Regulating protein degradation by ubiquitination

Do DNA copy number changes differentiate uterine from non-uterine leiomyosarcomas and predict metastasis?

DNA copy number changes were investigated in 51 (19 uterine and 32 nonuterine) primary leiomyosarcomas by comparative genomic hybridization. The aim was to evaluate whether true biological differences exist between uterine and nonuterine leiomyosarcoma and whether changes revealed by comparative genomic hybridization have prognostic value. Genomic imbalances were found in 48 (94%) cases. The most frequent DNA copy number changes were losses in 10q (35%), 13q (57%), and 16q (41%), gains in 1q (41%), and gains and high-level amplifications in 17p (39%). Gains were nearly as frequent as losses in both uterine and nonuterine leiomyosarcoma. Correlation-based tree modeling revealed two clusters that segregated significantly a group of uterine (gains at 1q11-q24) and a group of nonuterine (losses at 13q14-q34, 16q11.1-q24, and 10q21-q26) cases. The nonuterine cluster was associated with subcutaneous origin and a trend toward increased metastasis-free survival. Further explorative analyses identified aberrations associated with shorter metastasis-free survival time, including losses at 2q32.1-q37 and gains at 8q24.1-q24.3, whereas the cases with losses at 6cen-p25 showed longer metastasis-free survival time.

Src-like adaptor protein regulates TCR expression on thymocytes by linking the ubiquitin ligase c-Cbl to the TCR complex

The adaptor molecule SLAP and E3 ubiquitin ligase c-Cbl each regulate expression of T cell receptor (TCR)-CD3 on thymocytes. Here we provide genetic and biochemical evidence that both molecules function in the same pathway. TCR-CD3 expression was similar in the absence of SLAP and/or c-Cbl. SLAP and c-Cbl were found to interact, and their expression together downregulated CD3epsilon. This required multiple domains in SLAP and the ring finger of c-Cbl. Furthermore, expression of SLAP and c-Cbl together induced TCRzeta ubiquitination and degradation, preventing the accumulation of fully assembled recycling TCR complexes. These studies indicate that SLAP links the E3 ligase activity of c-Cbl to the TCR, allowing for stage-specific regulation of TCR expression.

The proteasome inhibitor MG-132 sensitizes PC-3 prostate cancer cells to ionizing radiation by a DNA-PK-independent mechanism

BACKGROUND

By modulating the expression levels of specific signal transduction molecules, the 26S proteasome plays a central role in determining cell cycle progression or arrest and cell survival or death in response to stress stimuli, including ionizing radiation. Inhibition of proteasome function by specific drugs results in cell cycle arrest, apoptosis and radiosensitization of many cancer cell lines. This study investigates whether there is also a concomitant increase in cellular radiosensitivity if proteasome inhibition occurs only transiently before radiation. Further, since proteasome inhibition has been shown to activate caspase-3, which is involved in apoptosis, and caspase-3 can cleave DNA-PKcs, which is involved in DNA-double strand repair, the hypothesis was tested that caspase-3 activation was essential for both apoptosis and radiosensitization following proteasome inhibition.

METHODS

Prostate carcinoma PC-3 cells were treated with the reversible proteasome inhibitor MG-132. Cell cycle distribution, apoptosis, caspase-3 activity, DNA-PKcs protein levels and DNA-PK activity were monitored. Radiosensitivity was assessed using a clonogenic assay.

RESULTS

Inhibition of proteasome function caused cell cycle arrest and apoptosis but this did not involve early activation of caspase-3. Short-time inhibition of proteasome function also caused radiosensitization but this did not involve a decrease in DNA-PKcs protein levels or DNA-PK activity.

CONCLUSION

We conclude that caspase-dependent cleavage of DNA-PKcs during apoptosis does not contribute to the radiosensitizing effects of MG-132.

Understanding the role of thyroid hormone in Sertoli cell development: a mechanistic hypothesis

More than a decade of research has shown that Sertoli cell proliferation is regulated by thyroid hormone. Neonatal hypothyroidism lengthens the period of Sertoli cell proliferation, leading to increases in Sertoli cell number, testis weight, and daily sperm production (DSP) when euthyroidism is re-established. In contrast, the neonatal Sertoli cell proliferative period is shortened under hyperthyroid conditions, but the mechanism by which thyroid hormone is able to negatively regulate Sertoli cell proliferation has been unclear. Recent progress in the understanding of the cell cycle has provided the opportunity to dissect the molecular targets responsible for thyroid-hormone-mediated effects on Sertoli cell proliferation. In this review, we discuss recent results indicating a critical role for the cyclin-dependent kinase inhibitors (CDKI) p27(Kip1) and p21(Cip1) in establishing Sertoli cell number, testis weight, and DSP, and the ability of thyroid hormone to modulate these CDKIs. Based on these recent results, we propose a working hypothesis for the way in which thyroid hormone regulates the withdrawal of the cell cycle by controlling CDKI degradation. Finally, although Sertoli cells have been shown to have two biologically active thyroid hormone receptor (TR) isoforms, TRalpha1 and TRbeta1, experiments with transgenic mice lacking TRalpha or TRbeta illustrate that only one TR mediates thyroid hormone effects in neonatal Sertoli cells. Although significant gaps in our knowledge still remain, advances have been made toward appreciation of the molecular sequence of events that occur when thyroid hormone stimulates Sertoli cell maturation.

Nicotine Inactivation of the Proapoptotic Function of Bax through Phosphorylation

Nicotine-induced cell survival is associated with chemoresistance of human lung cancer cells, but our understanding of the intracellular mechanism(s) is fragmentary. Bax is a major proapoptotic member of the Bcl2 family and a molecule required for apoptotic cell death. Growth factor (i.e. granulocyte-macrophage colony-stimulating factor)-induced phosphorylation of Bax has been reported to negatively regulate its proapoptotic function. Because Bax is ubiquitously expressed in both small cell lung cancer and non-small cell lung cancer cells, nicotine may mimic growth factor(s) to regulate the activity of Bax. We found that nicotine potently induces Bax phosphorylation at Ser-184, which results in abrogation of the proapoptotic activity of Bax and increased cell survival. AKT, a known physiological Bax kinase, is activated by nicotine, co-localizes with Bax in the cytoplasm, and can directly phosphorylate Bax in vitro. Treatment of cells with the phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 or specific depletion of AKT expression by RNA interference can block both nicotine-induced Bax phosphorylation and cell survival. Importantly, nicotine-induced Bax phosphorylation potently blocks stress-induced translocation of Bax from cytosol to mitochondria, impairs Bax insertion into mitochondrial membranes, and reduces the half-life of Bax protein (i.e. from 9-12 h to <6 h). Because knockdown of Bax expression by gene silencing results in prolonged cell survival following treatment with cisplatin in the absence or presence of nicotine, Bax may be an essential component in the nicotine survival signaling pathway. Thus, nicotine-induced survival and chemoresistance of human lung cancer cells may occur in a novel mechanism involving activation of PI3K/AKT that directly phosphorylates and inactivates the proapoptotic function of Bax.

