicopathological relevance of UbcH10 in breast cancer

Abrogated mitotic progression is a common hallmark of cancer. UbcH10, one of the components of the ubiquitin/proteasome pathway, plays a pivotal role in the regulation of mitotic progression. Abnormal UbcH10 activity is reported in certain types of human cancers; its overexpression is occasionally encountered in cancerous tissue compared with adjacent normal tissue. Current studies have suggested the critical role of UbcH10 in the spindle assembly checkpoint and the subsequent accurate separation of sister chromatids, which is orchestrated by a series of molecular interactions governed by the complex and diverse cell cycle machinery. To validate the potential role of UbcH10 in cell proliferation in cancer, we have analyzed the clinicopathological relevance of UbcH10 in progression of breast cancer using a combinatorial approach of human tumor arrays and biochemical analyses. Our results show that the percentage of tested samples which stained positive for UbcH10 in breast cancer tissues is significantly higher compared to the adjacent nonmalignant tissue. Furthermore, results from the clinicopathological analysis have revealed that elevated expression of UbcH10 is associated with higher histological grade tumors. In addition, depletion of UbcH10 by RNA interference in breast cancer cells resulted in decreased cellular proliferation, while overexpression of UbcH10 significantly enhanced cellular growth in breast cancer. Our results suggest a pathological correlation between UbcH10 and cell proliferation in breast cancer. Thus, aberrant UbcH10 activity may induce the dysfunction of proper cell cycle progression and result in the aggressive behavior of tumor cells in patients with breast cancer.

FOXP3+ regulatory T-cells in chronic kidney disease: molecular pathways and clinical implications

CD4+/FOXP3+ regulatory T-cells (Tregs) are essential for the maintenance of self-tolerance and Tregs deficiency results in spontaneous autoimmunity in both mice and humans. The forkhead box P3 (FOXP3) expression is required for both survival of Tregs precursors as well as their function. This suggests that Tregs may use multiple mechanisms to limit autoimmunity and may reflect functional heterogeneity among Tregs subsets that localize to distinct tissue environments. Both cell contact- and cytokine-based immunosuppressive mechanisms would require that Tregs be in close proximity to their targets. The fundamental regulatory activity that can be consistently demonstrated by Tregs in vivo and in vitro has stimulated great interest in developing novel strategies for treating ongoing inflammatory conditions. Patients with end-stage kidney disease (ESKD) are known to display a cellular immune dysfunction. Uremic solutes that accumulate during ESKD may be involved in these processes. In these patients, oxidative stress induced by oxLDL may increase Tregs sensitivity to Fas-mediated apoptosis in part as a consequence of 26S proteasome activation. The 26S proteasome, an ATP-dependent multisubunit protease complex found in the cytoplasm and in the nucleus of all eukaryotic cells, constitutes the central proteolytic machinery of the ubiquitin/proteasome system. Considering the effect of uremia and oxLDL, Tregs from patients with ESKD exhibit early cell-cycle arrest and become apoptotic. These phenomena are the consequence of the oxLDL inhibited proteasome proteolytic activity of p27(Kipl) and Bax proteins in Tregs. This may be one mechanistic explanation of the cellular immune dysfunction in patients with ESKD and may have important implications in clinics, since this response could contribute to the micro-inflammation and atherogenesis encountered in this population.

Imatinib mesylate induces quiescence in gastrointestinal stromal tumor cells through the CDH1-SKP2-p27Kip1 signaling axis

Gastrointestinal stromal tumors (GIST) are caused by activating mutations in the KIT or platelet-derived growth factor receptor alpha receptor tyrosine kinase genes. Approximately 85% of GIST patients treated with imatinib mesylate achieve disease stabilization, however, often in the presence of residual tumor masses. Complete remissions are rare and a substantial proportion of patients develop resistance to imatinib. Our study was designed to determine whether imatinib-associated responses may account for these clinical findings. We report here that imatinib stimulates cellular quiescence in a proportion of GIST cells as evidenced by up-regulation of the CDK inhibitor p27(Kip1), loss of cyclin A, and reduced BrdUrd incorporation. Mechanistically, these events are associated with an imatinib-induced modulation of the APC/CDH1 signaling axis. Specifically, we provide evidence that imatinib down-regulates SKP2 and that this event is associated with increased nuclear CDH1, an activator of the APC that has been shown to regulate SKP2 stability. We also show that those GIST cells that do not undergo apoptosis in response to imatinib overexpress nuclear p27(Kip1), indicating that they have withdrawn from the cell cycle and are quiescent. Lastly, we provide evidence that a fraction of primary GISTs with high SKP2 expression levels may have an increased risk of disease progression. Taken together, our results support a model in which GIST cells that do not respond to imatinib by apoptosis are removed from the proliferative pool by entering quiescence through modulation of the APC/CDH1-SKP2-p27(Kip1) signaling axis. These results encourage further studies to explore compounds that modulate this pathway as antitumor agents in GISTs.

G0 function of BCL2 and BCL-xL requires BAX, BAK, and p27 phosphorylation by Mirk, revealing a novel role of BAX and BAK in quiescence regulation

BCL2 and BCL-x(L) facilitate G(0) quiescence by decreasing RNA content and cell size and up-regulating p27 protein, but the precise mechanism is not understood. We investigated the relationship between cell cycle regulation and the anti-apoptosis function of BCL2 and BCL-x(L). Neither caspase inhibition nor abrogation of mitochondria-dependent apoptosis by BAX and BAK deletion fully recapitulated the G(0) effects of BCL2 or BCL-x(L), suggesting that mechanisms in addition to anti-apoptosis are involved in the cell cycle arrest function of BCL2 or BCL-x(L). We found that BCL2 and BCL-x(L) expression in bax(-/-) bak(-/-) cells did not confer cell cycle effects, consistent with the G(0) function of BCL2 and BCL-x(L) being mediated through BAX or BAK. Stabilization of p27 in G(0) in BCL2 or BCL-x(L) cells was due to phosphorylation of p27 at Ser(10) by the kinase Mirk. In bax(-/-) bak(-/-) cells, total p27 and p27 phosphorylated at Ser(10) were elevated. Re-expression of BAX in bax(-/-) bak(-/-) cells and silencing of BAX and BAK in wild type cells confirmed that endogenous BAX and BAK modulated p27. These data revealed a novel role for BAX and BAK in the regulation of G(0) quiescence.

