The RBCC gene RFP2 (Leu5) encodes a novel transmembrane E3 ubiquitin ligase involved in ERAD

RFP2, a gene frequently lost in various malignancies, encodes a protein with RING finger, B-box, and coiled-coil domains that belongs to the RBCC/TRIM family of proteins. Here we demonstrate that Rfp2 is an unstable protein with auto-polyubiquitination activity in vivo and in vitro, implying that Rfp2 acts as a RING E3 ubiquitin ligase. Consequently, Rfp2 ubiquitin ligase activity is dependent on an intact RING domain, as RING deficient mutants fail to drive polyubiquitination in vitro and are stabilized in vivo. Immunopurification and tandem mass spectrometry enabled the identification of several putative Rfp2 interacting proteins localized to the endoplasmic reticulum (ER), including valosin-containing protein (VCP), a protein indispensable for ER-associated degradation (ERAD). Importantly, we also show that Rfp2 regulates the degradation of the known ER proteolytic substrate CD3-delta, but not the N-end rule substrate Ub-R-YFP (yellow fluorescent protein), establishing Rfp2 as a novel E3 ligase involved in ERAD. Finally, we show that Rfp2 contains a C-terminal transmembrane domain indispensable for its localization to the ER and that Rfp2 colocalizes with several ER-resident proteins as analyzed by high-resolution immunostaining. In summary, these data are all consistent with a function for Rfp2 as an ERAD E3 ubiquitin ligase.     

The Transcription Map of the 13q14 Region Frequently Deleted in B-cell Chronic Lymphocytic Leukemia

Deletions in the region located between the STS markers D13S1168 and D13S25 on chromosome 13 are the most frequent genomic changes in patients with B-cell chronic lymphocytic leukemia (B-CLL). After sequencing of this region, two novel candidate genes were identified: C13orf1(chromosome 13 open reading frame 1) and PLCC (putative large CLL candidate). Analysis of the repeat distribution revealed two subregions differing in composition of repetitious DNA and gene organization. The interval D13S1168–D13S319 contains 131 Alu repeats accounting for 24.8% of its length, whereas the interval GCT16C05–D13S25, which is no more than 180 kb away from the former one is extremely poor in Alu repeats (4.1% of the total length). Both intervals contain almost the same amount of the LINE-type repeats L1 and L2 (20.3 and 21.24%, respectively). In the chromosomal region studied, 29 Alu repeats were found to belong to the evolutionary young subfamily Y, which is still capable of amplifying. A considerable proportion of repeats of this type with similar nucleotide sequences may contribute to the recombinational activity of the chromosomal region 13q14.3, which is responsible for its rearrangements in some tumors in humans.     

FBXO44 promotes DNA replication-coupled repetitive element silencing in cancer cells

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Interferon-alpha inhibits Stat5 DNA-binding in IL-2 stimulated primary T-lymphocytes.

It has previously been shown that IFN-alpha is a potent inhibitor of IL-2 induced proliferation in primary T-lymphocytes, by selectively abrogating the downstream effects of IL-2 on the core cell cycle machinery regulating the G1/S transition. Theoretically this could be mediated through cross-talk between the signalling cascades activated by these cytokines, as several signalling components are known to be shared. IL-2 activates multiple signalling pathways that are important for T-cell proliferation and differentiation. In the present study, the effects of IFN-alpha on IL-2 signal transduction was investigated. The IFN-alpha induced inhibition of IL-2 induced proliferation in activated T-lymphocytes, was associated with a suppressed Jak3 protein expression as well as an inhibited prolonged Stat5 DNA binding, and a partially reduced expression of the Stat5 inducible gene IL-2R alpha. Our results provide a possible molecular link between the prominent antiproliferative effects of IFN-alpha on IL-2 induced T-cell proliferation and the signal transduction pathways emerging from the IL-2 receptor.     

