Global analysis of Cdc14 phosphatase reveals diverse roles in mitotic processes.

Cdc14 phosphatase regulates multiple events during anaphase and is essential for mitotic exit in budding yeast. Cdc14 is regulated in both a spatial and temporal manner. It is sequestered in the nucleolus for most of the cell cycle by the nucleolar protein Net1 and is released into the nucleus and cytoplasm during anaphase. To identify novel binding partners of Cdc14, we used affinity purification of Cdc14 and mass spectrometric analysis of interacting proteins from strains in which Cdc14 localization or catalytic activity was altered. To alter Cdc14 localization, we used a strain deleted for NET1, which causes full release of Cdc14 from the nucleolus. To alter Cdc14 activity, we generated mutations in the active site of Cdc14 (C283S or D253A), which allow binding of substrates, but not dephosphorylation, by Cdc14. Using this strategy, we identified new interactors of Cdc14, including multiple proteins involved in mitotic events. A subset of these proteins displayed increased affinity for catalytically inactive mutants of Cdc14 compared with the wild-type version, suggesting they are likely substrates of Cdc14. We have also shown that several of the novel Cdc14-interacting proteins, including Kar9 (a protein that orients the mitotic spindle) and Bni1 and Bnr1 (formins that nucleate actin cables and may be important for actomyosin ring contraction) are specifically dephosphorylated by Cdc14 in vitro and in vivo. Our findings suggest the dephosphorylation of the formins may be important for their observed localization change during exit from mitosis and indicate that Cdc14 targets proteins involved in wide-ranging mitotic events.

Novel role for Cdc14 sequestration: Cdc14 dephosphorylates factors that promote DNA replication.

The phosphatase Cdc14 is required for mitotic exit in budding yeast. Cdc14 promotes Cdk1 inactivation by targeting proteins that, when dephosphorylated, trigger degradation of mitotic cyclins and accumulation of the Cdk1 inhibitor, Sic1. Cdc14 is sequestered in the nucleolus during most of the cell cycle but is released into the nucleus and cytoplasm during anaphase. When Cdc14 is not properly sequestered in the nucleolus, expression of the S-phase cyclin Clb5 is required for viability, suggesting that the antagonizing activity of Clb5-dependent Cdk1 specifically is necessary when Cdc14 is delocalized. We show that delocalization of Cdc14 combined with loss of Clb5 causes defects in DNA replication. When Cdc14 is not sequestered, it efficiently dephosphorylates a subset of Cdk1 substrates including the replication factors, Sld2 and Dpb2. Mutations causing Cdc14 mislocalization interact genetically with mutations affecting the function of DNA polymerase epsilon and the S-phase checkpoint protein Mec1. Our findings suggest that Cdc14 is retained in the nucleolus to support a favorable kinase/phosphatase balance while cells are replicating their DNA, in addition to the established role of Cdc14 sequestration in coordinating nuclear segregation with mitotic exit.

Experimental tests to definitively determine ubiquitylation of a substrate.

Ubiquitin-mediated proteolysis is a major pathway of protein degradation that regulates numerous cellular processes. An understanding of the circumstances that contribute to the ubiquitylation of a specific protein can yield vast insight into its regulation. This article examines multiple procedures that explain whether a protein is ubiquitylated and suggests methods to investigate the factors that specifically target the substrate for ubiquitylation, as well as the site of ubiquitin conjugation.

To be or not to be ubiquitinated?

Levels of p21, a cyclin-dependent kinase (CDK) inhibitor, are controlled in part at the post-translational level by protein degradation. Although the signaling pathways leading to p21 degradation have not yet been fully elucidated, it is evident that p21 ubiquitination is an essential factor in its degradation. We discuss that, with the only notable exception of ornithine decarboxylase, ubiquitination appears to be a prerequisite for proteasomal degradation rather than an unnecessary byproduct of such proteolysis.

Five-year survival in patients with cytokine-refractory metastatic renal cell carcinoma treated with axitinib.

BACKGROUND: In a phase II study of axitinib for cytokine-refractory metastatic renal cell carcinoma, median overall survival (OS) was 29.9 months (95% CI, 20.3 to not estimable months). PATIENTS AND METHODS: Long-term survival data were collected retrospectively from 52 patients with cytokine-refractory metastatic renal cell carcinoma who received axitinib in a completed phase II study (protocol 1), 11 of whom enrolled in a continuing access protocol (protocol 2), for the current observational study (protocol 3). In a post hoc analysis, the patients were grouped into quartiles based on cycle 1 day 1, 1- to 2-hour post-dose axitinib plasma levels to explore the impact of drug exposure on efficacy. RESULTS: The 5-year survival rate was 20.6% (95% CI, 10.9%-32.4%), with a median follow-up of 5.9 years. Frequent all-grade adverse events were fatigue (n = 38; 73.1%), diarrhea (n = 34; 65.4%), hypertension (n = 33; 63.5%), and nausea (n = 33; 63.5%). Quartile 3 patients (axitinib level, 45.2-56.4 ng/mL; n = 12) had the best clinical outcome: objective response rate 82%, median progression-free survival (PFS) 28.3 months, and median OS that was not reached after 5 years. CONCLUSIONS: Axitinib was well tolerated and provided an estimated 5-year survival rate of 20.6% for cytokine-refractory metastatic renal cell carcinoma. Exploratory analyses showed numerically higher objective response rate and longer OS and PFS in patients who achieved post-first-dose axitinib plasma concentrations within a specific range.