The role of α-synuclein in neurodegenerative diseases

Alpha-synuclein is a 140 amino acid neuronal protein that has been associated with several neurodegenerative diseases. A point mutation in the gene coding for the alpha-synuclein protein was the first discovery linking this protein to a rare familial form of Parkinson's disease (PD). Subsequently, other mutations in the alpha-synuclein gene have been identified in familial PD. The aggregated proteinaceous inclusions called Lewy bodies found in PD and cortical Lewy body dementia (LBD) were discovered to be predominantly alpha-synuclein. Aberrant aggregation of alpha-synuclein has been detected in an increasing number of neurodegenerative diseases, collectively known as synucleopathies. Alpha-synuclein exists physiologically in both soluble and membrane-bound states, in unstructured and alpha-helical conformations, respectively. The physiological function of alpha-synuclein appears to require its translocation between these subcellular compartments and interconversion between the 2 conformations. Abnormal processing of alpha-synuclein is predicted to lead to pathological changes in its binding properties and function. In this review, genetic and environmental risk factors for alpha-synuclein pathology are described. Various mechanisms for in vitro and in vivo alpha-synuclein aggregation and neurotoxicity are summarized, and their relevance to neuropathology is explored.

Molecular characterization of the nonspecific cytotoxic cell receptor (NCCRP-1) demonstrates gilthead seabream NCC heterogeneity

Teleost fish NCCs (nonspecific cytotoxic cells) are thought to be the evolutionary precursors of the mammalian NK cells. A novel mechanism mediating the NCC-mediated cytotoxicity has been described in teleosts. Now, this NCC receptor protein-1 (NCCRP-1) was characterized in gilthead seabream. The NCCRP-1 full-length sequence contains 1036 bp with an open reading frame of 702 bp. A comparison of the predicted 233-amino acid protein with several fish orthologues indicates a highly conserved sequence containing the F-box associated (FBA) domain and proline-rich motifs (PRM) characteristics of this family. The phylogenetical tree shows that seabream NCCRP-1 belongs to the NCCRP subfamily within the FBA family of proteins. This is a single copy gene with a constitutive and ubiquitous expression as determined by RT-PCR and flow cytometry. The results show that lymphocytes, monocyte/macrophages and acidophilic granulocytes from lymphoid tissues express the receptor, both at gene and protein level. Immunofluorescence microscopic observations confirm the previous results. The implications of this receptor on seabream NCC activity are discussed.

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.

Targeting of Bacillus anthracis interaction factors for human macrophages using two-dimensional gel electrophoresis

Bacillus anthracis, a gram-positive, endospore-forming, aerobic rod-shaped bacterium, interacts with macrophages at various stages of the disease. Spore germination and the outgrowth of vegetative bacilli are crucial steps enabling the bacteria to proliferate actively and to synthesize the virulence factors leading to a massive septicemia. In this study, we performed a proteomic analysis and MALDI-TOF/MS were carried out to identify proteins using human macrophages infected with the spores of B. anthracis live-Sterne or inactivated-Sterne. We identified 21 proteins which are related to the infection of B. anthracis spores on human macrophages at the early stage events. These proteins function in processes such as cytoskeleton regulation, apoptosis, cell division, and protein degradation. Proteins such as PAK 2 revealed a relationship to apoptosis in human macrophages. These proteins play an important role in the macrophage survival and death on human macrophages with infected B. anthracis spores.

WWP1-dependent ubiquitination and degradation of the lung Krüppel-like factor, KLF2

The zinc-finger transcription factor Krüppel-like factor-2 plays an important role in pulmonary development, inhibition of adipocyte differentiation, and maintaining quiescence in single-positive T cells. KLF2 levels rapidly decrease during adipogenesis and activation of T cells, but the pathways involved remain unclear. Previously, we identified WWP1, a HECT-domain E3-ubiquitin ligase, as an interacting partner of KLF2. This led us to speculate that KLF2 may be targeted for ubiquitination. Here, we demonstrate that WWP1 interacts with KLF2 in vivo and mediates both poly-ubiquitination and proteasomal degradation of KLF2. Deleting the inhibitory domain of KLF2 abrogated KLF2-WWP1 interactions and abolished WWP1-mediated poly-ubiquitination and down-regulation of KLF2. Furthermore, lysine-121 in the inhibitory domain of KLF2 is critical for ubiquitin-conjugation. Finally, the catalytic cysteine of WWP1 is not required for KLF2-ubiquitination. Our experiments demonstrate for the first time that WWP1 promotes ubiquitination and degradation of KLF2 and is not involved in the ubiquitin-transfer reaction.

Accelerated ubiquitination and proteasome degradation of a genetic variant of inducible nitric oxide synthase

Biochemical and pharmacological studies have suggested that NOS2 (inducible nitric oxide synthase) has a functional role in the blood pressure response to increases in dietary salt intake. On a high-salt diet, the Dahl/Rapp salt-sensitive (S) strain of rat, a genetic model of salt-sensitive hypertension, did not show increased nitric oxide production. NOS2 from S rats possesses a point mutation that results in substitution of proline for serine at position 714. In the present study, rat NOS2 was shown to be ubiquitinated in vitro and in vivo and to be degraded by the proteasome; this process was accelerated for the S714P mutant. Accelerated degradation of the S714P mutant enzyme accounted for the diminished enzyme activity of this mutant. Hsp90 (heat-shock protein 90) associated with NOS2 and modulated degradation, but was not responsible for the accentuated degradation of the S714P mutant enzyme. The combined findings demonstrate the integral role of ubiquitination and degradation by the proteasome in the regulation of NO production by rat NOS2. Demonstrating that this process is responsible for the abnormal function of the S714P mutant NOS2 in S rats confirms the physiological importance of the proteasome in NOS2 function.

The correlation between ubiquitin in the brainstem and sleep apnea in SIDS victims

BACKGROUND

The sudden infant death syndrome (SIDS) is still the main cause of postneonatal infant death and its etiology has stimulated many competing theories, among which is the role of hypoxia and brainstem abnormalities. One report claims an increased in ubiquitin in the liver of SIDS victims, ubiquitin being one of the heat-shock proteins. The correlation between ubiquitin in the brainstem and sleep apnea in SIDS was investigated here.

MATERIALS AND METHODS

Among 27,000 infants studied prospectively to characterize their sleep-wake behavior, 38 infants died under 6 months of age, including 26 cases of SIDS. All the infants had been recorded during one night in a pediatric sleep laboratory some 3-12 weeks before death. The frequency and duration of sleep apnea were analyzed. Brainstem material was collected at autopsy and examined immunohistochemically for ubiquitin. The density of ubiquitin-positive elements was measured semiquantitatively. Correlation analyses were carried out between the density of ubiquitin-positive elements and the data on sleep apnea.

RESULTS

In the victims of SIDS, a statistically significant positive correlation was found between the density of ubiquitin-positive neuronal factors in the pons and the frequency of obstructive apnea (p=0.001) and statistically significant negative correlations were seen between the density of ubiquitin-positive cells in the ependyma in the pons and the duration of obstructive apnea (p=0.044) and between the density of ubiquitin-positive cells in the subependyma in the medulla and the frequency of central apnea (p=0.024).

