**7. Role of** *COX-2* **methylation and** *H. pylori* **infection in gastric mucosal healing**

Nonselective COX inhibitors damage the gastrointestinal mucosa, and gastrointestinal injury represents the most significant side effect of chronic nonsteroidal anti-inflammatory drug (NSAID) use [34]. Thus, selective COX-2 inhibitors have been developed as ideal antiinflammatory drugs, devoid of GI toxicity, and clinical trials have suggested that selective COX-2 inhibitors produce less gastrointestinal injury than conventional NSAIDs [8, 9]. In support of this, animal studies have also suggested that, in contrast to the initial concept, the importance of COX-2 in the repair of gastric mucosal damage has been recognized [31, 35]. In rat gastric mucosa, markedly elevated levels of *COX-2* mRNA were observed after induction of damage by ischemia-reperfusion [35, 36]. COX-2 mRNA and protein expression increase during repair of gastric mucosal lesions, and selective COX-2 inhibitors delay mucosal healing in mice [31]. PGs derived from ulceration-induced COX-2 at the ulcer margin enhanced epithelial cell proliferation and increased the expression of growth factors, including hepatocyte growth factor, epidermal growth factor, transforming growth factor-α, and VEGF [16].

The release of inflammatory cytokines and recruitment of inflammatory cells have been considered the potential factors for delayed ulcer healing in *H. pylori* infection [2, 28]. Chronic inflammation, including *H. pylori* infection, is known to cause aberrant DNA methylation [37, 38]. Here, we focused on *COX-2* methylation in *H. pylori*-infected gastric mucosa. Human *COX-2* has CGIs in the promoter region, and methylation levels in this region are increased in *H. pylori*-infected gastric mucosa. In addition, *COX-2* promoter methylation levels in patients with successfully eradicated *H. pylori* infection are lower than the levels in *H. pylori*-positive cases. As mentioned above, experiments using rodents have shown that *COX-2* mRNA expression is enhanced during ulcer healing. Thus, we investigated the influence of *H. pylori* infection on COX-2 expression during stomach ulcer healing in MGs. Acetic acid-induced ulcer healing was delayed in *H. pylori*-infected MG stomachs when compared to the healing of ulcers without *H. pylori* infection [29]. The *COX-2* promoter region of MGs also has abundant CGIs, comparable to human *COX-2*. Thus, MGs are good models for investigating the role of *COX-2* methylation in gastric mucosal healing. While *COX-2* mRNA expression at the ulcer edge was increased 5 days after acetic acid injection in MG stomachs without *H. pylori* infection, as was observed in the mouse and rat models, such increases in *COX-2* mRNA expression were not observed in *H. pylori*-infected MG gastric mucosa. However, a demethylating agent restored COX-2 mRNA expression in both human and MG gastric carcinoma cell lines in which COX-2 is densely methylated. Accordingly, we demonstrated *in vitro* and *in vivo* that *COX-2* mRNA expression is regulated through an epigenetic mechanism in human and MG gastric mucosa. Aberrant DNA methylation has been extensively investigated in the context of the pathogenesis of various cancers, including stomach cancer. In addition, the involvement of epigenetic modifications has been reported in the pathogenesis of various chronic diseases, such as essential hypertension and cardiovascular disease [39]. Taken together, the epigenetic inhibition of COX-2 expression due to chronic inflammation induced by *H. pylori* infection seems to interfere with ulcer healing and increase the vulnerability of the gastric mucosa. The decrease in *COX-2* methylation levels after *H. pylori* eradication may restore gastric mucosal defense.
