Role of subtilisin-like convertases in cadherin processing or the conundrum to stall cadherin function by convertase inhibitors in cancer therapy

Design, Synthesis, and Characterization of Macrocyclic Inhibitors of the Proprotein Convertase Furin

The activation of viral glycoproteins by the host protease furin is an essential step in the replication of numerous pathogenic viruses. Thus, effective inhibitors of furin could serve as broad-spectrum antiviral drugs. A crystal structure of an inhibitory hexapeptide derivative in complex with furin served as template for the rational design of various types of new cyclic inhibitors. Most of the prepared derivatives are relatively potent furin inhibitors with inhibition constants in the low nanomolar or even sub-nanomolar range. For seven derivatives the crystal structures in complex with furin could be determined. In three complexes, electron density was found for the entire inhibitor. In the other cases the structures could be determined only for the P6/P5-P1 segments, which directly interact with furin. The cyclic derivatives together with two non-cyclic reference compounds were tested as inhibitors of the proteolytic activation and replication of respiratory syncytial virus in cells. Significant antiviral activity was found for both linear reference inhibitors, whereas a negligible efficacy was determined for the cyclic derivatives.

Furin processing dictates ectodomain shedding of human FAT1 cadherin

Fat1 is a single pass transmembrane protein and the largest member of the cadherin superfamily. Mouse knockout models and in vitro studies have suggested that Fat1 influences cell polarity and motility. Fat1 is also an upstream regulator of the Hippo pathway, at least in lower vertebrates, and hence may play a role in growth control. In previous work we have established that FAT1 cadherin is initially cleaved by proprotein convertases to form a noncovalently linked heterodimer prior to expression on the cell surface. Such processing was not a requirement for cell surface expression, since melanoma cells expressed both unprocessed FAT1 and the heterodimer on the cell surface. Here we further establish that the site 1 (S1) cleavage step to promote FAT1 heterodimerisation is catalysed by furin and we identify the cleavage site utilised. For a number of other transmembrane receptors that undergo heterodimerisation the S1 processing step is thought to occur constitutively but the functional significance of heterodimerisation has been controversial. It has also been generally unclear as to the significance of receptor heterodimerisation with respect to subsequent post-translational proteolysis that often occurs in transmembrane proteins. Exploiting the partial deficiency of FAT1 processing in melanoma cells together with furin-deficient LoVo cells, we manipulated furin expression to demonstrate that only the heterodimer form of FAT1 is subject to cleavage and subsequent release of the extracellular domain. This work establishes S1-processing as a clear functional prerequisite for ectodomain shedding of FAT1 with general implications for the shedding of other transmembrane receptors.

Intercellular junction assembly, dynamics, and homeostasis

Intercellular anchoring junctions are highly specialized regions of the plasma membrane where members of the cadherin family of transmembrane adhesion molecules on opposing cells interact through their extracellular domains, and through their cytoplasmic domains serve as a platform for organizing cytoskeletal anchors and remodelers. Here we focus on assembly of so-called "anchoring" or "adhering" junctions-adherens junctions (AJs) and desmosomes (DSMs), which associate with actin and intermediate filaments, respectively. We will examine how the assembly and function of AJs and DSMs are intimately connected during embryogenesis and in adult cells and tissues, and in some cases even form specialized "mixed" junctions. We will explore signaling and trafficking machineries that drive assembly and remodeling and how these mechanisms are co-opted in human disease.

Potent inhibitors of furin and furin-like proprotein convertases containing decarboxylated P1 arginine mimetics

Furin belongs to the family of proprotein convertases (PCs) and is involved in numerous normal physiological and pathogenic processes, such as viral propagation, bacterial toxin activation, cancer, and metastasis. Furin and related furin-like PCs cleave their substrates at characteristic multibasic consensus sequences, preferentially after an arginine residue. By incorporating decarboxylated arginine mimetics in the P1 position of substrate analogue peptidic inhibitors, we could identify highly potent furin inhibitors. The most potent compound, phenylacetyl-Arg-Val-Arg-4-amidinobenzylamide (15), inhibits furin with a K(i) value of 0.81 nM and has also comparable affinity to other PCs like PC1/3, PACE4, and PC5/6, whereas PC2 and PC7 or trypsin-like serine proteases were poorly affected. In fowl plague virus (influenza A, H7N1)-infected MDCK cells, inhibitor 15 inhibited proteolytic hemagglutinin cleavage and was able to reduce virus propagation in a long-term infection test. Molecular modeling revealed several key interactions of the 4-amidinobenzylamide residue in the S1 pocket of furin contributing to the excellent affinity of these inhibitors.

