Regeneration of gastric mucosa during ulcer healing is triggered by growth factors and signal transduction pathways.

An ulcer is a deep necrotic lesion penetrating through the entire thickness of the gastrointestinal mucosa and muscularis mucosae. Ulcer healing is a complex and tightly regulated process of filling the mucosal defect with proliferating and migrating epithelial and connective tissue cells. This process includes the re-establishment of the continuous surface epithelial layer, glandular epithelial structures, microvessels and connective tissue within the scar. Epithelial cells in the mucosa of the ulcer margin proliferate and migrate onto the granulation tissue to re-epithelialize the ulcer. Growth factors, such as epidermal growth factor (EGF), basic fibroblast growth factor (bFGF), trefoil peptides (TP), platelet derived growth factor (PDGF) and other cytokines produced locally by regenerating cells, control re-epithelialization and the reconstruction of glandular structures. These growth factors, most notably EGF, trigger epithelial cell proliferation via signal transduction pathways involving EGF-R- MAP (Erk1/Erk2) kinases. Granulation tissue, which develops at the ulcer base, consists of fibroblasts, macrophages and proliferating endothelial cells, which form microvessels under the control of angiogenic growth factors. These growth factors [bFGF, vascular endothelial growth factor (VEGF) and angiopoietins] promote angiogenesis--capillary vessel formation--thereby allowing for the reconstruction of microvasculature in the mucosal scar, which is essential for delivery of oxygen and nutrients to the healing site. The primary trigger to activate expression of angiogenic growth factors and their receptors appears to be hypoxia. During ulcer healing expression of growth factor genes is tightly regulated in a temporally and spatially ordered manner.

NSAIDs inhibit the activation of egr-1 gene in microvascular endothelial cells. A key to inhibition of angiogenesis?

Nonsteroidal anti-inflammatory drugs (NSAIDs) such as aspirin, indomethacin (IND), ibuprofen and newer cyclooxygenase-2 selective NSAIDs (e.g. celecoxib) delay gastric ulcer healing partly through the inhibition of angiogenesis, but the molecular mechanisms involved are not fully elucidated. Effective angiogenesis is required for ulcer healing to supply oxygen and nutrients to the healing site. The early growth response factor (Egr-1) is a transcription factor, which is rapidly activated by a variety of extracellular signals or tissue injury and is important for angiogenesis to occur. This study aimed to determine whether indomethacin (IND) and/or the selective COX-2 inhibitor, NS-398, interfere with egr-1 gene expression in human microvascular endothelial cells (HMVEC) in response to vascular endothelial growth factor (VEGF) stimulation. HMVEC were treated with 0.5 mM IND or 100 microM NS-398 for 16 h, and then VEGF (10 ng/ml) or vehicle was added. Egr-1 mRNA and protein expression levels were determined by RT-PCR and Western-blotting, respectively. VEGF treatment caused a significant elevation of Egr-1 mRNA (261+/-21%, P<0.001) and protein expression (174+/-15%, P<0.01) vs. vehicle. IND pre-treatment significantly inhibited VEGF-induced Egr-1 mRNA expression by 29+/-4% (P<0.01) and protein expression by 41+/-8% (P<0.05). NS-398 inhibited VEGF-induced Egr-1 mRNA and protein expression by 23+/-3% and 35+/-4%, respectively (both P<0.01). Since transcriptional activation of egr-1 is responsible for expression of proteins involved in proliferation of endothelial cells essential for angiogenesis, these results provide a new mechanism for NSAIDs' interference with angiogenesis.

Defective mitogen-activated protein kinase (ERK2) signaling in gastric mucosa of portal hypertensive rats: potential therapeutic implications.

