Deletion of the gene encoding proprotein convertase 5/6 causes early embryonic lethality in the mouse

Identifying Genetic Signatures Associated with Oncogene-Induced Replication Stress in Osteosarcoma and Screening for Potential Targeted Drugs

Osteosarcoma is the most common type of primary malignant bone tumor. Due to the lack of selectivity and sensitivity of chemotherapy drugs to tumor cells, coupled with the use of large doses, chemotherapy drugs often have systemic toxicity. The use of modern sequencing technology to screen tumor markers in a large number of tumor samples is a common method for screening highly specific and selective anti-tumor drugs. This study aims to identify potential biomarkers using the latest reported gene expression signatures of oncogene-induced replication stress (ORS) in aggressive cancers, and potential anti-osteosarcoma drugs were screened in different drug databases. In this study, we obtained 89 osteosarcoma-related samples in the TARGET database, all of which included survival information. According to the median expression of each of six reported ORS gene markers (NAT10/DDX27/ZNF48/C8ORF33/MOCS3/MPP6), we divided 89 osteosarcoma gene expression datasets into a high expression group and a low expression group and then performed a differentially expressed gene (DEG) analysis. The coexisting genes of 6 groups of DEGs were used as replication stress-related genes (RSGs) of osteosarcoma. Then, key RSGs were screened using LASSO regression, a Cox risk proportional regression prognostic model and a tenfold cross-validation test. GSE21257 datasets collected from the Gene Expression Omnibus (GEO) database were used to verify the prognostic model. The final key RSGs selected were used in the L1000PWD and DGIdb databases to mine potential drugs. After further validation by the prognostic model, we identified seven genes associated with ORS in osteosarcoma as key RSGs, including transcription factor 7 like 2 (TCF7L2), solute carrier family 27 member 4 (SLC27A4), proprotein convertase subtilisin/kexin type 5 (PCSK5), nucleolar protein 6 (NOL6), coiled-coil-coil-coil-coil-helix domain containing 4 (CHCHD4), eukaryotic translation initiation factor 3 subunit B (EIF3B), and synthesis of cytochrome C oxidase 1 (SCO1). Then, we screened the seven key RSGs in two drug databases and found six potential anti-osteosarcoma drugs (D GIdb database: repaglinide, tacrolimus, sirolimus, cyclosporine, and hydrochlorothiazide; L1000PWD database: the small molecule VU-0365117-1). Seven RSGs (TCF7L2, SLC27A4, PCSK5, NOL6, CHCHD4, EIF3B, and SCO1) may be associated with the ORS gene signatures in osteosarcoma. Repaglinide, tacrolimus, sirolimus, cyclosporine, hydrochlorothiazide and the small molecule VU-0365117-1 are potential therapeutic drugs for osteosarcoma.

Conserved Expression and Functionality of Furin between Chickens and Ducks as an Activating Protease of Highly Pathogenic Avian Influenza Virus Hemagglutinins

Highly pathogenic avian influenza viruses (HPAIVs) typically emerge from low-pathogenic avian influenza viruses (LPAIVs) of the H5 and H7 subtypes upon spillover from wild aquatic birds into poultry. The conversion from LPAIV to HPAIV is characterized by the acquisition of a multibasic cleavage site (MBCS) at the proteolytic cleavage site in the viral binding and fusion protein, hemagglutinin (HA), resulting in cleavage and activation of HA by ubiquitously expressed furin-like proteases. The ensuing HPAIVs disseminate systemically in gallinaceous poultry, are endotheliotropic, and cause hemorrhagic disease with high mortality. HPAIV infections in wild aquatic birds are generally milder, often asymptomatic, and generally not associated with systemic dissemination nor endotheliotropic. As MBCS cleavage by host proteases is the main virulence determinant of HPAIVs in poultry, we set out to determine whether cleavage of HPAIV HA by host proteases might influence the observed species-specific pathogenesis and tropism. Here, we sequenced, cloned, and characterized the expression and functionality of duck furin. The furin sequence was strongly conserved between chickens and ducks, and duck furin cleaved HPAIV and tetrabasic HA in an overexpression system, confirming its functionality. Furin was expressed ubiquitously and to similar extents in duck and chicken tissues, including in primary duck endothelial cells, which sustained multicycle replication of H5N1 HPAIV but not LPAIVs. In conclusion, differences in furin-like protease biology between wild aquatic birds and gallinaceous poultry are unlikely to largely determine the stark differences observed in species-specific pathogenesis of HPAIVs. IMPORTANCE HPAIV outbreaks are a global concern due to the health risks for poultry, wildlife, and humans and their major economic impact. The number of LPAIV-to-HPAIV conversions, which is associated with spillover from wild birds to poultry, has been increasing over recent decades. Furthermore, H5 HPAIVs from the A/goose/Guangdong/1/96 lineage have been circulating in migratory birds, causing increasingly frequent epizootics in poultry and wild birds. Milder symptoms in migratory birds allow for dispersion of HPAIVs over long distances, justifying the importance of understanding the pathogenesis of HPAIVs in wild birds. Here, we examined whether host proteases are a likely candidate to explain some differences in the degree of HPAIV systemic dissemination between avian species. This is the first report to show that furin function and expression is comparable between chickens and ducks, which renders the hypothesis unlikely that furin-like protease differences influence the HPAIV species-specific pathogenesis and tropism.

Proprotein Convertases and the Complement System

Proteins destined for secretion - after removal of the signal sequence - often undergo further proteolytic processing by proprotein convertases (PCs). Prohormones are typically processed in the regulated secretory pathway, while most plasma proteins travel though the constitutive pathway. The complement system is a major proteolytic cascade in the blood, serving as a first line of defense against microbes and also contributing to the immune homeostasis. Several complement components, namely C3, C4, C5 and factor I (FI), are multi-chain proteins that are apparently processed by PCs intracellularly. Cleavage occurs at consecutive basic residues and probably also involves the action of carboxypeptidases. The most likely candidate for the intracellular processing of complement proteins is furin, however, because of the overlapping specificities of basic amino acid residue-specific proprotein convertases, other PCs might be involved. To our surprise, we have recently discovered that processing of another complement protein, mannan-binding lectin-associated serine protease-3 (MASP-3) occurs in the blood by PCSK6 (PACE4). A similar mechanism had been described for the membrane protease corin, which is also activated extracellularly by PCSK6. In this review we intend to point out that the proper functioning of the complement system intimately depends on the action of proprotein convertases. In addition to the non-enzymatic components (C3, C4, C5), two constitutively active complement proteases are directly activated by PCs either intracellularly (FI), or extracellularly (MASP-3), moreover indirectly, through the constitutive activation of pro-factor D by MASP-3, the activity of the alternative pathway also depends on a PC present in the blood.

Effects of Maternal Diabetes and Diet on Gene Expression in the Murine Placenta

Adverse exposures during pregnancy have been shown to contribute to susceptibility for chronic diseases in offspring. Maternal diabetes during pregnancy is associated with higher risk of pregnancy complications, structural birth defects, and cardiometabolic health impairments later in life. We showed previously in a mouse model that the placenta is smaller in diabetic pregnancies, with reduced size of the junctional zone and labyrinth. In addition, cell migration is impaired, resulting in ectopic accumulation of spongiotrophoblasts within the labyrinth. The present study had the goal to identify the mechanisms underlying the growth defects and trophoblast migration abnormalities. Based upon gene expression assays of 47 candidate genes, we were able to attribute the reduced growth of diabetic placenta to alterations in the Insulin growth factor and Serotonin signaling pathways, and provide evidence for Prostaglandin signaling deficiencies as the possible cause for abnormal trophoblast migration. Furthermore, our results reinforce the notion that the exposure to maternal diabetes has particularly pronounced effects on gene expression at midgestation time points. An implication of these findings is that mechanisms underlying developmental programming act early in pregnancy, during placenta morphogenesis, and before the conceptus switches from histiotrophic to hemotrophic nutrition.

