Prohormone convertase PC5 is a candidate processing enzyme for prorenin in the human adrenal cortex

Proprotein convertases regulate trafficking and maturation of key proteins within the secretory pathway

Proprotein Convertases (PCs) are serine endoproteases that regulate the homeostasis of protein substrates in the cell. The PCs family counts 9 members-PC1/3, PC2, PC4, PACE4, PC5/6, PC7, Furin, SKI-1/S1P, and PCSK9. The first seven PCs are known as Basic Proprotein Convertases due to their propensity to cleave after polybasic clusters. SKI-1/S1P requires the additional presence of hydrophobic residues for processing, whereas PCSK9 is catalytically dead after autoactivation and exerts its functions using mechanisms alternative to direct cleavage. All PCs traffic through the canonical secretory pathway, reaching different compartments where the various substrates reside. Despite PCs members do not share the same subcellular localization, most of the cellular organelles count one or more Proprotein Convertases, including ER, Golgi stack, endosomes, secretory granules, and plasma membranes. The widespread expression of these enzymes at the systemic level speaks for their importance in the homeostasis of a large number of biological functions. Among others, PCs cleave precursors of hormones and growth factors and activate receptors and transcription factors. Notably, dysregulation of the enzymatic activity of Proprotein Convertases is associated to major human pathologies, such as cardiovascular diseases, cancer, diabetes, infections, inflammation, autoimmunity diseases, and Parkinson. In the current COVID-19 pandemic, Furin has further attracted the attention as a key player for conferring high pathogenicity to SARS-CoV-2. Here, we review the Proprotein Convertases family and their most important substrates along the secretory pathway. Knowledge about the complex functions of PCs is important to identify potential drug strategies targeting this class of enzymes.

Cleavage of the soluble (pro)renin receptor (sATP6AP2) in the placenta

INTRODUCTION

The (pro)renin receptor (ATP6AP2) is cleaved and released as soluble ATP6AP2 (sATP6AP2). The sATP6AP2 is detected in plasma and urine and is elevated in women with gestational diabetes and preeclampsia. The source and cleavage pathway of sATP6AP2 in pregnancy is unknown. The syncytiotrophoblast is the major placental secretory layer and is in direct contact with maternal blood. Both FURIN and Site 1 protease (MBTPS1) cleave sATP6AP2 in non-placental cells. We postulated that ATP6AP2 was cleaved by FURIN and/or MBTPS1 and that sATP6AP2 is secreted by the placental syncytiotrophoblast.

METHODS

Term primary trophoblast cells were transfected with FURIN siRNA, negative control siRNA or vehicle. In a separate experiment, primary trophoblasts were treated with a pro-protein convertase inhibitor (DEC-RVKR-CMK), an MBTPS1 inhibitor (PF 429242) or vehicle. Trophoblasts were left to spontaneously syncytialise before cells and supernatants were collected and intracellular and extracellular sATP6AP2 levels analysed by immunoblot.

RESULTS

sATP6AP2 is secreted by placental trophoblasts. Levels of intra and extra-cellular sATP6AP2 decrease with syncytialisation (P = 0.01 and P = 0.02, respectively), as do FURIN mRNA (P = 0.0003) and protein (P = 0.0007). FURIN siRNA decreased FURIN mRNA and protein levels (both P < 0.0001). Neither FURIN siRNA or PF 429242 affected sATP6AP2 levels. DEC-RVKR-CMK significantly decreased extracellular sATP6AP2 protein levels (P = 0.02).

DISCUSSION

Soluble ATP6AP2 is secreted by placental trophoblasts and levels decrease with syncytialisation. DEC-RVKR-CMK, a broad inhibitor of pro-protein convertases reduced extracellular sATP6AP2 levels, but FURIN siRNA and MBTPS1 inhibition had no effect. Hence, a convertase other than FURIN or MBTPS1 is most likely responsible for placental sATP6AP2 secretion.

