What Is Known/What Is New
What Is Known
- Alanylglutamine is a dipeptide that fuels enterocytes.
- Alanylglutamine restores intestinal barrier function and prevents deterioration of gut permeability.
What Is New
- Alanylglutamine healed gastric ulcers induced by indomethacin and its action was comparable to pantoprazole.
- Alanylglutamine has antisecretory potential mediated by reducing H+-K+-ATPase pump expression.
- Alanylglutamine enhances mucosal proliferation via the glucagon-like peptide-1/β-catenin/cyclin-D1 cue.
- Alanylglutamine inhibits apoptosis by repressing nuclear factor-kappa B/tumor necrosis factor-α/H2O2.
Glutamine or its more stable and soluble dipeptide, alanylglutamine (AG), are considered gut nutrients that restore the intestinal barrier function and promote the growth in malnourished enteropathic patients (1). AG is a primary fuel source for enterocytes, because it generates adenosine triphosphate (ATP) (2); it is a precursor of several amino acids as arginine and citrulline (3), besides being a source for alanine and glutamine. In addition to their utilization in protein synthesis, these amino acids possess coadjuvant roles during gut healing (2,4). In catabolic stress, the endogenous synthesis of glutamine is impaired and the body begins to mobilize glutamine from muscle stores (5); however, under the increased metabolic demand of rapidly proliferating cells, particularly those of the gastrointestinal tract mucosa, the endogenous supplementation of this amino acid may fail (6,7). In both clinical and experimental settings, enteral and/or parenteral glutamine has been reported to improve the gastrointestinal function, prevent the deterioration of gut permeability, and preserve the gut mucosal structure (8,9). In addition, this amino acid has been documented to increase the rate of intestinal mucosal healing, evident by the improved height of the villi (8,10), whereas deprivation of AG leads to villus atrophy, mucosal ulcerations, and cell necrosis (8,11).
The gastric ulcer healing cascades embrace a constellation of time-dependent events and comprise the activation of an array of genes (12). The healing process is related to prostaglandin E2, which induces angiogenesis (13) and maintains the gastric mucosal blood flow. Proper gastric mucosal perfusion delivers adequate oxygenation, nutrition, and bicarbonate along with mopping up excess acid and toxins (14). Aside from the documented role of the eicosanoid, prostaglandin-independent mechanisms have emerged, where several growth factors derived from epithelial and mesenchymal cells are involved in gastric ulcer healing (12,15). Moreover, reconstruction of the gastric mucosa occurs through the formation of granulation tissue at the ulcer base with the restoration of glandular tissue (12) via the proliferation and migration of epithelial cells and connective tissue to the denuded areas (16).
On the contrary, the commonly prescribed nonsteroidal anti-inflammatory drugs (NSAIDs) are well known for their gastropathy that limit their consumption. This adverse effect resides mainly on the inhibition of cyclooxygenase (COX) enzyme, which in turn suppresses the key protective autacoid, PGE2 (14). However, apart from the COX inhibition, NSAIDs mediate apoptotic cell death (17), repress the migration of endothelial cells to the denuded areas, and recruit inflammatory mediators to exacerbate injurious factors (18), events that delay gastric ulcer healing (19).
According to the above data, the present study was undertaken to delineate the conceivable effectiveness of AG in indomethacin-induced gastric ulceration in rats compared to the proton pump inhibitor pantoprazole, the reference drug as a primary objective and to unveil some of the potential molecular mechanisms of this dipeptide as a secondary objective.
Male Wistar rats (7-week old) weighing 160 ± 10 g were purchased from the Faculty of Veterinary Medicine (Cairo University, Cairo, Egypt) and habituated to the experimental conditions for a week before experimentation. They were maintained on 12/12-hour dark/light cycles, controlled laboratory conditions (room temperature of 25°C ± 2°C; humidity of 65% ± 5%), and proper diet chow and water ad libitum. Before the induction of gastric ulcers, rats were fasted overnight with free access to water and were kept individually in wide mesh bottom cages. Handling of animals complied with the Guide for the Care and Use of Laboratory Animals (NIH, 1978) and ARRIVE guidelines. The study was approved by the Research Ethics Committee at the Faculty of Pharmacy, Cairo University (Cairo, Egypt) with the permit number (PT) 1566.