Immunohistochemically detected expression of p27(Kip1) and Skp2 predicts survival in patients with intrahepatic cholangiocarcinomas

In intrahepatic cholangiocarcinomas (ICCs), the prognostic significance of p27(Kip1), a cyclin-dependent kinase inhibitor, remains controversial, and there have been no studies of degradation pathway associated proteins, S-phase kinase-interacting protein (Skp2), and Jun activation domain-binding protein-1 (Jab1). In the present study of 74 patients with ICC-mass forming type (ICC-MF) undergoing radical surgery, we determined immunohistochemical expression of p27(Kip1), Skp2, and Jab1 and examined relationships with clinicopathologic findings and patient survival. On the basis of the average of labeling indices, we set cutoff values to define high and low expressors and divided the cases into two groups. A statistically significant correlation was found between low p27(Kip1) expression and lymph node metastasis (P = .009). Patient survival in the low p27(Kip1) expression group (n = 25) was also significantly worse than that in the high p27(Kip1) expression group (n = 49, P = .0007). A significant inverse correlation was found between p27(Kip1) and Skp2 expression (P = .016). High Skp2 expression (n = 36) was significantly associated with poor prognosis (P = .0046). High Jab1 expression was observed in 32 cases, but there was no statistically significant relationship with clinicopathologic findings or patient survival. The multivariate analysis revealed that low p27(Kip1) and high Skp2 expression are independent and significant factors of poor prognosis. The results suggest that low p27(Kip1) and high Skp2 expression are associated with aggressive tumor behavior, and these cell-cycle regulators are useful markers to predict outcome of patients with ICC-MF.

USP19 deubiquitinating enzyme supports cell proliferation by stabilizing KPC1, a ubiquitin ligase for p27Kip1

p27(Kip1) is a cyclin-dependent kinase inhibitor that regulates the G(1)/S transition. Increased degradation of p27(Kip1) is associated with cellular transformation. Previous work demonstrated that the ubiquitin ligases KPC1/KPC2 and SCF(Skp2) ubiquitinate p27(Kip1) in G(1) and early S, respectively. The regulation of these ligases remains unclear. We report here that the USP19 deubiquitinating enzyme interacts with and stabilizes KPC1, thereby modulating p27(Kip1) levels and cell proliferation. Cells depleted of USP19 by RNA interference exhibited an inhibition of cell proliferation, progressing more slowly from G(0)/G1 to S phase, and accumulated p27(Kip1). This increase in p27(Kip1) was associated with normal levels of Skp2 but reduced levels of KPC1. The overexpression of KPC1 or the use of p27(-/-) cells inhibited significantly the growth defect observed upon USP19 depletion. KPC1 was ubiquitinated in vivo and stabilized by proteasome inhibitors and by overexpression of USP19, and it also coimmunoprecipitated with USP19. Our results identify USP19 as the first deubiquitinating enzyme that regulates the stability of a cyclin-dependent kinase inhibitor and demonstrate that progression through G(1) to S phase is, like the metaphase-anaphase transition, controlled in a hierarchical, multilayered fashion.

p27(Kip1)is overexpressed in very early stages of hepatocarcinogenesis

Hepatocellular carcinoma (HCC) associated with chronic liver disease evolves from precancerous lesions and early HCC to more malignant forms. Despite the demonstrated importance of cell-cycle regulators in tumor biology, there have been few studies of their role in multistep hepatocarcinogenesis. Expression of p27(Kip1) and a degradation pathway associated protein, S-phase kinase-interacting protein 2 (Skp2), was therefore evaluated in surgically resected specimens of eight adenomatous hyperplasias, 16 early HCC and 126 classical HCC. Immunohistochemistry revealed no p27(Kip1) expression in the majority of hepatocytes from normal and cirrhotic liver, whereas positive staining for p27(Kip1) protein was found in 75.0% and 93.8% of adenomatous hyperplasias and early HCC, respectively. The average p27(Kip1) labeling indices (LI) for adenomatous hyperplasias, early HCC, well differentiated HCC, moderately differentiated HCC and poorly differentiated HCC were 36.99, 43.59, 47.73, 49.24, and 30.21, respectively. Real-time quantitative reverse transcription-polymerase chain reaction (RT-PCR) analyses confirmed the increases. Skp2 LI were also significantly elevated in accordance with stepwise progression of hepatocarcinogenesis. Increased expression of Skp2 mRNA was observed most frequently in less differentiated tumors and Kaplan-Meier survival analysis showed a significantly association with a poor prognosis (P = 0.0496). In conclusion, a high level of p27(Kip1) expression is evident from early stages of hepatocarcinogenesis, indicating that this parameter could be a useful diagnostic marker for precancerous lesions and early HCC. In addition, Skp2 expression correlates with tumor dedifferentiation and may contribute to biological aggression in HCC.

The ubiquitin system, disease, and drug discovery

The ubiquitin system of protein modification has emerged as a crucial mechanism involved in the regulation of a wide array of cellular processes. As our knowledge of the pathways in this system has grown, so have the ties between the protein ubiquitin and human disease. The power of the ubiquitin system for therapeutic benefit blossomed with the approval of the proteasome inhibitor Velcade in 2003 by the FDA. Current drug discovery activities in the ubiquitin system seek to (i) expand the development of new proteasome inhibitors with distinct mechanisms of action and improved bioavailability, and (ii) validate new targets. This review summarizes our current understanding of the role of the ubiquitin system in various human diseases ranging from cancer, viral infection and neurodegenerative disorders to muscle wasting, diabetes and inflammation. I provide an introduction to the ubiquitin system, highlight some emerging relationships between the ubiquitin system and disease, and discuss current and future efforts to harness aspects of this potentially powerful system for improving human health.

