A previously uncharacterized gene, DBC2 (deleted in breast cancer), was cloned from a homozygously deleted region at human chromosome 8p21. DBC2 contains a highly conserved RAS domain and two putative protein interacting domains. Our analyses indicate that DBC2 is the best candidate tumor suppressor gene from this region. It lies within the epicenter of the deletions and is homozygously deleted in 3.5% (7͞200) of breast tumors. Mutation analysis of DBC2 led to discovery of two instances of somatic missense mutations in breast tumor specimens, whereas no missense mutations were found in other candidates from the region. Unlike other genes in the region, expression of DBC2 is often extinguished in breast cancer cells or tissues. Moreover, our functional analysis revealed that DBC2 expression in breast cancer cells lacking DBC2 transcripts causes growth inhibition. By contrast, expression of a somatic mutant discovered in a breast cancer specimen does not suppress the growth of breast cancer cells.

The regulated oscillation of protein expression is an essential mechanism of cell cycle control. The SCF class of E3 ubiquitin ligases is involved in this process by targeting cell cycle regulatory proteins for degradation by the proteasome, with the F-box subunit of the SCF specifically recruiting a given substrate to the SCF core. Here we identify NIPA (nuclear interaction partner of ALK) as a human F-box-containing protein that defines an SCF-type E3 ligase (SCF(NIPA)) controlling mitotic entry. Assembly of this SCF complex is regulated by cell-cycle-dependent phosphorylation of NIPA, which restricts substrate ubiquitination activity to interphase. We show nuclear cyclin B1 to be a substrate of SCF(NIPA). Inactivation of NIPA by RNAi results in nuclear accumulation of cyclin B1 in interphase, activation of cyclin B1-Cdk1 kinase activity, and premature mitotic entry. Thus, SCF(NIPA)-based ubiquitination may regulate S-phase completion and mitotic entry in the mammalian cell cycle.

Anaplastic large-cell lymphoma is a subtype of nonHodgkin lymphomas characterized by the expression of CD30. More than half of these lymphomas carry a chromosomal translocation t(2;5) leading to expression of the oncogenic tyrosine kinase nucleophosmin-anaplastic lymphoma kinase (NPM-ALK). NPM-ALK is capable of transforming fibroblasts and lymphocytes in vitro and of causing lymphomas in mice. Previously, we and others demonstrated phospholipase C-␥ and phosphatidylinositol 3-kinase as crucial downstream signaling mediators of NPM-ALK-induced oncogenicity. In this study, we used an ALK fusion protein as bait in a yeast twohybrid screen identifying NIPA (nuclear interacting partner of ALK) as a novel downstream target of NPM-ALK. NIPA encodes a 60-kDa protein that is expressed in a broad range of human tissues and contains a classical nuclear translocation signal in its C terminus, which directs its nuclear localization. NIPA interacts with NPM-ALK and other ALK fusions in a tyrosine kinasedependent manner and is phosphorylated in NPM-ALKexpressing cells on tyrosine and serine residues with serine 354 as a major phosphorylation site. Overexpression of NIPA in Ba/F3 cells was able to protect from apoptosis induced by IL-3 withdrawal. Mutations of the nuclear translocation signal or the Ser-354 phosphorylation site impaired the antiapoptotic function of NIPA. In NPM-ALK-transformed Ba/F3 cells, apoptosis triggered by wortmannin treatment was enhanced by overexpression of putative dominant-negative NIPA mutants. These results implicate an antiapoptotic role for NIPA in NPM-ALK-mediated signaling events.