Mouse Models of Human Proprotein Convertase Insufficiency

The kexin-like proprotein convertases perform the initial proteolytic cleavages that ultimately generate a variety of different mature peptide and proteins, ranging from brain neuropeptides to endocrine peptide hormones, to structural proteins, among others. In this review, we present a general introduction to proprotein convertase structure and biochemistry, followed by a comprehensive discussion of each member of the kexin-like subfamily of proprotein convertases. We summarize current knowledge of human proprotein convertase insufficiency syndromes, including genome-wide analyses of convertase polymorphisms, and compare these to convertase null and mutant mouse models. These mouse models have illuminated our understanding of the roles specific convertases play in human disease and have led to the identification of convertase-specific substrates; for example, the identification of procorin as a specific PACE4 substrate in the heart. We also discuss the limitations of mouse null models in interpreting human disease, such as differential precursor cleavage due to species-specific sequence differences, and the challenges presented by functional redundancy among convertases in attempting to assign specific cleavages and/or physiological roles. However, in most cases, knockout mouse models have added substantively both to our knowledge of diseases caused by human proprotein convertase insufficiency and to our appreciation of their normal physiological roles, as clearly seen in the case of the furin, proprotein convertase 1/3, and proprotein convertase 5/6 mouse models. The creation of more sophisticated mouse models with tissue- or temporally-restricted expression of specific convertases will improve our understanding of human proprotein convertase insufficiency and potentially provide support for the emerging concept of therapeutic inhibition of convertases.

Cells with Many Talents: Lymphatic Endothelial Cells in the Brain Meninges

The lymphatic system serves key functions in maintaining fluid homeostasis, the uptake of dietary fats in the small intestine, and the trafficking of immune cells. Almost all vascularized peripheral tissues and organs contain lymphatic vessels. The brain parenchyma, however, is considered immune privileged and devoid of lymphatic structures. This contrasts with the notion that the brain is metabolically extremely active, produces large amounts of waste and metabolites that need to be cleared, and is especially sensitive to edema formation. Recently, meningeal lymphatic vessels in mammals and zebrafish have been (re-)discovered, but how they contribute to fluid drainage is still not fully understood. Here, we discuss these meningeal vessel systems as well as a newly described cell population in the zebrafish and mouse meninges. These cells, termed brain lymphatic endothelial cells/Fluorescent Granular Perithelial cells/meningeal mural lymphatic endothelial cells in fish, and Leptomeningeal Lymphatic Endothelial Cells in mice, exhibit remarkable features. They have a typical lymphatic endothelial gene expression signature but do not form vessels and rather constitute a meshwork of single cells, covering the brain surface.

Effect of neurotensin on cultured mouse preimplantation embryos

Previously, we revealed that neurotensin (NTS) derived from the oviduct and uterus can function during fertilization. However, little is known about NTS actions on the pre-implantation embryo after fertilization. Here, we found that pro-Nts mRNA is expressed in the oviduct and uterus during when preimplantation embryos develop and an increase in mRNA level in the uterus is induced by human chorionic gonadotropin (hCG) treatment. Expression of mRNA for two NTS receptors, Ntr1 and Ntr3, was found throughout these stages, whereas Ntr2 mRNA was not detected, suggesting that NTS signaling occurred through NTR1 and NTR3. Supplementation of 1, 10, 100 or 1000 nM NTS to embryo culture medium after fertilization showed that 100 nM NTS significantly improved the blastocyst formation. In comparison, the total number of cells and inner cell mass ratio of blastocysts was not significant different between the 0 nM and 100 nM NTS treatment groups. These results indicate that NTS has a positive effect upon preimplantation embryo development in vitro.

Protease propeptide structures, mechanisms of activation, and functions

Proteases are a diverse group of hydrolytic enzymes, ranging from single-domain catalytic molecules to sophisticated multi-functional macromolecules. Human proteases are divided into five mechanistic classes: aspartate, cysteine, metallo, serine and threonine proteases, based on the catalytic mechanism of hydrolysis. As a protective mechanism against uncontrolled proteolysis, proteases are often produced and secreted as inactive precursors, called zymogens, containing inhibitory N-terminal propeptides. Protease propeptide structures vary considerably in length, ranging from dipeptides and propeptides of about 10 amino acids to complex multifunctional prodomains with hundreds of residues. Interestingly, sequence analysis of the different protease domains has demonstrated that propeptide sequences present higher heterogeneity compared with their catalytic domains. Therefore, we suggest that protease inhibition targeting propeptides might be more specific and have less off-target effects than classical inhibitors. The roles of propeptides, besides keeping protease latency, include correct folding of proteases, compartmentalization, liganding, and functional modulation. Changes in the propeptide sequence, thus, have a tremendous impact on the cognate enzymes. Small modifications of the propeptide sequences modulate the activity of the enzymes, which may be useful as a therapeutic strategy. This review provides an overview of known human proteases, with a focus on the role of their propeptides. We review propeptide functions, activation mechanisms, and possible therapeutic applications.

FURIN Inhibition Reduces Vascular Remodeling and Atherosclerotic Lesion Progression in Mice

Objective- Atherosclerotic coronary artery disease is the leading cause of death worldwide, and current treatment options are insufficient. Using systems-level network cluster analyses on a large coronary artery disease case-control cohort, we previously identified PCSK3 (proprotein convertase subtilisin/kexin family member 3; FURIN) as a member of several coronary artery disease-associated pathways. Thus, our objective is to determine the role of FURIN in atherosclerosis. Approach and Results- In vitro, FURIN inhibitor treatment resulted in reduced monocyte migration and reduced macrophage and vascular endothelial cell inflammatory and cytokine gene expression. In vivo, administration of an irreversible inhibitor of FURIN, α-1-PDX (α1-antitrypsin Portland), to hyperlipidemic Ldlr^-/- mice resulted in lower atherosclerotic lesion area and a specific reduction in severe lesions. Significantly lower lesional macrophage and collagen area, as well as systemic inflammatory markers, were observed. MMP2 (matrix metallopeptidase 2), an effector of endothelial function and atherosclerotic lesion progression, and a FURIN substrate was significantly reduced in the aorta of inhibitor-treated mice. To determine FURIN's role in vascular endothelial function, we administered α-1-PDX to Apoe^-/- mice harboring a wire injury in the common carotid artery. We observed significantly decreased carotid intimal thickness and lower plaque cellularity, smooth muscle cell, macrophage, and inflammatory marker content, suggesting protection against vascular remodeling. Overexpression of FURIN in this model resulted in a significant 67% increase in intimal plaque thickness, confirming that FURIN levels directly correlate with atherosclerosis. Conclusions- We show that systemic inhibition of FURIN in mice decreases vascular remodeling and atherosclerosis. FURIN-mediated modulation of MMP2 activity may contribute to the atheroprotection observed in these mice.

A single domain antibody against the Cys- and His-rich domain of PCSK9 and evolocumab exhibit different inhibition mechanisms in humanized PCSK9 mice

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a secreted protein that binds and escorts the low density lipoprotein receptor (LDLR) into the lysosomal degradation pathway. Prescribed monoclonal antibodies (mAbs) against PCSK9 prevent its binding to the LDLR, and result in ~60% lower LDL cholesterol (LDLc) levels. Although efficient, mAbs are expensive. Hence other PCSK9 inhibitors are needed. For screening purpose, we developed C57BL/6J mice expressing the human PCSK9 gene under the control of its own promoter, but lacking endogenous mouse PCSK9. All lines recapitulate the endogenous PCSK9 expression pattern. The Tg2 line that expresses physiological levels of human PCSK9 (hPCSK9) was selected to characterize the inhibitory properties of a previously reported single domain antibody (sdAb), PKF8-mFc, which binds the C-terminal domain of PCSK9. Upon intraveinous injection of 10 mg/kg, PKF8-mFc and the mAb evolocumab neutralized ~50% and 100% of the hPCSK9 impact on total cholesterol (TC) levels, respectively, but PKF8-mFc had a more sustained effect. PKF8-mFc barely affected hPCSK9 levels, whereas evolocumab promoted a 4-fold increase 3 days post-injection, suggesting very different inhibitory mechanisms. The present study also shows that the new transgenic mice are well suited to screen a variety of hPCSK9 inhibitors.