Republished from Current BioData's Targeted Proteins database (TPdb; ).

The CRL4Cdt2 ubiquitin ligase targets the degradation of p21Cip1 to control replication licensing

The faithful replication of genomic DNA is crucial for maintaining genome stability. In eukaryotes, DNA rereplication is prevented by the temporal regulation of replication licensing. Replication-licensing factors are required to form prereplicative complexes during G1 phase, but are inactivated in S phase to prevent rereplication. A vertebrate CUL4 CRL ubiquitin ligase (CRL4) complex containing Cdt2 as the substrate recognition subunit promotes proper DNA replication, in part, by degrading the replication-licensing factor Cdt1 during S phase. We show that the Caenorhabditis elegans CRL4(Cdt2) complex has a conserved role in degrading Cdt1. Furthermore, we show that CRL4(Cdt2) restrains replication licensing in both C. elegans and humans by targeting the degradation of the cyclin-dependent kinase (CDK) inhibitors CKI-1 and p21(Cip1), respectively. Human CRL4(Cdt2) targets the degradation of p21 in S phase, with the in vivo ubiquitylation of p21 by CRL4(Cdt2) dependent on p21 binding to PCNA. Inactivation of Cdt2 induces rereplication, which requires the presence of the CDK inhibitor p21. Strikingly, coinactivation of CRL4(Cdt2) and SCF(Skp2) (which redundantly targets p21 degradation) prevents the nuclear export of the replication-licensing factor Cdc6 during S phase, and the block on nuclear export is dependent on p21. Our work defines the degradation of p21 as a critical aspect of replication licensing in human cells.

Antitumor bioactivity of adenovirus-mediated p27mt in colorectal cancer cell line SW480

AIM: To explore the antitumor bioactivity of adenovirus-mediated mutant type p27^kip1 gene in a colorectal cancer cell line SW480.

METHODS: We constructed recombinant adenovirus vector expressing a mutant type p27^kip1 gene (ad-p27mt), with mutation of Thr-187/Pro-188 (ACGCCC) to Met-187/Ile-188 (ATGATC), and transduced into SW480 cells. Then we detected expression of p27, Bcl-2 and Bax protein in the transductants by Western blotting, cell cycle of transductants by a digital flow cytometric system, migrating potential with Boyden Chamber and SW480 tumor cell growth inhibition in vitro and in vivo.

RESULTS: We found that a recombinant adenovirus vector of expressing ad-p27mt, with mutation of Thr-187/Pro-188 (ACGCCC) to Met-187/Ile-188 (ATGATC) has potent inhibition of SW480 tumor cell growth in vitro and in vivo. Furthermore, ad-p27mt induced cell apoptosis via regulating bax and bcl-2 expressions, and G₁/S arrest in SW480 cells and inhibited cell migration.

CONCLUSION: ad-p27mt has a strong anti-tumor bioactivity and has the potential to develop into new therapeutic agents for colorectal cancer.

Mutant p27(Kip1) and its potential effect as hepatocellular gene therapy

BACKGROUND

The cyclin-dependent kinase (cdk) inhibitor p27(Kip1) is an important regulator of cell cycle progression as it negatively regulates G(0/1) progression and plays a major role in controlling the cell cycle. The screening of the p27(Kip1) sequence identified many potential phosphorylation sites. To investigate the effects of the overexpression of exogenous p27(Kip1) protein lacking the Thr157 sites on subcellular localization, cell cycle, and proliferation, a plasmid was constructed containing mutations of p27(Kip1) at Thr157 (T157A p27), and transfected into the SMMC7721 cell line with Lipofectamine. Wild-type and mutant p27 plasmids T157A were transfected separately as control groups.

METHODS

We detected the proliferation of SMMC7721 cells by the Cell Counting Kit and FACS/Calibur Flow Cytometer and analyzed the expression and localization of p27(Kip1) by Western blotting analysis and cell fractionation. The cdk2 dependent kinase activity was determined by in vitro kinase assay.

RESULTS

Proliferation of SMMC7721 cells was greatly inhibited and cell cycle was arrested in G(0/1) phase after exogenous p27(Kip1) mutant expression much more than wild-type p27(Kip1). The expressed T157A p27(Kip1) proteins were translocated from the cytoplasm into nucleus much more compare with wild-type. Compared with pcDNA3.1-Myc control, transient transfection of T157A p27(Kip1) decreased expression of cyclin D1 and the phosphorylated form of retinoblastoma protein.

CONCLUSIONS

These findings support the potential effectiveness of a PI3K/Akt-resistant phosphorylated form of p27 in hepatocellular carcinoma gene therapy.

S10 phosphorylation of p27 mediates atRA induced growth arrest in ovarian carcinoma cell lines

All trans retinoic acid (atRA) has been shown to inhibit the growth of CAOV3 ovarian carcinoma cells and to elevate the level of p27 cyclin-dependent kinase inhibitor. We report here that phosphorylation at S10 residue is an important event in mediating p27 role in atRA induced growth arrest. atRA treatment of atRA sensitive CAOV3 cells increases the levels of S10 phospho-p27 in both nuclear and cytoplasmic cell compartments. This increase is accompanied by a decrease in the levels of skp2 protein. This effect was not observed in SKOV3 cells which are resistant to atRA growth inhibitory effect. An A10-p27 mutant that cannot be phosphorylated at S10 induces a dominant negative effect on the atRA effect on the levels and activity of endogenous p27. Overexpression of A10-p27 mutant renders CAOV3 cells more resistant to atRA treatment and reverses the effect that atRA has on p27 binding to CDKs, on CDK activity, and on the expression of S phase genes.