Axitinib or bevacizumab plus FOLFIRI or modified FOLFOX-6 after failure of first-line therapy for metastatic colorectal cancer: a randomized phase II study.

OBJECTIVE: Axitinib, a potent and selective second-generation inhibitor of vascular endothelial growth factor receptors 1, 2, and 3, shows activity in multiple tumor types, including those refractory to previous antiangiogenic therapy. This randomized, multicenter, parallel-group, open-label phase II trial compared axitinib with bevacizumab each in combination with 5-fluorouracil/leucovorin/oxaliplatin (FOLFOX) or 5-fluorouracil/leucovorin/irinotecan (FOLFIRI) for second-line treatment of metastatic colorectal cancer. METHODS: Patients were randomized 1:1 to axitinib 5 mg twice daily or bevacizumab 5 mg/kg every 2 weeks plus modified FOLFOX-6 (if previously treated with irinotecan) or FOLFIRI (if previously treated with oxaliplatin) and were stratified by performance status and prior bevacizumab therapy. Primary endpoint was progression-free survival. RESULTS: In 171 patients, progression-free survival was 7.6 months with axitinib/FOLFOX vs. 6.4 months with bevacizumab/FOLFOX (hazard ratio [HR], 1.04; 95% confidence interval [CI], 0.55-1.96; 1-sided P = .55) and 5.7 months with axitinib/FOLFIRI vs. 6.9 months with bevacizumab/FOLFIRI (HR, 1.27; 95% CI, 0.77-2.11; 1-sided P = .83). Overall survival was 17.1 vs. 14.1 months with axitinib/FOLFOX and bevacizumab/FOLFOX (HR, 0.69; 95% CI, 0.37-1.27; 1-sided P = .12) and 12.9 vs. 15.7 months with axitinib/FOLFIRI and bevacizumab/FOLFIRI (HR, 1.36; 95% CI, 0.82-2.24; 1-sided P = .88). More grade ≥ 3 adverse events (eg, diarrhea, fatigue, decreased appetite) and treatment discontinuations due to adverse events occurred with axitinib. CONCLUSIONS: Compared with bevacizumab, axitinib did not improve outcomes when added to second-line chemotherapy for metastatic colorectal cancer. With current dosing regimens, axitinib plus FOLFOX or FOLFIRI seems to be less well tolerated than bevacizumab-based regimens.

Multiple levels of cyclin specificity in cell-cycle control.

Cyclins regulate the cell cycle by binding to and activating cyclin-dependent kinases (Cdks). Phosphorylation of specific targets by cyclin-Cdk complexes sets in motion different processes that drive the cell cycle in a timely manner. In budding yeast, a single Cdk is activated by multiple cyclins. The ability of these cyclins to target specific proteins and to initiate different cell-cycle events might, in some cases, reflect the timing of the expression of the cyclins; in others, it might reflect intrinsic properties of the cyclins that render them better suited to target particular proteins.

Modification of Cul1 regulates its association with proteasomal subunits.

BACKGROUND: Ubiquitylation targets proteins for degradation by the 26S proteasome. Some yeast and plant ubiquitin ligases, including the highly conserved SCF (Skp1/Cul1/F-box protein) complex, have been shown to associate with proteasomes. We sought to characterize interactions between SCF complexes and proteasomes in mammalian cells. RESULTS: We found that the binding of SCF complexes to proteasomes is conserved in higher eukaryotes. The Cul1 subunit associated with both sub-complexes of the proteasome, and high molecular weight forms of Cul1 bound to the 19S proteasome. Cul1 is ubiquitylated in vivo. Ubiquitylation of Cul1 promotes its binding to the S5a subunit of the 19S sub-complex without affecting Cul1 stability. CONCLUSION: The association of ubiquitylating enzymes with proteasomes may be an additional means to target ubiquitylated substrates for degradation.

The acidic tail domain of human Cdc34 is required for p27Kip1 ubiquitination and complementation of a cdc34 temperature sensitive yeast strain.