Osteopontin as a novel substrate for the proprotein convertase 5/6 (PCSK5) in bone

Seven proprotein convertases cleave the basic amino acid consensus sequence K/R-X_n-K/R↓ (where n=0, 2, 4 or 6 variable amino acids) to activate precursor proteins. Despite similarities in substrate specificity, basic amino acid-specific proprotein convertases have a distinct tissue distribution allowing for enzymatic actions on tissue-resident substrates. Proprotein convertase 5/6 (PC5/6) has two splice variants - soluble PC5/6A and membrane-bound PC5/6B - and is expressed during mouse development in many tissues including bone and tooth, but little is known about the substrates for PC5/6 therein. Osteopontin (OPN) is an abundant bone extracellular matrix protein with roles in mineralization, cell adhesion and cell migration, and it has putative consensus sequence sites for cleavage by PC5/6, which may modify its function in bone. Since PC5/6-knockout mouse embryos show developmental abnormalities, and reduced overall mineralization, we designed this study to determine whether OPN is a substrate of PC5/6. In silico analysis of OPN protein sequences identified four potential PC5/6 consensus cleavage sites in human OPN, and three sites - including a noncanonical sequence - in mouse OPN. Ex vivo co-transfections with human OPN revealed complete OPN cleavage reducing full-length OPN (~70kDa) to an N-terminal fragment migrating at ~50kDa and two C-terminal fragments at ~18kDa and ~16kDa. Direct cleavage of OPN by PC5/6A - the predominant isoform expressed in human osteoblast cells - was confirmed by cell-free enzyme-substrate assays and by mass spectrometry. The latter was also used to investigate potential cleavage sites. Co-transfections of PC5/6 and mouse OPN showed partial cleavage of OPN into a C-terminal OPN fragment migrating at ~30kDa and an N-terminal fragment migrating at ~29kDa. Micro-computed tomography of PC5/6-knockout embryos at E18.5 confirmed a reduction in mineralized bone, and in situ hybridization performed on cryo-sections of normal mouse bone using Pcsk5 and Opn anti-sense and control-sense cRNA probes indicated the co-localization of the expression of these genes in bone cells. This mRNA expression profile was supported by semi-quantitative RT-PCR using osteoblast primary cultures, and cultured MC3T3-E1 osteoblast and MLO-Y4 osteocyte cell lines. Immunoblotting for OPN from mouse bone extracts showed altered OPN processing in PC5/6-knockout mice compared to wildtype mice. OPN fragments migrated at ~25kDa and ~16kDa in wildtype bone and were not present in PC5/6-deficient bone. In conclusion, this study demonstrates that Pcsk5 is expressed in bone-forming cells, and that OPN is a novel substrate for PC5/6. Cleavage of OPN by PC5/6 may modify the function of OPN in bone and/or modulate other enzymatic cleavages of OPN, leading to alterations in the bone phenotype.

Spatiotemporal expression of proprotein convertase subtilisin/kexin type 5 in the development of anorectal malformations in fetal rats

This study examined the expression patterns of proprotein convertase subtilisin/kexin type 5 (Pcsk5) during anorectal development in normal and anorectal malformations (ARM) rat embryos, determine the possible role of Pcsk5 in the pathogenesis of ARM. An ARM rat model was developed by the administration of ethylenethiourea gestational day 10 (GD10). Embryos were harvested by surgical excision from GD13 to GD16, and the spatiotemporal expression of Pcsk5 was evaluated, using immunohistochemistry staining, Western blotting and real time RT-PCR. Immunohistochemistry staining in normal embryos revealed that Pcsk5 was abundantly expressed on the epithelium of the cloaca (CL) on GD13. On GD14 and GD15, positive cells were noted on the urorectal septum and the thin anal membrane. However, the epithelium of the CL of ARM embryos only faintly expressed Pcsk5 from GD13 to GD15. Western blotting and real time RT-PCR showed time-dependent increase of Pcsk5 expression in the developing hindgut. Pcsk5 expression levels were lower in the ARM group from GD14 to GD16 (p ≤ 0.05). These results indicate that downregulation of Pcsk5 during cloaca development into the rectum and urethra might be related to the formation of ARMs.

Thrombin activation of protein C requires prior processing by a liver proprotein convertase

Protein C, a secretory vitamin K-dependent anticoagulant serine protease, inactivates factors Va/VIIIa. It is exclusively synthesized in liver hepatocytes as an inactive zymogen (proprotein C). In humans, thrombin cleavage of the propeptide at PR²²¹↓ results in activated protein C (APC; residues 222-461). However, the propeptide is also cleaved by a furin-like proprotein convertase(s) (PCs) at KKRSHLKR¹⁹⁹↓ (underlined basic residues critical for the recognition by PCs), but the order of cleavage is unknown. Herein, we present evidence that at the surface of COS-1 cells, mouse proprotein C is first cleaved by the convertases furin, PC5/6A, and PACE4. In mice, this cleavage occurs at the equivalent site, KKRKILKR¹⁹⁸↓, and requires the presence of Arg¹⁹⁸ at P1 and a combination of two other basic residues at either P2 (Lys¹⁹⁷), P6 (Arg¹⁹³), or P8 (Lys¹⁹¹) positions. Notably, the thrombin-resistant R221A mutant is still cleaved by these PCs, revealing that convertase cleavage can precede thrombin activation. This conclusion was supported by the fact that the APC-specific activity in the medium of COS-1 cells is exclusively dependent on prior cleavage by the convertases, because both R198A and R221A lack protein C activity. Primary cultures of hepatocytes derived from wild-type or hepatocyte-specific furin, PC5/6, or complete PACE4 knock-out mice suggested that the cleavage of overexpressed proprotein C is predominantly performed by furin intracellularly and by all three proprotein convertases at the cell surface. Indeed, plasma analyses of single-proprotein convertase-knock-out mice showed that loss of the convertase furin or PC5/6 in hepatocytes results in a ∼30% decrease in APC levels, with no significant contribution from PACE4. We conclude that prior convertase cleavage of protein C in hepatocytes is critical for its thrombin activation.

Pcsk5 is required in the early cranio-cardiac mesoderm for heart development

Background

Loss of proprotein convertase subtilisin/kexin type 5 (Pcsk5) results in multiple developmental anomalies including cardiac malformations, caudal regression, pre-sacral mass, renal agenesis, anteroposterior patterning defects, and tracheo-oesophageal and anorectal malformations, and is a model for VACTERL/caudal regression/Currarino syndromes (VACTERL association - Vertebral anomalies, Anal atresia, Cardiac defects, Tracheoesophageal fistula and/or Esophageal atresia, Renal & Radial anomalies and Limb defects).

Results

Using magnetic resonance imaging (MRI), we examined heart development in mouse embryos with zygotic and cardiac specific deletion of Pcsk5. We show that conditional deletion of Pcsk5 in all epiblastic lineages recapitulates all developmental malformations except for tracheo-esophageal malformations. Using a conditional deletion strategy, we find that there is an essential and specific requirement for Pcsk5 in the cranio-cardiac mesoderm for cardiogenesis, but not for conotruncal septation or any other aspect of embryonic development. Surprisingly, deletion of Pcsk5 in cardiogenic or pharyngeal mesodermal progenitors that form later from the cranio-cardiac mesoderm does not affect heart development. Neither is Pcsk5 essential in the neural crest, which drives conotruncal septation.

Conclusions

Our results suggest that Pcsk5 may have an essential and early role in the cranio-cardiac mesoderm for heart development. Alternatively, it is possible that Pcsk5 may still play a critical role in Nkx2.5-expressing cardiac progenitors, with persistence of mRNA or protein accounting for the lack of effect of deletion on heart development.

Electronic supplementary material

The online version of this article (doi:10.1186/s12861-017-0148-y) contains supplementary material, which is available to authorized users.