Cyclin G2 is degraded through the ubiquitin-proteasome pathway and mediates the antiproliferative effect of activin receptor-like kinase 7

We have previously reported that Nodal, a member of the TGF-beta superfamily, acts through activin receptor-like kinase 7 (ALK7) to inhibit ovarian cancer cell proliferation. To determine the mechanism underlying their effects, a cell cycle gene array was performed and cyclin G2 mRNA was found to be strongly up-regulated by Nodal and ALK7. To study the function and regulation of cyclin G2 in ovarian cancer cells, expression constructs were generated. We found that cyclin G2 protein level decreased rapidly after transfection, and this decrease was prevented by 26S proteasome inhibitors. Immunoprecipitation and pull-down studies showed that ubiquitin, Skp1, and Skp2 formed complexes with cyclin G2. Knockdown of Skp2 by siRNA increased, whereas overexpression of Skp2 decreased cyclin G2 levels. Nodal and ALK7 decreased the expression of Skp1 and Skp2 and increased cyclin G2 levels. Overexpression of cyclin G2 inhibited cell proliferation whereas cyclin G2-siRNA reduced the antiproliferative effect of Nodal and ALK7. Taken together, these findings provide strong evidence that cyclin G2 is degraded by the ubiquitin-proteasome pathway and that Skp2 plays a role in regulating cyclin G2 levels. Furthermore, our results also demonstrate that the antiproliferative effect of Nodal/ALK7 on ovarian cancer cells is in part mediated by cyclin G2.

Anaphase-promoting complex/cyclosome-CDH1-mediated proteolysis of the forkhead box M1 transcription factor is critical for regulated entry into S phase

The forkhead box M1 (FoxM1) transcription factor is overexpressed in many cancers, and in mouse models it is required for tumor progression. FoxM1 activates expression of the cell cycle genes required for both S and M phase progression. Here we demonstrate that FoxM1 is degraded in late mitosis and early G(1) phase by the anaphase-promoting complex/cyclosome (APC/C) E3 ubiquitin ligase. FoxM1 interacts with the APC/C complex and its adaptor, Cdh1. Expression of Cdh1 stimulated degradation of the FoxM1 protein, and depletion of Cdh1 resulted in stabilization of the FoxM1 protein in late mitosis and in early G(1) phase of the cell cycle. Cdh1 has been implicated in regulating S phase entry. We show that codepletion of FoxM1 inhibits early S phase entry observed in Cdh1-depleted cells. The N-terminal region of FoxM1 contains both destruction box (D box) and KEN box sequences that are required for targeting by Cdh1. Mutation of either the D box sequence or the KEN box sequence stabilized FoxM1 and blocked Cdh1-induced proteolysis. Cells expressing a nondegradable form of FoxM1 entered S phase rapidly following release from M phase arrest. Together, our observations show that FoxM1 is one of the targets of Cdh1 in late M or early G(1) phase and that its proteolysis is important for regulated entry into S phase.

Activation of protein kinase G Increases the expression of p21CIP1, p27KIP1, and histidine triad protein 1 through Sp1

The anticancer role of cyclic guanosine 3',5'-monophosphate (cGMP)-dependent protein kinase G (PKG) has become of considerable interest, but the underlying mechanisms are not fully established. In this study, we examined the effects of activation of PKG on the expression of three tumor suppressor proteins in human SW480 colon cancer cells. Our results revealed that treatment with cell permeable cGMP derivatives, or the cGMP phosphodiesterase inhibitor sulindac sulfone (exisulind, aptosyn, hereafter called exisulind) led to increased expression of the tumor suppressor proteins p21(CIP1), p27(KIP1), and Histidine triad protein 1 (HINT1), and their corresponding mRNAs. Overexpression of PKG Ibeta also caused increased expression of the p21(CIP1), p27(KIP1), and HINT1 proteins. Both the p21(CIP1) and p27(KIP1) promoters contain Sp1 binding sites and they were activated by PKG in luciferase reporter assays. Specific Sp1 sites in the p21 and p27 promoters were sufficient to mediate PKG-induced luciferase reporter activity, suggesting an interaction between Sp1 and PKG. Indeed, we found that PKG can phosphorylate Sp1 on serine residue(s) and this resulted in transcriptional activation of Sp1. Knockdown of Sp1 expression with siRNA inhibited the increased expression of p21(CIP1), p27(KIP1), and HINT1 induced by the cGMP derivative 8-pCPT-cGMP in SW480 cells. These novel effects of PKG activation on the expression of three tumor suppressor genes may explain, at least in part, the anticancer effects of activation of PKG. They also provide a rationale for further developing activators of PKG for the prevention and treatment of cancer.

Dynamic changes of p27(kip1) and Skp2 expression in injured rat sciatic nerve

S phase kinase-associated protein 2 (Skp2), an F-box protein, is required for the ubiquitination and consequent degradation of p27(kip1). Previous reports have showed that p27(kip1 )played important roles in cell cycle regulation and neurogenesis in the developing central nervous system. But the distribution and function of p27(kip1 )and Skp2 in nervous system lesion and regeneration remains unclear. In this study, we observed that they were expressed mainly in both Schwann cells and axons in adult rat sciatic nerve. Sciatic nerve crush and transection resulted in a significant up-regulation of Skp2 and a down-regulation of p27(kip1). By immunochemistry, we found that in the distal stumps of transected nerve from the end to the edge, the appearance of Skp2 in the edge is coincided with the decrease in p27(kip1) levels. Changes of them were inversely correlated. Results obtained by coimmunoprecipitation and double labeling further showed their interaction in the regenerating process. Thus, these results indicate that p27(kip1 )and Skp2 likely play an important role in peripheral nerve injury and regeneration.