CONCLUSIONS

It was found that three significant associations existed between the pathological data referring to ubiquitin and physiological data in SIDS victims. These facts are in agreements with the association of sleep apnea in SIDS.

Preferential interaction of TIP120A with Cul1 that is not modified by NEDD8 and not associated with Skp1

The SCF complex, which consists of the invariable components Skp1, Cul1, and Rbx1 as well as a variable F-box protein, functions as an E3 ubiquitin ligase. The mechanism by which the activity of this complex is regulated, however, has been unclear. The application of tandem affinity purification has now resulted in the identification of a novel Cul1-binding protein: TATA-binding protein-interacting protein 120A (TIP120A, also called CAND1). Immunoprecipitation, immunoblot, and immunofluorescence analyses with mammalian cells revealed that TIP120A physically associates with Cul1 in the nucleus and that this interaction is mediated by a central region of Cul1 distinct from its binding sites for Skp1 and Rbx1. Furthermore, TIP120A was shown to interact selectively with Cul1 that is not modified by NEDD8. The Cul1-TIP120A complex does not include Skp1, raising the possibility that TIP120A competes with Skp1 for binding to Cul1. These observations thus suggest that TIP120A may function as a negative regulator of the SCF complex by binding to nonneddylated Cul1 and thereby preventing assembly of this ubiquitin ligase.

Characterization of the mouse gene for the U-box-type ubiquitin ligase UFD2a

UFD2a is a mammalian homolog of Saccharomyces cerevisiae Ufd2, originally described as an E4 ubiquitination factor. UFD2a belongs to the U-box family of ubiquitin ligases (E3s) and likely functions as both an E3 and E4. We have isolated and characterized the mouse gene (Ube4b) for UFD2a. A full-length (approximately 5700 bp) Ube4b cDNA was isolated and the corresponding gene spans >100 kb, comprising 27 exons. Luciferase reporter gene analysis of the 5(') flanking region of Ube4b revealed that nucleotides -1018 to -943 (relative to the translation initiation site) possess promoter activity. This functional sequence contains two putative Sp1 binding sites but not a TATA box. Immunoblot and immunohistochemical analyses revealed that UFD2a is expressed predominantly in the neuronal tissues. We also show that UFD2a interacts with VCP (a AAA-family ATPase) that is thought to mediate protein folding. These data implicate UFD2a in the degradation of neuronal proteins by the ubiquitin-proteasome pathway.

Macrophage interactions

B. anthracis virulence is the sum of the contributions of factors involved in toxicity, growth and persistence in the host. Recent data has revealed that the interactions between B. anthracis and macrophage is central to the B. anthracis pathogenesis. This review presents and describes tactics by which B. anthracis not only overcomes and avoids macrophages but also perverts the host defense immune system and defense-related products to its advantage. The understanding of the complex network of such interactions is likely to allow new therapeutic and preventative strategies to be developed.

A new subfamily of structurally related human F-box proteins

F-box proteins, a critical component of the evolutionary conserved ubiquitin-protein ligase complex SCF (Skp1/Cdc53-Cullin1/F-box), recruit substrates for ubiquitination and consequent degradation through their specific protein-protein interaction domains. Here, we report the identification of full-length cDNAs encoding three novel human F-box proteins named FBG3, FBG4 and FBG5 which display similarity with previously identified NFB42 (FBX2) and FBG2 (FBX6) proteins. All five proteins are characterized by an approximately 180-amino-acid (aa) conserved C-terminal domain and thus constitute a third subfamily of mammalian F-box proteins. Analysis of genomic organization of the five FBG genes revealed that all of them consist of six exons and five introns. FBG1, FBG2 and FBG3 genes are located in tandem on chromosome 1p36, and FBG4 and FBG5 are mapped to chromosome 19q13. FBG genes are expressed in a limited number of human tissues including kidney, liver, brain and muscle tissues. Expression of rat FBG2 gene was found related to differentiation/proliferation status of hepatocytes. Specifically, FBG2 mRNA was expressed in foetal liver, decreased after birth and re-accumulated in adult liver. Expression of FBG2 was strongly inhibited in hepatoma cells by okadaic acid.

Ubiquitin genes in rainbow trout (Oncorhynchus mykiss)

Ubiquitin is a small protein involved in intracellular proteolysis. It is highly conserved throughout eukaryotic phyla and has been detected in such diverse species as yeast, barley, Drosophila and man. A previous study showed that chromatin of rainbow trout testis contains free ubiquitin with a sequence similar to that of other phyla. In the present study, which focused on rainbow trout but included eleven other species, it is shown that fish ubiquitin genetic organisation and expression are similar to those of other phylogenetic groups through the following set of observations: (a) Multiple loci were detected, (b) These loci encode repeats of ubiquitin, (c) Although the DNA sequences are not conserved, the encoded amino acid sequences are fully conserved, (d) The expression of ubiquitin was influenced by cell culture conditions and viral infection.

N-acetyl-L-cysteine inhibits 26S proteasome function: implications for effects on NF-kappaB activation

Ionizing radiation shares with cytokines, such as TNF-alpha, an ability to generate free radicals in cells and activate downstream proinflammatory responses through NF-kappaB-dependent signal transduction pathways. Support for the role of free radicals in triggering such responses comes from the use of free radical scavengers like N-acetyl-L-cysteine (NAC). The nature of the link between free radical generation and NF-kappaB activation is, however, unclear. In this study, we explore the possibility that scavenging of free radicals by NAC might not be the mechanism by which it inhibits NF-kappaB activation, but rather that NAC acts through inhibition of proteasome function. The effect of NAC on the chymotryptic function of the 26s and 20s proteasome complex was measured in extracts from EVC 304 bladder carcinoma cells by assessing degradation of fluorogenic substrates. NAC inhibited 26s but not 20s proteasome activity, suggesting that it interferes with 19s regulatory subunit function. NAC blocked radiation-induced NF-kappaB activity in ECV 304 cells and RAW 264.7 macrophages, as measured by a gel shift assay, at doses that inhibited proteasome activity. This provides a possible mechanism whereby NAC could block NF-kappaB activation and affect the expression of other molecules that are dependent on the ubiquitin/proteasome system for their degradation, other than by scavenging free radicals.

Multiple Skp1-related proteins in Caenorhabditis elegans: diverse patterns of interaction with Cullins and F-box proteins

BACKGROUND

The ubiquitin-proteasome pathway of proteolysis controls the abundance of specific regulatory proteins. The SCF complex is a type of ubiquitin-protein ligase (E3) that contributes to this pathway in many biological systems. In yeast and mammals, the SCF complex consists of common components, including Skp1, Cdc53/Cul1, and Rbx1, as well as variable components known as F-box proteins. Whereas only one functional Skp1 gene is present in the human genome, the genome of Caenorhabditis elegans has now been shown to contain at least 21 Skp1-related (skr) genes. The biochemical properties, expression, and function of the C. elegans SKR proteins were examined.