Decreased PCSK9 expression in human hepatocellular carcinoma

BACKGROUND

The management of hepatocellular carcinoma (HCC) is limited by the lack of adequate screening biomarkers and chemotherapy. In response, there has been much interest in tumor metabolism as a therapeutic target. PCSK9 stimulates internalization of the LDL-receptor, decreases cholesterol uptake into hepatocytes and affects liver regeneration. Thus, we investigated whether PCSK9 expression is altered in HCC, influencing its ability to harness cholesterol metabolism.

METHODS

Thirty-nine patients undergoing partial hepatectomy or liver transplantation for HCC were consented for use of HCC tissue to construct a tissue microarray (TMA). The TMA was immunostained for PCSK9. Imagescope software was used to objectively determine staining, and assess for pathological and clinical correlations. PCSK9 and LDL receptor mRNA levels in flash-frozen HCC and adjacent liver tissue were determined by quantitative RT-PCR. Serum PCSK9 levels were determined by ELISA.

RESULTS

By immunohistochemistry, there was significantly lower expression of PCSK9 in HCC as compared to adjacent cirrhosis (p-value < 0.0001, wilcoxon signed-rank test). Significantly greater staining of PCSK9 was present in cirrhosis compared to HCC (p value <0.0001), and positivity (percentage of positive cells) was significantly greater in cirrhosis compared to HCC (p-value < 0.0001). Conversely, significantly higher expression of LDL-R was present in HCC as compared to the adjacent cirrhosis (p-value < 0.0001). There was no significant correlation of PCSK9 staining with grade of tumor, but there were significant correlations between PCSK9 staining and stage of fibrosis, according to spearman correlation test. PCSK9 mRNA levels were relatively less abundant within HCC compared to adjacent liver tissue (p-value =0.08) and normal control tissue (p-value =0.02). In contrast, serum PCSK9 levels were significantly increased among patients with HCC compared to those with chronic liver disease without HCC (p-value =0.029). LDL receptor mRNA was consistantly greater in HCC when compared to normal control tissue (p-value = 0.06) and, in general, was significantly greater in HCC when compared to adjacent liver (p-value = 0.04).

CONCLUSIONS

The decreased expression of PCSK9 and conversely increased LDL-R expression in HCC suggests that HCC modulates its local microenvironment to enable a constant energy supply. Larger-scale studies should be conducted to determine whether PCSK9 could be a therapeutic target for HCC.

Neuroinflammation-Induced Interactions between Protease-Activated Receptor 1 and Proprotein Convertases in HIV-Associated Neurocognitive Disorder

The proprotein convertases (PCs) furin, PC5, PACE4, and PC7 cleave secretory proteins after basic residues, including the HIV envelope glycoprotein (gp160) and Vpr. We evaluated the abundance of PC mRNAs in postmortem brains of individuals exhibiting HIV-associated neurocognitive disorder (HAND), likely driven by neuroinflammation and neurotoxic HIV proteins (e.g., envelope and Vpr). Concomitant with increased inflammation-related gene expression (interleukin-1β [IL-1β]), the mRNA levels of the above PCs are significantly increased, together with those of the proteinase-activated receptor 1 (PAR1), an inflammation-associated receptor that is cleaved by thrombin at ProArg41↓ (where the down arrow indicates the cleavage location), and potentially by PCs at Arg41XXXXArg46↓. The latter motif in PAR1, but not its R46A mutant, drives its interactions with PCs. Indeed, PAR1 upregulation leads to the inhibition of membrane-bound furin, PC5B, and PC7 and inhibits gp160 processing and HIV infectivity. Additionally, a proximity ligation assay revealed that furin and PC7 interact with PAR1. Reciprocally, increased furin expression reduces the plasma membrane abundance of PAR1 by trapping it in the trans-Golgi network. Furthermore, soluble PC5A/PACE4 can target/disarm cell surface PAR1 through cleavage at Arg46↓. PACE4/PC5A decreased calcium mobilization induced by thrombin stimulation. Our data reveal a new PC-PAR1-interaction pathway, which offsets the effects of HIV-induced neuroinflammation, viral infection, and potentially the development of HAND.

Amyloid Precursor-like Protein 2 and Sortilin Do Not Regulate the PCSK9 Convertase-mediated Low Density Lipoprotein Receptor Degradation but Interact with Each Other

Amyloid precursor-like protein 2 (APLP2) and sortilin were reported to individually bind the proprotein convertase subtilisin/kexin type 9 (PCSK9) and regulate its activity on the low-density lipoprotein receptor (LDLR). The data presented herein demonstrate that mRNA knockdowns of APLP2, sortilin, or both in the human hepatocyte cell lines HepG2 and Huh7 do not affect the ability of extracellular PCSK9 to enhance the degradation of the LDLR. Furthermore, mice deficient in APLP2 or sortilin do not exhibit significant changes in liver LDLR or plasma total cholesterol levels. Moreover, cellular overexpression of one or both proteins does not alter PCSK9 secretion, or its activity on the LDLR. We conclude that PCSK9 enhances the degradation of the LDLR independently of either APLP2 or sortilin both ex vivo and in mice. Interestingly, when co-expressed with PCSK9, both APLP2 and sortilin were targeted for lysosomal degradation. Using chemiluminescence proximity and co-immunoprecipitation assays, as well as biosynthetic analysis, we discovered that sortilin binds and stabilizes APLP2, and hence could regulate its intracellular functions on other targets.

PCSK5 mutation in a patient with the VACTERL association

BACKGROUND

The VACTERL association is a typically sporadic, non-random collection of congenital anomalies that includes vertebral defects, anal atresia, cardiac defects, tracheoesophageal fistula with esophageal atresia, renal anomalies, and limb abnormalities. Although several chromosomal aberrations and gene mutations have been reported as disease-causative, these findings have been sparsely replicated to date.

CASE PRESENTATION

In the present study, whole exome sequencing of a case with the VACTERL association uncovered a novel frameshift mutation in the PCSK5 gene, which has been reported as one of the causative genes for the VACTERL association. Although this mutation appears potentially pathogenic in its functional aspects, it was also carried by the healthy father. Furthermore, a database survey revealed several other deleterious variants in the PCSK5 gene in the general population.

CONCLUSIONS

Further studies are necessary to clarify the etiological role of the PCSK5 mutation in the VACTERL association.

PCSK6 regulated by LH inhibits the apoptosis of human granulosa cells via activin A and TGFβ2

Mammalian proprotein convertases (PCs) play an important role in folliculogenesis, as they proteolytically activate a variety of substrates such as the transforming growth factor beta (TGFβ) superfamily. PC subtilism/kexin 6 (PCSK6) is a member of the PC family and is ubiquitously expressed and implicated in many physiological and pathological processes. However, in human granulosa cells, the expression of the PC family members, their hormonal regulation, and the function of PCs are not clear. In this study, we found that PCSK6 is the most highly expressed PC family member in granulosa cells. LH increased PCSK6 mRNA level and PCSK6 played an anti-apoptosis function in KGN cells. Knockdown of PCSK6 not only increased the secretion of activin A and TGFβ2 but also decreased the secretion of follistatin, estrogen, and the mRNA levels of FSH receptor (FSHR) and P450AROM (CYP19A1). We also found that, in the KGN human granulosa cell line, TGFβ2 and activin A could promote the apoptosis of KGN cells and LH could regulate the follistatin level. These data indicate that PCSK6, which is regulated by LH, is highly expressed in human primary granulosa cells of pre-ovulatory follicles and plays important roles in regulating a series of downstream molecules and apoptosis of KGN cells.

Regulation of TGFβ and related signals by precursor processing

Secreted cytokines of the TGFβ family are found in all multicellular organisms and implicated in regulating fundamental cell behaviors such as proliferation, differentiation, migration and survival. Signal transduction involves complexes of specific type I and II receptor kinases that induce the nuclear translocation of Smad transcription factors to regulate target genes. Ligands of the BMP and Nodal subgroups act at a distance to specify distinct cell fates in a concentration-dependent manner. These signaling gradients are shaped by multiple factors, including proteases of the proprotein convertase (PC) family that hydrolyze one or several peptide bonds between an N-terminal prodomain and the C-terminal domain that forms the mature ligand. This review summarizes information on the proteolytic processing of TGFβ and related precursors, and its spatiotemporal regulation by PCs during development and various diseases, including cancer. Available evidence suggests that the unmasking of receptor binding epitopes of TGFβ is only one (and in some cases a non-essential) function of precursor processing. Future studies should consider the impact of proteolytic maturation on protein localization, trafficking and turnover in cells and in the extracellular space.

