Melittin as an epithelial permeability enhancer I: investigation of its mechanism of action in Caco-2 monolayers

Molecular Investigation of SNAC as an Oral Peptide Permeation Enhancer in Lipid Membranes via Solid-State NMR

Oral peptide therapeutics are increasingly favored in the pharmaceutical industry for their ease of use and better patient adherence. However, they face challenges with poor oral bioavailability due to their high molecular weight and surface polarity. Permeation enhancers (PEs) like salcaprozate sodium (SNAC) have shown promise in clinical trials, achieving about 1% bioavailability. One proposed mechanism for enhancing permeation is membrane perturbation or fluidization, though direct experimental proof and quantitative analysis of these effects are still needed. This study employs solid-state NMR (ssNMR) to investigate how SNAC interacts with hydrated DMPC liposomes, measuring enhancements in membrane fluidity across interfacial and transmembrane regions. The methodology involves analyzing phosphate lipid headgroups and acyl chains using static 31P chemical shift anisotropy and 2H quadrupolar coupling measurements alongside 1H and 13C magic angle spinning NMR for motional averaging of 1H-1H and 1H-13C dipolar couplings. Our findings indicate an overall increase in the uniaxial motion of phospholipids with SNAC in a PE concentration-dependent manner. It boosts lipid headgroup dynamics and enhancement plateaus at 25% between 24 and 72 mM concentrations. SNAC effectively enhances the fluidity of the hydrophobic center by 43% at 72 mM PE concentration, more significantly than the interfacial region. It is worth noting that the extent of liposome dissolution and conversion to micelles increases as SNAC concentration rises. Including a model peptide drug, octreotide, introduces a competitive equilibrium in this complex PE-lipid-peptide system, further influencing membrane dynamics for peptide permeation. Interestingly, the membrane enhancement does not show the expected plateau, and a less significant lipid mobility increase is observed in the presence of octreotide, suggesting a less substantial impact compared to peptide-free systems, which is likely due to peptide-PE interactions that consume monomeric SNAC, reducing its interaction with the lipid membrane. This study provides the first quantitative and site-specific ssNMR measurements of membrane mobility influenced by one representative PE as a snapshot of PE lipid interaction in a liposome model, demonstrating how peptide drugs modulate competitive equilibria and PE-induced lipid dynamics.

Interactions of oral permeation enhancers with lipid membranes in simulated intestinal environments

The oral bioavailability of therapeutic peptides is generally low. To increase peptide transport across the gastrointestinal barrier, permeation enhancers are often used. Despite their widespread use, mechanistic knowledge of permeation enhancers is limited. To address this, we here investigate the interactions of six commonly used permeation enhancers with lipid membranes in simulated intestinal environments. Specifically, we study the interactions of the permeation enhancers sodium caprate, dodecyl maltoside, sodium cholate, sodium dodecyl sulfate, melittin, and penetratin with epithelial cell-like model membranes. To mimic the molecular composition of the real intestinal environment, the experiments are performed with two peptide drugs, salmon calcitonin and desB30 insulin, in fasted-state simulated intestinal fluid. Besides providing a comparison of the membrane interactions of the studied permeation enhancers, our results demonstrate that peptide drugs as well as intestinal-fluid components may substantially change the membrane activity of permeation enhancers. This highlights the importance of testing permeation enhancement in realistic physiological environments and carefully choosing a permeation enhancer for each individual peptide drug.

A high-throughput gut-on-chip platform to study the epithelial responses to enterotoxins

Enterotoxins are a type of toxins that primarily affect the intestines. Understanding their harmful effects is essential for food safety and medical research. Current methods lack high-throughput, robust, and translatable models capable of characterizing toxin-specific epithelial damage. Pressing concerns regarding enterotoxin contamination of foods and emerging interest in clinical applications of enterotoxins emphasize the need for new platforms. Here, we demonstrate how Caco-2 tubules can be used to study the effect of enterotoxins on the human intestinal epithelium, reflecting toxins' distinct pathogenic mechanisms. After exposure of the model to toxins nigericin, ochratoxin A, patulin and melittin, we observed dose-dependent reductions in barrier permeability as measured by TEER, which were detected with higher sensitivity than previous studies using conventional models. Combination of LDH release assays and DRAQ7 staining allowed comprehensive evaluation of toxin cytotoxicity, which was only observed after exposure to melittin and ochratoxin A. Furthermore, the study of actin cytoskeleton allowed to assess toxin-induced changes in cell morphology, which were only caused by nigericin. Altogether, our study highlights the potential of our Caco-2 tubular model in becoming a multi-parametric and high-throughput tool to bridge the gap between current enterotoxin research and translatable in vivo models of the human intestinal epithelium.

