Transforming growth factor-alpha and beta-amyloid precursor protein share a secretory mechanism

Apical enrichment of human EGF precursor in Madin-Darby canine kidney cells involves preferential basolateral ectodomain cleavage sensitive to a metalloprotease inhibitor

EGF precursor (proEGF) is a member of the family of membrane-anchored EGF-like growth factors that bind with high affinity to the epidermal growth factor receptor (EGFR). In contrast to human transforming growth factor-alpha precursor (proTGFalpha), which is sorted basolaterally in Madin-Darby canine kidney (MDCK) cells (Dempsey, P., and R. Coffey, 1994. J. Biol. Chem. 269:16878-16889), we now demonstrate that human proEGF overexpressed in MDCK cells is found predominantly at the apical membrane domain under steady-state conditions. Nascent proEGF (185 kD) is not sorted but is delivered equally to the apical and basolateral membranes, where it is proteolytically cleaved within its ectodomain to release a soluble 170-kD EGF form into the medium. Unlike the fate of TGFalpha in MDCK cells, the soluble 170-kD EGF species accumulates in the medium, does not interact with the EGFR, and is not processed to the mature 6-kD peptide. We show that the rate of ectodomain cleavage of 185-kD proEGF is fourfold greater at the basolateral surface than at the apical surface and is sensitive to a metalloprotease inhibitor, batimastat. Batimastat dramatically inhibited the release of soluble 170-kD EGF into the apical and basal medium by 7 and 60%, respectively, and caused a concordant increase in the expression of 185-kD proEGF at the apical and basolateral cell surfaces of 150 and 280%, respectively. We propose that preferential ectodomain cleavage at the basolateral surface contributes to apical domain localization of 185-kD proEGF in MDCK cells, and this provides a novel mechanism to achieve a polarized distribution of cell surface membrane proteins under steady-state conditions. In addition, differences in disposition of EGF and TGFalpha in polarized epithelial cells offer a new conceptual framework to consider the actions of these polypeptide growth factors.

The carboxyl-terminal valine residues of proTGF alpha are required for its efficient maturation and intracellular routing

Soluble forms of transforming growth factor-alpha (TGF alpha) are derived by proteolytic processing of an integral membrane glycoprotein precursor (pro TGF alpha). Previous studies indicated that phorbol ester-induced cleavage of pro TGF alpha in CHO cells is dependent on the presence of a valine residue located at the carboxyl terminus of the precursor's cytoplasmic domain. We reassessed this requirement with epitope-tagged constructs introduced into transformed rat liver epithelial cells that normally express and process TGF alpha. We found that pro TGF alpha mutants lacking the terminal valine residues showed greatly reduced maturation to the fully glycosylated form. Additionally, they were present at substantially reduced levels on the cell surface and, instead, accumulated in the endoplasmic reticulum. Consistent with these results, enzyme-linked immunosorbent assay (ELISA) and Western blot analyses revealed little or no soluble TGF alpha in medium conditioned by cells expressing the mutant constructs. Finally, a truncated pro TGF alpha mutant lacking most of the cytoplasmic domain but retaining a carboxyl-terminal valine was processed and cleaved in a near-normal manner. These results, some of which were reproduced in CHO cells, indicate that the predominant effect of the carboxyl-terminal valines is to ensure normal maturation and routing of the precursor.

Role of the juxtamembrane domains of the transforming growth factor-alpha precursor and the beta-amyloid precursor protein in regulated ectodomain shedding

Although regulated ectodomain shedding is a well known process that affects a large group of transmembrane molecules, it is not clear how the shedding system selects its substrates. Here we investigate the structural requirements for the regulated shedding of two substrates of the general shedding system, the transforming growth factor-alpha precursor, pro-TGF-alpha, and the beta-amyloid precursor protein, beta-APP. The ability of different regions of pro-TGF-alpha or beta-APP to confer susceptibility to the shedding system was tested using as a reporter a transmembrane molecule that is not a substrate of this shedding system. For this purpose we chose the TGF-beta accessory receptor, betaglycan, since genetic and biochemical evidence showed that betaglycan is not a substrate of the shedding system. We determined that replacement of the 14 extracellular amino acids adjacent to the transmembrane region of betaglycan with the corresponding regions of TGF-alpha or beta-APP rendered betaglycan susceptible to ectodomain shedding. These domain swap constructs were cleaved in response to protein kinase C stimulation, and cleavage was prevented by the metalloprotease inhibitor TAPI, both effects being characteristic of the general shedding system. Domain swap constructs containing the transmembrane and/or the cytoplasmic domains of pro-TGF-alpha did not undergo regulated ectodomain cleavage. We conclude that despite a lack of sequence similarity, the extracellular regions of pro-TGF-alpha and beta-APP immediately preceding their transmembrane domains are key determinants of ectodomain shedding.