Induction of Indomethacin Gastric Ulceration and Experimental Design
Animals (n = 10 rats/group) were randomly allocated into 5 groups; viz, the control group, where rats received the vehicle (1% Tween 80), whereas those in the other groups received indomethacin (100 mg/kg; p.o.; Sigma-Aldrich, MO) suspended in the vehicle (20). Gastropathic animals were further subdivided into the indomethacin untreated group and those treated 4 hours after indomethacin with pantoprazole (30 mg/kg, i.p.; Sigma Pharmaceutical Industries, Cairo, Egypt (21)), AG (1.5 g/kg, i.p.; Fresenius Kabi, Cairo, Egypt (22)), or AG preceded 15 minutes by dexamethasone (DEXA; 5 mg/kg, i.p.; Sigma Pharmaceutical Industries (23)). The treatments were administered once again after 24 hours and animals were fasted overnight with free access to water that was removed 4 hours before euthanasia. In the first subset of the experiment (n = 6 rats/group), blood was collected from the femoral vein, under light anesthesia, to prepare sera for the estimation of tumor necrosis factor (TNF)-α and glucagon-like peptide (GLP)-1. Rats were euthanized with an overdose of thiopental (Egyptian International Pharmaceuticals Industries, Cairo, Egypt) and the stomachs were isolated, opened along the greater curvature. The gastric juice was collected and the glandular portion of the stomach was inspected for ulceration. The gastric mucosa was scraped and subdivided to be kept in RIPA buffer (Bio Basic, Ontario, Canada) for Western blot or RNA later solution (Thermo Fisher Scientific, MA) for reverse transcription polymerase chain reaction or homogenized in phosphate buffered saline for assessments of hydrogen peroxide (H2O2) and caspase-3 activity. Of note, the protein content of the gastric mucosa was estimated using Bradford technique (24). In the second experimental subset, the stomach of the remaining 4 animals/group was assessed for gastric ulceration and used for histopathological examination.
Determination Ulcer Index and Gastric Juice pH
The ulcer index was calculated by the summation of the length of each ulcer at its greater axis (25) and the pH of the gastric juice was determined using pH meter.
Determination of Gastric Mucosal Hydrogen Peroxide Content
H2O2 was assessed using the colorimetric reagent kit (Sigma-Aldrich; cat CS0270).
Determination of Gastric Mucosal Caspase-3 Activity
The ApoTarget selective colorimetric assay kit (Invitrogen Corporation, MA) was used for the assessment of caspase-3 activity according to the manufacturer's instructions. The test is based on incubating the sample with DEVD-p-nitroanilide (pNA) substrate that is a synthetic upstream tetrapeptide DEVD of the caspase-3 cleavage site linked to the chromophore pNA. When the substrate is cleaved by caspase-3, pNA is released to produce a yellow color that is measured at 400 nm. For the determination of the fold elevation in the caspase-3 activity, the absorbance of pNA from apoptotic sample is compared with the control.
Serum TNF-α (cat RTA00) and GLP-1 (cat MBS723782) were assessed using the corresponding ELISA kits (MyBioSource, CA) according to the manufacturer's protocols.