Curcumin affects proprotein convertase activity: elucidation of the molecular and subcellular mechanism

Proprotein convertases (PCs) form a group of serine endoproteases that are essential for the activation of proproteins into their active form. Some PCs have been proposed to be potential therapeutic targets for cancer intervention because elevated PC activity has been observed in many different cancer types and because many of the PC substrates, such as pro-IGF-1R, pro-TGF-beta, pro-VEGF, are involved in signaling pathways related to tumor development. Curcumin, reported to possess anticancer activity, also affects many of these pathways. We therefore investigated the effect of curcumin on PC activity. Our results show that curcumin inhibits PC activity in a cell lysate-based assay but not in vitro. PC zymogen maturation in the endoplasmic reticulum appears to be inhibited by curcumin. Treating cells with thapsigargin or cyclopiazonic acid, two structurally unrelated inhibitors of the sarco- and endoplasmic reticulum Ca(2+)ATPase (SERCA), also hampered both the PC zymogen maturation and the PC activity. Importantly, curcumin, like the SERCA inhibitors, impaired ATP-driven (45)Ca(2+) uptake in the endoplasmic reticulum. These results indicate that curcumin likely restrains PC activity by inhibiting SERCA-mediated Ca(2+)-uptake activity. Experiments in three colon cancer cell lines confirm that curcumin inhibits both the (45)Ca(2+) uptake and PC activity, notably the processing of pro-IGF-1R. Both curcumin and thapsigargin inhibit the anchorage-independent growth of these three colon carcinoma cell lines. In conclusion, our findings indicate that curcumin inhibits PC zymogen maturation and consequently PC activity and that its inhibitory effect on Ca(2+) uptake into the ER allows and is sufficient to explain this phenomenon.

Renin release: sites, mechanisms, and control

In the adult organism, systemically circulating renin almost exclusively originates from the juxtaglomerular cells in the afferent arterioles of the kidneys. These cells share similarities with pericytes and myofibro-blasts. They store renin in a vesicular network and granules and release it in a regulated fashion. The release mode of renin is not understood; in particular, the involvement of SNARE proteins is unknown. Renin release is acutely increased via the cAMP signaling pathway, which is triggered mainly by catecholamines and other G(s)-coupled agonists, and is inhibited by calcium-related pathways that are commonly activated by vasoconstrictors. Renin release from juxtaglomerular cells is directly modulated in an inverse fashion by the blood pressure inside the afferent arterioles and by the chloride content in the tubule fluid at the macula densa segment of the distal tubule. Renin release is stimulated by nitric oxide and by prostanoids released by neighboring endothelial and macula densa cells. Steady-state renin concentrations in the plasma are determined essentially by the number of renin-producing cells in the afferent arterioles, which changes in parallel with challenges to the renin-angiotensin-aldosterone system.

Neuropeptides, growth factors, and cytokines: a cohort of informational molecules whose expression is up-regulated by the stress-associated slow transmitter PACAP in chromaffin cells

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a co-transmitter with acetylcholine at the adrenomedullary synapse, mediating sustained hormone secretion and regulation of cellular plasticity in response to stress at the level of gene transcription. Here we have extended our investigation of PACAP-regulated neuroendocrine cell-specific genes from PC12 cells to PC12 cells expressing physiological levels of the PAC1hop receptor found on chromaffin cells in vivo. PACAP induces in these PC12_bPAC1hop cells an additional cohort of genes, compared to PC12 cells, enriched in informational molecules including cytokines, neuropeptides, and growth factors. Using two newly developed microarray platforms for expressed bovine transcripts, we further examined PACAP-induced genes in bovine chromaffin cells during a period of exposure (6 h) corresponding to a period of prolonged metabolic or psychogenic stress in vivo during which PACAP is released from the splanchnic nerve onto chromaffin cells. As in PC12_bPAC1hop cells, PACAP induced in bovine chromaffin cells a cohort of genes encoding secretory proteins, identified by tiling for cellular localization using Ingenuity Pathway Analysis, which were highly enriched in informational molecules (secreted proteins acting at extracellular receptors). These included cytokines, growth factors and hormones, as well as converting enzymes, or protease inhibitors modulating converting enzyme function. Several neuropeptide prohormone transcripts not previously shown to be PACAP-regulated in chromaffin cells, such as thyrotropin-releasing hormone, and tachykinin precursor 1, were identified. Identification of this cohort of informational molecule-encoding transcripts suggests a wider, more integrative role for PACAP as a co-transmitter specific to stress transduction in the adrenal medulla.