Ubiquitylation of cyclin E requires the sequential function of SCF complexes containing distinct hCdc4 isoforms

Cyclin E, an activator of cyclin-dependent kinase 2 (Cdk2), is targeted for proteasomal degradation by phosphorylation-dependent multiubiquitylation via the ubiquitin ligase SCF(hCdc4). SCF ubiquitin ligases are composed of a core of conserved subunits and one variable subunit (an F box protein) involved in substrate recognition. We show here that multiubiquitylation of cyclin E requires the sequential function of two distinct splice variant isoforms of the F box protein hCdc4 known as alpha and gamma. SCF(hCdc4alpha) binds a complex containing cyclin E, Cdk2, and the prolyl cis/trans isomerase Pin1 and promotes the activity of Pin1 without directly ubiquitylating cyclin E. However, due to the action of this SCF(hCdc4alpha)-Pin1 complex, cyclin E becomes an efficient ubiquitylation substrate of SCF(hCdc4gamma). Furthermore, in the context of Cdc4alpha and cyclin E, mutational data suggest that Pin1 isomerizes a noncanonical proline-proline bond, with the possibility that Cdc4alpha may serve as a cofactor for altering the specificity of Pin1.     

Interferon-alpha inhibits proliferation in human T lymphocytes by abrogation of interleukin 2-induced changes in cell cycle-regulatory proteins.

IFN-alpha exerts prominent regulatory functions on the immune system. One such effect is the inhibition of proliferation of in vitro stimulated T lymphocytes. The exact physiological function of this activity is not known, but it has been implicated in the antiviral effects of IFN, its antitumor action in T-cell malignancies, and the regulation of the in vivo T-cell response. Here, we have investigated the mechanism underlying the IFN-alpha-mediated growth inhibition of normal human PHA- and IL-2-stimulated T lymphocytes by an analysis of how IFN-alpha treatment influences known molecular events that normally accompany the transition from quiescence to proliferation in these cells. IFN-alpha treatment was found to profoundly block S-phase entry of stimulated T lymphocytes. This correlated with a strong inhibition of IL-2-induced changes in G1-regulatory proteins, including the prevented up-regulation of G1 cyclins and cyclin-dependent kinases as well as an abrogation of mitogen-induced reduction of p27Kip1 levels. This latter effect was due to a maintained stability of the p27Kip1 protein in the IFN-alpha-treated cells. In line with these findings, phosphorylation of the pocket proteins was abrogated in IFN-alpha-treated cells. Furthermore, our data indicate that IFN-alpha has selective effects on the pathways that emerge from the IL-2 receptor because IFN-alpha treatment does not block IL-2-induced up-regulation of c-myc or Cdc25A.     

Notch signaling induces SKP2 expression and promotes reduction of p27Kip1 in T-cell acute lymphoblastic leukemia cell lines

In T-cell acute lymphoblastic leukemia (T-ALL) NOTCH 1 receptors are frequently mutated. This leads to aberrantly high Notch signaling, but how this translates into deregulated cell cycle control and the transformed cell type is poorly understood. In this report, we analyze downstream responses resulting from the high level of NOTCH 1 signaling in T-ALL. Notch activity, measured immediately downstream of the NOTCH 1 receptor, is high, but expression of the canonical downstream Notch response genes HES 1 and HEY 2 is low both in primary cells from T-ALL patients and in T-ALL cell lines. This suggests that other immediate Notch downstream genes are activated, and we found that Notch signaling controls the levels of expression of the E3 ubiquitin ligase SKP2 and its target protein p27Kip1. We show that in T-ALL cell lines, recruitment of NOTCH 1 intracellular domain (ICD) to the SKP2 promoter was accompanied by high SKP2 and low p27Kip1 protein levels. In contrast, pharmacologically blocking Notch signaling reversed this situation and led to loss of NOTCH 1 ICD occupancy of the SKP2 promoter, decreased SKP2 and increased p27Kip1 expression. T-ALL cells show a rapid G1-S cell cycle transition, while blocked Notch signaling resulted in G0/G1 cell cycle arrest, also observed by transfection of p27Kip1 or, to a smaller extent, a dominant negative SKP2 allele. Collectively, our data suggest that the aberrantly high Notch signaling in T-ALL maintains SKP2 at a high level and reduces p27Kip1, leading to more rapid cell cycle progression.