Non-invasive assessment of intestinal permeability in healthy volunteers using transcutaneous fluorescence spectroscopy

The permeability of the intestinal barrier is altered in a multitude of gastrointestinal conditions such as Crohn's and coeliac disease. However, the clinical utility of gut permeability is currently limited due to a lack of reliable diagnostic tests. To address this issue, we report a novel technique for rapid, non-invasive measurement of gut permeability based on transcutaneous ('through-the-skin') fluorescence spectroscopy. In this approach, participants drink an oral dose of a fluorescent dye (fluorescein) and a fibre-optic fluorescence spectrometer is attached to the finger to detect permeation of the dye from the gut into the blood stream in a non-invasive manner. To validate this technique, clinical trial measurements were performed in 11 healthy participants. First, after 6 h of fasting, participants ingested 500 mg of fluorescein dissolved in 100 ml of water and fluorescence measurements were recorded at the fingertip over the following 3 h. All participants were invited back for a repeat study, this time ingesting the same solution but with 60 g of sugar added (known to transiently increase intestinal permeability). Results from the two study datasets (without and with sugar respectively) were analysed and compared using a number of analysis procedures. This included both manual and automated calculation of a series of parameters designed for assessment of gut permeability. Calculated values were compared using Student's T-tests, which demonstrated significant differences between the two datasets. Thus, transcutaneous fluorescence spectroscopy shows promise in non-invasively discriminating between two differing states of gut permeability, demonstrating potential for future clinical use.

S-layer protein 2 of vaginal Lactobacillus crispatus 2029 enhances growth, differentiation, VEGF production and barrier functions in intestinal epithelial cell line Caco-2

We have previously demonstrated the ability of the human vaginal strain Lactobacillus crispatus 2029 (LC2029) for strong adhesion to cervicovaginal epithelial cells, expression of the surface layer protein 2 (Slp2), and antagonistic activity against urogenital pathogens. Slp2 forms regular two-dimensional structure around the LC2029 cells,which is secreted into the medium and inhibits intestinal pathogen-induced activation of caspase-9 and caspase-3 in the human intestinal Caco-2 cells. Here, we elucidated the effects of soluble Slp2 on adhesion of proteobacteria pathogens inducing necrotizing enterocolitis (NEC), such as Escherichia coli ATCC E 2348/69, E. coli ATCC 31705, Salmonella Enteritidis ATCC 13076, Campylobacter jejuni ATCC 29428, and Pseudomonas aeruginosa ATCC 27853 to Caco-2 cells, as well as on growth promotion, differentiation, vascular endothelial growth factor (VEGF) production, and intestinal barrier function of Caco-2 cell monolayers. Slp2 acts as anti-adhesion agent for NEC-inducing proteobacteria, promotes growth of immature Caco-2 cells and their differentiation, and enhances expression and functional activity of sucrase, lactase, and alkaline phosphatase. Slp2 stimulates VEGF production, decreases paracellular permeability, and increases transepithelial electrical resistance, strengthening barrier function of Caco-2 cell monolayers. These data support the important role of Slp2 in the early postnatal development of the human small intestine enterocytes.

Antimicrobial Peptides in Gut Health: A Review

Animal antimicrobial peptides (AMPs), known as broad-spectrum and high-efficiency antibacterial activity, are important effector molecules in innate immune system. AMPs not only have antimicrobial, antiviral and antitumor effects but also exhibit important effects in vivo, such as anti-inflammatory response, recruiting immune cells, promoting epithelial damage repair, and promoting phagocytosis of bacteria. However, research on the application of AMPs is incomplete and controversial. This review mainly introduces the classification of AMPs, biological functions, as well as the mechanisms of action, expression rules, and nutrition regulation from three perspectives, aiming to provide important information for the application of AMPs.

Applications and evolution of melittin, the quintessential membrane active peptide

Melittin, the main venom component of the European Honeybee, is a cationic linear peptide-amide of 26 amino acid residues with the sequence: GIGAVLKVLTTGLPALISWIKRKRQQ-NH₂. Melittin binds to lipid bilayer membranes, folds into amphipathic α-helical secondary structure and disrupts the permeability barrier. Since melittin was first described, a remarkable array of activities and potential applications in biology and medicine have been described. Melittin is also a favorite model system for biophysicists to study the structure, folding and function of peptides and proteins in membranes. Melittin has also been used as a template for the evolution of new activities in membranes. Here we overview the rich history of scientific research into the many activities of melittin and outline exciting future applications.