RESULTS

Of the 17 SKR proteins examined, eight (SKR-1, -2, -3, -4, -7, -8, -9, and -10) were shown to interact with C. elegans CUL1 by yeast two-hybrid analysis or a coimmunoprecipitation assay in mammalian cells. Furthermore, SKR proteins exhibited diverse binding specificities for C. elegans F-box proteins. The tissue specificity of expression of the CUL1-interacting SKR proteins was also varied. Suppression of skr-1 or skr-2 genes by double-stranded RNA interference resulted in embryonic death, whereas that of skr-7, -8, -9, or -10 was associated with slow growth and morphological abnormalities.

CONCLUSIONS

The multiple C. elegans SKR proteins exhibit marked differences in their association with Cullins and F-box proteins, in tissue specificity of expression, and in phenotypes associated with functional suppression by RNAi. At least eight of the SKR proteins may, like F-box proteins, act as variable components of the SCF complex in C. elegans.

The human immunodeficiency virus type 1 accessory protein Vpu induces apoptosis by suppressing the nuclear factor kappaB-dependent expression of antiapoptotic factors

Human immunodeficiency virus (HIV) type 1 Vpu is an integral membrane protein with a unique affinity for betaTrCP (TrCP), a key member of the SkpI-Cullin-F-box E3 ubiquitin ligase complex that is involved in the regulated degradation of cellular proteins, including IkappaB. Remarkably, Vpu is resistant to TrCP-mediated degradation and competitively inhibits TrCP-dependent degradation of IkappaB, resulting in the suppression of nuclear factor (NF)-kappaB activity in Vpu-expressing cells. We now report that Vpu, through its interaction with TrCP, potently contributes to the induction of apoptosis in HIV-infected T cells. Vpu-induced apoptosis is specific and independent of other viral proteins. Mutation of a TrCP-binding motif in Vpu abolishes its apoptogenic property, demonstrating a close correlation between this property of Vpu and its ability to inhibit NF-kappaB activity. The involvement of NF-kappaB in Vpu-induced apoptosis is further supported by the finding that the levels of antiapoptotic factors Bcl-xL, A1/Bfl-1, and TNF receptor-associated factor (TRAF)1, all of which are expressed in an NF-kappaB-dependent manner, are reduced and, at the same time, levels of active caspase-3 are elevated. Thus, Vpu induces apoptosis through activation of the caspase pathway by way of inhibiting the NF-kappaB-dependent expression of antiapoptotic genes.

Regulation of rheumatoid synoviocyte proliferation by endogenous p53 induction

The p53 tumour suppressor protein protects cells from tumorigenic alterations by inducing either cell growth arrest or apoptosis. In the present study, we investigated the role of endogenous p53 expressed in rheumatoid arthritis synovial fibroblasts which show transformed-appearing phenotypes. Type B synovial cells (fibroblast-like synovial cells) were exposed to a proteasome inhibitor, carbobenzoxyl-leucinyl-leucinyl-leucinal (MG-132). During this process, the expressions of p53 and p21 were examined by Western blot. Cell cycle analysis of the synovial cells was determined by DNA staining using propidium iodide (PI). Inhibition of proteasome resulted in the accumulation of p53 which was followed by an increase in the amount of a cyclin-dependent kinase (CDK)-inhibitor, p21. As a consequence, the retinoblastoma gene product, Rb, remained in the hypophosphorylated state, thus preventing PDGF-stimulated synovial cells from progressing into S-phase. This study shows that endogenous p53, which is inducible in rheumatoid synovial cells, is functionally active based on the findings that its expression blocks the G1/S transition by inhibiting the CDK-mediated phosphorylation of Rb via p21 induction. Thus the induction of p53 using proteasome inhibitor may provide a new approach in the treatment of RA.

Src-catalyzed phosphorylation of c-Cbl leads to the interdependent ubiquitination of both proteins

The protooncogene c-Cbl has recently emerged as an E3 ubiquitin ligase for activated receptor tyrosine kinases. We report here that c-Cbl also mediates the ubiquitination of another protooncogene, the non-receptor tyrosine kinase c-Src, as well as of itself. The c-Cbl-dependent ubiquitination of Src and c-Cbl requires c-Cbl's RING finger, Src kinase activity, and c-Cbl's tyrosine phosphorylation, probably on Tyr-371. In vitro, c-Cbl forms a stable complex with the ubiquitin-conjugating enzyme UbcH7, but active Src destabilizes this interaction. In contrast, Src inhibition stabilizes the c-Cbl. UbcH7.Src complex. Finally, c-Cbl reduces v-Src protein levels and suppresses v-Src-induced STAT3 activation. Thus, in addition to mediating the ubiquitination of activated receptor tyrosine kinases, c-Cbl also acts as a ubiquitin ligase for the non-receptor tyrosine kinase Src, thereby down-regulating Src.

Involvement of the ubiquitin/proteasome system in sorting of the interleukin 2 receptor beta chain to late endocytic compartments

Down-regulation of cell surface growth factor receptors plays a key role in the tight control of cellular responses. Recent reports suggest that the ubiquitin system, in addition to participating in degradation by the proteasome of cytosolic and nuclear proteins, might also be involved in the down-regulation of various membrane receptors. We have previously characterized a signal in the cytosolic part of the interleukin 2 receptor beta chain (IL2Rbeta) responsible for its targeting to late endosomes/lysosomes. In this report, the role of the ubiquitin/proteasome system on the intracellular fate of IL2Rbeta was investigated. Inactivation of the cellular ubiquitination machinery in ts20 cells, which express a thermolabile ubiquitin-activating enzyme E1, leads to a significant decrease in the degradation rate of IL2Rbeta, with little effect on its internalization. In addition, we show that a fraction of IL2Rbeta can be monoubiquitinated. Furthermore, mutation of the lysine residues of the cytosolic region of a chimeric receptor carrying the IL2Rbeta targeting signal resulted in a decreased degradation rate. When cells expressing IL2Rbeta were treated either by proteasome or lysosome inhibitors, a significant decrease in receptor degradation was observed. Our data show that ubiquitination is required for the sorting of IL2Rbeta toward degradation. They also indicate that impairment of proteasome function might more generally affect intracellular routing.

U box proteins as a new family of ubiquitin-protein ligases

The U box is a domain of approximately 70 amino acids that is present in proteins from yeast to humans. The prototype U box protein, yeast Ufd2, was identified as a ubiquitin chain assembly factor that cooperates with a ubiquitin-activating enzyme (E1), a ubiquitin-conjugating enzyme (E2), and a ubiquitin-protein ligase (E3) to catalyze ubiquitin chain formation on artificial substrates. E3 enzymes are thought to determine the substrate specificity of ubiquitination and have been classified into two families, the HECT and RING finger families. Six mammalian U box proteins have now been shown to mediate polyubiquitination in the presence of E1 and E2 and in the absence of E3. These U box proteins exhibited different specificities for E2 enzymes in this reaction. Deletion of the U box or mutation of conserved amino acids within it abolished ubiquitination activity. Some U box proteins catalyzed polyubiquitination by targeting lysine residues of ubiquitin other than lysine 48, which is utilized by HECT and RING finger E3 enzymes for polyubiquitination that serves as a signal for proteolysis by the 26 S proteasome. These data suggest that U box proteins constitute a third family of E3 enzymes and that E4 activity may reflect a specialized type of E3 activity.

