A metalloprotease inhibitor blocks shedding of the 80-kD TNF receptor and TNF processing in T lymphocytes

Differential effects of total and partial neutralization of tumor necrosis factor on cell-mediated immunity to Mycobacterium bovis BCG infection

The effects of total and partial inhibition of tumor necrosis factor (TNF) on sensitivity to Mycobacterium bovis BCG infection were investigated by using transgenic mice in which hepatocytes produced different amounts of human soluble TNF receptor 1 (sTNFR1) fused to the Fc fragment of human immunoglobulin G3 that could be detected in the serum. Transgenic mice expressing high serum levels of sTNFR1, neutralizing all circulating TNF, failed to develop differentiated granulomas and bactericidal mechanisms, and they succumbed to BCG infection. sTNFR1 transgenic mice did not activate BCG-induced Th1-type cytokines early in infection, but uncontrolled cytokine release was found late in infection. In this work we also evaluated the effect of partial inhibition of TNF on resistance to BCG infection. Transgenic mice expressing low levels of sTNFR1 were protected against BCG infection, and they developed increased bactericidal mechanisms, such as enhanced inducible nitric oxide synthase activity, increased macrophage activation, and showed higher numbers of liver granulomas early in infection compared to their negative littermates. Our data suggest that while total inhibition of TNF prevented BCG-induced cell-mediated immune responses, partial inhibition of TNF could contribute to macrophage activation, induction of bactericidal mechanisms, and granuloma formation in the early phase of BCG infection.

Evidence for a role of ADAM17 (TACE) in the regulation of platelet glycoprotein V

Glycoprotein V (GPV) is a subunit of the GPIb-IX-V receptor for von Willebrand factor and thrombin and has been shown to modulate platelet responses to the two strongest physiological agonists, thrombin and collagen. Thrombin directly cleaves GPV from the platelet surface, yielding a 69-kDa fragment GPV f1 of unknown function. We show here that a approximately 82-kDa fragment of GPV is shed from the platelet surface upon cellular activation with phorbol 12-myristate 13-acetate or the collagen-related peptide. This shedding was inhibited by the broad range metalloproteinase inhibitor GM6001, the two potent ADAM17 inhibitors GW280264X and TAPI-2, and was absent in mice lacking functional ADAM17 (ADAM17 lacking Zn-binding domain; ADAM17(DeltaZn/DeltaZn)). Furthermore, we show that recombinant ADAM17 ectodomain efficiently releases GPV from the platelet surface. GPV is known to be associated with the intracellular regulatory protein calmodulin, which has previously been shown to be involved in ADAM17-mediated shedding of l-selectin from the surface of leukocytes. As in these reports, inhibition of calmodulin led to rapid GPV shedding from the platelet surface, a process that was again blocked by GM6001 or ADAM17 inhibitors and that was absent in ADAM17(DeltaZn/DeltaZn) mice. Inhibition of outside-in signaling through GPIIb/IIIa did not significantly affect GPV shedding, excluding an essential role of this pathway for the regulation of ADAM17 activity. These results demonstrate that GPV is cleaved upon agonist-induced platelet activation and show that ADAM17 is the major enzyme mediating this process.

Role of tumour necrosis factor-alpha and other cytokines in ischemia-reperfusion-induced injury in the heart

BACKGROUND

Several investigations have implicated cytokines such as tumour necrosis factor-alpha (TNF-alpha), interleukin (IL)-1, IL-6, IL-8 and transforming growth factor-beta in the pathophysiology of cellular dysfunction in ischemia-reperfusion (I/R). Although an increase in the production of these cytokines has been detected after myocardial infarction and cardiopulmonary bypass surgery, their exact role and mechanisms for inducing cardiac dysfunction are poorly understood.

OBSERVATIONS

TNF-alpha, transforming growth factor-beta, IL-1, IL-6 and IL-8 have frequently been studied in different cardiovascular diseases, including I/R injury in the heart. Low concentrations of TNF-alpha appear to exert cardioprotective effects, whereas high concentrations have been shown to produce deleterious actions in the heart. Some efforts have been made to explore the molecular mechanisms of cytokine actions; however, such information is insufficient to develop therapeutic strategies to combat their deleterious effects during the development of I/R injury in the heart.

CONCLUSIONS

In addition to a time-dependent response, the conflicting effects of cytokines seem to depend on their concentrations used in different experimental studies. It is also likely that both the beneficial and pathophysiological actions of cytokines occur concomitantly. On the basis of the existing literature, it is suggested that different ways need to be found to modify the synthesis as well as the cardiodepressant actions of cytokines to improve the therapy of ischemic heart disease.

Cutting edge: pulmonary immunopathology mediated by antigen-specific expression of TNF-alpha by antiviral CD8+ T cells

Respiratory virus infection results in considerable pulmonary immunopathology, a component of which results from the host immune responses. We have developed a murine model to specifically examine the lung injury due to CD8(+) T cell recognition of an influenza hemagglutinin (HA) transgene on lung epithelium in the absence of replicating virus, after adoptive transfer. Lung injury is largely mediated by chemokines expressed by the epithelial cells upon T cell recognition mediated by TNF-alpha. To determine the critical source of TNF-alpha, HA-specific TNF(-/-) CD8(+) T cells were transferred into HA transgenic animals, and lung injury was not observed, though these T cells exhibited no defect in antiviral activity in vivo. This indicates that the initiating event in the injury process is Ag-specific expression of TNF-alpha by antiviral CD8(+) T cells upon recognition of alveolar epithelial Ag, and that the effector activities responsible for viral clearance may be dissociable from those resulting in immunopathology.

Steatotic liver allografts up-regulate UCP-2 expression and suffer necrosis in rats

BACKGROUND

Fatty split-liver and living-related liver transplantation is associated with massive hepatocellular necrosis during acute rejection. Uncoupling protein (UCP)-2 is a potential regulator of energy expenditure and ATP production. We investigated the role of UCP-2 and the effects of a metalloprotease inhibitor, Y-39083, on hepatocellular injury in fatty liver allografts in rats.

MATERIALS AND METHODS

Rats were treated for 6 weeks with high-ethanol or isocalic dextrose-containing liquid diets that caused characteristic pericentral lipid accumulation. Alcoholic or nonalcoholic fatty livers from ACI (RT1a) rats were transplanted into LEW (RT1l) rats orthotopically. Hepatic necrosis was determined histologically following liver transplantation. UCP-2 mRNA levels in the hepatic allograft and in primary cultured hepatocytes from fatty liver stimulated by tumor necrosis factor (TNF)-alpha were determined. Y-39083 was administered to recipient rats continuously at 5 mg/kg/day using an osmotic infusion mini-pump.

RESULTS

The acute rejection index on day 5 posttransplant in alcoholic and nonalcoholic fatty donor livers was higher than in lean grafts. Massive hepatocyte necrosis was more prominent in alcoholic than nonalcoholic fatty liver allografts and was not seen in lean allografts. UCP-2 transcripts in both alcoholic and nonalcoholic fatty liver allografts were higher than in lean allografts. Serum TNF-alpha concentrations in recipient rats with either fatty liver allograft were greater than in animals with lean allografts. In vitro UCP-2 mRNA levels in primary cultured hepatocytes from both alcoholic and nonalcoholic fatty livers increased more after stimulation with TNF-alpha than those from lean livers. In vitro TNF-alpha production by Kupffer cells isolated from alcohol-induced fatty liver allografts on day 3 posttransplant was greater than those from lean allografts. Y-39083 significantly reduced serum concentrations of TNF-alpha and prevented massive hepatocellular necrosis in rats with both alcoholic and nonalcoholic fatty liver allografts.

CONCLUSION

Liver grafts with steatosis up-regulated UCP-2. TNF-alpha further enhanced UCP-2 transcripts, inducing massive hepatocellular necrosis during acute rejection. Posttransplantation necrosis may be prevented by metalloprotease inhibitors.

Association between serum levels of soluble tumor necrosis factor receptors/CA 125 and disease progression in patients with epithelial ovarian malignancy: a gynecologic oncology group study

BACKGROUND

A prospective study was undertaken within the Gynecologic Oncology Group to determine whether serum levels of soluble tumor necrosis factor receptors I (sTNFR-I) and II (sTNFR-II), alone or in combination with CA 125, were associated with clinicopathologic characteristics or outcome in patients with epithelial ovarian malignancies.