Portal hypertensive (PHT) gastropathy is a frequent, serious complication of liver cirrhosis. PHT gastric mucosa has numerous abnormalities such as reduced mucosal potential differences, reduced surface oxygenation, and increased susceptibility to injury caused by alcohol, aspirin, and other noxious factors. Because such mucosal injury is initially mediated by oxygen free radicals, and because mitogen-activated protein (MAP) kinase (ERK2) protects against cellular stress and induces cell proliferation, we postulated that oxidative stress-induced ERK2 activation is defective in PHT gastric mucosa. Here we show that in PHT gastric mucosa, ERK2 activation by oxidative stress is impaired. This impairment is mediated by overexpression of MAP kinase phosphatase-1 (MKP-1), which results from the underlying and continual oxidative state associated with portal hypertension, and is ameliorated by inhibiting MKP-1. Furthermore, we found that supplementing vitamin E, a free radical scavenger, reduces the oxidative state in PHT gastric mucosa, normalizes MKP-1 expression, and thereby reverses impairment of oxidative stress-induced ERK2 activation. Finally, we show that orally administered vitamin E completely reverses the increased susceptibility of PHT gastric mucosa to alcohol injury. Our findings point to a new molecular and mechanistic basis for PHT gastropathy and provide a new therapeutic modality for protection of PHT gastric mucosa.

Gene therapy for gastric ulcers with single local injection of naked DNA encoding VEGF and angiopoietin-1.

BACKGROUND & AIMS: Angiogenesis, formation of new capillary blood vessels, is crucial for gastroduodenal ulcer healing because it enables delivery of oxygen and nutrients to the healing site. Because angiogenesis is stimulated by vascular endothelial growth factor (VEGF) and angiopoietin-1 (Ang1), we studied whether local gene therapy with nonviral DNA encoding VEGF and/or Ang1 into the ulcer base could accelerate ulcer healing through enhanced angiogenesis. METHODS: Gastric ulcers were induced in rats by acetic acid applied to the serosal surface of the stomach, and the site around the ulcer was injected with nonviral plasmid-encoding full-length complementary DNA (cDNA) of human recombinant (rh) VEGF165, rhAng1, or their combination. For some studies, neutralizing anti-VEGF antibody was administered. RESULTS: Single local injection of plasmids encoding VEGF165 and Ang1 significantly increased neovascularization and accelerated ulcer healing. A neutralizing anti-VEGF antibody significantly reduced the acceleration of ulcer healing resulting from the treatment. Coinjection of both plasmids encoding rhVEGF165 and rhAng1 resulted in formation of more mature vessels and to more complete restoration of gastric glandular structures within the ulcer scar. However, this did not result in further reduction of ulcer size. CONCLUSIONS: VEGF and Ang1 gene therapy, with limited duration of target gene expression, significantly accelerates gastric ulcer healing. Coinjection of both plasmids leads to more complete structural restoration. Inhibition of accelerated healing by a neutralizing anti-VEGF antibody indicates an essential role for VEGF and enhanced angiogenesis in ulcer healing.

PGE(2) stimulates VEGF expression in endothelial cells via ERK2/JNK1 signaling pathways.

Vascular endothelial growth factor (VEGF) plays an essential role in the initiation and regulation of angiogenesis-a crucial component of wound healing and cancer growth. Prostaglandins (PGs) stimulate angiogenesis but the precise mechanisms of their pro-angiogenic actions remain unexplained. We investigated whether prostaglandin E(2) (PGE(2)) can induce VEGF expression in rat gastric microvascular endothelial cells (RGMEC) and the signaling pathway(s) involved. We demonstrated that PGE(2) significantly increased ERK2 and JNK1 activation and VEGF mRNA and protein expression. Incubation of RGMEC with PD 98059 (MEK kinase inhibitor) significantly reduced PGE(2)-induced ERK2 activity, VEGF mRNA and protein expression. Furthermore, PD 98059 treatment almost completely abolished JNK1 activation. Our data suggest that PGE(2)-stimulates VEGF expression in RGMEC via transactivation of JNK1 by ERK2. One potential implication of this finding is that increased PG levels in cancers could facilitate tumor growth by stimulating VEGF synthesis and angiogenesis.

Portal hypertensive gastric mucosa has reduced activation of MAP kinase (ERK2) in response to alcohol injury: a key to impaired healing?

