Defective processing of the insulin receptor in an endoprotease-deficient Chinese hamster cell strain is corrected by expression of mouse furin

An Overview of the Role of Furin in Type 2 Diabetes

Post-translational modifications (PTMs) play important roles in regulating several human diseases, like cancer, neurodegenerative disorders, and metabolic disorders. Investigating PTMs' contribution to protein functions is critical for modern biology and medicine. Proprotein convertases (PCs) are irreversible post-translational modifiers that have been extensively studied and are considered as key targets for novel therapeutics. They cleave proteins at specific sites causing conformational changes affecting their functions. Furin is considered as a PC model in regulating growth factors and is involved in regulating many pro-proteins. The mammalian target of the rapamycin (mTOR) signaling pathway is another key player in regulating cellular processes and its dysregulation is linked to several diseases including type 2 diabetes (T2D). The role of furin in the context of diabetes has been rarely explored and is currently lacking. Moreover, furin variants have altered activity that could have implications on overall health. In this review, we aim to highlight the role of furin in T2D in relation to mTOR signaling. We will also address furin genetic variants and their potential effect on T2D and β-cell functions. Understanding the role of furin in prediabetes and dissecting it from other confounding factors like obesity is crucial for future therapeutic interventions in metabolic disorders.

Endoplasmic reticulum stress causes insulin resistance by inhibiting delivery of newly synthesized insulin receptors to the cell surface

Accumulation of unfolded proteins in the endoplasmic reticulum (ER) causes ER stress and activates a signaling network known as the unfolded protein response (UPR). Here we characterize how ER stress and the UPR inhibit insulin signaling. We find that ER stress inhibits insulin signaling by depleting the cell surface population of the insulin receptor. ER stress inhibits proteolytic maturation of insulin proreceptors by interfering with transport of newly synthesized insulin proreceptors from the ER to the plasma membrane. Activation of AKT, a major target of the insulin signaling pathway, by a cytosolic, membrane-bound chimera between the AP20187-inducible FV2E dimerization domain and the cytosolic protein tyrosine kinase domain of the insulin receptor was not affected by ER stress. Hence, signaling events in the UPR, such as activation of the JNK mitogen-activated protein (MAP) kinases or the pseudokinase TRB3 by the ER stress sensors IRE1α and PERK, do not contribute to inhibition of signal transduction in the insulin signaling pathway. Indeed, pharmacologic inhibition and genetic ablation of JNKs, as well as silencing of expression of TRB3, did not restore insulin sensitivity or rescue processing of newly synthesized insulin receptors in ER-stressed cells. [Media: see text].

The beta secretase BACE1 regulates the expression of insulin receptor in the liver

Insulin receptor (IR) plays a key role in the control of glucose homeostasis; however, the regulation of its cellular expression remains poorly understood. Here we show that the amount of biologically active IR is regulated by the cleavage of its ectodomain, by the β-site amyloid precursor protein cleaving enzyme 1 (BACE1), in a glucose concentration-dependent manner. In vivo studies demonstrate that BACE1 regulates the amount of IR and insulin signaling in the liver. During diabetes, BACE1-dependent cleavage of IR is increased and the amount of IR in the liver is reduced, whereas infusion of a BACE1 inhibitor partially restores liver IR. We suggest the potential use of BACE1 inhibitors to enhance insulin signaling during diabetes. Additionally, we show that plasma levels of cleaved IR reflect IR isoform A expression levels in liver tumors, which prompts us to propose that the measurement of circulating cleaved IR may assist hepatic cancer detection and management.

Insulin-induced translocation of IR to the nucleus in insulin responsive cells requires a nuclear translocation sequence

Insulin binding to its cell surface receptor (IR) activates a cascade of events leading to its biological effects. The Insulin-IR complex is rapidly internalized and then is either recycled back to the plasma membrane or sent to lysosomes for degradation. Although most of the receptor is recycled or degraded, a small amount may escape this pathway and migrate to the nucleus of the cell where it might be important in promulgation of receptor signals. In this study we explored the mechanism by which insulin induces IR translocation to the cell nucleus. Experiments were performed cultured L6 myoblasts, AML liver cells and 3T3-L1 adipocytes. Insulin treatment induced a rapid increase in nuclear IR protein levels within 2 to 5 min. Treatment with WGA, an inhibitor of nuclear import, reduced insulin-induced increases nuclear IR protein; IR was, however, translocated to a perinuclear location. Bioinformatics tools predicted a potential nuclear localization sequence (NLS) on IR. Immunofluorescence staining showed that a point mutation on the predicted NLS blocked insulin-induced IR nuclear translocation. In addition, blockade of nuclear IR activation in isolated nuclei by an IR blocking antibody abrogated insulin-induced increases in IR tyrosine phosphorylation and nuclear PKCδ levels. Furthermore, over expression of mutated IR reduced insulin-induced glucose uptake and PKB phosphorylation. When added to isolated nuclei, insulin induced IR phosphorylation but had no effect on nuclear IR protein levels. These results raise questions regarding the possible role of nuclear IR in IR signaling and insulin resistance.