Physiology of Kidney Renin

The protease renin is the key enzyme of the renin-angiotensin-aldosterone cascade, which is relevant under both physiological and pathophysiological settings. The kidney is the only organ capable of releasing enzymatically active renin. Although the characteristic juxtaglomerular position is the best known site of renin generation, renin-producing cells in the kidney can vary in number and localization. (Pro)renin gene transcription in these cells is controlled by a number of transcription factors, among which CREB is the best characterized. Pro-renin is stored in vesicles, activated to renin, and then released upon demand. The release of renin is under the control of the cAMP (stimulatory) and Ca2+(inhibitory) signaling pathways. Meanwhile, a great number of intrarenally generated or systemically acting factors have been identified that control the renin secretion directly at the level of renin-producing cells, by activating either of the signaling pathways mentioned above. The broad spectrum of biological actions of (pro)renin is mediated by receptors for (pro)renin, angiotensin II and angiotensin-( 1 – 7 ).

Regulation of renin release by local and systemic factors

The renin-angiotensin system (RAS) is critically involved in the regulation of the salt and volume status of the body and blood pressure. The activity of the RAS is controlled by the protease renin, which is released from the renal juxtaglomerular epithelioid cells into the circulation. Renin release is regulated in negative feedback-loops by blood pressure, salt intake, and angiotensin II. Moreover, sympathetic nerves and renal autacoids such as prostaglandins and nitric oxide stimulate renin secretion. Despite numerous studies there remained substantial gaps in the understanding of the control of renin release at the organ or cellular level. Some of these gaps have been closed in the last years by means of gene-targeted mice and advanced imaging and electrophysiological methods. In our review, we discuss these recent advances together with the relevant previous literature on the regulation of renin release.

Renin Release

The aspartyl-protease renin is the key regulator of the renin-angiotensin-aldosterone system, which is critically involved in salt, volume, and blood pressure homeostasis of the body. Renin is mainly produced and released into circulation by the so-called juxtaglomerular epithelioid cells, located in the walls of renal afferent arterioles at the entrance of the glomerular capillary network. It has been known for a long time that renin synthesis and secretion are stimulated by the sympathetic nerves and the prostaglandins and are inhibited in negative feedback loops by angiotensin II, high blood pressure, salt, and volume overload. In contrast, the events controlling the function of renin-secreting cells at the organ and cellular level are markedly less clear and remain mysterious in certain aspects. The unravelling of these mysteries has led to new and interesting insights into the process of renin release.

The proprotein convertases are potential targets in the treatment of dyslipidemia

The family of the secretory proprotein convertases (PCs) comprises seven basic amino acid (aa)-specific subtilisin-like serine proteinases known as PC1/3, PC2, furin, PC4, PC5/6, PACE4 and PC7, and two other PCs, SKI-1 (subtilisin-kexin isozyme-1)/S1P (site-1 protease) and PCSK9 (proprotein convertase subtilisin kexin 9) that cleave at nonbasic residues. Except for the testicular PC4, all the other convertases are expressed in brain and peripheral organs and play a critical role in various functions including the production of diverse neuropeptides as well as growth factors and receptors, the regulation of cellular adhesion/migration, cholesterol and fatty acid homeostasis, and growth/differentiation of progenitor cells. Some of these convertases process proteins that are implicated in pathologies, including cancer malignancies, tissue regeneration, and viral infections. The implication of some of these convertases in sterol/lipid metabolism has only recently been appreciated. SKI-1/S1P activates the synthesis of cholesterol and fatty acids as well as the LDL receptor (LDLR), whereas PCSK9 inactivates the LDLR. Moreover, furin, PC5 and/or, PACE4 inactivates endothelial and lipoprotein lipases. Humans and mice exhibiting either a gain or loss of function of PCSK9 through specific point mutations or knockouts develop hypercholesterolemia and hypocholesterolemia phenotypes, respectively. A PCSK9 inhibitor in combination with statins offers a most promising therapeutic target to treat cardiovascular disorders including dyslipidemias. Specific inhibitors/modulators of the other PCs should find novel therapeutic applications in the control of PC-regulated pathologies.