Proprotein convertases subtilisin/kexin type 9, an enzyme turned escort protein: hepatic and extra hepatic functions

Proprotein Convertases Subtilisin/Kexin Type 9 (PCSK9) is a serine endoproteinase. Biosynthesized as a zymogen, it cleaves itself once, and then turns into an escort protein for transmembrane proteins, leading them into lysosomes for degradation. It is primarily produced and secreted by the liver. It attaches to the low-density lipoprotein receptor (LDLR) at the surface of hepatocytes and, after co-endocytosis, directs it into lysosomes where it is degraded. By downregulating LDLR, PCSK9 reduces hepatic clearance of LDL-cholesterol. Inborn or induced increase of this function causes hypercholesterolemia; its decrease causes hypocholesterolemia. This has been experimentally demonstrated ex vivo and in vivo, and corroborated by epidemiological studies associating PCSK9 genetic variations with plasma cholesterol levels. PCSK9 is now a proven target for inactivation in the treatment of hypercholesterolemia and associated atherosclerosis. However, it is still uncertain whether its severe or complete inactivation, combined with other predispositions, will be without undesirable side-effects. Some experimental data suggest that PCSK9 could contribute positively to the physiology of non-hepatic cells such as pancreatic islets β cells, adipocytes and macrophages, protecting them from excessive lipid uptake, in an endocrine, autocrine, or paracrine manner. Genetic variations that attenuate PCSK9 anti-LDLR activity are common in human populations. Their evolutionary significance still needs to be evaluated on the background of environmental pressures, such as infectious diseases, cold weather and famine, which have threatened survival and reproduction in the course of human prehistory and history.

Proprotein convertase subtilisin/kexin type 7 (PCSK7) is essential for the zebrafish development and bioavailability of transforming growth factor β1a (TGFβ1a)

Proprotein convertase subtilisin/kexin (PCSK) enzymes convert proproteins into bioactive end products. Although other PCSK enzymes are known to be essential for biological processes ranging from cholesterol metabolism to host defense, the in vivo importance of the evolutionarily ancient PCSK7 has remained enigmatic. Here, we quantified the expressions of all pcsk genes during the 1st week of fish development and in several tissues. pcsk7 expression was ubiquitous and evident already during the early development. To compare mammalian and zebrafish PCSK7, we prepared homology models, which demonstrated remarkable structural conservation. When the PCSK7 function in developing larvae was inhibited, we found that PCSK7-deficient fish have defects in various organs, including the brain, eye, and otic vesicle, and these result in mortality within 7 days postfertilization. A genome-wide analysis of PCSK7-dependent gene expression showed that, in addition to developmental processes, several immune system-related pathways are also regulated by PCSK7. Specifically, the PCSK7 contributed to the mRNA expression and proteolytic cleavage of the cytokine TGFβ1a. Consequently, tgfβ1a morphant fish displayed phenotypical similarities with pcsk7 morphants, underscoring the importance of this cytokine in the zebrafish development. Targeting PCSK activity has emerged as a strategy for treating human diseases. Our results suggest that inhibiting PCSK7 might interfere with normal vertebrate development.

Modulation of protease activated receptor 1 influences human metapneumovirus disease severity in a mouse model

Human metapneumovirus (hMPV) infection causes acute respiratory tract infections (RTI) which can result in hospitalization of both children and adults. To date, no antiviral or vaccine is available for this common viral infection. Immunomodulators could represent an interesting strategy for the treatment of severe viral infection. Recently, the role of protease-activated receptors (PAR) in inflammation, coagulation and infection processes has been of growing interest. Herein, the effects of a PAR1 agonist and a PAR1 antagonist on hMPV infection were investigated in BALB/c mice. Intranasal administration of the PAR1 agonist resulted in increased weight loss and mortality of infected mice. Conversely, the PAR1 antagonist was beneficial to hMPV infection by decreasing weight loss and clinical signs and by significantly reducing pulmonary inflammation, pro-inflammatory cytokine levels (including IL-6, KC and MCP-1) and recruitment of immune cells to the lungs. In addition, a significant reduction in pulmonary viral titers was also observed in the lungs of PAR1 antagonist-treated mice. Despite no apparent direct effect on virus replication during in vitro experiments, an important role for PAR1 in the regulation of furin expression in the lungs was shown for the first time. Further experiments indicated that the hMPV fusion protein can be cleaved by furin thus suggesting that PAR1 could have an effect on viral infectivity in addition to its immunomodulatory properties. Thus, inhibition of PAR1 by selected antagonists could represent an interesting strategy for decreasing the severity of paramyxovirus infections.

Opposite roles of furin and PC5A in N-cadherin processing

We recently demonstrated that lack of Furin-processing of the N-cadherin precursor (proNCAD) in highly invasive melanoma and brain tumor cells results in the cell-surface expression of a nonadhesive protein favoring cell migration and invasion in vitro. Quantitative polymerase chain reaction analysis of malignant human brain tumor cells revealed that of all proprotein convertases (PCs) only the levels of Furin and PC5A are modulated, being inversely (Furin) or directly (PC5A) correlated with brain tumor invasive capacity. Intriguingly, the N-terminal sequence following the Furin-activated NCAD site (RQKR↓DW(161), mouse nomenclature) reveals a second putative PC-processing site (RIRSDR↓DK(189)) located in the first extracellular domain. Cleavage at this site would abolish the adhesive functions of NCAD because of the loss of the critical Trp(161). This was confirmed upon analysis of the fate of the endogenous prosegment of proNCAD in human malignant glioma cells expressing high levels of Furin and low levels of PC5A (U343) or high levels of PC5A and negligible Furin levels (U251). Cellular analyses revealed that Furin is the best activating convertase releasing an ~17-kDa prosegment, whereas PC5A is the major inactivating enzyme resulting in the secretion of an ~20-kDa product. Like expression of proNCAD at the cell surface, cleavage of the NCAD molecule at RIRSDR↓DK(189) renders the U251 cancer cells less adhesive to one another and more migratory. Our work modifies the present view on posttranslational processing and surface expression of classic cadherins and clarifies how NCAD possesses a range of adhesive potentials and plays a critical role in tumor progression.

Laser-capture microdissection and transcriptional profiling of the dorsomedial nucleus of the hypothalamus

Identifying neuronal molecular markers with restricted patterns of expression is a crucial step in dissecting the numerous pathways and functions of the brain. While the dorsomedial nucleus of the hypothalamus (DMH) has been implicated in a host of physiological processes, current functional studies have been limited by the lack of molecular markers specific for DMH. Identification of such markers would facilitate the development of mouse models with DMH-specific genetic manipulations. Here we used a combination of laser-capture microdissection (LCM) and gene expression profiling to identify genes that are highly expressed within the DMH relative to adjacent hypothalamic regions. Six of the most highly expressed of these genes, Gpr50, 4930511J11Rik, Pcsk5, Grp, Sulf1, and Rorβ, were further characterized by real-time polymerase chain reaction (PCR) analysis and in situ hybridization histochemistry. The genes identified in this article will provide the basis for future gene-targeted approaches for studying DMH function.