METHODS

Quantitative immunoassays were performed on valid pretreatment serum specimens obtained from patients with epithelial ovarian malignancies to assess levels of sTNFR-I, sTNFR-II, and CA 125. The authors then analyzed the results of these immunoassays for potential correlations with clinicopathologic characteristics and outcome.

RESULTS

The median age of the 139 women evaluated was 59 years. Seventy-eight percent had Stage III or IV disease, and 58% had serous carcinomas. sTNFR-II was associated with age (P = 0.013), and CA 125 was associated with histologic subtype (P = 0.0009). In addition, sTNFR-I (P = 0.037) and CA 125 (P < 0.0001) were associated with extent of disease. After adjusting for patient age, histologic subtype, and extent of disease, all three biomarkers were predictive of progression-free survival, but not overall survival, when the combination was included in the model. The authors observed a 51% reduction (hazard ratio [HR], 0.49; 95% confidence interval [CI], 0.24-0.99), a 2.9-fold increase (HR, 2.87; 95% CI, 1.15-7.20), and a 22% increase (HR, 1.22; 95% CI, 0.99-1.51) in the risk of progression for each unit increase in the log-transformed levels of sTNFR-I, sTNFR-II, and CA 125, respectively.

CONCLUSIONS

The observations made in the current study-that among patients with low or high CA 125 levels, those with high sTNFR-I levels and low sTNFR-II levels had the lowest risk, that patients with low-low or high-high sTNFR-I and sTNFR-II levels, respectively, had an intermediate risk, and that patients with low sTNFR-I levels and high sTNFR-II levels had the highest risk of progression-suggested the potential value of simultaneous assessment of all three biomarkers in patients with epithelial ovarian malignancies.

LIGHT Expression by Mucosal T Cells May Regulate IFN-γ Expression in the Intestine

The TNF superfamily of cytokines play an important role in T cell activation and inflammation. Sustained expression of lymphotoxin-like inducible protein that competes with glycoprotein D for binding herpesvirus entry mediator on T cells (LIGHT) (TNFSF14) causes a pathological intestinal inflammation when constitutively expressed by mouse T cells. In this study, we characterized LIGHT expression on activated human T cell subsets in vitro and demonstrated a direct proinflammatory effect on regulation of IFN-gamma. LIGHT was induced in memory CD45RO CD4+ T cells and by IFN-gamma-producing CD4+ T cells. Kinetic analysis indicated rapid induction of LIGHT by human lamina propria T cells, reaching maximal levels by 2-6 h, whereas peripheral blood or lymph node-derived T cells required 24 h. Further analysis of intestinal specimens from a 41 patient cohort by flow cytometry indicated membrane LIGHT induction to higher peak levels in lamina propria T cells from the small bowel or rectum but not colon, when compared with lymph node or peripheral blood. Independent stimulation of the LIGHT receptor, herpesvirus entry mediator, induced IFN-gamma production in lamina propria T cells, while blocking LIGHT inhibited CD2-dependent induction of IFN-gamma synthesis, indicating a role for LIGHT in the regulation of IFN-gamma and as a putative mediator of proinflammatory T-T interactions in the intestinal mucosa. Taken together, these findings suggest LIGHT-herpesvirus entry mediator mediated signaling as an important immune regulatory mechanism in mucosal inflammatory responses.

The proteolytic processing of the amyloid precursor protein gene family members APLP-1 and APLP-2 involves alpha-, beta-, gamma-, and epsilon-like cleavages: modulation of APLP-1 processing by n-glycosylation

Amyloid precursor protein (APP) processing is of major interest in Alzheimer's disease research, since sequential cleavages by beta- and gamma-secretase lead to the formation of the 4-kDa amyloid Abeta protein peptide that accumulates in Alzheimer's disease brain. The processing of APP involves proteolytic conversion by different secretases leading to alpha-, beta-, gamma-, delta-, and epsilon-cleavages. Since modulation of these cleavages represents a rational therapeutic approach to control amyloid formation, its interference with the processing of the members of the APP gene family is of considerable importance. By using C-terminally tagged constructs of APLP-1 and APLP-2 and the untagged proteins, we have characterized their proteolytic C-terminal fragments produced in stably transfected SH-SY5Y cells. Pharmacological manipulation with specific protease inhibitors revealed that both homologues are processed by alpha- and gamma-secretase-like cleavages, and that their intracellular domains can be released by cleavage at epsilon-sites. APLP-2 processing appears to be the most elaborate and to involve alternative cleavage sites. We show that APLP-1 is the only member of the APP gene family for which processing can be influenced by N-glycosylation. Additionally, we were able to detect p3-like fragments of APLP-1 and p3-like and Abeta-like fragments of APLP-2 in the media of stably transfected SH-SY5Y cells.

Identification and Characterization of 4-[[4-(2-Butynyloxy)phenyl]sulfonyl]-N-hydroxy-2,2-dimethyl-(3S)thiomorpholinecarboxamide (TMI-1), a Novel Dual Tumor Necrosis Factor-α-Converting Enzyme/Matrix Metalloprotease Inhibitor for the Treatment of Rheumatoid Arthritis

Tumor necrosis factor (TNF)-alpha is a well validated therapeutic target for the treatment of rheumatoid arthritis. TNF-alpha is initially synthesized as a 26-kDa membrane-bound form (pro-TNF) that is cleaved by a Zn-metalloprotease named TNF-alpha-converting enzyme (TACE) to generate the 17-kDa, soluble, mature TNF-alpha. TACE inhibitors that prevent the secretion of soluble TNF-alpha may be effective in treating rheumatoid arthritis (RA) patients. Using a structure-based design approach, we have identified a novel dual TACE/matrix metalloprotease (MMP) inhibitor 4-[[4-(2-butynyloxy)phenyl]sulfonyl]-N-hydroxy-2,2-dimethyl-(3S)thiomorpholinecarboxamide (TMI-1). This molecule inhibits TACE and several MMPs with nanomolar IC(50) values in vitro. In cell-based assays such as monocyte cell lines, human primary monocytes, and human whole blood, it inhibits lipopolysaccharide (LPS)-induced TNF-alpha secretion at submicromolar concentrations, whereas there is no effect on the TNF-alpha mRNA level as judged by RNase protection assay. The inhibition of LPS-induced TNF-alpha secretion is selective because TMI-1 has no effect on the secretion of other proinflammatory cytokines such as interleukin (IL)-1beta, IL-6, and IL-8. Importantly, TMI-1 potently inhibits TNF-alpha secretion by human synovium tissue explants of RA patients. In vivo, TMI-1 is highly effective in reducing clinical severity scores in mouse prophylactic collagen-induced arthritis (CIA) at 5, 10, and 20 mg/kg p.o. b.i.d. and therapeutic CIA model at 100 mg/kg p.o. b.i.d. In summary, TMI-1, a dual TACE/MMP inhibitor, represents a unique class of orally bioavailable small molecule TNF inhibitors that may be effective and beneficial for treating RA.

TACE/ADAM-17 enzymatic activity is increased in response to cellular stimulation

Tumor necrosis factor-alpha converting enzyme (TACE/ADAM-17) is a metalloprotease disintegrin that cleaves a variety of membrane proteins, releasing ("shedding") their extracellular domains from cells. Most TACE-mediated shedding events occur at low basal rates that are enhanced by treatment of cells with a variety of stimuli. To study the mechanism of induced shedding, we developed a peptide-cleavage assay that measures the cellular TACE activity. In unstimulated cells, cleavage of a TNFalpha processing-site peptide was mediated mainly by enzymes other than TACE. However, stimulation of cells with phorbol-12-myristate-13-acetate (PMA) increased peptide cleavage in a TACE-dependent manner. PMA treatment did not increase the amount of TACE on the cell surface. Moreover, the cytoplasmic domain of TACE was not required for the induced activity. Based on these observations, induction of TACE-mediated shedding events occurs at least in part via an increase in the enzymatic activity of cellular TACE, independent of its cytoplasmic domain.