Regulated shedding of syndecan-1 and -4 ectodomains by thrombin and growth factor receptor activation

The syndecan family of transmembrane heparan sulfate proteoglycans is abundant on the surface of all adherent mammalian cells. Syndecans bind and modify the action of various growth factors/cytokines, proteases/antiproteases, cell adhesion molecules, and extracellular matrix components. Syndecan expression is highly regulated during wound repair, a process orchestrated by many of these effectors. Each syndecan ectodomain is shed constitutively by cultured cells, but the mechanism and significance of this shedding are not understood. Therefore, we examined (i) whether physiological agents active during wound repair influence syndecan shedding, and (ii) whether wound fluids contain shed syndecan ectodomains. Using SVEC4-10 endothelial cells we find that certain proteases and growth factors accelerate shedding of the syndecan-1 and -4 ectodomains. Protease-accelerated shedding is completely inhibited by serum-containing media. Thrombin activity is duplicated by the 14-amino acid thrombin receptor agonist peptide that directly activates the thrombin receptor and is not inhibited by serum. Epidermal growth factor family members accelerate shedding but FGF-2, platelet-derived growth factor-AB, transforming growth factor-beta, tumor necrosis factor-alpha, and vascular endothelial cell growth factor 165 do not. Shed ectodomains are soluble, stable in the conditioned medium, have the same size core proteins regardless whether shed at a basal rate, or accelerated by thrombin or epidermal growth factor-family members and are found in acute human dermal wound fluids. Thus, shedding is accelerated by activation of at least two distinct receptor classes, G protein-coupled (thrombin) and protein tyrosine kinase (epidermal growth factor). Proteases and growth factors active during wound repair can accelerate syndecan shedding from cell surfaces. Regulated shedding of syndecans suggests physiological roles for the soluble proteoglycan ectodomains.

Proteolytic release of membrane proteins: studies on a membrane-protein-solubilizing activity in CHO cells

Diverse membrane proteins are solubilized by a specific proteolytic cleavage in the stalk sequence adjacent to the membrane anchor, with release of the extracellular domain. Examples are the amyloid precursor protein, membrane-bound growth factors and angiotensin-converting enzyme (ACE). The identities and characteristics of the responsible proteases remain elusive. We have studied this process in Chinese hamster ovary (CHO) cells stably expressing wild-type ACE (WT-ACE) or juxtamembrane (stalk) deletion or chimaera mutants. Determination of the C termini (i.e. the cleavage sites) of released, soluble wild-type and mutant ACE by MALDI-TOF mass spectrometry indicated that the membrane-protein-solubilizing protease (MPSP) in CHO cells is not constrained by a particular cleavage site motif or by a specific distance from the membrane, but instead may position itself with respect to the putative proximal, folded extracellular domain adjacent to the stalk. Nevertheless, kinetic analyses of release rates indicated that a minimum distance from the membrane must be preserved. Interestingly, soluble full-length (anchor-plus) WT-ACE incubated with fractions of, or intact, CHO cells was not cleaved. In all cases, release was stimulated by a media change or by the addition of phorbol ester, with rate enhancements of 5- and 50-fold, respectively, for WT-ACE. The phorbol ester effect was abolished by staurosporine, a protein kinase C (PKC) inhibitor. We propose that the CHO cell MPSP that solubilizes ACE: (1) only cleaves proteins embedded in a membrane; (2) requires an accessible stalk and cleaves at a minimum distance from both the membrane and proximal extracellular domain; (3) positions itself primarily with respect to the proximal extracellular domain and (4) is regulated in part by a PKC-dependent mechanism.

Cleavage-secretion of angiotensin I-converting enzyme in yeast

Angiotensin I-converting enzyme (ACE) is a type I transmembrane protein composed of two domains (N and C domains) which undergoes a post-translational proteolytic cleavage in mammalian cells to release the soluble ectodomain. The protease involved in ACE cleavage-secretion (ACE-secretase) is not well characterised and eludes isolation: the presence of a yeast homologue, thus more amenable to genetic manipulation, would facilitate its identification. We have expressed a secreted form of the ACE C domain, lacking the C-terminal membrane anchor (C domain(deltaCOOH)), and the membrane-anchored C domain (C domain) in the yeast Pichia pastoris by fusion to prepro-alpha-factor. Immunofluorescent labelling localises the ACE C domain to the periphery of yeast cells but not C domain(deltaCOOH), however, expression of both C domain and C domain(deltaCOOH) produced soluble enzymes in the culture medium. Immunocharacterisation of the two soluble forms of the C domain indicates a proteolytic cleavage of the membrane-bound C domain to produce the soluble counterpart. Thus ACE undergoes a proteolytic cleavage in yeast.

