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

Role for tumour necrosis factor-alpha receptors in ultraviolet-induced skin tumours

The biological effects of tumour necrosis factor (TNF)-alpha are mediated through either the TNFR1 or the TNFR2 receptor. In the present study, the effects of ultraviolet (UV) irradiation on skin pathology and tumour promotion were studied in hairless mice deficient in either the TNFR1 or the TNFR2 receptor. SKH-1 hairless mice were crossed with either TNFR1 knockout (KO) mice or TNFR2 KO mice to develop a strain of hairless mice deficient in either of these receptors. Elastosis and other pathological indications of UVB irradiation were not affected by the loss of either receptor. The absence of either receptor, however, resulted in a highly significant reduction in skin tumours in response to UVB irradiation. Inflammatory cell influx following chronic UV irradiation was virtually eliminated in the TNFR1 KO mice, while the TNFR2 KO mice responded to UV irradiation with the normal increase in inflammatory cells throughout the lower and upper dermis. Contact hypersensitivity responses were eliminated in the TNFR2 KO mice, whereas the TNFR1 KO mice retained normal contact hypersensitivity reactions. These studies suggest that TNF-alpha plays no part in the accumulation of excessive elastin in the skin during chronic UVB exposure. However, there appears to be an important role for TNF-alpha in mediating tumorigenesis, distinct from its role in initiating cutaneous immune responses.

Stimulation-induced down-regulation of tumor necrosis factor-alpha converting enzyme

The extracellular domains of many proteins, including growth factors, cytokines, receptors, and adhesion molecules, are proteolytically released from cells, a process termed "shedding." Tumor necrosis factor-alpha converting enzyme (TACE/ADAM-17) is a metalloprotease-disintegrin that sheds tumor necrosis factor-alpha and other proteins. To study the regulation of TACE-mediated shedding, we examined the effects of stimulation of cells on TACE localization and expression. Immunofluorescence microscopy revealed a punctate distribution of TACE on the surface of untreated cells, and stimulation of monocytic cells with lipopolysaccharide did not affect TACE staining. Phorbol 12-myristate 13-acetate (PMA), a potent inducer of shedding, decreased cell-surface staining for TACE. Surface biotinylation experiments confirmed and extended this observation; PMA decreased the half-life of surface-biotinylated TACE without increasing the turnover of total cell-surface proteins. Soluble fragments of TACE were not detected in the medium of cells that had down-regulated TACE, and TACE was not down-regulated when endocytosis was inhibited. Antibody uptake experiments suggested that cell-surface TACE was internalized in response to PMA. Surprisingly, a metalloprotease inhibitor prevented the PMA-induced turnover of TACE. Thus, PMA activates shedding and causes the down-regulation of a major "sheddase," suggesting that induced shedding may be regulated by a mechanism that decreases the amount of active TACE on the cell surface.

Functional analysis of the domain structure of tumor necrosis factor-alpha converting enzyme

Many membrane-bound proteins, including cytokines, receptors, and growth factors, are proteolytically cleaved to release a soluble form of their extracellular domain. The tumor necrosis factor (TNF)-alpha converting enzyme (TACE/ADAM-17) is a transmembrane metalloproteinase responsible for the proteolytic release or "shedding" of several cell-surface proteins, including TNF and p75 TNFR. We established a TACE-reconstitution system using TACE-deficient cells co-transfected with TACE and substrate cDNAs to study TACE function and regulation. Using the TACE-reconstitution system, we identified two additional substrates of TACE, interleukin (IL)-1R-II and p55 TNFR. Using truncations and chimeric constructs of TACE and another ADAM family member, ADAM-10, we studied the function of the different domains of TACE in three shedding activities. We found that TACE must be expressed with its membrane-anchoring domain for phorbol ester-stimulated shedding of TNF, p75 TNFR, and IL-1R-II, but that the cytoplasmic domain is not required for the shedding of these substrates. The catalytic domain of ADAM-10 could not be functionally substituted for that of TACE. IL-1R-II shedding required the cysteine-rich domain of TACE as well as the catalytic domain, whereas TNF and p75 TNFR shedding required only the tethered TACE catalytic domain.

Shedding of interleukin-6 receptor and tumor necrosis factor alpha. Contribution of the stalk sequence to the cleavage pattern of transmembrane proteins

A functionally and structurally diverse group of transmembrane proteins including transmembrane forms of mediators or receptors can be proteolytically cleaved to form soluble growth factors or receptors. Recently, the proteolytic activity responsible for pro-tumor necrosis factor alpha (proTNFalpha) processing has been identified and named TACE (TNFalpha converting enzyme). In experiments with TACE deficient (TACE-/-) fibroblasts we found that 4beta-phorbol 12-myristate 13-acetate (PMA)-induced shedding of the interleukin-6 receptor (IL-6R) is strongly reduced. A basal hydroxamate sensitive release of IL-6R, however, could still be detected. This result demonstrates that TACE plays a role in IL-6R processing and that additional metalloproteases might be involved. PMA-induced shedding of IL-6R in TACE deficient mouse fibroblasts could be restored by stable transfection of a TACE cDNA. To characterize differences between shedding of IL-6R and proTNFalpha we generated chimeric IL-6R and proTNFalpha proteins wherein the endogenous cleavage sites (CS) had been replaced by the corresponding region of proTNFalpha and IL-6R, respectively. Interestingly, proTNFalpha chimeric proteins showed only minimal shedding. In contrast, IL-6R chimeras containing the proTNFalpha CS were shed spontaneously, processing was not further induced by PMA. Thus, the cleavage pattern transferred by the introduction of the proTNFalpha CS is similar to that of proTNFalpha itself. We conclude that the amino-acid sequence at the proteolytic CS contributes to the cleavage characteristics of a protein. However, this information alone is not sufficient to transfer cleavability as seen with proTNFalpha chimeras containing the IL-6R CS and which were resistant to shedding.

LPS-induced liver injury in D-galactosamine-sensitized mice requires secreted TNF-alpha and the TNF-p55 receptor

Lipopolysaccharide and D-galactosamine induced lethality and apoptotic liver injury is dependent on endogenously produced tumor necrosis factor (TNF)-alpha. The present study was undertaken to determine whether membrane-associated or secreted TNF-alpha signaling through the p55 or p75 receptor was responsible for survival and hepatic injury after lipopolysaccharide administration in D-galactosamine-sensitized mice. Transgenic mice expressing null forms of TNF-alpha, the p55 and p75 receptor, and mice expressing only a cell-associated form of TNF-alpha were challenged with 8 mg D-galactosamine and 100 ng lipopolysaccharide. Mortality and apoptotic liver injury were only seen in wild-type and p75 knockout mice. p75 Knockout mice had significantly higher concentrations of plasma TNF-alpha than any other experimental group (P </= 0.05) and tended to have the highest mortality and liver injury. In contrast, p55 and TNF-alpha knockout mice and animals expressing only a cell-associated form of TNF-alpha exhibited no mortality or liver injury. We conclude that survival and apoptotic liver injury in response to lipopolysaccharide and D-galactosamine are dependent exclusively on secreted TNF-alpha signaling through the p55 receptor.

The Drosophila tumor necrosis factor receptor-associated factor-1 (DTRAF1) interacts with Pelle and regulates NFkappaB activity

A member of the tumor necrosis factor (TNF) receptor-associated factor (TRAF) family was identified in Drosophila. DTRAF1 contains 7 zinc finger domains followed by a TRAF domain, similar to mammalian TRAFs and other members of the family identified in data bases from Caenorhabditis elegans, Arabidopsis, and Dictyostelium. Analysis of DTRAF1 binding to different members of the human TNF receptor family showed that this protein can interact through its TRAF domain with the p75 neurotrophin receptor and weakly with the lymphotoxin-beta receptor. DTRAF1 can also self-associate and binds to human TRAF1, TRAF2, and TRAF4. Interestingly, DTRAF1 interacts with human cIAP-1 and cIAP-2 but not with Drosophila DIAP-1 and -2. By itself, DTRAF1 did not induce significant NFkappaB activation when overexpressed in mammalian cells, although it specifically increased NFkappaB induction by TRAF6. In contrast, TRAF2-mediated NFkappaB induction was partially inhibited by DTRAF1. Mutants of DTRAF1 lacking the N-terminal region inhibited NFkappaB induction by either TRAF2 or TRAF6. DTRAF1 specifically associated with the regulatory N-terminal domain of Pelle, a Drosophila homolog of the human kinase interleukin-1 receptor-associated kinase (IRAK). Interestingly, though Pelle and DTRAF1 individually were unable to induce NFkappaB in a human cell line, co-expression of Pelle and DTRAF1 resulted in significant NFkappaB activity. Interactions of DTRAF1 with human TRAF-, TNF receptor-, and IAP-family proteins imply strong evolutionary conservation of TRAF protein structure and function throughout Metazoan evolution.