Matrix metalloproteinases (MMP) and cathepsin K contribute differently to osteoclastic activities

The best established proteolytic event of osteoclasts is bone matrix solubilization by the cysteine proteinase cathepsin K. Here, however, we draw the attention on osteoclastic activities depending on matrix metalloproteinases (MMPs). We discuss the observations supporting that MMPs contribute significantly to bone matrix solubilization in specific areas of the skeleton and in some developmental and pathological situations. Our discussion takes into account (1) the characteristics of the bone remodeling persisting in the absence of cathepsin K, (2) the ultrastructure of the resorption zone in response to inactivation of MMPs and of cathepsin K in different bone types, (3) bone resorption levels in MMP knockout mice compared to wild-type mice, (4) the identification of MMPs in osteoclasts and surrounding cells, and (5) the effect of different bone pathologies on the serum concentrations of specific collagen fragments believed to discriminate between cathepsin K and MMP cleavage. Next, we provide evidence that MMPs are very critical for osteoclast migration, thereby controlling also the cell-matrix interactions required for cell attachment/detachment. The evidence supporting this role is based on a model of osteoclast recruitment in primitive long bones, an assay of osteoclast invasion through collagen gel, and the effect of proteinase inhibitors/knockouts in these models. Furthermore, we mention observations indicating a role of MMPs in initiation of bone resorption. Finally, we emphasize the many distinct ways MMPs may alter focally the extracellular environment thereby regulating the osteoclast behavior. Although the understanding of MMPs in osteoclast biology is rapidly expanding, it is suspected that important roles remain to be discovered.

Furin-mediated processing of Pro-C-type natriuretic peptide

C-type natriuretic peptide (CNP) is a member of the natriuretic peptide family that is involved in a variety of homeostatic processes. Here we characterize the processing essential for the conversion of the precursor, human pro-CNP, to the biologically active hormone. In human embryonic kidney 293 and chondrosarcoma SW 1353 cells, recombinant pro-CNP was converted into a mature peptide intracellularly as detected by Western analysis. Expression of recombinant human corin, a proatrial natriuretic peptide convertase, did not enhance the processing of pro-CNP in these cells. The processing of pro-CNP was inhibited in the presence of an inhibitor of the endoprotease furin but was not affected by inhibitors of matrix metalloproteinases and tumor necrosis factor-alpha convertase. In furin-deficient human colon adenocarcinoma LoVo cells, no conversion of recombinant pro-CNP to CNP was detected. Expression of recombinant human furin in LoVo cells restored the ability of these cells to process pro-CNP. Furthermore, incubation of purified recombinant human furin with LoVo cell lysate containing pro-CNP led to the conversion of the precursor to a mature peptide. The furin-processed CNP was shown to be biologically active in a cell-based cGMP assay. These results demonstrate that furin is a critical enzyme for the processing of human pro-CNP.

TLR4-dependent lipopolysaccharide-induced shedding of tumor necrosis factor receptors in mouse bone marrow granulocytes

We reported previously that bone marrow granulocytes respond to small amounts of enterobacterial lipopolysaccharide (LPS) via a CD14-independent and TLR4-mediated mechanism by de novo expression of an inducible receptor (CD14) and by down-modulation of a constitutive receptor (L-selectin). In this report we address another effect of LPS: the down-regulation of receptors for tumor necrosis factor-alpha. In mouse bone marrow cells (BMC), this down-regulation is detectable soon (20 min) after exposure of the cells to low levels (0.5 ng/ml) of LPS. This temperature-dependent effect is rather selective for LPS and requires the presence of a conventional lipid A structure in the LPS molecule and a functional TLR4 molecule in the cells. The down-modulation, due to a shedding of the receptors, is blocked by p38 MAPK inhibitors, by a furin inhibitor, and by three metalloproteinase inhibitors (BB-3103, TIMP-2, and TIMP-3). In contrast, inhibitors of MEK, protein kinase C, cAMP-dependent protein kinase, and kinases of the Src family do not block the shedding. Analysis of BMC from mice lacking tumor necrosis factor receptor-1 (CD120a-/-) or tumor necrosis factor receptor-2 (CD120b-/-) indicates that the LPS-induced shedding is specific for CD120b. Thus, exposure of BMC to LPS triggers a rapid shedding of CD120b via a protein kinase C- and Src-independent pathway mediated by p38 MAPK, furin, and metalloproteinase. The additive effects of furin and metalloproteinase inhibitors suggest that these enzymes are involved in parallel shedding pathways.

Characterization of growth factor-induced serine phosphorylation of tumor necrosis factor-alpha converting enzyme and of an alternatively translated polypeptide

Tumor necrosis factor-alpha converting enzyme (TACE) is a prototype member of the adamalysin family of transmembrane metalloproteases that effects ectodomain cleavage and release of many transmembrane proteins, including transforming growth factor-alpha. Growth factors that act through tyrosine kinase receptors, as well as other stimuli, induce shedding through activation of the Erk mitogen-activated protein (MAP) kinase pathway without the need of new protein synthesis. How MAP kinase regulates shedding by TACE is not known. We now report that the cytoplasmic domain of TACE is phosphorylated in response to growth factor stimulation. We also identified a naturally expressed smaller polypeptide corresponding to most of the cytoplasmic domain of TACE. This protein, which we named SPRACT, is derived through alternative translation of the TACE-coding sequence and is, similarly to TACE, phosphorylated in response to growth factor and phorbol 12-myristate 13-acetate stimulation. Phosphoamino acid analysis revealed that growth factor-induced phosphorylation of TACE occurs only on serine and not on threonine or tyrosine. Tryptic mapping experiments coupled with site-directed mutagenesis identified Ser(819) as the major target of growth factor-induced phosphorylation, whereas Ser(791) undergoes dephosphorylation in response to growth factor stimulation. The phosphorylation of Ser(819), but not the dephosphorylation of Ser(791), depends on activation of the Erk MAP kinase pathway. Increased SPRACT expression or mutation of the TACE cytoplasmic domain to inactivate growth factor-induced phosphorylation did not detectably affect growth factor-induced shedding of transmembrane transforming growth factor-alpha by TACE. The roles of SPRACT and the cytoplasmic phosphorylation of TACE remain to be defined.

An MMP-2/MMP-9 inhibitor, 5a, enhances apoptosis induced by ligands of the TNF receptor superfamily in cancer cells

Several studies have shown that matrix metalloproteases (MMPs) promote tumor growth, invasion, and metastasis. Consequently, MMP inhibitors have been developed as a new class of anticancer drugs, many of which are in clinical trials. The exact mechanism of the antineoplastic activity of MMP antagonists is unknown. To investigate the mechanism, we hypothesized that MMP inhibitors enhance the actions of apoptosis-inducing agents. To test this hypothesis, we treated breast, melanoma, leukemia, osteosarcoma, and normal breast epithelial cells with (2R)-2-[(4-biphenylsulfonyl)amino]-3-phenylproprionic acid (compound 5a), an organic inhibitor of MMP-2/MMP-9, alone or in combination with TNFalpha or other apoptotic agents. FACS analysis showed that 5a interacted synergistically with ligands of the TNF receptor superfamily, including TNFalpha and TNF receptor-like apoptosis-inducing ligand (TRAIL), and with a Fas-cross-linking antibody (CH11), UV, paclitaxel, thapsigargin, and staurosporin, to induce apoptosis in a cell-type-specific manner. Other MMP inhibitors did not synergize with TNFalpha. Compound 5a did not act directly on the mitochondrion or via changes in protein synthesis. Instead, the mechanism requires ligand-receptor interaction and caspase 8 activation. Investigation of the effect of 5a on tumor growth in vivo revealed that continuous treatment of subcutaneous melanoma with a combination of 5a plus TRAIL reduced tumor growth and angiogenesis in nude mice. Our data demonstrate that 5a possesses a novel proapoptotic function, thus providing an alternative mechanism for its antineoplastic action. These observations have important implications for combination cancer therapy.

CD4(+)CD3(-) accessory cells costimulate primed CD4 T cells through OX40 and CD30 at sites where T cells collaborate with B cells

In this report we identify an accessory cell that interacts with primed and memory T cells at sites where they collaborate with B cells. These cells are distinguished from conventional dendritic cells by their lack of response to Flt3 ligand and their inability to process antigen. Unlike dendritic cells, the CD4(+)CD3(-) cells have little CD80 or CD86 expression but do express high levels of the TNF ligands, OX40 ligand and CD30 ligand. We show that Th2-primed cells express the receptors for these TNF ligands and preferentially survive when cocultured with these cells. Furthermore, we show that the preferential survival of OX40(+) T cells and support of memory T cell help for B cells are linked to their association with CD4(+)CD3(-) cells in vivo.