The paired basic amino acid-cleaving enzyme 4 (PACE4) is involved in the maturation of insulin receptor isoform B: an opportunity to reduce the specific insulin receptor-dependent effects of insulin-like growth factor 2 (IGF2)

Gaining the full activity of the insulin receptor (IR) requires the proteolytic cleavage of its proform by intra-Golgi furin-like activity. In mammalian cells, IR is expressed as two isoforms (IRB and IRA) that are responsible for insulin action. However, only IRA transmits the growth-promoting and mitogenic effects of insulin-like growth factor 2. Here we demonstrate that the two IR isoforms are similarly cleaved by furin, but when this furin-dependent maturation is inefficient, IR proforms move to the cell surface where the proprotein convertase PACE4 selectively supports IRB maturation. Therefore, in situations of impaired furin activity, the proteolytic maturation of IRB is greater than that of IRA, and accordingly, the amount of phosphorylated IRB is also greater than that of IRA. We highlight the ability of a particular proprotein convertase inhibitor to effectively reduce the maturation of IRA and its associated mitogenic signaling without altering the signals emanating from IRB. In conclusion, the selective PACE4-dependent maturation of IRB occurs when furin activity is reduced; accordingly, the pharmacological inhibition of furin reduces IRA maturation and its mitogenic potential without altering the insulin effects.

Ebola virus envelope glycoprotein derived peptide in human Furin-bound state: computational studies

Ebola virus (EboV) is currently ravaging West Africa with estimated case fatality rate of 52%. Currently, no drug treatment is available and immunoglobulin therapy is still at the rudimentary stage. For anti-EboV drug development, druggable viral and host protein targets, including human Furin are under intense investigation. Here, molecular dynamics simulation was performed on Apo-Furin, meta-guanidinomethyl-Phac-RVR-Amba-bound, and two EboV glycoprotein (GP) 494-TGGRRTRREA-503/Furin complexes (Accurate and one amino acid shift alignment). The results of the simulation established ligand-induced desolvation of Furin active site and structural compactness. Accurately aligned EboV-GP peptide exhibited a tighter binding mode with Furin and showed 1.5- and 3.0-fold MMPBSA binding free energy estimate compared with the displaced peptide and inhibitor, respectively. The difference in free energy was traced to the difference in contribution of threonine residues of the peptides. Furthermore, Furin subsites I conferred substrate specificity and ligand binding accuracy. Accurately aligned peptide trapped active site His194 side chain into gauche (-) (+60(o)) χ1-dihedral compared with gauche+ (-60(o)) in other biosystems while Asp153 is trapped in gauche+ (-60(o)) in ligand bound not Apo state. Ramachandran plot showed that the scissile Arg8 of the accurately aligned peptide showed β conformation distribution as apposed to 310R, αL, and 310L. Finally, the active site proximal Na(+) binding is dependent on substrate peptide occupancy of the active site but detaches in the absence of a ligand. In conclusion, Furin might represent candidate drug target for Ebola virus disease treatment via therapeutic target of the active site and Na(+) binding pocket.

High-resolution analysis and functional mapping of cleavage sites and substrate proteins of furin in the human proteome

Background

There is a growing appreciation of the role of proteolytic processes in human health and disease, but tools for analysis of such processes on a proteome-wide scale are limited. Furin is a ubiquitous proprotein convertase that cleaves after basic residues and transforms secretory proproteins into biologically active proteins. Despite this important role, many furin substrates remain unknown in the human proteome.

Methodology/Principal Findings

We devised an approach for proteinase target identification that combines an in silico discovery pipeline with highly multiplexed proteinase activity assays. We performed in silico analysis of the human proteome and identified over 1,050 secretory proteins as potential furin substrates. We then used a multiplexed protease assay to validate these tentative targets. The assay was carried out on over 3,260 overlapping peptides designed to represent P7-P1’ and P4-P4’ positions of furin cleavage sites in the candidate proteins. The obtained results greatly increased our knowledge of the unique cleavage preferences of furin, revealed the importance of both short-range (P4-P1) and long-range (P7-P6) interactions in defining furin cleavage specificity, demonstrated that the R-X-R/K/X-R↓ motif alone is insufficient for predicting furin proteolysis of the substrate, and identified ∼490 potential protein substrates of furin in the human proteome.

Conclusions/Significance

The assignment of these substrates to cellular pathways suggests an important role of furin in development, including axonal guidance, cardiogenesis, and maintenance of stem cell pluripotency. The novel approach proposed in this study can be readily applied to other proteinases.

Plasminogen activator inhibitor 1 is an intracellular inhibitor of furin proprotein convertase

Proprotein convertases (PCs) are a family of serine proteases that are involved in the post-translational processing and activation of a wide range of regulatory proteins. The upstream role of PCs in the control of many physiological and pathological processes generates a growing interest in understanding their regulation. Here, we demonstrate that the serine protease inhibitor plasminogen activator inhibitor 1 (PAI-1) forms an SDS-stable complex with the PC furin, which leads to the inhibition of the intra-Golgi activity of furin. It is known that elevated PAI-1 plasma levels are correlated with the occurrence of the metabolic syndrome and type 2 diabetes, and we show that PAI-1 reduces the furin-dependent maturation and activity of the insulin receptor and ADAM17: two proteins involved in the onset of these metabolic disorders. In addition to demonstrating that PAI-1 is an intracellular inhibitor of furin, this study also provides arguments in favor of an active role for PAI-1 in the development of metabolic disorders.

