Characterization of pneumolysin from Streptococcus pneumoniae,interacting with carbohydrate moiety and cholesterol as a component of cell membrane

The cytolytic mechanism of cholesterol-dependent cytolysins (CDCs) requires the presence of cholesterol in the target cell membrane. Membrane cholesterol was thought to serve as the common receptor for these toxins, but not all CDCs require cholesterol for binding. One member of this toxin family, pneumolysin (PLY) is a major virulence factor of Streptococcus pneumoniae, and the mechanism via which PLY binds to its putative receptor or cholesterol on the cell membrane is still poorly understood. Here, we demonstrated that PLY interacted with carbohydrate moiety and cholesterol as a component of the cell membrane, using the inhibitory effect of hemolytic activity. The hemolytic activity of PLY was inhibited by cholesterol-MβCD, which is in a 3β configuration at the C3-hydroxy group, but is not in a 3α-configuration. In the interaction between PLY and carbohydrate moiety, the mannose showed a dose-dependent increase in the inhibition of PLY hemolytic activity. The binding ability of mannose with truncated PLYs, as determined by the pull-down assay, showed that mannose might favor binding to domain 4 rather than domains 1–3. These studies provide a new model for the mechanism of cellular recognition by PLY, as well as a foundation for future investigations into whether non-sterol molecules can serve as receptors for other members of the CDC family of toxins.     

Characterization of a streptococcal cholesterol-dependent cytolysin with a lewis y and b specific lectin domain

The cholesterol-dependent cytolysins (CDCs) are a large family of pore-forming toxins that often exhibit distinct structural changes that modify their pore-forming activity. A soluble platelet aggregation factor from Streptococcus mitis (Sm-hPAF) was characterized and shown to be a functional CDC with an amino-terminal fucose-binding lectin domain. Sm-hPAF, or lectinolysin (LLY) as renamed herein, is most closely related to CDCs from Streptococcus intermedius (ILY) and Streptococcus pneumoniae (pneumolysin or PLY). The LLY gene was identified in strains of S. mitis, S. pneumoniae, and Streptococcus pseudopneumoniae. LLY induces pore-dependent changes in the light scattering properties of the platelets that mimic those induced by platelet aggregation but does not induce platelet aggregation. LLY monomers form the typical large homooligomeric membrane pore complex observed for the CDCs. The pore-forming activity of LLY on platelets is modulated by the amino-terminal lectin domain, a structure that is not present in other CDCs. Glycan microarray analysis showed the lectin domain is specific for difucosylated glycans within Lewis b (Le (b)) and Lewis y (Le (y)) antigens. The glycan-binding site is occluded in the soluble monomer of LLY but is apparently exposed after cell binding, since it significantly increases LLY pore-forming activity in a glycan-dependent manner. Hence, LLY represents a new class of CDC whose pore-forming mechanism is modulated by a glycan-binding domain.     

Membrane assembly of the cholesterol-dependent cytolysin pore complex

The cholesterol-dependent cytolysins (CDCs) are a large family of pore-forming toxins that are produced, secreted and contribute to the pathogenesis of many species of Gram-positive bacteria. The assembly of the CDC pore-forming complex has been under intense study for the past 20years. These studies have revealed a molecular mechanism of pore formation that exhibits many novel features. The CDCs form large β-barrel pore complexes that are assembled from 35 to 40 soluble CDC monomers. Pore formation is dependent on the presence of membrane cholesterol, which functions as the receptor for most CDCs. Cholesterol binding initiates significant secondary and tertiary structural changes in the monomers, which lead to the assembly of a large membrane embedded β-barrel pore complex. This review will focus on the molecular mechanism of assembly of the CDC membrane pore complex and how these studies have led to insights into the mechanism of pore formation for other pore-forming proteins. This article is part of a Special Issue entitled: Protein Folding in Membranes.     