Cytokine signaling--regulation of the immune response in normal and critically ill states

Cytokines are produced during the activation of innate and acquired immunity, and are the principal means for intercellular communication of a microbial invasion. Cytokines serve to initiate the inflammatory response and to define the magnitude and the nature of the acquired immune response. The response of critically ill patients to their injury and/or invading pathogens is dependent, in large part, on the pattern of cytokines which are produced. The immunologic response of critically ill patients can vary from a strongly proinflammatory response, characterized by increased production of tumor necrosis factor-alpha, interleukin (IL)-1, interferon (IFN)-gamma, and IL-12 to one predominantly of anergy, characterized by increased production of T(H)2 cytokines, like IL-10 and to IL-4. Therapeutic efforts to modify the host immune response in critical illness will require a more thorough understanding of the cytokine milieu and the factors that determine their production.

Increased T cell cytotoxicity by Betathine-induced upregulation of TNFalpha

Betathine (BT) is a low molecular weight disulfide that has previously been shown to exhibit in vivo antitumor activity in murine myeloma and melanoma models. We have shown that BT treatment of both human T cells and monocytes is associated with an increase in surface tumor necrosis alpha (TNFalpha) expression. Further, in T cells and monocytes that have been stimulated with PMA and ionomycin, the addition of BT results in a dose and time dependent increase in the percentage of high TNFalpha-expressing cells. Unlike TNFalpha upregulation produced by the commonly used thiol antioxidant N-acetyl-L-cysteine (NAC), the BT-induced increase in TNFalpha is observed consistently in different donors. This increase in surface TNFalpha is associated with elevated levels of TNFalpha mRNA. In addition, expression of TNFalpha receptor I is also significantly enhanced by BT treatment. The upregulation of surface TNFalpha by BT has functional consequences, in that, BT-treated T cells exhibit enhanced cytotoxic activity. Thus, increased TNFalpha expression may be one mechanism responsible for the antineoplastic activity of BT.

Cutting Edge: A Dominant Negative Form of TNF-α Converting Enzyme Inhibits ProTNF and TNFRII Secretion

TNF-α converting enzyme (TACE) is the protease responsible for processing proTNF from the 26-kDa membrane-anchored precursor to the secreted 17-kDa TNF-α. We show here that a deletion mutant of TACE (dTACE), lacking the pro and catalytic domains of the protease, acts as a dominant negative for proTNF processing in transfected HEK293 cells. We used the same system to test the effect of dTACE on TNFRII processing. Overexpression of dTACE with TNFRII resulted in &gt;80% inhibition of TNFRII shedding. Although significant inhibition of TNF-α and TNFRII shedding was achieved with dTACE, we could not detect a cell surface accumulation of the noncleaved substrates above that observed in the absence of dTACE. Our results suggest that TNFRII is a substrate for TACE, and that dTACE is capable of interfering with the function of endogenous TACE, either by binding and sequestering TACE substrates via the disintegrin domain, transmembrane domain, or cytoplasmic tail, or by some other mechanism that has yet to be determined.

The shedding of membrane-anchored heparin-binding epidermal-like growth factor is regulated by the Raf/mitogen-activated protein kinase cascade and by cell adhesion and spreading

Heparin-binding epidermal-like growth factor (HB-EGF) is synthesized as a transmembrane precursor (HB-EGF(TM)). The addition of phorbol ester (PMA, phorbol 12-myristate 13-acetate) to cells expressing HB-EGF(TM) results in the metalloproteinase-dependent release (shedding) of soluble HB-EGF. To analyze mechanisms that regulate HB-EGF shedding, a stable cell line was established expressing HB-EGF(TM) in which the ectodomain and the cytoplasmic tail were tagged with hemagglutinin (HA) and Myc epitopes, respectively (HB-EGF(TM)HA/Myc). HB-EGF(TM)HA/Myc cleavage was followed by the appearance of soluble HB-EGFHA in conditioned medium, the loss of biotinylated cell-surface HB-EGF(TM)HA/Myc, and the appearance of a Myc-tagged cytoplasmic tail fragment in cell lysates. By using this approach, several novel metalloproteinase-dependent regulators of HB-EGF(TM) shedding were identified as follows. (i) HB-EGF(TM)HA/Myc shedding induced by PMA was blocked by the mitogen-activated protein (MAP) kinase kinase inhibitor, PD98059. PMA activated MAP kinase within 5 min, but HB-EGF(TM)HA/Myc shedding did not occur until 20 min, suggesting that MAP kinase activation was a necessary step in the pathway of PMA-induced HB-EGF(TM) cleavage. (ii) Activation of an inducible Raf-1 kinase, DeltaRaf-1:estrogen receptor, resulted in a rapid MAP kinase activation within 10 min and shedding of HB-EGF(TM)HA/Myc within 20-40 min. (iii) Serum induced MAP kinase activation and HB-EGF(TM)HA/Myc shedding that were inhibited by PD98059. (iv) Whereas PMA induced HB-EGF(TM)HA/Myc shedding in attached cells, no shedding occurred when the cells were placed in suspension. Shedding was fully restored shortly after cells were allowed to spread on fibronectin, and the extent of PMA-induced shedding increased with the extent of cell spreading. PMA induced the same level of MAP kinase activation whether the cells were attached or in suspension suggesting that although MAP kinase activation might be necessary for shedding, it was not sufficient. Taken together, these results suggest that there are two components of cell regulation that contribute to the shedding process, not previously recognized, the Raf-1/MAP kinase signal transduction pathway and cell adhesion and spreading.

The soluble CD40 ligand sCD154 in systemic lupus erythematosus

We found that the plasma of patients with active systemic lupus erythematosus (SLE) could induce a human B-cell line (Ramos) to express high levels of immune accessory molecules that are commonly found on blood B cells of patients with active SLE. The ability of SLE plasma to induce such phenotypic changes could be abrogated by neutralizing antibodies specific for the CD40 ligand (CD154) but not by antibodies to TNF-alpha. Immunoprecipitation studies with anti-CD154 identified a 20-kDa protein in the plasma of SLE patients with active disease, but not in plasma of normal donors, indicating that such plasma contained soluble CD154 (sCD154). Using a quantitative ELISA method, we found that the plasma of patients with active disease had levels of sCD154 that were significantly higher than those found in plasma of normal donors. Levels of CD154 transcripts in SLE blood lymphocytes correlated with the relative concentrations of sCD154 found in SLE plasma. Furthermore, plasma levels of sCD154 correlated with the titers of anti-double-stranded DNA autoantibody and with clinical disease activity. These studies indicate that sCD154 of patients with SLE may act as a functional ligand for CD40 that is associated with SLE disease activity.

The Effect of a Metalloproteinase Inhibitor (GI5402) on Tumor Necrosis Factor- (TNF-) and TNF- Receptors During Human Endotoxemia

Tumor necrosis factor- (TNF-) is released from the cell surface by cleavage of pro–TNF- by metalloproteinases (MPs). In cell cultures, inhibition of MPs has been found not only to reduce the release of TNF-, but also to enhance the surface expression of TNF- and TNF- receptors, which might lead to a proinflammatory effect. To determine the effect of MP inhibition during inflammation in humans, 7 healthy subjects were studied after intravenous injection of lipopolysaccharide (LPS; 4 ng/kg) preceded (−20 minutes) by an oral dose of the MP inhibitor GI5402 (100 mg) or matching placebo. GI5402 strongly reduced LPS-induced TNF- release (P &lt; .001), but did not influence the increase in monocyte-bound TNF-. In addition, GI5402 attenuated the rise in plasma-soluble TNF- receptors types I and II after LPS injection (both P &lt; .001), but did not change the LPS-induced decreases in granulocyte and monocyte TNF- receptor expression. These data suggest that MP inhibitors may be useful as a treatment modality in diseases in which excessive production of TNF- is considered to play an important role. Furthermore, unlike in vitro, no evidence has been found in vivo with MP inhibition for a potential proinflammatory effect due to increases in membrane-bound TNF- and TNF- receptor number.

