**8. Conclusions**

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

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].

**Figure 8.** DNA methylation levels at *COX-2* in the MG gastric mucosa in the presence or absences of *H. pylori* infection. Methylation levels at *COX-2* in the MG gastric mucosa were analyzed by quantitative bisulfite pyrosequencing.

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

**mucosal healing**

216 Chromatin and Epigenetics

*H. pylori* infection, the leading cause of peptic ulcer disease, induces sustained inflammation in the gastric mucosa. This chronic inflammation causes aberrant DNA methylation in various genes. The *COX-2* promoter regions in both MGs and humans contain abundant CGIs. COX plays critical roles in peptic ulcer development and healing, and both COX-1 and COX-2 are necessary for the gastric mucosal defense system. COX-1 is a housekeeping enzyme, which maintains microcirculation and mucous production. During ulcer healing, COX-2 is expressed at the margin of an ulcer, and COX-2-derived PGs induce various growth factors to promote mucosal healing and angiogenesis. Ulcer healing is delayed in *H. pylori* infected-MG stomachs, which is the best model of *H. pylori* infection in humans. We showed that *COX-2* mRNA induction during ulcer healing was impaired in *H. pylori*-infected-MG stomachs. The *COX-2* promoter region is methylated in both *H. pylori*-infected human and MG gastric mucosa. These CGIs are also methylated in human Kato III cells and MGC2 gastric carcinoma cells. *COX-2* mRNA expression in these cells is restored by treatment with a demethylating agent. In *H. pylori*-infected gastric mucosa, *COX-2* promoter methylation appears to be involved in the impaired *COX-2* mRNA induction typically observed during ulcer healing, which leads to delayed ulcer healing.
