Proprotein convertase activity contributes to the processing of the Alzheimer's beta-amyloid precursor protein in human cells: evidence for a role of the prohormone convertase PC7 in the constitutive alpha-secretase pathway

The physiological maturation of the beta-amyloid precursor protein (betaAPP) leads to the secretion of a fragment termed APPalpha, after cleavage by a proteolytic enzyme called-secretase. In Alzheimer's disease, betaAPP undergoes exacerbated proteolytic attacks by beta- and gamma-secretases, which liberate a readily aggregatable 40-42-amino acid peptide called AP. We show here that overexpression of the prohormone convertase PC7 triggers increased secretion of APPalpha and lowers both Abeta40 and Abeta42 recoveries. Overexpression of alpha1-antitrypsin Portland (alpha1-PDX), which blocks mammalian precursor convertases of the constitutive secretory pathway, reverses the PC7-induced APPalpha increase as well as the decrease of Abeta40/42 in HEK293 cells. It is interesting that alpha1-PDX also lowers the level of APPalpha endogenously produced by mock-transfected HEK293 cells. Finally, a Jurkat clone stably expressing alpha1-PDX produces noticeably lower amounts of APPalpha. Therefore, this serpin affects endogenous a-secretase activity/pathway in distinct cell types. By contrast, alpha1-PDX does not alter the processing of presenilin 1 or its mutated congeners linked to some familial forms of Alzheimer's disease. Altogether, we demonstrate that a prohormone convertase participates in the alpha-secretase pathway of betaAPP maturation in human cells and concomitantly contributes to slowing the pathogenic route leading to the formation of Abeta. Our data strongly suggest that PC7 could fulfill such a role.

The subtilisin/kexin family of precursor convertases. Emphasis on PC1, PC2/7B2, POMC and the novel enzyme SKI-1

Proopiomelanocortin (POMC) is a precursor to various, bioactive peptides including ACTH, beta LPH, alpha MSH, and beta endorphin (beta END). Processing of POMC at dibasic residues is tissue-specific and is performed by either PC1 alone (resulting in ACTH and beta LPH, anterior pituitary corticotrophes) or by a combination of PC1 and PC2 (yielding alpha MSH and beta END, pituitary neurointermediate lobe and hypothalamus). The PC2-specific binding protein 7B2 is intimately involved in the zymogen activation of proPC2 into PC2. Structure-function studies of these enzymes demonstrated the presence of N- and C-terminal domains, as well as specific amino acids within the catalytic segment that influence the degree of activity of each enzyme and the interaction of PC2 with 7B2. The tissue distribution, plasticity of expression, and the multiple precursors that are differentially cleaved by PC1 and/or PC2, predict a wide array of combinatorial activities of these convertases within the endocrine and neuroendocrine system. The phenotypic consequences of the absence of genetic expression of either PC1 or PC2 are now explored using knockout mice and in human patients suffering from obesity and diabetes.

Enzymic characterization in vitro of recombinant proprotein convertase PC4

Proprotein convertase PC4A, a member of the subtilisin/kexin family of serine proteases, was obtained in enzymically active form following expression of vaccinia virus recombinant rat (r)PC4A in GH4C1 cells. It displayed maximal activity at pH 7.0 and a Ca(2+) concentration of 2.0 mM. Using PC4-specific antibodies, Western blot analysis of the medium revealed a major band at approximately 54 kDa, corresponding to the molecular size of mature rPC4A. Among the various peptidyl-[4-methylcoumarin 7-amide (MCA)] substrates tested, the one that was preferred the most by rPC4A was acetyl (Ac)-Arg-Lys-Lys-Arg-MCA, which is cleaved 9 times faster (as judged from V(max)/K(m) measurements) than the best furin and PC1 substrate, pGlu-Arg-Thr-Lys-Arg-MCA. Recombinant rPC4A, along with human (h)furin and hPC1, cleaved a 17-amino-acid synthetic peptide, YQTLRRRVKR downward arrowSLVVPTD (where downward arrow denotes site of cleavage, and the important basic residues are shown in bold), encompassing the junction between the putative pro-segment of rPC4A and the active enzyme, suggesting a possible auto-activation of the enzyme. In an effort to identify potential physiological substrates for PC4, studies were performed with pro-[insulin-growth-factor (IGF)]-derived synthetic peptides, namely Ac-PAKSAR downward arrowSVRA (IGF-I(66-75)) and Ac-PAKSER downward arrowDVST (IGF-II(63-72)), as well as two lysine mutants [(IGF-I(66-75)Lys(70)) and (IGF-II(63-72)Lys(67))]. Unlike PC1 and furin, rPC4A cleaved efficiently both IGF-I(66-75) and IGF-II(63-72), suggesting a possible role of PC4 in the maturation of IGF-I and -II. In contrast, the peptides with a position 2 (P2) lysine mutation, IGF-I(66-75)Lys(70) and IGF-II(63-72)Lys(67), were cleaved more efficiently by PC1 and furin compared with rPC4A. Furthermore, using synthetic peptides containing the processing sites of pituitary adenylate-cyclase-activating polypeptide (PACAP)-38, we were able to confirm that, of the two testicular enzymes PC4 and PC7, PC4 is the best candidate enzyme for maturation of PACAP. Our data suggest that rPC4A is a functionally active convertase, with a substrate specificity somewhat different from that of other convertases, namely KXXR downward arrow (where X denotes any other residue). As expected, p-chloromercuribenzoic acid and metal chelators such as EDTA, EGTA and trans-1,2-diaminocyclohexane-N,N,N', N'-tetraacetic acid inhibit the proteolytic activity of rPC4A, whereas it is activated by dithiothreitol. PC4A was also inhibited by transition-metal ions (Cu(2+)>Hg(2+)>Zn(2+) Ni(2+)>Co(2+)), as well as by small peptide semicarbazones (SCs), such as Arg-Lys-Lys-Arg-SC (K(i) 0.75 microM) and Arg-Ser-Lys-Arg-SC (K(i) 11.4 microM).