RNA-based pharmacotherapy for tumors: From bench to clinic and back

RNA therapy is a treatment that regulates cell proteins and cures diseases by affecting the metabolism of mRNAs in cells, which has cut a figure in the studies on various incurable illnesses like hereditary diseases, tumors, etc. In this review, we introduced the discovery and development of RNA therapy and discussed its classification, mechanisms, advantages, and challenges. Moreover, we highlighted how RNA therapy works in killing tumor cells as well as what progresses it has made in related researches. And the development of RNA anti-tumor drugs and the clinical trial process were also included.

Dual Action of the PN159/KLAL/MAP Peptide: Increase of Drug Penetration across Caco-2 Intestinal Barrier Model by Modulation of Tight Junctions and Plasma Membrane Permeability

The absorption of drugs is limited by the epithelial barriers of the gastrointestinal tract. One of the strategies to improve drug delivery is the modulation of barrier function by the targeted opening of epithelial tight junctions. In our previous study the 18-mer amphiphilic PN159 peptide was found to be an effective tight junction modulator on intestinal epithelial and blood–brain barrier models. PN159, also known as KLAL or MAP, was described to interact with biological membranes as a cell-penetrating peptide. In the present work we demonstrated that the PN159 peptide as a penetration enhancer has a dual action on intestinal epithelial cells. The peptide safely and reversibly enhanced the permeability of Caco-2 monolayers by opening the intercellular junctions. The penetration of dextran molecules with different size and four efflux pump substrate drugs was increased several folds. We identified claudin-4 and -7 junctional proteins by docking studies as potential binding partners and targets of PN159 in the opening of the paracellular pathway. In addition to the tight junction modulator action, the peptide showed cell membrane permeabilizing and antimicrobial effects. This dual action is not general for cell-penetrating peptides (CPPs), since the other three CPPs tested did not show barrier opening effects.

The In Vitro Protective Role of Bovine Lactoferrin on Intestinal Epithelial Barrier

The intestinal epithelial barrier plays a key protective role in the gut lumen. Bovine lactoferrin (bLF) has been reported to improve the intestinal epithelial barrier function, but its impact on tight junction (TJ) proteins has been rarely described. Human intestinal epithelial crypt cells (HIECs) were more similar to those in the human small intestine, compared with the well-established Caco-2 cells. Accordingly, both HIECs and Caco-2 cells were investigated in this study to determine the effects of bioactive protein bLF on their growth promotion and intestinal barrier function. The results showed that bLF promoted cell growth and arrested cell-cycle progression at the G2/M-phase. Moreover, bLF decreased paracellular permeability and increased alkaline phosphatase activity and transepithelial electrical resistance, strengthening barrier function. Immunofluorescence, western blot and quantitative real-time polymerase chain reaction revealed that bLF significantly increased the expression of three tight junction proteins-claudin-1, occludin, and ZO-1-at both the mRNA and protein levels, and consequently strengthened the barrier function of the two cell models. bLF in general showed higher activity in Caco-2 cells, however, HIECs also exhibited desired responses to barrier function. Therefore, bLF may be incorporated into functional foods for treatment of inflammatory bowel diseases which are caused by loss of barrier integrity.

The epithelial barrier-protecting properties of a soy hydrolysate

Enhancing the epithelial barrier function could be a possible strategy to prevent food allergy or reduce its symptoms. Soy hydrolysates containing bioactive peptides could be instrumental in this. In this study, the protective effects of pretreatment with 6 soy hydrolysates on calcium ionophore A23187-induced TEER reduction were studied in T84 cells. The effects of the most potent soy hydrolysate on tight junction gene expression were studied. In order to identify the underlying pathways involved, the barrier disruptor specificity of the effect was studied by comparing the protective effects on TEER and Lucifer Yellow flux after the exposure to barrier disruptors that work via different intracellular pathways, i.e. the disruptors A23187, mellitin, and deoxynivalenol (DON). Preincubation with one of the six hydrolysates protected the epithelial cells from a decrease in TEER induced by A23187 (restored to 105% of the starting point, while A23187 alone decreased to 53% of the starting value) and mellitin (restored to 11% of the starting point, while mellitin alone decreased to 3.8% of the starting value). This soy hydrolysate was found to increase claudin-1 and decrease claudin-2 expression. The protective effect of the hydrolysate on TEER was specific for the barrier disruptors A23187 and mellitin, but was not observed for DON. This observation suggests that the soy hydrolysate may act via PKC isoforms, which are known to lead to changes in the expression of claudin-1 and 2. Our data suggest that specific soy hydrolysates may be designed to strengthen the epithelial barrier which might be instrumental in the management of the barrier function in individuals at risk of developing food allergy.