A method to increase the number of growth hormone receptors at the surface of cells

The number of growth hormone receptors (GHRs) per cell are regulated and this feature plays a major role in the hormone responsiveness of the body. We previously observed in transfected Chinese hamster lung cells that GHR availability is determined by three factors: endocytosis (75%), shedding (10%), and other undetermined mechanisms (15%). The endocytosis depends on an active ubiquitin conjugation system. In addition, this process is ligand-independent. Here, we show that this principle is useful to increase the abundance of GHRs at the cell surface of cells using a combination of inhibitors. In theory, an inhibitor that targets the ubiquitin conjugation specific for the GHR, would suffice, as almost 80% of the removal rate depends on this mechanism. As the molecular mechanism is unknown yet, we used a general inhibitor of proteasome action. Unfortunately, such an inhibitor stimulates the shedding process severalfold. Our data show that the combination of a general proteasome inhibitor and a matrix metalloprotease inhibitor results in an almost twofold increase in functional GHRs at the cell surface, and generate new perspectives to increase the sensitivity of cells for growth hormone.

Divergent roles for p55 and p75 TNF-alpha receptors in the induction of plasminogen activator inhibitor-1

Tumor necrosis factor-alpha (TNF-alpha) is elevated in obesity and in acute inflammatory states, and contributes to the elevated plasminogen activator inhibitor-1 (PAI-1) levels associated with these conditions. Mice genetically deficient in the p55 and p75 TNF-alpha receptors were used to study the roles of these receptors in the expression of PAI-1 in obese (ob/ob) mice, and in lean mice following acute stimulation with TNF-alpha. In ob/ob mice, p55 and p75 tumor necrosis factor-alpha receptors (TNFRs) act cooperatively to induce PAI-1 mRNA in most tissues, including the adipose tissue, kidney, heart, and liver. However, in lean mice, TNF-alpha-induced PAI-1 expression is mediated primarily by the p55 TNFR. Interestingly, PAI-1 mRNA expression in all tissues of the TNF-alpha-treated p75-deficient lean mice was significantly higher than that observed in TNF-alpha-treated wild-type mice. These observations suggest that the p75 TNFR may play a role in attenuating TNF-alpha-induced PAI-1 mRNA expression in acute inflammatory conditions. Our observation that soluble p75 TNFR was elevated in the plasma of TNF-alpha-treated mice in comparison to untreated mice supports this hypothesis. These studies thus provide insights into the TNF-alpha receptors involved in mediating and modulating the expression of PAI-1 in acute and chronic (eg, obesity) inflammatory states associated with elevated TNF-alpha.

Lack of tumor necrosis factor alpha induces impaired proliferation of hepatitis B virus-specific cytotoxic T lymphocytes

Recent studies have shown that tumor necrosis factor alpha (TNF-alpha) plays critical roles in not only viral clearance but also lymphoid tissue development and stem cell differentiation. In this study, we attempted to induce hepatitis B virus (HBV)-specific cytotoxic T lymphocytes (CTLs) by immunization of TNF-alpha knockout (TNF-alpha(-/-)) mice with HBsAg-encoding plasmid DNA. An immunization with the HBV plasmid failed to induce CTL responses in TNF-alpha(-/-) mice, although CTLs were readily induced in wild-type mice by the same protocol. Weak CTL responses were produced in TNF-alpha(-/-) mice after two sessions of immunization with the HBV plasmid; however, TNF-alpha was required to maintain the responses of these CTL lines to in vitro stimulation and, even then, the responses were lost after 3 weeks. Interestingly, a limiting dilution of a CTL line showed that HBV-specific CTL clones with high specific cytotoxicity were present in TNF-alpha(-/-) mice, but these clones again failed to proliferate for more than 3 weeks. Furthermore, since exogenously added TNF-alpha enhanced the proliferation of a TNF-alpha(-/-) clone but suppressed that of a TNF-alpha(+/+) clone in vitro, TNF-alpha also has a direct effect on the proliferation of CTLs. In conclusion, TNF-alpha is essential rather than important for the proliferation of HBV-specific CTLs both in vivo and in vitro and this effect is not only due to the activation of dendritic cells but is also induced by the direct effect on CTLs.

Putative function of ADAM9, ADAM10, and ADAM17 as APP alpha-secretase

The putative alpha-secretase cleaves the amyloid precursor protein (APP) of Alzheimer's disease in the middle of the amyloid beta peptide (Abeta) domain. It is generally thought that the alpha-secretase pathway mitigates Abeta formation in the normal brain. Several studies have suggested that ADAM9, ADAM10, and ADAM17 are candidate alpha-secretases belonging to the ADAM (a disintegrin and metalloprotease) family, which are membrane-anchored cell surface proteins. In this comparative study of ADAM9, ADAM10, and ADAM17, we examined the physiological role of ADAMs by expressing these ADAMs in COS-7 cells, and both "constitutive" and "regulated" alpha-secretase activities of these ADAMs were determined. We tried to suppress the expression of these ADAMs in human glioblastoma A172 cells, which contain large amounts of endogenous alpha-secretase, by lipofection of the double-stranded RNA (dsRNA) encoding each of these ADAMs. The results indicate that ADAM9, ADAM10, and ADAM17 catalyze alpha-secretory cleavage and therefore act as alpha-secretases in A172 cells. This is the first report that to suggest the endogenous alpha-secretase is composed of several ADAM enzymes.

Autolytic processing at Glu586-Ser587 within the cysteine-rich domain of human adamalysin 19/disintegrin-metalloproteinase 19 is necessary for its proteolytic activity

We investigated the regulation of the proteolytic activity of human adamalysin 19 (a disintegrin and metalloproteinase 19, hADAM19). It was processed at Glu(586)(P1)-Ser(587)(P1') site in the cysteine-rich domain as shown by protein N-terminal sequencing. This truncation was autolytic as illustrated by its R199A/R200A or E346A mutation that prevented the zymogen activation by furin or abolished the catalytic activity. Reagents that block furin-mediated activation of pro-hADAM19, decRVKR-CMK, and brefeldin A abrogated this processing. The sizes of the side chains of the P1 and P1' residues are critical for the processing of hADAM19. The amount of processing product in the E586Q or S587A mutant with a side chain almost the same size as that in the wild type was almost equal. Conversely, very little processing was observed when the size of the side chain was changed significantly, such as in the E586A, E586G, or S587F mutants. Two mutants with presumably subtle structural distinctions from wild type hADAM19, E586D and S587T, displayed rare or little processing and had very low capacities to cleave alpha2-macroglobulin and a peptide substrate. Therefore, this processing is necessary for hADAM19 to exert its proteolytic activities. Moreover, a new peptide substrate, Ac-RPLE-SNAV, which is identical to the processing site sequence, was cleaved at the E-S bond by soluble hADAM19 containing the catalytic and disintegrin domains. This enzyme cleaved the substrate with K(m), k(cat), and k(cat)/K(m) of 2.0 mm, 2.4/min, and 1200 m(-1) min(-1), respectively, using a fluorescamine assay. Preliminary studies showed that a protein kinase C activator, phorbol 12-myristate 13-acetate, promoted the cellular processing of hADAM19; however, three calmodulin antagonists, trifluoperazine, W7, and calmidazolium, impaired this cleavage, indicating complex signal pathways may be involved in the processing.