Proparathyroid hormone processing by the proprotein convertase-7: comparison with furin and assessment of modulation of parathyroid convertase messenger ribonucleic acid levels by calcium and 1,25-dihydroxyvitamin D3

We previously showed that the processing of proparathyroid hormone (proPTH) to PTH was accomplished most efficiently by furin (17). Colocalization studies demonstrated that furin is expressed in the parathyroid, whereas proprotein convertase (PC)1 and PC2 are not. Since that time, another member of the PC family, called PC7, has been identified. Here we show, using coinfection studies, that PC7, as well as furin, can appropriately cleave PTH from proPTH. ProPTH and PTH were purified from cell extracts by reversed-phase HPLC and were identified by Western blot analysis and delayed extraction matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Colocalization studies, using Northern blot and reverse transcriptase-PCR analyses, showed that PC7 messenger RNA (mRNA) is expressed in the parathyroid gland. Therefore, PC7, like furin, has the potential to be involved in the physiological processing of proPTH to PTH. The two major regulators of parathyroid cell synthetic and secretory activity are the extracellular fluid calcium and 1,25-dihydroxyvitamin D [1,25(OH)2D] levels. We investigated whether either of these agents might modulate processing of proPTH to PTH by altering parathyroid convertase gene expression. In both in vitro and in vivo systems in which regulation of PTH mRNA levels were clearly apparent, there was no effect of either calcium or 1,25(OH)2D3 on parathyroid furin or PC7 mRNA levels. This is in contrast to the processing of proinsulin to insulin in the pancreatic beta-cell, which is up-regulated by glucose stimulation of PC1 and PC2 synthesis.

Occurrence of an HIV-1 gp160 endoproteolytic activity in low-density vesicles and evidence for a distinct density distribution from endogenously expressed furin and PC7/LPC convertases

Human immunodeficiency virus (HIV) glycoprotein (gp) 160 processing by host cell proteinases is an essential step for viral fusion and infectivity. We have identified a rat liver subcellular fraction which specifically processes gp160 into gp120 and gp41. Using equilibration of microsomes in sucrose gradients, the gp160 cleavage activity was associated with particles equilibrating at low densities, well-separated from the endoplasmic reticulum, cis-Golgi network, Golgi stacks, lysosomes and plasma membrane. Its density distribution was compatible with light secretory vesicles derived from the trans-Golgi network (TGN) or to endosomes, but association with endosomes was not supported by free flow electrophoresis. Although furin and pro-protein convertase (PC) 7/LPC have been proposed as the major gp160 processing convertases, the rat liver microsomal gp160 processing activity was essentially resolved from furin and only partially overlapped PC7/LPC. These data suggest that proteinase(s) other than furin and PC7/LPC, presumably located in TGN-derived vesicles, may participate in the gp160 processing into gp120 and gp41.

