Biomedical subjects

Markus Welcker

Publications and source records attributed to Markus Welcker.

10 recordsLinked to original sources

PubMed2007

Kinase-independent function of cyclin E.

E-type cyclins are thought to drive cell-cycle progression by activating cyclin-dependent kinases, primarily CDK2. We previously found that cyclin E-null cells failed to incorporate MCM helicase into DNA prereplication complex during G(0) --> S phase progression. We now report that a kinase-deficient cyclin E mutant can partially restore MCM loading and S phase entry in cyclin E-null cells. We found that cyclin E is loaded onto chromatin during G(0) --> S progression. In the absence of cyclin E, CDT1 is normally loaded onto chromatin, whereas MCM is not, indicating that cyclin E acts between CDT1 and MCM loading. We observed a physical association of cyclin E with CDT1 and with MCMs. We propose that cyclin E facilitates MCM loading in a kinase-independent fashion, through physical interaction with CDT1 and MCM. Our work indicates that-in addition to their function as CDK activators-E cyclins play kinase-independent functions in cell-cycle progression.

Animals↗

PubMed2005

Zcchc8 is a glycogen synthase kinase-3 substrate that interacts with RNA-binding proteins.

Phosphorylation of c-Myc on threonine 58 (T58) stimulates its degradation by the Fbw7-SCF ubiquitin ligase. We used a phosphorylation-specific antibody raised against the c-Myc T58 region to attempt to identify other proteins regulated by the Fbw7 pathway. We identified two predominant proteins recognized by this antibody. The first is Ebna1 binding protein 2, a nucleolar protein that, in contrast with a previous report, is likely responsible for the nucleolar staining exhibited by this antibody. The second is Zcchc8, a nuclear protein that is highly phosphorylated in cells treated with nocodazole. We show that Zcchc8 is directly phosphorylated by GSK-3 in vitro and that GSK-3 inhibition prevents Zcchc8 phosphorylation in vivo. Moreover, we found that Zcchc8 interacts with proteins involved in RNA processing/degradation. We suggest that Zcchc8 is a GSK-3 substrate with a role in RNA metabolism.

Carrier Proteins↗

PubMed2005

Cell cycle: how cyclin E got its groove back.

CDK1 has long been known to orchestrate the passage of mammalian cells into and through mitosis. Recent work revisits the idea that CDK1, in conjunction with cyclin E, participates in S-phase entry as well. The new results shed light on a recent cell-cycle mystery, and provide another dramatic example of apparent functional redundancy among cyclins and cyclin-dependent kinases.

Animals↗

PubMed2005

Ras activity regulates cyclin E degradation by the Fbw7 pathway.

The Skp1-Cullin1 F-box protein-Fbw7 ubiquitin ligase regulates phosphorylation-dependent cyclin E degradation, and disruption of this pathway is associated with genetic instability and tumorigenesis. Fbw7 is a human tumor suppressor that is targeted for mutation in primary cancers. However, mechanisms other than mutation of Fbw7 may also disrupt cyclin E proteolysis in cancers. We show that oncogenic Ha-Ras activity regulates cyclin E degradation by the Fbw7 pathway. Activated Ras impairs Fbw7-driven cyclin E degradation, and, conversely, inhibition of normal Ras activity decreases cyclin E abundance. Moreover, activation of the mitogen-activated protein kinase pathway is the essential Ras function that inhibits cyclin E turnover, and activated Ha-Ras expression inhibits both the binding of cyclin E to Fbw7 and cyclin E ubiquitination. Last, we found that oncogenic Ras activity potentiates cyclin E-induced genetic instability but only when cyclin E is susceptible to degradation by Fbw7. Thus, we conclude that Ras activity regulates Fbw7-mediated cyclin E proteolysis and suggest that impaired cyclin E proteolysis is a mechanism through which Ras mutations promote tumorigenesis.

Animals↗

PubMed2004

The SV40 large T antigen contains a decoy phosphodegron that mediates its interactions with Fbw7/hCdc4.