Inhibition of insulin receptor isoform-A signalling restores sensitivity to gefitinib in previously de novo resistant colon cancer cells

Resistance to antiepidermal growth factor (EGFR) strategies is an emerging clinical problem. Using human colorectal cancer (CRC) cells, we evaluated the involvement of the insulin receptor isoform-A (InsR-A) in de novo resistance to gefitinib, an EGFR tyrosine kinase inhibitor. Challenging the EGFR positive LoVo cells with gefitinib (1 microM) resulted in a small ( approximately 18%) inhibition of cell growth and although a modest reduction in phospho (p)EGFR Tyr845 was seen, pEGFR at residues -Tyr1068 and -Tyr1173 were unchanged. LoVo cells produced unprocessed pro-IGF-1R protein, substantial levels of IGF-II mRNA and mature InsR protein, consisting mainly of the InsR-A isoform. Insulin and IGF-II promoted cell growth and pEGFR Tyr845, Tyr1068 and Tyr1173 activity and conversely, the insulin-like growth factor-1 receptor (IGF-1R)/InsR inhibitor ABDP (1 muM) inhibited growth and reduced pEGFR activity at all three tyrosine residues. pInsR and pAkt levels were increased after gefitinib treatment. Blocking of pInsR with ABDP enabled gefitinib to markedly reduce pEGFR Tyr845, Tyr1068 and Tyr1173. Short-term gefitinib/ABDP dual treatment was more effective than either agent alone and chronic exposure to this combination resulted in total cell loss after 9 weeks, preventing acquisition of resistance to ABDP. LoVo cells with acquired resistance to ABDP were acutely sensitive to gefitinib. We concluded that InsR-A reduces sensitivity to gefitinib in LoVo CRC cells, thus its co-targeting alongside EGFR can improve the anti-tumour effect of gefitinib.

Limited redundancy of the proprotein convertase furin in mouse liver

Furin is an endoprotease of the family of mammalian proprotein convertases and is involved in the activation of a large variety of regulatory proteins by cleavage at basic motifs. A large number of substrates have been attributed to furin on the basis of in vitro and ex vivo data. However, no physiological substrates have been confirmed directly in a mammalian model system, and early embryonic lethality of a furin knock-out mouse model has precluded in vivo verification of most candidate substrates. Here, we report the generation and characterization of an interferon inducible Mx-Cre/loxP furin knock-out mouse model. Induction resulted in near-complete ablation of the floxed fur exon in liver. In sharp contrast with the general furin knock-out mouse model, no obvious adverse effects were observed in the transgenic mice after induction. Histological analysis of the liver did not reveal any overt deviations from normal morphology. Analysis of candidate substrates in liver revealed complete redundancy for the processing of the insulin receptor. Variable degrees of redundancy were observed for the processing of albumin, alpha(5) integrin, lipoprotein receptor-related protein, vitronectin and alpha(1)-microglobulin/bikunin. None of the tested substrates displayed a complete block of processing. The absence of a severe phenotype raises the possibility of using furin as a local therapeutic target in the treatment of pathologies like cancer and viral infections, although the observed redundancy may require combination therapy or the development of a more broad spectrum convertase inhibitor.

The kindest cuts of all: crystal structures of Kex2 and furin reveal secrets of precursor processing

Pro-hormone or pro-protein convertases are a conserved family of eukaryotic serine proteases found in the secretory pathway. These endoproteases mature precursors for peptides and proteins that perform a wide range of physiologically important and clinically relevant functions. The first member of this family to be identified was Kex2 in the yeast Saccharomyces cerevisiae. One mammalian member of this family - furin - is responsible for processing substrates that include insulin pro-receptor, human immunodeficiency virus gp160 glycoprotein, Ebola virus glycoprotein, and anthrax protective antigen. Recent determination of the crystal structures for the catalytic core domains of both Kex2 and furin - the first for any members of this family - provide remarkable insights and a new level of understanding of substrate specificity and catalysis by the pro-protein convertases.

Proprotein convertases in tumor progression and malignancy: novel targets in cancer therapy

The mammalian subtilisin/kexin-like proprotein convertase (PC) family has been implicated in the activation of a wide spectrum of proteins. These proteins are usually synthesized as inactive precursors before their conversion to fully mature bioactive forms. A large majority of these active proteins such as matrix metalloproteases, growth factors, and adhesion molecules are crucial in the processes of cellular transformation, acquisition of the tumorigenic phenotype, and metastases formation. Inhibition of PCs significantly affects the malignant phenotype of various tumor cells. In addition to direct tumor cell proliferation and migration blockade, PC inhibitors can also be used to target tumor angiogenesis. In this Review article we discuss a number of recent findings on the clinical relevance of PCs in cancer patients, their implication in the regulation of multiple cellular functions that impact on the invasive/metastatic potential of cancer cells. Thus, PC inhibitors may constitute new promising agents for the treatment of multiple tumors and/or in adjuvant therapy to prevent recurrence.