Immunohistochemical distribution of the prohormone convertase PC5-A in rat brain

Prohormone convertase 5 is an endoprotease of the kexin/subtilisin-like family, which has been postulated to play a role in the proteolytic maturation of a variety of pro-peptides in the mammalian brain. In order to gain insight into the functional role of prohormone convertase 5 in the central nervous system, the regional, cellular and subcellular distributions of the enzyme were investigated by immunohistochemistry in rat brain using an N-terminal-directed specific antibody shown previously to recognize both the mature and unprocessed forms of the enzyme. Throughout the brain, prohormone convertase 5 immunoreactivity was concentrated within nerve cell bodies and proximal dendrites. No prohormone convertase 5 immunoreactivity was associated with astrocytes, as confirmed by the absence of prohormone convertase 5 immunolabeling in cells immunopositive for the glial protein S-100alpha. Within neurons, prohormone convertase 5 immunoreactivity was concentrated within the Golgi apparatus, as revealed immunohistochemically within the same sections using antibodies against the medial cisternae protein MG-160. It was also present within small vesicular-like elements distributed throughout the cytoplasm of perikarya and dendrites, but not of axons, as confirmed by its lack of co-localization with the synaptic terminal marker Dynamin-1. These results suggest that prohormone convertase 5 is active within early compartments of the neuronal regulated secretory pathway and that it is unlikely to be released with its processed substrates. At the regional level, prohormone convertase 5-immunoreactive perikarya were distributed extensively throughout the forebrain. The most numerous and intensely labeled were detected in the olfactory bulb, cerebral cortex, globus pallidus, endopeduncular and subthalamic nuclei, septum, diagonal band of Broca, magnocellular and medial preoptic areas, supraoptic and arcuate nuclei of the hypothalamus, and anterodorsal, laterodorsal, paraventricular and reticular nuclei of the thalamus. Moderate to dense neuronal labeling was also evident in the olfactory tubercle, caudate-putamen, claustrum, bed nucleus of the stria terminalis, substantia innominata, hippocampus, amygdala, and remaining thalamic and hypothalamic nuclei. This widespread distribution suggests that prohormone convertase 5 is involved in the processing of a variety of neuropeptide and/or neurotrophin precursors in mammalian brain.

Distribution and regulation of proconvertases PC1 and PC2 in human pituitary adenomas

Pituitary adenomas are members of the family of neuroendocrine cells and tumors which have secretory granules containing chromogranins/secretogranins and other proteins. Pituitary adenomas express the neuroendocrine specific proconvertases PC1 (also known as PC3) and PC2, which are important for the proteolytic processing of chromogranins/secretogranins molecules. We examined the distribution of PC1 and PC2 in primary cultures of 20 pituitary adenomas and analyzed the regulation of the proconvertase mRNAs and proteins by various secretagogues including hypothalamic hormones and phorbol ester to determine the role of PC1 and PC2 in CgA processing in pituitary adenomas. Although PC2 was present in all adenomas, there was a differential distribution of PC1 with PRL adenomas expressing lower levels of PC1 compared to other adenoma types by RT-PCR analysis, in situ hybridization and immunostaining. Treatment of primary cultures of pituitary adenomas with phorbol 12-myristrate 13-acetate (PMA) resulted in an increase in pancreastatin (PST) secretion in most pituitary adenomas and increased PC1 mRNA and protein expression in gonadotroph adenomas, but not in other types of adenomas. Analysis of a human pituitary adenoma cell line, immortalized by recombinant defective adenovirus (HP75), which expressed chromogranin A, FSH, PC1 and PC2 showed that PST was secreted by these immortalized cells. Treatment with TGF beta 1 resulted in an increase in PST secretion and in PC1 mRNA and protein. These results indicate that a) there is a differential distribution of PC1 in human pituitary adenomas with PRL adenomas expressing very little PC1 mRNA and protein and b) that PC1 expression in gonadotropin hormone-producing adenomas is regulated by PMA and TGF beta 1. These findings support the observation that chromogranin A is a substrate for the endoproteinase PC1 in human pituitary adenoma cells.

The RGD motif and the C-terminal segment of proprotein convertase 1 are critical for its cellular trafficking but not for its intracellular binding to integrin alpha5beta1

Cellular trafficking of subtilisin/kexin-like precursor convertases (PCs) may be regulated by a number of motifs, some of which are present within the P-domain and in the C-terminal sequence. Six of the seven known PCs contain a conserved RGD sequence within the P domain. In order to investigate the functional importance of this motif, we generated mutants of PC1 that contain a Myc tag epitope inserted between the prosegment and the catalytic subunit. Cellular expression of vaccinia virus recombinants revealed that this tag did not seem to influence the autocatalytic conversion of proPC1 into PC1 or its bioactivity. The two PC1 variants produced possess either the wild type RGD sequence or its RGE mutant. Stable transfectants of these variants in AtT20 cells revealed that similar to the wild type enzyme, PC1-RGD-Myc is sorted to secretory granules. In contrast, PC1-RGE-Myc exits the cell via the constitutive secretory pathway. In vitro, a 14-mer peptide spanning the RGD sequence of PC1, but not its RGE mutant, binds to cell surface vitronectin-binding integrins of Chinese hamster ovary cells. However, within the endoplasmic reticulum and in an RGD-independent fashion, integrin alpha5beta1 associates primarily with the zymogens proPC1, proPC1-DeltaC (missing the C-terminal 137 residues), as well as proPC2. Thus, the observed discrimination between the secretion routes of PC1-RGD and PC1-RGE does not implicate integrins such as alpha5beta1.

