Ubiquitin-dependent degradation of IkappaBalpha is mediated by a ubiquitin ligase Skp1/Cul 1/F-box protein FWD1

Abnormal retinal development in the Btrc null mouse

Previous microarray analysis revealed beta-transducin repeat containing (Btrc) down-regulation in the retina of mouse embryos specifically lacking cholinergic amacrine cells (CACs) as a result of the absence of skeletal musculature and fetal ocular movements. To investigate the role of Btrc in the determination of retinal cell fate, the present study examined retinal morphology in Btrc-/- mouse fetuses. The Btrc-/- retina showed a normal number of cell layers and number of cells per layer with normal cell proliferation and apoptosis. However, there was a complete absence of CACs and a decrease in tyrosine hydroxylase-expressing amacrine cells. The population of other amacrine cell subtypes was normal, whereas that of the precursor cells was decreased. There was also a reduction in the number of retinal ganglion cells, whereas their progenitors were increased. These findings suggest a role for Btrc in regulating the eventual ratio of resulting differentiated retinal cell types.

Adenovirus E1A inhibits SCF(Fbw7) ubiquitin ligase

The SCF(Fbw7) ubiquitin ligase complex plays important roles in cell growth, survival, and differentiation via the ubiquitin-proteasome-mediated regulation of protein stability. Fbw7 (also known as Fbxw7, Sel-10, hCdc4, or hAgo), a substrate recognition subunit of SCF(Fbw7) ubiquitin ligase, facilitates the degradation of several proto-oncogene products by the proteasome. Given that mutations in Fbw7 are found in various types of human cancers, Fbw7 is considered to be a potent tumor suppressor. In the present study, we show that E1A, an oncogene product derived from adenovirus, interferes with the activity of the SCF(Fbw7) ubiquitin ligase. E1A interacted with SCF(Fbw7) and attenuated the ubiquitylation of its target proteins in vivo. Furthermore, using in vitro purified SCF(Fbw7) component proteins, we found that E1A directly bound to Roc1/Rbx1 and CUL1 and that E1A inhibited the ubiquitin ligase activity of the Roc1/Rbx1-CUL1 complex but not that of another RING-type ubiquitin ligase, Mdm2. Ectopically expressed E1A interacted with cellular endogenous Roc1/Rbx1 and CUL1 and decelerated the degradation of several protooncogene products that were degraded by SCF(Fbw7) ubiquitin ligase. Moreover, after wild-type adenovirus infection, adenovirus-derived E1A interacted with endogenous Roc1/Rbx1 and decelerated degradation of the endogenous target protein of SCF(Fbw7). These observations demonstrated that E1A perturbs protein turnover regulated by SCF(Fbw7) through the inhibition of SCF(Fbw7) ubiquitin ligase. Our findings may help to explain the mechanism whereby adenovirus infection induces unregulated proliferation.

Ubiquitin-mediated control of oncogene and tumor suppressor gene products

Cellular levels of products from both oncogenes and tumor suppressor genes in normal cells need to be critically regulated to avoid malignant transformation. These products are often controlled by the ubiquitin proteasome pathway, the specific degradation mechanism in the cell. E3 ubiquitin ligases polyubiquitylate their specific substrates by collaborating with E1 and E2, and then the modified substrates are degraded in the proteasome. Mdm2 targets p53 and retinoblastoma protein, two major tumor suppressor gene products, for ubiquitin-dependent degradation. SCF(Skp2) targets other tumor suppressor gene products and CDK inhibitors such as p130, Tob1, p27(Kip1), p57(Kip2), and p21(Cip1). Therefore, both E3 ligases act like oncogene products. In contrast, degradation of several oncogene products, such as Cyclin E, Notch, c-Myc, c-Jun, and c-Myb, are mediated by SCF(Fbw7). Fbw7 is often deleted or mutated in human cancers and acts like a tumor suppressor. As well as growth factor receptors and signal transduction regulators, DNA repair-related proteins are also regulated via the ubiquitin-proteasome pathway mediated by their specific E3 ligases. The stabilization of oncogene products and enhanced degradation of tumor suppressor gene products or DNA repair proteins might be associated with carcinogenesis and malignant progression, due to defects or the abnormal expression of their E3 ligases.

Fbw7 promotes ubiquitin-dependent degradation of c-Myb: involvement of GSK3-mediated phosphorylation of Thr-572 in mouse c-Myb

Expression of oncoprotein c-Myb oscillates during hematopoiesis and hematological malignancies. Its quantity is not only regulated through transcriptional control but also through the ubiquitin-proteasome pathway, accompanied by phosphorylation, although the mechanisms are poorly understood. In this report, we tried to identify an E3 ubiquitin ligase, which targets c-Myb for ubiquitin-dependent degradation. We found that an F-box protein, Fbw7, interacted with c-Myb, which is mutated in numerous cancers. Fbw7 facilitated ubiquitylation and degradation of c-Myb in intact cells. Moreover, depletion of Fbw7 by RNA interference delayed turnover and increased the abundance of c-Myb in myeloid leukemia cells concomitantly, and suppressed the transcriptional level of gamma-globin, which receives transcriptional repression from c-Myb. In addition, we analysed sites required for both ubiquitylation and degradation of c-Myb. We found that Thr-572 is critical for Fbw7-mediated ubiquitylation in mouse c-Myb using site-directed mutagenesis. Fbw7 recognized the phosphorylation of Thr-572, which was mediated by glycogen synthase kinase 3 (GSK3). In consequence, the c-Myb protein was markedly stabilized by the substitution of Thr-572 to Ala. These observations suggest that SCF(Fbw7) ubiquitin ligase regulates phosphorylation-dependent degradation of c-Myb protein.

Age-related decrease in proteasome expression contributes to defective nuclear factor-kappaB activation during hepatic ischemia/reperfusion

Hepatic ischemia/reperfusion (I/R) leads to liver injury and dysfunction through the initiation of a biphasic inflammatory response that is regulated by the transcription factor nuclear factor kappaB (NF-kappaB). We have previously shown that there is an age-dependent difference in the injury response to hepatic I/R in mice that correlates with divergent activation of NF-kappaB such that young mice have greater NF-kappaB activation, but less injury than old mice. In this study, we investigated the mechanism by which age alters the activation of NF-kappaB in the liver during I/R. Young (4-5 weeks) and old (12-14 months) mice underwent partial hepatic I/R. Livers were obtained for RNA microarray analysis and protein expression assays. Using microarray analysis, we identified age-dependent differences in the expression of genes related to protein ubiquitinylation and the proteasome. In old mice, genes that are involved in the ubiquitin-proteasome pathway were significantly down-regulated during I/R. Consistent with these findings, expression of a critical proteasome subunit, non-adenosine triphosphatase 4 (PSMD4), was reduced in old mice. Expression of the NF-kappaB inhibitory protein, IkappaB alpha, was increased in old mice and was greatly phosphorylated and ubiquitinylated. The data provide strong evidence that the age-related defect in hepatic NF-kappaB signaling during I/R is a result of decreased expression of PSMD4, a proteasome subunit responsible for recognition and recruitment of ubiquitinylated substrates to the proteasome. It appears that decreased PSMD4 expression prevents recruitment of phosphorylated and ubiquitinylated IkappaB alpha to the proteasome, resulting in a defect in NF-kappaB activation.