Identification and characterization of SNX15, a novel sorting nexin involved in protein trafficking

Sorting nexins are a family of phox homology domain containing proteins that are homologous to yeast proteins involved in protein trafficking. We have identified a novel 342-amino acid residue sorting nexin, SNX15, and a 252-amino acid splice variant, SNX15A. Unlike many sorting nexins, a SNX15 ortholog has not been identified in yeast or Caenorhabditis elegans. By Northern blot analysis, SNX15 mRNA is widely expressed. Although predicted to be a soluble protein, both endogenous and overexpressed SNX15 are found on membranes and in the cytosol. The phox homology domain of SNX15 is required for its membrane association and for association with the platelet-derived growth factor receptor. We did not detect association of SNX15 with receptors for epidermal growth factor or insulin. However, overexpression of SNX15 led to a decrease in the processing of insulin and hepatocyte growth factor receptors to their mature subunits. Immunofluorescence studies showed that SNX15 overexpression resulted in mislocalization of furin, the endoprotease responsible for cleavage of insulin and hepatocyte growth factor receptors. Based on our data and the existing findings with yeast orthologs of other sorting nexins, we propose that overexpression of SNX15 disrupts the normal trafficking of proteins from the plasma membrane to recycling endosomes or the trans-Golgi network.

Evidence that furin is an authentic transforming growth factor-beta1-converting enzyme

Transforming growth factor (TGF)-beta1 plays an essential role in cell growth and differentiation. It is also considered as a gatekeeper of immune homeostasis with gene disruption leading to autoimmune and inflammatory diseases. TGF-beta1 is produced as an inactive precursor polypeptide that can be efficiently secreted but correct proteolytic cleavage is an essential step for its activation. Assessment of the cleavage site has revealed a unique R-H-R-R sequence reminiscent of proprotein convertase (PC) recognition motifs and has previously demonstrated that this PC-like cleavage site is correctly cleaved by furin, a member of the PC family. Here we report that among PC members, furin more closely satisfies the requirements needed to fulfill the role of a genuine TGF-beta1 convertase. Even though six members of the PC family have the ability to cleave TGF-beta1, ectopic expression of alpha(1)-antitrypsin Portland (alpha(1)-AT-PDX), a potent furin inhibitor, blocked 80% of TGF-beta1 processing mediated by endogenous enzymes as demonstrated in an in vitro digestion assay. Genetic complementation of a furin-deficient LoVo cell line with the wild-type gene restores the production of mature and bioactivable TGF-beta1. Moreover, both furin and TGF-beta are coordinately expressed and regulated in vitro and in vivo in the hematopoietic and immune system, an important tissue target. These results demonstrate for the first time that furin is an authentic and adaptive TGF-beta1-converting enzyme whereas other members of the PC family might substitute or supplement furin activity. Our study advances our comprehension of the complexity of the TGF-beta system and should facilitate the development of therapeutically useful TGF-beta inhibitors.

Furin-mediated processing in the early secretory pathway: sequential cleavage and degradation of misfolded insulin receptors

Improperly folded membrane proteins are retained in the endoplasmic reticulum and then diverted to a degradative pathway by a network of molecular chaperones and intracellular proteases. Here we report that mutant insulin proreceptors (Pro(62)) retained in the early secretory pathway undergo proteolytic cleavage at a tetrabasic concensus site for the subtilisin-like protease furin (SPC 1), generating two unstable proteolytic intermediates of 80/120 kDa corresponding to alpha (135 kDa) and beta (90 kDa) subunits. These are degraded more rapidly than the uncleaved proreceptor protein. Site-directed mutagenesis of the normal RKRR processing site prevented cleavage. Use of inhibitors and furin-deficient cell lines confirmed that furin is responsible for proreceptor cleavage; furin overexpression increased the degradation of mutant but not wild-type receptors. Together, these results suggest that processing and degradation occur sequentially for mutant proreceptors.

Quantitative characterization of furin specificity. Energetics of substrate discrimination using an internally consistent set of hexapeptidyl methylcoumarinamides

Furin, an essential mammalian proprotein processing enzyme of the kexin/furin family of subtilisin-related eukaryotic processing proteases, is implicated in maturation of substrates involved in development, signaling, coagulation, and pathogenesis. We examined the energetics of furin specificity using a series of peptidyl methylcoumarinamide substrates. In contrast to previous reports, we found that furin can cleave such substrates with kinetics comparable to those observed with extended peptides and physiological substrates. With the best of these hexapeptidyl methylcoumarinamides, furin displayed k(cat)/K(m) values greater than 10(6) M(-1) s(-1). Furin exhibited striking substrate inhibition with hexapeptide but not tetrapeptide substrates, an observation of significance to the evaluation of peptide-based furin inhibitors. Quantitative comparison of furin and Kex2 recognition at P(1), P(2), and P(4) demonstrates that whereas interactions at P(1) make comparable contributions to catalysis by the two enzymes, furin exhibited a approximately 10-fold lesser dependence on P(2) recognition but a 10-100-fold greater dependence on P(4) recognition. Furin has recently been shown to exhibit P(6) recognition and we found that this interaction contributes approximately 1.4 kcal/mol toward catalysis independent of the nature of the P(4) residue. We have also shown that favorable residues at P(2) and P(6) will compensate for less than optimal residues at either P(1) or P(4). The quantitative analysis of furin and Kex2 specificity sharply distinguish the nature of substrate recognition by the processing and degradative members of subtilisin-related proteases.

