Deletion of the Cul1 gene in mice causes arrest in early embryogenesis and accumulation of cyclin E

Ubiquitin-mediated proteolysis of vertebrate G1- and S-phase regulators

Cell-cycle progression in all eukaryotes is driven by cyclin-dependent kinases (CDKs) and their cyclin partners. In vertebrates, the proper and timely duplication of the genome during S-phase relies on the coordinated activities of positive regulators such as CDK-cyclins and E2F, and negative regulators such as CDK inhibitors of the Cip/Kip and INK4 families. Recent and ongoing work indicates that many important regulators of G1- and S-phases are targeted for ubiquitination and subsequent degradation by the 26S proteasome. The proteolysis of key proteins during G1- and S-phases appears to be central for proper custodial regulation of DNA replication and the maintenance of cellular homeostasis in general. This review highlights the current literature regarding ubiquitin-mediated proteolysis of G1- and S-phase regulators and the control of events during the initiation and completion of DNA replication in vertebrates.

A CDK-independent function of mammalian Cks1: targeting of SCF(Skp2) to the CDK inhibitor p27Kip1

The Cks/Suc1 proteins associate with CDK/cyclin complexes, but their precise function(s) is not well defined. Here we demonstrate that Cks1 directs the ubiquitin-mediated proteolysis of the CDK-bound substrate p27Kip1 by the protein ubiquitin ligase (E3) SCF(Skp2). Cks1 associates with the F box protein Skp2 and is essential for recognition of the p27Kip1 substrate for ubiquitination in vivo and in vitro. Using purified recombinant proteins, we reconstituted p27Kip1 ubiquitination activity and show that it is dependent on Cks1. CKS1-/- mice are abnormally small, and cells derived from them proliferate poorly, particularly under limiting mitogen conditions, possibly due to elevated levels of p27Kip1.

Regulation of the G1 to S transition by the ubiquitin pathway

This year the most prestigious prize in medical sciences, the Lasker Award, has been presented to the three scientists who discovered the ubiquitin pathway: Aaron Ciechanover, Avram Hershko, and Alexander Varshavsky [Nature Med. 6 (2000) 1073-1081]. During a time when the scientific community was focused on understanding how proteins were synthesized, they intently pursued the novel idea that cells were programmed to selectively destroy proteins. Their work led to the identification of an elaborate system of protein degradation targeting a myriad of cellular substrates. A small protein called ubiquitin is at the center of this process. Although the ubiquitin pathway was first described in the early 1980s, it has only more recently advanced to the forefront of basic research as a significant regulatory network within the cell. The field continues to grow as new ubiquitination enzymes and novel functions of this system are identified. Scientists are focused on elucidating the mechanisms by which cells deploy the ubiquitin pathway to control levels of selected proteins, such as cell cycle regulatory proteins, transcription factors and signaling molecules. Accelerated or decelerated rates of degradation of particular substrates participate in the genesis of many human diseases. Thus, understanding the mechanisms that confer specificity to the ubiquitin system will allow the development of novel therapeutic approaches to target aberrations in this pathway underlying tumorigenesis and other human pathologies.

Regulation of G(1) cyclin-dependent kinases in the mammalian cell cycle

Cyclin-dependent kinases are the key regulators of cell-cycle transitions. In mammalian cells, Cdk2, Cdk4, Cdk6 and associated cyclins control the G(1) to S phase transition. Because proper regulation of this transition is critical for an organism's survival, these protein kinases are exquisitely regulated at different mechanistic levels and in response to a large variety of intrinsic and extrinsic signals.

Myc-enhanced expression of Cul1 promotes ubiquitin-dependent proteolysis and cell cycle progression

The c-Myc oncoprotein plays an important role in the growth and proliferation of normal and neoplastic cells. To execute these actions, c-Myc is thought to regulate functionally diverse sets of genes that directly govern cellular mass and progression through critical cell cycle transitions. Here, we provide several lines of evidence that c-Myc promotes ubiquitin-dependent proteolysis by directly activating expression of the Cul1 gene, encoding a critical component of the ubiquitin ligase SCF(SKP2). The cell cycle inhibitor p27(kip1) is a known target of the SCF(SKP2) complex, and Myc-induced Cul1 expression matched well with the kinetics of declining p27(kip1) protein. Enforced Cul1 expression or antisense neutralization of p27(kip1) was capable of overcoming the slow-growth phenotype of c-Myc null primary mouse embryonic fibroblasts (MEFs). In reconstitution assays, the addition of in vitro translated Cul1 protein alone was able to restore p27(kip1) ubiquitination and degradation in lysates derived from c-myc(-/-) MEFs or density-arrested human fibroblasts. These functional and biochemical data provide a direct link between c-Myc transcriptional regulation and ubiquitin-mediated proteolysis and together support the view that c-Myc promotes G(1) exit in part via Cul1-dependent ubiquitination and degradation of the CDK inhibitor, p27(kip1).