Differential sorting of nerve growth factor and brain-derived neurotrophic factor in hippocampal neurons

Nerve growth factor (NGF) is released through the constitutive secretory pathway from cells in peripheral tissues and nerves where it can act as a target-derived survival factor. In contrast, brain-derived neurotrophic factor (BDNF) appears to be processed in the regulated secretory pathway of brain neurons and secreted in an activity-dependent manner to play a role in synaptic plasticity. To determine whether sorting differences are intrinsic to the neurotrophins or reflect differences between cell types, we compared NGF and BDNF processing in cultured hippocampal neurons using a Vaccinia virus expression system. Three independent criteria (retention or release from cells after pulse-chase labeling, depolarization-dependent release, and immunocytochemical localization) suggest that the bulk of newly synthesized NGF is sorted into the constitutive pathway, whereas BDNF is primarily sorted into the regulated secretory pathway. Similar results occurred with AtT 20 cells, including those transfected with cDNAs encoding neurotrophin precursor-green fluorescent protein fusions. The NGF precursor, but not the BDNF precursor, is efficiently cleaved by the endoprotease furin in the trans-Golgi network (TGN). Blocking furin activity in AtT 20 cells with alpha1-PDX as well as increasing the expression of NGF precursor partially directed NGF into the regulated secretory pathway. Therefore, neurotrophins can be sorted into either the constitutive or regulated secretory pathways, and sorting may be regulated by the efficiency of furin cleavage in the TGN. This mechanism may explain how neuron-generated neurotrophins can act both as survival factors and as neuropeptides.

Molecular cloning, characterization of cDNA, and distribution of mRNA encoding the frog prohormone convertase PC1

Prohormone convertases (PCs) are calcium-dependent serine endoproteases of the subtilisin/kexin family that play a key role in the posttranslational processing of precursors for biologically active peptides. In this study, we have characterized the cDNA encoding PC1 in the European green frog Rana ridibunda. A frog brain cDNA library was screened by using a heterologous probe at low stringency, and a 2.3-kb cDNA clone encoding PC1 was isolated. This cDNA encodes a 736-residue protein with a 26-amino-acid signal peptide. Comparative structural analysis revealed that frog PC1 exhibits a high degree of amino acid identity with its mammalian counterparts, in particular in the subtilisin-like catalytic domain. Northern blot analysis resolved two major transcripts of 3.0 kb and 5.0 kb that were expressed differentially in the brain and pituitary. In situ hybridization studies showed that, in the frog brain, the highest densities of PC1 mRNA are present in the amygdala, the hypothalamus, and the anterior preoptic area. High concentrations of PC1 mRNA also were found in the pars distalis and pars intermedia of the pituitary, whereas the pars nervosa was devoid of hybridization signal. The wide distribution of PC1 mRNA in the brain and pituitary suggests that, in frog, PC1 is involved in the processing of a number of hormone and neuropeptide precursors.

Cellular localization and role of prohormone convertases in the processing of pro-melanin concentrating hormone in mammals

Melanin concentrating hormone (MCH) and neuropeptide EI (NEI) are two peptides produced from the same precursor in mammals, by cleavage at the Arg145-Arg146 site and the Lys129-Arg130 site, respectively. We performed co-localization studies to reveal simultaneously the expression of MCH mRNA and proconvertases (PCs) such as PC1/3 or PC2. In the rat hypothalamus, PC2 was present in all MCH neurons, and PC1/3 was present in about 15-20% of these cells. PC1/3 or PC2 was not found in MCH-positive cells in the spleen. In GH4C1 cells co-infected with vaccinia virus (VV):pro-MCH along with VV:furin, PACE4, PC1/3, PC2, PC5/6A, PC5/6B, or PC7, we observed only efficient cleavage at the Arg145-Arg146 site to generate mature MCH. Co-expression of pro-MCH together with PC2 and 7B2 resulted in very weak processing to NEI. Comparison of pro-MCH processing patterns in PC1/3- or PC2-transfected PC12 cells showed that PC2 but not PC1/3 generated NEI. Finally, we analyzed the pattern of pro-MCH processing in PC2 null mice. In the brain of homozygotic mutants, the production of mature NEI was dramatically reduced. In contrast, MCH content was increased in the hypothalamus of PC2 null mice. In the spleen, a single large MCH-containing peptide was identified in both wild type and PC2 null mice. Together, our data suggest that pro-MCH is processed differently in the brain and in peripheral organs of mammals. PC2 is the key enzyme that produces NEI, whereas several PCs may cleave at the Arg145-Arg146 site to generate MCH in neuronal cell types.

