**1. Introduction**

*Helicobacter pylori* (*H. pylori*) is a Gram-negative bacterium that selectively colonizes the gastric epithelium of humans and is the leading cause of peptic ulcers [1]. Although the majority of

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

individuals infected with *H. pylori* remain asymptomatic throughout their life, essentially all infected individuals develop chronic inflammation. Patients with antral-predominant gastritis are predisposed to duodenal ulcers, while patients with corpus-predominant gastritis and multifocal atrophy are more likely to have gastric ulcers. Eradication of *H. pylori* drastically lowers the recurrence of *H. pylori*-associated peptic ulcers. In addition, the observed delayed ulcer healing has been reported to involve *H. pylori*-induced inflammation, increased apoptosis of epithelial cells at the ulcer margin, overexpression of inflammatory cytokines, and reduced gastric microcirculation [2]. Cyclooxygenase (COX) is a membrane-bound glycoprotein that functions as the rate-limiting enzyme in prostaglandin (PG) synthesis. PGs increase the resistance of the gastric mucosa to injury by downregulating inflammatory responses. Two major COX isoforms have been identified, COX-1, which is constitutively expressed and considered a housekeeping enzyme, and COX-2, whose mRNA levels rise rapidly in response to inflammatory and mitogenic stimuli. There is a known synergism between *H. pylori* infection and nonsteroidal anti-inflammatory drug (NSAID) use in the development of peptic ulcers and ulcer bleeding [3]. These findings indicate possible interactions between *H. pylori* infection, COX activity, and ulcerogenesis. Aberrant methylation of 5′-CpG islands (CGIs) has been implicated in the transcriptional silencing of a wide range of genes involved in various diseases, such as cancer. Human *COX-2* has CG-rich CGIs in its promoter region, which suggests epigenetic regulation. Here, we report and discuss our recent results on the epigenetic regulation of *COX-2* activity in *H. pylori*-infected gastric mucosa of humans and Mongolian gerbils (MGs), and the possible relationship between *COX-2* methylation and delayed gastric ulcer healing.

by specific inhibitors or genetically by gene targeting [10], does not cause gastric mucosal injury. It has been shown that a combination of selective COX-1 and COX-2 inhibitors is required to cause hemorrhagic erosion of the gastric mucosa, which is comparable to that

Role of *COX-2* Promoter Methylation and *Helicobacter pylori* Infection in Impaired Gastric Ulcer…

to be mediated via the EP4 receptor [12]. In addition, COX-2-derived PG stimulates vascular endothelial growth factor (VEGF) release from gastric fibroblasts, which is an important contributor to ulcer healing [13, 14], likely via stimulation of new blood vessel growth. The increase in COX-2 immunoreactivity that is observed in monocytes, macrophages, fibroblasts, and endothelial cells at the ulcer margin is closely correlated, both temporally and spatially, with the increase in cell proliferation [15]. COX-2 appears to represent a second line of defense that is activated during ulcer healing to compensate for the temporary loss of COX-1 in the mucosa adjacent to the ulcer and assists COX-1 in protecting gastric mucosal integrity. The healing-impairment effect of NSAIDs is also observed with selective COX-2 inhibitors [16].

The pathophysiological roles of COX-2 in *H. pylori*-infected gastric mucosa are intriguing. *H. pylori* has been implicated as an inducer of COX-2 in the stomach [17–19]. In addition, COX-2 expression is elevated within *H. pylori*-induced gastritis and malignant lesions [20],

carcinoma cell line. Both *H. pylori* infection and NSAID use independently and significantly increase the risk of peptic ulcers and ulcer bleeding. While COX-2 is necessary for gastric mucosal healing [21], *H. pylori* infection is the leading cause of gastric ulceration. To understand the bimodal effects of *H. pylori* infection on COX-2 induction during ulcer healing, we explored the effects of epigenetic regulation and *H. pylori* infection on the induction of COX-2

*H. pylori* infection causes aberrant DNA methylation of various genes in the gastric mucosa, including *COX-2* [22–25]. Human *COX-2* has CGIs in its promoter region. Thus, we compared *COX-2* promoter methylation levels in the gastric mucosa in *H. pylori*-positive and *H. pylori*-negative cases [26]. As mentioned above, in qualitative experiments, *COX-2* gene promoter methylation levels were significantly higher in *H. pylori*-positive cases than in *H. pylori*-negative cases (**Figure 1**). *COX-2* promoter methylation levels were significantly lower in patients with *H. pylori* eradication than in those with *H. pylori* infection. We then investigated the effects of *COX-2* promoter methylation on *COX-2* mRNA expression *in vitro* using the human gastric adenocarcinoma cell line Kato III, in which the *COX-2* promoter is densely methylated [22]. *COX-2* mRNA expression was not observed in these cells, despite the addition of the protein kinase C stimulator α-phorbol 12,13-dibutyrate (PDBu). However, *COX-2* expression was observed after the addition of the demethylating agent 5-Aza-dC, and expression was enhanced by adding PDBu (**Figure 2**) [26]. These results indicate that *H. pylori* infection causes reversible *COX-2* promoter methylation in the gastric mucosa, and that

*COX-2* mRNA expression is regulated through an epigenetic mechanism.

, enhance

209

on gastric ulcer healing in rodents appear

http://dx.doi.org/10.5772/intechopen.79973

production in a human gastric

observed with indomethacin [11]. Prostanoids produced by COX-2, especially PGE<sup>2</sup>

cell proliferation. The beneficial effects of PGE<sup>2</sup>

**3.** *H. pylori* **infection and COX-2 in gastric mucosa**

and *H. pylori* induced COX-2 expression and enhanced PGE<sup>2</sup>

*in vivo* and *in vitro.*