Fbxo45 forms a novel ubiquitin ligase complex and is required for neuronal development

Fbxo45 is an F-box protein that is restricted to the nervous system. Unlike other F-box proteins, Fbxo45 was found not to form an SCF complex as a result of an amino acid substitution in the consensus sequence for Cul1 binding. Proteomics analysis revealed that Fbxo45 specifically associates with PAM (protein associated with Myc), a RING finger-type ubiquitin ligase. Mice deficient in Fbxo45 were generated and found to die soon after birth as a result of respiratory distress. Fbxo45(-)(/)(-) embryos show abnormal innervation of the diaphragm, impaired synapse formation at neuromuscular junctions, and aberrant development of axon fiber tracts in the brain. Similar defects are also observed in mice lacking Phr1 (mouse ortholog of PAM), suggesting that Fbxo45 and Phr1 function in the same pathway. In addition, neuronal migration was impaired in Fbxo45(-)(/)(-) mice. These results suggest that Fbxo45 forms a novel Fbxo45-PAM ubiquitin ligase complex that plays an important role in neural development.

Inhibition of NF-kappaB signaling via tyrosine phosphorylation of Ymer

Cytoplasmic zinc finger protein A20 functionally dampens inflammatory signals and apoptosis via inhibition of NF-kappaB activation. We have reported that Ymer interacts with A20 and lysine (K)-63-linked polyubiquitin chain and that Ymer inhibits NF-kappaB signaling in collaboration with A20. It has also been reported that Ymer is phosphorylated by EGF stimulation. We found that Ymer was considerably phosphorylated on tyrosine residues also via Src family kinases such as Lck. A luciferase reporter assay showed that mutation of tyrosines on Ymer (YmerY217/279/304F) results in loss of the inhibitory activity for NF-kappaB signaling. Furthermore, a soft agar colony formation assay showed that the combination of SrcY527F and YmerY217/279/304F has no ability for anchorage-independent growth, suggesting that tyrosine phosphorylation of Ymer is important for inhibition of the NF-kappaB-mediated apoptotic pathway. These findings demonstrate that Ymer is likely to be a negative regulator for the NF-kappaB signaling pathway.

Membrane androgen receptor activation triggers down-regulation of PI-3K/Akt/NF-kappaB activity and induces apoptotic responses via Bad, FasL and caspase-3 in DU145 prostate cancer cells

Background

Recently we have reported membrane androgen receptors-induced apoptotic regression of prostate cancer cells regulated by Rho/ROCK/actin signaling. In the present study we explored the specificity of these receptors and we analyzed downstream effectors controlling survival and apoptosis in hormone refractory DU145-prostate cancer cells stimulated with membrane androgen receptor-selective agonists.

Results

Using membrane impermeable conjugates of serum albumin covalently linked to testosterone, we show here down-regulation of the activity of pro-survival gene products, namely PI-3K/Akt and NF-κB, in DU145 cells. Testosterone-albumin conjugates further induced FasL expression. A FasL blocking peptide abrogated membrane androgen receptors-dependent apoptosis. In addition, testosterone-albumin conjugates increased caspase-3 and Bad protein activity. The actin cytoskeleton drug cytochalasin B and the ROCK inhibitor Y-27632 inhibited FasL induction and caspase-3 activation, indicating that the newly identified Rho/Rock/actin signaling may regulate the downstream pro-apoptotic effectors in DU145 cells. Finally, other steroids or steroid-albumin conjugates did not interfere with these receptors indicating testosterone specificity.

Conclusion

Collectively, our results provide novel mechanistic insights pointing to specific pro-apoptotic molecules controlling membrane androgen receptors-induced apoptotic regression of prostate cancer cells and corroborate previously published observations on the potential use of membrane androgen receptor-agonists as novel anti-tumor agents in prostate cancer.

Deregulated proteolysis by the F-box proteins SKP2 and beta-TrCP: tipping the scales of cancer

The maintenance and preservation of distinct phases during the cell cycle is a highly complex and coordinated process. It is regulated by phosphorylation--through the activity of cyclin-dependent kinases (CDKs)--and protein degradation, which occurs through ubiquitin ligases such as SCF (SKP1-CUL1-F-box protein) complexes and APC/C (anaphase-promoting complex/cyclosome). Here, we explore the functionality and biology of the F-box proteins, SKP2 (S-phase kinase-associated protein 2) and beta-TrCP (beta-transducin repeat-containing protein), which are emerging as important players in cancer biogenesis owing to the deregulated proteolysis of their substrates.

Identification of ubiquitin ligase activity of RBCK1 and its inhibition by splice variant RBCK2 and protein kinase Cbeta

We previously identified a RING-IBR protein, RBCK1, as a protein kinase C (PKC) beta- and zeta-interacting protein, and its splice variant, RBCK2, lacking the C-terminal half including the RING-IBR domain. RBCK1 has been shown to function as a transcriptional activator whose nuclear translocation is prevented by interaction with the cytoplasmic RBCK2. We here demonstrate that RBCK1, like many other RING proteins, also possesses a ubiquitin ligase (E3) activity and that its E3 activity is inhibited by interaction with RBCK2. Moreover, RBCK1 has been found to undergo efficient phosphorylation by PKCbeta. The phosphorylated RBCK1 shows no self-ubiquitination activity in vitro. Overexpression of PKCbeta leads to significant increases in the amounts of intracellular RBCK1, presumably suppressing the proteasomal degradation of RBCK1 through self-ubiquitination, whereas coexpression with PKCalpha, PKCepsilon, and PKCzeta shows no or little effect on the intracellular amount of RBCK1. Taken together, the E3 activity of RBCK1 is controlled by two distinct manners, interaction with RBCK2 and phosphorylation by PKCbeta. It is possible that other RING proteins, such as Parkin, BRCA1, and RNF8, having the E3 activity, are also down-regulated by interaction with their RING-lacking splice variants and/or phosphorylation by protein kinases.

Clusterin, a novel modulator of TGF-beta signaling, is involved in Smad2/3 stability

Clusterin (CLU) is known as a multifunctional protein involved in a variety of physiological processes including lipid transport, epithelial cell differentiation, tumorigenesis, and apoptosis. It is known that CLU interacts with TGF-beta type ll receptor (TbetaRll). However, the relationship of CLU and TGF-beta signaling is unclear. Here we present that CLU is a novel modulator of TGF-beta signaling by regulating Smad2/3 proteins. Overexpression of CLU enhanced TGF-beta-induced transcriptional activity and increased the amount of Smad2/3 proteins, while CLU siRNA repressed TGF-beta-induced transcriptional activity and decreased the amount of Smad2/3 proteins in Hep3B cells. We also found that CLU was involved in Smad2/3 stability at the protein level. These findings suggest that CLU regulates TGF-beta signaling pathway by modulating the stability of Smad2/3 proteins.