The COP9 signalosome negatively regulates proteasome proteolytic function and is essential to transcription

The COP9 signalosome (CSN) is an evolutionarily conserved protein complex formed by eight subunits (CSN1 through CSN8). Deneddylating cullin family proteins is considered the bona fide function of the CSN. It has been proposed that the CSN regulates the assembly and disassembly of the cullin-based ubiquitin ligases via its deneddylation activity. Here we report that down-regulation of CSN8 by RNA interference destabilized differentially other CSN subunits and reduced the amount of CSN holo-complexes, leading to increases in neddylated cullin proteins and reduction of F-box protein Skp2 in HEK293 cells. Moreover, suppression of CSN8 enhanced the degradation of a proteasome surrogate substrate and cyclin kinase inhibitor p21(cip). Reduced transcript levels of cyclin kinase inhibitor p21(cip) and p27(kip) were also observed upon down-regulation of CSN8. These data suggest that the homeostatic level of CSN8/CSN suppresses proteasome proteolytic function and regulates transcription.

Over-expression of Skp2 is associated with resistance to preoperative doxorubicin-based chemotherapy in primary breast cancer

INTRODUCTION

Preoperative chemotherapy is often used in patients with locally advanced breast cancer. However, commonly used clinical and pathological parameters are poor predictors of response to this type of therapy. Recent studies have suggested that altered regulation of the cell cycle in cancer may be involved in resistance to chemotherapy. Over-expression of the ubiquitin ligase Skp2 results in loss of the cell cycle inhibitor p27Kip1 and is associated with poor prognosis in early breast cancer. The purpose of the present study was to examine the role of these proteins as predictors of clinical outcome and response to chemotherapy in locally advanced breast cancer.

METHODS

The expression levels of Skp2 and p27Kip1 were determined by immunohistochemistry both before and after preoperative chemotherapy in 40 patients with locally advanced breast cancer. All patients were treated with cyclophosphamide/doxorubicin (adriamycin)/5-fluorouracil (CAF) and some patients received additional treatment with docetaxel. Expression data were compared with patients' clinical and pathological features, clinical outcome, and response to chemotherapy.

RESULTS

Skp2 expression before preoperative chemotherapy was inversely related to p27Kip1 levels, tumor grade, and expression of estrogen and progesterone receptors. Both Skp2 and p27Kip1 were found to be accurate prognostic markers for disease-free and overall survival. High preoperative expression of Skp2 was associated with resistance to CAF therapy in 94% of patients (P < 0.0001) but not with resistance to docetaxel.

CONCLUSION

Skp2 expression may be a useful marker for predicting response to doxorubicin-based preoperative chemotherapy and clinical outcome in patients with locally advanced breast cancer.

Argyrin a reveals a critical role for the tumor suppressor protein p27(kip1) in mediating antitumor activities in response to proteasome inhibition

A reduction in the cellular levels of the cyclin kinase inhibitor p27(kip1) is frequently found in many human cancers and correlates directly with patient prognosis. In this work, we identify argyrin A, a cyclical peptide derived from the myxobacterium Archangium gephyra, as a potent antitumoral drug. All antitumoral activities of argyrin A depend on the prevention of p27(kip1) destruction, as loss of p27(kip1) expression confers resistance to this compound. We find that argyrin A exerts its effects through a potent inhibition of the proteasome. By comparing the cellular responses exerted by argyrin A with siRNA-mediated knockdown of proteasomal subunits, we find that the biological effects of proteasome inhibition per se depend on the expression of p27(kip1).

Skp2 inversely correlates with p27 and tuberin in transformed cells

The cyclin-dependent kinase inhibitor p27Kip1 (p27) is a major gatekeeper of the mammalian cell cycle progression known to be regulated by both, its subcellular localization and its degradation. To allow entrance into S phase and thereby mammalian cell cycle progression p27 must be degraded by a skp2-containing E3 ubiquitin ligase whose task is to target p27 for degradation by the proteasome. The tumor suppressor gene product tuberin directly binds to p27 and protects it from degradation via skp2. Whereas, p27 and tuberin are known to be localized to both, the cytoplasm and the nucleus, the localization of skp2 remained elusive. Here we demonstrate that skp2 is a cytoplasmic and nuclear protein. In addition we found an inverse correlation of the endogenous protein levels of skp2 with p27 and tuberin in different transformed cells and under different growth conditions. These data allow new important insights into this molecular network of cell cycle control.

Distinct modes of activation of phosphatidylinositol 3-kinase in response to cyclic adenosine 3', 5'-monophosphate or insulin-like growth factor I play different roles in regulation of cyclin D1 and p27Kip1 in FRTL-5 cells

Bioactivities of IGFs in various cells are often potentiated in the presence of other hormones. In previous studies we showed that pretreatment of rat FRTL-5 thyroid cells with TSH or other cAMP-generating agents markedly potentiated DNA synthesis induced by IGF-I. Under these conditions we found that phosphatidylinositol (PI) 3-kinase was activated in response to either cAMP or IGF stimulus, and both activation modes were indispensable for the potentiation of DNA synthesis. The present studies were undertaken to elucidate how cAMP and/or IGF-I stimulus regulated the G1 cyclin-cyclin dependent kinase (CDK)-inhibitor system, and to determine the roles of PI 3-kinase activation by cAMP or IGF-I stimulus in this system. We found that cAMP pretreatment enhanced IGF-I-dependent increases in cyclin D1, due to synergistic increases in mRNA and elevation of translation rates. Furthermore, cAMP pretreatment enhanced IGF-I-induced protein degradation of the CDK inhibitor, p27(Kip1). These changes well explained an increase in cyclin E, leading to marked activation of G1 CDKs, followed by retinoblastoma protein phosphorylation. Our results using a PI 3-kinase inhibitor showed that cAMP-dependent PI 3-kinase activation plays an important role in the increase in cyclin D1 translation. In contrast, IGF-I-dependent PI 3-kinase activation was required for the increase in cyclin D1 mRNA levels and degradation of p27(Kip1). Together, the present study elucidates the role of cAMP and IGF-I in differentially activating PI 3-kinase as a mediator of multiple molecular events. These events converge in the regulation of cyclin D1 and p27(Kip1), leading to cAMP-dependent potentiation of IGF-I-dependent CDK activation and DNA synthesis.

