4E-BP3, a new member of the eukaryotic initiation factor 4E-binding protein family

Translation initiation in eukaryotes is mediated by the cap structure (m7GpppN, where N is any nucleotide) present at the 5' end of all cellular mRNAs, except organellar. The cap is recognized by eukaryotic initiation factor 4F (eIF4F), which consists of three polypeptides, including eIF4E, the cap-binding protein subunit. The interaction of the cap with eIF4E facilitates the binding of the ribosome to the mRNA. eIF4E activity is regulated in part by two translational repressors, 4E-BP1 and 4E-BP2, which bind to it and prevent its assembly into eIF4F. We report here the isolation of 4E-BP3, a new member of the 4E-BP family. 4E-BP3 is homologous to 4E-BP1 and 4E-BP2, exhibiting 57 and 59% identity, respectively. The homology is most striking in the middle region of the protein, which contains the eIF4E binding motif and residues that are phosphorylated in 4E-BP1. 4E-BP3 is a heat stable protein that binds to eIF4E in vitro as well as in vivo. Further, 4E-BP3 overexpression specifically reduces eIF4E-dependent translation. The overlapping function and expression of the different 4E-BP family members imply that there is redundancy in this translational control mechanism, underscoring its importance.

CLF associates with CLC to form a functional heteromeric ligand for the CNTF receptor complex

Ciliary neurotrophic factor (CNTF) is a cytokine supporting the differentiation and survival of various cell types in the peripheral and central nervous systems. Its receptor complex consists of a non-signaling alpha chain, CNTFR, and two signaling beta chains, gp130 and the leukemia inhibitory factor receptor (LIFR). Striking phenotypic differences between CNTF- and CNTFR-deficient mice suggest that CNTFR serves as a receptor for a second, developmentally important ligand. We have identified this factor as a stable secreted complex of cardiotrophin-like cytokine (CLC) and the soluble receptor cytokine-like factor-1 (CLF). CLF expression was required for CLC secretion, and the complex acted only on cells expressing functional CNTF receptors. The CLF/CLC complex activated gp130, LIFR and signal transducer and activator of transcription 3 (STAT3) and supported motor neuron survival. Our results indicate that the CLF/CLC complex is a second ligand for CNTFR with potentially important implications in nervous system development.

Focal adhesion kinase (pp125FAK) expression, activation and association with paxillin and p50CSK in human metastatic prostate carcinoma

pp125FAK, a protein tyrosine kinase (PTK) co-localized with integrins in focal adhesion plaques, is known to transduce signals involved in the regulation of cell adhesion and motility as well as the anchorage-independent growth of transformed cells. We investigated whether pp125FAK could be part of a signalling pathway that contributes to the progression of human prostate carcinoma (PCa). Up-regulation of pp125FAK expression, its activation by phosphorylation on tyrosine and its association with paxillin and p50csk were preferentially observed in PCa tissues from patients with metastases, whereas normal and hyperplastic prostates and localized PCa tissues showed undetectable or low levels of both FAK mRNA and protein and an absence of pp125FAK signalling complexes. The increase in expression and activation of pp125FAK in metastatic PCa tissues was also corroborated by our findings in human PCa cell lines. Indeed, higher levels of pp125FAK and FAK mRNA were observed in highly tumorigenic PC-3 cells as was the presence of activated pp125FAK, as opposed to an inactive form in LNCaP cells, which have a lower tumorigenic ability. In addition, pp125FAK formed signalling complexes with both paxillin and p50csk in PC-3 cells as in metastatic PCa tissues. Together, our results show that an increase in FAK mRNA and protein, as well as pp125FAK activation and association with signalling proteins, correlates with progression and invasion in human PCa tissues and cells.

Early events in DNA replication require cyclin E and are blocked by p21CIP1

Using immunodepletion of cyclin E and the inhibitor protein p21WAF/CIP1, we demonstrate that the cyclin E protein, in association with Cdk2, is required for the elongation phase of replication on single-stranded substrates. Although cyclin E/Cdk2 is likely to be the major target by which p21 inhibits the initiation of sperm DNA replication, p21 can inhibit single-stranded replication through a mechanism dependent on PCNA. While the cyclin E/Cdk2 complex appears to have a role in the initiation of DNA replication, another Cdk kinase, possibly cyclin A/Cdk, may be involved in a later step controlling the switch from initiation to elongation. The provision of a large maternal pool of cyclin E protein shows that regulators of replication are constitutively present, which explains the lack of a protein synthesis requirement for replication in the early embryonic cell cycle.