Processing of human intestinal prolactase to an intermediate form by furin or by a furin-like proprotein convertase

Human lactase-phlorizin hydrolase (human-LPH) is synthesized as a large precursor (prepro-LPH), then cleaved to a pro-LPH of 220 kDa which is further cut to a "mature-like LPH" of a size close to that of mature LPH, i.e. about 150 kDa (in the processing of rabbit pro-LPH the intermediate has a mass of approximately 180 kDa). By coexpression of human prepro-LPH with furin in COS-7 cells we show that furin generates a mature-like LPH. Radioactive amino acid sequence analysis reveals that furin recognizes the motif R-T-P-R832, a protein convertase consensus, to generate a NH2 terminus located 36 amino acids upstream of the NH2 terminal found in vivo at Ala869. This intermediate is ultimately cleaved to the mature LPH form by other proteases including the pancreatic ones. These data demonstrate that human pro-LPH, like the rabbit enzyme, is processed to the mature enzyme by furin or furin-like enzymes through at least an intermediate form that has, however, an apparent mass close to that of the mature enzyme.

Deficient processing and activity of type I insulin-like growth factor receptor in the furin-deficient LoVo-C5 cells

To investigate endoproteolytic processing of the type I insulin-like growth factor receptor (IGF-IR), we have examined its structure and activity in the furin-deficient LoVo-C5 cell line. Immunoprecipitation experiments using the monoclonal anti-IGF-IR antibody (alpha-IR3) showed that LoVo-C5 cells expressed a major high molecular mass receptor (200 kDa) corresponding to the unprocessed alpha/beta pro-receptor. A small amount of successfully cleaved alpha/beta heterodimers was also produced, indicating a residual endoproteolytic cleavage activity in these cells. In vitro, a soluble form of recombinant furin was able to cleave the pro-IGF-IR (200 kDa) into alpha-subunit (130 kDa) and beta-subunit (97 kDa). Measurement of IGF binding parameters in LoVo-C5 cells indicated a low number of typical type I IGF-binding sites (binding capacity, 5 x 10(3) sites/cell; Kd, 1.9 nM for IGF-I and 7.0 nM for IGF-II). These findings in LoVo-C5 contrast with those in HT29-D4 cells, which have active furin, and where IGF-IR (2.8 x 10(4) sites/cell) was fully processed. Moreover, the 200-kDa pro-IGF-IR of LoVo-C5 was unable to induce intracellular signaling, such as beta-subunit tyrosine autophosphorylation and insulin-related substrate-1 tyrosine phosphorylation. Flow immunocytometry analysis using alpha-IR3 antibody indicated that LoVo-C5 cells expressed 40% more receptors than HT29-D4 cells, suggesting that in LoVo-C5 cells only the small amount of mature type I IGF-IR binds IGFs with high affinity. To provide evidence for this idea, we showed that mild trypsin treatment of living LoVo-C5 cells partially restored alpha/beta cleavage of IGF-IR, and greatly enhanced (6-fold) the IGF-I binding capacity of LoVo-C5 cells, but did not restore IGF-IR signaling activity. Moreover, LoVo-C5 cells were totally unresponsive to IGF-I in terms of cell migration, in contrast to fully processed IGF-IR-HT29-D4 cells. Our data indicate that furin is involved in the endoproteolytic processing of the IGF-IR and suggest that this posttranslational event might be crucial for its ligand binding and signaling activities. However, our data do not exclude that other proprotein convertases could participate to IGF-IR maturation.

Folding of insulin receptor monomers is facilitated by the molecular chaperones calnexin and calreticulin and impaired by rapid dimerization

Many complex membrane proteins undergo subunit folding and assembly in the ER before transport to the cell surface. Receptors for insulin and insulin-like growth factor I, both integral membrane proteins and members of the family of receptor tyrosine kinases (RTKs), are unusual in that they require homodimerization before export from the ER. To better understand chaperone mechanisms in endogenous membrane protein assembly in living cells, we have examined the folding, assembly, and transport of the human insulin receptor (HIR), a dimeric RTK. Using pulse-chase labeling and nonreducing SDS-PAGE analysis, we have explored the molecular basis of several sequential maturation steps during receptor biosynthesis. Under normal growth conditions, newly synthesized receptor monomers undergo disulfide bond formation while associated with the homologous chaperones calnexin (Cnx) and calreticulin (Crt). An inhibitor of glucose trimming, castanospermine (CST), abolished binding to Cnx/Crt but also unexpectedly accelerated receptor homodimerization resulting in misfolded oligomeric proreceptors whose processing was delayed and cell surface expression was also decreased by approximately 30%. Prematurely-dimerized receptors were retained in the ER and more avidly associated with the heat shock protein of 70 kD homologue binding protein. In CST-treated cells, receptor misfolding followed disordered oligomerization. Together, these studies demonstrate a chaperone function for Cnx/Crt in HIR folding in vivo and also provide evidence that folding efficiency and homodimerization are counterbalanced.