Shedding of the transferrin receptor is mediated constitutively by an integral membrane metalloprotease sensitive to tumor necrosis factor alpha protease inhibitor-2

The transferrin receptor (TfR) is a transmembrane protein that mediates cellular uptake of iron. Although the serum concentration of the soluble TfR (sTfR) is altered in several diseases and used for diagnostic purposes, the identity and regulation of the shedding protease is unknown. In this study we quantified sTfR release from microsomal membranes and leukocytic cell lines in the presence of numerous protease inhibitors and cell activating compounds. We show that sTfR release is mediated by an integral membrane metalloprotease and can be inhibited by matrix metalloproteinase inhibitor 2 and tumor necrosis factor alpha protease inhibitor-2 (TAPI-2). Cleavage is also inhibited by a specific furin inhibitor, indicating that the protease is activated by a furin-like proprotein convertase. Whereas stimulation of the cells by the ectodomain shedding activator phorbol 12-N-myristate 13-acetate did not alter sTfR release significantly, the phosphatase inhibitor pervanadate led to an increase of TfR shedding in several leukocytic cell lines. Our results suggest that TfR shedding is constitutively mediated by a member of the metalloprotease family known as ADAM (for a disintegrin and metalloprotease).

TACE inhibition: a new approach to treating inflammation

Clinical trials showing the benefits of reducing the effects of TNF- alpha in rheumatoid arthritis have highlighted the key role of the cytokine TNF- alpha in this inflammatory condition. A new approach to reducing the effects of TNF- alpha is to decrease its synthesis by inhibiting TNF- alpha converting enzyme with GW3333. In rat models of arthritis, GW3333 has some beneficial effects. Further longer-term studies of GW3333 in animal models are required to determine whether its benefit is maintained. TACE inhibition may represent a new approach to treating inflammation.

TNF receptor subtype signalling: differences and cellular consequences

Tumour necrosis factor-alpha (TNF alpha) is a multifunctional cytokine belonging to a family of ligands with an associated family of receptor proteins. The pleiotropic actions of TNF range from proliferative responses such as cell growth and differentiation, to inflammatory effects and the mediation of immune responses, to destructive cellular outcomes such as apoptotic and necrotic cell death mechanisms. Activated TNF receptors mediate the association of distinct adaptor proteins that regulate a variety of signalling processes including kinase or phosphatase activation, lipase stimulation, and protease induction. Moreover, the cytokine regulates the activities of transcription factors, heterotrimeric or monomeric G-proteins and calcium ion homeostasis in order to orchestrate its cellular functions. This review addresses the structural basis of TNF signalling, the pathways employed with their cellular consequences, and focuses on the specific role played by each of the two TNF receptor isotypes, TNFR1 and TNFR2.

A novel mutation in the third extracellular domain of the tumor necrosis factor receptor 1 in a Finnish family with autosomal-dominant recurrent fever

OBJECTIVE

To investigate the presence of TRAPS (tumor necrosis factor receptor-associated periodic syndrome), which is a recently defined, dominantly inherited autoinflammatory syndrome caused by mutations in the tumor necrosis factor receptor superfamily 1A gene (TNFRSF1A, CD120a), in a Finnish family with recurrent fever.

METHODS

The TNFRSF1A gene was sequenced in both affected and unaffected family members. Flow cytometry and enzyme-linked immunosorbent assay analyses were used to assess membrane expression and serum levels of the TNFRSF1A protein, respectively.

RESULTS

A missense mutation in exon 4, located in the third extracellular domain of TNFRSF1A and resulting in an amino acid substitution (F112I) close to a conserved cysteine, was found in all 4 affected family members and in 1 asymptomatic individual. The mutation was clearly associated with low levels of soluble TNFRSF1A as well as with the clinical symptoms of recurrent fever and abdominal pain. Impaired shedding of TNFRSF1A after phorbol myristate acetate stimulation was detected in blood granulocytes and monocytes from the 3 adult family members with the mutation, but in the child bearing the mutation and showing clinical symptoms of recent onset, the shedding defect was less marked.

CONCLUSION

TRAPS should be suspected in any patient who presents with a history of intermittent fever accompanied by unexplained abdominal pain, arthritis, or skin rash, particularly in the presence of a family history of such symptoms. Screening for low serum levels of soluble TNFRSF1A identifies individuals who are likely to have TNFRSF1A mutations.

Chloroquine decreases cell-surface expression of tumour necrosis factor receptors in human histiocytic U-937 cells

Proinflammatory cytokine tumour necrosis factor (TNF) mediates its diverse effects through cell surface receptors. A variety of inflammatory signals are known to modulate TNF activities by changing expression and shedding of cell-surface TNF receptors. We have examined the effects of anti-rheumatic drug chloroquine on the expression of cell surface and soluble TNF receptors in human histiocytic U-937 cells. Chloroquine partially reduced production of soluble p55 and p75 TNF receptors in cells stimulated with phorbol 12-myristate 13-acetate (PMA). In these cells, induction of both TNF receptor mRNA was not changed and the levels of cell-associated TNF receptors were rather increased by chloroquine. Flow cytometric analysis revealed that chloroquine does not inhibit the PMA-triggered shedding of TNF receptors from cell surface, while it was suppressed by a metalloproteinase inhibitor BB-3103. Treatment of U-937 cells with chloroquine significantly reduced the level of cell surface TNF receptors and a similar effect was observed with human peripheral blood monocytes. Other weak-base amines, including hydroxychloroquine, ammonium chloride and methylamine, also induced reduction of cell surface TNF receptors, whereas lysosomal proteinase inhibitor, leupeptin, and BB-3013 were without effect. Our results suggest that chloroquine down-regulates cell surface TNF receptors by retarding their transport to the cell surface, while cleavage of cell surface receptors is not inhibited by chloroquine.

Transmodulation of cell surface regulatory molecules via ectodomain shedding

Cell responses to exogenous stimuli often result in a rapid decrease of cell surface density of a wide range of diverse regulatory proteins, receptor and adhesion molecules in particular. This decrease may occur in a ligand-dependent fashion (down-regulation), following endocytosis and degradation by lysosomal proteases, or by down-modulation, where molecules are targeted by endoproteases directly on cell surface. These proteases are recruited by trans-modulating agents, different from ligand, which act via their own receptors and the related intracellularly-generated signals. Endoproteolytic activity determines the release of large portions (shedding) of substrate proteins, called ectodomains, which are usually not ligand-bound, and therefore represent biologically-active molecules. Ectodomain shedding is involved in a number of pathophysiological processes, such as inflammation, cell degeneration and apoptosis, and oncogenesis. Common features of the process, such as the involvement of protein kinase C and of transmembrane metalloproteases, have been identified. In this review, we summarize basic concepts on down-modulation and ectodomain shedding, and provide an update of the issue with respect to: (i) new entries to the list of molecules found involved in the process; (ii) current views about the upstream control of shedding, i.e. the pathways linking the signals triggered by the trans-modulating agents to the activation of endoproteolytic activity on the cell surface.

Tumor necrosis factor-alpha-converting enzyme mediates the inducible cleavage of fractalkine

Fractalkine (FK, CX3CL1) is a novel multidomain protein expressed on the surface of endothelial cells. As a full-length transmembrane protein, FK binds cells expressing CX3CR1, its cognate receptor, with high affinity. Proteolytic cleavage of FK releases a soluble form that is a potent chemoattractant for monocytes, T cells, and natural killer cells. Activation of protein kinase C dramatically increases the rate of this cleavage. Regulation of FK cleavage is critical for maintaining the balance between the immobilized and soluble forms, but the protease responsible has not been identified. Here we report that tumor necrosis factor-alpha-converting enzyme (TACE) is primarily responsible for the inducible cleavage of FK. After transfection into host cells, the proteolytic cleavage of FK was blocked by TACE-specific inhibitors and was not detected in cells genetically altered to remove TACE activity. In contrast, the constitutive cleavage of FK was not mediated by TACE and proceeded normally in TACE-null fibroblasts. We conclude that TACE is primarily responsible for the inducible cleavage of FK. These studies identify a potentially important link between local generation of potent cytokines and control of the balance between the cell adhesion and chemotactic properties of FK.

Genomic characterization of LIGHT reveals linkage to an immune response locus on chromosome 19p13.3 and distinct isoforms generated by alternate splicing or proteolysis

LIGHT is a member of the TNF cytokine superfamily that signals through the lymphotoxin (LT)beta receptor and the herpesvirus entry mediator. LIGHT may function as a costimulatory factor for the activation of lymphoid cells and as a deterrent to infection by herpesvirus, which may provide significant selective pressure shaping the evolution of LIGHT. Here, we define the molecular genetics of the human LIGHT locus, revealing its close linkage to the TNF superfamily members CD27 ligand and 4-1BB ligand, and the third complement protein (C3), which positions LIGHT within the MHC paralog on chromosome 19p13.3. An alternately spliced isoform of LIGHT mRNA that encodes a transmembrane-deleted form is detected in activated T cells and gives rise to a nonglycosylated protein that resides in the cytosol. Furthermore, membrane LIGHT is shed from the cell surface of human 293 T cells. These studies reveal new mechanisms involved in regulating the physical forms and cellular compartmentalization of LIGHT that may contribute to the regulation and biological function of this cytokine.