Secretory processing of amyloid precursor protein is inhibited by increase in cellular cholesterol content

Plasma-membrane composition plays a crucial role in most of the cellular functions that depend on membrane processes. In virtually all cell types the proteolytic processing of Alzheimer amyloid precursor protein (APP) to generate soluble APP (sAPP) is believed to occur at the plasma membrane or in its immediate proximity. Alteration of this metabolic pathway has been linked to the pathogenesis of Alzheimer's disease. We analysed the effect of membrane cholesterol enrichment on APP metabolism. Incubation of COS cells with increasing concentrations of non-esterified cholesterol carried by rabbit beta-very low-density lipoprotein caused a dose-dependent inhibition of sAPP release: 70% inhibition with 10 microg/ml non-esterified cholesterol. A less pronounced inhibitory effect was observed on treatment with human low-density lipoprotein. Inhibition of sAPP release was independent of receptor-mediated lipoprotein metabolism since simultaneous treatment with chloroquine did not modify the effect of lipoprotein treatment. In addition, treatment with cholesterol dissolved in either ethanol or methyl-beta-cyclodextrin elicited the same effect. Excess non-esterified cholesterol did not cause cell toxicity. Cell cholesterol mass inversely correlated with sAPP release. Progesterone, which inhibits shuttling of non-esterified cholesterol between the plasma membrane and intracellular pools, had no effect on the inhibition of sAPP release from cholesterol-loaded cells, providing indirect evidence that cholesterol may act at the plasma membrane.

Alpha 1-antitrypsin blocks the release of transforming growth factor-alpha from MCF-7 human breast cancer cells

Human breast cancer cells synthesize and release a variety of growth-modulating substances in response to estrogen stimulation, and it is generally accepted that the growth-promoting effects of estrogens are due at least in part to this autocrine/paracrine mechanism. Several of these growth-modulating substances, including transforming growth factor-alpha (TGF alpha) and its analogs, have been shown to require pericellular proteolysis for activation or release. Recently, we reported that MCF-7 human breast cancer cells are able to synthesize alpha 1-antitrypsin (alpha 1-AT), the major elastase inhibitor in human serum, and that there is a negative correlation between anchorage-independent growth of MCF-7 cells in soft agar and synthesis of alpha 1-AT. The studies we present here were undertaken to gain an understanding of the mechanisms responsible for this observation. We show that release of TGF alpha from its membrane-bound precursor on MCF-7 cells is blocked by alpha 1-AT whether the cells were maintained in the presence or absence of estradiol and that there is a clear dose-response relationship between the alpha 1-AT concentration and both the release of TGF alpha and growth in soft agar. Consistent with this, TGF alpha release was increased in the presence of antibody to alpha 1-AT. In contrast, TGF alpha release and growth in soft agar were not blocked by peptide inhibitors specific for trypsin- or chymotrypsin-like enzymes. The alpha 1-AT concentration required for a half-maximal effect is lower for inhibition of TGF alpha release than it is for inhibition of colony formation (0.4 vs. 1.5 mumol/L). However, both values are in the range of concentrations one might expect at the cell surface in vivo. A new MCF-7 cell subline producing 10-fold higher levels of alpha 1-AT than its parent cell line was constructed by stable transfection of MCF-7 ML cells (a subline producing low levels of alpha 1-AT) with an alpha 1-AT complementary DNA. Growth in soft agar and release of TGF alpha were significantly decreased in cells transfected with the alpha 1-AT complementary DNA compared to those in cells transfected with vector alone, although, TGF alpha expression was the same. The above observations support a model for growth regulation in human breast ductal epithelial cells in which growth factor activation and release are dependent on the coordinate action of proteases and protease inhibitors. This model would predict that alpha 1-AT can act as a tumor suppressor in inhibiting the growth of breast cancer cells.

Phorbol esters affect multiple steps in beta-amyloid precursor protein trafficking and amyloid beta-protein production

BACKGROUND

Amyloid beta-protein (A beta), the major constituent of amyloid deposits found in Alzheimer's disease, is derived from the beta-amyloid precursor protein (beta PP). Constitutive proteolysis by alpha-secretase and secretion of soluble beta PP (beta PPs) are stimulated by protein kinase C (PKC) activation, whereas A beta production and release are inhibited. The cellular mechanism that underlies the PKC-mediated down-regulation of A beta generation is unclear. Because endocytic processing of beta PP from the cell surface is a major pathway of A beta production, the effect of PKC activation by phorbol 12,13-dibutyrate (PDBu) on endocytic trafficking of beta PP was examined.

MATERIALS AND METHODS

In this study, trafficking of beta PP was assayed in Chinese hamster ovary cells (CHO) cells stably transfected with full-length beta PP751.

RESULTS

Treatment with PDBu resulted in a rapid and striking reduction of up to 80% in the amount of beta PP at the cell surface. This loss of cell-surface molecules could not be accounted for by changes in the trafficking of cell-surface beta PP molecules, as determined by a radiolabeled antibody assay. Rather, the decrease in beta PP was due primarily to a reduction in the sorting of beta PP to the cell surface. This alteration was correlated with accelerated intracellular alpha-secretase-mediated beta PP cleavage and accelerated beta PP trafficking in the exocytic pathway.

CONCLUSIONS

The data suggest that the displacement of beta PP away from the cell surface after phorbol ester treatment reduces the substrate available for endocytic processing and in turn, results in the inhibition of A beta production.