Ubiquitination and degradation of Syk and ZAP-70 protein tyrosine kinases in human NK cells upon CD16 engagement

Syk and ZAP-70 nonreceptor protein tyrosine kinases (PTKs) are essential elements in several cascades coupling immune receptors to intracellular responses. The critical role of these kinases in promoting the propagation of intracellular signaling requires a tight regulation of their activity, thus the existence of a negative feedback loop regulating their expression can be hypothesized. Herein, we have investigated whether ubiquitin-dependent proteolysis could be a mechanism responsible for controlling the fate of Syk and ZAP-70 after their immunoreceptor-induced activation. We found that both Syk and ZAP-70 become ubiquitinated in response to aggregation of the low affinity Fc receptor for IgG (CD16) on human natural killer cells. We confirmed the identity of the major in vivo ubiquitinated kinase species by performing an in vitro ubiquitination assay. In addition, we found that after CD16 stimulation, ubiquitinated forms of Syk and ZAP-70 associate with the receptor complex. After CD16 engagement, we also observed a decrease in the stability of Syk and ZAP-70 PTKs that is counteracted by pretreatment with either proteasome or lysosomal inhibitors. Moreover, in the presence of the proteasome inhibitor, epoxomicin, we observed an accumulation of ubiquitinated forms of both kinases. Our findings provide evidence of ligand-induced ubiquitination of nonreceptor PTKs belonging to the Syk family and propose the ubiquitin-dependent proteasome-mediated degradation pathway as a mechanism for attenuating the propagation of intracellular signaling initiated by immune receptor engagement.

Proteasomes are a target of the anti-tumour drug vinblastine

Proteasomes, the proteolytic machinery of the ubiquitin/ATP-dependent pathway, have a relevant role in many processes crucial for cell physiology and cell cycle progression. Proteasome inhibitors are used to block cell cycle progression and to induce apoptosis in certain cell lines. Here we examine whether proteasomal function is affected by the anti-tumour drug vinblastine, whose cytostatic action relies mainly on the disruption of mitotic spindle dynamics. The effects of vinblastine on the peptidase activities of human 20 S and 26 S proteasomes and on the proteolytic activity of 26 S proteasome were assessed in the presence of specific fluorogenic peptides and (125)I-lysozyme-ubiquitin conjugates respectively. The assays of ubiquitin-protein conjugates and of inhibitory kappa B alpha (I kappa B alpha), which are characteristic intracellular proteasome substrates, by Western blotting on lysates from HL60 cells incubated with or without vinblastine, illustrated the effects of vinblastine on proteasomes in vivo. We also evaluated the effects of vinblastine on the signal-induced degradation of I kappa B alpha. Vinblastine at 3--110 microM reversibly inhibited the chymotrypsin-like activity of the 20 S proteasome and the trypsin-like and peptidyl-glutamyl-peptide hydrolysing activities of both proteasomes, but only at 110 microM vinblastine was the chymotrypsin-like activity of the 26 S proteasome inhibited; furthermore, at 25--200 microM the drug inhibited the degradation of ubiquitinated lysozyme. In HL60 cells exposed for 6 h to 0.5--10 microM vinblastine, the drug-dose-related accumulation of polyubiquitinated proteins, as well as that of a high-molecular-mass form of I kappa B alpha, occurred. Moreover, vinblastine impaired the signal-induced degradation of I kappa B alpha. Cell viability throughout the test was approx. 95%. Proteasomes can be considered to be a new and additional vinblastine target.

Conjugation of phenylalanine hydroxylase with polyubiquitin chains catalysed by rat liver enzymes

Phenylalanine hydroxylase (PAH, EC 1.14.16.1) is a highly regulated liver enzyme which catalyses the conversion of L-phenylalanine to L-tyrosine, the rate-limiting step in the catabolic pathway of this amino acid. Among the approx. 400 different mutations of human (h) PAH, frequently associated with the metabolic disease phenylketonuria, a low stability is a characteristic property when expressed in eucaryotic cells. In this study, the pathway of hPAH degradation is addressed with focus on its conjugation with polyubiquitin chains catalysed by the ubiquitin-conjugating enzyme system (E1, E2, E3) isolated from rat liver by covalent affinity chromatography on ubiquitin-Sepharose. In the reconstituted in vitro ubiquitination assay, the enzyme system catalysed both the formation of free polyubiquitin chains and the polyubiquitination of wild-type (wt) hPAH and its 'catalytic domain' (DeltaN102/DeltaC24-hPAH) as visualized by two-dimensional electrophoresis. The ubiquitination of wt-PAH may play a role in the degradation of this liver enzyme notably of its many unstable disease-associated mutant forms. The present approach may also have a more general application in the study of liver proteins as possible targets for ubiquitination.

Involvement of proteasomes in gene induction by interferon and double-stranded RNA

Cytokine induced gene expression is mediated through the ligand-dependent activation of the janus kinase (jak)/signal transducer and activator of transcription (STAT) signal transduction pathway. The ubiquitin proteasome pathway functions in the controlled degradation of cellular proteins, and regulates cytokine signal transduction through the degradation of specific signaling components. Interferon (IFN) treatment induces genes that function in ubiquitin conjugation, suggesting a reciprocal regulation of proteasome activity and IFN action; however, a role for the proteasome in IFN-alpha-induced gene expression has not been examined. In this report, we find that proteasome inhibitors markedly reduce the induction of interferon-stimulated-gene 15 (ISG15), ISG43, and STAT1 by IFN-alpha and double-stranded RNA (dsRNA). The reduction in gene expression by proteasome inhibitors was dose-dependent, and was specific for ISGs. Neither STAT1 phosphorylation nor ISGF-3 activation was affected by proteasome inhibition at early times post-IFN treatment. Cycloheximide treatment diminished the effect of proteasome inhibitors on ISG induction, implicating an IFN/dsRNA-induced protein in this activity. These findings demonstrate that a functional proteasome is required for optimal ISG induction, and are consistent with a model in which IFN and dsRNA induce a proteasome-sensitive repressor of ISG expression.