Human Cdc34 is an ubiquitin conjugating enzyme or E2 that ubiquitinates substrates including p27(Kip1), IkappaBalpha, Wee1, and MyoD. Cdc34 possesses a core catalytic domain encoding the active site cysteine and an acidic tail domain within the carboxyl terminal 36 amino acids. Studies suggest that Cdc34 is phosphorylated in mammalian cells at 5 potential residues within the tail domain. In order to study the biological significance of the Cdc34 acidic tail domain and the possible significance of phosphorylation within this region, we tested the ability of human Cdc34 mutants to complement the cdc34-2 temperature sensitive (ts) strain of Saccharomyces cerevisiae. Our studies indicated that complementation of the cdc34-2 ts strain was critically dependent upon the carboxyl-terminal 36 amino acids of human Cdc34, but did not require phosphorylation of human Cdc34 residues S203, S222, S231, T233, and S236. Further studies demonstrated that although a Cdc34 mutant bearing a deletion of the C-terminal 36 amino acids (Cdc34 1-200) was efficiently charged with ubiquitin by E1, it was severely reduced for the ability to ubiquitinate p27(Kip1) in vitro compared to wildtype Cdc34. Both in vivo and in vitro binding studies indicated that Cdc34 1-200 bound to the E3-SCF components, Cul1 and Roc1, at levels comparable to the wildtype Cdc34. These studies suggest that the 36 amino acid acidic tail domain of human Cdc34 is critical for its ability to transfer ubiquitin to a substrate and is dispensable for the association of Cdc34 with Cul1 and Roc1. We postulate that the tail domain of Cdc34 may be important for its efficient dissociation from Cul1 and Roc1, an essential requirement for ubiquitination by the budding yeast Cdc34p, or it may be required more directly for ubiquitin transfer to the substrate.

Deregulated degradation of the cdk inhibitor p27 and malignant transformation.

p27 acts as a critical negative regulator of the cell cycle by inhibiting the activity of cyclin/cdk complexes during G0 and G1. Degradation of p27 is a critical event for the G1/S transition and occurs through ubiquitination by SCF(Skp2) and subsequent degradation by the 26S-proteasome. A tumor suppressing function of p27 has been demonstrated in mouse models and studies of human tumors. More recent evidence suggests that Skp2, the specific recognition factor for p27 ubiquitination, has oncogenic properties. This review will focus on the regulation of p27 proteolysis and its consequences for tumorigenesis.

Proteasome-mediated degradation of p21 via N-terminal ubiquitinylation.

We examined the mechanism responsible for the degradation of p21, a negative regulator of the cell division cycle. We found that p21 proteolysis requires functional ubiquitin and Nedd8 systems. Ubiquitinylated forms of p21 and p21(K0), a p21 mutant missing all lysines, are detected in vivo and in vitro, showing that the presence of lysines is dispensable for p21 ubiquitinylation. Instead, the free amino group of the N-terminal methionine of p21 is a site for ubiquitinylation in vivo. Although wild-type p21 is more abundantly ubiquitinylated than p21(K0) mutant due to the presence of internal lysine residues, their rates of proteolysis are indistinguishable. These results demonstrate that proteasomal degradation of p21 is regulated by the ubiquitin pathway and suggest that the site of the ubiquitin chain is critical in making p21 a competent substrate for the proteasome.

An Rb-Skp2-p27 pathway mediates acute cell cycle inhibition by Rb and is retained in a partial-penetrance Rb mutant.

It is believed that Rb blocks G1-S transition by inhibiting expression of E2F regulated genes. Here, we report that the effects of E2F repression lag behind the onset of G1 cell cycle arrest in timed Rb reexpression experiments. In comparison, kinase inhibitor p27Kip1 protein accumulates with a faster kinetics. Conversely, Rb knockout leads to faster p27 degradation. Rb interacts with the N terminus of Skp2, interferes with Skp2-p27 interaction, and inhibits ubiquitination of p27. Disruption of p27 function or expression of the Skp2 N terminus prevents Rb from causing G1 arrest. A full-penetrance, inactive Rb mutant fails to interfere with Skp2-p27 interaction but, interestingly, a partial-penetrance Rb mutant that is defective for E2F binding retains full activity in inhibiting Skp2-p27 interaction and can induce G1 cell cycle arrest with wild-type kinetics. These results identify an Rb-Skp2-p27 pathway in Rb function, which may be involved in inhibition of tumor progression.

Role of the SCFSkp2 ubiquitin ligase in the degradation of p21Cip1 in S phase.

The cyclin-dependent kinase inhibitor p21Cip1 has important roles in the control of cell proliferation, differentiation, senescence, and apoptosis. It has been observed that p21 is a highly unstable protein, but the mechanisms of its degradation remained unknown. We show here that p21 is a good substrate for an SCF (Skp1-Cullin1-F-box protein) ubiquitin ligase complex, which contains the F-box protein Skp2 (S phase kinase-associated protein 2) and the accessory protein Cks1 (cyclin kinase subunit 1). A similar ubiquitin ligase complex has been previously shown to be involved in the degradation of a related cyclin-dependent kinase inhibitor, p27Kip1. The levels of Skp2 oscillate in the cell cycle, reaching a maximum in S phase. The ubiquitylation of p21 in vitro required the supplementation of all components of the SCF complex as well as of Cks1 and Cdk2-cyclin E. The protein kinase Cdk2-cyclin E acts both by the phosphorylation of p21 on Ser-130 and by the formation of a complex with p21, which is required for its presentation to the ubiquitin ligase. As opposed to the case of p27, the phosphorylation of p21 stimulates its ubiquitylation but is not absolutely required for this process. Levels of p21 are higher in Skp2-/- mouse embryo fibroblasts than in wild-type fibroblasts in the S phase, and the rates of the degradation of p21 are slower in cells that lack Skp2. It is suggested that SCFSkp2 participates in the degradation of p21 in the S phase.