Involvement of Ymer in suppression of NF-kappaB activation by regulated interaction with lysine-63-linked polyubiquitin chain

It is known that the cytoplasmic zinc finger protein A20 functionally dampens inflammatory signals and apoptosis via inhibition of NF-kappaB activation and biochemically acts as a unique ubiquitin-modifying protein with deubiquitinating activity and ubiquitin ligase activity. However, the molecular mechanisms of A20-modulated signal transduction that influence normal immune responses or tumor immunity have not been fully elucidated. Using a yeast two-hybrid system to search for proteins interacting with A20, we identified one novel binding protein, Ymer. Ymer, which has been reported to be highly phosphorylated on tyrosine residues via EGF stimulation, bound to lysine (K)-63-linked polyubiquitin chain on receptor-interacting serine/threonine-protein kinase 1 (RIP1), which is essential for NF-kappaB signaling in collaboration with A20. A luciferase assay showed that NF-kappaB signaling was down-regulated by overexpression of Ymer, whereas knock-down of Ymer up-regulated NF-kappaB signaling even without stimulation. These findings demonstrate that Ymer is likely to be a negative regulator for the NF-kappaB signaling pathway.

Critical and functional regulation of CHOP (C/EBP homologous protein) through the N-terminal portion

C/EBP homologous protein (CHOP) is an endoplasmic reticulum stress-inducible protein that plays a critical role in the regulation of programmed cell death; however, the regulation of its function has not been well characterized. We have previously demonstrated that CHOP is regulated by the ubiquitin-proteasome system. In this study, during the process of clarifying the mechanism of the degradation of CHOP, we identified a novel regulation domain of CHOP in its N-terminal portion that is involved in various regulations and functions. The CHOP N-terminal domain is necessary not only for protein degradation but also for its transactivity and interaction with p300. In addition, trichostatin A, a histone deacetylase inhibitor, repressed the degradation of CHOP protein via the N-terminal domain. TRB3, a mammalian tribbles homolog that functions as a repressor of CHOP, also interacted with CHOP via the N-terminal portion and significantly blocked the association of p300 with CHOP. These results suggest that the N-terminal portion of CHOP plays a crucial role in its functional regulation and enable us to identify a novel function of TRB3 as an intracellular antagonist of the p300-binding domain of CHOP.

UXT is a novel and essential cofactor in the NF-kappaB transcriptional enhanceosome

As a latent transcription factor, nuclear factor κB (NF-κB) translocates from the cytoplasm into the nucleus upon stimulation and mediates the expression of genes that are important in immunity, inflammation, and development. However, little is known about how it is regulated inside the nucleus. By a two-hybrid approach, we identify a prefoldin-like protein, ubiquitously expressed transcript (UXT), that is expressed predominantly and interacts specifically with NF-κB inside the nucleus. RNA interference knockdown of UXT leads to impaired NF-κB activity and dramatically attenuates the expression of NF-κB–dependent genes. This interference also sensitizes cells to apoptosis by tumor necrosis factor-α. Furthermore, UXT forms a dynamic complex with NF-κB and is recruited to the NF-κB enhanceosome upon stimulation. Interestingly, the UXT protein level correlates with constitutive NF-κB activity in human prostate cancer cell lines. The presence of NF-κB within the nucleus of stimulated or constitutively active cells is considerably diminished with decreased endogenous UXT levels. Our results reveal that UXT is an integral component of the NF-κB enhanceosome and is essential for its nuclear function, which uncovers a new mechanism of NF-κB regulation.

Experimental selection for Drosophila survival in extremely low O(2) environment

BACKGROUND

Cellular hypoxia, if severe enough, results usually in injury or cell death. Our research in this area has focused on the molecular mechanisms underlying hypoxic tissue injury to explore strategies to prevent injury or enhance tolerance. The current experiments were designed to determine the genetic basis for adaptation to long term low O(2) environments.

METHODOLOGY/PRINCIPAL FINDINGS

With long term experimental selection over many generations, we obtained a Drosophila melanogaster strain that can live perpetually in extremely low, normally lethal, O(2) condition (as low as 4% O(2)). This strain shows a dramatic phenotypic divergence from controls, including a decreased recovery time from anoxic stupor, a higher rate of O(2 )consumption in hypoxic conditions, and a decreased body size and mass due to decreased cell number and size. Expression arrays showed that about 4% of the Drosophila genome altered in expression and about half of the alteration was down-regulation. The contribution of some altered transcripts to hypoxia tolerance was examined by testing the survival of available corresponding P-element insertions (and their excisions) under extremely low O(2) conditions. We found that down-regulation of several candidate genes including Best1, broad, CG7102, dunce, lin19-like and sec6 conferred severe hypoxia tolerance in Drosophila.

CONCLUSIONS/SIGNIFICANCE

We have identified a number of genes that play an important role in the survival of a selected Drosophila strain in extremely low O(2) conditions, selected by decreasing O(2) availability over many generations. Because of conservation of pathways, we believe that such genes are critical in hypoxia adaptation in physiological or pathological conditions not only in Drosophila but also in mammals.

Establishment of a newly improved detection system for NF-κB activity

The transcription factor nuclear factor-kappaB (NF-kappaB) plays roles in apoptosis, inflammation and oncogenesis. It is important for biological and medical research to understand when proteins of interest are activated in cells, leading to the establishment of a luciferase/EGFP assay to monitor the activation of transcription factors. Here, we describe an improved reporter system for NF-kappaB, the NF-kappaB-activated transgene (NAT) system that can detect NF-kappaB signalling with high sensitivity and specificity. The NAT system consists of large copy numbers of NF-kappaB consensus sequence and a minimal promoter derived from the mouse interleukin-2 (IL-2) gene. Furthermore, we generated NAT systems with stable or unstable luciferase/EGFP proteins. Stable and unstable types of luciferase/EGFP are suitable for analyzing the accumulation of and the real-time activity of NF-kappaB signal, respectively. Our findings suggest that the NAT system is effective for in vivo imaging of NF-kappaB signalling using cells or animals.