Nuclear interaction partner of ALK (NIPA) is an F-box-containing protein that defines a nuclear skp1 cullin F-box (SCF)-type ubiquitin E3 ligase (SCF NIPA ) implicated in the regulation of mitotic entry. The SCF NIPA complex targets nuclear cyclin B1 for ubiquitination in interphase, whereas phosphorylation of NIPA in late G 2 phase and mitosis inactivates the complex to allow for accumulation of cyclin B1. Here, we identify the region of NIPA that mediates binding to its substrate cyclin B1. In addition to the recently described serine residue 354, we specify 2 new residues, Ser-359 and Ser-395, implicated in the phosphorylation process at G 2 /M within this region. Moreover, we found cyclin B1/Cdk1 to phosphorylate NIPA at Ser-395 in mitosis. Mutation of both Ser-359 and Ser-395 impaired effective inactivation of the SCF NIPA complex, resulting in reduced levels of mitotic cyclin B1. These data are compatible with a process of sequential NIPA phosphorylation where cyclin B1/Cdk1 amplifies phosphorylation of NIPA once an initial phosphorylation event has dissociated the SCF NIPA complex. Thus, cyclin B1/Cdk1 may contribute to the regulation of its own abundance in early mitosis.The SCF 3 family of E3 ubiquitin ligases essentially regulates the abundance of key cell cycle regulatory proteins. The F-box protein is the only variable component of the SCF complex that mediates substrate binding and thus determines specificity of the respective SCF complex (1-4). To bind the respective ubiquitylation target, each F-box protein contains a protein interaction domain. Depending on the type of the interaction domain, F-box proteins have been classified into three major classes; two classes of F-box proteins contain WD40 repeats and leucine-rich repeats (5, 6), whereas a third class contains other types of protein interaction domains or no canonical interaction domain (reviewed in Ref. (4). These classes of F-box proteins have been termed FBXWs, FBXLs and FBXOs, respectively (4, 7).We previously reported the cloning of NIPA (nuclear interaction partner of ALK) and subsequently characterized NIPA as an F-box-containing protein that defines an ubiquitin E3 ligase (SCF NIPA ) that targets nuclear cyclin B1 in interphase, thus contributing to the timing of mitotic entry (8 -10). The oscillating activity of the SCF NIPA complex is governed by cell cycle-dependent inhibitory phosphorylation of NIPA in late G 2 phase that dissociates NIPA from the SCF core. This phosphorylation event was found to occur, at least in part, on Ser-354 (9). The identity of the kinase(s) responsible for NIPA phosphorylation at G 2 /M and during mitosis has so far remained elusive.The substrate of NIPA, cyclin B1, together with its associated kinase Cdk1, form the maturation-promoting factor that essentially regulates the transition from G 2 phase into mitosis. Although mitotic entry requires activity of cyclin B1/Cdk1, mitotic exit requires its destruction. At mitotic entry, regulation of the maturation-promoting factor occurs at two distinct levels:...

SUMMARYNIPA (nuclear interaction partner of ALK) is an F-box-like protein that monitors the timing of mitotic entry. Constitutively active NIPA delays mitotic entry by preventing accumulation of nuclear cyclin B1. Here, we have investigated the consequences of Nipa inactivation by using a conditional knockout strategy. Nipa-deficient animals are viable but show a lower birth rate and reduced body weight. Furthermore, Nipa-deficient males are sterile owing to a block of spermatogenesis during meiotic prophase. Whereas Nipa -/-mouse embryonic fibroblasts show no severe phenotype, Nipa -/-spermatocytes arrest during stage IV of the epithelial cycle with subsequent TUNEL-positive apoptosis resulting from improper synapsis, defects in the repair of DNA double-stranded breaks and synaptonemal complex formation. Moreover, we show nuclear accumulation of cyclin B1 with a subsequent premature increase in G 2 /M kinase activity in Nipa -/-spermatocytes. Together, these results reveal a novel role for NIPA in meiosis.

NIPA (Nuclear Interaction Partner of Alk kinase) is an F-box like protein that targets nuclear Cyclin B1 for degradation. Integrity and therefore activity of the SCFNIPA E3 ligase is regulated by cell-cycle-dependent phosphorylation of NIPA, restricting substrate ubiquitination to interphase. Here we show that phosphorylated NIPA is degraded in late mitosis in an APC/CCdh1-dependent manner. Binding of the unphosphorylated form of NIPA to Skp1 interferes with binding to the APC/C-adaptor protein Cdh1 and therefore protects unphosphorylated NIPA from degradation in interphase. Our data thus define a novel mode of regulating APC/C-mediated ubiquitination.