Candida albicans-Induced Epithelial Damage Mediates Translocation through Intestinal Barriers

Life-threatening systemic infections often occur due to the translocation of pathogens across the gut barrier and into the bloodstream. While the microbial and host mechanisms permitting bacterial gut translocation are well characterized, these mechanisms are still unclear for fungal pathogens such as Candida albicans, a leading cause of nosocomial fungal bloodstream infections. In this study, we dissected the cellular mechanisms of translocation of C. albicans across intestinal epithelia in vitro and identified fungal genes associated with this process. We show that fungal translocation is a dynamic process initiated by invasion and followed by cellular damage and loss of epithelial integrity. A screen of >2,000 C. albicans deletion mutants identified genes required for cellular damage of and translocation across enterocytes. Correlation analysis suggests that hypha formation, barrier damage above a minimum threshold level, and a decreased epithelial integrity are required for efficient fungal translocation. Translocation occurs predominantly via a transcellular route, which is associated with fungus-induced necrotic epithelial damage, but not apoptotic cell death. The cytolytic peptide toxin of C. albicans, candidalysin, was found to be essential for damage of enterocytes and was a key factor in subsequent fungal translocation, suggesting that transcellular translocation of C. albicans through intestinal layers is mediated by candidalysin. However, fungal invasion and low-level translocation can also occur via non-transcellular routes in a candidalysin-independent manner. This is the first study showing translocation of a human-pathogenic fungus across the intestinal barrier being mediated by a peptide toxin.IMPORTANCECandida albicans, usually a harmless fungus colonizing human mucosae, can cause lethal bloodstream infections when it manages to translocate across the intestinal epithelium. This can result from antibiotic treatment, immune dysfunction, or intestinal damage (e.g., during surgery). However, fungal processes may also contribute. In this study, we investigated the translocation process of C. albicans using in vitro cell culture models. Translocation occurs as a stepwise process starting with invasion, followed by epithelial damage and loss of epithelial integrity. The ability to secrete candidalysin, a peptide toxin deriving from the hyphal protein Ece1, is key: C. albicans hyphae, secreting candidalysin, take advantage of a necrotic weakened epithelium to translocate through the intestinal layer.

Using peptides to increase transport across the intestinal barrier

The oral route is the preferred for the administration of drugs; however, it has some serious limitations. One of the main disadvantages is poor permeability across the intestinal barrier. Various approaches are currently being adopted to overcome this issue. In this review, we describe the alternatives that use peptides to enhance intestinal absorption. First, we define the various sources of peptide enhancers followed by the analysis of the absorption mechanism used. We then comment on the possible toxic effects derived from their use as permeation enhancers, as well as potential formulation strategies. Finally, the advantages and drawbacks of peptides as intestinal enhancers are examined.

The effect of phytic acid on tight junctions in the human intestinal Caco-2 cell line and its mechanism

This study investigated the effect of phytic acid (IP6), a potential absorption enhancer of flavonoid components, on tight junction (TJ) integrity in Caco-2 cell monolayers and its possible mechanisms. Transepithelial electrical resistance (TEER) across the monolayers decreased rapidly, and the flux of fluorescein sodium (a paracellular marker) increased after treating with IP6 in a concentration-dependent manner. Confocal microscopy results showed that IP6 produced a concentration-dependent attenuation in the distribution of occludin, ZO-1, and claudin-1. Immunoblot analysis revealed that IP6 could down-regulate the expression level of these TJ proteins, which resulted in the opening of TJ. Additionally, the divalent cations Ca(2+) and Mg(2+) influenced the IP6-induced distribution of occludin, ZO-1, and claudin-1 in different directions, which enhanced barrier function. In conclusion, IP6 can decrease the integrity of Caco-2 cell monolayers by modulating the TJ proteins' localization and down-regulating the expression levels of TJ proteins including claudin-1, occludin, and ZO-1; the reduction effects of divalent cations such as Ca(2+) and Mg(2+) on the regulation of TJ induced by IP6 should be addressed. The present work will offer some useful guidance for the application of IP6 in drug delivery area.