The proprotein convertase SKI-1/S1P. In vitro analysis of Lassa virus glycoprotein-derived substrates and ex vivo validation of irreversible peptide inhibitors

Herein we designed, synthesized, tested, and validated fluorogenic methylcoumarinamide (MCA) and chloromethylketone-peptides spanning the Lassa virus GPC cleavage site as substrates and inhibitors for the proprotein convertase SKI-1/S1P. The 7-mer MCA (YISRRLL-MCA) and 8-mer MCA (IYISRRLL-MCA) are very efficiently cleaved with respect to both the 6-mer MCA (ISRRLL-MCA) and point mutated fluorogenic analogues, except for the 7-mer mutant Y253F. The importance of the P7 phenylic residue was confirmed by digestions of two 16-mer non-fluorogenic peptidyl substrates that differ by a single point mutation (Y253A). Because NMR analysis of these 16-mer peptides did not reveal significant structural differences at recognition motif RRLL, the P7 Tyr residue is likely important in establishing key interactions within the catalytic pocket of SKI-1. Based on these data, we established through analysis of pro-ATF6 and pro-SREBP-2 cellular processing that decanoylated chloromethylketone 7-mer, 6-mer, and 4-mer peptides containing the core RRLL sequence are irreversible and potent ex vivo SKI-1 inhibitors. Although caution must be exercised in using these inhibitors in in vitro reactions, as they can also inhibit the basic amino acid-specific convertase furin, within cells and when used at concentrations < or = 100 microM these inhibitors are relatively specific for inhibition of SKI-1 processing events, as opposed to those performed by furin-like convertases.

Identification of novel endometrial targets for contraception

Successful embryo implantation is a critical step in establishing pregnancy and requires appropriate preparation of the endometrium to provide a transient state of "uterine receptivity." The most essential of the molecular events determining receptivity may therefore provide potential targets for postcoital contraception. Using the mouse as a model, we identified molecules specifically regulated in the endometrium at very early implantation: these were monoclonal nonspecific suppressor factor beta (MNSFbeta), splicing factor SC35, a novel protease of the HtrA family, termed HtrA3, calcium-binding protein (CaBP)-d9k (calbindin d9k) and proprotein convertase 6 (PC6). All of these molecules were also expressed in human endometrium, with the exception of CaBP-d9k, which was represented by the functionally similar CaBP-d28k. Appropriate spatial and temporal expressions of mRNA and protein were demonstrated for all five candidate molecules in mouse and primate (human and rhesus monkey) endometrium during the menstrual cycle and early pregnancy. Functional studies in mice established that blocking production of the CaBPs and PC6 within the endometrium completely prevented implantation and thus provided proof of principle that these molecules are potential contraceptive targets.

Small-intestinal dysfunction accompanies the complex endocrinopathy of human proprotein convertase 1 deficiency