Mammalian subtilisin/kexin isozyme SKI-1: A widely expressed proprotein convertase with a unique cleavage specificity and cellular localization

Using reverse transcriptase-PCR and degenerate oligonucleotides derived from the active-site residues of subtilisin/kexin-like serine proteinases, we have identified a highly conserved and phylogenetically ancestral human, rat, and mouse type I membrane-bound proteinase called subtilisin/kexin-isozyme-1 (SKI-1). Computer databank searches reveal that human SKI-1 was cloned previously but with no identified function. In situ hybridization demonstrates that SKI-1 mRNA is present in most tissues and cells. Cleavage specificity studies show that SKI-1 generates a 28-kDa product from the 32-kDa brain-derived neurotrophic factor precursor, cleaving at an RGLT downward arrowSL bond. In the endoplasmic reticulum of either LoVo or HK293 cells, proSKI-1 is processed into two membrane-bound forms of SKI-1 (120 and 106 kDa) differing by the nature of their N-glycosylation. Late along the secretory pathway some of the membrane-bound enzyme is shed into the medium as a 98-kDa form. Immunocytochemical analysis of stably transfected HK293 cells shows that SKI-1 is present in the Golgi apparatus and within small punctate structures reminiscent of endosomes. In vitro studies suggest that SKI-1 is a Ca2+-dependent serine proteinase exhibiting a wide pH optimum for cleavage of pro-brain-derived neurotrophic factor.

Inhibition of proprotein convertases-1, -7 and furin by diterpines of Andrographis paniculata and their succinoyl esters

Studies were performed to investigate the prohormone/proprotein convertase (PC)-inhibitory properties of chemical constituents of the medicinally active plant Andrographis paniculata (AP; from the family Acanthaceae), also known as 'King of Bitters'. Among the individual components tested against the clinically important convertases, furin and PC1, neoandrographolide (a C3 O-glucoside derivative of the major constituent andrographolide) exhibited the highest inhibitory action with an IC50 of 53.5 microM against furin. The data further revealed that although andrographolide, the major bitter principle of AP, exhibited a relatively small enzyme inhibition (IC50=1.0 mM and Ki=200 microM against furin), upon succinoylation, its inhibitory action against the above convertases was enhanced significantly with a Ki in the low micromolar range (<30 microM), suggesting that a specific structural modification of the andrographolide skeleton may be exploited to develop a new class of non-peptide inhibitors of PCs. When tested against PC7, these succinoylated derivatives of andrographolide also displayed strong inhibitory action, with Ki values again in the low micromolar range. This potentially interesting observation may be attributed to the reported anti-HIV property of 14-dehydroandrographolide succinic acid monoester (DASM). It is suggested here that DASM, by virtue of this protease inhibitory property, possibly acts by suppressing the proteolytic cleavage of envelope glycoprotein gp160 of HIV, which is known to be PC-mediated, particularly by furin and PC7.

The pore-forming toxin proaerolysin is activated by furin

Aerolysin is secreted as an inactive dimeric precursor by the bacterium Aeromonas hydrophila. Proteolytic cleavage within a mobile loop near the C terminus of the protoxin is required for oligomerization and channel formation. This loop contains the sequence KVRRAR432, which should be recognized by mammalian proprotein convertases such as furin, PACE4, and PC5/6A. Here we show that these three proteases cleave proaerolysin after Arg-432 in vitro, yielding active toxin. We also investigated the potential role of these enzymes in the in vivo activation of the protoxin. We found that Chinese hamster ovary cells were able to convert the protoxin to aerolysin in the absence of exogenous proteases and that activation did not require internalization of the toxin. The furin inhibitor alpha1-antitrypsin Portland reduced the rate of proaerolysin activation in vivo, and proaerolysin processing was even further reduced in furin-deficient FD11 Chinese hamster ovary cells. The cells were also less sensitive to proaerolysin than wild type cells; however, transient transfection of FD11 cells with the cDNA encoding furin conferred normal sensitivity to the protoxin. Together these findings argue that furin catalyzes the cell-surface activation of proaerolysin in vivo.