UVC activation of the HeLa cell membrane "TGF alpha ase," a metalloenzyme

We have investigated the effect of UVC irradiation on "TGF alpha ase" activity using both intact HeLa cells and isolated membrane fragments with an assay based on the previously described nonapeptide substrate method [Brown et al. (1992): J Cell Biochem 48:411-423]. This method allows recognition of cleavage at the scissile bond cognate with that of the TGF alpha N-terminal cleavage site from its membrane-bound precursor. The level of ectoendopeptidase (including "TGF alpha ase") activity observed on intact cells was lower than that of ectoaminopeptidases. Addition of foetal bovine serum (FBS) enhanced aminopeptidase and dipeptidyl peptidase activity but inhibited "TGF alpha ase" activity, while phorbol 12-myristate 13-acetate (PMA) had no significant effect on the ectopeptidases monitored, expect for "TGF alpha ase," which was also inhibited, in contradistinction to their effects in other cell systems. Sublethal UVC irradiation (10 Jm-2) of the cultures resulted in activation of the ectoaminopeptidase and ectoendopeptidases which was maximal 16 and 20-24 h after irradiation, respectively. The addition of FBS to these irradiated cells appeared to reduce the increase in endopeptidase products, due in part to increased aminopeptidase activity but also to the direct inhibitory effect of FBS on the "TGF alpha ase." The activation of these proteases by UVC, even at high fluences (500 Jm-2), was not observed within the first 30 min after the cells were irradiated. Purified plasma membrane fragments were prepared from suspension cultures of HeLa cells and displayed high levels of "TGF alpha ase" activity. The rate of "TGF alpha ase" activity using 140 nM peptide substrate (P9) was 5.6 pmol/min/mg membrane protein, which was elevated to 13.7 pmol/min/mg membrane protein, 20 h after the cells had been irradiated with 10 Jm-2 UVC. Inhibition studies indicate that the plasma membrane "TGF alpha ase is a metalloenzyme as it was inhibited by EDTA, EGTA, and 1,10-phenanthroline but not by elastase or serine protease inhibitors. "TGF alpha ase" activity on intact cells was shown to be inhibited by 1,10-phenanthroline, which further supports this suggestion. Treatment of the membranes with Triton X-100 resulted in a loss of "TGF alpha ase" activity, raising the possibility that this enzyme may require a cofactor to be fully functional. We show that in purified membrane preparations of HeLa cells there is evidence for the presence of a "TGF alpha ase" which can be activated by UV irradiation, which differs from the putative "TGF alpha ase" described in various other cell lines, and which does not seem dependent on protein kinase C (PKC) activity.

Cellular signaling roles of TGF beta, TNF alpha and beta APP in brain injury responses and Alzheimer's disease

beta-Amyloid precursor protein (beta APP), transforming growth factor beta (TGF beta), and tumor necrosis factor-alpha (TNF alpha) are remarkably pleiotropic neural cytokines/neurotrophic factors that orchestrate intricate injury-related cellular and molecular interactions. The links between these three factors include: their responses to injury; their interactive effects on astrocytes, microglia and neurons; their ability to induce cytoprotective responses in neurons; and their association with cytopathological alterations in Alzheimer's disease. Astrocytes and microglia each produce and respond to TGF beta and TNF alpha in characteristic ways when the brain is injured. TGF beta, TNF alpha and secreted forms of beta APP (sAPP) can protect neurons against excitotoxic, metabolic and oxidative insults and may thereby serve neuroprotective roles. On the other hand, under certain conditions TNF alpha and the fibrillogenic amyloid beta-peptide (A beta) derivative of beta APP can promote damage of neuronal and glial cells, and may play roles in neurodegenerative disorders. Studies of genetically manipulated mice in which TGF beta, TNF alpha or beta APP ligand or receptor levels are altered suggest important roles for each factor in cellular responses to brain injury and indicate that mediators of neural injury responses also have the potential to enhance amyloidogenesis and/or to interfere with neuroregeneration if expressed at abnormal levels or modified by strategic point mutations. Recent studies have elucidated signal transduction pathways of TGF beta (serine/threonine kinase cascades), TNF alpha (p55 receptor linked to a sphingomyelin-ceramide-NF kappa B pathway), and secreted forms of beta APP (sAPP; receptor guanylate cyclase-cGMP-cGMP-dependent kinase-K+ channel activation). Knowledge of these signaling pathways is revealing novel molecular targets on which to focus neuroprotective therapeutic strategies in disorders ranging from stroke to Alzheimer's disease.

Membrane protein secretases

A diverse range of membrane proteins of Type 1 or Type II topology also occur as a circulating, soluble form. These soluble forms are often derived from the membrane form by proteolysis by a group of enzymes referred to collectively as 'secretases' or 'sheddases'. The cleavage generally occurs close to the extracellular face of the membrane, releasing physiologically active protein. This secretion process also provides a mechanism for down-regulating the protein at the cell surface. Examples of such post-translational proteolysis are seen in the Alzheimer's amyloid precursor protein, the vasoregulatory enzyme angiotensin converting enzyme, transforming growth factor-alpha, the tumour necrosis factor ligand and receptor superfamilies, certain cytokine receptors, and others. Since the proteins concerned are involved in pathophysiological processes such as neurodegeneration, apoptosis, oncogenesis and inflammation, the secretases could provide novel therapeutic targets. Recent characterization of these individual secretases has revealed common features, particularly sensitivity to certain metalloprotease inhibitors and upregulation of activity by phorbol esters. It is therefore likely that a closely related family of metallosecretases controls the surface expression of multiple integral membrane proteins. Current knowledge of the various secretases are compared in this Review, and strategies for cell-free assays of such proteases are outlined as a prelude to their ultimate purification and cloning.