Early development of mouse embryos null mutant for the cyclin A2 gene occurs in the absence of maternally derived cyclin A2 gene products

Progression through the mammalian cell cycle is regulated by the sequential activation and inactivation of the cyclin-dependent kinases. In adult cells, cyclin A2-dependent kinases are required for entry into S and M phases, completion of S phase, and centrosome duplication. However, mouse embryos lacking the cyclin A2 gene nonetheless complete preimplantation development, but die soon after implantation. In this report, we investigated whether a contribution of maternal cyclin A2 mRNA and protein to early embryonic cell cycles might explain these conflicting observations. Our data show that a maternal stock of cyclin A2 mRNA is present in the oocyte and persists after fertilization until the second mitotic cell cycle, when it is degraded to undetectable levels coincident with transcriptional activation of the zygotic genome. A portion of maternally derived cyclin A2 protein is stable during the first mitosis and persists in the cytoplasm, but is completely degraded at the second mitosis. The ability of cyclin A2-null mutants to develop normally from the four-cell to the postimplantation stage in the absence of detectable cyclin A2 gene product indicates therefore that cyclin A2 is dispensable for cellular progression during the preimplantation nongrowth period of mouse embryo development.

Characterization of Cdk2-cyclin E complexes in plasma membrane and endosomes of liver parenchyma. Insulin-dependent regulation

Rat liver parenchyma Golgi/endosomes fractions harbor a tyrosine-phosphorylated 34-kDa protein. Screening of Golgi, endosomes (ENs), plasmalemma (PM), and cytosolic (Cyt) fractions revealed the presence of the mitotic kinase Cdk2 in ENs, PM, and Cyt. The fluid phase endocytic marker horseradish peroxidase gained access to the endosomal Cdk2, confirming its localization. Cdk2 was shown to be associated to cyclin E and was active in ENs and PM fractions. The administration of a single dose of insulin (1.5 microgram/100 g, body weight) induced a time-dependent activation of the insulin receptor kinase in these structures. Insulin receptor-kinase activation was followed by the inhibition of immunoprecipitated Cdk2-cyclin E kinase activity in PM and the progressive disappearance of cyclin E. In marked contrast, no such effect was observed in ENs. The injection of a phosphotyrosyl phosphatase inhibitor (bpV(phen)) increased the levels of cyclin E in ENs and PM. A massive recruitment of p27(kip1) was observed in the Cdk2-cyclin E complexes isolated from PM and Cyt but not from ENs. In vitro, Cdk2-cyclin E complexes have the capacity to inhibit the formation of hybrid structures containing horseradish peroxidase and radioiodinated epidermal growth factor. Therefore, in the PM and ENs of adult rat liver, an active and regulated pool of the mitotic kinase Cdk2-cyclin E and some yet to be defined effectors are present. Cdk2 may contribute to the modulation of transport events and/or maintenance of the topology of endocytic elements.

Targeted disruption of Skp2 results in accumulation of cyclin E and p27(Kip1), polyploidy and centrosome overduplication

The ubiquitin-proteasome pathway plays an important role in control of the abundance of cell cycle regulators. Mice lacking Skp2, an F-box protein and substrate recognition component of an Skp1-Cullin-F-box protein (SCF) ubiquitin ligase, were generated. Although Skp2(-/-) animals are viable, cells in the mutant mice contain markedly enlarged nuclei with polyploidy and multiple centrosomes, and show a reduced growth rate and increased apoptosis. Skp2(-/-) cells also exhibit increased accumulation of both cyclin E and p27(Kip1). The elimination of cyclin E during S and G(2) phases is impaired in Skp2(-/-) cells, resulting in loss of cyclin E periodicity. Biochemical studies showed that Skp2 interacts specifically with cyclin E and thereby promotes its ubiquitylation and degradation both in vivo and in vitro. These results suggest that specific degradation of cyclin E and p27(Kip1) is mediated by the SCF(Skp2) ubiquitin ligase complex, and that Skp2 may control chromosome replication and centrosome duplication by determining the abundance of cell cycle regulators.