Capacitation is associated with tyrosine phosphorylation and tyrosine kinase-like activity of pig sperm proteins

Capacitation represents the final maturational steps that render mammalian sperm competent to fertilize, either in vivo or in vitro. Capacitation is defined as a series of events that enables sperm to bind the oocyte and undergo the acrosome reaction in response to the zona pellucida. Although the molecular mechanisms involved are not fully understood, sperm protein phosphorylation is associated with capacitation. The hypothesis of this study is that protein tyrosine phosphorylation and kinase activity mediate capacitation of porcine sperm. Fresh sperm were incubated in noncapacitating or capacitating media for various times. Proteins were extracted with SDS, subjected to SDS-PAGE, and immunoblotted with an antiphosphotyrosine antibody. An M(r) 32 000 tyrosine-phosphorylated protein (designated as p32) appeared only when the sperm were incubated in capacitating medium and concomitant with capacitation as assessed by the ionophore-induced acrosome reaction. The p32 was soluble in Triton X-100. Fractionation of sperm proteins with Triton X-114 demonstrated that after capacitation, this tyrosine phosphoprotein is located in both the cytosol and the membrane. Enzyme renaturation of sperm proteins was conducted in gels with or without either poly glu:tyr (a tyrosine kinase substrate) or kemptide (a protein kinase A substrate). An M(r) 32 000 enzyme with kinase behavior was observed in all gels but was preferentially phosphorylated on tyrosine, as assessed by phosphorimagery and by thin layer chromotography to identify the phosphoamino acids. Indirect immunolocalization showed that the phosphotyrosine residues redistribute to the acrosome during capacitation, which is an appropriate location for a protein involved in the acquisition of fertility.

Cell cycle regulation of the replication licensing system: involvement of a Cdk-dependent inhibitor

The replication licensing factor (RLF) is an essential initiation factor that is involved in preventing re-replication of chromosomal DNA in a single cell cycle. In Xenopus egg extracts, it can be separated into two components: RLF-M, a complex of MCM/P1 polypeptides, and RLF-B, which is currently unpurified. In this paper we investigate variations in RLF activity throughout the cell cycle. Total RLF activity is low in metaphase, due to a lack of RLF-B activity and the presence of an RLF inhibitor. RLF-B is rapidly activated on exit from metaphase, and then declines during interphase. The RLF inhibitor present in metaphase extracts is dependent on the activity of cyclin-dependent kinases (Cdks). Affinity depletion of Cdks from metaphase extracts removed the RLF inhibitor, while Cdc2/cyclin B directly inhibited RLF activity. In metaphase extracts treated with the protein kinase inhibitor 6-dimethylaminopurine (6-DMAP), both cyclin B and the RLF inhibitor were stabilized although the extracts morphologically entered interphase. These results are consistent with studies in other organisms that invoke a key role for Cdks in preventing re-replication of DNA in a single cell cycle.

Both cyclin A and cyclin E have S-phase promoting (SPF) activity in Xenopus egg extracts

Extracts of activated Xenopus eggs in which protein synthesis has been inhibited support a single round of chromosomal DNA replication. Affinity-depletion of cyclin dependent kinases (Cdks) from these extracts blocks the initiation of DNA replication. We define ‘S-phase promoting factor’ (SPF) as the Cdk activity required for DNA replication in these Cdk-depleted extracts. Recombinant cyclins A and E, but not cyclin B, showed significant SPF activity. High concentrations of cyclin A promoted entry into mitosis, which inhibited DNA replication. In contrast, high concentrations of cyclin E1 promoted neither nuclear envelope disassembly nor full chromosome condensation. In the early embryo cyclin E1 complexes exclusively with Cdk2 and cyclin A is complexed predominantly with Cdc2; only later in development does cyclin A associate with Cdk2. We show that baculovirus-produced complexes of cyclin A-Cd2, cyclin A-Cdk2 and cyclin E-Cdk2 could each provide SPF activity. These results suggest that although in the early Xenopus embryo cyclin E1-Cdk2 is sufficient to support entry into S-phase, cyclin A-Cdc2 provides a significant additional quantity of SPF as its levels rise during S phase.