Proteolytic release of membrane-bound angiotensin-converting enzyme: role of the juxtamembrane stalk sequence

Many structurally and functionally diverse membrane proteins are solubilized by a specific proteolytic cleavage in the stalk sequence adjacent to the membrane anchor, with release of the extracellular domain. Examples are the amyloid precursor protein, membrane-bound growth factors, and angiotensin-converting enzyme (ACE). The identities and characteristics of the responsible proteases remain elusive. We have studied this process in Chinese hamster ovary (CHO) cells stably expressing wild-type ACE (WT-ACE; human testis isozyme) or one of four juxtamembrane (stalk) mutants containing either deletions of 17, 24, and 47 residues (ACE-JM delta 17, -JM delta 24, and -JM delta 47, respectively) or a substitution of 26 stalk residues with a 20-residue sequence from the stalk of the low-density lipoprotein receptor (ACE-JMLDL). The C termini of released, soluble WT-ACE and ACE-JM delta 17 and -JMLDL were determined by MALDI-TOF mass spectrometry analyses of C-terminal peptides generated by CNBr cleavage. Observed masses of 4264 (WT-ACE) and 4269 (ACE-JM delta 17) are in good agreement with an expected mass of 4262 for the C-terminal CNBr peptide ending at Arg-627, indicating cleavage at the Arg-627/Ser-628 bond in both WT-ACE and ACE-JM delta 17, at distances of 24 and 10 residues from the membrane, respectively. Data for ACE-JM delta 24 are also consistent with cleavage at or near Arg-627. For ACE-JMLDL, in which the native cleavage site is absent, observed masses of 4372 and 4542 are in close agreement with expected masses of 4371 and 4542 for peptides ending at Ala-628 and Gly-630, respectively, indicating cleavages at 17 or 15 residues from the membrane. These data indicate that the membrane-protein-solubilizing protease (MPSP) in CHO cells is not constrained by a particular cleavage site motif or by a specific distance from the membrane but instead may position itself with respect to the putative proximal, folded extracellular domain adjacent to the stalk. Nevertheless, cleavage at a distance of 10 residues from the membrane is more favorable, as ACE-JM delta 17 is cleaved 12-fold faster than WT-ACE. In contrast, ACE-JM delta 24 is released 17-fold slower, suggesting that a minimum distance from the membrane must be preserved. This is supported by results with the ACE-JM delta 47 mutant, which is membrane-bound but not cleaved, likely because the entire stalk has been deleted. Finally, soluble full-length (anchor-plus) WT-ACE is not cleaved when incubated with various CHO cell fractions or intact CHO cells. On the basis of these and other data, we propose that the CHO cell MPSP that solubilizes ACE (1) only cleaves proteins embedded in a membrane; (2) requires an accessible stalk and cleaves at a minimum distance from both the membrane and proximal extracellular domain; (3) positions itself primarily with respect to the proximal extracellular domain; and (4) may have a weak preference for cleavage at Arg/Lys-X bonds.

Pore-forming toxins trigger shedding of receptors for interleukin 6 and lipopolysaccharide

Cleavage of membrane-associated proteins with the release of biologically active macromolecules is an emerging theme in biology. However, little is known about the nature and regulation of the involved proteases or about the physiological inducers of the shedding process. We here report that rapid and massive shedding of the interleukin 6 receptor (IL-6R) and the lipopolysaccharide receptor (CD14) occurs from primary and transfected cells attacked by two prototypes of pore-forming bacterial toxins, streptolysin O and Escherichia coli hemolysin. Shedding is not induced by an streptolysin O toxin mutant which retains cell binding capacity but lacks pore-forming activity. The toxin-dependent cleavage site of the IL-6R was mapped to a position close to, but distinct from, that observed after stimulation with phorbol myristate acetate. Soluble IL-6R that was shed from toxin-treated cells bound its ligand and induced an IL-6-specific signal in cells that primarily lacked the IL-6R. Transsignaling by soluble IL-6R and soluble CD14 is known to dramatically broaden the spectrum of host cells for IL-6 and lipopolysaccharide, and is thus an important mechanism underlying their systemic inflammatory effects. Our findings uncover a novel mechanism that can help to explain the long-range detrimental action of pore-forming toxins in the host organism.