Biology of TACE inhibition

Studies conducted over the past decade have demonstrated a central role for tumour necrosis factor alpha (TNFalpha) in inflammatory diseases. As a result of this work, a number of biological agents that neutralise the activity of this cytokine have entered the clinic. The recent clinical data obtained with etanercept and infliximab highlight the relevance of this strategy. TNFalpha converting enzyme (TACE) is the metalloproteinase that processes the 26 kDa membrane bound precursor of TNFalpha (proTNFalpha) to the 17 kDa soluble component. Although a number of proteases have been shown to process proTNFalpha, none do so with the efficiency of TACE. A series of orally bioavailable, selective, and potent TACE inhibitors are currently in clinical development. These inhibitors effectively block TACE mediated processing of proTNFalpha and can reduce TNF production by lipopolysaccharide stimulated whole blood by >95%. Through a series of studies it is shown here that >80% of the unprocessed proTNFalpha is degraded intracellularly. The remainder appears to be transiently expressed on the cell surface. Although, in vitro, TACE inhibition has also been implicated in shedding of p55 and p75 surface TNFalpha receptors, the in vivo data cast doubt on the consequences of this finding. In a mouse model of collagen-induced arthritis, the inhibitors are efficacious both prophylactically and therapeutically. The efficacy seen is equivalent to strategies that neutralise TNFalpha. In many studies greater efficacy is observed with the TACE inhibitors, presumably owing to greater penetration to the site of TNFalpha production.

Regulation of Membrane Metalloproteolytic Cleavage of L-selectin (CD62L) by the Epidermal Growth Factor Domain

The adhesion molecule L-selectin is cleaved rapidly from the surface of activated leukocytes by tumor necrosis factor-alpha converting enzyme, a cell surface metalloprotease, and also undergoes slower constitutive shedding in unactivated cells. The structural features that render it susceptible to shedding are poorly understood. We therefore analyzed the shedding of a series of mutant and chimeric L-selectin molecules. Although murine L-selectin is cleaved at a specific location in the juxtamembrane region 11 amino acids distal to the cell membrane, this cleavage has little sequence specificity. However, proline substitution at the P2' or P3' position or deletion of the epidermal growth factor (EGF) domain completely blocks the rapid phorbol ester-induced cleavage, but does not affect the slower basal proteolytic shedding. Insertion of the 15-residue membrane-proximal region (MPR) of L-selectin into the heterologous protein B7.2 results in a molecule that undergoes constitutive proteolytic turnover. In contrast, insertion of both the EGF domain and the MPR confers susceptibility to both slow constitutive shedding and the rapid proteolytic cleavage induced by phorbol 12-myristate 13-acetate. These results demonstrate that constitutive and induced L-selectin cleavage are separable processes and that the rapid phorbol ester-induced shedding requires the presence of the EGF domain, a sequence that is remote from the cleavage site.

Evidence of a role for a non-matrix-type metalloproteinase activity in the shedding of syndecan-1 from human myeloma cells

Syndecan-1 is a cell surface proteoglycan that is expressed on human myeloma cells and is thought to act as a co-receptor for certain extracellular matrix proteins and growth factors. The ectodomain of syndecan-1 is thought to be shed from the surface of myeloma cells, although the exact mechanism of release remains unclear. In this study, we used a panel of inhibitors to identify the class of proteinase responsible for shedding the soluble syndecan-1 ectodomain from human myeloma cells. Using enzyme-linked immunosorbent assay, flow cytometry and immunocytochemistry, we demonstrated that myeloma cell lines expressed syndecan-1 on their surface and that this was shed constitutively, but to a varying extent. In addition, phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C, stimulated a marked loss of cell surface syndecan-1 from each of the cell lines and this was associated with a corresponding increase in soluble syndecan-1. Inhibitors of serine and cysteine proteinases, and matrix-type metalloproteinases, did not inhibit constitutive or PMA-stimulated syndecan-1 shedding from JJN3 and RPMI 8226 cells. However, BB-94, a hydroxamate-based, broad-spectrum, metalloproteinase inhibitor, substantially suppressed constitutive and PMA-stimulated syndecan-1 loss from myeloma cells. These data indicate that a non-matrix-type metalloproteinase is responsible for syndecan-1 shedding from the surface of myeloma cells.

Antibody-induced shedding of CD44 from adherent cells is linked to the assembly of the cytoskeleton

CD44 is a widely expressed integral membrane glycoprotein that serves as a specific adhesion receptor for the extracellular matrix glycosaminoglycan hyaluronan. CD44 participates in a variety of physiological and pathological processes through its role in cell adhesion. Under appropriate conditions, the ectodomain of CD44 is proteolytically removed from the cell surface. In this study we show that excessive CD44 shedding can be induced in mouse fibroblasts and monocytes upon exposure of these cells to a CD44-specific Ab immobilized on plastic, whereas treatment with phorbol ester induces significantly enhanced CD44 release from the monocytes only. CD44 shedding proceeds normally in fibroblasts and monocytes deficient in TNF-alpha converting enzyme (TACE), a sheddase involved in the processing of several substrates. Conversely, activation of the CD44 protease has no effect on the release of TNF-alpha from TACE-expressing cells, although the same metalloprotease inhibitor effectively blocks both TACE and the CD44 sheddase. Concomitant with anti-CD44 Ab- or phorbol ester-induced CD44 shedding, dramatic changes are observed in cell morphology and the structure of the actin cytoskeleton. Disruption of actin assembly with cytochalasin reduces CD44 shedding, but not the release of TNF-alpha. Moreover, pharmacological activation of Rho family GTPases Rac1 and Cdc42, which regulate actin filament assembly into distinct cytoskeletal structures, has a profound effect on CD44 release. We conclude that the CD44 sheddase and TACE are distinct enzymes, and that Ab- and phorbol ester-enhanced cleavage of CD44 is controlled in a cell type-dependent fashion by Rho GTPases through the cytoskeleton.

Biochemical and pharmacological criteria define two shedding activities for TRANCE/OPGL that are distinct from the tumor necrosis factor alpha convertase

A number of structurally and functionally diverse membrane proteins are released from the plasma membrane in a process termed protein ectodomain shedding. Ectodomain shedding may activate or inactivate a substrate or change its properties, such as converting a juxtacrine into a paracrine signaling molecule. Here we have characterized the activities involved in protein ectodomain shedding of the tumor necrosis factor family member TRANCE/OPGL in different cell types. The criteria used to evaluate these activities include (a) cleavage site usage, (b) response to activators and inhibitors of intracellular signaling pathways, and (c) sensitivity to tissue inhibitors of metalloproteases (TIMPs). At least two TRANCE shedding activities emerged, both of which are distinct from the tumor necrosis factor alpha convertase. One of the TRANCE sheddases is induced by the tyrosine phosphatase inhibitor pervanadate but not by phorbol esters, whereas the other is refractory to both of these stimuli. Furthermore, the pervanadate-regulated sheddase activity is sensitive to TIMP-2 but not TIMP-1, which is consistent with the properties of a membrane type matrix metalloprotease. This study provides insights into the properties of different activities involved in protein ectodomain shedding and has implications for the functional regulation of TRANCE by potentially more than one protease.

Matrix-dependent mechanism of neutrophil-mediated release and activation of matrix metalloproteinase 9 in myocardial ischemia/reperfusion

BACKGROUND

A key component of reperfusion of myocardial infarction is an immediate inflammatory response, which enhances tissue repair. Matrix turnover is crucial to tissue repair, and matrix metalloproteinases (MMPs) are key enzymes involved in matrix degradation. The hypothesis tested is that one inflammation-based effector of tissue repair is the secretion and activation of MMP-9 by infiltrating neutrophils.