Modulation of juxtamembrane cleavage ("shedding") of angiotensin-converting enzyme by stalk glycosylation: evidence for an alternative shedding protease

The role of juxtamembrane stalk glycosylation in modulating stalk cleavage and shedding of membrane proteins remains unresolved, despite reports that proteins expressed in glycosylation-deficient cells undergo accelerated proteolysis. We have constructed stalk glycosylation mutants of angiotensin-converting enzyme (ACE), a type I ectoprotein that is vigorously shed when expressed in Chinese hamster ovary cells. Surprisingly, stalk glycosylation did not significantly inhibit release. Introduction of an N-linked glycan directly adjacent to the native stalk cleavage site resulted in a 13-residue, proximal displacement of the cleavage site, from the Arg-626/Ser-627 to the Phe-640/Leu-641 bond. Substitution of the wild-type stalk with a Ser-/Thr-rich sequence known to be heavily O-glycosylated produced a mutant (ACE-JGL) in which this chimeric stalk was partially O-glycosylated; incomplete glycosylation may have been due to membrane proximity. Relative to levels of cell-associated ACE-JGL, rates of basal, unstimulated release of ACE-JGL were enhanced compared with wild-type ACE. ACE-JGL was cleaved at an Ala/Thr bond, 14 residues from the membrane. Notably, phorbol ester stimulation and TAPI (a peptide hydroxamate) inhibition of release-universal characteristics of regulated ectodomain shedding-were significantly blunted for ACE-JGL, as was a formerly undescribed transient stimulation of ACE release by 3, 4-dichloroisocoumarin. These data indicate that (1) stalk glycosylation modulates but does not inhibit ectodomain shedding; and (2) a Ser-/Thr-rich, O-glycosylated stalk directs cleavage, at least in part, by an alternative shedding protease, which may resemble an activity recently described in TNF-alpha convertase null cells [Buxbaum, J. D., et al. (1998) J. Biol. Chem. 273, 27765-27767].

Expression pattern of tumour necrosis factor receptors in subcutaneous and omental human adipose tissue: role of obesity and non-insulin-dependent diabetes mellitus

BACKGROUND

Tumour necrosis factor alpha (TNF) mRNA expression has been reported to be up-regulated in adipose tissue from several rodent models of obesity and diabetes and from obese humans. This elevated expression has been assumed to be associated with the development of insulin resistance. However, the biological signal of TNF may be influenced by the expression of the two TNF receptors: the p60 TNF receptor, TNFR60, and the p80 TNF receptor, TNFR80.

DESIGN

The aim of this study was to investigate the mRNA expression pattern of the two TNF receptors and their ligand in two adipose tissue depots of glucose-tolerant obese women [n = 18, body mass index (BMI) 48.2 +/- 8.4 kg m-2], obese women with impaired glucose tolerance or overt non-insulin-dependent diabetes mellitus (NIDDM) (n = 10, BMI 49.1 +/- 11.6 kg m-2) and healthy non-obese control subjects (n = 12, BMI 25.8 +/- 2.7 kg m-2). RNA expression was assessed by a semiquantitative reverse transcriptase polymerase chain reaction (RT-PCR) technique.

RESULTS

The mean mRNA levels of both TNF receptors were two- to threefold higher in adipose tissue samples from the obese than from the non-obese women (P < 0.01 for each). Although TNFR60 mRNA did not vary within each obese group, there was a wide variation in the levels of TNFR80 mRNA and of TNF mRNA. A comparison of the expression levels between the subcutaneous abdominal and the omental adipose tissue depots showed significantly higher expression in the former. The TNFR60 expression level was positively correlated with BMI and fat cell size, whereas TNFR80 and TNF mRNA levels showed positive associations with serum insulin and triglyceride concentrations. No significant differences in the expression levels were observed between obese individuals with and without impaired glucose tolerance/NIDDM.

CONCLUSION

These results indicate that severe obesity in women is characterized by increased amounts of the two TNF receptor mRNAs. The role of this up-regulation for the development of obesity-associated insulin resistance remains to be elucidated.

Tumor necrosis factor receptors encoded by poxviruses

Poxviruses encode several cytokine response modifying (Crm) proteins. The Crm proteins possess sequence homology to several human proteins important in immunity. This homology and the conservation of Crm proteins among poxvirus strains suggest an immunomodulatory function that provides a survival advantage to the virus. Cowpox virus encodes several tumor necrosis factor (TNF) receptor family homologues: CrmB, CrmC, and CrmD. CrmB and CrmD encode a similar approximately 155 amino acid COOH-terminus region distal to their TNF ligand-binding portions. These C-terminus regions contain no significant homology with sequences in public databases. It is not known whether the C-terminus regions have a complementary function to the TNF ligand-binding domains, or an unrelated function. Myxoma virus, a rabbit poxvirus, encodes a protein termed T2 which is homologous to CrmB and CrmD. Deletion of the T2 gene results in decreased pathogenicity of myxoma in rabbits. T2 has also been shown to interfere with TNF-induced apoptosis in vitro. Understanding the role viral TNF receptor homologues play in altering host immune responses may suggest ways to develop specific anti-inflammatory therapeutics.

ADAM family proteins in the immune system

CD156 is a member of a family proteins characterized by a disintegrin and a metalloprotease domain (ADAM). These molecules are phylogenically conserved but have individual roles in a variety of cells. Here, Shunsuke Yamamoto and colleagues discuss data suggesting that ADAM family proteins have important roles in the immune system.

Effects of culture conditions and exposure to catabolic stimulators (IL-1 and retinoic acid) on the expression of matrix metalloproteinases (MMPs) and disintegrin metalloproteinases (ADAMs) by articular cartilage chondrocytes

The chondrocytes of articular cartilage synthesize a number of proteinases which are capable of degrading the component molecules of this specialized extracellular matrix. The use of class-specific proteinase inhibitors indicates that major activities responsible for catabolism of proteoglycan (aggrecan) and collagen are attributable to zinc-dependent metalloproteinases. In this study, we have compared the mRNA expression profiles of two matrix metalloproteinases (MMP-3 and MMP-13) and five disintegrin-metalloproteinases (ADAM-10, ADAM-9, ADAM-15, TNF-alpha-converting enzyme and decysin) by chondrocytes (human, porcine and bovine) from fresh cartilage and in cartilage explant cultures and isolated cells cultured in monolayer or in agarose gels. Such cultures were maintained in the presence or absence of interleukin-1 (IL-1) or all-trans-retinoic acid, two agents which promote cartilage matrix degradation in vitro. Whereas transcripts for all metalloproteinases examined were detected in chondrocytes from human osteoarthritic cartilage in monolayer cultures, mRNAs for ADAM-15 and decysin were not present in fresh osteoarthritic human cartilage or explant cultures. Similarly, expression of porcine and bovine metalloproteinase mRNAs varied with different culture conditions. Novel cDNA sequences obtained for porcine and bovine MMP-3 and MMP-13, porcine ADAM-10, porcine and bovine ADAM-9 and porcine TACE confirmed expression of mRNAs for these molecules by articular chondrocytes. Quantitative RT-PCR analysis was used to determine the effects of IL-1 and retinoic acid on metalloproteinase mRNA levels in human chondrocytes cultured in monolayer and in porcine chondrocytes cultured in agarose. For the MMPs, IL-1 treatment resulted in an approximately two to threefold increase in human and porcine MMP-3 and MMP-13 mRNAs, while retinoic acid treatment caused a statistically significant increase in human MMP-3 mRNA levels, but no significant change in transcript levels for porcine MMP-3 nor human or porcine MMP-13. The mRNA levels for ADAM-15 were elevated in human monolayer chondrocytes exposed to IL-1 or retinoic acid, while transcripts levels for TNF-alpha converting enzyme were increased in response to retinoic acid. In contrast, ADAM-9 mRNA levels were decreased in human monolayer chondrocytes exposed to IL-1 or retinoic acid. The results demonstrate that chondrocyte metalloproteinase expression can vary dependent on cell environment in situ and in vitro, and information on chondrocyte MMP and ADAM gene expression following cytokine (IL-1) or retinoid stimulation.