A biosynthetic regulated secretory pathway in constitutive secretory cells

It has frequently been proposed that while the constitutive secretory pathway is present in all cells, the regulated secretory pathway is found only in specialized cells such as neuronal, endocrine, or exocrine types. In this study we provide evidence that suggests that this distinction is not as restrictive as proposed. We have identified a population of post-Golgi storage vesicles in several constitutive secretory cells using [35S]SO4-labeled glycosaminoglycan (GAG) chains as a marker. A fraction of this pool of vesicles can undergo exocytosis in response to stimuli such as cytoplasmic Ca2+ and phorbol esters. The effect of Ca2+ was demonstrated both in intact cells in the presence of the ionophore A23187 and in streptolysin-O-permeabilized semi-intact cells. N-ethylmaleiimide, under conditions known to block regulated and constitutive secretion, inhibited the stimulated secretion from these cells, suggesting that the observed release of labeled GAG chains was not due to a leakage artefact. Subcellular fractionation revealed that the stored GAG chains were in low-density membrane granules (d approximately 1.12 g/ml), whose size was greater than that of synaptic-like vesicles found in PC12 cells. In addition, in CHO cells that express epitope-tagged rab 3D, the labeled GAG chains were found to cofractionate with the exogenous rab protein. When expressed in the regulated cell line AtT-20, this tagged rab protein was found to colocalize with ACTH-containing dense-core granules by indirect immunofluorescence. Taken together, these results provide evidence for the presence of a cryptic regulated secretory pathway in "constitutive" cells and suggest that the regulated secretory pathway is more widespread amongst different cell types than previously believed.

Diverse cell surface protein ectodomains are shed by a system sensitive to metalloprotease inhibitors

The extracellular domains of a diverse group of membrane proteins are shed in response to protein kinase C activators such as phorbol 12-myristate 13-acetate (PMA). The lack of sequence similarity in the cleavage sites suggests the involvement of many proteases of diverse specificity in this process. However, a mutant Chinese hamster ovary cell line recently isolated for being defective in PMA-activated shedding of the membrane-anchored growth factor transforming growth factor alpha precursor (proTGF-alpha) is concomitantly defective in the shedding of many other unrelated membrane proteins. Here we show that independent mutagenesis and selection experiments yield shedding mutants having the same recessive phenotype and belonging to the same genetic complementation group. Furthermore, two structurally distinct agents, TAPI-2 and 1,10-phenanthroline, which are known to inhibit metalloproteases, block PMA-activated shedding of proTGF-alpha, cell adhesion receptor L-selectin, interleukin 6 receptor alpha subunit, beta-amyloid precursor protein, and an entire set of anonymous Chinese hamster ovary cell surface proteins. Certain serine protease inhibitors prevent release of these proteins by interfering with their maturation and transport to the cell surface but do not inhibit ectodomain shedding from the cell surface. The results suggest the existence of a common system for membrane protein ectodomain shedding involving one or several proteolytic activities sensitive to metalloprotease inhibitors, whose ability to act can be disrupted by recessive mutations in a single gene.

Shedding of the lymphocyte L-selectin adhesion molecule is inhibited by a hydroxamic acid-based protease inhibitor. Identification with an L-selectin-alkaline phosphatase reporter

Expression of the L-selectin adhesion molecule can be rapidly down-modulated by regulated proteolysis at a membrane-proximal site. The L-selectin secretase has remained undefined, and the secretase activity is resistant to a broad panel of common protease inhibitors. We have developed an L-selectin-alkaline phosphatase reporter, consisting of the ectodomain of human placental alkaline phosphatase fused to the membrane-proximal cleavage, transmembrane, and cytoplasmic domains of L-selectin, to aid in the screening for L-selectin secretase inhibitors. A hydroxamic acid-based metalloprotease inhibitor, KD-IX-73-4, inhibited release of the L-selectin-alkaline phosphatase reporter in a dose-dependent manner. The hydroxamic acid-based peptide was also found to inhibit wild type L-selectin down-regulation from the surfaces of phorbol myristate acetate-activated peripheral blood lymphocytes and phytohemagglutinin-stimulated lymphoblasts. Analysis of the proteolytic cleavage fragments of L-selectin confirmed that KD-IX-73-4 inhibited L-selectin proteolysis. Lymphocyte L-selectin was not down-regulated when co-cultured with formylmethionylleucylphenylalanine-stimulated neutrophils, suggesting that the putative secretase acts in cis with the membrane-bound L-selectin. These results suggest that the L-selectin secretase activity may involve a cell surface, zinc-dependent metalloprotease, although L-selectin shedding is not affected by EDTA and may be related to the recently described activity involved in processing of membrane-bound TNF-alpha.

Autocrine regulation of membrane transforming growth factor-alpha cleavage

Transforming growth factor alpha (TGF-alpha) is biosynthesized as a membrane-bound precursor protein, pro-TGF-alpha, that undergoes sequential endoproteolytic cleavages to release a soluble form of the factor. In the present study, we have analyzed the biosynthesis and regulation of TGF-alpha production in human tumor-derived cell lines that endogenously express pro-TGF-alpha and the epidermal growth factor (EGF) receptor. These cells biosynthesized membrane-anchored forms of the TGF-alpha that accumulated on the cell surface. Membrane-bound pro-TGF-alpha interacted with the EGF receptor, and complexes of receptor and pro-TGF-alpha contained tyrosine-phosphorylated receptor. Activation of the EGF receptor by soluble EGF or TGF-alpha had a dual effect on TGF-alpha production: an increase in pro-TGF-alpha mRNA levels and an increase in pro-TGF-alpha cleavage. These effects were largely prevented by preincubation with an anti-EGF receptor monoclonal antibody that blocked ligand binding. Growth factor autoinduction of cleavage could be stimulated by several second messenger pathways that are activated by the EGF receptor, including protein kinase C and intracellular calcium, and by other alternative mechanisms. EGF-stimulated cleavage of pro-TGF-alpha could be partially blocked by inhibition of these second messenger pathways. These results suggest that juxtacrine stimulation takes place in human tumor cells that coexpress both the EGF receptor and membrane-anchored TGF-alpha and that TGF-alpha is able to induce its own endoproteolytic cleavage by activating the EGF receptor.