Structure and molecular mechanism of a functional form of pneumolysin: a cholesterol-dependent cytolysin from Streptococcus pneumoniae

One of the key steps in understanding human disease arising from gram-positive bacteria lies in the mechanisms of the cholesterol-dependent cytolysins (CDCs). Pneumolysin (PLY), a CDC from Streptococcus pneumoniae, is of special importance due to the severe impacts of pneumococcal infections on mortality and morbidity worldwide. We have overexpressed, purified, and characterized PLY in its fully functional complex form with the enzyme bound to its receptor activator on target cells, cholesterol. The circular dichroism studies of PLY in solution with an excess of cholesterol show a change in the far UV spectrum consistent with a decrease in the beta-sheet and an increase in the random coil structures of the enzyme. Pore formation in membranes leading to cell lysis is the functional target for this cytolysin. The sedimentation velocity and equilibrium analyses of the cholesterol-bound enzyme show hydrodynamic properties different from those of the cholesterol-free form. The soluble form of the cholesterol-free enzyme exists in solution as a mixture of monomers and dimers, whereas the cholesterol-bound form exists only as a monomer. A mechanism of formation of PLY pores in the lipid bilayer of the target cells is discussed.     

The Cholesterol-Dependent Cytolysin Signature Motif: A Critical Element in the Allosteric Pathway that Couples Membrane Binding to Pore Assembly

The cholesterol-dependent cytolysins (CDCs) constitute a family of pore-forming toxins that contribute to the pathogenesis of a large number of Gram-positive bacterial pathogens.The most highly conserved region in the primary structure of the CDCs is the signature undecapeptide sequence (ECTGLAWEWWR). The CDC pore forming mechanism is highly sensitive to changes in its structure, yet its contribution to the molecular mechanism of the CDCs has remained enigmatic. Using a combination of fluorescence spectroscopic methods we provide evidence that shows the undecapeptide motif of the archetype CDC, perfringolysin O (PFO), is a key structural element in the allosteric coupling of the cholesterol-mediated membrane binding in domain 4 (D4) to distal structural changes in domain 3 (D3) that are required for the formation of the oligomeric pore complex. Loss of the undecapeptide function prevents all measurable D3 structural transitions, the intermolecular interaction of membrane bound monomers and the assembly of the oligomeric pore complex. We further show that this pathway does not exist in intermedilysin (ILY), a CDC that exhibits a divergent undecapeptide and that has evolved to use human CD59 rather than cholesterol as its receptor. These studies show for the first time that the undecapeptide of the cholesterol-binding CDCs forms a critical element of the allosteric pathway that controls the assembly of the pore complex.     

Trypsin/acrosin inhibitor activity of rat and guinea pig caltrin proteins. Structural and functional studies

Dramatic inhibition of trypsin activity by rat caltrin and guinea pig caltrin I was spectrophotometrically demonstrated using the artificial substrate benzoylarginyl ethyl ester. Approximately 6% and 21% of residual proteolytic activity was recorded after preincubating the enzyme with 0.22 and 0.27 microM rat caltrin and guinea pig caltrin I, respectively. Reduction and carboxymethylation of the cysteine residues abolished the inhibitor activity of both caltrin proteins. Rat caltrin and guinea pig caltrin I show structural homology with secretory trypsin/acrosin inhibitor proteins isolated from boar and human seminal plasma and mouse seminal vesicle secretion and share a fragment of 13 amino acids of almost identical sequence (DPVCGTDGH/K/ITYG/AN), which is also present in the structure of Kazal-type trypsin inhibitor proteins from different mammalian tissues. Bovine, mouse, and guinea pig caltrin II, three caltrin proteins that have no structural homology with rat caltrin or guinea pig caltrin I, lack trypsin inhibitor activity. Rat caltrin, guinea pig caltrin I, and the mouse seminal vesicle trypsin inhibitor protein P12, which also inhibits Ca(2+) uptake into epididymal spermatozoa (mouse caltrin I), bound specifically to the sperm head, on the acrosomal region, as detected by indirect immunofluorescence. They also inhibited the acrosin activity in the gelatin film assay. Caltrin I may play an important role in the control of sperm functions such as Ca(2+) influx in the acrosome reaction and activation of acrosin and other serine-proteases at the proper site and proper time to ensure successful fertilization.     