Samples were washed twice with RIBA lysis buffer PL005 (Bio Basic; cat L3R 8T4) composed of NaCl (150 mmol/L), Triton X-100 (0.1%), sodium deoxycholate (0.5%), sodium dodecyl sulfate (0.1%), Tris-HCl (50 mmol/L), protease inhibitor buffer, and phosphatase inhibitor buffer. The cell lysates were centrifuged and equal amounts of protein extracted from the gastric mucosa were separated by 4% sodium dodecyl sulfate polyacrylamide gel electrophoresis TGX Stain-Free FastCast Acrylamide kit (Bio-Rad, CA; cat 161-0181) and transferred to nitrocellulose membranes. Blots were blocked for 1 hour in Tris-buffered saline that contained 3% bovine serum albumin, then incubated using the primary antibodies (Thermo Fisher Scientific) for pS133-cAMP response element-binding protein (CREB; 1:2000; cat 700129), pS473-protein kinase B (Akt; 1:2000; cat 700256), pS9-glucagon synthase kinase 3 beta (GSK3β; 1:2000; cat MA5-15023), pS536-nuclear factor kappa B (NFκB; 1:2000; cat 51-0500), pS45-β catenin (1:2000; cat PA5-17685), and β-actin (1:2000; cat MA1-300). Incubation was done with the horseradish peroxidase-conjugated goat anti-rabbit immunoglobulin secondary antibody (Novus Biologicals, CO; cat NBP2-30348H), then visualized by the enhanced chemiluminescent substrate (Clarity TM Western ECL; Bio-Rad; cat 170-5060). For the quantitative analysis, the mean intensity of each band (mean pixel) was compared with β-actin band using stain-free blot technology and ChemiDoc MP Imager (Bio-Rad).
Gene Expression Using Real-time Polymerase Chain Reaction Analysis
In alliance with the manufacturer's protocol, the total RNA was extracted using SV Total RNA Isolation System (Promega, WI) that was determined at 260 nm. RNA was reverse transcribed into cDNA using random primers (250 ng/μL), deoxyribonucleotide triphosphate (10 mmol/L), and the SuperScript II Reverse Transcriptase kit (Invitrogen). The following primer set was used for the quantitative determination of the gene expression of claudin-1 (forward primer: 5′-CTATCGCTCCGTTACTGGCAT-3 and reverse primer: 5′-ACAGCTGGGACCGTATGTGG-3), proton pump H+/K+ ATPase (forward primer: 5′-GCAGCGAGTGACTGGAACA-3′, reverse primer:5′-CCAGCCACGTTGCATTGTAG-3′), cyclin-D1 (forward primer: 5′-TGGAGCCCCTGAAGAAGAG-3′ and reverse primer: 5′-AAGTGCGTTGTGCGGTAGC-3′), and the housekeeping gene β-actin (forward primer: 5′-GGTCGGTGTGAACGGATTTGG-3 and reverse primer: 5′- ATGTAGGCCATGAGGTCCACC-3). The cDNA synthesis was performed at 37°C for 60 minutes, then 95°C for 10 minutes for reverse transcriptase inactivation. The resulting cDNA was mixed with SYBR green and other polymerase chain reaction mix components, then incubated in programmed thermocycler that was set at 40 cycles of denaturation step (95°C for 15 seconds), primer annealing (60°C for 1 minute), and extension step (72°C for 1 minute). The relative quantitation was calculated using an Applied Biosystem with software version 3.1 (StepOne, Thermo Fisher Scientific).
The stomach from 4 representative animals from each group was fixed in 10% formalin and embedded in paraffin. The blocks were sectioned (5 μm) and stained with hematoxylin and eosin and examined under a light microscope blindly.
Data are expressed as a mean of 6 to 10 rats ± standard deviation. The GraphPad Prism v7.0 (GraphPad Prism, CA) was used to analyze and present all data. Multiple comparisons were performed using 1-way analysis of variance, followed by Tukey's multiple comparison post-hoc test; P < 0.05 was considered as the significance limit for all comparisons.
Effect of Different Treatments on the Indomethacin-induced Gastric Ulceration
As depicted in Figure 1A, AG post-treatment prohibited the indomethacin-induced gastric ulceration in a manner comparable to that of pantoprazole; however, DEXA in combination with AG partly reverted its protective effect. Compared to the (Fig. 1B) control rat stomach photomicrograph, (Fig. 1C–E) indomethacin reveals ulceration of the gastric mucosa (u) with the presence of necrotic mucosal cells, edema, congested capillaries (c), and dense inflammatory infiltrate (i). Moreover, inflammatory cellular infiltrate were noted in the submucosa in gastropathic rats. Section of (Fig. 1F) pantoprazole shows an average mucosal thickness, intact mucus covering, and normal gastric pits, mucosal neck cells, parietal and peptic cells in the mucosa, with very few inflammatory cells (i) in the submucosa. Section of (Fig. 1G) AG shows gastric mucosa with average mucosal thickness and intact mucus covering with normal crowded gastric pits ending by branched tubular glands. Finally, the gastric mucosal section of rats treated with (Fig. 1H) DEXA and AG shows patchy mucosal ulceration (u) indicating its antagonistic effect.