[Apoptosis--what is it? Significance in coronary heart disease and myocardial infarct]

Apoptosis is a physiological, highly conserved program of cellular suicide, characterized by nuclear condensation with DNA-fragmentation, by alterations in the distribution of cell membrane phospholipids, and by cellular shrink-age. Apoptotic cellular remnants engulfed by cell membranes are phagocytized largely without activation of inflammatory reactions. The apoptotic program is executed by a cascade of highly specific caspases, activated by complexation of initiatorcaspases in cytosolic signalling complexes at receptors of the TNF family or at impaired mitochondria. In many forms of cellular stress with damage of nuclear DNA and mitochondria, mixed forms of cell death are triggered with regulated activation of the apoptotic program and concomitantly, with induction of catastrophic necrosis. Such a mixed form of myocyte death is observed in myocardial ischemia and reperfusion. Antiapoptotic interventions can delay ischemic myocardial damage in experiments. Therefore, those interventions appear conceivable as future strategy for acutely enhancing the available time interval for therapeutic reperfusion. However, chronic inhibition of apoptosis for ongoing prevention of myocardial ischemic damage may not become a plausible strategy because of disturbances of the immune system, because of putatively infavorable effects on arteriosclerotic lesions and because of likely disturbances in the physiologic elimination of damaged mitochondria.

Involvement of endothelin-1 in up-regulation of gastric mucosal inflammatory responses to Helicobacter pylori lipopolysaccharide

In this study, we investigated gastric mucosal inflammatory responses during Helicobacter pylori lipopolysaccharide-induced gastritis by analyzing the interplay between mucosal expression of endothelin-1 (ET-1), interleukin-4 (IL-4) and tumor necrosis factor-alpha (TNF-alpha). The assays conducted 4 days after intragastric dose of H. pylori lipopolysaccharide demonstrated a pattern of acute mucosal reaction characterized by the inflammatory infiltration of the lamina propria, hyperemia, and epithelial hemorrhage. This was accompanied by a 3.1-fold increase in the mucosal expression of ET-1 and a 9-fold enhancement in TNF-alpha, while the level of IL-4 showed a 20.8% decline. The results implicate ET-1 in gastric mucosal responses to H. pylori, and suggest that an increase in its level, combined with a loss of compensatory action by IL-4, may be responsible for the induction of TNF-alpha and triggering apoptotic events that exacerbate the inflammatory process.

Germline mutations in the extracellular domains of the 55 kDa TNF receptor, TNFR1, define a family of dominantly inherited autoinflammatory syndromes

Autosomal dominant periodic fever syndromes are characterized by unexplained episodes of fever and severe localized inflammation. In seven affected families, we found six different missense mutations of the 55 kDa tumor necrosis factor receptor (TNFR1), five of which disrupt conserved extracellular disulfide bonds. Soluble plasma TNFR1 levels in patients were approximately half normal. Leukocytes bearing a C52F mutation showed increased membrane TNFR1 and reduced receptor cleavage following stimulation. We propose that the autoinflammatory phenotype results from impaired downregulation of membrane TNFR1 and diminished shedding of potentially antagonistic soluble receptor. TNFR1-associated periodic syndromes (TRAPS) establish an important class of mutations in TNF receptors. Detailed analysis of one such mutation suggests impaired cytokine receptor clearance as a novel mechanism of disease.

Metalloprotease-disintegrin MDC9: intracellular maturation and catalytic activity

Metalloprotease disintegrins are a family of membrane-anchored glycoproteins that are known to function in fertilization, myoblast fusion, neurogenesis, and ectodomain shedding of tumor necrosis factor (TNF)-alpha. Here we report the analysis of the intracellular maturation and catalytic activity of the widely expressed metalloprotease disintegrin MDC9. Our results suggest that the pro-domain of MDC9 is removed by a furin-type pro-protein convertase in the secretory pathway before the protein emerges on the cell surface. The soluble metalloprotease domain of MDC9 cleaves the insulin B-chain, a generic protease substrate, providing the first evidence that MDC9 is catalytically active. Soluble MDC9 appears to have distinct specificities for cleaving candidate substrate peptides compared with the TNF-alpha convertase (TACE/ADAM17). The catalytic activity of MDC9 can be inhibited by hydroxamic acid-type metalloprotease inhibitors in the low nanomolar range, in one case with up to 50-fold selectivity for MDC9 versus TACE. Peptides mimicking the predicted cysteine-switch region of MDC9 or TACE inhibit both enzymes in the low micromolar range, providing experimental evidence for regulation of metalloprotease disintegrins via a cysteine-switch mechanism. Finally, MDC9 is shown to become phosphorylated when cells are treated with the phorbol ester phorbol 12-myristate 13-acetate, a known inducer of protein ectodomain shedding. This work implies that removal of the inhibitory pro-domain of MDC9 by a furin-type pro-protein convertase in the secretory pathway is a prerequisite for protease activity. After pro-domain removal, additional steps, such as protein kinase C-dependent phosphorylation, may be involved in regulating the catalytic activity of MDC9, which is likely to target different substrates than the related TNF-alpha-convertase.

Enhancement of the surface expression of tumor necrosis factor alpha (TNFalpha) but not the p55 TNFalpha receptor in the THP-1 monocytic cell line by matrix metalloprotease inhibitors

The monocytic cell line THP-1 can be induced to express and release tumor necrosis factor alpha (TNFalpha) and both TNFalpha receptors (p55 and p75) upon exposure to bacterial lipopolysaccharide (LPS). The broad-spectrum matrix metalloprotease (MMP) inhibitors [4-(N-hydroxyamino)-2R-isobutyl-3S-(phenylthiomethyl)succinyl]-L-p henylalanine-N-methylamide (GI-129471) and marimastat [2S-[N4(R*),2R*,3S*]]-N4[2,2-dimethyl-1-[(methylamino)carbonyl]propyl]-N 1,2-dihydroxy-3-(2-methylpropyl)butanediamide (BB-2516) were effective inhibitors of LPS-induced TNFalpha (soluble) release with IC50 values of 0.2 and 4.0 microM, respectively. Upon LPS stimulation, the expression of pro-TNFalpha (membrane associated) on the cell surface (FACS analysis) could not be observed. However, in the presence of GI-129471, a concentration-dependent increase in TNFalpha surface expression was observed. Peak expression (percentage of cells expressing pro-TNFalpha and mean fluorescence units) in the presence of GI-129471 was at 2 hr, and steadily declined to return to near control levels by 8 hr. This time course was similar to TNFalpha release, which also peaked at 2-4 hr after LPS exposure and then declined. Stimulation of THP-1 cells with LPS + phorbol myristate acetate increased the percentage of cells expressing pro-TNFalpha by 10-fold. In the presence of GI-129471, these increases were augmented further and peaked between 2 and 4 hr, but also returned to near control levels of expression by 24 hr. This was in contrast to the release of soluble TNFalpha, which continued to accumulate over a 24-hr time course. TNFalpha receptor I (p55, TNFRI) and II (p75, TNFRII) shedding was also inhibited by GI-129471 (IC50 = 1.5 and 3.1 microM, respectively) and BB-2516 (IC50 = 14 and 15 microM, respectively). Unlike pro-TNFalpha surface expression, surface expression of both TNFalpha receptors steadily increased over 72 hr. In contrast to pro-TNFalpha surface expression, TNFRI surface expression was not augmented by these MMP inhibitors in THP-1 cells after LPS stimulation. Surface expression of TNFRII was augmented by these MMP inhibitors. These results suggest that even in the continued presence of LPS stimulation and an inhibitor of TNFalpha processing, the augmented surface expression of TNFalpha is transient. The potential "deleterious" implications of high levels of surface pro-TNFalpha expression in the presence of these inhibitors may be lessened by its transient nature.