In silico modelling of permeation enhancement potency in Caco-2 monolayers based on molecular descriptors and random forest

Structural traits of permeation enhancers are important determinants of their capacity to promote enhanced drug absorption. Therefore, in order to obtain a better understanding of structure-activity relationships for permeation enhancers, a Quantitative Structural Activity Relationship (QSAR) model has been developed. The random forest-QSAR model was based upon Caco-2 data for 41 surfactant-like permeation enhancers from Whitehead et al. (2008) and molecular descriptors calculated from their structure. The QSAR model was validated by two test-sets: (i) an eleven compound experimental set with Caco-2 data and (ii) nine compounds with Caco-2 data from literature. Feature contributions, a recent developed diagnostic tool, was applied to elucidate the contribution of individual molecular descriptors to the predicted potency. Feature contributions provided easy interpretable suggestions of important structural properties for potent permeation enhancers such as segregation of hydrophilic and lipophilic domains. Focusing on surfactant-like properties, it is possible to model the potency of the complex pharmaceutical excipients, permeation enhancers. For the first time, a QSAR model has been developed for permeation enhancement. The model is a valuable in silico approach for both screening of new permeation enhancers and physicochemical optimisation of surfactant enhancer systems.

Bioactive self-assembling lipid-like peptides as permeation enhancers for oral drug delivery

Amphiphilic, lipid-like, self-assembling peptides are functional biomaterials with surfactant properties. In this work, lipid-like peptides were designed to have a hydrophilic head composed of aspartic acid or lysine and a six alanine residue hydrophobic domain and have a length similar to that of biological lipids. The aim of this work was to examine the potential of using ac-A6 K-CONH2 , KA6 -CONH2 , ac-A6 D-COOH, and DA6 -COOH lipid-like peptides as permeability enhancers to facilitate transport through the intestinal barrier. In vitro transport studies of the macromolecular fluorescent marker fluorescein isothiocyanate (FITC)-dextran (4.4 kDa) through Caco-2 cell monolayers show the permeation enhancement ability of the lipid-like peptides. We observed increased FITC-dextran transport across the epithelial monolayer up to 7.6-fold in the presence of lipid-like peptides. Furthermore, we monitored the transepithelial resistance and performed immunofluorescence studies of the cell tight junctions. Ex vivo studies showed increased mucosal to serosal absorption of FITC-dextran in rat jejunum in the presence of the ac-A6 D-COOH peptide. Furthermore, a small increase in the serosal transport of bovine serum albumin was observed upon addition of ac-A6 D-COOH. Lipid-like peptides are biocompatible and they do not affect epithelial cell viability and epithelial monolayer integrity. Our results suggest that short, lipid-like peptides may be used as permeation enhancers to facilitate oral delivery of diagnostic and therapeutic molecules.

Homoharringtonine increases intestinal epithelial permeability by modulating specific claudin isoforms in Caco-2 cell monolayers

Homoharringtonine (HHT), a natural alkaloid produced by various Cephalotaxus species, has antileukemic activity in acute and chronic myelogenous leukemia. However, HHT can also induce unanticipated effects in the gastrointestinal tract, such as diarrhea and nausea/vomiting, but the mechanism behind these adverse effects has not been clarified. In the present study, we show that HHT affects the epithelial permeability of intestinal Caco-2 cell monolayers. HHT reduced the transepithelial electrical resistance (TER) of Caco-2 cells in a dose- and time-dependent manner. The HHT effect was reversible and no cytotoxicity was observed at the concentrations used. HHT simultaneously increased the paracellular flux of the 4 kDa and 40 kDa FITC-dextrans associated with the TER reduction. Immunoblotting analysis revealed that HHT decreased the protein expression of TJ components such as claudin-3, -5, and -7. However, the transcription levels of these claudins were not repressed by HHT treatment. HHT also disturbed the cellular localization of claudin-1 and -4. These changes coincided with the reduced barrier function. Our findings suggest that HHT enhances the paracellular permeability of Caco-2 cell monolayers by modulating the protein expression and localization of claudin isoforms; these actions might be responsible for the gastrointestinal effects of HHT.