METHODS AND RESULTS

Cardiac lymph and tissue were assayed for atent and active MMP-2 and MMP-9 by zymography and immunochemistry. Dual-labeling immunofluorescence determined the cellular source of MMP-9 protein. Isolated canine neutrophils were incubated with preischemic and postischemic cardiac lymph in the presence and absence of collagen-fibronectin pads, and the supernatants were assayed for latent and active MMP-9. MMP-9 increased during the first hours of reperfusion in both lymph supernatants and myocardial extracts, and this increase was of neutrophil origin. MMP-9 in the cardiac lymph remained latent but was activatable. In contrast, MMP-9 in the myocardium was in both latent and active forms. In situ zymography demonstrated that activated MMP-9 surrounded the infiltrated neutrophils. When postischemic cardiac lymph was incubated with neutrophils in vitro, MMP-9 secretion and activation occurred only in the presence of a collagen-fibronectin substrate; preischemic cardiac lymph did not induce significant secretion or activation.

CONCLUSIONS

Infiltrating neutrophils are an early source of MMP-9 after reperfusion, and a portion of MMP-9 in the myocardium is active. Infiltrating neutrophils may localize MMP-9 activation by secreting MMP-9 and as a source of activating proteases.

Fractalkine (CX3CL1) as an amplification circuit of polarized Th1 responses

Fractalkine (FKN, CX3CL1) is a membrane-bound CX3C chemokine induced by primary proinflammatory signals in vascular endothelial cells (ECs). Here we examined the role of FKN in polarized Th1 or Th2 responses. Proinflammatory signals, including LPS, IL-1, TNF, and CD40 ligand, induced FKN, as did IFN-gamma, which had synergistic activity with TNF. IL-4 and IL-13 did not stimulate the expression of FKN and markedly reduced induction by TNF and IFN-gamma. TNF alone or combined with IFN-gamma also induced release of soluble FKN, which was inhibited by IL-4 and IL-13. In light of this differential regulation of FKN by the master cytokines that control polarized responses, we analyzed the interaction of FKN with natural killer (NK) cells and polarized T-cell populations. NK cells expressed high levels of the FKN receptor CX3CR1 and responded to FKN. CX3CR1 was preferentially expressed in Th1 compared with Th2 cells. Th1 but not Th2 cells responded to FKN. By immunohistochemistry, FKN was expressed on ECs in psoriasis, a Th1-dominated skin disorder, but not in Th2-driven atopic dermatitis. Similarly, ECs in Mycobacterium tuberculosis granulomatous lymphadenitis, but not those in reactive lymph node hyperplasia or in Castelman's disease, showed immunoreactive FKN. These results indicate that regulated expression of FKN in ECs participates in an amplification circuit of polarized type I responses.

Toxic effector molecules in the pathogenesis of immune-mediated disorders of the central nervous system

A growing body of evidence supports the notion that inflammatory reactions in the central nervous system (CNS) are not only restricted to established immune mediated disorders, such as multiple sclerosis, but also contribute to the pathogenesis of Alzheimer's disease and other types of neurodegenerative disorders. The biological roles of toxic mediators, such as nitric oxide, reactive oxygen species, as well as complement and proteases in the genesis of inflammatory reactions in the CNS are reviewed within the context of demyelination and neuronal damage.

Recognition sequences and structural elements contribute to shedding susceptibility of membrane proteins

Although regulated ectodomain shedding affects a large panel of structurally and functionally unrelated proteins, little is known about the mechanisms controlling this process. Despite a lack of sequence similarities around cleavage sites, most proteins are shed in response to the stimulation of protein kinase C by phorbol esters. The signal-transducing receptor subunit gp130 is not a substrate of the regulated shedding machinery. We generated several chimaeric proteins of gp130 and the proteins tumour necrosis factor alpha (TNF-alpha), transforming growth factor alpha (TGF-alpha) and interleukin 6 receptor (IL-6R), which are known to be subject to shedding. By exchanging small peptide sequences of gp130 for cleavage-site peptides of TNF-alpha, TGF-alpha and IL-6R we showed that these short sequences conferred susceptibility to spontaneous and phorbol-ester-induced shedding of gp130. Importantly, these chimaeric gp130 proteins were functional, as shown by the phosphorylation of gp130 and the activation of signal transduction and activators of transcription 3 ('STAT3') on stimulation with cytokine. To investigate minimal requirements for shedding, truncated cleavage-site peptides of IL-6R were inserted into gp130. The resulting chimaeras were susceptible to shedding and showed the same cleavage pattern as observed in the chimaeras containing the complete IL-6R cleavage site. Surprisingly, we could also generate cleavable chimaeras by exchanging the juxtamembrane sequence of gp130 for the corresponding region of leukaemia inhibitory factor ('LIF') receptor, a protein that like gp130 is not subject to regulated or spontaneous shedding. Thus it seems that there is no minimal consensus shedding sequence. We speculate that structural changes allow the access of the protease to a membrane-proximal region, leading to shedding of the protein.

TNF-alpha-converting enzyme cleaves the macrophage colony-stimulating factor receptor in macrophages undergoing activation

We previously reported that macrophage activators such as LPS, IL-2, and IL-4 down-modulate the M-CSFR via a mechanism involving protein kinase C and phospholipase C. In this study, we showed that M-CSFR is shed from macrophage surface and identified the protease responsible for M-CSFR cleavage and down-modulation. The shedding of M-CSFR elicited by phorbol esters (tetradecanoylphorbol myristate acetate (TPA)) or LPS in murine BAC.1-2F5 macrophages was prevented by cation chelators, as well as hydroxamate-based competitive inhibitors of metalloproteases. We found that the protease cleaving M-CSFR is a transmembrane enzyme and that its expression is controlled by furin-like serine endoproteases, which selectively process transmembrane metalloproteases. M-CSFR down-modulation was inhibited by treating cells in vivo, before TPA stimulation, with an Ab raised against the extracellular, catalytic domain of proTNF-converting enzyme (TACE). TACE expression was confirmed in BAC.1-2F5 cells and found inhibited after blocking furin-dependent processing. Using TACE-negative murine Dexter-ras-myc cell monocytes, we found that in these cells TPA is unable to down-modulate M-CSFR expression. These data indicated that TACE is required for the TPA-induced M-CSFR cleavage. The possibility that the cleavage is indirectly driven by TACE via the release of TNF was excluded by treating cells in vivo with anti-TNF Ab. Thus, we concluded that TACE is the protease responsible for M-CSFR shedding and down-modulation in mononuclear phagocytes undergoing activation. The possible physiological relevance of this mechanism is discussed.

Endocytosis of uncleaved tumor necrosis factor-alpha in macrophages

Activated monocytes and macrophages secrete the inflammatory cytokine tumor necrosis factor-alpha (TNF-alpha). TNF-alpha is produced as a 26 kd transmembrane protein that is cleaved to release a 17 kd soluble protein. TNF-alpha in both forms is biologically active. The intracellular trafficking of membrane-associated TNF-alpha in lipopolysaccharide-activated mouse macrophages was assessed after treatment with the metalloprotease inhibitor BB-3103, which prevents the cleavage of pro-TNF-alpha. Immunoprecipitation and immunofluorescence studies showed sustained expression of cell-associated TNF-alpha in the presence of the inhibitor. Cell immunoreactivity and surface biotinylation revealed that uncleaved TNF-alpha accumulated on the cell surface and was endocytosed, appearing in intracellular vesicles. Perturbation of post-Golgi traffic blocked the surface expression of 26 kd TNF-alpha. Tracking a bolus of TNF-alpha over time in cycloheximide-treated cells confirmed that uncleaved TNF-alpha is first transported to the cell surface and subsequently endocytosed. Vesicular structures immunoreactive for TNF-alpha were identified as endosomes by double labeling. The secretory and membrane-associated endocytic trafficking of TNF-alpha provides a mechanism for modulating the quantity of biologically active 26 kd TNF-alpha expressed on macrophages, allowing regulation of paracrine and autocrine responses.