Effects of proteasomal inhibitors on the maturation of the insulin proreceptor: an anatomical paradox

Inhibitors of proteasomal functions Carbobenzoxy-L-leucyl-L-leucyl-L-leucinal (MG132) and Carbobenzoxy-L-isoleucyl-gamma-t-butyl-L-alanyl-L-leucinal (PSI) were found to inhibit the conversion of the Insulin proreceptor to its mature alpha and beta subunits. By contrast no effect of these inhibitors was found on 125-I insulin binding, internalization and degradation. Since the insulin proreceptor is an integral membrane protein that is compartmentally separated from the cytoplasmic 26S proteasome, the inhibition of the normal biosynthetic processing of the insulin proreceptor presents an anatomical paradox.

Functional expression of the chicken low density lipoprotein receptor-related protein in a mutant chinese hamster ovary cell line restores toxicity of Pseudomonas exotoxin A and degradation of alpha2-macroglobulin

The low density lipoprotein receptor-related protein (LRP) is responsible for the clearance of several physiological ligands including a complex of proteinase and alpha2-macroglobulin (alpha2M) and for the entrance of Pseudomonas exotoxin A (PEA) into cells. We have prepared expression plasmids for the full-length chicken LRP (designated LRP100) and two intermediates encoding 25 and 67% of the receptor (designated LRP25 and LRP67, respectively) using overlapping cDNA fragments. LRP25 and LRP67 encode the N-terminal 22 and 64%, respectively, of LRP100 plus the transmembrane and intracellular domains. Transient transfection of these plasmids into COS-7 cells yielded recombinant proteins of expected molecular mass and immunoreactivity. However, LRP100 was incompletely processed into alpha- (515-kDa) and beta- (85-kDa) chains and was poorly transported from the endoplasmic reticulum to the Golgi compartment. Stable transformants of LRP100, LRP67, and LRP25 were generated in a mutant Chinese hamster ovary cell line that lacked expression of endogenous LRP and was resistant to PEA. All forms of recombinant LRP proteins were transported from the endoplasmic reticulum to the Golgi apparatus in Chinese hamster ovary cells as shown by their sensitivity to endoglycosidase H and resistance to neuraminidase. Cell surface iodination and subcellular fractionation studies indicated that all three LRP variants were expressed on the plasma membrane. Furthermore, expression of the three LRP variants restored, to various degrees, sensitivity to PEA and the ability to degrade methylamine-activated alpha2M (alpha2M*). These data suggest that deletion of large internal portions of LRP, including the processing site, does not prevent transport of LRP to the plasma membrane, nor does it abolish the interaction of LRP with alpha2M* or PEA. This LRP expression system may allow for the characterization of domains within LRP responsible for its multifunctionality.

Furin-mediated cleavage of Pseudomonas exotoxin-derived chimeric toxins

Pseudomonas exotoxin (PE) requires proteolytic cleavage to generate a 37-kDa C-terminal fragment that translocates to the cytosol and ADP-ribosylates elongation factor 2. Cleavage within cells is mediated by furin, occurs between arginine 279 and glycine 280, and requires an arginine at both P1 and P4 residues. To study the proteolytic processing of PE-derived chimeric toxins, TGFalpha-PE38 (transforming growth factor fused to the domains II and III of PE) and a mutant form, TGFalpha-PE38gly279, were each produced in Escherichia coli. When assessed on various epidermal growth factor (EGF) receptor-positive cell lines, TGFalpha-PE38 was 100-500-fold more toxic than TGFalpha-PE38gly279. In contrast to PE, where cleavage by furin is only evident at pH 5.5, furin cleaved TGFalpha-PE38 over a broad pH range, while TGFalpha-PE38gly279 was resistant to cleavage. TGFalpha-PE38 was poorly toxic for furin-deficient LoVo cells, unless it was first pretreated in vitro with furin. Furin treatment produced a nicked protein that was 30-fold more toxic than its unnicked counterpart. Using the single chain immunotoxin HB21scFv-PE40 as a substrate, furin-mediated processing of an antibody-based immunotoxin was also evaluated. HB21scFv-PE40, which targets cells expressing the transferrin receptor, was cleaved in a similar fashion to that of TGFalpha-PE38 and nicked HB21scFv-PE40 exhibited increased toxicity for LoVo cells. In short-term experiments, the rate of reduction in protein synthesis by furin-nicked immunotoxins was increased compared with unnicked protein, indicating that cleavage by furin can be a rate-limiting step. We conclude that furin-mediated cleavage of PE-derived immunotoxins is important for their cytotoxic activity.