Dissection of the APCCdh1-Skp2 cascade in breast cancer

PURPOSE

Skp2 is a subunit of the SCF ubiquitin protein ligase, which plays a vital role in the control of tumorigenesis via its regulation of G(1)-S transition. Deregulation of Skp2 in various types of cancers correlates with aggressive clinical behavior and poor prognosis. Recent studies suggest that cell cycle-dependent fluctuation of Skp2 is governed by APC(Cdh1), another important E3 ligase, thereby preventing premature entry into S phase. To assess the potential role of APC(Cdh1) in tumorigenesis through proteolysis of Skp2, we have dissected the APC(Cdh1)-Skp2 cascade.

EXPERIMENTAL DESIGN

We manipulated the APC(Cdh1)-Skp2 cascade and examined its cellular behavior using both breast cancer and normal breast epithelial cells. Furthermore, applying immunohistochemistry, we analyzed the clinicopathologic significance of these molecules in patients with breast cancer.

RESULTS

Analysis of tissue arrays indicated that the percentage of samples positive for Cdh1 in breast cancer was significantly lower compared with normal breast tissues (P=0.004). Conversely, the percentage of samples scored as positive for Skp2 in cancer was significantly higher than in normal tissues (P<0.001). Moreover, prognostic studies revealed that relatively high levels of Cdh1 are associated with survivability in patients with breast cancer. In addition, depletion of Cdh1 by small interfering RNA in normal breast cells resulted in increased cellular proliferation, whereas knockdown of Skp2 significantly suppressed growth in breast cancer cells.

CONCLUSIONS

This study shows a correlation between Skp2 and APC(Cdh1) in breast cancer. Thus, Cdh1 may act as an important component in tumor suppression and could be considered as a novel biomarker in breast cancer.

Bortezomib (Velcade) induces p27Kip1 expression through S-phase kinase protein 2 degradation in colorectal cancer

S-phase kinase protein 2 (SKP2), an F-box protein, targets cell cycle regulators including cycle-dependent kinase inhibitor p27Kip1 via ubiquitin-mediated degradation. SKP2 is frequently overexpressed in a variety of cancers. We investigated the role of SKP2 and its ubiquitin-proteasome pathway in colorectal carcinoma using a panel of cell lines, clinical samples, and the NUDE mouse model. Using immunohistochemical analysis on a large tissue microarray of 448 samples, an inverse association of SKP2 expression with p27Kip1 protein levels was seen. A colorectal cancer (CRC) subset with high level of SKP2 and low level of p27Kip1 showed a decreased overall survival (P = 0.0057). Treatment of CRC cell lines with bortezomib or expression of small interfering RNA of SKP2 causes down-regulation of SKP2 and accumulation of p27Kip1. Furthermore, treatment of CRC cells with bortezomib causes apoptosis by involving the mitochondrial pathway and activation of caspases. In addition, treatment of CRC cells with bortezomib down-regulated the expression of XIAP, cIAP1, and survivin. Finally, treatment of CRC cell line xenografts with bortezomib resulted in growth inhibition of tumors in NUDE mice via down-regulation of SKP2 and accumulation of p27Kip1. Altogether, our results suggest that SKP2 and the ubiquitin-proteasome pathway may be potential targets for therapeutic intervention for treatment of CRC.

Regulation of Skp2-p27 axis by the Cdh1/anaphase-promoting complex pathway in colorectal tumorigenesis

Abrogated entry into S phase is a common hallmark of cancer cells. Skp2, a subunit of ubiquitin ligase, is critical for regulating the G(1)/S transition. Uncontrolled Skp2 activity is detected frequently in human tumors, often correlated with poor prognosis. Current studies have suggested that the regulation of Skp2 turnover is mediated by another critical ubiquitin ligase, the anaphase-promoting complex (APC), in association with its substrate-specific factor Cdh1. To dissect the potential role of Cdh1/APC in tumorigenesis through the degradation of Skp2, we analyzed the Cdh1/APC-Skp2-p27 axis in colorectal tumorigenesis using a human tumor array and biochemical analyses. Our results show that the percentage of Cdh1- and p27-positive samples in colon cancer tissues was significantly lower than that in adjacent nonmalignant tissue. Conversely, the percentage of Skp2-positive colon cancer samples was significantly higher than that in normal tissue. Furthermore, results from clinicopathological analysis revealed that elevated Cdh1 expression was associated with lower histological grade tumors. In addition, depletion of Cdh1 by RNA interference in nonmalignant colon cells resulted in increased cellular proliferation, whereas knockdown of Skp2 significantly suppressed cancer cell growth. Our result suggests a pathological correlation between Skp2 and Cdh1/APC in colorectal cancer. Thus, Cdh1 may function as a component in tumor suppression via proteolysis of Skp2 in colorectal tumorigenesis and may serve as a prognostic marker in colon cancer patients.

Effect of S-phase kinase-associated protein 2 expression on distant metastasis and survival in nasopharyngeal carcinoma patients

PURPOSE

The S-phase kinase-associated protein 2 (Skp2) oncoprotein is an E3 ubiquitin ligase targeting the p27(Kip1) tumor suppressor for degradation. We evaluated the prognostic utility of Skp2 in nasopharyngeal carcinoma (NPC), with an emphasis on distant metastasis-free (DMF) survival.

METHODS AND MATERIALS

Immunohistochemical expression of Skp2 was assessed by H-score for 233 NPC patients without initial distant metastasis and correlated with the clinicopathologic features, therapeutic modalities, locoregional control rate, DMF survival, and overall survival (OS). No selection bias in essential parameters was shown between these and another 113 control patients.