The role of cholesterol in the activity of pneumolysin, a bacterial protein toxin

The mechanism via which pneumolysin (PLY), a toxin and major virulence factor of the bacterium Streptococcus pneumoniae, binds to its putative receptor, cholesterol, is still poorly understood. We present results from a series of biophysical studies that shed light on the interaction of PLY with cholesterol in solution and in lipid bilayers. PLY lyses cells whose walls contain cholesterol. Using standard hemolytic assays we have demonstrated that the hemolytic activity of PLY is inhibited by cholesterol, partially by ergosterol but not by lanosterol and that the functional stoichiometry of the cholesterol-PLY complex is 1:1. Tryptophan (Trp) fluorescence data recorded during PLY-cholesterol titration studies confirm this ratio, reveal a significant blue shift in the Trp fluorescence peak with increasing cholesterol concentrations indicative of increasing nonpolarity in the Trp environment, consistent with cholesterol binding by the tryptophans, and provide a measure of the affinity of cholesterol binding: K(d) = 400 +/- 100 nM. Finally, we have performed specular neutron reflectivity studies to observe the effect of PLY upon lipid bilayer structure.     

Human L-ficolin,a Recognition Molecule of the Lectin Activation Pathway of Complement,Activates Complement by Binding to Pneumolysin,the Major Toxin of Streptococcus pneumoniae

The complement system is an essential component of the immune response, providing a critical line of defense against different pathogens including S. pneumoniae. Complement is activated via three distinct pathways: the classical (CP), the alternative (AP) and the lectin pathway (LP). The role of Pneumolysin (PLY), a bacterial toxin released by S. pneumoniae, in triggering complement activation has been studied in vitro. Our results demonstrate that in both human and mouse sera complement was activated via the CP, initiated by direct binding of even non-specific IgM and IgG3 to PLY. Absence of CP activity in C1q^−/− mouse serum completely abolished any C3 deposition. However, C1q depleted human serum strongly opsonized PLY through abundant deposition of C3 activation products, indicating that the LP may have a vital role in activating the human complement system on PLY. We identified that human L-ficolin is the critical LP recognition molecule that drives LP activation on PLY, while all of the murine LP recognition components fail to bind and activate complement on PLY. This work elucidates the detailed interactions between PLY and complement and shows for the first time a specific role of the LP in PLY-mediated complement activation in human serum.     

Metabolism of prostaglandin endoperoxide by microsomes from human lung parenchyma and comparison with metabolites produced by pig, bovine, rat, mouse and guinea-pig

The metabolism of PGH2 by human lung parenchymal microsomes was characterized by radiometric high performance liquid chromatography and compared with metabolism by pig, bovine, rat, mouse, and guinea pig lung microsomes. Microsomes from human lung synthesized 0.74 nmoles/mg protein and 0.72 nmoles/mg protein, PGI2 (6-Keto-PGF1 alpha) and TxA2 (TxB2) respectively, upon incubation with 4.0 nmoles of PGH2. Pig, bovine, rat, mouse, and guinea pig microsomes respectively synthesized 1.0, 1.0, 0.9, 0.4, and 0.1 nmoles of PGI2/mg protein, and 0.9, 1.0, 0.7, 0.3, 1.8 nmoles of TxA2/mg protein, and preparations formed some PGE2, PGF2 alpha, and PGD2. Mouse lung microsomes were unique in synthesizing PGE2 as the major prostaglandin. The thromboxane synthetase inhibitor 1-benzylimidazole was a specific inhibitor in these six species.     

Acrogranin,an acrosomal cysteine-rich glycoprotein,is the precursor of the growth-modulating peptides,granulins, and epithelins,and is expressed in somatic as well as male germ cells

Spermatogenesis is a unique system of differentiation involving cellular remodeling and the biogenesis of sperm-specific organelles. To study the biogenesis of one such organelle, the acrosome, we have been examining the gene expression, biosynthesis, and targeting of specific acrosomal proteins during mammalian spermatogenesis. An acrosomal marker that we recently purified and began characterizing is acrogranin, a 67,000-molecular-weight glycoprotein originally isolated from guinea pig testes. This glycoprotein is detected in pachytene spermatocytes and is found later in the acrosomes of developing spermatids and sperm. Immunoblotting of several tissues and immunofluorescent localization in frozen sections of guinea pig testes suggested that acrogranin was a germ cell-specific glycoprotein that was expressed meiotically and post-meiotically. However, Northern blot analysis demonstrated that the mRNA for acrogranin was ubiquitously expressed in all guinea pig and mouse tissues examined. Furthermore, the primary structures of guinea pig and mouse acrogranins, deduced from the cDNA sequences, reveal that this glycoprotein is a cysteine-rich molecule with a motif that is tandemly repeated seven times, very similar to that of the human epithelin/granulin precursor. We conclude that guinea pig and mouse acrogranins are homologues of the precursor of the human and rat epithelin/granulin peptides previously demonstrated to have growth-modulating properties. © 1993 Wiley-Liss, Inc.     

