We have previously shown heregulin (HRG)- expression in human gastric fibroblasts, and its stimulation of gastric epithelial cell growth. Although cyclooxygenase (COX)-2 has also been shown to stimulate growth factor production in these cells, the interaction between COX-2 and HRG remains unknown. Conditioned media (CM) from gastric fibroblasts incubated with prostaglandin E2 (PGE2) or interleukin (IL)-1, a well known COX-2 inducer, were analyzed for their effect on erbB3 tyrosine phosphorylation in MKN28 gastric epithelial cells. HRG protein expression in fibroblast lysates and CM was also examined by western blot. HRG- and HRG- mRNA expression in gastric fibroblasts and human gastric tissue was examined by real-time quantitative PCR. HRG and COX-2 expressions in surgical resections of human gastric ulcer tissue were examined immunohistochemically. CM from fibroblasts incubated with PGE2, or IL-1, stimulated erbB3 phosphorylation in MKN28 cells. Pre-incubation of the fibroblasts with celecoxib, a selective COX-2 inhibitor, suppressed CM-induced erbB3 phosphorylation. This inhibition was reversed by exogenous PGE2. As with erbB3 phophorylation, IL-1 stimulated both HRG- and HRG- mRNA expression, as well as HRG release into gastric fibroblast CM. IL-1-stimulated HRG expression and release were also inhibited by celecoxib, and exogenous PGE2 restored this inhibitory effect; suggesting the activation of an IL-1-COX-2-PGE2 pathway that culminates in the release of HRG from fibroblasts. HRG- and HRG- mRNA levels were significantly higher in gastric ulcer tissue than in normal gastric mucosa. HRG immunoreactivity was found in interstitial cells of the gastric ulcer bed, and coexpressed with COX-2. These results suggest that HRG might be a new member of the growth factor family involved in the COX-2-dependent ulcer repair process.