PC2/CPE-mediated pro-protein processing in tumor cells and its differentiated cells or tissues

Pro-protein convertase-2 (PC2) and carboxypeptidase-E (CPE) proteins are two major members of the pro-protein convertases that involve in the maturation of protein precursor. By using PC2 activity, immunocytochemistry (ICC) and Western blot method, PC2, CPE and preproNPY protein expression levels were compared among mature retina tissue, RGC-5 cells and its differentiated cells, or brain cortex tissue, NS20Y tumor cells and its differentiated cells, or mature breast tissue, breast tumor cell RM1 and breast adenocarcinoma tissue. The experimental results indicated that the differentiated cells or tissues had higher or highest PC2 activity. In the comparative experiments, more PC2 protein expression in the mature tissues and more CPE and preproNPY protein expression in the tumor cells or tumor tissue were observed, but no expression of preproNPY protein was observed in the mature tissues. Compared with NS20Y or RGC-5 undifferentiated cells, its differentiated cells showed less proPC2, more proCPE and more preproNPY protein expressions. The results demonstrated that the mature tissues showed stronger PC2/CPE-mediated pro-protein processing ability than the tumor cells or tissue. The results also showed that the artificial differentiation of RGC-5 or NS20Y cells was different from maturation of its corresponding normal tissue.

Pro-protein convertase-2/carboxypeptidase-E mediated neuropeptide processing of RGC-5 cell after in vitro ischemia

OBJECTIVE

To observe the change of the neuropeptide pro-protein processing system in the ischemic retina ganglion cell-5 (RGC-5) cells, pro-protein convertase-2 (PC2), carboxypeptidase-E (CPE) and preproneuropeptide Y (preproNPY) protein levels in the ischemic RGC-5 cells and conditioned medium were analyzed.

METHODS

The RGC-5 cell was differentiated in 0.1 mumol/L staurosporine for 24 h and then stressed by different doses of oxygen and glucose deprivation (OGD). The acute or chronic OGD-induced cell death rates were obtained by using PI or TUNEL staining. The protein expression levels were determined by using the Western blot method and PC2 activity analysis.

RESULTS

The ischemia caused substantial cell death in an OGD dose-dependent manner. In the cells, proPC2 and preproNPY protein levels gradually increased whereas proCPE gradually decreased. After OGD, PC2 activity was decreased. In the conditioned medium, proPC2 and PC2 proteins gradually decreased whereas proCPE, CPE, and preproNPY proteins gradually increased.

CONCLUSION

These results demonstrated that OGD inhibited the neuropeptide pro-protein processing system by reducing PC2 activity and the maturation of proPC2. The aggregation of the pro-proteins and the increase of the active CPE excision adversely exacerbated the cell injury. The pro-protein processing system might play a critical role in the ischemic stress of RGC-5 cells.

Peptide processing and biology in human disease

PURPOSE OF REVIEW

To describe recent advances in the processing of gastrointestinal hormones, and the consequences for human disease of mutations in the enzymes involved.

RECENT FINDINGS

Although gastrointestinal prohormones were long regarded as devoid of biological activity, recent data indicate that the prohormones for both gastrin and gastrin-releasing peptide are bioactive, through different receptors from the mature hormones. Mutations in the family of prohormone convertases responsible for the initial steps in the processing of gastrointestinal hormones are associated with several different pathophysiological conditions in humans.

SUMMARY

Human mutational studies, when taken together with the phenotypes observed in mice deficient in the prohormone convertases, emphasize the crucial importance of the processing enzymes in mammalian biology. Although the phenotypes may often be ascribed to defective production of a mature hormone or growth factor, the recognition that the precursors are independently bioactive suggests that the increased precursor concentrations may also contribute to the symptoms. The observation that the precursors often act through different receptors from the mature hormones may permit the development of precursor-selective antagonists for therapeutic use.

Potential opportunity in the development of new therapeutic agents based on endogenous and exogenous inhibitors of the proprotein convertases

The proprotein convertases (PCs) are responsible for the endoproteolytic processing of various protein precursors (e.g., growth factors, receptors, adhesion molecules, and matrix metalloproteinases) implicated in several diseases such as obesity, diabetes, atherosclerosis, cancer, and Alzheimer disease. The potential clinical and pharmacological role of the PCs has fostered the development of various PC‐inhibitors. In this review we summarized the recent findings on PCs inhibitors, their mode of actions and potential use in the therapy of various diseases. © 2006 Wiley Periodicals, Inc. Med Res Rev, 27, No. 5, 631–648, 2007