Signaling of the cardiotrophin-1 receptor. Evidence for a third receptor component

Cardiotrophin-1 (CT-1) is a recently isolated cytokine belonging to the interleukin-6 cytokine family. In the present study we show that CT-1 activates its receptor expressed at the surface of a human neural cell line by recruiting gp130 and gp190/leukemia inhibitory factor receptor beta, as shown by analyzing their tyrosine phosphorylation level. Neutralizing antibody directed against gp130 and reconstitution experiments performed in the COS-7 cell line demonstrate that gp130-gp190 heterocomplex formation is essential for CT-1 signaling. Analysis of the subsequent activation events revealed that CT-1 induces and utilizes Jak1-, Jak2-, and Tyk2-associated tyrosine kinases, which are in turn relayed by STAT-3 transcription factor. Cross-linking of iodinated CT-1 to the cell surface led to the identification of a third alpha component in addition to gp130 and gp190, with an apparent molecular mass of 80 kDa. Removal of N-linked carbohydrates from the protein backbone of the alpha component resulted in a protein of 45 kDa. Our results provide evidence that the CT-1 receptor is composed of a tripartite complex, a situation similar to the high affinity receptor for ciliary neurotrophic factor.

Elevations of plasma methylarginines in obesity and ageing are related to insulin sensitivity and rates of protein turnover

AIMS/HYPOTHESIS

Increased circulating methylarginines (MA) have been linked to the metabolic syndrome to explain endothelial dysfunction and cardiovascular disease risk. Proteins that contain MA are regulatory and release them during catabolism. We hypothesised that increased protein turnover in insulin-resistant states contributes to an increase in circulating MA. MATWERIALS AND METHODS: We performed hyperinsulinaemic, euglycaemic, and isoaminoacidaemic experiments on 49 lean, obese and elderly subjects, with measurements of the kinetics of glucose and protein metabolism. Plasma MA, i.e. asymmetrical dimethylarginine (ADMA), symmetrical dimethylarginine (SDMA), and N -monomethyl-L-arginine (NMMA), lipids and body composition were measured.

RESULTS

Insulin resistance of glucose and protein metabolism occurred in obese and elderly subjects. ADMA concentrations were 29 to 120% higher in obese and 34% higher in elderly than in lean subjects. SDMA were 34 and 20% higher in obese than in lean and than in elderly subjects, respectively. NMMA were 32% higher in obese than in lean subjects. ADMA differed by sex, being higher in men, namely by 1.75x in obese men and by 1.27x in elderly men. Postabsorptive ADMA (r=0.71), SDMA (r=0.46), and NMMA (r=0.31) correlated (all p<0.05) with rates of protein flux. All three MA correlated negatively with clamp glucose infusion rates and uptake (p<0.001). ADMA and SDMA correlated negatively with net protein synthesis and clamp amino acid infusion rates (p<0.05). All MA also correlated with adiposity indices and fasting insulin and triglycerides (p<0.05).

CONCLUSIONS/INTERPRETATION

Obesity, sex and ageing affect MA. Elevations of the three MA in obese, and of ADMA in elderly men, are related to increased protein turnover and to lesser insulin sensitivity of protein metabolism. These interrelationships might amplify insulin resistance and endothelial dysfunction.

Bombesin stimulates the motility of human prostate-carcinoma cells through tyrosine phosphorylation of focal adhesion kinase and of integrin-associated proteins

Bombesin-like peptides, including the mammalian homologue gastrin-releasing peptides, are highly expressed and secreted by neuroendocrine cells in prostate carcinoma (PCa) tissues and are likely to be related to the progression of this disease. In the present study, we show that bombesin enhances the migration of androgen-independent PCa cells (PC-3) in vitro, while not affecting their adhesion to extracellular matrix proteins. The bombesin-increased motility of PC-3 cells occurs through its receptor, and, as shown with inhibitors, it likely requires activation of both protein tyrosine kinases (PTKs) and protein kinases C (PKCs). Because the focal adhesion kinase pp125FAK plays a key role in adhesion/motility and is highly expressed in advanced PCa, we examined whether in PC-3 cells bombesin signal transduction triggers the tyrosine phosphorylation of this PTK and of associated integrins and signaling proteins likely to be present in focal adhesion plaques. pp125FAK tyrosine phosphorylation was stimulated by bombesin and mimicked by PKC activation with the tumor-promotor phorbol 12-myristate-13-acetate (PMA). Moreover, this effect of bombesin on pp125FAK tyrosine phosphorylation requires the presence of both active PKC and cytoskeleton integrity since this signal was abolished by down-regulating PKCs induced by prolonged PMA treatment or by PKC inhibition with GF 109203X, as well as by disruption of the cytoskeleton with cytochalasin D. We also show that bombesin increases the tyrosine phosphorylation of a 95-kDa protein (pp95) which was co-immunoprecipitated with the alpha v and beta (3 and 5) subunits, forming integrin receptors with alpha v in PC-3 cells. The protein pp95 is distinct from the endogenously tyrosine-phosphorylated beta3 subunit. In addition, upon bombesin treatment, the beta1, beta3 and beta5 integrin subunits co-immunoprecipitated with pp125FAK and major phosphotyrosine (pY)-containing proteins of 125 and 68-70 kDa, likely corresponding to pp125FAK and paxillin. Together our data suggest that, in addition to PKC activation, tyrosine phosphorylation of pp125FAK and integrin-associated proteins may play an important role in bombesin signaling, triggering the processes of PCa cell motility and invasion.