Portal hypertensive (PHT) gastric mucosa has increased susceptibility to injury and impaired mucosal healing. Because our previous study showed that ulcer-induced activation of mitogen-activated protein (MAP) kinase (ERK) plays a pivotal role in gastric mucosal healing, we investigated whether ERK activation is altered in PHT gastric mucosa following alcohol injury. We studied ERK2 phosphorylation and activity and expression of MAP kinase phosphatase-1 (MKP-1) in gastric mucosa of PHT and sham-operated (SO) normal rats both at baseline and following alcohol injury. In SO gastric mucosa, ERK2 phosphorylation and activity were significantly increased time-dependently following alcohol injury: by 221% and 137%, respectively at 24 h vs. baseline. In contrast, in PHT gastric mucosa following alcohol injury, neither ERK2 phosphorylation nor activity was increased versus baseline. In PHT gastric mucosa, MKP-1 mRNA and protein expression were increased at baseline versus SO rats and were increased further following alcohol injury with values higher by 20%-40% at each study time versus SO rats. Because ERK2 is crucial for mucosal healing, reduced ERK2 activation resulting from the overexpression of MKP-1 might be the basis for the impaired mucosal healing in PHT gastric mucosa.

Despite activation of EGF-receptor-ERK signaling pathway, epithelial proliferation is impaired in portal hypertensive gastric mucosa: relevance of MKP-1, c-fos, c-myc, and cyclin D1 expression.

Portal hypertensive (PHT) gastric mucosa has increased susceptibility to injury and impaired mucosal healing. Our previous study demonstrated increased ERK activation and MAP kinase phosphatase-1 (MKP-1) overexpression in PHT gastric mucosa. However, it remains unknown which tyrosine kinase receptors are involved in ERK activation and whether ERK activation results in increased cell proliferation. We examined whether EGF receptor (EGF-R) is involved in ERK activation and whether ERK activation triggers epithelial proliferation in PHT gastric mucosa. In gastric mucosa of PHT and sham-operated (SO) rats we studied: (1) EGF-R mRNA and protein expression as well as phosphorylation and membrane protein tyrosine kinase (PTK) activity; (2) ERK2 phosphorylation and activity; (3) MKP-1 mRNA and protein; (4) c-fos, c-myc and cyclin D1 mRNAs, and gastric epithelial proliferation. In PHT gastric mucosa: (1) EGF-R mRNA, protein and phosphorylation and membrane PTK activity were all significantly increased by 38%, 49%, 43% and 49%, respectively; (2) ERK2 phosphorylation and activity were significantly increased by 40% and 50 %, respectively; (3) MKP-1 mRNA and protein expression were significantly increased by 27% and 34%, respectively. In contrast, (4) c-fos, c-myc, and cyclin D1 mRNAs expression were all significantly decreased in PHT gastric mucosa by 36%, 33%, and 49%, respectively, and cell proliferation was significantly lower that in SO rats (11% in PHT vs. 18% in SO). These results suggest that in PHT gastric mucosa, ERK activation is mediated through EGF-R upregulation, but the gastric epithelial proliferation is impaired, possibly by MKP-1 overexpression, leading to reduction of c-fos, c-myc and cyclin D1.

Activation of hypoxia inducible factor-1alpha in gastric mucosa in response to ethanol injury: a trigger for angiogenesis?