SAG/ROC-SCF beta-TrCP E3 ubiquitin ligase promotes pro-caspase-3 degradation as a mechanism of apoptosis protection

Skp1-cullin-F-box protein (SCF) is a multicomponent E3 ubiquitin (Ub) ligase that ubiquitinates a number of important biologic molecules such as p27, beta-catenin, and IkappaB for proteasomal degradation, thus regulating cell proliferation and survival. One SCF component, SAG/ROC2/Rbx2/Hrt2, a RING finger protein, was first identified as a redox-inducible protein, which, when overexpressed, inhibited apoptosis both in vitro and in vivo. We report here that sensitive to apoptosis gene (SAG), as well as its family member ROC1/Rbx1, bound to the proinactive form of caspase-3 (pro-caspase-3). Binding was likely mediated through F-box protein, beta-transducin repeat-containing protein (beta-TrCP), which binds to the first 38 amino acids of pro-caspase-3. Importantly, beta-TrCP1 expression significantly shortened the protein half-life of pro-caspase-3, whereas expression of a dominant-negative beta-TrCP1 mutant with the F-box domain deleted extended it. An in vitro ubiquitination assay showed that SAG/ROC-SCF(beta-TrCP) promoted ubiquitination of pro-caspase-3. Furthermore, endogenous levels of pro-caspase-3 were decreased by overexpression of SAG/ROC-SCF(beta-TrCP) E3 Ub ligases, but increased on siRNA silencing of SAG, regulator of cullin-1 (ROC1), or beta-TrCPs, leading to increased apoptosis by etoposide and TNF-related apoptosis-inducing ligand through increased activation of caspase-3. Thus, pro-caspase-3 appears to be a substrate of SAG/ROC-SCF(beta-TrCP) E3 Ub ligase, which protects cells from apoptosis through increased apoptosis threshold by reducing the basal level of pro-caspase-3.

FBXO11 promotes the Neddylation of p53 and inhibits its transcriptional activity

The p53 tumor suppressor is regulated by post-translational modification, including ubiquitination, phosphorylation and acetylation. It has previously been shown that the ubiquitin ligase Mdm2 also promotes the conjugation of Nedd8, a ubiquitin-like protein, to p53, inhibiting its transcriptional activity. We report the identification of FBXO11, a member of the F-box protein family and a component of the Skp1.Cullin1.F-box (SCF) complex, as a new p53-interacting protein. We show that FBXO11 promotes the neddylation of p53 both in vitro and in vivo. In addition to the C-terminal lysine residues, FBXO11 can also promote Nedd8 conjugation to Lys-320 and Lys-321, and neddylation of p53 leads to suppression of p53 function. This is consistent with recent studies showing that a lysine to arginine mutation at Lys-320 significantly enhances p53 function, although Lys-320 was originally identified as an acetylation site involving PCAF-mediated activation of p53. Our study provides an example of an F-box protein acting as an adaptor protein that can mediate the neddylation of a non-cullin substrate.

E3 ubiquitin ligases as cancer targets and biomarkers

E3 ubiquitin ligases are a large family of proteins that are engaged in the regulation of the turnover and activity of many target proteins. Together with ubiquitin-activating enzyme E1 and ubiquitin-conjugating enzyme E2, E3 ubiquitin ligases catalyze the ubiquitination of a variety of biologically significant protein substrates for targeted degradation through the 26S proteasome, as well as for nonproteolytic regulation of their functions or subcellular localizations. E3 ubiquitin ligases, therefore, play an essential role in the regulation of many biologic processes. Increasing amounts of evidence strongly suggest that the abnormal regulation of some E3 ligases is involved in cancer development. Furthermore, some E3 ubiquitin ligases are frequently overexpressed in human cancers, which correlates well with increased chemoresistance and poor clinic prognosis. In this review, E3 ubiquitin ligases (such as murine double minute 2, inhibitor of apoptosis protein, and Skp1-Cullin-F-box protein) will be evaluated as potential cancer drug targets and prognostic biomarkers. Extensive study in this field would lead to a better understanding of the molecular mechanism by which E3 ligases regulate cellular processes and of how their deregulations contribute to carcinogenesis. This would eventually lead to the development of a novel class of anticancer drugs targeting specific E3 ubiquitin ligases, as well as the development of sensitive biomarkers for cancer treatment, diagnosis, and prognosis.

Fbxw8 is essential for Cul1-Cul7 complex formation and for placental development

Cullin-based ubiquitin ligases (E3s) constitute one of the largest E3 families. Fbxw8 (also known as Fbw6 or Fbx29) is an F-box protein that is assembled with Cul7 in an SCF-like E3 complex. Here we show that Cul7 forms a heterodimeric complex with Cul1 in a manner dependent on Fbxw8. We generated mice deficient in Fbxw8 and found that Cul7 did not associate with Cul1 in cells of these mice. Two-thirds of Fbxw8-/- embryos die in utero, whereas the remaining one-third are born alive and grow to adulthood. Fbxw8-/- embryos show intrauterine growth retardation and abnormal development of the placenta, characterized by both a reduced thickness of the spongiotrophoblast layer and abnormal vessel structure in the labyrinth layer. Although the placental phenotype of Fbxw8-/- mice resembles that of Cul7-/- mice, other abnormalities of Cul7-/- mice are not apparent in Fbxw8-/- mice. These results suggest that the Cul7-based SCF-like E3 complex has both Fbxw8-dependent and Fbxw8-independent functions.

Fbxw7 contributes to tumor suppression by targeting multiple proteins for ubiquitin-dependent degradation

Fbxw7 (also known as Sel-10, hCdc4 or hAgo) is the F-box protein component of a Skp1-Cul1-F-box protein (SCF) ubiquitin ligase. Fbxw7 contributes to the ubiquitin-mediated degradation of cyclin E, c-Myc, Aurora-A, Notch and c-Jun, all of which appear to function as cell-cycle promoters and oncogenic proteins. Loss of Fbxw7 results in elevated expression of its substrates, which may lead to oncogenesis. However, it remains largely unclear which accumulating substrate is most related to cancer development in Fbxw7-mutant cancer cells. In the present study, we examined the abundance of cyclin E, c-Myc and Aurora-A in seven cancer cell lines, which harbor wild-type (three lines) or mutant (four lines) Fbxw7. Although these three substrates accumulated in the Fbxw7-mutant cells, the extent of increase in the expression of these proteins varied in each line. Forced expression of Fbxw7 reduced the levels of cyclin E, c-Myc and Aurora-A in the Fbxw7-mutant cells. In contrast, a decrease in the expression of cyclin E, c-Myc or Aurora-A by RNA interference significantly suppressed the rate of proliferation and anchorage-independent growth of the Fbxw7-mutant cells. These findings thus suggest that the loss of Fbxw7 results in accumulation of cyclin E, c-Myc and Aurora-A, all of which appear to be required for growth promotion of cancer cells. Fbxw7 seems to regulate the levels of multiple targets to suppress cancer development.

Negative regulation of protein phosphatase 2Cbeta by ISG15 conjugation

ISG15, an interferon-upregulated ubiquitin-like protein, is covalently conjugated to various cellular proteins (ISGylation). In this study, we found that protein phosphatase 2Cbeta (PP2Cbeta), which functions in the nuclear factor kappaB (NF-kappaB) pathway via dephosphorylation of TGF-beta-activated kinase, was ISGylated, and analysis by NF-kappaB luciferase reporter assay revealed that PP2Cbeta activity was suppressed by co-expression of ISG15, UBE1L, and UbcH8. We determined the ISGylation sites of PP2Cbeta and constructed its ISGylation-resistant mutant. In contrast to the wild type, this mutant suppressed the NF-kappaB pathway even in the presence of ISG15, UBE1L, and UbcH8. Thus, we propose that ISGylation negatively regulates PP2Cbeta activity.