Comparative effects of five bisphosphonates on apoptosis of macrophage cells in vitro

Bisphosphonates (BPs) inhibits bone resorption by reducing osteoclastic activity; they induce osteoclast apoptosis. Pathophysiology of prostheses loosening is complex and implies an inflammatory reaction secondary to the phagocytosis of wear debris by macrophages with a secondary increased bone resorption by osteoclasts. BPs inhibit proliferation and cause cell death in macrophages by induction of apoptosis. We have used mouse macrophage-like J774.1 cells to evaluate the effects of five BPs. J774A.1 cells were cultured in a standard culture medium for 2-days. BPs (alendronate, pamidronate, etidronate, risedronate, zoledronic acid) were added in the medium at concentration of 10(-6) to 10(-4)M during 3 days. Cells were studied by fluorescence microscopy after staining with the fluorescent dye Hoescht H33342 and the percentage of apoptotic cells was determined on 300 nuclei. Cells were analyzed by flow cytofluorometry after staining with annexin V-FITC (for counting apoptotic cells) and propidium iodide (for necrotic/late-apoptotic cells) on 2000 cells. Etidronate did not cause significant apoptosis or necrosis, at any concentration. Alendronate and pamidronate caused apoptosis and death only at very high concentration [10(-4)M]. On the contrary, apoptotic and necrotic cells were evidenced with risedronate or zoledronic acid at lower concentrations. These effects were dose-dependant and occurred when concentration reached [10(-5)M]. The number of apoptotic cells was higher with zoledronic acid and then with risedronate. Cytofluorometry appeared superior to cytologic analysis in the investigation of macrophage apoptosis, since necrotic cells loose contact with the glass slides and are not identifiable in cytological counts. Some amino-BPs appear to induce apoptosis in macrophages.

Uptake of an oviductal antigen by the hamster zona pellucida

Using an antiserum raised against hamster oviductal zona pellucida, we observed specific immunogenic components of the reproductive tract on the zonae of oviductal eggs and in oviductal fluid. Results of immunohistochemical studies suggested that these oviductal components may originate from epithelial cells of the isthmus and, to a lesser extent, of the ampulla and fimbria. The oviductal immunogenic components have also been observed within the bursal cavity, which contains the ovary. These observations suggest that these oviductal components may play an important role in the first steps of the hamster reproductive process.

Signaling pathways recruited by the cardiotrophin-like cytokine/cytokine-like factor-1 composite cytokine: specific requirement of the membrane-bound form of ciliary neurotrophic factor receptor alpha component

Ciliary neurotrophic factor (CNTF) is a cytokine supporting the differentiation and survival of a number of neural cell types. Its receptor complex consists of a ligand-binding component, CNTF receptor (CNTFR), associated with two signaling receptor components, gp130 and leukemia inhibitory factor receptor (LIFR). Striking phenotypic differences between CNTF- and CNTFR-deficient mice suggest that CNTFR serves as a receptor for a second developmentally important ligand. We recently demonstrated that cardiotrophin-like cytokine (CLC) associates with the soluble orphan receptor cytokine-like factor-1 (CLF) to form a heterodimeric cytokine that displayed activities only on cells expressing the tripartite CNTF receptor on their surface. In this present study we examined the membrane binding of the CLC/CLF composite cytokine and observed a preferential interaction of the cytokine with the CNTFR subunit. Signaling pathways recruited by the CLC/CLF complex in human neuroblastoma cell lines were also analyzed in detail. The results obtained showed an activation of Janus kinases (JAK1, JAK2, and TYK2) leading to a tyrosine phosphorylation of the gp130 and LIFR. The phosphorylated signaling receptors served in turn as docking proteins for signal transducing molecules such as STAT3 and SHP-2. In vitro analysis revealed that the gp130-LIFR pathway could also stimulate the phosphatidylinositol 3-kinase and the mitogen-activated protein kinase pathways. In contrast to that reported before for CNTF, soluble CNTFR failed to promote the action CLC/CLF, and an absolute requirement of the membrane form of CNTFR was required to generate a functional response to the composite cytokine. This study reinforces the functional similarity between CNTF and the CLC/CLF composite cytokine defining the second ligand for CNTFR.