We have previously described the only reported case of human proprotein convertase 1 (PC1) deficiency, in a female (Subject A) with obesity, hypogonadism, hypoadrenalism, and reactive hypoglycemia. We now report the second case of human PC1 deficiency (Subject B), also due to compound heterozygosity for novel missense and nonsense mutations. While both subjects shared the phenotypes of obesity, hypoadrenalism, reactive hypoglycemia, and elevated circulating levels of certain prohormones, the clinical presentation of Subject B was dominated by severe refractory neonatal diarrhea, malabsorptive in type. Subsequent investigation of Subject A revealed marked small-intestinal absorptive dysfunction, which was not previously clinically suspected. We postulate that PC1, presumably in the enteroendocrine cells, is essential for the normal absorptive function of the human small intestine. The differences in the nature and severity of presentation between the two cases cannot readily be explained on the basis of allelic heterogeneity, as the nonsense and missense mutations from both subjects had comparably severe effects on the catalytic activity of PC1. Despite Subject A's negligible PC1 activity, some mature ACTH and glucagon-like peptide 17-36(amide) were detectable in her plasma, suggesting that the production of these hormones, at least in humans, does not have an absolute dependence on PC1. The presence of severe obesity and the absence of growth retardation in both subjects contrast markedly with the phenotype of mice lacking PC1 and suggest that the precise physiological repertoire of this enzyme may vary between mammalian species.

Structure-function analysis of the prosegment of the proprotein convertase PC5A

To investigate if some residues within the prosegment of PC5A are important for its optimal proteolytic function, various PC5A mutants were cellularly expressed, and their processing activities were compared using pro-vascular endothelial growth factor C (pro-VEGF-C) as a substrate. Although wild type PC5A almost completely processes pro-VEGF-C, a prosegment deletion as well as both P1 mutants of the primary (R116A) and secondary (R84A) autocatalytic cleavage sites are inactive. The in vitro inhibitory potency of various decapeptides mimicking the C-terminal sequence of PC5 prosegment (pPC5) revealed that the native (107)QQVVKKRTKR(116) peptide is a nanomolar inhibitor, whereas its P6 mutant K111H is more selective toward PC5A than Furin. In vitro activity assays using the bacterially expressed pPC5 and its mutants revealed them to be very potent nanomolar inhibitors (IC(50)) and only approximately 6-fold more selective inhibitors of PC5A versus Furin. Expression of the preprosegment of PC5 (ppPC5) and its mutants in Chinese hamster ovary FD11 cells overexpressing pro-VEGF-C with either PC5A or Furin showed them to be as good inhibitors of PC5A as the serpin alpha1-antitrypsin Portland (alpha1-PDX), ppFurin, or ppPACE4 but less potent toward overexpressed Furin. In conclusion, cleavages of the prosegment of PC5A at both Arg(116) and Arg(84) are required for PC5A cellular activity, and ppPC5 is a very potent but modestly selective cellular inhibitor of PC5A.

Molecular characterization of the cDNA and localization of the mRNA encoding the prohormone convertase PC5-A in the European green frog

The structure and distribution of PC5-A, a prohormone convertase that is thought to be involved in post-translational processing of peptide hormone and neuropeptide precursors, have not been investigated in submammalian vertebrates. In the present study, we characterized the cDNA encoding PC5-A in the European green frog Rana esculenta. The frog PC5-A cDNA encodes a 913-amino acid protein that encompasses a 28-amino acid signal peptide, the Asp/His/Ser catalytic triad found in all serine proteinases of the subtilisin family, and two potential N-linked glycosylation sites located in a C-terminal cysteine-rich domain. Reverse transcriptase polymerase chain reaction amplification showed that PC5-A mRNA is expressed in various organs including the brain, spinal cord, pituitary, lung, liver, intestine, and testis, but not in the stomach and pancreas. The distribution of PC5-A mRNA in the frog brain was studied by in situ hybridization histochemistry. Intense expression was observed in the mitral cellular layer of the olfactory bulb, the nucleus of the diagonal band of Broca, the anterior preoptic area, and the suprachiasmatic and ventral hypothalamic nuclei. The expression pattern of PC5-A mRNA in the central nervous system of anuran amphibians was consistent with the implication of this prohormone convertase in the processing of various neuropeptide precursors.