Ubiquitination of free cyclin D1 is independent of phosphorylation on threonine 286

Cyclin D1 binds and regulates the activity of cyclin-dependent kinases (CDKs) 4 and 6. Phosphorylation of the retinoblastoma protein by cyclin D1.CDK4/6 complexes during the G(1) phase of the cell cycle promotes entry into S phase. Cyclin D1 protein is ubiquitinated and degraded by the 26 S proteasome. Previous studies have demonstrated that cyclin D1 ubiquitination is dependent on its phosphorylation by glycogen synthase kinase 3beta (GSK-3beta) on threonine 286 and that this phosphorylation event is greatly enhanced by binding to CDK4 (Diehl, J. A., Cheng, M. G., Roussel, M. F., and Sherr, C. J. (1998) Genes Dev. 12, 3499-3511). We now report an additional pathway for the ubiquitination of free cyclin D1 (unbound to CDKs). We show that, when unbound to CDK4, a cyclin D1-T286A mutant is ubiquitinated. Further, we show that a mutant of cyclin D1 that cannot bind to CDK4 (cyclin D1-KE) is also ubiquitinated in vivo. Our results demonstrate that free cyclin D1 is ubiquitinated independently of its phosphorylation on threonine 286 by GSK-3beta, suggesting that, as has been shown for cyclin E, distinct pathways of ubiquitination lead to the degradation of free and CDK-bound cyclin D1. The pathway responsible for ubiquitination of free cyclin D1 may be important in limiting the effects of cyclin D1 overexpression in a variety of cancers.

Periodic expression of the cyclin-dependent kinase inhibitor p57(Kip2) in trophoblast giant cells defines a G2-like gap phase of the endocycle

Endoreduplication is an unusual form of cell cycle in which rounds of DNA synthesis repeat in the absence of intervening mitoses. How G1/S cyclin-dependent kinase (Cdk) activity is regulated during the mammalian endocycle is poorly understood. We show here that expression of the G1/S Cdk inhibitor p57(Kip2) is induced coincidentally with the transition to the endocycle in trophoblast giant cells. Kip2 mRNA is constitutively expressed during subsequent endocycles, but the protein level fluctuates. In trophoblast giant cells synchronized for the first few endocycles, the p57(Kip2) protein accumulates only at the end of S-phase and then rapidly disappears a few hours before the onset of the next S-phase. The protein becomes stabilized by mutation of a C-terminal Cdk phosphorylation site. As a consequence, introduction of this stable form of p57(Kip2) into giant cells blocks S-phase entry. These data imply that p57(Kip2) is subject to phosphorylation-dependent turnover. Surprisingly, although this occurs in endoreduplicating giant cells, p57(Kip2) is stable when ectopically expressed in proliferating trophoblast cells, indicating that these cells lack the mechanism for protein targeting and/or degradation. These data show that the appearance of p57(Kip2) punctuates the completion of DNA replication, whereas its turnover is subsequently required to initiate the next round of endoreduplication in trophoblast giant cells. Cyclical expression of a Cdk inhibitor, by terminating G1/S Cdk activity, may help promote the resetting of DNA replication machinery.

Proteolysis and the cell cycle: with this RING I do thee destroy

The ubiquitin system drives the cell division cycle by the timely destruction of numerous regulatory proteins. Remarkably, the two main activities that catalyze substrate ubiquitination in the cell cycle, the Skp1-Cdc53/cullin-F-box protein (SCF) complexes and the anaphase-promoting complex/cyclosome (APC/C), define a new superfamily of E3 ubiquitin ligases, all based on related cullin and RING-H2 finger protein subunits. The circuits that interconnect the SCF, APC/C and cyclin-dependent kinase activities form a master oscillator that coordinates the replication and segregation of the genome.

Proteasomal turnover of p21Cip1 does not require p21Cip1 ubiquitination

The Cdk inhibitor p21Cip1 is an unstable protein. Pharmacologic inhibition of the proteasome increases the half-life of p21 from less than 30 min to more than 2 hr and results in the accumulation of p21-ubiquitin conjugates. To determine whether ubiquitination was required for proteasomal degradation of p21, we constructed mutant versions of p21 that were not ubiquitinated in vivo. Remarkably, these mutants remained unstable and increased in abundance upon proteasome inhibition, indicating that direct ubiquitination of p21 is not necessary for its turnover by the proteasome. The frequently observed correlation between protein ubiquitination and proteasomal degradation is insufficient to conclude that ubiquitination is a prerequisite for degradation.