Lung Krüppel-like factor contains an autoinhibitory domain that regulates its transcriptional activation by binding WWP1, an E3 ubiquitin ligase

Lung Krüppel-like factor (LKLF/Krüppel-like factor 2), a member of the Krüppel-like factor family of transcription factors, is expressed predominantly in the lungs, with low levels of expression in other organs such as heart, spleen, skeletal muscle, and testis. LKLF is essential during pulmonary development and single-positive T-cell development and is indispensable during mouse embryogenesis. In this study, we performed a series of experiments to define the activation domain of LKLF as a means to further advance the understanding of the molecular mechanisms underlying transcriptional regulation by this transcription factor. Using deletion analysis, it is shown that LKLF contains a transcriptional activation domain as well as a strong autoinhibitory subdomain. The inhibitory subdomain is able to independently suppress transcriptional activation of other strong activators such as viral protein 16, VP16. This occurs either when the inhibitory domain is fused directly to VP16 or when the inhibitory domain is independently bound to DNA by GAL4 DNA-binding domain independent of the VP16 activator. Overexpression of the LKLF autoinhibitory domain alone potentiates transactivation by wild type LKLF, suggesting that the inhibitory domain binds a cofactor that prevents LKLF from transactivating. A yeast-two hybrid screen identified WWP1, an E3 ubiquitin ligase that binds specifically to the LKLF inhibitory domain but not to other transcription factors. In mammalian cells, WWP1 functions as a cofactor by binding LKLF and suppressing transactivation. These data demonstrate that LKLF contains multiple domains that either potentiate or inhibit the ability of this factor to function as an activator of transcription; moreover, regulation of LKLF transactivation is attenuated by an E3 ubiquitin ligase, WWP1.

Sequence of events in the assembly of Mallory body components in mouse liver: clues to the pathogenesis and significance of Mallory body formation

BACKGROUND/AIMS

Chronic intoxication of mice with 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC) or griseofulvin (GF) results in appearance of Mallory bodies (MBs) and alterations of the keratin cytoskeleton, which are reversible upon drug withdrawal but recur after readministration within 2-3 days.

METHODS

DDC- or GF-treated and recovered mice were reintoxicated with the original drugs but also colchicine and lumicolchicine. Cytoskeletal alterations of hepatocytes and MB formation were monitored by immunofluorescence microscopy using keratin, MB-specific antibodies, antibodies to phosphoepitopes and to HSP70. Keratin 8/18 mRNA expression and protein levels were determined by competitive reverse transcription-polymerase chain reaction, in situ-hybridization and western blotting.

RESULTS

Duration of pretreatment was important for the efficiency of MB triggering. Rapid increase of keratin 8/18 mRNA and proteins was found in all reintoxicated mice concomitant with MB formation, whereby keratin 8 prevailed over keratin 18. Keratins and a protein with heat shock characteristics (M(M) 120-1 antigen) were the earliest detectable MB components, whereas ubiquitination and phosphorylation followed later.

CONCLUSIONS

Overproduction of keratins is a major but not the only step responsible for MB formation. Additional components (e.g. M(M) 120-1 antigen) and excess of keratin 8 over keratin 18 are essential.

Proteasome inhibitors sensitize human vascular smooth muscle cells to Fas (CD95)-mediated death

It was investigated whether proteasome activity was implicated in susceptibility of human vascular smooth muscle cells (VSMCs) to Fas-mediated death. Human fetal aorta smooth muscle cells were treated with agonistic anti-Fas antibody (CH11) and proteasome inhibitors (MG115 or MG132) and then cell death was determined by morphology, viability, and DNA fragmentation. The present study reports that: (a) crosslinking of Fas receptor with anti-Fas antibody in the presence of proteasome inhibitor-induced death and DNA degradation in human VSMCs that were blocked by caspases inhibitor z-DEVD.fmk; (b) cotreatment with anti-Fas antibody and proteasome inhibitor activated caspase-3; (c) proteasome inhibitors did not influence expression of procaspase-8, procaspase-3, c-FLIP, and Bcl-2; and (d) proteasome inhibitors up-regulated Fas and FADD. The data indicate that proteasome activity is important in survival of VSMCs and provide the first evidence that proteasome is involved in Fas signal transduction. The present study proposes novel mechanism(s) by which VSMCs become susceptible to FasL.

Ubiquitination of Lyn-kinase in rat basophilic leukemia RBL-2H3 cells

Receptor-mediated signal transduction pathways of cells involved in allergy and inflammations are extremely significant. Lyn is a member of the Src family of non-receptor protein tyrosine kinases and is associated with a number of cell surface receptors, including the B-cell antigen receptor and immunoglobulin E receptor (FcepsilonRI). Lyn is necessary for FcepsilonRI-mediated mast cell activation. To investigate how the level of Lyn is maintained in mast cell activation, it was studied whether Lyn binds to ubiquitin and is ubiquitinated for proteasomal degradation in cells. In the yeast two hybrid system, Lyn specifically interacted with ubiquitin in vivo. Furthermore, Lyn bound to ubiquitin-conjugated Sepharose beads in vitro and was efficiently competed by soluble ubiquitin. Pulse-chase experiments indicated intracellular degradation of Lyn was associated with the generation of a high molecular weight complex in the presence of proteasome-specific inhibitor, lactacystin. This high molecular weight complex cross-reacted with anti-Lyn and anti-ubiquitin demonstrating the ubiquitination Lyn. Overexpression of Lyn and ubiquitin in COS 7.2 cells also resulted in the ubiquitination of Lyn in the presence of lactacystin, supporting the ubiquitination of Lyn by a proteasome specific pathway.

Properties of the nuclear proteasome activator PA28gamma (REGgamma)

PA28 or 11S REG is a proteasome activator composed of homologous alpha- and beta-subunits and predominantly found in the cytosol. A homologous protein originally known as the Ki antigen but now called PA28gamma or REGgamma is predominantly localized in the nucleus. To further characterize the biochemical properties of PA28gamma, we expressed and purified homogenous recombinant human protein with and without an N-terminal 6-His extension. PA28gamma is a heptamer based on the molecular masses of the native and monomeric proteins. The heptameric 6-His fusion protein can dimerize. Recombinant PA28y stimulates the proteasome-mediated hydrolysis of synthetic substrates containing hydrophobic, basic, and acidic amino acids in the P1 position. Stimulation is dependent on substrate size. PA28y only minimally stimulates degradation of the oxidized B chain of insulin. PA28gamma may facilitate the later stages of protein metabolism in the nucleus and/or have a more specialized role in controlling the levels of biologically active peptides in the nucleus.

The IAP family: endogenous caspase inhibitors with multiple biological activities

IAPs (inhibitors of apoptosis) are a family of proteins containing one or more characteristic BIR domains. These proteins have multiple biological activities that include binding and inhibiting caspases, regulating cell cycle progression, and modulating receptor-mediated signal transduction. Our recent studies found the IAP family members XIAP and c-IAP1 are ubiquitinated and degraded in proteasomes in response to apoptotic stimuli in T cells, and their degradation appears to be important for T cells to commit to death. In addition to three BIR domains, each of these IAPs also contains a RING finger domain. We found this region confers ubiquitin protease ligase (E3) activity to IAPs, and is responsible for the auto-ubiquitination and degradation of IAPs after an apoptotic stimulus. Given the fact that IAPs can bind a variety of proteins, such as caspases and TRAFs, it will be of interest to characterize potential substrates of the E3 activity of IAPs and the effects of ubiquitination by IAPs on signal transduction, cell cycle, and apoptosis.