A 10-amino acid domain within human T-cell leukemia virus type 1 and type 2 tax protein sequences is responsible for their divergent subcellular distribution

Human T-cell leukemia virus type 1 and type 2 (HTLV-1/2) are related retroviruses that infect T-lymphocytes. Whereas HTLV-1 infection can cause leukemia, HTLV-2 has not been demonstrated to be the agent of a hematological malignant disease. Nevertheless, the virally encoded Tax-1 and Tax-2 transactivators display a high percentage of similarity. Tax-1 is a shuttling protein that contains a noncanonical nuclear localization signal as well as a nuclear export signal. The presence of the nuclear localization signal and the nuclear export signal domains in the Tax-2 sequence has not been determined. The distribution of Tax-2 in infected cells is not known but has been assumed to be similar to that of Tax-1. By using a Tax-2-specific antibody, we report here that Tax-2 is located predominantly in the cytoplasm of the HTLV-2 immortalized or transformed infected T-cells. These results were confirmed after transient transfection of untagged Tax-1 and Tax-2 constructs, histidine tag Tax1/Tax2, GFP-Tax, and Tax-GFP fusion constructs in several cell lines. We show that this unanticipated localization is not due to a default in the Tax-2 nuclear localization signal functions nor to major differences in Tax-2 versus Tax-1 binding to the IKKgamma/NEMO protein. In addition, we demonstrate that inhibiting the proteasome results in a relocalization of Tax-1 in the cytoplasm, similar to that of Tax-2. By using a series of Tax-1/Tax-2 chimeras, we determined that the minimal domain that is necessary for Tax-2 peculiar distribution encompasses amino acids 90-100. Finally, we show a high correlation between intracellular localization of Tax and their NF-kappaB or CREB transactivating ability.

NRP/Optineurin Cooperates with TAX1BP1 to potentiate the activation of NF-kappaB by human T-lymphotropic virus type 1 tax protein

Nuclear factor (NF)-κB is a major survival pathway engaged by the Human T-Lymphotropic Virus type 1 (HTLV-1) Tax protein. Tax1 activation of NF-κB occurs predominantly in the cytoplasm, where Tax1 binds NF-κB Essential Modulator (NEMO/IKKγ) and triggers the activation of IκB kinases. Several independent studies have shown that Tax1-mediated NF-κB activation is dependent on Tax1 ubiquitination. Here, we identify by co-immunoprecipitation assays NEMO-Related Protein (NRP/Optineurin) as a binding partner for Tax1 in HTLV-1 infected and Tax1/NRP co-expressing cells. Immunofluorescence studies reveal that Tax1, NRP and NEMO colocalize in Golgi-associated structures. The interaction between Tax1 and NRP requires the ubiquitin-binding activity of NRP and the ubiquitination sites of Tax1. In addition, we observe that NRP increases the ubiquitination of Tax1 along with Tax1-dependent NF-κB signaling. Surprisingly, we find that in addition to Tax1, NRP interacts cooperatively with the Tax1 binding protein TAX1BP1, and that NRP and TAX1BP1 cooperate to modulate Tax1 ubiquitination and NF-κB activation. Our data strongly suggest for the first time that NRP is a critical adaptor that regulates the assembly of TAX1BP1 and post-translationally modified forms of Tax1, leading to sustained NF-κB activation.