Secretion of human meprin from intestinal epithelial cells depends on differential expression of the alpha and beta subunits

Human meprin (N-benzoyl-l-tyrosyl-p-aminobenzoic acid hydrolase, EC 3.4.24.18), an astacin-type metalloprotease, is expressed by intestinal epithelial cells as a dimeric protein complex of alpha and beta subunits. In transfected cells, intracellular proteolytic removal of the membrane anchor from the alpha subunit results in its secretion, while the beta subunit and alpha/beta heterodimers are retained at the cell membrane. We investigated the consequence of differential intracellular processing of alpha and beta subunits in the human small and large intestine using subunit-specific immunohistochemistry, in situ hybridization and biosynthetic studies in organ culture. In the ileum, both subunits localize to the brush-border membrane of villus enterocytes. In contrast, the beta subunit is not expressed in the colon, which leads to the secretion of the alpha subunit. We conclude that differential expression of meprin alpha and beta subunits is a unique means of targeting the proteolytic activity of the alpha subunit either to the brush-border membrane in the ileum or to the lumen in the colon, suggesting dual functions of cell-associated and luminal meprin. Meprin alpha and beta subunits are also coexpressed in distinct lamina propria leukocytes, suggesting an additional role for this protease in leukocyte function in the intestinal mucosa.

Interleukin-4 down-regulates both forms of tumor necrosis factor receptor and receptor-mediated apoptosis, NF-kappaB, AP-1, and c-Jun N-terminal kinase. Comparison with interleukin-13

The activity of tumor necrosis factor (TNF), a proinflammatory cytokine, is regulated by a number of other cytokines, including interleukin (IL)-4. How IL-4 regulates various activities of TNF is not fully understood. In the present report, we investigated the effect of IL-4 on the cell surface TNF receptors in human histiocytic lymphoma U-937 cells. Pretreatment of cells with IL-4 down-regulated TNF receptors in a dose- and time-dependent manner; an almost 90% decrease occurred with 10 ng/ml IL-4 treatment for 24 h. Scatchard analysis revealed that the decrease was due to receptor number and not affinity. IL-13, which shares a common receptor subunit and various biological activities with IL-4, had no effect on TNF receptors. IL-4's effect on TNF receptors was not cell type-specific, since decreases also occurred on various epithelial and T cells. Both the p60 and p80 forms of the TNF receptor were down-regulated to the same extent. Western blot showed that IL-4 induced shedding of the TNF receptors. The decrease of TNF receptors by IL-4 was accompanied by down-regulation of TNF-induced activities, including cytotoxicity, caspase-3 activation, NF-kappaB and AP-1 activation, and c-Jun N-terminal kinase induction. Wortmannin reversed the IL-4-induced TNF receptor down-regulation and all other measured cellular responses, indicating a critical role of phosphatidylinositol 3-kinase. Rapamycin also blocked the effect of IL-4-induced regulation, thus suggesting the role of p70 S6 kinase. Overall, our results suggest that TNF receptor down-regulation by IL-4 plays a critical role in the antagonistic effects of IL-4 on TNF-induced cellular responses and that this mechanism differs from that of IL-13.

An essential role for ectodomain shedding in mammalian development

The ectodomains of numerous proteins are released from cells by proteolysis to yield soluble intercellular regulators. The responsible protease, tumor necrosis factor-alpha converting enzyme (TACE), has been identified only in the case when tumor necrosis factor-alpha (TNFalpha) is released. Analyses of cells lacking this metalloproteinase-disintegrin revealed an expanded role for TACE in the processing of other cell surface proteins, including a TNF receptor, the L-selectin adhesion molecule, and transforming growth factor-alpha (TGFalpha). The phenotype of mice lacking TACE suggests an essential role for soluble TGFalpha in normal development and emphasizes the importance of protein ectodomain shedding in vivo.

Tissue inhibitors of metalloproteinases and metalloproteinases in atherosclerosis

The ability of the metalloproteinases to degrade extracellular matrix proteins is essential for the matrix remodelling that occurs during infiltration of inflammatory cells, intimal thickening, angiogenesis and plaque rupture which are a result of atherosclerosis. Increased metalloproteinase activity therefore requires stimulation of metalloproteinase expression by cytokines and growth factors, activation of metalloproteinases, and downregulation of tissue inhibitors of metalloproteinases. In addition, metalloproteinases may influence atherosclerosis by processing of proteins involved in inflammation and cell growth and death and the tissue inhibitors of metalloproteinases may also play a less inhibitory role by influencing cell growth and apoptosis.

Pro-tumor necrosis factor-alpha processing activity is tightly controlled by a component that does not affect notch processing

The extracellular domain of a heterogeneous group of transmembrane proteins can be proteolytically released from the cell surface, a process known as protein ectodomain shedding. Despite the biomedical importance of several substrates of the shedding system, such as the beta-amyloid precursor protein (betaAPP), little is known about the regulation of protein ectodomain shedding, and the only protease known to be involved is the metalloprotease disintegrin, tumor necrosis factor-alpha converting enzyme (TACE). Here, we show that previously described pro-transforming growth factor-alpha shedding-defective cell mutants (M2 cells), known to be defective in ectodomain shedding of several molecules, that include betaAPP, fail to shed the ectodomain of pro-TNF-alpha. The target of the mutation is a component required for TACE activity, since transfection of TACE into M2 cells has no effect on the shedding of pro-TNF-alpha and somatic cell fusions between M2 cells and TACE null cells recover the ability to shed pro-TNF-alpha, pro-transforming growth factor-alpha, and betaAPP. Furthermore, we show that TACE is also necessary for the shedding of betaAPP since TACE null cells show defective betaAPP shedding. Biochemical evidence shows that the component that controls TACE is different from protein kinase C, the only known activator of protein ectodomain shedding, and that this component does not affect biosynthesis or processing of TACE or other metalloprotease disintegrins. The component mutated in M2 cells is likely to control only a subset of metalloprotease disintegrins involved in regulated ectodomain shedding, since Notch processing, a process known to be dependent on the activity of another metalloprotease disintegrin, Kuzbanian, is normal in M2 cells.

A functional soluble form of the murine mannose receptor is produced by macrophages in vitro and is present in mouse serum

A soluble form of the mannose receptor (sMR) has been found in conditioned medium of primary macrophages in vitro and in mouse serum. sMR was released as a single species, had a smaller size than the cell-associated form, and accumulated in macrophage-conditioned medium, in a cytokine-regulated manner, to levels comparable with those found for cell-associated mannose receptor. Pulse-chase experiments showed that sMR production in culture occurred by constitutive cleavage of pre-existing full-length protein. A binding assay was developed to determine the sugar specificity of sMR and its ability to interact with pathogens and particulate antigens (i.e. Candida albicans and zymosan). Protease inhibitor studies suggested that sMR was produced by cleavage of an intact mannose receptor by a matrix metalloprotease or ADAM metalloprotease. A role for sMR in the immune response is proposed based on its binding properties, regulation by cytokines, and the previous discovery of putative ligands for the cysteine-rich domain of the mannose receptor in lymph nodes and spleen.

Regulation of monocyte MMP-9 production by TNF-alpha and a tumour-derived soluble factor (MMPSF)

The matrix metalloprotease MMP-9 localizes to tumour-associated macrophages in human ovarian cancer but little is known of its regulation. Co-culture of human ovarian cancer cells (PEO-1) and a monocytic cell line (THP-1) led to production of 92-kDa proMMP-9. PEO-1-conditioned medium (CM) also stimulated THP-1 cells or isolated peripheral blood monocytes to produce proMMP-9. Expression of TIMP-1, however, remained unaffected. There was evidence that tumour necrosis factor alpha (TNF-alpha) was involved in tumour-stimulated monocytic proMMP-9 production. Antibody to TNF-alpha inhibited proMMP-9 production, and synthesis of TNF-alpha mRNA and protein preceded proMMP-9 release. In addition, the synthetic matrix metalloprotease inhibitor (MMPI) BB-2116, which blocks TNF-alpha shedding, inhibited proMMP-9 release in the co-cultures and from CM-stimulated monocytic cells. Further experiments suggested that the stimulating factor present in CM was not TNF-alpha, but acted synergistically with autocrine monocyte-derived TNF-alpha to release monocytic proMMP-9. Thus, ovarian cancer cells can stimulate monocytic cells in vitro to make proMMP-9 without affecting the expression of its inhibitor TIMP-1. This induction is mediated via a soluble factor (provisionally named MMPSF) that requires synergistic action of autocrine or paracrine TNF-alpha.