Cell cycle-dependent expression of mammalian E2-C regulated by the anaphase-promoting complex/cyclosome

Progression through mitosis requires the precisely timed ubiquitin-dependent degradation of specific substrates. E2-C is a ubiquitin-conjugating enzyme that plays a critical role with anaphase-promoting complex/cyclosome (APC/C) in progression of and exit from M phase. Here we report that mammalian E2-C is expressed in late G(2)/M phase and is degraded as cells exit from M phase. The mammalian E2-C shows an autoubiquitinating activity leading to covalent conjugation to itself with several ubiquitins. The ubiquitination of E2-C is strongly enhanced by APC/C, resulting in the formation of a polyubiquitin chain. The polyubiquitination of mammalian E2-C occurs only when cells exit from M phase. Furthermore, mammalian E2-C contains two putative destruction boxes that are believed to act as recognition motifs for APC/C. The mutation of this motif reduced the polyubiquitination of mammalian E2-C, resulting in its stabilization. These results suggest that mammalian E2-C is itself a substrate of the APC/C-dependent proteolysis machinery, and that the periodic expression of mammalian E2-C may be a novel autoregulatory system for the control of the APC/C activity and its substrate specificity.

Ubiquitin protein ligase activity of IAPs and their degradation in proteasomes in response to apoptotic stimuli

To determine why proteasome inhibitors prevent thymocyte death, we examined whether proteasomes degrade anti-apoptotic molecules in cells induced to undergo apoptosis. The c-IAP1 and XIAP inhibitors of apoptosis were selectively lost in glucocorticoid- or etoposide-treated thymocytes in a proteasome-dependent manner before death. IAPs catalyzed their own ubiquitination in vitro, an activity requiring the RING domain. Overexpressed wild-type c-IAP1, but not a RING domain mutant, was spontaneously ubiquitinated and degraded, and stably expressed XIAP lacking the RING domain was relatively resistant to apoptosis-induced degradation and, correspondingly, more effective at preventing apoptosis than wild-type XIAP. Autoubiquitination and degradation of IAPs may be a key event in the apoptotic program.

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

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

Identification of a family of noncanonical ubiquitin-conjugating enzymes structurally related to yeast UBC6

Ubiquitin-conjugating enzymes (UBCs) selectively target proteins for proteasomal degradation by the covalent attachment of ubiquitin moieties. Yeast UBC6 is unusual in having an active site distinct from all other UBCs and in possessing a transmembrane domain that anchors it to the cytoplasmic surface of the endoplasmic reticulum. During a differential display analysis on chick growth plate chondrocytes we isolated a cDNA encoding a noncanonical ubiquitin-conjugating enzyme (NCUBE1) structurally similar to yeast UBC6. Chick NCUBE1 transcripts were detected in all tissues examined and decreased threefold during chondrocyte terminal differentiation. Database searches identified other related proteins; the human and mouse orthologues of NCUBE1, a second human homologue of yeast UBC6 (NCUBE2), and related proteins from S. pombe, C. elegans, and P. mariana. Together with yeast UBC6 these proteins constitute a distinct family of UBCs sharing a conserved noncanonical active site sequence and a C-terminal transmembrane domain. By analogy with yeast UBC6 they are likely to be localised to the endoplasmic reticulum where they may be involved in targeting retrotranslocated, ER-associated proteins for proteasomal degradation.

Integration of signaling networks that regulate Dictyostelium differentiation

In Dictyostelium amoebae, cell-type differentiation, spatial patterning, and morphogenesis are controlled by a combination of cell-autonomous mechanisms and intercellular signaling. A chemotactic aggregation of approximately 10(5) cells leads to the formation of a multicellular organism. Cell-type differentiation and cell sorting result in a small number of defined cell types organized along an anteroposterior axis. Finally, a mature fruiting body is created by the terminal differentiation of stalk and spore cells. Analysis of the regulatory program demonstrates a role for several molecules, including GSK-3, signal transducers and activators of transcription (STAT) factors, and cAMP-dependent protein kinase (PKA), that control spatial patterning in metazoans. Unexpectedly, two component systems containing histidine kinases and response regulators also play essential roles in controlling Dictyostelium development. This review focuses on the role of cAMP, which functions intracellularly to mediate the activity of PKA, an essential component in aggregation, cell-type specification, and terminal differentiation. Cytoplasmic cAMP levels are controlled through both the regulated activation of adenylyl cyclases and the degradation by a phosphodiesterase containing a two-component system response regulator. Extracellular cAMP regulates G-protein-dependent and -independent pathways to control aggregation as well as the activity of GSK-3 and the transcription factors GBF and STATa during multicellular development. The integration of these pathways with others regulated by the morphogen DIF-1 to control cell fate decisions are discussed.

Degradation of alpha-synuclein by proteasome

Mutations in alpha-synuclein are known to be associated with Parkinson's disease (PD). The coexistence of this neuronal protein with ubiquitin and proteasome subunits in Lewy bodies in sporadic disease suggests that alterations of alpha-synuclein catabolism may contribute to the pathogenesis of PD. The degradation pathway of alpha-synuclein has not been identified nor has the kinetics of this process been described. We investigated the degradation kinetics of both wild-type and A53T mutant 6XHis-tagged alpha-synuclein in transiently transfected SH-SY5Y cells. Degradation of both isoforms followed first-order kinetics over 24 h as monitored by the pulse-chase method. However, the t((1)/(2)) of mutant alpha-synuclein was 50% longer than that of the wild-type protein (p < 0.01). The degradation of both recombinant proteins and endogenous alpha-synuclein in these cells was blocked by the selective proteasome inhibitor beta-lactone (40 microM), indicating that both wild-type and A53T mutant alpha-synuclein are degraded by the ubiquitin-proteasome pathway. The slower degradation of mutant alpha-synuclein provides a kinetic basis for its intracellular accumulation, thus favoring its aggregation.

Structure and expression of the gene encoding mouse F-box protein, Fwd2

A novel class of ubiquitin ligases, termed the SCF complex, consists of invariable components, Skp1 and Cullin, and variable components called F-box proteins, which have a primary role in determining substrate specificity. We have isolated a cDNA encoding the mouse F-box protein Fwd2 (also known as MD6) as a possible constituent of an SCF-type ubiquitin ligase. Fwd2 cDNA contains 1890 bp with a 1362-bp open reading frame and encodes an approximately 51.5-kDa protein. Fwd2 is expressed predominantly in liver and, to a lesser extent, in the testis, lung, heart, and skeletal muscle. Immunofluorescence staining for Fwd2 protein shows a pattern with the cytoplasm. A coimmunoprecipitation assay has revealed the in vivo interaction between Skp1 and Fwd2 through the F-box domain. Fwd2 also interacts with Cul1 through Skp1, suggesting that Skp1, Cul1, and the F-box protein Fwd2 form an SCF complex (SCF(Fwd2)). We have also isolated and determined the nucleotide sequence and genomic organization of the gene that encodes mouse Fwd2. This gene spans approximately 17 kb and consists of six exons and five introns. Our results suggest that Fwd2 is an F-box protein that constitutes an SCF ubiquitin ligase complex and that it plays a critical role in the ubiquitin-dependent degradation of proteins expressed in the liver.