Production of antibacterial peptide from bee venom via a new strategy for heterologous expression

Honey bee is important economic insect that not only pollinates fruits and crops but also provides products with various physiological activities. Bee venom is a functional agent that is widely applied in clinical treatment and pharmacy. Secapin is one of these agents that have a significant role in therapy. The functions of secapin from the bee venom have been documented, but little information is known about its heterologous expression under natural condition. Moreover, few scholars verified experimentally the functions of secapin from bee venom in vitro. In this study, we successfully constructed a heterologous expression vector, which is different from conventional expression system. A transgenic approach was established for transformation of secapin gene from the venom of Apis mellifera carnica (Ac-sec) into the edible fungi, Coprinus cinereus. Ac-sec was encoded by a 234 bp nucleotide that contained a signal peptide domain and two potential phosphorylation sites. The sequence exhibited highly homology with various secapins characterized from honey bee and related species. Southern blot data indicated that Ac-sec was present as single or multiple copy loci in the C. cinereus genome. By co-transformation and double-layer active assay, Ac-sec was expressed successfully in C. cinereus and the antibacterial activity of the recombinants was identified, showing notable antibacterial activities on different bacteria. Although Ac-sec is from the venom of Apidae, phylogenetic analysis demonstrated that Ac-sec was more closely related to that of Vespid than to bee species from Apidae. The molecular characteristics of Ac-sec and the potential roles of small peptides in biology were discussed.

Fucoidan enhances intestinal barrier function by upregulating the expression of claudin-1

AIM: To evaluate the protective effects of fucoidan on oxidative stress-induced barrier disruption in human intestinal epithelial cells.

METHODS: In Caco-2 cell monolayer models, the disruption of barrier function by oxidative stress is mediated by H₂O_2. The integrity of polarized Caco-2 cell monolayers was determined by measuring the transepithelial resistance (TER) and permeability was estimated by measuring the paracellular transport of FITC-labeled 4-kDa dextran (FD4). The protective effects of fucoidan on epithelial barrier functions on polarized Caco-2 cell monolayers were evaluated by TER and FD4 flux. The expression of tight junction (TJ) proteins was assessed using reverse-transcription polymerase chain reaction (RT-PCR) and immunofluorescence staining.

RESULTS: Without H₂O₂ treatment, fucoidan significantly increased the TER compared to control (P < 0.05), indicating a direct enhancement of intestinal epithelial barrier function. Next, H₂O₂ disrupted the epithelial barrier function in a time-dependent manner. Fucoidan prevented the H₂O₂-induced destruction in a dose-dependent manner. Fucoidan significantly decreased H₂O₂-induced FD4 flux (P < 0.01), indicating the prevention of disruption in paracellular permeability. RT-PCR showed that Caco-2 cells endogenously expressed claudin-1 and -2, and occludin and that H₂O₂ reduced the mRNA expression of these TJ proteins. Treatment with fucoidan attenuated the reduction in the expressions of claudin-1 and claudin-2 but not occludin. Immunofluorescence staining revealed that the expression of claudin-1 was intact and high on the cell surface. H₂O₂ disrupted the integrity of claudin-1. Treatment with fucoidan dramatically attenuated the expression of claudin-1.

CONCLUSION: Fucoidan enhanced intestinal epithelial barrier function by upregulating the expression of claudin-1. Thus, fucoidan may be an appropriate therapy for the treatment of inflammatory bowel diseases.

Oral drug delivery systems using chemical conjugates or physical complexes

Oral delivery of therapeutics is extremely challenging. The digestive system is designed in a way that naturally allows the degradation of proteins or peptides into small molecules prior to absorption. For systemic absorption, the intact drug molecules must traverse the impending harsh gastrointestinal environment. Technologies, such as enteric coating, with oral dosage formulation strategies have successfully provided the protection of non-peptide based therapeutics against the harsh, acidic condition of the stomach. However, these technologies showed limited success on the protection of therapeutic proteins and peptides. Importantly, inherent permeability coefficient of the therapeutics is still a major problem that has remained unresolved for decades. Addressing this issue in the context, we summarize the strategies that are developed in enhancing the intestinal permeability of a drug molecule either by modifying the intestinal epithelium or by modifying the drug itself. These modifications have been pursued by using a group of molecules that can be conjugated to the drug molecule to alter the cell permeability of the drug or mixed with the drug molecule to alter the epithelial barrier function, in order to achieve the effective drug permeation. This article will address the current trends and future perspectives of the oral delivery strategies.