Oncogenic viruses (i.e., viruses that can induce cancer) have usually been found to deregulate several cellular signaling pathways controlling cell survival and proliferation. Among those, the NF-κB pathway is particularly important. In this study, we focus on the Human T-Lymphotropic Virus type 1 (HTLV-1), which infects immune T cells, and is associated with the development of a severe hematological disease, termed adult T cell leukemia. The viral Tax oncoprotein is known to activate the NF-κB pathway, but the precise mechanism is still under investigation. In cells, proteins can undergo modifications that can modulate their function. In the case of Tax, a modified form of the protein (ubiquitinated Tax) is able to activate the NF-κB pathway. Our aim was to identify cellular proteins that participate in the modification of Tax, and in turn in the regulation of its function. We show for the first time that the cellular protein NRP/Optineurin interacts with Tax and increases its ubiquitination, thus leading to an enhanced NF-κB activation. We further demonstrate that TAX1BP1, another cellular protein that had been previously identified as a partner of Tax, also participates in this regulation. Thus, this study uncovers new actors of the virally induced cell signaling.

Bombesin specifically induces intracellular calcium mobilization via gastrin-releasing peptide receptors in human prostate cancer cells

Bombesin and gastrin-releasing peptide (GRP) are potent neuropeptides expressed by prostate cancer neuroendocrine cells and are related to the progression of this malignancy. This study characterizes bombesin receptors in human prostate cancer cell lines (PC-3, DU-145, LNCaP) and assesses the in vitro effect of bombesin on signal transduction and cell proliferation. [125I]Tyr4-bombesin binding assays (37 degrees C) and Scatchard analyses revealed the presence of a single class of high-affinity receptors with similar Kd values (1.5, 1.1 and 3.6 x 10(-10) M in PC-3, DU-145 and LNCaP cells respectively) but with significant differences in the number of binding sites per cell (47.6, 1.5 and 0.1 x 10(3) in PC-3, DU-145 and LNCaP cells respectively). Molecular characterization of the binding sites performed in PC-3 cells by cross-linking experiments and SDS/PAGE revealed a single radioactive band of 85 kDa. To determine which of the three known bombesin receptor subtypes (GRP receptor (GRP-R), neuromedin B receptor, bombesin receptor subtype-3) were expressed in the cell lines, reverse transcription/PCR analysis of cellular RNA followed by hybridization with receptor-specific cDNA was performed. This revealed the presence of GRP-R transcript in all cell lines, while neither of the other two receptor transcripts were expressed. When intracellular calcium mobilization was measured by Fura-2/AM cell labeling and spectrofluorometric monitoring, bombesin (100 nM) induced rapid calcium mobilization in both PC-3 (> 200% of baseline) and DU-145 (> 100% of baseline) cells, but not in LNCaP cells. However, as measured by 3-(4,5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide assay and [3H]thymidine incorporation, no growth modulation was observed with bombesin or bombesin receptor antagonist at various concentrations (0-500 nM). Our data indicate that bombesin is a potent inducer of signal transduction via GRP-R receptors in androgen-insensitive PC-3 and DU-145 prostate cancer cells. This suggests that the bombesin/GRP family of neuropeptides may play a regulatory role in the biology of androgen-independent prostate cancer.

PPAR alpha and the regulation of cell division and apoptosis

Peroxisome proliferators (PPs) such as the hypolipidaemic drug, nafenopin and the phthalate plasticiser 2-diethylhexylphthalate induce rodent hepatocyte cell proliferation and suppress apoptosis leading to tumours. PPs act via the nuclear hormone receptor peroxisome proliferator activated receptor alpha (PPAR alpha) which directly regulates genes implicated in the response to PPs such as the peroxisomal gene acyl CoA oxidase. As expected for xenobiotics that perturb proliferation, PPs alter expression of cell cycle regulatory proteins. However, the ability to alter expression of cyclins and cyclin-dependent kinases is shared by physiological hepatic mitogens such as epidermal growth factor and is thus unlikely to be specific to the PP-induced aberrant growth associated with hepatocarcinogenesis. Recent evidence suggests that the response of hepatocytes to PPs is not only dependent upon PPAR alpha but also on the trophic environment provided by nonparenchymal cells and by cytokines such as tumour necrosis factor alpha. Additionally, the ability of PPs to suppress apoptosis and induce proliferation depends upon survival signalling mediated by p38 mitogen activated protein kinase. The cross talk between PPAR alpha-mediated transcription, survival signalling and cell cycle will be discussed with particular emphasis on relevance to toxicology.