Variations in induction of drug-metabolizing enzymes by trans-Stilbene oxide in rodent species

trans-Stilbene oxide (400 mg/kg) produced a 500% increase in the microsomal in the microsomal epoxide hydratase activity in rat and mouse with little change in the soluble enzyme activity. However, in guinea pig, the soluble epoxide hydratase activity increased by about 33% with only a small increase (47.6%) in the microsomal enzyme activity. The soluble glutathione S-transferase activities were also induced in both rat and mouse, with little change in that of the guinea pig. Increasing dosage of trans-stilbene oxide from 400 mg/kg to 1000 mg/kg had little effect on the above enzyme activities. That the guinea pig was not relatively refractory to all inducing agents was shown by the fact phenobarbital (100 mg/kg) and 3-methylcholanthrene (25 mg/kg) produced relatively similar increases in the activities of aniline hydroxylase and P-aminopyrineP-demethylase in rat, mouse and guinea pig. However, these inducers produced only a 15–20% stimulation in the soluble glutathione S-transferase and microsomal epoxide hydratase activities in guinea pig, when compared to a 50–80% increase in rat and mouse, suggesting a general resistance to induction by the phase II enzymes in guinea liver. In all animal models, the inducer markedly increased th emicrosomal total phospholipid content, although the sphingomyelin content itself was decreased. In both rat and mouse, the microsomal cholesterol content was significantly decreased while that in guinea pig was unaffected. Possible factors responsible for the observed species differences are discussed.     

PKC-Dependent Phosphorylation of eNOS at T495 Regulates eNOS Coupling and Endothelial Barrier Function in Response to G+ -Toxins

Gram positive (G⁺) infections make up ∼50% of all acute lung injury cases which are characterized by extensive permeability edema secondary to disruption of endothelial cell (EC) barrier integrity. A primary cause of increased permeability are cholesterol-dependent cytolysins (CDCs) of G⁺-bacteria, such as pneumolysin (PLY) and listeriolysin-O (LLO) which create plasma membrane pores, promoting Ca²⁺-influx and activation of PKCα. In human lung microvascular endothelial cells (HLMVEC), pretreatment with the nitric oxide synthase (NOS) inhibitor, ETU reduced the ability of LLO to increase microvascular cell permeability suggesting an endothelial nitric oxide synthase (eNOS)-dependent mechanism. LLO stimulated superoxide production from HLMVEC and this was prevented by silencing PKCα or NOS inhibition suggesting a link between these pathways. Both LLO and PLY stimulated eNOS T495 phosphorylation in a PKC-dependent manner. Expression of a phosphomimetic T495D eNOS (human isoform) resulted in increased superoxide and diminished nitric oxide (NO) production. Transduction of HLMVEC with an active form of PKCα resulted in the robust phosphorylation of T495 and increased peroxynitrite production, indicative of eNOS uncoupling. To determine the mechanisms underlying eNOS uncoupling, HLMVEC were stimulated with LLO and the amount of hsp90 and caveolin-1 bound to eNOS determined. LLO stimulated the dissociation of hsp90, and in particular, caveolin-1 from eNOS. Both hsp90 and caveolin-1 have been shown to influence eNOS uncoupling and a peptide mimicking the scaffolding domain of caveolin-1 blocked the ability of PKCα to stimulate eNOS-derived superoxide. Collectively, these results suggest that the G⁺ pore-forming toxins promote increased EC permeability via activation of PKCα, phosphorylation of eNOS-T495, loss of hsp90 and caveolin-1 binding which collectively promote eNOS uncoupling and the production of barrier disruptive superoxide.     