Gastric mucosal injury triggers angiogenesis and activation of VEGF expression, but the mechanism(s) of VEGF gene activation are not known. In some tissues (e.g. myocardium), hypoxia triggers activation of hypoxia-inducible factor-1alpha (HIF-1alpha), a transcription factor known to activate VEGF gene expression. This study was aimed to determine whether hypoxia and/or alcohol injury may induces HIF-1alpha in gastric mucosa. Normal rat gastric tissue was incubated in organ culture under either hypoxic or normoxic conditions for 6hrs. Rats received, intragastrically, either saline or alcohol and gastric mucosa bordering necrosis was obtained at 1-24hrs. HIF-1alpha mRNA and protein were determined by RT-PCR and Western-blot analysis. HIF-1alpha and VEGF proteins were localized by immunostaining. Incubation of normal gastric mucosa under hypoxia caused a significant elevation of HIF-1alpha mRNA (20+/-2%, p<0.05) and protein (262+/-15%, p<0.005) vs. normoxia. Following alcohol injury, gastric mucosa bordering necrosis demonstrated a significant increase in HIF-1alpha mRNA at 3 and 6hrs (40+/-4%, 19+/-2%; p<0.05), and protein (>300+/-16%; p<0.02 at all time points; highest at 1-3hrs). HIF-1alpha signal was detected in regenerating mucosal microvessels, where it co-localized with VEGF. Since HIF-1alpha initiates transcription of VEGF mRNA, HIF-1alpha activation by ethanol-induced injury is likely responsible for activation of VEGF gene and induction of angiogenesis.

MAPK (ERK2) kinase--a key target for NSAIDs-induced inhibition of gastric cancer cell proliferation and growth.

UNLABELLED: Limited clinical and experimental studies indicate that nonsteroidal anti-inflammatory drugs (NSAIDs) may inhibit gastric cancer growth. However, the mechanisms involved are not completely understood and cannot be explained by COX-2 inhibition alone. MAPK signaling pathway is essential for cell proliferation, but the effect of NSAIDs on MAPK activity and phosphorylation in gastric cancer has never been studied. Since increased and unregulated cell proliferation and reduced cell apoptosis are important features of cancer growth, we studied whether NS-398, a selective COX-2 inhibitor and/ or indomethacin (IND), a non-selective NSAID: 1) inhibit gastric cancer cell proliferation, 2) whether this inhibition is mediated via MAPK (ERK2), and 3) whether NSAIDs enhance apoptosis in gastric cancer cells. Human gastric epithelial cells (MKN28) derived from gastric tubular adenocarcinoma were cultured and treated with either vehicle, IND (0.25-0.5mM) or NS-398 (50-100 microM) for 6, 16, 24 and 48h. STUDIES: 1) Cellular proliferation was determined by 3H-thymidine uptake. 2) MAPK activity was measured by incorporation of radiolabeled phosphate into myelin basic protein. 3) Apoptosis was evaluated using TUNEL assay. IND and NS-398 significantly inhibited the proliferation of MKN28 cells at 24h by 3.5 - 5 fold (p<0.002) and at 48h by 2.5 - 10 fold (p<0.02). Both NSAIDs also significantly inhibited ERK2 activity: IND >53% inhibition, NS-398, 100 microM >72% inhibition; all p<0.05. Both IND and NS-398 significantly increased apoptotic index. In conclusion, IND and NS-398 significantly inhibit proliferation and growth of human gastric cancer cell line MKN28. This effect is mediated by NSAID-induced inhibition of MAPK (ERK2) kinase signaling pathway, essential for cell proliferation. NSAIDs also increase apoptosis in MKN28 cells. In addition to inhibiting cyclooxygenase, NSAIDs inhibit phosphorylating enzymes--kinases essential for signaling cell proliferation.

Nonsteroidal anti-inflammatory drugs inhibit re-epithelialization of wounded gastric monolayers by interfering with actin, Src, FAK, and tensin signaling.

Re-epithelialization is essential for gastrointestinal ulcer and cutaneous wound healing. It requires epithelial cell migration and proliferation, processes that are stimulated by epidermal growth factor (EGF), and dependent on the cell cytoskeleton. Activation of Src and focal adhesion kinase (FAK) has been implicated in EGF-stimulated cell migration. Nonsteroidal anti-inflammatory drugs (NSAIDs) (both nonselective and Cox2-selective) interfere with ulcer healing and re-epithelialization in vitro and in vivo, but the cellular targets and mechanisms remain unexplored forming the basis of this study. Using a wounded gastric epithelial cell monolayer model, we demonstrated that NSAIDs reduce both basal and epidermal growth factor (EGF)-induced re-epithelialization, and that this action involves disruption of actin stress fiber formation, reduced c-Src activity, decreased phosphorylation of focal adhesion kinase (FAK), tensin and their cellular re-distribution. There was a strong correlation between NSAIDs-mediated inhibitory effect on re-epithelialization and loss of stress fibers and reduced tensin signal. Furthermore, NSAIDs significantly reduced EGF-stimulated c-Src association with FAK. These findings suggest that NSAIDs can directly affect the cell cytoskeleton and signaling pathways essential for re-epithelialization.