A mouse diversity panel approach reveals the potential for clinical kidney injury due to DB289 not predicted by classical rodent models

DB289 is the first oral drug shown in clinical trials to have efficacy in treating African trypanosomiasis (African sleeping sickness). Mild liver toxicity was noted but was not treatment limiting. However, development of DB289 was terminated when several treated subjects developed severe kidney injury, a liability not predicted from preclinical testing. We tested the hypothesis that the kidney safety liability of DB289 would be detected in a mouse diversity panel (MDP) comprised of 34 genetically diverse inbred mouse strains. MDP mice received 10 days of oral treatment with DB289 or vehicle and classical renal biomarkers blood urea nitrogen (BUN) and serum creatinine (sCr), as well as urine biomarkers of kidney injury were measured. While BUN and sCr remained within reference ranges, marked elevations were observed for kidney injury molecule-1 (KIM-1) in the urine of sensitive mouse strains. KIM-1 elevations were not always coincident with elevations in alanine aminotransferase (ALT), suggesting that renal injury was not linked to hepatic injury. Genome-wide association analyses of KIM-1 elevations indicated that genes participating in cholesterol and lipid biosynthesis and transport, oxidative stress, and cytokine release may play a role in DB289 renal injury. Taken together, the data resulting from this study highlight the utility of using an MDP to predict clinically relevant toxicities, to identify relevant toxicity biomarkers that may translate into the clinic, and to identify potential mechanisms underlying toxicities. In addition, the sensitive mouse strains identified in this study may be useful in screening next-in-class compounds for renal injury.

The biology and therapeutic targeting of the proprotein convertases

The mammalian proprotein convertases constitute a family of nine secretory serine proteases that are related to bacterial subtilisin and yeast kexin. Seven of these (proprotein convertase 1 (PC1), PC2, furin, PC4, PC5, paired basic amino acid cleaving enzyme 4 (PACE4) and PC7) activate cellular and pathogenic precursor proteins by cleavage at single or paired basic residues, whereas subtilisin kexin isozyme 1 (SKI-1) and proprotein convertase subtilisin kexin 9 (PCSK9) regulate cholesterol and/or lipid homeostasis via cleavage at non-basic residues or through induced degradation of receptors. Proprotein convertases are now considered to be attractive targets for the development of powerful novel therapeutics. In this Review, we summarize the physiological functions and pathological implications of the proprotein convertases, and discuss proposed strategies to control some of their activities, including their therapeutic application and validation in selected disease states.

The proprotein convertases, 20 years later

The proprotein convertases (PCs) are secretory mammalian serine proteinases related to bacterial subtilisin-like enzymes. The family of PCs comprises nine members, PC1/3, PC2, furin, PC4, PC5/6, PACE4, PC7, SKI-1/S1P, and PCSK9 (Fig. 3.1). While the first seven PCs cleave after single or paired basic residues, the last two cleave at non-basic residues and the last one PCSK9 only cleaves one substrate, itself, for its activation. The targets and substrates of these convertases are very varied covering many aspects of cellular biology and communication. While it took more than 22 years to begin to identify the first member in 1989-1990, in less than 14 years they were all characterized. So where are we 20 years later in 2011? We have now reached a level of maturity needed to begin to unravel the mechanisms behind the complex physiological functions of these PCs both in health and disease states. We are still far away from comprehensively understanding the various ramifications of their roles and to identify their physiological substrates unequivocally. How do these enzymes function in vivo? Are there other partners to be identified that would modulate their activity and/or cellular localization? Would non-toxic inhibitors/silencers of some PCs provide alternative therapies to control some pathologies and improve human health? Are there human SNPs or mutations in these PCs that correlate with disease, and can these help define the finesses of their functions and/or cellular sorting? The more we know about a given field, the more questions will arise, until we are convinced that we have cornered the important angles. And yet the future may well reserve for us many surprises that may allow new leaps in our understanding of the fascinating biology of these phylogenetically ancient eukaryotic proteases (Fig. 3.2) implicated in health and disease, which traffic through the cells via multiple sorting pathways (Fig. 3.3).

Disruption of proprotein convertase 1/3 (PC1/3) expression in mice causes innate immune defects and uncontrolled cytokine secretion

The proprotein convertase 1/3 is expressed in the regulated secretory pathway of neural and endocrine cells. Its major function is in the post-translational processing and activation of precursor proteins. The PC1/3 knock-out (KO) mouse model has allowed us to elucidate its physiological functions in studies focused primarily on neuroendocrine tissues. However, PC1/3 is also expressed in cells of the immune system, mainly in macrophages. The present study explores the effects of innate immune challenge in the PC1/3 KO mouse. PC1/3 KO mice have an enlarged spleen with marked disorganization of the marginal zone and red pulp. Immunohistochemical studies using various markers demonstrate a depletion of dendritic cells in PC1/3 KO spleens. When challenged with lipopolysaccharide, PC1/3 KO mice are more susceptible to septic shock than wild-type controls or other PC KO mice, such as PC2 and PC7 null mice. Plasma levels of proinflammatory cytokines (IL-6, IL-1β, and TNF-α) were very significantly elevated in PC1/3 KO mice, consistent with a hypercytokinemia, i.e. indicative of a major systemic uncontrolled inflammatory response or cytokine storm. Peritoneal macrophages isolated from PC1/3 KO mice also demonstrate elevated cytokine secretion when treated with LPS. Electron micrographs show morphological features indicating a prolonged activation of these cells following LPS stimulation. We also present evidence that the proinflammatory T(h)1 pathway is dominant in the PC1/3 KO mouse model. We conclude that aside from its important role in neuroendocrine functions PC1/3 also has an important role in the regulation of the innate immune system, most likely through the regulation of cytokine secretion in macrophages.

Internalization of proprotein convertase PC7 from plasma membrane is mediated by a novel motif

Proprotein convertase 7 (PC7) is a member of the subtilisin-like proprotein convertase family, which is involved in the endoproteolysis of a variety of precursor proteins. Under steady state conditions, PC7 is mainly localized in the trans-Golgi network, but a small fraction is found at the cell surface. So far, no sorting signals for membrane trafficking have been identified in PC7. In this study, we have examined the internalization of PC7 from the plasma membrane. Our results show that internalization of PC7 is mediated by clathrin-coated vesicles. After inhibition of clathrin-mediated endocytosis using hypertonic conditions or the small molecule inhibitor, Pitstop 2, PC7 accumulated at the plasma membrane. Furthermore, PC7 was present in isolated clathrin-coated vesicles. To determine the internalization motif, constructs were generated in which parts of the N and C terminus of the cytoplasmic tail of PC7 were deleted, and chimeric proteins were constructed consisting of the luminal and transmembrane domains of Tac (CD25) and parts of the cytoplasmic domain of PC7. Antibody uptake experiments as well as surface biotinylation experiments demonstrated that the region between Ala(713) and Cys(726) in the cytoplasmic domain of PC7 is essential and sufficient for the internalization of PC7 but not for trans-Golgi network localization. Individual amino acids in this region were substituted with alanine, which identified Pro, Leu, and Cys as the essential amino acids. In conclusion, internalization of PC7 depends on a short transferable sequence in the cytoplasmic tail, which contains the three crucial amino acids PLC.

On the cutting edge of proprotein convertase pharmacology: from molecular concepts to clinical applications

There is increasing interest in the therapeutic targeting of proteases for the treatment of important diseases. Additionally new protein-based therapeutic strategies have the potential to widen the available treatments against these pathologies. In the last decade, accumulated evidence has confirmed that the family of proteases known as proprotein convertases (PCs) are potential targets for viral infections, osteoarthritis, cancer and cardiovascular disease, among others. Nevertheless, there are still many unanswered questions about the relevance of targeting PCs in a therapeutic context, especially regarding the anticipated secondary effects of treatment, considering the observed embryonic lethality of some PC knockout mice. In this review, the benefits of PCs as pharmacological targets will be discussed, with focus on concepts and strategies, as well as on the state of advancement of actual and future inhibitors.