Cholesterol exchange in platelets, erythrocytes and megakaryocytes

Cholesterol exchange between plasma and human platelets and erythrocytes and guinea pig platelets, erythrocytes and megakaryocytes was studied. The characteristics of exchange of cholesterol between [3H]cholesterol-labeled plasma and human platelets and erythrocytes were similar: exchange per cell was independent of cell concentration in whole plasma, decreased only 2-fold over a wide range of cell concentrations in low concentrations of plasma and approached a plateau at 1/3 normal plasma cholesterol concentration, and there was no net change in the cholesterol content of either cell. The activation energy for exchange for both cells was 47 kJ/mol. In all experiments, erythrocyte cholesterol was labeled to approximately twice the specific activity of platelet cholesterol. Guinea pig megakaryocyte cholesterol exchanged at 25-33% of the rate of guinea pig platelet cholesterol in vitro. Similarly, when guinea pigs were fed [3H]cholesterol, erythrocyte cholesterol specific activity after 24 h was 90%, platelet 50-65%, and megakaryocyte 20-26% that of plasma. Guinea pig platelets incubated with plasma radiolabeled in free and esterified cholesterol incorporated radioactivity from free but not esterified cholesterol. The similarity of free cholesterol exchange in platelets and erythrocytes in vitro and in vivo and the apparent inability of platelets to take up cholesterol esters from lipoproteins suggest that the interaction between normal platelets and normocholesterolemic plasma is limited to cholesterol exchange.     

Rat, Mouse, and Guinea Pig Brain Development and Microtubule Assembly

The development of in vitro microtubule assembly and of tubulin concentration have been studied during brain maturation in the mouse and the rat, two species which have postnatal brain development, and in one species which is mature at birth, the guinea pig. (a) The rate of tubulin assembly is very slow soon after birth in both the mouse and rat; it increases progressively with age until adulthood. In contrast, in the guinea pig this rate is maximal at birth and slower rates are seen only at foetal stages. (b) Postnatal changes in the lag period of assembly and in the minimal concentration of tubulin (Cc) required to obtain in vitro assembly are seen in the mouse and the rat; in contrast these parameters are constant at all postnatal stages in the guinea pig with longer lag periods and lower Cc values being seen only at foetal stages. (c) Maximal rates of assembly, minimal lag periods, and minimal Cc values are restored after addition of microtubule-associated proteins to foetal guinea pig or young mouse and rat preparations, suggesting that the difference in the kinetic parameters of assembly between these species depends on differences in the concentration or activity of these proteins. (d) Maximal tubulin concentrations are observed before birth in the guinea pig and approximately at day 10 in the rat and mouse. Lennon A. M. et al. Rat, mouse, and guinea pig brain development and microtubule assembly. J. Neurochem. 35, 804–813 (1980).     

Cytochemical and Western Blot Analysis of the Subcompartmentalization of the Acrosome in Rodents using Soybean Lectin

In the present study, the formation and development of the acrosome during spermiogenesis in four different rodent species (rat, mouse, hamster and guinea pig) was compared by means of cytochemical and blotting techniques using a lectin from soybean (SBA). This lectin recognizes specifically the acrosome of the four species at all steps of formation. At the ultrastructural level, SBA-binding pattern was similar in the acrosome of the rat, mouse and hamster. SBA preferentially labelled the electron-lucent area of the acrosome in early spermatids (Golgi and cap phases) and the outer region of the acrosome in mature spermatids (acrosome and maturation phase). The lectin binding pattern was more complex in the guinea pig acrosome. Three different subdomains can be established in the early acrosome of the guinea pig. The lectin bound the three subdomains but mainly a thin fold which spreads over the nucleus during the cap phase. In the acrosome phase, SBA strongly reacted with the principal segment. In contrast, no reactivity was observed in most of this segment in maturation phase spermatids. In this phase, SBA bound preferentially a thin area covering the dorsal region of the apical segment. Lectin blots of detergent-extracted testes indicated that SBA only recognizes proteins of high molecular weight (>100kD) in the four species studied. The results obtained in the present study suggest that the development of acrosomal subdomains is very similar in the mouse, rat and hamster but shows a more complex pattern in the guinea pig.     