Gene expression profiling in Salmonella Choleraesuis-infected porcine lung using a long oligonucleotide microarray

Understanding the transcriptional response to pathogenic bacterial infection within food animals is of fundamental and applied interest. To determine the transcriptional response to Salmonella enterica serovar Choleraesuis (SC) infection, a 13,297-oligonucleotide swine array was used to analyze RNA from control, 24-h postinoculation (hpi), and 48-hpi porcine lung tissue from pigs infected with SC. In total, 57 genes showed differential expression (p < 0.001; false discovery rate = 12%). Quantitative real-time PCR (qRT-PCR) of 61 genes was used to confirm the microarray results and to identify pathways responding to infection. Of the 33 genes identified by microarray analysis as differentially expressed, 23 were confirmed by qRT-PCR results. A novel finding was that two transglutaminase family genes (TGM1 and TGM3) showed dramatic increases in expression postinoculation; combined with several other apoptotic genes, they indicated the induction of apoptotic pathways during SC infection. A predominant T helper 1-type immune response occurred during infection, with interferon gamma (IFNG) significantly increased at 48 hpi. Genes induced by IFNs (GBP1, GBP2, C1S, C1R, MHC2TA, PSMB8, TAP1, TAP2) showed increased expression during porcine lung infection. These data represent the first thorough investigation of gene regulation pathways that control an important porcine respiratory and foodborne bacterial infection.

A comprehensive map of the toll-like receptor signaling network

Recognition of pathogen-associated molecular signatures is critically important in proper activation of the immune system. The toll-like receptor (TLR) signaling network is responsible for innate immune response. In mammalians, there are 11 TLRs that recognize a variety of ligands from pathogens to trigger immunological responses. In this paper, we present a comprehensive map of TLRs and interleukin 1 receptor signaling networks based on papers published so far. The map illustrates the possible existence of a main network subsystem that has a bow-tie structure in which myeloid differentiation primary response gene 88 (MyD88) is a nonredundant core element, two collateral subsystems with small GTPase and phosphatidylinositol signaling, and MyD88-independent pathway. There is extensive crosstalk between the main bow-tie network and subsystems, as well as feedback and feedforward controls. One obvious feature of this network is the fragility against removal of the nonredundant core element, which is MyD88, and involvement of collateral subsystems for generating different reactions and gene expressions for different stimuli.

Knockdown of Skp2 by siRNA inhibits melanoma cell growth in vitro and in vivo

BACKGROUND

Low levels of p27Kip1 expression are associated with poor prognosis in various malignancies including malignant melanoma. Recently, it has been reported that S phase kinase-interacting protein 2 (Skp2), the specific ubiquitin ligase subunit that targets p27Kip1 for degradation, was overexpressed and was inversely related to p27Kip1 levels in malignant melanoma with poor prognosis.

OBJECTIVE

We investigated whether small interfering RNA (siRNA)-mediated gene silencing of Skp2 can be employed in order to inhibit p27Kip1 down-regulation and suppress melanoma cell growth as a consequence in vitro and in vivo.

METHODS

We constructed a plasmid vector, which synthesizes siRNAs to determine the effects of decreasing the high constitutive levels of Skp2 protein in melanoma cells. Western blot and real-time RT-PCR were performed to examine the decreases of Skp2 protein and mRNA in vitro. Furthermore, melanoma cells were injected into the back of nude mice subcutaneously to examine the suppression of tumorigenicity targeting Skp2 gene silencing in vivo.

RESULTS

Skp2 protein was decreased and the p27Kip1 protein was accumulated in Skp2 siRNA transfected melanoma cells. Skp2 siRNA inhibited the cell growth of melanoma cells in vitro. Moreover, Skp2 siRNA also suppressed tumor proliferation in vivo.

CONCLUSION

Our results suggest that siRNA-mediated gene silencing of Skp2 can be a potent tool of cancer gene therapy for suppression of p27Kip1 degradation in malignant melanoma.

Suppression of p65 phosphorylation coincides with inhibition of IkappaBalpha polyubiquitination and degradation

Transcription factor nuclear factor-kappaB (NF-kappaB) is held in the cytoplasm in an inactive state by IkappaB inhibitors. Oncogenic activation of NF-kappaB is achieved by stimulus-induced ubiquitination and subsequent proteasome-mediated degradation of IkappaBalpha. Once released from the inhibitor, NF-kappaB/p65 enters the nucleus. A pre-requisite for cytokine-induced IkappaBalpha ubiquitination and degradation is the phosphorylation of IkappaBalpha at S32/S36. Phosphorylation of IkappaBalpha alone, however, is not sufficient to trigger its degradation, suggesting other events must be required for regulating IkappaBalpha degradation. In this study, we tested the hypothesis that phosphorylation of p65 at 536 is required for TNF-alpha induced IkappaBalpha proteolysis that in turn controls p65 nuclear translocation. We observed that, without affecting IkappaBalpha phosphorylation, MEK1 inhibitor U0126 treatment inhibited not only p65-S536 phosphorylation but also TNF-alpha-induced polyubiquitination of IkappaBalpha thereby inhibiting IkappaBalpha degradation. With p65 S536 phosphorylation mutants and mimics, we further observed that the structural mutation of p65 serine 536 to alanine inhibited the recruitment of ubiquitin to the p65-containing complex. As a consequence of suppressing polyubiquitination of the p65-containing complex, degradation of p65 phosphorylation mutant-bound IkappaBalpha was also inhibited. Accordingly, the nuclear translocation of phosphorylation-impaired p65 was significantly reduced. These findings suggest that p65 phosphorylation plays a key role in stimulus-induced IkappaBalpha ubiquitination.

SCF(beta-TrCP1) controls Smad4 protein stability in pancreatic cancer cells

Smad4, also known as deleted in pancreatic carcinoma locus 4 (DPC4), is a critical co-factor in signal transduction pathways activated by transforming growth factor (TGF)-beta-related ligands that regulate cell growth and differentiation. Mutations in Smad4/DPC4 have been identified in approximately 50% of pancreatic adenocarcinomas. Here we report that SCF(beta-TrCP1), a ubiquitin (E3) ligase, is a critical determinant for Smad4 protein degradation in pancreatic cancer cells. We found that F-box protein beta-TrCP1 in this E3 ligase interacted with Smad4 and that SCF(beta-TrCP1) inhibited TGF-beta biological activity in pancreatic cancer cells by decreasing Smad4 stability. Very low Smad4 protein levels in human pancreatic ductal adenocarcinoma cells were observed by immunohistochemistry. By analyzing pancreatic tumor-derived Smad4 mutants, we found that most point-mutated Smad4 proteins, except those within or very close to a mutation cluster region, exhibited higher interaction affinity with beta-TrCP1 and significantly elevated protein ubiquitination by SCF(beta-TrCP1). Furthermore, AsPC-1 and Caco-2, two cancer cell lines harboring Smad4 point mutations, exhibited rapid Smad4 protein degradation due to the effect of SCF(beta-TrCP1). Both Smad4 levels and TGF-beta signaling were elevated by retrovirus-delivered beta-TrCP1 siRNA in pancreatic cancer cells. Therefore, inhibition of Smad4-specific E3 ligase might be a target for therapeutic intervention in pancreatic cancer.