TrkA receptor ectodomain cleavage generates a tyrosine-phosphorylated cell-associated fragment

The extracellular domain of several membrane-anchored proteins can be released as a soluble fragment by the action of a cell surface endoproteolytic system. This cleavage results in the generation of a soluble and a cell-bound fragment. In the case of proteins with signaling capability, such as tyrosine kinase receptors, the cleavage process may have an effect on the kinase activity of the cell-bound receptor fragment. By using several cell lines that express the TrkA neurotrophin receptor, we show that this receptor tyrosine kinase is cleaved by a proteolytic system that mimics the one that acts at the cell surface. TrkA cleavage is regulated by protein kinase C and several receptor agonists (including the TrkA ligand NGF), occurs at the ectodomain in a membrane-proximal region, and is independent of lysosomal function. TrkA cleavage results in the generation of a cell-associated fragment that is phosphorylated on tyrosine residues. Tyrosine phosphorylation of this fragment is not detected in TrkA mutants devoid of kinase activity, suggesting that phosphorylation requires an intact TrkA kinase domain, and is not due to activation of an intermediate intracellular tyrosine kinase. The increased phosphotyrosine content of the cell-bound fragment may thus reflect higher catalytic activity of the truncated fragment. We postulate that cleavage of receptor tyrosine kinases by this naturally occurring cellular mechanism may represent an additional mean for the regulation of receptor activity.

Human betacellulin, a member of the EGF family dominantly expressed in pancreas and small intestine, is fully active in a monomeric form

Betacellulin (BTC) was found to be expressed mainly in human pancreas and small intestine. This finding suggests that BTC possesses some specific function distinguished from the other members of epidermal growth factor (EGF) family. To clarify this function, the released form of human BTC has been expressed in E.coli, purified, and characterized. The recombinant human BTC was produced as an inclusion body. This material was dissolved in guanidine-HCl under reducing conditions, refolded, and purified through sequential liquid chromatography. Purified BTC was electrophoresed under reducing conditions and a molecular size of 18 kDa was determined, which is the supposed size of a dimer of the peptide. However, chemical analysis failed to show a covalently linked dimer. The molecular mass of BTC analyzed by mass spectrometry revealed it to be 9 kDa, which is consistent with theoretical value for a monomer. Recombinant BTC showed growth promoting activity for mouse fibroblasts and rat aortic smooth muscle cells which was equivalent to EGF On the other hand, BTC was found to exhibit a growth inhibitory effect on the cells overexpressing EGF receptor.

Exchanging the extracellular domain of amyloid precursor protein for horseradish peroxidase does not interfere with alpha-secretase cleavage of the beta-amyloid region, but randomizes secretion in Madin-Darby canine kidney cells

Secretory processing and polarized sorting of horseradish peroxidase fused to the amyloid precursor protein transmembrane domain were compared with those of wild-type amyloid precursor protein in COS and polarized Madin-Darby canine kidney (MDCK) cells. The cellular and secreted forms of the chimeric protein were enzymatically active in colorimetric and cytochemical assays after reconstitution with hemin and Ca2+. The peroxidase enzyme was secreted by a proteolytic process, similar to the parent amyloid precursor protein. In polarized MDCK cells, amyloid precursor protein was secreted exclusively in the basolateral compartment, while the peroxidase chimeric protein was secreted in both compartments. The basolateral sorting determinant for secretion must therefore be located in the extracellular domain of amyloid precursor protein. On the other hand, cell surface-associated peroxidase chimeric protein was similar to cell surface-associated wild-type amyloid precursor protein, mainly expressed at the basolateral side. The basolateral cell-surface expression, in contrast to the basolateral secretion, is therefore controlled by determinants in the cytoplasmic domain. Methylamine inhibited and bafilomycin slightly increased the basolateral secretion of both proteins, but both drugs strongly increased apical secretion. The default secretory pathway of COS cells and the basolateral (but not the apical) secretory pathway of MDCK cells are therefore comparably sensitive to methylamine and not to bafilomycin.