Endoglin expression on human microvascular endothelial cells association with betaglycan and formation of higher order complexes with TGF-beta signalling receptors

Transforming growth factor-beta (TGF-beta) plays an important role in angiogenesis and vascular function. Endoglin, a transmembrane TGF-beta binding protein, is highly expressed on vascular endothelial cells and is the target gene for the hereditary haemorrhagic telangiectasia type I (HHT1), a dominantly inherited vascular disorder. The specific function of endoglin responsible for HHT1 is believed to involve alterations in TGF-beta responses. The initial interactions on the cell surface between endoglin and TGF-beta receptors may be an important mechanism by which endoglin modulates TGF-beta signalling, and thereby responses. Here it is shown that on human microvascular endothelial cells, endoglin is co-expressed and is associated with betaglycan, a TGF-beta accessory receptor with which endoglin shares limited amino acid homology. This complex formation may occur in either a ligand-dependent or a ligand-independent manner. In addition, the occurrence of three higher order complexes containing endoglin, type II and/or type I TGF-beta receptors, on these cells is demonstrated. Our findings suggest that endoglin may modify TGF-beta signalling by interacting with both betaglycan and the TGF-beta signalling receptors at physiological receptor concentrations and ratios.

Interleukin-6 family of cytokines induced activation of different functional sites expressed by gp130 transducing protein

Gp130 transducing protein was shown to be involved in the formation of the high affinity receptors for interleukin 6 (IL-6), interleukin-11 (IL-11), leukemia inhibitory factor, oncostatin M (OSM), ciliary neurotrophic factor (CNTF), and cardiotrophin-1. In the present study we have characterized the functional properties of antibodies directed against this protein and identified a group of monoclonal antibodies able to antagonize the biological activities of all the cytokines belonging to the IL-6 cytokine family. The B-R3 pan-blocking antibody weakly interfered with the binding of the radiolabeled ligands (with the exception of OSM, whose binding was abrogated in the presence of B-R3 monoclonal antibody) but inhibited the gp130 homodimerization or its association with gp190/leukemia inhibitory factor receptor, as well as the subsequent tyrosine phosphorylation events. In addition we identified antibodies that were able to neutralize only one single cytokine of the IL-6 family. This was the case for the B-K5 antibody, which antagonized the binding of OSM to gp130 but did not interfere with the signals provided by the related cytokines triggering the proliferation of the TF1 erythroleukemia cell line or the induction of haptoglobin synthesis in the HepG2 hepatoma cell line. Similarly, we also characterized two additional antibodies B-P8 and B-P4, which inhibited the TF1 cell proliferation observed in the presence of CNTF and IL-11, respectively. B-P8 antibody only faintly interfered with the binding of the gp130-ligands and might modulate the signal transduction pathways. This study indicates that in addition to functional site(s) required by the whole family of IL-6 type cytokines to transduce the signal insight the cell, specific cognate functional sites were recruited by OSM, CNTF, or IL-11.

Species differences in sequence and activity of the peroxisome proliferator response element (PPRE) within the acyl CoA oxidase gene promoter

In rats and mice, peroxisome proliferators (PP) cause liver enlargement, hepatocarcinogenesis and peroxisome proliferation associated with induction of enzymes such as acyl CoA oxidase (ACO). However, humans appear to be non-responsive to the adverse effects of PPs such as ACO induction. PPs activate the peroxisome proliferator activated receptor alpha (PPARalpha) that binds to DNA at peroxisome proliferator response elements (PPREs) within the promoters of PP-responsive genes. When the human ACO promoter was cloned previously (Varanasi et al., 1996. Journal of Biological Chemistry, 271, 2147-2155), it was reported to contain a PPRE (5' AGGTCA C TGGTCA 3') that bound PPARalpha and could be activated in vitro by Wyeth-14,643 (at >1 mM) or DEHP (at > 1.5 mM). In contrast, when we cloned the ACO gene promoter from a human liver biopsy, it was non-responsive to PPs and differed at three positions (5' AGGTCA G CTGTCA 3') from that reported previously (Woodyatt et al., 1999. Carcinogenesis, 20, 369-375). Subsequent to this, Varanasi et al. re-sequenced their constructs and obtained the same sequence as we have described (Varanasi et al., 1998. Journal of Biological Chemistry, 273, 30832). However, the observation that the errant sequence (5' AGGTCA C TGGTCA 3') was able to bind PPARalpha still remained since it appears that this sequence was used by Varanasi et al. (1996) to design oligonucleotides for their DNA binding analyses. Thus, if the 5' AGGTCA C TGGTCA 3' sequence did exist in some individuals, it could be active. To address this, we used site-directed mutagenesis to create a promoter fragment that contained the errant sequence. This reporter gene was transfected into NIH3T3 cells together with a plasmid expressing mPPARalpha, and assessed for its ability to drive PP-mediated gene transcription using a non-toxic concentration of Wyeth-14,643 (100 microM). This human ACO promoter was also inactive, unlike the equivalent rat ACO promoter fragment used as a positive control. Next, we used site directed mutagenesis to convert the PPRE found in the active rat ACO promoter (3' AGGACA A AGGTCA 5') to our inactive human sequence (AGGTCA G CTGTCA). This human PPRE was unable to drive PP-induced gene transcription even in the context of the rat ACO promoter suggesting that the activity of the rat promoter is conferred principally by the PPRE sequence, even though it may be enhanced by flanking sequences. These data confirm that neither the native nor the errant human ACO gene PPRE can respond to PPs. The absence of a responsive PPRE contributes to our understanding of the lack of response of humans to some of the adverse effects of the PP class of non-genotoxic hepatocarcinogens.