Indomethacin inhibits endothelial cell proliferation by suppressing cell cycle proteins and PRB phosphorylation: a key to its antiangiogenic action?

Nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit angiogenesis in vivo and in vitro, but the mechanism of this action is unclear. Angiogenesis-formation of new capillary vessels-requires endothelial proliferation, migration, and tube formation. It is stimulated by basic fibroblast growth factor (bFGF) and vascular endothelial growth factor (VEGF). The cell cycle is regulated positively by cyclins and negatively by cyclin-dependent kinase inhibitors (CKI) and the retinoblastoma protein (pRb). Since the effects of NSAIDs on cell cycle-regulatory proteins in endothelial cells remain unknown, we examined the effect of indomethacin on bFGF-stimulated endothelial cell proliferation and on cell cycle regulatory proteins in rat primary aortic endothelial cells (RAEC). Indomethacin significantly inhibited basal and bFGF-stimulated endothelial cell proliferation. This inhibition correlated significantly with reduced cyclin D1 and increased p21 protein expression. Furthermore, indomethacin reduced pRb phosphorylation. These findings suggest that indomethacin arrests endothelial cell proliferation essential for angiogenesis by modulating cell cycle protein levels.

ATP-induced CA2+-signaling enhances rat gastric microvascular endothelial cell migration.

The effects of exogenous ATP on Ca2+ signaling and wound healing were investigated in rat gastric microvascular endothelial cells (RGMEC). ATP (10 microM) triggered a significant rise in intracellular Ca2+ concentration ([Ca2+]i) from 46+/-2 nM at baseline to peak values averaging 283+/-31 nM (n = 5 experiments, 132 cells). Return to the basal [Ca2+]i was delayed by slowly declining plateau phase that persisted for 200+/-30 s. Removal of extracellular Ca2+ did not significantly affect the peak rise in [Ca2+]i, but reduced the plateau. ATP (10 microM) also significantly increased the migration of RGMEC in a wounded monolayer. Addition of the non-subtype selective purinergic receptor antagonist, suramin, abrogated the effects of ATP on [Ca2+]i and migration. We conclude that local elevation of ATP acting through purinergic receptors induce Ca2+ signals in RGMEC and may contribute to endothelial cell migration.

Isolation and characterization of rat gastric microvascular endothelial cells as a model for studying gastric angiogenesis in vitro.

We have previously characterized morphologic features of wounding-induced angiogenesis that occurs in response to acute and chronic gastric mucosal injury. As a means of investigating the molecular mechanisms underlying gastric angiogenesis, microvascular endothelial cells were isolated from stomachs of normal (non-injured) rats. The isolation procedure adapted and combined aspects of previous methods and employed positive selection using magnetic beads coated with monoclonal antibody specific for rat CD31 (PECAM-1), a cell surface marker restricted to platelets, monocytes, T lymphocytes and endothelial cells. The isolated microvascular endothelial cells expressed vascular endothelium-specific antigen and the endothelial-specific receptors, Tie2 and flt-1 (VEGFR1). When plated on growth factor-reduced matrigel, the isolated microvascular endothelial cells formed capillary-like structures reflecting in vitro angiogenesis. These cells were also responsive to vascular endothelial growth factor, VEGF, further verifying their endothelial nature. The rat microvascular endothelial cells isolated by this procedure should be useful in delineating molecular mechanisms and regulation of the angiogenesis that is essential for the healing of acute and chronic gastric injury.