Inhibitory potency and specificity of subtilase-like pro-protein convertase (SPC) prodomains

The SPCs (subtilisin-like pro-protein convertases) are a family of enzymes responsible for the proteolytic processing of numerous precursor proteins of the constitutive and regulated secretory pathways. SPCs are themselves synthesized as inactive zymogens. Activation of SPCs occurs via the intramolecular autocatalytic removal of the prodomain. SPC prodomains have been proposed as templates in the development of potent and specific SPC inhibitors. In this study, we investigated the specificity and potency of complete prodomains and short C-terminal prodomain peptides of each SPC on highly purified, soluble enzyme preparations of human SPC1, SPC6, and SPC7. Progress curve kinetic analysis of prodomain peptides and complete prodomains showed competitive inhibitory profiles in the low nanomolar range. Complete prodomains were 5-100 times more potent than C-terminal prodomain peptides, suggesting that N-terminal determinants are involved in the recognition process. However, complete prodomains and prodomain peptides exhibit only a partial specificity toward their cognate enzyme. Ala-scan structure activity studies indicated the importance of basic residues in the P(4), P(5), and P(6) positions for inhibition of SPC1. In contrast, hydrophobic residues in P(6) and P(7), as well as basic residues in P(4) and P(5), were critical for inhibition of SPC7. Our data demonstrated that the use of prodomains as specific inhibitors acting in trans would be of limited usefulness, unless modified into more specific compounds.

Post-translational processing of beta-secretase (beta-amyloid-converting enzyme) and its ectodomain shedding. The pro- and transmembrane/cytosolic domains affect its cellular activity and amyloid-beta production

Processing of the beta-amyloid precursor protein (betaAPP) by beta- and gamma-secretases generates the amyloidogenic peptide Abeta, a major factor in the etiology of Alzheimer's disease. Following the recent identification of the beta-secretase beta-amyloid-converting enzyme (BACE), we herein investigate its zymogen processing, molecular properties, and cellular trafficking. Our data show that among the proprotein convertase family members, furin is the major converting enzyme of pro-BACE into BACE within the trans-Golgi network of HK293 cells. While we demonstrate that the 24-amino acid prosegment is required for the efficient exit of pro-BACE from the endoplasmic reticulum, it may not play a strong inhibitory role since we observe that pro-BACE can produce significant quantities of the Swedish mutant betaAPP(sw) beta-secretase product C99. BACE is palmitoylated at three Cys residues within its transmembrane/cytosolic tail and is sulfated at mature N-glycosylated moieties. Data with three different antibodies show that a small fraction of membrane-bound BACE is shed into the medium and that the extent of ectodomain shedding is palmitoylation-dependent. Overexpression of full-length BACE causes a significant increase in the production of C99 and a decrease in the alpha-secretase product APPsalpha. Although there is little increase in the generation of Abeta by full-length BACE, overexpression of either a soluble form of BACE (equivalent to the shed form) or one lacking the prosegment leads to enhanced Abeta levels. These findings suggest that the shedding of BACE may play a role in the amyloidogenic processing of betaAPP.

A furin-like convertase mediates propeptide cleavage of BACE, the Alzheimer's beta -secretase

The novel transmembrane aspartic protease BACE (for Beta-site APP Cleaving Enzyme) is the beta-secretase that cleaves amyloid precursor protein to initiate beta-amyloid formation. As such, BACE is a prime therapeutic target for the treatment of Alzheimer's disease. BACE, like other aspartic proteases, has a propeptide domain that is removed to form the mature enzyme. BACE propeptide cleavage occurs at the sequence RLPR downward arrowE, a potential furin recognition motif. Here, we explore the role of furin in BACE propeptide domain processing. BACE propeptide cleavage in cells does not appear to be autocatalytic, since an inactive D93A mutant of BACE is still cleaved appropriately. BACE and furin co-localize within the Golgi apparatus, and propeptide cleavage is inhibited by brefeldin A and monensin, drugs that disrupt trafficking through the Golgi. Treatment of cells with the calcium ionophore, leading to inhibition of calcium-dependent proteases including furin, or transfection with the alpha(1)-antitrypsin variant alpha(1)-PDX, a potent furin inhibitor, dramatically reduces cleavage of the BACE propeptide. Moreover, the BACE propeptide is not processed in the furin-deficient LoVo cell line; however, processing is restored upon furin transfection. Finally, in vitro digestion of recombinant soluble BACE with recombinant furin results in complete cleavage only at the established E46 site. Taken together, our results strongly suggest that furin, or a furin-like proprotein convertase, is responsible for cleaving the BACE propeptide domain to form the mature enzyme.