Clinorotation prevents differentiation of rat myoblastic L6 cells in association with reduced NF-kappa B signaling

In this study, we examined effects of the three-dimensional (3D)-clinorotation, a simulated-model of microgravity, on proliferation/differentiation of rat myoblastic L6 cells. Differentiation of L6 cells into myotubes was significantly disturbed in the 3D-clinorotation culture system, although the 3D-clinorotation had no effect on the proliferation. The 3D-clinorotation also suppressed the expression of myogenesis marker proteins, such as myogenin and myosin heavy chain (MHC), at the mRNA level. In association with this reduced differentiation, we found that the 3D-clinorotation prevented accumulation of ubiquitinated proteins, compared with non-rotation control cells. Based on these findings, we focused on the ubiquitin-dependent degradation of I kappa B, a myogenesis inhibitory protein, to clarify the mechanism of this impaired differentiation. A decline in the amount of I kappa B protein in L6 cells was significantly prevented by the rotation, while the amount of the protein in the non-rotated cells decreased along with the differentiation. Furthermore, the 3D-clinorotation reduced the NF-kappaB-binding activity in L6 cells and prevented the ubiquitination of I kappa B proteins in the I kappa B- and ubiquitin-expressing Cos7 cells. Other myogenic regulatory factors, such as deubiquitinases, cyclin E and oxygen, were not associated with the differentiation impaired by the clinorotation. Our present results suggest that simulated microgravity such as the 3D-clinorotation may disturb skeletal muscle cell differentiation, at least in part, by inhibiting the NF-kappa B pathway.

A Novel Protein Specifically Interacting with Homer2 Regulates Ubiquitin-Proteasome Systems

Homer family proteins are encoded by three genes, homer1, 2 and 3. Most of these proteins are expressed constitutively in nervous systems and accumulated in postsynaptic regions. However, the functional significance of these proteins, especially the significance of the distinction among the proteins encoded by homer1, 2 and 3, is still obscure. In the present study, we isolated a cDNA clone encoding a novel protein by two-hybrid system screening using the C-terminal half of Homer2b as the bait. This protein, termed 2B28, has 297 amino acid residues and contains three major domains: a UBA domain, a coiled-coil region, and a UBX domain. When expressed in HEK293T cells, 2B28 showed colocalization with uniquitin and enhanced the expression levels of IkappaB or Homer1a proteins, which are known to be degraded by proteasomes, indicating that 2B28 is involved in ubiquitin-proteasome functions. 2B28 specifically interacted and colocalized with Homer2 proteins, but not with Homer1 proteins. So far, we have identified no counterpart of 2B28 for Homer1 experimentally or in the protein databases. These results suggest that the specific interaction of 2B28 with Homer2 may play a role in regulation of protein degradation by ubiquitin-proteasome systems and that this function may be specific to Homer2 proteins among Homer family proteins.

Enhanced Mdm2 activity inhibits pRB function via ubiquitin-dependent degradation

Retinoblastoma gene product (pRB) plays critical roles in regulation of the cell cycle and tumor suppression. It is known that downregulation of pRB can stimulate carcinogenesis via abrogation of the pRB pathway, although the mechanism has not been elucidated. In this study, we found that Mdm2, a ubiquitin ligase for p53, promoted ubiquitin-dependent degradation of pRB. pRB was efficiently ubiquitinated by wild-type Mdm2 in vivo as well as in vitro, but other RB family proteins were not. Mutant Mdm2 with a substitution in the RING finger domain showed dominant-negative stabilization of pRB. Both knockout and knockdown of Mdm2 caused accumulation of pRB. Moreover, Mdm2 inhibited pRB-mediated flat formation of Saos-2 cells. Downregulation of pRB expression was correlated with a high level of expression of Mdm2 in human lung cancers. These results suggest that Mdm2 regulates function of pRB via ubiquitin-dependent degradation of pRB.

Transcriptional response of avian cells to infection with Newcastle disease virus

Newcastle disease virus (NDV) causes widespread disease in poultry and wild-birds throughout the world. cDNA microarray analysis was used to examine the effect of NDV infection on host cell transcription. The results show that NDV infection causes an apparent suppression of the interferon response genes during the early stages of infection. In addition, the results reveal transcriptional silencing of cytoskeletal proteins such as the alpha, beta, and gamma types of actin, and a downregulation of the thioredoxin gene, a likely mediator of apoptosis with possible implications in NDV pathogenesis. Comparative analyses show that a majority of genes that were transcriptionally regulated during infection with another common respiratory pathogen of poultry, the avian pneumovirus, remained unaltered during NDV infection, suggesting that even phylogenetically related viruses elicit unique or "signature" patterns of host transcriptional profiles during infection of host cells.

Role of serine 10 phosphorylation in p27 stabilization revealed by analysis of p27 knock-in mice harboring a serine 10 mutation

The inhibition of cyclin-dependent kinase activity by p27 contributes to regulation of cell cycle progression. Serine 10 is the major phosphorylation site of p27, and its phosphorylation has been shown to affect the stability and nuclear export of p27 at the G0-G1 transition in transfected cultured cells. To investigate the physiological relevance of p27 phosphorylation on Ser10, we generated p27 "knock-in" mice that harbor an S10A mutation in this protein. Mice homozygous for the mutation (p27(S10A/S10A) mice) were normal in body size, but the abundance of p27 was decreased in many organs, including brain, thymus, spleen, and testis. The stability of p27 in G0 phase was markedly reduced in lymphocytes of p27(S10A/S10A) mice compared with that in wild-type cells, whereas p27 stability in S phase was similar in cells of the two genotypes. The degradation of p27 in cells of the mutant mice at G0 phase was prevented by a proteasome inhibitor. These data indicate that the physiological role of p27 phosphorylation on Ser10 is to stabilize the protein in G0 phase. Unexpectedly, the nuclear export of p27 at the G0-G1 transition occurred normally in p27(S10A/S10A) mouse embryonic fibroblasts, indicating that phosphorylation of Ser10 is dispensable for this process.

Signaling to NF-kappaB

The transcription factor NF-kappaB has been the focus of intense investigation for nearly two decades. Over this period, considerable progress has been made in determining the function and regulation of NF-kappaB, although there are nuances in this important signaling pathway that still remain to be understood. The challenge now is to reconcile the regulatory complexity in this pathway with the complexity of responses in which NF-kappaB family members play important roles. In this review, we provide an overview of established NF-kappaB signaling pathways with focus on the current state of research into the mechanisms that regulate IKK activation and NF-kappaB transcriptional activity.