RESULTS

Skp2 was detectable in most patients (95%) but displayed a wide range of expression. Despite no correlation between Skp2 and any clinicopathologic factor, greater Skp2 expression (H-score >80) significantly portended inferior DMF survival (p = 0.01) and OS (p = 0.02) when categorically dichotomizing the study cohort. The associations with DMF survival (p = 0.003) and OS (p = 0.003) became even stronger when the H-score was expressed as a continuous variable. In the multivariate model, greater Skp2 expression remained significantly predictive of poorer DMF survival and OS (p = 0.01 for both), along with T stage (p = 0.04 for DMF survival; p = 0.005 for OS), N stage (p = 0.008 for DMF survival; p = 0.02 for OS), and/or age (p = 0.001 for OS). In contrast, T stage (p = 0.01) was the single independent prognosticator of LCR.

CONCLUSIONS

The results of our study have shown that Skp2 expression is frequent in NPC and has a wide range of distribution in H-score. Skp2 overexpression is significantly predictive of DMF survival and OS, independent of the T stage and/or older age. Therefore, Skp2 might represent a useful prognostic adjunct to risk stratify NPC patients for appropriate allocation of adjuvant therapy.

Deregulated proteolysis by the F-box proteins SKP2 and beta-TrCP: tipping the scales of cancer

The maintenance and preservation of distinct phases during the cell cycle is a highly complex and coordinated process. It is regulated by phosphorylation--through the activity of cyclin-dependent kinases (CDKs)--and protein degradation, which occurs through ubiquitin ligases such as SCF (SKP1-CUL1-F-box protein) complexes and APC/C (anaphase-promoting complex/cyclosome). Here, we explore the functionality and biology of the F-box proteins, SKP2 (S-phase kinase-associated protein 2) and beta-TrCP (beta-transducin repeat-containing protein), which are emerging as important players in cancer biogenesis owing to the deregulated proteolysis of their substrates.

Oncogenic properties and prognostic implications of the ubiquitin ligase Skp2 in cancer

The expression of Skp2, the ubiquitin ligase subunit that targets p27(Kip1) for degradation, is commonly overexpressed in human cancers. p27(Kip1) is a negative regulator of the cell cycle that plays an important role in tumor suppression. Loss of p27(Kip1) secondary to enhanced ubiquitin-mediated degradation results in uncontrolled proliferation and promotes tumor progression. In the present study the prognostic implications of Skp2 are reviewed and the mechanisms that regulate its expression in different human cancers. A review and analysis of the English literature was undertaken. Overexpression of Skp2 mRNA and protein levels was observed in many aggressive cancers and was commonly associated with down-regulation of p27(Kip1) levels and loss of tumor differentiation. Skp2 is an independent prognostic marker for disease-free and overall survival and may provide additional predictive information to that provided by p27(Kip1) alone. Targeting Skp2 in experimental models resulted in up-regulation of p27(Kip1) and arrested cellular proliferation. Alterations in Skp2 expression have profound effects on cancer progression and may serve as an accurate and independent prognostic marker. Thus, determination of levels of Skp2 and p27(Kip1) by readily available immunohistochemical studies may be a useful tool in the assessment of prognosis, especially in patients with intermediate disease, and may potentially assist in the planning of adjuvant therapy. Skp2 may be an attractive target for the development of novel interventional therapy.

Molecular mechanism underlying the functional loss of cyclindependent kinase inhibitors p16 and p27 in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is one of the most common human cancers, and its incidence is still increasing in many countries. The prognosis of HCC patients remains poor, and identification of useful molecular prognostic markers is required. Many recent studies have shown that functional alterations of cell-cycle regulators can be observed in HCC. Among the various types of cell-cycle regulators, p16 and p27 are frequently inactivated in HCC and are considered to be potent tumor suppressors. p16, a G1-specific cell-cycle inhibitor that prevents the association of cyclindependent kinase (CDK) 4 and CDK6 with cyclin D1, is frequently inactivated in HCC via CpG methylation of its promoter region. p16 may be involved in the early steps of hepatocarcinogenesis, since p16 gene methylation has been detected in subsets of pre-neoplastic liver cirrhosis patients. p27, a negative regulator of the G1-S phase transition through inhibition of the kinase activities of Cdk2/cyclin A and Cdk2/cyclin E complexes, is now considered to be an adverse prognostic factor in HCC. In some cases of HCC with increased cell proliferation, p27 is overexpressed but inactivated by sequestration into cyclin D1-CDK4-containing complexes. Since loss of p16 is closely related to functional inactivation of p27 in HCC, investigating both p16 and p27 may be useful for precise prognostic predictions in individuals with HCC.

Degradation of the cyclin-dependent kinase inhibitor KRP1 is regulated by two different ubiquitin E3 ligases

In animals and fungi, a group of proteins called the cyclin-dependent kinase inhibitors play a key role in cell cycle regulation. However, comparatively little is known about the role of these proteins in plant cell cycle regulation. To gain insight into the mechanisms by which the plant cell cycle is regulated, we studied the cyclin-dependent kinase inhibitor KRP1 in Arabidopsis. KRP1 interacts with the CDKA;1/CYCD2;1 complex in planta and functions in the G1-S transition of the cell cycle. Furthermore, we show that KRP1 is a likely target of the ubiquitin/proteasome pathway. Two different ubiquitin protein ligases, SCF(SKP2) and the RING protein RKP, contribute to its degradation. These results suggest that SCF(SKP2b) and RPK play an important role in the cell cycle through regulating KRP1 protein turnover.