Hydrolysis by plasma kallikrein of fluorogenic peptides derived from prorenin processing site

Human plasma kallikrein (HPK) activates plasma prorenin to renin, and the physiological significance of this activation is still unknown. In this paper we investigated the efficiency and the cleavage pattern of the hydrolysis by HPK of the internally quenched fluorescent peptides (qf-peptides) derived from the amino acid sequence of human prorenin cleavage site. The peptide Abz-F-S-Q-P-M-K-R-L-T-L-G-N-T-T-Q-EDDnp (Abz=ortho-aminobenzoic acid, and EDDnp=N-[2,4-dinitrophenyl]-ethylene diamine), that corresponds to the amino acid sequence P(7) to P(7)' of human prorenin cleavage site, is hydrolyzed at the correct processing site (R-L bond) with k(cat)/K(m)=85 mM(-1) s(-1). Alanine was scanned in all positions from P(5) to P(5)' in order to investigate the substrate specificity requirements of HPK. The qf-peptides derived from the equivalent segment of rat prorenin, that has Lys-Lys as basic amino acid pair, and the peptide Abz-NVTSPVQ-EDDnp that contains the proposed cleavage site of rat prorenin have very low susceptibility to hydrolysis by rat plasma kallikrein. These data are according to the previously reported absence of rat plasma prorenin activation by rat plasma kallikrein (RPK), and with the view that prorenin activation in rat requires alternative enzymes and/or mechanism. All the obtained peptides described in this paper were also assayed with bovine trypsin that was taken as a reference protease because it is commonly used to activate prorenin.

The Kex2p proregion is essential for the biosynthesis of an active enzyme and requires a C-terminal basic residue for its function

The Saccharomyces cerevisiae prohormone-processing enzyme Kex2p is biosynthesized as an inactive precursor extended by its N-terminal proregion. Here we show that deletion of the proregion renders Kex2p inactive both in vivo and in vitro. Absence of the proregion impaired glycosylation and stability and resulted in the retention of the enzyme in the endoplasmic reticulum. These phenotypes were partially complemented by expression of the proregion in trans. Trans complementation was specific to Kex2p proregion because expression of any of the seven mammalian prohormone convertase propeptides had no effect. These data are consistent with a model whereby Kex2p proregion functions as an intramolecular chaperone and indicate that covalent linkage to the protein is not an absolute requirement for proregion function. Furthermore, extensive mutagenesis revealed that, in addition to their function as proteolytic recognition sites, C-terminal basic residues play an active role in proregion-dependent Kex2p activation.

Transmural pressure inhibits prorenin processing in juxtaglomerular cell

Pressure control of renin secretion involves a complex integration of shear stress, stretch, and transmural pressure (TP). This study was designed to delineate TP control of renin secretion with minimal influence of shear stress or stretch and to determine its mechanism. Rat juxtaglomerular (JG) cells were applied to a TP-loading apparatus for 12 h. In cells conditioned with atmospheric pressure or atmospheric pressure + 40 mmHg, renin secretion rate (RSR) averaged 29.6 +/- 3.7 and 14.5 +/- 3.3% (P < 0.05, n = 8 cultures), respectively, and active renin content (ARC) averaged 47.3 +/- 4.6 and 38.4 +/- 3.4 ng of ANG I. h(-1). million cells(-1) (P < 0.05, n = 10 cultures), respectively. Total renin content and renin mRNA levels were unaffected by TP. The TP-induced decrease in RSR was prevented by Ca(2+)-free medium and the Ca(2+) channel blocker verapamil and was attenuated by thapsigargin and caffeine, which deplete intracellular Ca(2+) stores. Thapsigargin and caffeine, but not Ca(2+)-free medium or verapamil, prevented TP-induced decreases in ARC. The adenylate cyclase activator forskolin did not modulate TP-induced decreases in RSR or ARC. These findings suggest that TP not only stimulates Ca(2+) influx but also inhibits prorenin processing through an intracellular Ca(2+) store-dependent mechanism and thus inhibits active renin secretion by JG cells.