Early embryonic death in mice lacking the beta-catenin-binding protein Duplin

The Wnt signaling pathway plays a pivotal role in vertebrate early development and morphogenesis. Duplin (axis duplication inhibitor) interacts with beta-catenin and prevents its binding to Tcf, thereby inhibiting downstream Wnt signaling. Here we show that Duplin is expressed predominantly from early- to mid-stage mouse embryogenesis, and we describe the generation of mice deficient in Duplin. Duplin(-/-) embryos manifest growth retardation from embryonic day 5.5 (E5.5) and developmental arrest accompanied by massive apoptosis at E7.5. The mutant embryos develop into an egg cylinder but do not form a primitive streak or mesoderm. Expression of beta-catenin target genes, including those for T (brachyury), Axin2, and cyclin D1, was not increased in Duplin(-/-) embryos, suggesting that the developmental defect is not simply attributable to upregulation of Wnt signaling caused by the lack of this inhibitor. These results suggest that Duplin plays an indispensable role, likely by a mechanism independent of inhibition of Wnt signaling, in mouse embryonic growth and differentiation at an early developmental stage.

Role of F-box protein betaTrcp1 in mammary gland development and tumorigenesis

The F-box protein betaTrcp1 controls the stability of several crucial regulators of proliferation and apoptosis, including certain inhibitors of the NF-kappaB family of transcription factors. Here we show that mammary glands of betaTrcp1(-/-) female mice display a hypoplastic phenotype, whereas no effects on cell proliferation are observed in other somatic cells. To investigate further the role of betaTrcp1 in mammary gland development, we generated transgenic mice expressing human betaTrcp1 targeted to epithelial cells under the control of the mouse mammary tumor virus (MMTV) long terminal repeat promoter. Compared to controls, MMTV betaTrcp1 mammary glands display an increase in lateral ductal branching and extensive arrays of alveolus-like protuberances. The mammary epithelia of MMTV betaTrcp1 mice proliferate more and show increased NF-kappaB DNA binding activity and higher levels of nuclear NF-kappaB p65/RelA. In addition, 38% of transgenic mice develop tumors, including mammary, ovarian, and uterine carcinomas. The targeting of betaTrcp1 to lymphoid organs produces no effects on these tissues. In summary, our results support the notion that betaTrcp1 positively controls the proliferation of breast epithelium and indicate that alteration of betaTrcp1 function and expression may contribute to malignant behavior of breast tumors, at least in part through NF-kappaB transactivation.

Functional regulation of FEZ1 by the U-box-type ubiquitin ligase E4B contributes to neuritogenesis

E4B (also known as UFD2a) is a mammalian homolog of Saccharomyces cerevisiae Ufd2, which was originally described as a ubiquitin chain assembly factor (E4). E4B is a U-box-type ubiquitin-protein isopeptide ligase (E3) and likely functions as either an E3 or an E4. With a yeast two-hybrid screen, we have now identified FEZ1 (fasciculation and elongation protein zeta 1) as a protein that interacts with E4B. FEZ1 is implicated in neuritogenesis when phosphorylated by protein kinase Czeta (PKCzeta). Interaction between E4B and FEZ1 in mammalian cells was enhanced by coexpression of constitutively active PKCzeta. E4B mediated the polyubiquitylation of FEZ1 but did not affect its intracellular stability, suggesting that such modification of FEZ1 is not a signal for its proteolysis. Polyubiquitylation of FEZ1 by E4B required Lys(27) of ubiquitin. Expression of a dominant-negative mutant of E4B in rat pheochromocytoma PC12 cells resulted in inhibition of neurite extension induced either by nerve growth factor or by coexpression of FEZ1 and constitutively active PKCzeta. These findings indicate that E4B serves as a ubiquitin ligase for FEZ1 and thereby regulates its function but not its degradation.

Cloning and characterization of cellular senescence-associated genes in human fibroblasts by suppression subtractive hybridization

Cellular senescence marks the end of the proliferative life span of normal cells in tissue culture and occurs after cells have undergone a certain number of population doublings (PDLs). It is accompanied by alterations in the pattern of gene expression. A specific human embryonic lung diploid fibroblast cell line, 2BS, has been studied as a model of senescence in our laboratory. Here, we report a set of cellular senescence-associated genes identified from suppression subtractive cDNA libraries from senescent and young 2BS cells. They include three novel genes and six previously identified genes of unknown function. The genes whose functions are known belong to various functional pathways that have been reported to change with the onset of senescence. These include three pre-mRNA splicing factors with reduced expression in senescent cells, indicating that the regulation of mRNA splicing is altered during cell senescence. In addition, the expression of the gene TOM1 (target of Myb 1), which has not previously been associated with cellular senescence, is shown to increase in senescent cells, and we demonstrate that the expression of antisense TOM1 gene in 2BS cells can delay the progress of senescence.

E3 ubiquitin ligases as T cell anergy factors

E3 ubiquitin ligases have emerged as key molecular regulators of immune cell function. Three families of proteins with ubiquitin ligase activity have been described (the HECT, RING and U-box proteins), and each may be involved in the regulation of immune responses during infection by targeting specific inhibitory molecules for proteolytic destruction. Several HECT and RING E3 proteins have now also been linked to the induction and maintenance of immune self-tolerance: c-Cbl, Cbl-b, GRAIL, Itch and Nedd4 each negatively regulate T cell growth factor production and proliferation. This review will discuss the relationship between the ubiquitination of select components of the antigen-sensing signaling apparatus in T cells and the development and maintenance of the clonal anergy state.

Role of AIP and its homologue the blindness-associated protein AIPL1 in regulating client protein nuclear translocation

Mutations in the AIPL1 (aryl hydrocarbon receptor interacting protein-like 1) cause the blinding disease Leber's congenital amaurosis. AIPL1 is a homologue of the AIP. AIP functions as part of a chaperone heterocomplex to facilitate signalling by the AhR and plays an important role in regulating the nuclear translocation of the receptor. We review the evidence for the role of AIP in protein translocation and compare the potential functions of AIPL1 in the translocation of its interacting partner the NEDD8 ultimate buster protein 1.

Skp2-mediated degradation of p27 regulates progression into mitosis

Although Skp2 has been thought to mediate the degradation of p27 at the G(1)-S transition, Skp2(-/-) cells exhibit accumulation of p27 in S-G(2) phase with overreplication. We demonstrate that Skp2(-/-)p27(-/-) mice do not exhibit the overreplication phenotype, suggesting that p27 accumulation is required for its development. Hepatocytes of Skp2(-/-) mice entered the endoduplication cycle after mitogenic stimulation, whereas this phenotype was not apparent in Skp2(-/-)p27(-/-) mice. Cdc2-associated kinase activity was lower in Skp2(-/-) cells than in wild-type cells, and a reduction in Cdc2 activity was sufficient to induce overreplication. The lack of p27 degradation in G(2) phase in Skp2(-/-) cells may thus result in suppression of Cdc2 activity and consequent inhibition of entry into M phase. These data suggest that p27 proteolysis is necessary for the activation of not only Cdk2 but also Cdc2, and that Skp2 contributes to regulation of G(2)-M progression by mediating the degradation of p27.