Protection of islets of Langerhans from antibodies by microencapsulation with alginate-poly-L-lysine membranes

Microencapsulation of islets has been proposed to prevent their immune destruction following transplantation. An indirect immunofluorescence technique has been developed and used to study the permeability of the alginate-poly-L-lysine microcapsules to antibodies. Wistar rat islets were incubated with the R2D6 monoclonal mouse IgM antibody against rat islets, microencapsulated, and incubated with fluorescein-labeled goat IgG antibodies against mouse IgG and IgM. For the negative controls, the first antibody was omitted or both antibodies were omitted. The positive controls included islets incubated with both antibodies before they were encapsulated. Our study demonstrated that the alginate-poly-L-lysine membranes are not permeable to IgG when poly-L-lysine of molecular weights ranging from 21,000 to 390,000 are used. This simple immunofluorescence technique demonstrated the nonpermeability of the microcapsules to IgG, and could be useful for the initial evaluation of new types of membranes.

Links between Fer tyrosine kinase expression levels and prostate cell proliferation

In our cloning strategy to identify tyrosine kinases implicated in the regulation of prostate growth, the dog fer cDNA was obtained and shown to be highly homologous to known fer cDNAs. Using a polyclonal Fer antibody directed against a C-terminal peptide, we studied its associations with cortactin, beta-catenin and p120Cas in human prostate carcinoma PC-3 cells. In contrast to previous reports, no interactions were observed. To assess its functional role, fer cDNA constructs were transfected in PC-3 cells. Antisense clones exhibiting a marked diminution of Fer expression had a reduced growth rate (doubling time of 29 vs. 42 h) and were unable to form colonies in soft agar. In agreement with these results, Fer protein expression was linked to human prostatic proliferative diseases, with enhanced levels in extracts from cancer tissues as compared to those from normal and hyperplastic ones, and was also expressed in the human prostate carcinoma cell lines DU145 and LNCaP. In the dog model, Fer expression was up-regulated in dividing versus resting prostate epithelial cells in vitro, and also in vivo when basal cell hyperplasia and metaplasia was induced by estrogen after castration. Minimal effects were observed when renewing the luminal epithelium with androgens. Taken together, these results show that Fer expression is associated with prostate cell proliferation and enhanced in prostate cancer.

Involvement of gp130/interleukin-6 receptor transducing component in interleukin-11 receptor

The recently cloned interleukin (IL)-11 displays many biological properties in common with those reported for IL-6. In order to analyze the nature and the functionality of the IL-11 receptor we developed a proliferative assay using the human multifactor-dependent cell line TF1. We showed that a blocking monoclonal antibody GPX7 raised against the gp130/IL-6 receptor transducing subunit was also able to inhibit the IL-11-triggered TF1 line proliferation. In addition, involvement of gp130 in IL-11 signaling was demonstrated by an induction of the transducing protein phosphorylation in response to IL-11, as observed for IL-6. In contrast, the blocking monoclonal antibody B-R6, which recognized the gp80/IL-6 binding subunit failed to interfere with the IL-11 proliferative signal in the TF1 cell line. Similarly, we did not observe any competition between IL-6 and IL-11 for a putative common binding site on the cell surface. These results suggest that the IL-11 binding component is different from the gp80/IL-6 receptor. In conclusion, IL-11, along with IL-6, leukemia inhibitory factor, oncostatin M and ciliary neurotrophic factor, belongs to the same family of cytokines, using gp130 as a transducing protein.