Delta-1 activation of notch-1 signaling results in HES-1 transactivation

The Notch receptor is involved in many cell fate determination events in vertebrates and invertebrates. It has been shown in Drosophila melanogaster that Delta-dependent Notch signaling activates the transcription factor Suppressor of Hairless, leading to an increased expression of the Enhancer of Split genes. Genetic evidence has also implicated the kuzbanian gene, which encodes a disintegrin metalloprotease, in the Notch signaling pathway. By using a two-cell coculture assay, we show here that vertebrate Dl-1 activates the Notch-1 cascade. Consistent with previous data obtained with active forms of Notch-1 a HES-1-derived promoter construct is transactivated in cells expressing Notch-1 in response to Dl-1 stimulation. Impairing the proteolytic maturation of the full-length receptor leads to a decrease in HES-1 transactivation, further supporting the hypothesis that only mature processed Notch is expressed at the cell surface and activated by its ligand. Furthermore, we observed that Dl-1-induced HES-1 transactivation was dependent both on Kuzbanian and RBP-J activities, consistent with the involvement of these two proteins in Notch signaling in Drosophila. We also observed that exposure of Notch-1-expressing cells to Dl-1 results in an increased level of endogenous HES-1 mRNA. Finally, coculture of Dl-1-expressing cells with myogenic C2 cells suppresses differentiation of C2 cells into myotubes, as previously demonstrated for Jagged-1 and Jagged-2, and also leads to an increased level of endogenous HES-1 mRNA. Thus, Dl-1 behaves as a functional ligand for Notch-1 and has the same ability to suppress cell differentiation as the Jagged proteins do.

Molecular characterization, enzymatic analysis, and purification of murine proprotein convertase-1/3 (PC1/PC3) secreted from recombinant baculovirus-infected insect cells

A cDNA coding for the murine proprotein convertase-1 (mPC1 also known as mPC3 or mSPC3) was inserted into the Autographa californica nuclear polyhedrosis virus. Following infection of Spodoptera frugiperda cells, the recombinant N-glycosylated protein is secreted into the cell culture medium from which it can be purified to homogeneity as a fully enzymatically active enzyme. Two major secreted molecular forms of mPC1 with apparent molecular weights of 85 and 71 kDa, respectively, and a minor one of 75 kDa are immunodetected in the medium. Automated NH2-terminal sequencing reveals that all three forms result from processing at the predicted zymogen activation site whereas both the 75- and the 71-kDa forms are truncated at their COOH-terminus. Labeling by an active-site titrant demonstrates that the 85-kDa form is optimally labeled at near neutral pH whereas the COOH-truncated forms are optimally labeled at acidic pH. Additionally it is shown that the 85-kDa mPC1 is transformed into the COOH-truncated forms following in vitro incubation at acidic pH levels and in presence of calcium. Concomitantly, the transformation from 85 to 71 kDa is accompanied by a 10- to 40-fold increase in enzymatic activity upon assaying at pH 6.0. The 71-kDa form can be recovered after purification at a level of 1 to 1.5 mg per liter of cell culture medium and is enzymatically stable only in the pH range from 5.0 to 6.5. Cells treated with tunicamycin show a drastically reduced secretion of the convertase in the medium but are not affected by swainsonine and deoxymannojirimycin. Finally, the 85-kDa secreted mPC1 is shown to be sulfated.

The vasopressin precursor is not processed in the hypothalamus of Wolfram syndrome patients with diabetes insipidus: evidence for the involvement of PC2 and 7B2

Wolfram syndrome (WS) is characterized by optic atrophy, insulin-dependent diabetes mellitus, vasopressin (VP)-sensitive diabetes insipidus, and neurosensory hearing loss. Here we report a disturbance in VP precursor processing in the supraoptic and paraventricular nuclei of WS patients. In these patients with diabetes insipidus we could hardly detect any cellular immunoreactivity for processed VP in the supraoptic and paraventricular nuclei. On the other hand, in the paraventricular nucleus a considerable number of cells immunoreactive for the VP precursor were present. In addition, the proprotein convertase PC2 and the molecular chaperone 7B2 were absent. As expression of PC2 and 7B2 was detected in the nearby nucleus basalis of Meynert of one WS patient and in the anterior lobe of the other WS patient, the absence of the two proteins in the paraventricular nucleus was not due to mutations in their genes. These results indicate that in WS patients with diabetes insipidus, not only does VP neuron loss occur in the supraoptic nucleus, but there is also a defect in VP precursor processing.