Latent transforming growth factor beta-binding proteins-2 and -3 inhibit the proprotein convertase 5/6A

The basic amino acid-specific proprotein convertase 5/6 (PC5/6) is an essential secretory protease, as knock-out mice die at birth and exhibit multiple homeotic transformation defects, including impaired bone morphogenesis and lung structure. Some of the observed defects were attributed to impaired processing of the TGFβ-like growth differentiating factor 11 precursor (proGdf11). In this work we present evidence that the latent TGFβ-binding proteins 2 and 3 (LTBP-2 and -3) inhibit the extracellular processing of proGdf11 by PC5/6A. This is partly due to the binding of LTBPs in the endoplasmic reticulum to the zymogen proPC5/6A, thus allowing the complex to exit the endoplasmic reticulum and be sequestered as an inactive zymogen in the extracellular matrix but not at the cell surface. This results in lower levels of PC5/6A in the media, without affecting those of PACE4, Furin, or a soluble form of PC7. The secreted soluble protease-specific activity of PC5/6A or a variant lacking the C-terminal Cys-rich domain (PC5/6-ΔCRD) is significantly decreased when co-expressed with LTBPs in cells. A similar enzymatic inhibition seems to apply to PACE4 and Furin. In situ hybridization analyses revealed extensive co-localization of PC5/6 and LTBP-3 mRNAs in mice at embryonic day 15.5 and post partum day 1. In conclusion, this is the first time that a zymogen of the proprotein convertases was shown to exit the endoplasmic reticulum in the presence of LTBPs, representing a potential novel mechanism for the regulation of PC5/6A activity, e.g. in tissues such as bone and lung where LTBP-3 and PC5/6 co-localize.

Role of PCSK5 expression in mouse ovarian follicle development: identification of the inhibin α- and β-subunits as candidate substrates

Inhibin and activin are essential dimeric glycoproteins belonging to the transforming growth factor-beta (TGFβ) superfamily. Inhibin is a heterodimer of α- and β-subunits, whereas activin is a homodimer of β-subunits. Production of inhibin is regulated during the reproductive cycle and requires the processing of pro-ligands to produce mature hormone. Furin is a subtilisin-like proprotein convertase (proconvertase) that activates precursor proteins by cleavage at basic sites during their transit through the secretory pathway and/or at the cell surface. We hypothesized that furin-like proconvertases are central regulators of inhibin α- and β-subunit processing within the ovary. We analyzed the expression of the proconvertases furin, PCSK5, PCSK6, and PCSK7 in the developing mouse ovary by real-time quantitative RT-PCR. The data showed that proconvertase enzymes are temporally expressed in ovarian cells. With the transition from two-layer secondary to pre-antral follicle, only PCSK5 mRNA was significantly elevated. Activin A selectively enhanced expression of PCSK5 mRNA and decreased expression of furin and PCSK6 in cultured two-layer secondary follicles. Inhibition of proconvertase enzyme activity by dec-RVKR-chloromethylketone (CMK), a highly specific and potent competitive inhibitor of subtilisin-like proconvertases, significantly impeded both inhibin α- and β-subunit maturation in murine granulosa cells. Overexpression of PC5/6 in furin-deficient cells led to increased inhibin α- and β_B-subunit maturation. Our data support the role of proconvertase PCSK5 in the processing of ovarian inhibin subunits during folliculogenesis and suggest that this enzyme may be an important regulator of inhibin and activin bioavailability.

The proprotein convertase PC5/6 is protective against intestinal tumorigenesis: in vivo mouse model

Background

The secretory basic amino acid-specific proprotein convertases (PCs) have often been associated with cancer/metastasis. By controlling the cleavage of cancer-associated proteins, PCs play key roles in multiple steps of cancer development. Most analyses of the implication of PCs in cancer/metastasis relied on the use of in vitro overexpression systems or inhibitors that can affect more than one PC. Aside from the role of furin in salivary gland tumorigenesis, no other in vivo genetic model of PC-knockout was reported in relation to cancer development.

Results

Since PC5/6 is highly expressed in the small intestine, the present study examined its in vivo role in intestinal tumorigenesis. Analysis of human intestinal tumors at various stages showed a systematic down-regulation of PC5/6 expression. Since gene inactivation of PC5/6 leads to lethality at birth, we generated mice lacking PC5/6 in enterocytes and analyzed the impact of the presence or absence of this PC in the mouse Apc^Min/+ model that develops numerous adenocarcinomas along the intestinal tract. This resulted in viable mice with almost no expression of PC5/6 in small intestine, but with no overt phenotype. The data showed that by themselves Apc^Min/+ tumors express lower levels of PC5/6 mRNA, and that the lack of PC5/6 in enterocytes results in a significantly higher tumor number in the duodenum, with a similar trend in other intestinal segments. Finally, the absence of PC5/6 is also associated with a premature mortality of Apc^Min/+ mice.

Conclusion

Overall, these data suggest that intestinal PC5/6 is protective towards tumorigenesis, especially in mouse duodenum, and possibly in human colon.

Genetic variation at the proprotein convertase subtilisin/kexin type 5 gene modulates high-density lipoprotein cholesterol levels

BACKGROUND

A low level of plasma high-density lipoprotein cholesterol (HDL-C) is a risk factor for cardiovascular disease. HDL particles are modulated by a variety of lipases, including endothelial lipase, a phospholipase present on vascular endothelial cells. The proprotein convertase subtilisin/kexin type 5 (PCSK5) gene product is known to directly inactivate endothelial lipase and indirectly cleave and activate angiopoetin-like protein 3, a natural inhibitor of endothelial lipase. We therefore investigated the effect of human PCSK5 genetic variants on plasma HDL-C levels.

METHODS AND RESULTS

Haplotypes at the PCSK5 locus were examined in 9 multigenerational families that included 60 individuals with HDL-C <10th percentile. Segregation with low HDL-C in 1 family was found. Sequencing of the PCSK5 gene in 12 probands with HDL-C <5th percentile identified 7 novel variants. Using a 2-stage design, we first genotyped these single-nucleotide polymorphisms (SNPs) along with 163 tagSNPs and 12 additional SNPs (n=182 total) in 457 individuals with documented coronary artery disease. We identified 9 SNPs associated with HDL-C (P<0.05), with the strongest results for rs11144782 and rs11144766 (P=0.002 and P=0.005, respectively). The SNP rs11144782 was also associated with very low-density lipoprotein (P=0.039), triglycerides (P=0.049), and total apolipoprotein levels (P=0.022). In stage 2, we replicated the association of rs11144766 with HDL-C (P=0.014) in an independent sample of Finnish low HDL-C families. In a combined analysis of both stages (n=883), region-wide significance of rs11144766 and low HDL-C was observed (unadjusted P=1.86x10(-4) and Bonferroni-adjusted P=0.031).

CONCLUSIONS

We conclude that variability at the PCSK5 locus influences HDL-C levels, possibly through the inactivation of endothelial lipase activity, and, consequently, atherosclerotic cardiovascular disease risk.

Processing of peptide and hormone precursors at the dibasic cleavage sites

Many functionally important cellular peptides and proteins, including hormones, neuropeptides, and growth factors, are synthesized as inactive precursor polypeptides, which require post-translational proteolytic processing to become biologically active polypeptides. This is achieved by the action of a relatively small number of proteases that belong to a family of seven subtilisin-like proprotein convertases (PCs) including furin. In view of this, this review focuses on the importance of privileged secondary structures and of given amino acid residues around basic cleavage sites in substrate recognition by these endoproteases. In addition to their participation in normal cell functions, PCs are crucial for the initiation and progress of many important diseases. Hence, these proteases constitute potential drug targets in medicine. Accordingly, this review also discusses the approaches used to shed light on the cleavage preference and the substrate specificity of the PCs, a prerequisite to select which PCs are promising drug targets in each disease.

An activin/furin regulatory loop modulates the processing and secretion of inhibin alpha- and betaB-subunit dimers in pituitary gonadotrope cells

Of all ligands of the transforming growth factor beta superfamily, inhibins and activins are a physiologically relevant pair that are functional antagonists of each other. Activin stimulates whereas inhibin blocks follicle-stimulating hormone biosynthesis and secretion from pituitary gonadotrope cells, and together, inhibin and activin control the pituitary gonadal axis essential for normal reproductive function. Sharing a similar beta-subunit, the secretion of inhibin heterodimers (alpha/beta) or activin homodimers (beta/beta) as mature bioactive ligands depends, in part, on the proteolytic processing of precursor proteins. A short loop regulatory pathway controlling precursor processing and dimer secretion was discovered. Activin stimulates endogenous inhibin alpha- and betaB-subunit mRNA, protein, and proteolytic processing. Simultaneously, activin stimulated the proconvertase furin through a Smad2/3-dependent process. The data provide a mechanism where the regulation of furin and inhibin subunits cooperates in an important positive short feedback loop. This regulatory loop augments the secretion of bioactive mature activin B, as well as inhibin B dimers, necessary for local follicle-stimulating hormone beta regulation.