The soluble interleukin 6 receptor: mechanisms of production and implications in disease

Interleukin 6 (IL-6) performs a prominent role during disease and has been described as both a pro- and anti-inflammatory cytokine. A key feature in the regulation of IL-6 responses has been the identification of a soluble interleukin 6 receptor (sIL-6R), which forms a ligand-receptor complex with IL-6 that is capable of stimulating a variety of cellular responses including proliferation, differentiation and activation of inflammatory processes. Elevated sIL-6R levels have been documented in numerous clinical conditions indicating that its production is coordinated as part of a disease response. Thus, sIL-6R has the potential to regulate both local and systemic IL-6-mediated events. This review will outline the central role of sIL-6R in the coordination of IL-6 responses. Details relating to the mechanisms of sIL-6R production will be provided, while the potential significance of sIL-6R during the development of clinical conditions will be emphasized. We want to convey, therefore, that when thinking about the inflammatory capability of IL-6, it is essential to consider not only the action of IL-6 itself, but also the effect sIL-6R may have on cellular processes.

Metalloprotease-mediated shedding of enzymatically active mouse ecto-ADP-ribosyltransferase ART2.2 upon T cell activation

T cells proteolytically shed the ectodomains of several cell surface proteins and, thereby, can alter their responsiveness and can release soluble intercellular regulators. ART2.2 is a GPI-anchored ecto-ADP-ribosyltransferase (ART) related to ADP-ribosylating bacterial toxins. ART2.2 is expressed exclusively by mature T cells. Here we show that ART2.2 is shed from the cell surface in enzymatically active form upon activation of T cells. Shedding of ART2.2 resembles that of L-selectin (CD62L) in dose response, kinetics of release, and sensitivity to the metalloprotease inhibitor Immunex Compound 3, suggesting that ART2.2, like CD62L, is cleaved by TNF-alpha-converting enzyme or by another metalloprotease. ART2.2 shed from activated T cells migrates slightly faster in SDS-PAGE analyses than does ART2.2 released upon cleavage of the GPI anchor. This indicates that shedding of ART2.2 is mediated by proteolytic cleavage close to its membrane anchor. Shed ART2.2 is enzymatically active and ADP-ribosylates several substrates in vitro. Thus, shedding of ART2.2 releases a potential intercellular regulator. Finally, using a new FACS assay for monitoring ADP-ribosylation of cell surface proteins, we demonstrate that shedding of ART2.2 correlates with a reduced sensitivity of T cell surface proteins to ADP-ribosylation. Our findings suggest that by shedding ART2.2 the activated T cell not only releases a potential intercellular regulator but also may alter its responsiveness to immune regulation by ART2.2-mediated ADP-ribosylation of cell surface proteins.

TNF-Induced shedding of TNF receptors in human polymorphonuclear leukocytes: role of the 55-kDa TNF receptor and involvement of a membrane-bound and non-matrix metalloproteinase

A down-modulation of both the 55-kDa (TNF-R55) and the 75-kDa (TNF-R75) TNF receptors is observed in neutrophils exposed to a variety of stimuli. Proteolytic cleavage of the extracellular region of both receptors (shedding) and, with TNF, internalization of TNF-R55 and shedding of TNF-R75 are the proposed mechanisms. We have characterized the TNF-induced shedding of TNF receptors in neutrophils and determined the nature of the involved proteinase. Neutrophils exposed to TNF release both TNF receptors. A release of TNF receptors comparable to that observed with TNF was induced with TNF-R55-specific reagents (mAbs and a mutant of TNF) but not with the corresponding TNF-R75-specific reagents. A hydroxamic acid compound (KB8301) almost completely inhibited shedding of TNF-R55 and to a lesser degree shedding of TNF-R75. KB8301 also inhibited FMLP-induced shedding to a similar extent. Shedding was also inhibited by 1,10-phenanthroline, but this effect was considered nonspecific as the compound, at variance with KB8301, almost completely inhibited TNF and FMLP-induced PMN activation. Diisopropylfluorophosphate partially inhibited shedding of TNF-R75, suggesting the contribution of a serine proteinase to the release of this receptor. Shedding activity was not affected by matrix metalloproteinases inhibitors nor was it released in the supernatants of FMLP-stimulated neutrophils. These results suggest that TNF induces release of its receptors, that such a release is mediated via TNF-R55, and that a membrane-bound and non-matrix metalloproteinase is involved in the process. The possibility that ADAM-17, which we show to be expressed in neutrophils, might be the involved proteinase is discussed.

TNFRSF1A mutations and autoinflammatory syndromes

The autoinflammatory syndromes are systemic disorders characterized by apparently unprovoked inflammation in the absence of high-titer autoantibodies or antigen-specific T lymphocytes. One such illness, TNF-receptor-associated periodic syndrome (TRAPS), presents with prolonged attacks of fever and severe localized inflammation. TRAPS is caused by dominantly inherited mutations in TNFRSF1A (formerly termed TNFR1), the gene encoding the 55 kDa TNF receptor. All known mutations affect the first two cysteine-rich extracellular subdomains of the receptor, and several mutations are substitutions directly disrupting conserved disulfide bonds. One likely mechanism of inflammation in TRAPS is the impaired cleavage of TNFRSF1A ectodomain upon cellular activation, with diminished shedding of the potentially antagonistic soluble receptor. Preliminary experience with recombinant p75 TNFR-Fc fusion protein in the treatment of TRAPS has been favorable.

Regulation of matrix metalloproteinases and their inhibitor genes in lipopolysaccharide-induced endotoxemia in mice

An imbalance between matrix metalloproteinases (MMPs) and inhibitors of MMPs (TIMPs) may contribute to tissue destruction that is found in various inflammatory disorders. To determine in an in vivo experimental setting whether the inflammatory reaction in the course of lipopolysaccharide (LPS)-induced endotoxemia causes an altered balance in the MMP/TIMP system, we analyzed the expression of a number of MMP and TIMP genes as well as MMP enzymatic activity in the liver, kidney, spleen, and brain at various time points after systemic injection of different doses of LPS in mice. Injection of sublethal doses of LPS led to an organ- and time-specific pattern of up-regulation of several MMP genes and the TIMP-1 gene in the liver, spleen, and kidney, whereas in the brain only TIMP-1 was induced. Injection of a lethal dose of LPS caused similar but more prolonged expression of these MMP genes as well as the induction of additional MMP genes in all organs. In LPS-treated mice in situ hybridization revealed collagenase 3 gene induction in cells resembling macrophages whereas TIMP-1 RNA was detected predominantly in parenchymal cells. Finally, gelatin zymography revealed increased gelatinolytic activity in all organs after LPS treatment. These observations highlight a dramatic shift in favor of increased expression of the MMP genes over the TIMP genes during LPS-induced endotoxemia, and suggest that MMPs may contribute to the development of organ damage in endotoxemia.

Nitric oxide enhances expression and shedding of tumor necrosis factor receptor I (p55) in endothelial cells

The biological actions of tumor necrosis factor-alpha (TNF-alpha) are mediated by 2 distinct receptors, TNF-RI (p55) and TNF-RII (p75). The extracellular domains of both receptors are shed in soluble form (sTNF-RI and sTNF-RII). The soluble receptors are involved in regulating TNF-alpha activities and may have therapeutic potential as TNF-neutralizing agents. However, it remains unclear as to what kind of physiological molecule can regulate TNF receptors. Nitric oxide (NO) mediates a variety of biological and pathophysiological functions. We hypothesized that NO may modulate the expression and shedding of TNF-RI. An NO donor, diethylamine/NO complex (NOC 5), increased sTNF-RI in the supernatants of ECV304, a human umbilical vein cell line, in a dose-dependent manner. TNF-RI mRNA in these cells was upregulated by NOC 5. 8-Br-cGMP and peroxynitrate had no effect on sTNF-RI release. Genistein and herbimycin A, inhibitors of tyrosine kinase, inhibited sTNF-RI release. Herbimycin A inhibited the levels of TNF-RI mRNA enhanced by NOC 5, which downregulated the surface expression of TNF-RI, indicating that NO is also involved in the shedding process of TNF-RI. The shedding of TNF-RI was abolished by a synthetic inhibitor of matrix metalloproteinase, KB-R8301. In conclusion, NO enhanced the release of sTNF-RI from endothelial cells by a cGMP-independent mechanism. Dual pathways suggested for NO-induced sTNF-RI release include (1) enhanced expression of TNF-RI, at least partially, by a tyrosine kinase-dependent mechanism and (2) increased shedding of TNF-RI by a type of metalloproteinase.