The LIM homeobox protein mLIM3/Lhx3 induces expression of the prolactin gene by a Pit-1/GHF-1-independent pathway in corticotroph AtT20 cells

mLIM3, a member of the LIM homeobox family, was recently demonstrated to be critical for proliferation and differentiation of the pituitary cell lineage. Using a pool of degenerate oligonucleotides we determined the DNA sequence ANNAGGAAA(T/C)GA(CIG)AA as the set preferentially recognized by mLIM3. A nearly identical sequence is found in the prolactin (PRL) promoter, within a 15-mer stretch from nucleotides (nts) -218 to -204 which is highly conserved between human, rat, and bovine. In order to test the hypothesis of a transcriptional effect of mLIM3 on the prolactin promoter, stable transfectants of mLIM3 cDNA in AtT20 tumor cells revealed that PRL mRNA expression was induced in 3 separate stable clones. Gel retardation experiments performed using nuclear extracts isolated from one of the AtT20/mLIM3 stable transfectants revealed affinity towards the 15-mer element of the PRL promoter. From these results, we propose that the PRL promoter element (nts -218 to -204) could be functional in vivo. Finally, we demonstrate that in AtT20 cells prolactin mRNA expression is not induced by the Pit-1/GHF-1 pathway and that growth hormone mRNA is not detected concomitantly with prolactin. We conclude that mLIM3 may play a key role in inducing PRL gene expression in lactotrophs by binding to a conserved motif close to a Pit-1/GHF-1 site within the proximal PRL promoter.

The Notch1 receptor is cleaved constitutively by a furin-like convertase

The Notch receptor, which is involved in numerous cell fate decisions in invertebrates and vertebrates, is synthesized as a 300-kDa precursor molecule (p300). We show here that proteolytic processing of p300 is an essential step in the formation of the biologically active receptor because only the cleaved fragments are present at the cell surface. Our results confirm and extend recent reports indicating that the Notch receptor exists at the plasma membrane as a heterodimeric molecule, but disagree as to the nature of the protease that is responsible for the cleavage that takes place in the extracellular region. We report here that constitutive processing of murine Notch1 involves a furin-like convertase. We show that the calcium ionophore A23187 and the alpha1-antitrypsin variant, alpha 1-PDX, a known inhibitor of furin-like convertases, inhibit p300 processing. When expressed in the furin-deficient Lovo cell line, p300 is not processed. In vitro digestion of a recombinant Notch-derived substrate with purified furin allowed mapping of the processing site to the carboxyl side of the sequence RQRR (amino acids 1651-1654). Mutation of these four amino acids (and of two secondary dibasic furin sites located nearby) completely abolished processing of the Notch1 receptor.

Residues unique to the pro-hormone convertase PC2 modulate its autoactivation, binding to 7B2 and enzymatic activity

The prohormone convertase PC2 is one of the major subtilisin/kexin-like enzymes responsible for the formation of small bioactive peptides in neural and endocrine cells. This convertase is unique among the members of the subtilisin/kexin-like mammalian serine proteinase family in that it undergoes zymogen processing of its inactive precursor proPC2 late along the secretory pathway and requires the help of a PC2-specific binding protein known as 7B2. We hypothesized that some of these unique properties of PC2 are dictated by the presence of PC2-specific amino acids, which in the six other known mammalian convertases are otherwise conserved but distinct. Accordingly, six sites were identified within the catalytic segment of PC2. Herein we report on the site-directed mutagenesis of Tyr194 and of the oxyanion hole Asp309 and the consequences of such mutations on the cellular expression and enzyme activity of PC2. The data show that the Y194D mutation markedly increases the ex vivo ability of PC2 to process proopiomelanocortin (POMC) into beta-endorphin in cells devoid of 7B2, e.g. BSC40 cells. In these cells, expression of native PC2 does not result in the secretion of measurable in vitro activity against a pentapeptide fluorogenic substrate. In contrast, secreted Y194D-PC2 exhibited significant enzymatic activity, even in the absence of 7B2. Based on co-immunoprecipitations and Western blots, binding assays indicate that Tyr194 participates in the interaction of PC2 with 7B2, and that the oxyanion hole Asp309 is critical for the binding of proPC2 with pro7B2.

In vitro cleavage of internally quenched fluorogenic human proparathyroid hormone and proparathyroid-related peptide substrates by furin. Generation of a potent inhibitor