Cell transformation by simian virus 40 (SV40) results mostly from the highly oncogenic activities of the large T antigen (LT), which corrupts the cellular checkpoint mechanisms that guard cell division and the transcription, replication, and repair of DNA. The most prominent LT targets are the retinoblastoma protein (pRb) and p53. Here we report that LT binds directly to Fbw7, the substrate recognition component of the SCF(Fbw7) ubiquitin ligase and a human tumor suppressor. LT binding mislocalizes the nucleolar Fbw7gamma isoform to the nucleoplasm. Interestingly, the binding of LT to Fbw7 occurs via a decoy phospho-epitope within the C terminus of LT that closely mimics the consensus Cdc4 phospho-degron found within Fbw7 substrates. We demonstrate that, using this mode of interaction, LT can interfere with Fbw7-driven cyclin E turnover in vivo and causes increased cyclin E-associated kinase activity. Our data suggest that LT competes with cellular proteins for Fbw7 binding in a substrate-like fashion.

Amino Acid Sequence↗

PubMed2004

A nucleolar isoform of the Fbw7 ubiquitin ligase regulates c-Myc and cell size.

The human tumor suppressor Fbw7/hCdc4 functions as a phosphoepitope-specific substrate recognition component of SCF ubiquitin ligases that catalyzes the ubiquitination of cyclin E , Notch , c-Jun and c-Myc . Fbw7 loss in cancer may thus have profound effects on the pathways that govern cell division, differentiation, apoptosis, and cell growth. Fbw7-inactivating mutations occur in human tumor cell lines and primary cancers , and Fbw7 loss in cultured cells causes genetic instability . In mice, deletion of Fbw7 leads to embryonic lethality associated with defective Notch and cyclin E regulation . The human Fbw7 locus encodes three protein isoforms (Fbw7alpha, Fbw7beta, and Fbw7gamma) . We find that these isoforms occupy discrete subcellular compartments and have identified cis-acting localization signals within each isoform. Surprisingly, the Fbw7gamma isoform is nucleolar, colocalizes with c-Myc when the proteasome is inhibited, and regulates nucleolar c-Myc accumulation. Moreover, we find that knockdown of Fbw7 increases cell size consistent with its ability to control c-Myc levels in the nucleolus. We suggest that interactions between c-Myc and Fbw7gamma within the nucleolus regulate c-Myc's growth-promoting function and that c-Myc activation is likely to be an important oncogenic consequence of Fbw7 loss in cancers.

Cell Cycle Proteins↗

PubMed2004

Recognition of phosphodegron motifs in human cyclin E by the SCF(Fbw7) ubiquitin ligase.

Turnover of cyclin E is controlled by SCF(Fbw7). Three isoforms of Fbw7 are produced by alternative splicing. Whereas Fbw7alpha and -gamma are nuclear and the beta-isoform is cytoplasmic in 293T cells, all three isoforms induce cyclin E destruction in an in vivo degradation assay. Cyclin E is phosphorylated on Thr(62), Ser(88), Ser(372), Thr(380), and Ser(384) in vivo. To examine the roles of phosphorylation in cyclin E turnover, a series of alanine point mutations in each of these sites were analyzed for Fbw7-driven degradation. As expected, mutation of the previously characterized residue Thr(380) to alanine led to profound defects of cyclin E turnover, and largely abolished association with Fbw7. Mutation of Thr(62) to alanine led to a dramatic reduction in the extent of Thr(380) phosphorylation, suggesting an indirect effect of this mutation on cyclin E turnover. Nevertheless, phosphopeptides centered at Thr(62) associated with Fbw7, and residual binding of cyclin E(T380A) to Fbw7 was abolished upon mutation of Thr(62), suggesting a minor role for this residue in direct association with Fbw7. Mutation of Ser(384) to alanine also rendered cyclin E resistant to degradation by Fbw7, with the largest effects being observed with Fbw7beta. Cyclin E(S384A) associated more weakly with Fbw7alpha and -beta isoforms but was not defective in Thr(380) phosphorylation. Analysis of the localization of cyclin E mutant proteins indicated selective accumulation of cyclin E(S384A) in the nucleus, which may contribute to the inability of cytoplasmic Fbw7beta to promote turnover of this cyclin E mutant protein.