Effect of Different Treatments on Gastric Acid Secretion in Indomethacin-induced Gastric Ulceration
As presented in Figure 2, indomethacin increased the basal hydrogen ion concentration, as depicted by a reduction in the (Fig. 2A) gastric juice pH and (Fig. 2B) an upregulation of the mRNA expression of H+/K+- ATPase. These effects were almost equally hindered by AG and pantoprazole, but reverted when DEXA was combined with AG. Administration of indomethacin caused 9.3- and 11.5-fold increase in (Fig. 2C) pS9-GSK3β, and (Fig. 2D) pS133-CREB, respectively, relative to the control rats. Treatment with AG, however, brought them back to normal, an effect that surpassed that of pantoprazole and was partially hindered by the coadministration of DEXA.
Effect of Different Treatments on Indomethacin-induced Impairment of Gastric Mucosal Proliferation and Tight Junction
Figure 3 illustrates the suppressing effect of indomethacin on (Fig. 3A) pS473-Akt, (Fig. 3B) cyclin-D1, and (Fig. 3E) GLP-1, effects that were associated with the marked increase in (Fig. 3C) pS45-β catenin and (Fig. 3D) the tight junction protein claudin-1, as compared to the control rats. These alterations were reverted by AG and pantoprazole in a comparable pattern; however, coadministration of DEXA with AG abolished its beneficial effects on all assessed parameters.
Effect of Different Treatments on Indomethacin-induced Gastric Mucosal Inflammation, Oxidative Stress, and Apoptosis
As shown in Figure 4, indomethacin increased the contents/activity of (Fig. 4A) pS536-NF-κB p65, (Fig. 4B) H2O2, (Fig. 4C) caspase-3, and (Fig. 4D) TNF-α, and, as compared to the control rats. AG and pantoprazole, abated the inflammatory, oxidative, and apoptotic markers, whereas combination with DEXA, obliterated the effect of AG to different degrees.
The current study verified that AG has an ulcer-healing activity that was comparable to the reference proton pump inhibitor. The merit of the dipeptide in healing indomethacin-induced gastric ulceration and some of the potential molecular mechanisms underlying its effect are addressed for the first time in this study. The AG healing capacity relies on the interplay between multiple pathways.
In the present work, sundry mechanisms gather to substantiate the AG antisecretory character; the first is its ability to increase GLP-1 level, which in turn reduces acetylcholine release and consequently gastric acid secretion (26) by virtue of increasing glutamine concentration (27).
Meanwhile, GLP-1 activates its subsequent downstream phosphatidylinositol 3 kinase/Akt signaling pathway, which plays also a role in suppressing acid secretion via stimulating the serum and glucocorticoid regulated kinase 3 (28).
Although GLP-1 is reported to activate Akt, which in turn phosphorylates/inactivates GSK-3β, findings of the present study showed an opposite result. AG has activated GSK-3β instead, to consequently reduce gastric acidity. Notably, GSK-3β phosphorylation at S9 residue was reported to enhance histamine release to increase acid secretion by binding to the H2 receptor (29). The nonsynchronized finding of GSK-3β points to the interplay of other players, such as indomethacin-mediated phosphorylation/inactivation of the protein phosphatase (PP)-2A (30). Furthermore, AG also abated indomethacin-induced p-CREB, possibly by inhibiting TNF-α and H2O2 formation, as documented in the present work. Both mediators are reported to activate p38 MAPK (31) that can phosphorylate CREB at the serine 133 residue (32). Noteworthy, p-CREB has been reported to play a role in the transcription of the α2 subunit of the proton pump (33). The inhibitory effect of AG on p-CREB protein expression, can explain the current downregulation of the H+/K+-ATPase pump to afford an explanation of the elevated gastric juice pH value. The aforementioned molecular alterations induced by the NSAID point to additional new mechanisms of indomethacin ulcerogenic activity, beside its known COX-2 inhibition.