Epigallocatechin gallate inhibits Streptococcus pneumoniae virulence by simultaneously targeting pneumolysin and sortase A

Streptococcus pneumoniae (pneumococcus), the causative agent of several human diseases, possesses numerous virulence factors associated with pneumococcal infection and pathogenesis. Pneumolysin (PLY), an important virulence factor, is a member of the cholesterol‐dependent cytolysin family and has cytolytic activity. Sortase A (SrtA), another crucial pneumococcal virulence determinate, contributes greatly to the anchoring of many virulence‐associated surface proteins to the cell wall. In this study, epigallocatechin gallate (EGCG), a natural compound with little known antipneumococcal activity, was shown to directly inhibit PLY‐mediated haemolysis and cytolysis by blocking the oligomerization of PLY and simultaneously reduce the peptidase activity of SrtA. The biofilm formation, production of neuraminidase A (NanA, the pneumococcal surface protein anchored by SrtA), and bacterial adhesion to human epithelial cells (Hep2) were inhibited effectively when S. pneumoniae D39 was cocultured with EGCG. The results from molecular dynamics simulations and mutational analysis confirmed the interaction of EGCG with PLY and SrtA, and EGCG binds to Glu277, Tyr358, and Arg359 in PLY and Thr169, Lys171, and Phe239 in SrtA. In vivo studies further demonstrated that EGCG protected mice against S. pneumoniae pneumonia. Our results imply that EGCG is an effective inhibitor of both PLY and SrtA and that an antivirulence strategy that directly targets PLY and SrtA using EGCG is a promising therapeutic option for S. pneumoniae pneumonia.     

Identification, evolution, and regulation of expression of Guinea pig trappin with an unusually long transglutaminase substrate domain

Trappins are found in human, bovine, hippopotamus, and members of the pig family, but not in rat and mouse. To clarify the evolution of the trappin genes and the functional significance of their products, we isolated the trappin gene in guinea pig, a species belonging to a rodent family distinct from rat and mouse. Guinea pig trappin was confirmed to encode the same domain structure as trappin, consisting of a signal sequence, an extra large transglutaminase substrate domain, and a whey acidic protein motif. Northern blot analysis and in situ hybridization histochemistry as well as immunohistochemistry demonstrated that guinea pig trappin is expressed solely in the secretory epithelium of the seminal vesicle and that its expression is androgen-dependent. We confirmed that guinea pig trappin is cross-linked by prostate transglutaminase and that the whey acidic protein motif derived from guinea pig trappin has an inhibitory activity against leukocyte elastase. Genome sequence analysis showed that guinea pig trappin belongs to the family of REST (rapidly evolving seminal vesicle transcribed) genes.     

T-cell-activating monoclonal antibodies, reacting with both leukocytes and erythrocytes, recognize the guinea pig Thy-1 differentiation antigen: characterization and cloning of guinea pig CD90

A glycophosphatidylinositol (GPI)-linked differentiation antigen expressed on guinea pig T and B lymphocytes was identified by several monoclonal antibodies; it has been shown previously that this membrane protein induced strong polyclonal T cell proliferation upon antibody binding and costimulation by PMA. Purification by immunoadsorption and microsequencing revealed that this T-cell-activating protein is the homologue of Thy-1 or CD90. In contrast to the Thy-1 antigen of most other species, guinea pig Thy-1 has a much higher molecular weight, which is due to a more extensive N-linked glycosylation, bringing the molecular weight of the total antigen up to 36 kDa. Molecular cloning of guinea pig Thy-1 indicated that the deduced molecular weight of the protein backbone is 12,777 after removal of an N-terminal 19-amino-acid leader peptide and cleavage of the 31 amino acids for GPI anchoring the C-terminal end. Sequence comparison showed that guinea pig Thy-1 has an 82% homology to human and a 72% homology to mouse Thy-1 on the amino acid level. Immunohistological staining of cryostat sections revealed intensive staining with the monoclonal antibody H154 on fibroblasts, fibrocytes, Kupffer cells, alveolar macrophages, and mesangial cells. As observed in the human, mouse, and rat, Thy-1 is abundant in the guinea pig brain. Unlike Thy-1 expression in other species, guinea pig Thy-1 is strongly expressed on most resting, nonactivated B cells and, to a lesser extent, on erythrocytes. While treatment of erythrocytes and lymphocytes with GPI-specific phospholipase C largely decreased reactivity with mAb H154, T cells retained the proliferative response to antibody and phorbol esters.     