Common pathway for the ubiquitination of IkappaBalpha, IkappaBbeta, and IkappaBepsilon mediated by the F-box protein FWD1

FWD1 (the mouse homolog of Drosophila Slimb and Xenopus betaTrCP, a member of the F-box- and WD40 repeat-containing family of proteins, and a component of the SCF ubiquitin ligase complex) was recently shown to interact with IkappaBalpha and thereby to promote its ubiquitination and degradation. This protein has now been shown also to bind to IkappaBbeta and IkappaBepsilon as well as to induce their ubiquitination and proteolysis. FWD1 was shown to recognize the conserved DSGPsiXS motif (where Psi represents the hydrophobic residue) present in the NH(2)-terminal regions of these three IkappaB proteins only when the component serine residues are phosphorylated. However, in contrast to IkappaBalpha and IkappaBbeta, the recognition site in IkappaBepsilon for FWD1 is not restricted to the DSGPsiXS motif; FWD1 also interacts with other sites in the NH(2)-terminal region of IkappaBepsilon. Substitution of the critical serine residues in the NH(2)-terminal regions of IkappaBalpha, IkappaBbeta, and IkappaBepsilon with alanines also markedly reduced the extent of FWD1-mediated ubiquitination of these proteins and increased their stability. These data indicate that the three IkappaB proteins, despite their substantial structural and functional differences, all undergo ubiquitination mediated by the SCF(FWD1) complex. FWD1 may thus play an important role in NF-kappaB signal transduction through regulation of the stability of multiple IkappaB proteins.

Ligand-induced ubiquitination of the epidermal growth factor receptor involves the interaction of the c-Cbl RING finger and UbcH7

c-Cbl plays a negative regulatory role in tyrosine kinase signaling by an as yet undefined mechanism. We demonstrate here, using the yeast two-hybrid system and an in vitro binding assay, that the c-Cbl RING finger domain interacts with UbcH7, a ubiquitin-conjugating enzyme (E2). UbcH7 interacted with the wild-type c-Cbl RING finger domain but not with a RING finger domain that lacks the amino acids that are deleted in 70Z-Cbl, an oncogenic mutant of c-Cbl. The in vitro interaction was enhanced by sequences on both the N- and C-terminal sides of the RING finger. In vivo and in vitro experiments revealed that c-Cbl and UbcH7 synergistically promote the ligand-induced ubiquitination of the epidermal growth factor receptor (EGFR). In contrast, 70Z-Cbl markedly reduced the ligand-induced, UbcH7-mediated ubiquitination of the EGFR. MG132, a proteasome inhibitor, significantly prolonged the ligand-induced phosphorylation of both the EGFR and c-Cbl. Thus, c-Cbl plays an essential role in the ligand-induced ubiquitination of the EGFR by a mechanism that involves an interaction of the RING finger domain with UbcH7. This mechanism participates in the down-regulation of tyrosine kinase receptors and loss of this function, as occurs in the naturally occurring 70Z-Cbl isoform, probably contributes to oncogenic transformation.

Molecular dissection of the interactions among IkappaBalpha, FWD1, and Skp1 required for ubiquitin-mediated proteolysis of IkappaBalpha

The SCF complex containing Skp1, Cul1, and the F-box protein FWD1 (the mouse homologue of Drosophila Slimb and Xenopus beta-TrCP) functions as the ubiquitin ligase for IkappaBalpha. FWD1 associates with Skp1 through the F-box domain and also recognizes the conserved DSGXXS motif of IkappaBalpha. The structural requirements for the interactions of FWD1 with IkappaBalpha and with Skp1 have now been investigated further. The D31A mutation (but not the G33A mutation) in the DSGXXS motif of IkappaBalpha abolished the binding of IkappaBalpha to FWD1 and its subsequent ubiquitination without affecting the phosphorylation of IkappaBalpha. The IkappaBalpha mutant D31E still exhibited binding to FWD1 and underwent ubiquitination. These results suggest that, in addition to site-specific phosphorylation at Ser(32) and Ser(36), an acidic amino acid at position 31 is required for FWD1-mediated ubiquitination of IkappaBalpha. Deletion analysis of Skp1 revealed that residues 61-143 of this protein are required for binding to FWD1. On the other hand, the highly conserved residues Pro(149), Ile(160), and Leu(164) in the F-box domain of FWD1 were dispensable for binding to Skp1. Together, these data delineate the structural requirements for the interactions among IkappaBalpha, FWD1, and Skp1 that underlie substrate recognition by the SCF ubiquitin ligase complex.

Tyrosine kinase-dependent ubiquitination of CD16 zeta subunit in human NK cells following receptor engagement

We investigated whether aggregation of the low-affinity immunoglobulin G receptor (CD16) on human NK cells results in receptor ubiquitination. We found that the CD16 zeta subunit becomes ubiquitinated in response to receptor engagement. We then investigated whether protein tyrosine kinase (PTK) activation is required for CD16-mediated receptor ubiquitination. Pretreatment with the PTK inhibitor genistein substantially decreased ligand-induced zeta ubiquitination, suggesting a requirement for PTK activation in receptor ubiquitination. We further analyzed PTK involvement in controlling receptor ubiquitination by using the vaccinia virus expression system. Overexpression of wild-type active lck, but not a kinase-deficient mutant, enhanced both ligand-induced tyrosine phosphorylation and ubiquitination of the CD16 zeta subunit. Taken together, our data demonstrate that CD16 engagement induces zeta chain ubiquitination and strongly suggest a role for lck in regulating this modification.

Targeting of hepatitis C virus core protein for MHC I or MHC II presentation does not enhance induction of immune responses to DNA vaccination

We analyzed different vaccine approaches aimed at enhancing CD4(+)- and CD8(+)-dependent responses against hepatitis C virus (HCV) core antigen. Specific DNA vectors expressing various forms of the core in fusion with the ubiquitin or the lysosome-associated membrane protein (LAMP) were generated. These expressed the full-length wildtype core; the full-length core expressed as a covalent fusion with the ubiquitin; the full-length core expressed as a noncovalent fusion with the ubiquitin and containing a N-stabilizing or N-destabilizing residue; and the full-length core expressed as a fusion with the LAMP sequence. In vitro expression levels of the different plasmids differed by as much as tenfold. After injection into mice, none of the plasmids yielded a detectable antibody response, whereas core-specific cytotoxic T-lymphocyte (CTL) activity could be observed with all plasmids as long as 21 weeks postimmunization. No increase in CTL activity (ranging from 7% to 34% specific lysis) was observed with the ubiquitin-fusion-expressed core antigens compared with the wildtype core. The lowest CTL activity (< 5% specific lysis) was observed with the LAMP fusion. This vector was nonetheless unable to induce a detectable proliferative response. Screening of 10 different putative CTL peptide epitopes failed to reveal newly targeted epitopes when the core-fusion plasmids were used compared with the wildtype core-expressing plasmid. These data underline the difficulty in optimizing anti-core cellular immune response using molecular targeting strategies in DNA-based vaccination.