Endoprotease activities other than furin and PACE4 with a role in processing of HIV-I gp160 glycoproteins in CHO-K1 cells

We addressed the question of whether furin is the endoprotease primarily responsible for processing the human immunodeficiency virus type I (HIV-I) envelope protein gp160 in mammalian cells. The furin-deficient Chinese hamster ovary (CHO)-K1 strain RPE.40 processed gp160 as efficiently as wild-type CHO-K1 cells in vivo. Although furin can process gp160 in vitro, this processing is probably not physiologically relevent, because it occurs with very low efficiency. PACE4, a furin homologue, allowed processing of gp160 when both were expressed in RPE.40 cells. Further, PACE4 participated in the activation of a calcium-independent protease activity in RPE.40 cells, which efficiently processed the gp160 precursor in vitro. This calcium-independent protease activity was not found in another furin-deficient cell strain, 7.P15, selected from the monkey kidney cell line COS-7.

Furin/PACE/SPC1: a convertase involved in exocytic and endocytic processing of precursor proteins

One of the most exciting breakthroughs of the 90's in the fields of biochemistry, cell biology and neuroendocrinology is the identification of a novel family of proteolytic enzymes called mammalian subtilisin-like convertases. This family is comprised so far of seven distinct endoproteases responsible for the proteolytic excision of biologically active polypeptides from inactive precursor proteins. Six years after the initial observation of a structural conservation between a characterized yeast enzyme (kexin) and a human gene product (furin), it is now well accepted that one of these convertases, furin, has the enzymatic capabilities to efficiently and correctly process a great variety of precursors. Furin's ability to cleave precursors within both the exocytic and endocytic pathways will require sustained efforts in order to delineate all of its physiological roles.

Processing of mutated human proinsulin to mature insulin in the non-endocrine cell line, CHO

Heterologous genes encoding proproteins, including proinsulin, generally produce mature protein when expressed in endocrine cells while unprocessed or partially processed protein is produced in non-endocrine cells. Proproteins, which are normally processed in the regulated pathway restricted to endocrine cells, do not always contain the recognition sequence for cleavage by furin, the endoprotease specific to the constitutive pathway, the principal protein processing pathway in non-endocrine cells. Human proinsulin consists of B-Chain-C-peptide-A-Chain and cleavage at the B/C and C/A junctions is required for processing. The B/C, but not the C/A junction, is recognised and cleaved in the constitute pathway. We expressed a human proinsulin and a mutated proinsulin gene with an engineered furin recognition sequence at the C/A junction and compared the processing efficiency of the mutant and native proinsulin in Chinese Hamster Ovary cells. The processing efficiency of the mutant proinsulin was 56% relative to 0.7% for native proinsulin. However, despite similar levels of mRNA being expressed in both cell lines, the absolute levels of immunoreactive insulin, normalized against mRNA levels, were 18-fold lower in the mutant proinsulin-expressing cells. As a result, there was only a marginal increase in absolute levels of insulin produced by these cells. This unexpected finding may result from preferential degradation of insulin in non-endocrine cells which lack the protection offered by the secretory granules found in endocrine cells.

Mutational analysis of the insulin-like growth factor I prohormone processing site

Insulin-like growth factor I (IGF-I) is a mitogenic peptide that is produced in most tissues and cell lines and plays an important role in embryonic development and postnatal growth. IGF-I is initially synthesized as a prohormone precursor that is converted to mature IGF-I by endoproteolytic removal of the carboxyl-terminal E-domain. Regulation of the conversion of proIGF-I to mature IGF-I is a potential mechanism by which the biological activity of this growth factor might be modulated. Endoproteolysis of the IGF-I prohormone occurs at the unique pentabasic motif Lys-X-X-Lys-X-X-Arg71-X-X-Arg-X-X-Arg. Recently, a family of enzymes which cleave prohormone precursors at sites containing multiple basic residues has been discovered. The goals of this study were 1) to determine which basic residues in the pentabasic proIGF-I processing site were necessary for proper cleavage and 2) to examine the role that subtilisin-related proprotein convertase 1 (SPC1/furin) might play in proIGF-I processing. We have shown that an expression vector coding for an epitope-tagged proIGF-I directs synthesis and secretion of mature IGF-I-(1-70), extended IGF-I-(1-76), proIGF-I, and N-glycosylated proIGF-I in human embryonic kidney 293 cells. Extended IGF-I-(1-76) is produced by cleavage at Arg77 and requires both Arg74 (P4) and Arg77 (P1). Cleavage at Arg77 does not occur in the SPC1-deficient cell lines RPE.40 and LoVo, suggesting that processing at this site is mediated by SPC1. Mature IGF-I-(1-70) is produced by cleavage at Arg71 and requires both Lys68 (P4) and Arg71 (P1). Lys65 in the P7 position is important for efficient cleavage. SPC1 is not required for processing at Arg71 since this cleavage occurs in RPE.40 and LoVo cells. These data suggest the existence of a processing enzyme which is specific for the Lys-X-X-Arg motif of proIGF-I.