Plakoglobin-dependent disruption of the desmosomal plaque in pemphigus vulgaris

We recently reported that the pathogenesis of pemphigus vulgaris (PV), an autoimmune blistering skin disorder, is driven by the accumulation of c-Myc secondary to abrogation of plakoglobin (PG)-mediated transcriptional c-Myc suppression. PG knock-out mouse keratinocytes express high levels of c-Myc and resemble PVIgG-treated wild-type keratinocytes in most respects. However, they fail to accumulate nuclear c-Myc and loose intercellular adhesion in response to PVIgG-treatment like wild-type keratinocytes. This suggested that PG is also required for propagation of the PVIgG-induced events between augmented c-Myc expression and acantholysis. Here, we addressed this possibility by comparing PVIgG-induced changes in the desmosomal organization between wild-type and PG knock-out keratinocytes. We found that either bivalent PVIgG or monovalent PV-Fab (known to trigger blister formation in vivo) disrupt the linear organization of all major desmosomal components along cell borders in wild-type keratinocytes, simultaneously with a reduction in intercellular adhesive strength. In contrast, PV-Fab failed to affect PG knock-out keratinocytes while PVIgG cross-linked their desmosomal cadherins without significantly affecting desmoplakin. These results identify PG as a principle effector of the PVIgG-induced signals downstream of c-Myc that disrupt the desmosomal plaque at the plasma membrane.

Short polybasic peptide sequences are potent inhibitors of PC5/6 and PC7: Use of positional scanning-synthetic peptide combinatorial libraries as a tool for the optimization of inhibitory sequences

Positional scanning-synthetic peptide combinatorial libraries (PS-SPCLs) are powerful molecular tools to identify enzyme substrate and potent inhibitory sequences and also to provide crucial information about active site determinants. PS-SPCLs have been surveyed for furin, proprotein convertase (PC)2, PC1/3, and PACE4 and proven efficient to identify potent peptidyl inhibitors in the low nanomolar range for furin and PC1/3. We report herein the screenings of nonamidated and acetylated hexapeptide PS-SPCLs for PC5/6A and PC7. The L-configuration library surveys distinctively revealed that L-Arg, L-Lys, and sometimes L-His in all six positions would generate the most potent inhibitors for both enzymes. Based on this clear polybasic preference, L-poly-Arg peptides ranging from four to nine residues were assayed. Inhibitory potency of these polybasic peptides increased with chain length, making nona-l-Arg a potent nanomolar inhibitor of PC5/6A and PC7 (Ki of 150 and 120 nM). PC5/6 and PC7 inhibition by nona-l-arginine was equivalent to that of furin (Ki of 114 nM) (J Biol Chem 275: 36741-36749, 2000). Nona-d-arginine was a more potent inhibitor of PC5/6 and PC7 than its levorotatory version (Ki of 19 and 81 nM), reminiscent of furin (Ki of 1.3 nM) (J Biol Chem 279:36788-36794, 2004). Our data indicate that certain poly-arginine peptides represent potent inhibitors targeting PCs of the constitutive secretory pathway (furin, PC5/6, and PC7). We conclude that basic residues within PC peptide inhibitors might be responsible for targeting PCs in general and for inhibitory potency, but that select amino acid changes will be necessary to acquire true specificity toward a single PC.

Mechanisms of plakoglobin-dependent adhesion: desmosome-specific functions in assembly and regulation by epidermal growth factor receptor

Plakoglobin (PG) is a member of the Armadillo family of adhesion/signaling proteins that can be incorporated into both adherens junctions and desmosomes. Loss of PG results in defects in the mechanical integrity of heart and skin and decreased adhesive strength in keratinocyte cultures established from the skin of PG knock-out (PG-/-) mice, the latter of which cannot be compensated for by overexpressing the closely related beta-catenin. In this study, we examined the mechanisms of PG-regulated adhesion in murine keratinocytes. Biochemical and morphological analyses indicated that junctional incorporation of desmosomal, but not adherens junction, components was impaired in PG-/- cells compared with PG+/- controls. Re-expression of PG, but not beta-catenin, in PG-/- cells largely reversed these effects, indicating a key role for PG in desmosome assembly. Epidermal growth factor (EGF) receptor activation resulted in Tyr phosphorylation of PG, which was accompanied by a loss of desmoplakin from desmosomes and decreased adhesive strength following 18-h EGF treatment. Importantly, introduction of a phosphorylation-deficient PG mutant into PG null cells prevented the EGF receptor-dependent loss of desmoplakin from junctions, attenuating the effects of long term EGF treatment on cell adhesion. Therefore, PG is essential for maintaining and regulating adhesive strength in keratinocytes largely through its contributions to desmosome assembly and structure. As a target for modulation by EGF, regulation of PG-dependent adhesion may play an important role during wound healing and tumor metastasis.