VACTERL/caudal regression/Currarino syndrome-like malformations in mice with mutation in the proprotein convertase Pcsk5

We have identified an ethylnitrosourea (ENU)-induced recessive mouse mutation (Vcc) with a pleiotropic phenotype that includes cardiac, tracheoesophageal, anorectal, anteroposterior patterning defects, exomphalos, hindlimb hypoplasia, a presacral mass, renal and palatal agenesis, and pulmonary hypoplasia. It results from a C470R mutation in the proprotein convertase PCSK5 (PC5/6). Compound mutants (Pcsk5(Vcc/null)) completely recapitulate the Pcsk5(Vcc/Vcc) phenotype, as does an epiblast-specific conditional deletion of Pcsk5. The C470R mutation ablates a disulfide bond in the P domain, and blocks export from the endoplasmic reticulum and proprotein convertase activity. We show that GDF11 is cleaved and activated by PCSK5A, but not by PCSK5A-C470R, and that Gdf11-deficient embryos, in addition to having anteroposterior patterning defects and renal and palatal agenesis, also have a presacral mass, anorectal malformation, and exomphalos. Pcsk5 mutation results in abnormal expression of several paralogous Hox genes (Hoxa, Hoxc, and Hoxd), and of Mnx1 (Hlxb9). These include known Gdf11 targets, and are necessary for caudal embryo development. We identified nonsynonymous mutations in PCSK5 in patients with VACTERL (vertebral, anorectal, cardiac, tracheoesophageal, renal, limb malformation OMIM 192350) and caudal regression syndrome, the phenotypic features of which resemble the mouse mutation. We propose that Pcsk5, at least in part via GDF11, coordinately regulates caudal Hox paralogs, to control anteroposterior patterning, nephrogenesis, skeletal, and anorectal development.

Hormonal regulation of proprotein convertase subtilisin/kexin type 5 expression during ovarian follicle development in the rat

The proprotein convertase subtilisin/kexin (PCSKs), a family of subtilisin-like proteases, is the processing enzymes for the activation of many hormone precursors. The present study was designed to identify the PCSK isoform expressed in the ovary and to examine its expression in gonadotropin-stimulated rat ovary. Northern blot analysis of ovaries obtained from prepubertal rats revealed an increased expression of Pcsk5 messenger RNA (mRNA) during development with the highest levels at 21 days of age. Treatment of immature rats with PMSG further increased ovarian Pcsk5 expression, and in situ hybridization analysis revealed the localization of Pcsk5 mRNA in theca-interstitial cells of follicles in different sizes. Interestingly, treatment of PMSG-primed rats with hCG resulted in a transient stimulation of ovarian Pcsk5 mRNA levels within 3-6 h. In addition to theca-interstitial cells, hCG treatment induced the expression of Pcsk5 in granulosa cells of preovulatory follicles. Pcsk1, 2 and 4 mRNAs were not detected whereas Pcsk7 mRNA was slightly expressed. Injection of a progestin antagonist RU486 or an inhibitor of 3beta-hydroxysteroid dehydrogenase epostane at 1h before hCG treatment inhibited hCG-induced Pcsk5 mRNA levels. Treatment with LH stimulated both Pcsk5 mRNA and protein levels in preovulatory follicles cultured in vitro. In addition, forskolin but not TPA stimulated Pcsk5 mRNA levels. RNase protection assay revealed that the soluble Pcsk5A variant was the predominant form stimulated by gonadotropins in the ovary. Finally, the predicted proprotein substrates cleaved by PCSK5 were analyzed in preovulatory follicles using regular expressions. The present study demonstrates PCSK5A as the gonadotropin-regulated PCSK isoform in the ovary, and its possible contribution to ovulation by processing pro-TGFbeta and matrix metalloproteinase family.

In vivo functions of the proprotein convertase PC5/6 during mouse development: Gdf11 is a likely substrate

The proprotein convertase PC5/6 cleaves protein precursors after basic amino acids and is essential for implantation in CD1/129/Sv/C57BL/6 mixed-background mice. Conditional inactivation of Pcsk5 in the epiblast but not in the extraembryonic tissue bypassed early embryonic lethality but resulted in death at birth. PC5/6-deficient embryos exhibited Gdf11-related phenotypes such as altered anteroposterior patterning with extra vertebrae and lack of tail and kidney agenesis. They also exhibited Gdf11-independent phenotypes, such as a smaller size, multiple hemorrhages, collapsed alveoli, and retarded ossification. In situ hybridization revealed overlapping PC5/6 and Gdf11 mRNA expression patterns. In vitro and ex vivo analyses showed that the selectivity of PC5/6 for Gdf11 essentially resides in the presence of a P1' Asn in the RSRR downward arrowN cleavage motif. This work identifies Gdf11 as a likely in vivo specific substrate of PC5/6 and opens the way to the identification of other key substrates of this convertase.

Dual mechanisms for the fibrate-mediated repression of proprotein convertase subtilisin/kexin type 9

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is associated with familial autosomal dominant hypercholesterolemia and is a natural inhibitor of the LDL receptor (LDLr). PCSK9 is degraded by other proprotein convertases: PC5/6A and furin. Both PCSK9 and the LDLr are up-regulated by the hypocholesterolemic statins. Thus, inhibitors or repressors of PCSK9 should amplify their beneficial effects. In the present study, we showed that PPARalpha activation counteracts PCSK9 induction by statins by repressing PCSK9 promoter activity and by increasing PC5/6A and furin expression. Quantification of mRNA and protein levels showed that various fibrates decreased PCSK9 and increased PC5/6A and furin expression. Fenofibric acid (FA) reduced PCSK9 protein content in immortalized human hepatocytes (IHH) as well as its cellular secretion. FA suppressed PCSK9 induction by statins or by the liver X receptor agonist TO901317. PCSK9 repression is occurring at the promoter level. We showed that PC5/6A and furin fibrate-mediated up-regulation is PPARalpha-dependent. As a functional test, we observed that FA increased by 30% the effect of pravastatin on the LDLr activity in vitro. In conclusion, fibrates simultaneously decreased PCSK9 expression while increasing PC5/6A and furin expression, indicating a broad action of PPARalpha activation in proprotein convertase-mediated lipid homeostasis. Moreover, this study validates the functional relevance of a combined therapy associating PCSK9 repressors and statins.

Regulation of Pcsk6 Expression During the Preantral to Antral Follicle Transition in Mice: Opposing Roles of FSH and Oocytes1

Several secreted products of the TGFbeta superfamily have important roles during follicular development and are produced by both oocytes and somatic cells (granulosa and theca) in the follicle. The proprotein convertases are a family of seven known proteins that process TGFbeta ligands and other secreted products to their mature active form. The present study examined the regulation of steady-state levels of Pcsk6 mRNA, which encodes a convertase protein known to process members of the TGFbeta superfamily, during mouse follicular development. Pcsk6 mRNA and protein were expressed in preantral but not cumulus or mural granulosa cells. Pcsk6 mRNA levels in preantral granulosa cells were not regulated by growing oocytes of preantral follicles, but were elevated by FSH. Furthermore, Pcsk6 mRNA in preantral granulosa cells was potently suppressed by factor(s) secreted by fully grown oocytes from antral follicles, in part through SMAD2/3-mediated pathways. Oocytes acquired the ability to suppress the steady-state levels of Pcsk6 mRNA in granulosa cells during the preantral to antral follicle transition. Suppression of Pcsk6 mRNA by oocytes could reflect a change in the mechanism(s) regulating the activity of members of the TGFbeta superfamily.