Pathogenic Mycobacterium tuberculosis Evades Apoptosis of Host Macrophages by Release of TNF-R2, Resulting in Inactivation of TNF-α

Infection by Mycobacterium tuberculosis (MTB) induces human alveolar macrophage (AMφ) apoptosis by a TNF-α-dependent mechanism. The apoptotic response is postulated to be a defense mechanism, limiting the growth of this intracellular pathogen. Consistent with that model, recent studies showed that the virulent MTB strain H37Rv induces substantially less AMφ apoptosis than the attenuated strain H37Ra. We now report that AMφ infection with either H37Rv or H37Ra induces comparable levels of TNF-α measured by ELISA but that TNF-α bioactivity is reduced in supernatants of H37Rv-infected AMφ. Differential release of soluble TNFR2 (sTNFR2), with formation of inactive TNF-α-TNFR2 complexes accounted for the difference in TNF-α bioactivity in these cultures. Release of sTNFR2 by H37Rv-infected AMφ was IL-10 dependent since it was inhibited by neutralizing anti-IL-10 Ab. Thus, the effect of TNF-α produced by AMφ following infection can be modulated by virulent MTB, using IL-10 as an upstream mediator.

Efficient adenoviral infection with IkappaB alpha reveals that macrophage tumor necrosis factor alpha production in rheumatoid arthritis is NF-kappaB dependent

Tumor necrosis factor (TNF) alpha has been shown to be a major therapeutic target in rheumatoid arthritis with the success of anti-TNFalpha antibody clinical trials. Although signaling pathways leading to TNFalpha expression have been studied in some detail, there is evidence for considerable differences between individual cell types. This prompted us to investigate the intracellular signaling pathways that result in increased TNFalpha synthesis from macrophages in the diseased synovial joint tissue. Using an adenoviral system in vitro we report the successful delivery of genes to more than 95% of normal human macrophages. This permitted us to show, by using adenoviral transfer of IkappaB alpha, the natural inhibitor of NF-kappaB, that induction of TNFalpha in normal human macrophages by lipopolysaccharide, but not by some other stimuli, was inhibited by 80%. Furthermore the spontaneous production of TNFalpha from human rheumatoid joint cell cultures was inhibited by 75%, indicating that the NF-kappaB pathway is an essential step for TNFalpha synthesis in synovial macrophages and demonstrating that NF-kappaB should be an effective therapeutic target in this disease.

Two distinct forms of soluble MHC class I molecules synthesized by different mechanisms in normal rat cells in vitro

Rat soluble MHC class I synthesis was studied at both RNA and protein levels to determine whether multiple forms of soluble MHC class I molecules are produced by different mechanisms. RT-PCR and sequencing of MHC class I transcripts identified an alternatively spliced nonclassical MHC class I gene product, lacking both exon 5 and 6, in both spleen and liver. Immunoprecipitation and SDS-PAGE identified two distinct soluble MHC class I proteins in both splenocyte- and hepatocyte-culture supernatants. The 36Kd classical soluble MHC class I protein (RT1.Aa) was precipitated by both allele-specific (MN4.91.6, R3/13, R2/15S) and pan-reactive (OX18) mAbs. The 39Kd non-RT1.A soluble MHC class I protein was precipitated only by OX18. The production of soluble RT1.Aa was inhibited by a metalloproteinase inhibitor, but not by serine/thiol protease inhibitors. None of these protease inhibitors interfered with the soluble non-RT1.A production, suggesting that it might be derived from an alternatively spliced MHC class I transcript. The soluble non-RT1.A was always associated with beta2m. However, soluble RT1.Aa molecule was cleaved in beta2m-free form and was reassociated with beta2m in culture supernatants. Thus two soluble MHC class I molecules, classical (36Kd RT1.Aa) and nonclassical (the alternatively spliced transcript), were produced from rat cells. Alternative splicing led to the nonclassical soluble MHC class I synthesis. Proteolytic cleavage by metalloproteinase led to the classical soluble MHC class I synthesis.

Human myeloma cells shed the interleukin-6 receptor: inhibition by tissue inhibitor of metalloproteinase-3 and a hydroxamate-based metalloproteinase inhibitor

Interleukin-6 (IL-6) is the major growth factor for human myeloma cells, exerting its effect through the IL-6 receptor (IL-6R). A soluble form of IL-6R (sIL-6R) has been identified, which increases the sensitivity of myeloma cells to IL-6. In patients with multiple myeloma (MM), serum concentrations of sIL-6R are elevated and associated with poor prognosis. The present study was undertaken to determine whether proteolytic cleavage of IL-6R could contribute to sIL-6R release from human myeloma cells, and also to identify the class of proteinase responsible for this event. Human myeloma cell lines were shown to express IL-6R upon their surface and also to release sIL-6R into culture supernatants. In addition, phorbol 12-myristate 13-acetate (PMA) stimulated a loss of IL-6R from the cell surface, with a corresponding increase in the concentration of sIL-6R in the supernatant. Inhibitors of serine and cysteine proteinases, and tissue inhibitor of metalloproteinase (TIMP) -1 and TIMP-2, were shown to have no effect on the magnitude of sIL-6R release. In contrast, TIMP-3 and a hydroxamate-based metalloproteinase inhibitor (BB-94), inhibited both constitutive and PMA-induced release of sIL-6R. Myeloma cells freshly isolated from the bone marrow of a patient with MM were also shown to express IL-6R upon their surface, and to shed this receptor in response to PMA. These data demonstrate that increased proteolytic cleavage of IL-6R, mediated by a non-matrix-type metalloproteinase, is likely to contribute to the elevated concentrations of sIL-6R found in the serum of patients with MM. Inhibition of sIL-6R release by hydroxamate-based metalloproteinase inhibitors may represent a novel therapeutic approach to the treatment of MM.

Cleavage of beta-catenin and plakoglobin and shedding of VE-cadherin during endothelial apoptosis: evidence for a role for caspases and metalloproteinases

Growth factor deprivation of endothelial cells induces apoptosis, which is characterized by membrane blebbing, cell rounding, and subsequent loss of cell-matrix and cell-cell contacts. In this study, we show that initiation of endothelial apoptosis correlates with cleavage and disassembly of intracellular and extracellular components of adherens junctions. beta-Catenin and plakoglobin, which form intracellular links between vascular endothelial cadherin (VE-cadherin) and actin-binding alpha-catenin in adherens junctions, are cleaved in apoptotic cells. In vitro incubations of cell lysates and immunoprecipitates with recombinant caspases indicate that CPP32 and Mch2 are involved, possibly by initiating proteolytic processing. Cleaved beta-catenin from lysates of apoptotic cells does not bind to endogenous alpha-catenin, whereas plakoglobin retains its binding capacity. The extracellular portion of the adherens junctions is also altered during apoptosis because VE-cadherin, which mediates endothelial cell-cell interactions, dramatically decreases on the surface of cells. An extracellular fragment of VE-cadherin can be detected in the conditioned medium, and this "shedding" of VE-cadherin can be blocked by an inhibitor of metalloproteinases. Thus, cleavage of beta-catenin and plakoglobin and shedding of VE-cadherin may act in concert to disrupt structural and signaling properties of adherens junctions and may actively interrupt extracellular signals required for endothelial cell survival.