Ubiquitin-mediated destruction of regulatory proteins marks the vital means of controlling cell cycle progresssion. The E3 ubiquitin-ligases are prominent in this process, as they allow the transfer of ubiquitin to the target protein and mediate substrate binding specificity. Recently, a new class of E3 ligases referred to as SCF complexes has been identified that consists of four subunits:SKP1, Cul1, Roc1 and an F-box protein, the latter of which determines substrate specifity. We previously reported the cloning of NIPA (nuclear interaction partner of ALK) in complex with constitutively-active oncogenic fusions of ALK, which contribute to the development of certain lymphomas and sarcomas. Subsequently we characterized NIPA as a human F-box protein that determines a novel SCF complex (SCFNIPA) whose cell cycle regulated activity is restricted to interphase to allow for substrate expression at G2/M and mitosis. Phosphorylation of NIPA in late S-phase was found to be the underlying mechanism of SCFNIPA inactivation. We have now identified the key mitotic regulator cyclin B1 to serve as the relevant substrate of the SCFNIPA complex. This targeting process is restricted to interphase and directed towards the nuclear pool of cyclin B1. Inactivation of NIPA by siRNAs results in nuclear accumulation of cyclin B1 in interphase and an elevation of cells in S-phase and mitosis. In contrast, expression of a phosphorylation deficient NIPA mutant that retains constitutive SCFNIPA activity throughout the cell cycle arrests cells at early prophase thus delaying mitotic entry. Both effects are likely attributable to either cyclin B1 accumulation in the case of NIPA inactivation by siRNA or untimely cyclin B degradation at G2/M upon expression of the constitutively active SCFNIPA complex. Cyclin B1 is physiologically kept cytoplasmic during interphase and premature nuclear accumulation has been associated with untimely mitotic entry, loss of checkpoint control and genomic instability. Our data provides a mechanism to inhibit premature nuclear accumulation of cyclin B1 in the mammalian cell cycle. NIPAs association with NPM-ALK of ALCL has been shown to be associated with NIPA phosphorylation and thus to the inactivation of the SCFNIPA complex. The mechanism described above may therefore provide a framework for understanding how this oncogene interferes with the physiologic regulation of cyclin B - a potential mechanism by which NPM-ALK transforms hematopoietic cells.

Regulated oscillation of protein expression is an essential mechanism of cell cycle control. The SCF class of E3 ubiquitin ligases is involved in this process by targeting cell cycle regulatory proteins for degradation by the proteasome. We previously reported the cloning of NIPA (Nuclear Interaction Partner of ALK) in complex with constitutively active oncogenic fusions of ALK, which contributes to the development of lymphomas and sarcomas. Subsequently we characterized NIPA as a F-Box protein that defines an oscillating ubiquitin E3 ligase targeting nuclear cyclin B1 in interphase thus contributing to the timing of mitotic entry. Using a conditional knockout strategy we inactivated the gene encoding Nipa. Nipa-deficient animals are viable, but show a lower birth rate and a reduced body weight. Furthermore, Nipa-deficient males were sterile due to a block of spermatogenesis during meiotic prophase. Virtually no spermatocytes progress beyond a late-zygotene to early-pachytene stage and reach an aberrant stage, with synaptonemal complex disassembly and incomplete synapsis. Nipa-/- females are sub-fertile with an early and severe meiotic defect during embryogenesis with extensive apoptosis in early prophase (E13.5-E14.5). Here we report, that Nipa-/- meiocytes exhibit persistent cytological markers for DNA double strand break repair proteins (like DMC1, RAD51) in meiotic prophase with more than twice as many DMC1 foci as control animals. Kinetic analysis of the first wave of spermatogenesis showed increased DMC1/RAD51 foci in Nipa-/- cells as soon as early-pachynema cells appear (13-14 days post partum). Moreover, we show that Nipa deficiency does not lead to a defect in meiotic sex chromosome inactivation despite epithelial stage IV apoptosis. Nipa-deficient spermatocytes exhibit numerous abnormalities in staining of chromosome axis associated proteins (like SYCP3 and STAG3) indicating that chromosome axis defects were associated with compromised chromosome axis integrity leading to overt chromosome fragmentation. Further in vitro analyses with bleomycin treated MEFs displayed high pH2AX levels in cells lacking NIPA. Repair of DNA DSB seemed to be abolished in these cells as the pH2AX-level were sustained and still visible after 90 min of timecourse, where wildtype cells already repaired sides of DNA Damage. Consistent with these findings NIPA-deficient spleen cells showed compromised DNA Damage repair measured in a comet assay with a significantly longer olive tail moment in NIPA knockout cells under repair conditions. Taken together, the phenotype of Nipa-knockout mice is a definitive proof of the meiotic significance of NIPA and our results show a new, unsuspected role of NIPA in chromosome stability and the repair of DNA double strand breaks. Disclosures: No relevant conflicts of interest to declare.