Melittin modulates keratinocyte function through P2 receptor-dependent ADAM activation

Melittin, the major component of the bee venom, is an amphipathic, cationic peptide with a wide spectrum of biological properties that is being considered as an anti-inflammatory and anti-cancer agent. It modulates multiple cellular functions but the underlying mechanisms are not clearly understood. Here, we report that melittin activates disintegrin-like metalloproteases (ADAMs) and that downstream events likely contribute to the biological effects evoked by the peptide. Melittin stimulated the proteolysis of ADAM10 and ADAM17 substrates in human neutrophil granulocytes, endothelial cells and murine fibroblasts. In human HaCaT keratinocytes, melittin induced shedding of the adhesion molecule E-cadherin and release of TGF-α, which was accompanied by transactivation of the EGF receptor and ERK1/2 phosphorylation. This was followed by functional consequences such as increased keratinocyte proliferation and enhanced cell migration. Evidence is provided that ATP release and activation of purinergic P2 receptors are involved in melittin-induced ADAM activation. E-cadherin shedding and EGFR phosphorylation were dose-dependently reduced in the presence of ATPases or P2 receptor antagonists. The involvement of P2 receptors was underscored in experiments with HEK cells, which lack the P2X7 receptor and showed strikingly increased response to melittin stimulation after transfection with this receptor. Our study provides new insight into the mechanism of melittin function which should be of interest particularly in the context of its potential use as an anti-inflammatory or anti-cancer agent.

In vivo effects of dietary (1→3), (1→4)-β-D-glucans from oat on mucosal immune responses in man and mice

OBJECTIVE

Antimicrobial peptides and tight junction proteins are crucial to maintain mucosal immunity. It is known that oat β-glucan may affect intestinal immunity. Therefore, the aim of the present study was to evaluate the effect of oat β-glucan on the presence of antimicrobial peptides and tight junction protein.

MATERIAL AND METHODS

We analyzed antimicrobial peptide levels in fecal water prepared from 24 h ileostomic bag contents obtained from ileostomic patients consuming oat β-glucan enriched or control diets in a cross-over design. In addition, intestinal sections of mice, which received oat β-glucan via oral gavages for 3.5 days, were analyzed for lysozyme and zonula occludens-1 expression.

RESULTS

We observed a trend toward lower lysozyme (-23%; p = 0.076) and bactericidal/permeability-increasing protein (-17%; p = 0.098) levels in oat β-glucan enriched fecal water as compared with placebo. Additionally, mice receiving oat β-glucan showed a lower lysozyme expression in stained distal small intestinal sections (p = 0.011). Staining of zonula occludens-1 was decreased in β-glucan treated mice indicating disruption of the tight junction integrity.

CONCLUSIONS

In conclusion, the consumption of oat β-glucan seems to decrease the levels of antimicrobial peptides in fecal water from human ileostomy patients and its expression in distal small intestine sections in mice. The decreased intestinal integrity in mice could be explained by the drop in antimicrobial peptides.

Omeprazole induces gastric transmucosal permeability to the peptide bradykinin

AIM: To investigate omeprazole-induced transepithelial gastric leak and its effects on the permeability of the peptides bradykinin and oxytocin.

METHODS: Rat gastric corpus tissue was isolated and mounted in an Ussing chamber apparatus to evaluate the permeability of ³H-bradykinin, ³H-oxytocin, and ¹⁴C-EDTA in the presence or absence of omeprazole. Thin-layer chromatography was performed to identify any metabolic breakdown products of the peptides resulting from permeation through the gastric tissue, and thereby calculate the true flux of the peptide.

RESULTS: The flux rate of intact ³H-bradykinin increased substantially after omeprazole addition (109.5%) compared to the DMSO vehicle control (14%). No corresponding change in flux of intact ³H-oxytocin was observed under the same conditions (11.9% and 6.4% in the DMSO- and omeprazole-treated conditions, respectively). After exposure to omeprazole, the flux rate of ¹⁴C-EDTA also increased dramatically (122.3%) compared to the DMSO condition (36.3%).

CONCLUSION: The omeprazole-induced gastric leak allows for transmucosal permeability to charged molecules as well as non-electrolytes. This induced leak will allow certain peptides to permeate.

HPMCs Induce Greater Intercellular Delocalization of Tight Junction-Associated Proteins Due to a Higher Susceptibility to H2O2 Compared with HUVECs

Background

Reactive oxygen species (ROS) have been speculated as possible inducers of structural or functional changes that lead to a hyperpermeable state in patients on long-term peritoneal dialysis. This study aimed to compare localization of tight junction-associated proteins (TJPs), which relate to solute permeability characteristics, between human peritoneal mesothelial cell (HPMC) monolayers and human umbilical vein endothelial cell (HUVEC) monolayers under oxidative stress.