Targeting the p27 E3 ligase SCF(Skp2) results in p27- and Skp2-mediated cell-cycle arrest and activation of autophagy

Decreased p27(Kip1) levels are a poor prognostic factor in many malignancies, and can occur through up-regulation of SCF(Skp2) E3 ligase function, resulting in enhanced p27 ubiquitination and proteasome-mediated degradation. While proteasome inhibitors stabilize p27(Kip1), agents inhibiting SCF(Skp2) may represent more directly targeted drugs with the promise of enhanced efficacy and reduced toxicity. Using high-throughput screening, we identified Compound A (CpdA), which interfered with SCF(Skp2) ligase function in vitro, and induced specific accumulation of p21 and other SCF(Skp2) substrates in cells without activating a heat-shock protein response. CpdA prevented incorporation of Skp2 into the SCF(Skp2) ligase, and induced G(1)/S cell-cycle arrest as well as SCF(Skp2)- and p27-dependent cell killing. This programmed cell death was caspase-independent, and instead occurred through activation of autophagy. In models of multiple myeloma, CpdA overcame resistance to dexamethasone, doxorubicin, and melphalan, as well as to bortezomib, and also acted synergistically with this proteasome inhibitor. Importantly, CpdA was active against patient-derived plasma cells and both myeloid and lymphoblastoid leukemia blasts, and showed preferential activity against neoplastic cells while relatively sparing other marrow components. These findings provide a rational framework for further development of SCF(Skp2) inhibitors as a novel class of antitumor agents.

Differential effects of all-trans retinoic acid on the growth of human keratinocytes and mouth carcinoma epidermoid cultures. Involvement of GRIM-19 and complex I of the respiratory chain

Squamous cell carcinoma (SSC) is the most frequent malignant tumor of the oral cavity. A study on the effect of all-trans retinoic acid (RA) on cell growth, expression of GRIM-19 and content and activity of complex I of the mitochondrial respiratory chain in normal human keratinocytes (NHEK) and mouth carcinoma cells with low (HN) and high (KB) transformation grade was carried out. In NHEK cells, RA treatment resulted in growth suppression, significant overexpression of GRIM-19 protein, enhanced content of complex I but depressed activity of NADH-UQ oxidoreductase activity of the complex. In HN cells, RA treatment depressed cell growth, inhibited the enzymatic activity of complex I but had no significant effect on the levels of GRIM-19 and complex I. In KB cells RA had no effect on cell growth, GRIM-19 expression, content and activity of complex I.

Expression of S-phase kinase-asociated protein 2 mRNA in large intestine adenoma tissue

AIM: To study the expression of S-phase kinase-asociated protein 2 (Skp2) in large intestine adenoma tissue.

METHODS: Expression of Skp2 was detected in 20 cases of tubular adenoma, 15 cases of villous adenoma, 18 cases of mixed type adenoma and 20 cases of normal tissue using RT-PCR and immunohistochemical technique.

RESULTS: The expression level of Skp2 mRNA was significantly higher in villous adenoma and mixed type adenoma than in normal tissue (0.71 ± 0.34, 0.63 ± 0.20 vs 0.34 ± 0.20, P < 0.01). However, there was no difference in the relative amount of Skp2 mRNA between tubular adenoma and normal tissue. However, there was a significant difference in villous adenoma, mixed type adenoma and tubular adenoma (0.71 ± 0.34, 0.63 ± 0.20 vs 0.43 ± 0.22, P < 0.05). There was no difference in villous adenoma and mixed type adenoma. The positive rate for the expression of Skp2 in tubular adenoma, villous adenoma, mixed type adenoma and normal tissue was 15% (3/20), 46.7% (7/15), 27.8% (5/18) and 5.0% (1/20), respectively(P < 0.05)

CONCLUSION: Positive expression of Skp2 in large intestine adenoma is related with its histology type and may promote the development of large intestine adenoma.

CSN5 isopeptidase activity links COP9 signalosome activation to breast cancer progression

CSN5 has been implicated as a candidate oncogene in human breast cancers by genetic linkage with activation of the poor-prognosis, wound response gene expression signature. CSN5 is a subunit of the eight-protein COP9 signalosome, a signaling complex with multiple biochemical activities; the mechanism of CSN5 action in cancer development remains poorly understood. Here, we show that CSN5 isopeptidase activity is essential for breast epithelial transformation and progression. Amplification of CSN5 is required for transformation of primary human breast epithelial cells by defined oncogenes. The transforming effects of CSN5 require CSN subunits for assembly of the full COP9 signalosome and the isopeptidase activity of CSN5, which potentiates the transcriptional activity of MYC. Transgenic inhibition of CSN5 isopeptidase activity blocks breast cancer progression evoked by MYC and RAS in vivo. These results highlight CSN5 isopeptidase activity in breast cancer progression, suggesting it as a therapeutic target in aggressive human breast cancers.

Cyclin E and SV40 small T antigen cooperate to bypass quiescence and contribute to transformation by activating CDK2 in human fibroblasts

Cyclin E overexpression is observed in multiple human tumors and linked to poor prognosis. We have previously shown that ectopic expression of cyclin E is sufficient to induce mitogen-independent cell cycle entry in a variety of tumor/immortal cell lines. Here we have investigated the rate-limiting step leading to cell cycle entry in quiescent normal human fibroblasts (NHF) ectopically expressing cyclin E. We found that in serum-starved NHF, cyclin E forms inactive complexes with CDK2 and fails to induce DNA synthesis. Coexpression of SV40 small t antigen (st), but not other tested oncogenes, efficiently induces mitogen-independent CDK2 phosphorylation on Thr-160, CDK2 activation, and DNA synthesis. Additionally, in contact-inhibited NHF ectopically expressing cyclin E, st induces cell cycle entry, continued proliferation, and foci formation. Coexpression of cyclin E and st also bypasses G(0)/G(1) arrests induced by CDK inhibitors. Although CDK2 is dispensable for G(0)/G(1) cell cycle entry and normal proliferation in mammals, CDK2 activity is an essential rate-limiting step in NHF with deregulated cyclin E expression and altered PP2A activity, which endows primary cells with transformed features. Consequently, CDK2 could be targeted therapeutically in tumors that involve these alterations. These data also suggest that alterations prior to cyclin E deregulation facilitate proliferation of tumor cells by bypassing mitogenic requirements and negative regulation by adjacent cells.