Apart from its antisecretory effect, AG increased gastric mucosal proliferation by activating the GLP-1/β-catenin/cyclin-D1 cue to verify its gastric healing capacity. In the present work, AG decreased the phosphorylation of β-catenin at the S45 to prevent its ubiquitination and to permit its nuclear translocation. As a transcription factor, salvaged β-catenin enhances the formation of cyclin-D1, a Wnt target gene (34) leading to a pronounced increase in cellular division (30). The effect of GLP-1 here is believed to be through a noncanonical mechanism, possibly by GLP-1/protein kinase A/β-catenin signaling to harmonize with previous studies (34,35). In support to our study, Palorini et al (36) stated that cancer cells survived deprivation of glucose by glutamine utilization that was associated by activation of protein kinase A and cell survival.
In the present work, AG succeeded to inactivate NF-κB p65 to signify its anti-inflammatory and antiapoptotic properties that play a role in enhancing the healing of gastric ulcer. The indomethacin-induced activation of NF-κB p65 is again linked to the previously mentioned protein, PP-2A, as well as to increased TNF-α and H2O2(30,37,38). Both PP-2A and TNF-α mark the inhibitory kappa B for degradation to set NF-κB p65 free and to aid in its activation/phosphorylation at the serine 536 residue to be translocated into the nucleus (30,39). As a transcription factor, active NF-κB p65 transcribes several inflammatory mediators, such as TNF-α (40,41) and overly produce H2O2. On the contrary, in the present work AG had curtailed the excessive production of H2O2 and TNF-α in the indomethacin model, which in turn inactivate NF-κB p65, nailing down its anti-inflammatory effect. In addition, these markers entail their effect to initiate apoptosis in the indomethacin untreated rats, where NF-κB/TNF-α/H2O2 cycle ends up with the activation of caspase-3, as reported here and hitherto (41). Therefore, inhibition of these mediators is, in part, responsible for the dipeptide antiapoptotic effect.
In addition, the current activation of NF-κB upregulated the gene expression of the tight junction protein claudin-1 through TNF-α activation (42) in the indomethacin treated rats. This effect may inhibit the cellular migration to prohibit the mucosal healing process (43). Therefore, downregulation of claudin-1 gene expression by AG explains its ability to initiate mucosal healing by facilitating cell migration in the denuded areas. The decreased claudin-1 can prime the proliferation process triggered by the GLP-1/β-catenin/cyclin D1 cue, seen with AG post-treatment to verify its gastric healing capacity.
In the existing work, DEXA was able to oppose AG healing ability to reach that of indomethacin alone on most of the examined markers. Corticosteroids are known to act mainly through inhibiting phospholipase A2 to curtail the production of PGE2 (44). DEXA coadministration with AG opposed its antisecretory mechanism, where it activated the proton pump and decreased the pH value by enhancing p-GSK-3β and p-CREB, while decreasing p-Akt and GLP-1. Unexpectedly, in the present model, DEXA also has canceled the AG anti-inflammatory/-apoptotic and its proliferative effects, where NF-κB/TNF-α/H2O2, caspase3, and claudin-1 were elevated once again, whereas cyclin-D1 was abated. Indeed, previous studies using different experimental models, showed that DEXA reduced GLP-1 (45) and pS473-Akt (46), whereas it increased acid secretion (47), TNF-α (48), H2O2(49) and caspase-3 (46), findings that hence, support the present results.
In conclusion, AG proved its healing efficacy against indomethacin-induced gastric ulceration via manipulating several intersecting pathways; viz, NF-κB/TNF-α/H2O2/claudin-1, GLP-1/β-catenin/cyclin-D1, and p-CREB- p-Akt/ H+-K+-ATPase pump and p-GSK-3β–GLP-1/pH. These mechanisms were reverted, when DEXA was coadministered with AG, results that shed some light on the involvement of other mechanisms in the ulcerogenic effects of both indomethacin and DEXA.
The authors would like to thank Prof Dr. Kawkab Ahmed (Pathology Department, Faculty of Veterinary Medicine, Cairo University, Cairo, Egypt) for her valuable assistance in the histopathological examination.
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