Phosphorylation of the cytosolic domain of peptidylglycine alpha-amidating monooxygenase

Peptidylglycine alpha-amidating monooxygenase (PAM) is a bifunctional enzyme that catalyzes the COOH-terminal alpha-amidation of neural and endocrine peptides through a two-step reaction carried out sequentially by its monooxygenase and lyase domains. PAM occurs in soluble and integral membrane forms. Metabolic labeling of stably transfected hEK-293 and AtT-20 cells showed that [32P]PO4(3-) was efficiently incorporated into Ser and Thr residues of membrane PAM but not into soluble PAM. Truncation of integral membrane PAM proteins (which terminate with Ser976) at Tyr936 eliminated their phosphorylation, suggesting that the COOH-terminal region of the protein was the site of phosphorylation. Recombinant PAM COOH-terminal domain was phosphorylated on Ser932 and Ser937 by protein kinase C (PKC). PAM-1 protein recovered from different subcellular fractions of stably transfected AtT-20 cells was differentially susceptible to calcium-dependent, staurosporine-inhibitable phosphorylation catalyzed by endogenous cytosolic protein kinase(s). Although phorbol ester treatment of hEK-293 cells expressing PAM-1 stimulated the cleavage/release of a bifunctional 105-kDa PAM protein, the effect was an indirect one since it was also observed in hEK-293 cells expressing a truncated PAM-1 protein that was not phosphorylated. AtT-20 cells expressing PAM-1 lacking one of the PKC sites (PAM-1/Ser937-->Ala) exhibited an altered pattern of PAM.PAM antibody internalization, with the mutant protein targeted to lysosomes upon internalization. Thus, phosphorylation of Ser937 in the COOH-terminal cytosolic domain of membrane PAM plays a role in a specific step in the targeting of this protein.

Cell surface localization of proteolysis of human endothelial angiotensin I-converting enzyme. Effect of the amino-terminal domain in the solubilization process

Angiotensin-converting enzyme (ACE) belongs to the type I class of ectoproteins and is solubilized by Chinese hamster ovary cells transfected with the full-length human ACE cDNA. ACE release in Chinese hamster ovary cells involves a proteolytic cleavage occurring in the carboxyl-terminal region, between Arg-1137 and Leu-1138. The subcellular localization of ACE proteolysis was established by pulse-chase experiments, cell surface immunolabeling, and biotinylation of radiolabeled mature proteins. The proteolysis of ACE takes place primarily at the plasma membrane. The solubilization of ACE is less than 2% within 1 h, is increased 2.4-fold by phorbol esters, but is not influenced by ionophores. An ACE mutant lacking the transmembrane domain and the cytosolic part (ACE delta COOH), is secreted at a faster rate without a carboxyl-terminal cleavage, and phorbol esters or ionophores have no effect on its rate of production in the medium. Therefore, the proteolysis of ACE is dependent on the presence of the membrane anchor and suggests that the secretase(s) involved is also membrane-associated. An ACE mutant lacking the amino-terminal domain (ACECF) is secreted 10-fold faster compared with wild-type ACE. The solubilization of ACECF occurs at the plasma membrane and is stimulated 2.7-fold by phorbol esters, and the cleavage site is localized between Arg-1227 and Val-1228. The amino-terminal domain of ACE slows down the proteolysis and seems to act as a "conformational inhibitor" of the proteolytic process, possibly via interactions with the "stalk" of ACE and the secretase(s) itself.

Regulated formation of Golgi secretory vesicles containing Alzheimer beta-amyloid precursor protein

Phorbol esters, activators of protein kinase C (PKC), regulate the relative utilization of alternative processing pathways for the Alzheimer beta-amyloid precursor protein (beta-APP) in intact cells, increasing the production of nonamyloidogenic soluble beta-APP (s beta-APP) and decreasing that of neurotoxic beta-amyloid (A beta) peptide. The molecular and cellular bases of PKC-regulated beta-APP cleavage are poorly understood. Here we demonstrate in a reconstituted cell-free system that activation of endogenous PKC increases formation from the trans-Golgi network of secretory vesicles containing beta-APP and that this effect can be mimicked by purified PKC. The results demonstrate directly that PKC is involved in regulation of secretory vesicle formation and provide a mechanism by which PKC may reduce the formation of the A beta peptide characteristic of Alzheimer disease.

Amino-terminal region of the beta-amyloid precursor protein activates mitogen-activated protein kinase

The secreted form of the beta-amyloid precursor protein (beta-APP) has previously been shown to stimulate mitogen-activated protein (MAP) kinases in PC-12 pheochromocytoma cells. The amino-terminal half of secreted beta-APP contains a region rich in cysteine residues reminiscent of cysteine-rich binding regions in other families of extracellular proteins. We found that reductive alkylation of disulfide linkages eliminated the ability of secreted beta-APP to activate MAP kinase. To confirm the role of the cysteine-rich amino-terminal region, fragments representing the amino- and carboxyl-terminal halves of secreted beta-APP were expressed in bacteria as fusion proteins and purified. Ten-minute treatment with the amino-terminal segment of beta-APP activated MAP kinase approximately 15-fold, while the carboxyl segment had no effect. The amino-terminal fragment, like intact secreted beta-APP, was substantially inactivated by reduction of sulfhydryl groups. These results suggest that the amino-terminal region of beta-APP is responsible for activation of MAP kinase and that it requires structural loops created by disulfide linkages for activity.