Interaction of U-box-type ubiquitin-protein ligases (E3s) with molecular chaperones

Members of the U-box family of proteins constitute a class of ubiquitin-protein ligases (E3s) distinct from the HECT-type and RING finger-containing E3 families. Two representative mammalian U-box proteins, UFD2a and CHIP, interact with the molecular chaperones VCP and either Hsp90 or Hsc70, respectively, and are implicated in the degradation of damaged proteins. We have now investigated the roles of mammalian U-box proteins by performing a comprehensive screen for molecules that interact with these proteins in the yeast two-hybrid system. All mammalian U-box proteins tested were found to interact with molecular chaperones or cochaperones, including Hsp90, Hsp70, DnaJc7, EKN1, CRN, and VCP. These observations suggest that the function of U box-type E3s is to mediate the degradation of unfolded or misfolded proteins in conjunction with molecular chaperones as receptors that recognize such abnormal proteins.

Negative regulation of prolactin receptor stability and signaling mediated by SCF(beta-TrCP) E3 ubiquitin ligase

Ubiquitin-dependent degradation of hormone receptors is emerging as a key mechanism that regulates the magnitude and duration of hormonal effects on cells and tissues. The pituitary hormone prolactin (PRL) is involved in regulating cell differentiation, proliferation, and survival. PRL engages its receptor (PRLR) to initiate various signaling cascades, including the phosphorylation and activation of Stat5. We found that PRL promotes interaction between PRLR and the F-box protein beta-TrCP2, which functions as a substrate recognition subunit of the SCF(beta-TrCP) E3 ubiquitin ligase. This interaction requires PRLR phosphorylation and the integrity of serine 349 within a conserved motif, which is similar to conserved motifs present in other substrates of SCF(beta-TrCP). The PRLR(S349A) mutant is resistant to ubiquitination and is more stable than its wild-type counterpart. Phosphorylated PRLR undergoes ubiquitination by SCF(beta-TrCP) in vitro. Knockdown of beta-TrCP expression inhibits the ubiquitination and degradation of PRLR and promotes PRL-dependent phosphorylation of Stat5 as well as Stat5-dependent transcription in cells. Furthermore, the activation of Stat5 and the stimulation of cell growth by PRL are augmented in cells expressing the PRLR(S349A) mutant. These data indicate that PRLR is a novel SCF(beta-TrCP) substrate and implicate beta-TrCP as an important negative regulator of PRL signaling and cellular responses to this hormone.

Phosphorylation-dependent degradation of c-Myc is mediated by the F-box protein Fbw7

The F-box protein Skp2 mediates c-Myc ubiquitylation by binding to the MB2 domain. However, the turnover of c-Myc is largely dependent on phosphorylation of threonine-58 and serine-62 in MB1, residues that are often mutated in cancer. We now show that the F-box protein Fbw7 interacts with and thereby destabilizes c-Myc in a manner dependent on phosphorylation of MB1. Whereas wild-type Fbw7 promoted c-Myc turnover in cells, an Fbw7 mutant lacking the F-box domain delayed it. Furthermore, depletion of Fbw7 by RNA interference increased both the abundance and transactivation activity of c-Myc. Accumulation of c-Myc was also apparent in mouse Fbw7-/- embryonic stem cells. These observations suggest that two F-box proteins, Fbw7 and Skp2, differentially regulate c-Myc stability by targeting MB1 and MB2, respectively.

Oncogenic aberrations of cullin-dependent ubiquitin ligases

Accumulating evidence points to a key role of the ubiquitin-proteasome pathway in oncogenesis. Aberrant proteolysis of substrates involved in cellular processes such as the cell division cycle, gene transcription, the DNA damage response and apoptosis has been reported to contribute significantly to neoplastic transformation. Cullin-dependent ubiquitin ligases (CDLs) form a class of structurally related multisubunit enzymes central to the ubiquitin-mediated proteolysis of many important biological substrates. In this review, we describe the role of CDLs in the ubiquitinylation of cancer-related substrates and discuss how altered ubiquitinylation by CDLs may contribute to tumor development.

The many faces of β-TrCP E3 ubiquitin ligases: reflections in the magic mirror of cancer

Beta-transducin repeats-containing proteins (beta-TrCP) serve as the substrate recognition subunits for the SCFbeta-TrCP E3 ubiquitin ligases. These ligases ubiquitinate specifically phosphorylated substrates and play a pivotal role in the regulation of cell division and various signal transduction pathways, which, in turn, are essential for many aspects of tumorigenesis. We review the functions of the SCFbeta-TrCP ligases in the light of their relevance to cell growth, survival and transformation. Mechanisms underlying beta-TrCP regulation and their aberration in human and animal cancer as well as prospective of targeting beta-TrCP as a means of anticancer therapy are also discussed.

Ubiquitination-mediated regulation of biosynthesis of the adhesion receptor SHPS-1 in response to endoplasmic reticulum stress

Misfolding of proteins during endoplasmic reticulum (ER) stress results in the formation of cytotoxic aggregates. The ER-associated degradation pathway counteracts such aggregation through the elimination of misfolded proteins by the ubiquitin-proteasome system. We now show that SHP substrate-1 (SHPS-1), a transmembrane glycoprotein that regulates cytoskeletal reorganization and cell-cell communication, is a physiological substrate for the Skp1-Cullin1-NFB42-Rbx1 (SCF(NFB42)) E3 ubiquitin ligase, a proposed mediator of ER-associated degradation. SCF(NFB42) mediated the polyubiquitination of immature SHPS-1 and its degradation by the proteasome. Ectopic expression of NFB42 both suppressed the formation of aggresome-like structures and the phosphorylation of the translational regulator eIF2alpha induced by overproduction of SHPS-1 as well as increased the amount of mature SHPS-1 at the cell surface. An NFB42 mutant lacking the F box domain had no such effects. Our results suggest that SCF(NFB42) regulates SHPS-1 biosynthesis in response to ER stress.

Mouse Fbw7/Sel-10/Cdc4 is required for notch degradation during vascular development

Mammalian Fbw7 (also known as Sel-10, hCdc4, or hAgo) is the F-box protein component of an SCF (Skp1-Cul1-F-box protein-Rbx1)-type ubiquitin ligase, and the mouse Fbw7 is expressed prominently in the endothelial cell lineage of embryos. We generated mice deficient in Fbw7 and found that the embryos died in utero at embryonic day 10.5-11.5, manifesting marked abnormalities in vascular development. Vascular remodeling was impaired in the brain and yolk sac, and the major trunk veins were not formed. In vitro para-aortic splanchnopleural explant cultures from Fbw7(-/-) embryos also manifested an impairment of vascular network formation. Notch4, which is the product of the proto-oncogene Int3 and an endothelial cell-specific mammalian isoform of Notch, accumulated in Fbw7(-/-) embryos, resulting in an increased expression of Hey1, which encodes a transcriptional repressor that acts downstream of Notch signaling and is implicated in vascular development. Expression of Notch1, -2, or -3 or of cyclin E was unaffected in Fbw7(-/-) embryos. Mammalian Fbw7 thus appears to play an indispensable role in negative regulation of the Notch4-Hey1 pathway and is required for vascular development.