Prorenin processing by cathepsin B in vitro and in transfected cells

Renin, which catalyzes the initial proteolytic cleavage reaction in the production of angiotensins, is first synthesized as a zymogen, prorenin, and requires the proteolytic removal of an amino-terminal prosegment for activation in vivo. The lysosomal hydrolase cathepsin B has been proposed as a prorenin processing enzyme based on reports of its co-localization with renin in the secretory granules of certain tissues and its ability to activate prorenin in vitro. In the current study, scanning mutagenesis was used to identify the amino acids which determine the site selectivity of prorenin cleavage by human cathepsin B in vitro. Co-expression assays in AtT-20 cells were also used to test for the ability of cathepsin B to cleave human prorenin within cells. Our results suggest that a basic lysine residue at the -2 position from the cleavage site is required for cathepsin B cleavage of prorenin in vitro and that the structure of prorenin itself may account for the selection of the proper cleavage site. In addition, although cathepsin B appears to be correctly sorted to lysosomes, the enzyme exhibits prorenin processing activity in transfected AtT-20 cells, raising the question of the cellular localization in which the processing event occurs.

Two activation states of the prohormone convertase PC1 in the secretory pathway

PC1, a neuroendocrine member of the prohormone convertase family of serine proteinases, is implicated in the processing of proproteins in the secretory pathway. PC1 is synthesized as a zymogen and cleaves not only its own profragment in the endoplasmic reticulum, but a subset of protein substrates in the Golgi apparatus and in the Golgi-distal compartments of the regulated secretory pathway. Likewise, mouse PC1 (mPC1) has previously been shown to cleave human prorenin in GH4 cells (that contain secretory granules) while being unable to cleave prorenin in cells, such as Chinese hamster ovary (CHO) or BSC-40, which are devoid of secretory granules. In the current study, we show that removal of a C-terminal tail of mPC1 allows the efficient cleavage of prorenin in the constitutive secretory pathway of CHO cells. The C-terminal tail thus appears to act as an inhibitor of PC1 activity against certain substrates in the endoplasmic reticulum and Golgi apparatus, and its removal, which occurs naturally in secretory granules, may explain the observed granule-specific processing of certain proproteins. These results also demonstrate that PC1 is present in a partially active state prior to the secretory granules where it is processed to a maximally active state.

Prorenin activation and prohormone convertases in the mouse As4.1 cell line

The precise identification of prorenin-processing enzymes has been hampered by the very low abundance of juxtaglomerular cells in the kidney. Recently, an immortalized renin-producing renal tumor cell line (As4.1) has been proposed as a model to carry out such studies. Despite the fact that they contain secretory granules, we found no evidence (on the basis of enzymatic assays of renin activity in the supernatant of the cells and of immunoprecipitations experiments) that the As4.1 cells can secrete active renin through the regulated pathway. As4.1 cells produce only renin-1, as they derive from a strain of mice expressing only one renin gene. However, stable transfection of these cells with a renin-2 expression plasmid increased the capacity of this cell line to secrete active renin in the regulated pathway. Northern blot and reverse transcriptase-polymerase chain reaction amplification (RT-PCR) assays revealed that furin, PACE4 and PC5 were the only members of the proprotein convertase (PC) family to be present in these cells. As PC5 is the only such enzyme with the demonstrated ability to process mouse prorenin 2, it may constitute a candidate enzyme for the processing of prorenin-2 in mouse juxtaglomerular cells. However, it is not likely to be involved in the processing of mouse prorenin 1.