Methods

HPMCs and HUVECs were cultured on a polymer mesh until transepithelial electrical resistance reached a plateau. Solute permeation tests were conducted using FITC-labeled dextrans. Localization of TJPs was observed under a confocal laser scanning microscope. These experiments were carried out with/without 0.1 mmol/L H2O2. In addition, ROS production as well as the amounts of intracellular reductive glutathione (GSH) and oxidative glutathione were measured.

Results

When the monolayers were exposed to 0.1 mmol/L H2O2/medium for 2 hours, the HPMC monolayer revealed a significant reduction in transepithelial electrical resistance (from 32.5 ± 3.4 to 17.4 ± 4.9 Ω cm ) with delocalization of TJPs, particularly occludins. The HUVEC monolayer remained stable and exhibited an unremarkable change in TJP organization. Compared to the HUVEC monolayer, the HPMC monolayer exhibited two- to threefold higher 2′,7′–dichlorofluorescein intensities that increased in a dose-dependent manner. HUVECs contained approximately 2.5-times more GSH than HPMCs. This supported the lesser production of ROS when exposed to 0.1 mmol/L H2O2 for 24 hours. HUVECs used 8.03 nmol/mg GSH protein to maintain TJP localization, while only 3.75 nmol/mg GSH protein was available for the HPMCs.

Conclusion

The HUVEC monolayer, which was less permeable to middle-to-high molecular weight solutes, was more tolerant against ROS stress than the HPMC monolayer. Availability of intracellular GSH is an important issue in maintaining the integrity of the mesothelium.

Melittin exhibits necrotic cytotoxicity in gastrointestinal cells which is attenuated by cholesterol

Melittin, a cationic antimicrobial peptide isolated from the venom of Apis mellifera, has shown potential as a permeability enhancer, transiently increasing intestinal permeability and enhancing the absorption of paracellular markers. Although it is cytotoxic to eukaryotic cells, its cytotoxicity is significantly lower in polarised epithelia compared to non-polarised cells. The aim of this study was to explore the mechanism of melittin cytotoxicity in gastrointestinal cells and to determine whether cytotoxicity was mediated by a necrotic or an apoptotic pathway. The role of cholesterol in melittin cytotoxicity was also examined. Using four distinct assays for apoptosis, phosphatidylserine translocation, caspase activation, DNA ladder formation and cell cycle analysis, no evidence of apoptotic pathway for cell death was observed with any of these approaches. It can therefore be concluded that cytotoxicity was likely to be mediated by necrosis in gastrointestinal epithelial cells. However, at low concentrations of melittin (<1 microM), BRDU uptake was enhanced, demonstrating proliferative effects of melittin at sub-lethal concentrations. Furthermore, melittin cytotoxicity was further enhanced by depletion of cholesterol, using methyl-beta-cyclodextrin, indicating that cholesterol depleting agents could be contradictory to its potential as an enhancer. Overall, although melittin appears to stimulate necrosis, with careful dosage selection the peptide could be considered for the oral delivery of poorly bioavailable drugs.

Melittin as a permeability enhancer II: in vitro investigations in human mucus secreting intestinal monolayers and rat colonic mucosae

PURPOSE

Melittin has shown potential as a non-cytotoxic absorption enhancer in Caco-2 monolayers. Our objectives were to assess in vitro efficacy and cytotoxicity of melittin in two intestinal permeability models and investigate the potential mechanism by which melittin might enhance gastrointestinal absorption.

MATERIALS AND METHODS

The effects of melittin were examined in the mucus-secreting intestinal cell monolayers, HT29-MTX-E12 (E12), using transepithelial electrical resistance (TER), transmission electron microscopy (TEM) and the MTT viability assay. The effects of melittin on TER, permeability and short circuit current (Isc) were also investigated in rat colon mucosae mounted in Ussing chambers. Ion transporting capacity of tissue was measured in response to secretagogues as surrogate markers of cytotoxicity. Melittin stability was examined by a means of a hemolytic assay. The mechanism by which melittin decreases TER across the rat mucosa was examined with a range of enzymatic inhibitors.

RESULTS

Apical addition of melittin resulted in a reversible non-cytotoxic concentration-dependent decrease in TER across E12 monolayers, which was independent of the presence of mucus. Apical addition of melittin reduced TER and increased the permeability of [(14)C]-mannitol across rat colonic mucosae. The melittin-induced drop in TER in rat colon was significantly attenuated by W7 suggesting partial mediation by calmodulin.

CONCLUSIONS

The rapid and reversible nature of melittin's permeation enhancing properties and its limited cytotoxicity in polarized intestinal epithelia, suggests a potential drug delivery role for the peptide in oral formulations of poorly absorbed drugs.