ADAMTS-1 protein anchors at the extracellular matrix through the thrombospondin type I motifs and its spacing region

Cellular disintegrin and metalloproteinases (ADAMs) are a family of genes with a sequence similar to those of snake venom metalloproteinases and disintegrins. The ADAMTS-1 gene encodes a new type of ADAM protein with respect to possessing the thrombospondin (TSP) type I motifs. Expression of the gene is induced in kidney and heart by in vivo administration of lipopolysaccharide, suggesting a possible role in the inflammatory reaction. In this study, we characterized the ADAMTS-1 gene product by using a transient expression system in COS-7 cells. We found that the precursor and processed forms of ADAMTS-1 were secreted from cells. Under normal growth conditions, little or none of both forms was detected in the cell culture medium, and instead the majority was found associated with the extracellular matrix (ECM). In addition, when cells were cultured in the presence of heparin, the mature form of ADAMTS-1 protein was detected in the cell culture medium, suggesting that binding of ADAMTS-1 to the ECM is mediated through sulfated glycosaminoglycans such as heparan sulfate. Analyses of deletion mutants of the ADAMTS-1 protein revealed that the spacer region as well as three TSP type I motifs in the carboxyl-terminal region of the ADAMTS-1 protein are important for a tight interaction with the ECM. These results suggest that the ADAMTS-1 is a unique ADAM family protein that anchors at the ECM.

Blockade of growth hormone receptor shedding by a metalloprotease inhibitor

GH, an important growth-promoting and metabolic hormone, exerts its biological effects by interacting with cell surface GH receptors (GHRs). The GHR is a single membrane-spanning protein that binds GH via its extracellular domain. The high affinity GH-binding protein (GHBP), which corresponds to a soluble form of the GHR extracellular domain, carries a substantial fraction of the GH in the circulation of various species and probably has a role in modulation of the hormone's bioavailability. Although in rodents, it is believed that the GHBP is largely derived by translation of an alternatively spliced GHR messenger RNA, in humans and rabbits, proteolytic cleavage of the membrane-anchored receptor releases the GHR extracellular domain, which is believed to thereby become the GHBP. In this study, we used human IM-9 lymphocytes and GHR antibodies to study this proteolytic shedding of the GHBP. As determined by immunoblotting with anti-GHR cytoplasmic domain serum, addition of phorbol 12-myristate 13-acetate (PMA; 1 microg/ml) to serum-starved cells led to rapid loss (roughly 60% decline after 1 h; t(1/2) = approximately 5 min) of mature GHRs (115-140 kDa) from either total cell or detergent-soluble extracts. Loss of full-length GHRs was accompanied by accumulation of four proteins (65-68 kDa), each reactive with the cytoplasmically directed antiserum. The pattern of appearance of these GHR ctyoplasmic domain proteins, the electrophoretic and immunological characteristics of which are similar to those of a recombinant rabbit GHR mutant that lacks the extracellular domain, was such that progressively faster migrating forms were evident between 5-60 min of PMA exposure. Treatment with N-ethylmaleimide (NEM; 5 mM), an agent known to cause GHBP shedding from IM-9 cells, promoted a similar rapid loss of full-length GHRs and an accumulation of GHR cytoplasmic domain remnant proteins. PMA-induced, but not NEM-induced, GHR proteolysis was blocked by the protein kinase C inhibitor, GF109203X. Both PMA- and NEM-induced receptor proteolysis were, however, inhibited by the metalloprotease inhibitor, Immunex Compound 3 (minimum effective concentration, 10 microM). Notably, PMA and NEM also promoted shedding of GHBP into the conditioned medium of the cells, as determined by a chromatographic [125I]human GH binding assay; this GHBP shedding was also inhibited by Immunex Compound 3. These results strongly implicate a member(s) of the metalloprotease family as a potential GHBP-generating enzyme.

Crystal structure of the catalytic domain of human tumor necrosis factor-alpha-converting enzyme

Tumor necrosis factor-alpha (TNFalpha) is a cytokine that induces protective inflammatory reactions and kills tumor cells but also causes severe damage when produced in excess, as in rheumatoid arthritis and septic shock. Soluble TNFalpha is released from its membrane-bound precursor by a membrane-anchored proteinase, recently identified as a multidomain metalloproteinase called TNFalpha-converting enzyme or TACE. We have cocrystallized the catalytic domain of TACE with a hydroxamic acid inhibitor and have solved its 2.0 A crystal structure. This structure reveals a polypeptide fold and a catalytic zinc environment resembling that of the snake venom metalloproteinases, identifying TACE as a member of the adamalysin/ADAM family. However, a number of large insertion loops generate unique surface features. The pro-TNFalpha cleavage site fits to the active site of TACE but seems also to be determined by its position relative to the base of the compact trimeric TNFalpha cone. The active-site cleft of TACE shares properties with the matrix metalloproteinases but exhibits unique features such as a deep S3' pocket merging with the S1' specificity pocket below the surface. The structure thus opens a different approach toward the design of specific synthetic TACE inhibitors, which could act as effective therapeutic agents in vivo to modulate TNFalpha-induced pathophysiological effects, and might also help to control related shedding processes.

Adherence to endothelial cells induces release of soluble tumor necrosis factor (TNF) receptor forms from neutrophil granulocytes

The TNF receptors, TNF-R55 and TNF-R75, may undergo proteolytic cleavage and form soluble receptor forms, TNF-R55-BP and TNF-R75-BP. Neutrophils are abundant with both forms of TNF-receptors, while endothelial cells (ECV 304) only express TNF-R55. Human neutrophils were allowed to interact with an unstimulated or a IL-1 beta stimulated endothelium followed by determination of TNF-R75-BP with ELISA. Neutrophils in suspension or in contact with an unstimulated endothelium released only low amounts of TNF-R75-BP. However, neutrophils released significant amounts of TNF-R75-BP after adherence to an endothelium stimulated with IL-1 beta. Neutrophils were not generally activated during adherence since concomitant release of lactoferrin from neutrophils only reached levels of 1-5% compared with incubation with phorbolesters. Blocking integrins with antibodies to CD11/CD18 resulted in inhibition of both neutrophil adherence to an endothelium and shedding of TNF-R75. In addition, TNF-R55-BP decreased the production of TNF from IL-1 beta stimulated endothelial cells, suggesting that soluble TNF receptor forms are able to inhibit TNF production.

Endothelial cell contact potentiates release of soluble tumor necrosis factor (TNF) receptors from the monocyte-like cell line THP-1

The two tumor necrosis factor (TNF) receptors can undergo proteolytic cleavage to form soluble receptors, TNF-R55-BP and TNF-R75-BP, that can neutralize TNF. The aim was to study the release of soluble TNF receptor forms during monocyte-endothelial cell interaction. Monocytic THP-1 cells were used, and their release of TNF-R75-BP was determined. Contact between THP-1 cells and confluent endothelial cells induced fourfold higher release of TNF-R75-BP from the THP-1 cells than with these cells in suspension. The release was further increased up to eightfold after prestimulation of the endothelial cells with interleukin-1beta (IL-1beta). Prestimulation for 10 min gave maximal release of TNF-R75-BP from the attached THP-1 cells. IL-1beta by itself did not induce shedding of soluble TNF receptors in THP-1 cells. Blocking antibodies against the endothelial cell adhesion molecules VCAM, ICAM, and E-selectin did not affect the release of TNF-R75-BP from THP-1 cells attached to the endothelium. Conditioned medium from IL-1beta-stimulated endothelial cells increased the production of TNF-R75-BP from THP-1 cells in suspension. However, surface contact between endothelial cells and THP-1 cells was necessary for maximal production of TNF-R75-BP. TNF-alpha released from endothelial cells on IL-1beta stimulation did not promote shedding of TNF-R75 from THP-1 cells. Thus, endothelial cell contact potentiates the production of TNF-R75-BP in a monocyte-like cell line. The shedding of soluble TNF receptors observed in this case seems to be a result of both cell attachment and soluble factors.

Transforming the cell surface through proteolysis

Protein shedding, or the proteolytic cleavage of a protein from the surface of a cell, is emerging as an important mechanism in the regulation of cellular activity but it is poorly understood. Growing evidence suggesting that protein shedding and protein function are closely linked may lead to new strategies for the treatment of a wide range of diseases.