The cleavage of parathyroid hormone (PTH) from its precursor proparathyroid hormone (pro-PTH) is accomplished efficiently by the proprotein convertase furin (Hendy, G. N., Bennett, H. P. J., Gibbs, B. F., Lazure, C., Day, R., and Seidah, N. G. (1995) J. Biol. Chem. 270, 9517-9525). We also showed that a synthetic peptide comprising the -6 to +7 sequence of human pro-PTH is appropriately cleaved by purified furin in vitro. The human pro-PTH processing site Lys-Ser-Val-Lys-Lys-Arg differs from the consensus furin site Arg-Xaa-(Lys/Arg)-Arg that is represented by Arg-Arg-Leu-Lys-Arg in the cleavage site of pro-PTH-related peptide (pro-PTHrP). An earlier study demonstrated that an internally quenched fluorogenic substrate bearing an O-aminobenzoyl fluorescent donor at the NH2 terminus and an acceptor 3-nitrotyrosine near the COOH terminus was appropriately cleaved by the convertases furin and PC1 (Jean, F., Basak, A., DiMaio, J., Seidah, N. G., and Lazure, C. (1995) Biochem. J. 307, 689-695). Here, we have synthesized a series of internally quenched fluorogenic substrates based upon the pro-PTH and pro-PTHrP sequences to determine which residues are important for furin cleavage. Purified recombinant furin and PC1 cleaved the human pro-PTH internally quenched substrate at the appropriate site in an identical manner to that observed with the nonfluorescent peptide. Several substitutions in the P6-P3 sequence were well tolerated; however, replacement of the Lys at the P6 position with Gly and replacement of the P3 Lys by an acidic residue led to markedly compromised cleavage by furin. Furin activity was very sensitive to substitution in P' positions. Replacement of Ser at P1' with Gly and Val at P2' with Ala generated substrates that were less well cleaved. Substitution at the P1' position of Val for Ser in conjunction with Ala for Val at P2', as well as a single substitution of Lys for Val at P2', generated specific inhibitors of furin cleavage. The findings of this study open the way to the rational design of inhibitors of furin with therapeutic potential.

Attenuation of the polypeptide 7B2, prohormone convertase PC2, and vasopressin in the hypothalamus of some Prader-Willi patients: indications for a processing defect

7B2 is a neuroendocrine chaperone interacting with the prohormone convertase PC2 in the regulated secretory pathway. Its gene is located near the Prader-Willi syndrome (PWS) region on chromosome 15. In a previous study we were able to show 7B2 immunoreactivity in the supraoptic nucleus (SON) or the paraventricular nucleus (PVN) in only three of five PWS patients. Here we report that in contrast with five other PWS patients, the neurons in the hypothalamic SON and PVN of the two 7B2-immunonegative PWS patients also failed to show any reaction using two antibodies directed against processed vasopressin (VP). On the other hand, even these two cases reacted normally with five antibodies that recognize different parts of the VP precursor. This finding pointed to a processing defect. Indeed, the same patients had no PC2 immunoreactivity in the SON or PVN, whereas PC1 immunoreactivity was only slightly diminished. In conclusion, in the VP neurons of two PWS patients, greatly reduced amounts of 7B2 and PC2 are present, resulting in diminished VP precursor processing.

Immunocytochemical localization of the prohormone convertases PC1 and PC2 in rat prolactin cells

The prohormone convertases PC1 and PC2 are subtilisin-related endopeptidases that process prohormone and neuropeptide precursors. Using different ultrastructural immunocytochemical approaches, we have investigated their intracellular distribution in a neuroendocrine cell type that has not been examined thus far, the rat anterior pituitary lactotrope. These cells secrete mainly prolactin and also express the neuroendocrine-specific protein secretogranin II, which is considered a peptide precursor. Our study provides evidence for the expression of PC1 and PC2 in rat lactotropes and provides new information on their subcellular localization. Apart from their presence in the secretory granules, PC1 and PC2 displayed different major localization along the secretory pathway. PC1 immunoreactivity was concentrated in the Golgi apparatus, whereas PC2 immunoreactivity was prominent in the rough endoplasmic reticulum (RER). These observations provide morphological support for previous biochemical analysis of proPC1 and proPC2 post-translational processing, which has demonstrated that PC1 exits very rapidly from the RER, whereas PC2 is retained much longer in this compartment. (J Histochem Cytochem 46:101-108, 1998)

Proteolytic processing of chromogranin B and secretogranin II by prohormone convertases

Two experimental approaches were used to study the processing of chromogranin B and secretogranin II by prohormone convertases. In GH3 cells various prohormone convertases were overexpressed together with the substrate chromogranin B by use of a vaccinia virus infection system. PC1 appeared to be by far the most active enzyme and converted chromogranin B to several smaller molecules, including the peptide PE-11. In brain this peptide is cleaved physiologically from chromogranin B. Some processing of chromogranin B and formation of free PE-11 were also observed with PC2 and PACE4. Furin produced larger fragments, whereas PC5-A and PC5-B had negligible effects. As a second model, PC12 cells were stably transfected with PC1 or PC2 to investigate the processing of endogenous chromogranins. Both enzymes effectively cleaved chromogranin B and secretogranin II, liberating the peptides PE-11 and secretoneurin, respectively. However, in transfection experiments the ability to generate the free peptides was more pronounced with PC2 than with PC1. The extent of proprotein processing achieved by prohormone convertases apparently differed depending on the experimental system applied. This suggests that in vivo mechanisms to support and fine-tune the activity of the processing enzymes exist, which might be overlooked by using only one methodological approach.