Processing of transforming growth factor beta 1 precursor by human furin convertase

Proteolytic processing of the transforming growth factor beta precursor (pro-TGF beta) is an essential step in the formation of the biologically active TGF beta homodimeric protein (TGF beta). The 361-amino-acid precursor pro-TGF beta 1 has within its primary structure the R-H-R-R processing signal found in many constitutively secreted precursor proteins and potentially recognized by members of the mammalian convertase family of endoproteases. To determine whether cleavage of pro-TGF beta 1 can be achieved by the furin convertase in vitro, purified precursor was incubated in the presence of a truncated/secreted form of the enzyme. Immunoblots showed that the 55-kDa pro-TGF beta 1 was converted into the 44 and 12.5 kDa bands corresponding to the pro-region and the mature monomer, respectively. Treatment of pro-TGF beta 1 with furin resulted in a 5-fold increase in the production of biologically active TGF beta 1. Furthermore, when expressed in the furin-deficient LoVo cells, no processing of pro-TGF beta 1 was observed. In contrast, efficient processing was observed when pro-TGF beta was coexpressed with the furin convertase. Collectively, these results provide evidence that in our experimental systems the TGF beta 1 precursor is efficiently and correctly processed by human furin thus permitting release of the biologically active peptide.

Processing of proendothelin-1 by human furin convertase

Endothelin-1 (ET-1) is the most potent vasoactive peptide known to date. The peptide is initially synthesized as an inactive precursor (proET-1) which undergoes proteolysis at specific pairs of basic amino acids to yield bigET-1. Production of ET-1 then proceeds by cleavage of bigET-1 by the endothelin converting enzyme (ECE). Here, we demonstrate that the in vitro cleavage of proET-1 by furin, a mammalian convertase involved in precursor processing, produced bigET-1. Upon further processing, bigET-1 was converted to biologically active ET-1. Furthermore, we demonstrate that the furin inhibitor, decanoyl-Arg-Val-Lys-Arg chloromethylketone, abolished production of ET-1 in endothelial cells.

Expression of insulin receptor on clonal pancreatic alpha cells and its possible role for insulin-stimulated negative regulation of glucagon secretion

In pancreatic alpha cells, the existence and function of the insulin receptor has not yet been fully established. In this study, to confirm the expression of functional insulin receptors in pancreatic alpha cells, we performed: 1) insulin receptor binding assay, 2) Northern blot analysis and RT-PCR (reverse transcription-polymerase chain reaction) amplification of insulin receptor mRNA, 3) immunocytochemical staining, 4) biosynthetic labelling of insulin receptor protein using [35S]methionine, 5) analysis of insulin-stimulated autophosphorylation of the insulin receptor in glucagon secreting cell lines, In-R1-G9 and alpha TC clone 6 cells. Glucagon secretion decreased with the addition of insulin in both cells. The receptor binding studies using [125I-Tyr-A14] insulin revealed that both cells possessed a significant number of insulin receptors (In-R1-G9:K1 = 2.1 x 10(9) mol/l-1, K2 = 6.2 x 10(7) mol/l-1, R1 = 0.27 x 10(4), R2 = 1.86 x 10(4) sites/cell; alpha TC clone 6: K1 = 2.1 x 10(9) mol/l-1, K2 = 7.3 x 10(7) mol/l-1, R1 = 0.27 x 10(4), R2 = 1.95 x 10(4) sites/cell). Northern blot analysis as well as RT-PCR amplification showed the mRNA specific for insulin receptor in both cells. By immunocytochemical staining using anti-insulin receptor alpha-subunit antibody, positive immunostaining for insulin receptor was observed in both cells. [35S]Methionine labelling of both cells followed by immunoprecipitation using anti-insulin receptor antibody showed the correct size of the insulin receptor protein. The insulin receptor expressed in these cells underwent autophosphorylation by insulin stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of mutations in alleles of the fur gene from an endoprotease-deficient Chinese hamster ovary cell strain

RPE.40 mutant cells differ from wild-type Chinese hamster ovary (CHO-K1) cells in their increased resistance to Pseudomonas exotoxin A and their inability to process the insulin proreceptor and certain viral envelope proproteins. Northern analysis revealed that RPE.40 cells maintained a substantially lower steady-state level of 4.0 kb fur mRNA than did CHO-K1 cells. Analysis of fur cDNAs showed that RPE.40 cells were diploid at the fur locus, and RPE.40 cells had a Cys (TGC) to Tyr (TAC) mutation in codon 196 of one allele (allele I). Approximately 25-30% of the CHO-K1 cells were also heterozygous (Tyr/Cys) at codon 196, and pre-mRNAs transcribed from the second allele (allele II) in RPE.40 cells were defectively spliced. All other pre-mRNAs were correctly spliced. Rapid turnover of defectively spliced transcripts may account for the reduced steady-state level of fur mRNA observed in RPE.40 cells. Our results provide a mechanistic basis for the endoprotease-deficient phenotype of RPE.40 cells.