Mutation of Proline 211 Reduces Shedding of the Human p75 TNF Receptor

We have identified a residue necessary for the cleavage of human p75 TNF-R from the cell surface by deletion and mutagenesis analysis of the membrane-proximal domain between amino acids 147 and 221. Deletion analysis of this area showed that residues between amino acids 207 and 216 are required for shedding. Site-directed mutagenesis of proline 211 to glycine reduced PMA-induced shedding of human p75 TNF-R from COS-7 or Raw 264.7 cells. Mutation of glycine 210 to aspartic acid did not affect receptor shedding. Mutation of serine 212 to leucine did not affect the PMA-induced shedding from the surface of COS-7 cells, but reduced the efficiency of shedding from the surface of Raw 264.7 macrophages by PMA or LPS. Deletion of putative elastase cleavage sites at amino acids 151 to 153, 162 to 163, and 177 to 178 (also a putative metalloprotease site similar to the cleavage site of TNF-α) or mutation of a serine-threonine-serine motif resembling a similar motif at the site of L-selectin cleavage at amino acids 202 to 204 did not reduce shedding of p75 TNF-R after treatment of cells with PMA. This work shows that a single amino acid mutation at proline 211 of human p75 TNF-R can prevent shedding from the cell surface, and that deletion of other previously proposed putative cleavage sites of the human p75 TNF-R does not prevent its shedding.

Cleavage of membrane-associated ICAM-1 from astrocytes: involvement of a metalloprotease

Disease of the central nervous system (CNS) with immune-mediated pathogenesis is frequently associated with enhanced expression of intercellular adhesion molecule-1 (ICAM-1) on resident glial cells, including astrocytes. Recently, a soluble form of ICAM-1 (sICAM-1) has been demonstrated within the CNS and cerebrospinal fluid (CSF), arising from an intrathecal source. In this study, we investigated the ability of TNF-alpha treated astrocytes to generate sICAM-1 from a population of membrane-associated ICAM-1. To determine the ability of ICAM-1 to be released from the cell surface, generating sICAM-1, cell cultures were treated with TNF-alpha for 21 h prior to cell surface protein iodination or biotinylation. We show that the membrane-associated form of ICAM-1 (approximately 90 KD) is converted to a soluble form (approximately 83 KD) in cell culture supernatants. The half-life of TNF-alpha induced membrane-associated ICAM-1 on rat astrocytes is approximately 5 h. The proteolytic cleavage process for the conversion of membrane-associated ICAM-1 to sICAM-1 was sensitive to Batimastat (BB94) and phosphoramidon, two inhibitors of metalloproteases, whereas inhibitors of serine-, cysteine-, aspartic-, and chymotrypsin-like proteases had no effect on this process. These results indicate that astrocytes can be induced to produce sICAM-1, and this process involves a metalloprotease that is induced/activated in a TNF-alpha-dependent fashion. It is proposed that astrocytes may be a source of intrathecal sICAM-1 under inflammatory conditions.

A matrix metalloproteinase gene expressed in human T lymphocytes is identical with collagenase 3 from breast carcinomas

The response of human T lymphocytes to various stimuli includes the expression of the matrix metalloproteinase (MMP) genes stromelysin 2, gelatinase A and gelatinase B. The proteins encoded by these genes could confer the capacity to degrade macromolecular components of the extracellular matrix (ECM), and to shed transmembrane proteins such as tumor necrosis factor (TNF), TNF receptor, Interleukin-6 receptor and Fas ligand. To identify further MMP genes transcribed in T lymphocytes exposed to phorbol 12-myristate 13-acetate and a calcium ionophore, we combined reverse transcription and polymerase chain reaction using primers specific for conserved domains and detected collagenase 3 transcripts, first described in a human breast cancer. However, when the sequence of the complementary DNA was compared, additional 23 nucleotides were found in the 5' nontranslated region of the lymphocyte messenger RNA (mRNA). Northern blot analysis revealed 2 major inducible mRNA species of 1.9 and 2.8 kilobases, whose levels were lower than those of stromelysin 2. The observation that activated T lymphocytes transcribe several MMP genes, including a collagenase, indicates that the effector functions of these cells include enzymatic activities towards most constituents of the ECM, as well as some transmembrane proteins relevant to inflammation and apoptosis.

TNF Receptor-Deficient Mice Reveal Divergent Roles for p55 and p75 in Several Models of Inflammation

The pleiotropic activities of the potent proinflammatory cytokine TNF are mediated by two structurally related, but functionally distinct, receptors, p55 and p75, that are coexpressed on most cell types. The majority of biologic responses classically attributed to TNF are mediated by p55. In contrast, p75 has been proposed to function as both a TNF antagonist by neutralizing TNF and as a TNF agonist by facilitating the interaction between TNF and p55 at the cell surface. We have examined the roles of p55 and p75 in mediating and modulating the activity of TNF in vivo by generating and examining mice genetically deficient in these receptors. Selective deficits in several host defense and inflammatory responses are observed in mice lacking p55 or both p55 and p75, but not in mice lacking p75. In these models, the activity of p55 is not impaired by the absence of p75, arguing against a physiologic role for p75 as an essential element of p55-mediated signaling. In contrast, exacerbated pulmonary inflammation and dramatically increased endotoxin induced serum TNF levels in mice lacking p75 suggest a dominant role for p75 in suppressing TNF-mediated inflammatory responses. In summary, these data help clarify the biologic roles of p55 and p75 in mediating and modulating the biologic activity of TNF and provide genetic evidence for an antagonistic role of p75 in vivo.

Cell biology of autoimmune diseases

Autoimmune diseases such as insulin-dependent diabetes mellitus, rheumatoid arthritis, and multiple sclerosis are common in the western world and are often devastating diseases which pose serious health problems. The key feature of such diseases is the development and persistence of inflammatory processes in the apparent absence of pathogens, leading to chronic breakdown of selected tissues. To date, no comprehensive explanation can be given for the onset or persistence of autoimmunity. As a rule, the chronic activation of helper T lymphocytes reactive against self proteins appears to be crucial for fueling the destructive autoimmune process, but why this occurs remains to be established. In this review, we present an overview on the rules that govern activation of T lymphocytes and on the factors that control it. The contribution of both genetic and environmental factors are discussed, clarifying that most autoimmune disease are of multifactorial origin. Special emphasis is given to the contribution of infectious events and the role of stress proteins in the process. In attempts to dissect the mechanisms involved in autoimmunity and to develop ways of blocking disease, experimental animal models are widely employed. We describe the various experimental models that exist for the study of multiple sclerosis, diabetes, and other autoimmune diseases and on the experience that has been gained in such models with experimental therapies to block the activation of self-reactive T lymphocytes. The lessons that can be drawn from these studies provide hope that continued efforts will lead to the successful development of antigen-specific strategies which block the development of autoimmunity also in humans.

A metalloprotease-inhibitor reduces pain associated behavior in mice with experimental neuropathy

Tumor necrosis factor-alpha (TNF) is involved in the generation of inflammatory and neuropathic pain. The synthetic hydroxamic acid based metalloprotease inhibitor TAPI blocks cleavage of cell surface TNF and thus reduces levels of the mature 17-kDa TNF polypeptide in activated macrophages and T-cells. We have previously shown that pharmacologic inhibition of TNF production reduces pain related behaviors in mice with chronic constriction injury (CCI). Here we investigated whether blockage of TNF shedding by administration of TAPI would diminish hyperalgesia in animals with partial nerve injury. We injected 0.5 mg of the inhibitor epineurially once daily to mice with CCI for 7 days. The animals were tested for withdrawal thresholds to heat to test for thermal hyperalgesia and to von Frey hairs to assess mechanical allodynia. Mice with CCI developed thermal hyperalgesia and mechanical allodynia by day 3 after the injury. In mice treated with TAPI, a reduction of thermal hyperalgesia and mechanical allodynia of up to 50% occurred. Endoneurial TNF-immunoreactivity was reduced, but not immunoreactivity for IL-1alpha or IL-1beta. The numbers of degenerating axons and endoneurial macrophages were not affected by the treatment as compared to controls. We conclude that the metalloprotease inhibitor TAPI specifically reduces endoneurial TNF-levels after nerve injury and thereby may diminish neuropathic pain in the CCI-model.