Apolipoprotein B binding domains: evidence that they are cell-penetrating peptides that efficiently deliver antigenic peptide for cross-presentation of cytotoxic T cells

Low-density lipoproteins (LDLs) are a good source of cholesterol, which is important in cellular homeostasis and production of steroids. Apolipoprotein B-100 (ApoB-100), the sole protein component of LDL, is known to bind to cell surface LDL receptor (LDLR) or cell surface-bound proteoglycans and to be internalized into cells. We found that APCs, consisting of macrophages and dendritic cells, upregulate LDLR on culture in vitro without obvious stimulation. In contrast, T cell populations only upregulate LDLR on activation. Thus, we strategized that tagging immunogens to ApoB-100 might be a useful means to target Ag to APCs. We generated fusion proteins consisting of receptor binding sites in ApoB-100, coupled to OVA peptide (ApoB-OVA), as Ag delivery vehicles and demonstrated that this novel delivery method successfully cross-presented OVA peptides in eliciting CTL responses. Surprisingly, internalization of ApoB-OVA peptide occurred via cell surface proteoglycans rather than LDLRs, consistent with evidence that structural elements of ApoB-100 indicate it to have cell-penetrating peptide properties. Finally, we used this strategy to assess therapeutic vaccination in a tumor setting. OVA-expressing EL-4 tumors grew progressively in mice immunized with ApoB-100 alone but regressed in mice immunized with ApoB-OVA fusion protein, coinciding with development of OVA-specific CTLs. Thus, to our knowledge, this is the first article to describe the cell-penetrating properties of a conserved human origin cell penetrating peptide that may be harnessed as a novel vaccination strategy as well as a therapeutics delivery device.     

Degradation of apolipoprotein B-100 in human chylomicrons

The purpose of this study was to investigate the molecular forms of apolipoprotein B (ApoB) in human chylomicrons under well-preserved conditions. To this end, plasma and serum were collected from the same normal subjects after ingestion of a fatty meal. The samples were divided into three or four aliquots before the addition of various preservative mixtures, including antibiotics, antioxidants and proteinase inhibitors. The chylomicrons were isolated immediately, and all steps were carried out at or below 4 degrees C. Changes in the molecular weight of ApoB in chylomicrons were followed by a time study using 3.3% polyacrylamide gel electrophoresis containing SDS. ApoB from chylomicrons analyzed within 5 h of blood collection showed a single band with mobility identical to that of ApoB (ApoB-100) in low-density lipoproteins. When analyzed after 1-2 days, satellite bands smaller than ApoB-100 were observed, and a very faint band with Mr 200,000 appeared, which comigrated with intestinal ApoB (ApoB-48). Upon storage, the molecular weight of ApoB was smaller in chylomicrons subjected to a higher number of reflotations than those in chylomicrons washed less frequently, suggesting that purified chylomicrons degrade faster. A longer storage time at 4 degrees C (i.e., 7 or 14 days) revealed a stepwise degradation of ApoB, yielding Mr 200,000 band as the prominent form. The degradation of ApoB-100 was slower when both proteinase inhibitors, leupeptin and epsilon-amino caproic acid, were employed, and the appearance of Mr 200,000 band was quicker when the chylomicrons were processed at higher temperature (15-25 degrees C) in the absence of a proteinase inhibitor. Immunoblotting shows that the segment removed from ApoB-100 was the carboxyl-terminal portion. These results suggest the possible presence of a proteinase(s), which copurified with chylomicrons, and which converts ApoB-100 from a large to a smaller molecular form. Although the stop codon has been discovered recently in intestinal ApoB mRNA, which explains the mechanism for direct synthesis of ApoB-48, apparently ApoB-100 is also synthesized in the intestine of all eight subjects studied here, and the ApoB-100 degrades to a form which is ApoB-48-like.