Amino Acid Motifs↗

PubMed2004

The Fbw7 tumor suppressor regulates glycogen synthase kinase 3 phosphorylation-dependent c-Myc protein degradation.

Myc proteins regulate cell growth and division and are implicated in a wide range of human cancers. We show here that Fbw7, a component of the SCF(Fbw7) ubiquitin ligase and a tumor suppressor, promotes proteasome-dependent c-Myc turnover in vivo and c-Myc ubiquitination in vitro. Phosphorylation of c-Myc on threonine-58 (T58) by glycogen synthase kinase 3 regulates the binding of Fbw7 to c-Myc as well as Fbw7-mediated c-Myc degradation and ubiquitination. T58 is the most frequent site of c-myc mutations in lymphoma cells, and our findings suggest that c-Myc activation is one of the key oncogenic consequences of Fbw7 loss in cancer. Because Fbw7 mediates the degradation of cyclin E, Notch, and c-Jun, as well as c-Myc, the loss of Fbw7 is likely to elicit profound effects on cell proliferation during tumorigenesis.

Base Sequence↗

PubMed2003

Multisite phosphorylation by Cdk2 and GSK3 controls cyclin E degradation.

Autophosphorylation-triggered ubiquitination has been proposed to be the major pathway regulating cyclin E protein abundance: phosphorylation of cyclin E on T380 by its associated CDK allows binding to the receptor subunit, Fbw7, of the SCFFbw7 ubiquitin ligase. We have tested this model in vivo and found it to be an inadequate representation of the pathways that regulate cyclin E degradation. We show that assembly of cyclin E into cyclin E-Cdk2 complexes is required in vivo for turnover by the Fbw7 pathway; that Cdk2 activity is required for cyclin E turnover in vivo because it phosphorylates S384; that phosphorylation of T380 in vivo does not require Cdk2 and is mediated primarily by GSK3; and that two additional phosphorylation sites, T62 and S372, are also required for turnover. Thus, cyclin E turnover is controlled by multiple biological inputs and cannot be understood in terms of autophosphorylation alone.

Animals↗

PubMed2002

p53 and p21 form an inducible barrier that protects cells against cyclin E-cdk2 deregulation.

BACKGROUND: Cyclin E, in conjunction with its catalytic partner cdk2, is rate limiting for entry into the S phase of the cell cycle. Cancer cells frequently contain mutations within the cyclin D-Retinoblastoma protein pathway that lead to inappropriate cyclin E-cdk2 activation. Although deregulated cyclin E-cdk2 activity is believed to directly contribute to the neoplastic progression of these cancers, the mechanism of cyclin E-induced neoplasia is unknown. RESULTS: We studied the consequences of deregulated cyclin E expression in primary cells and found that cyclin E initiated a p53-dependent response that prevented excess cdk2 activity by inducing expression of the p21Cip1 cdk inhibitor. The increased p53 activity was not associated with increased expression of the p14ARF tumor suppressor. Instead, cyclin E led to increased p53 serine15 phosphorylation that was sensitive to inhibitors of the ATM/ATR family. When either p53 or p21cip1 was rendered nonfunctional, then the excess cyclin E became catalytically active and caused defects in S phase progression, increased ploidy, and genetic instability. CONCLUSIONS: We conclude that p53 and p21 form an inducible barrier that protects cells against the deleterious consequences of cyclin E-cdk2 deregulation. A response that restrains cyclin E deregulation is likely to be a general protective mechanism against neoplastic transformation. Loss of this response may thus be required before deregulated cyclin E can become fully oncogenic in cancer cells. Furthermore, the combination of excess cyclin E and p53 loss may be particularly genotoxic, because cells cannot appropriately respond to the cell cycle anomalies caused by excess cyclin E-cdk2 activity.

CDC2-CDC28 Kinases↗