Gastroesophageal reflux disease (GERD) has been established as a strong risk factor for esophageal adenocarcinoma. GERD can be complicated by esophagitis, and by replacement of esophageal squamous mucosa with the metaplastic mucosa of Barrett's esophagus (BE). In the setting of continued peptic injury, BE can give rise to esophageal adenocarcinoma. It is not known why only a minority of individuals with GERD develop BE, or why only a minority of individuals with BE develop esophageal adenocarcinoma. One reason for this void is that the molecular events triggered by acid reflux which mediate the development and neoplastic progression of BE are poorly characterized. Preliminary data from our laboratory demonstrate that acid activates the mitogen activated protein kinase (MAPK) pathways in the metaplastic mucosa of patients with BE, and in the squamous mucosa of normal subjects and of patients with GERD who do not develop BE. In contrast, we found that acid fails to activate the MAPK pathways in the squamous mucosa of patients with BE. We hypothesize that acid activates the MAPK-dependent signal transduction pathways which increase expression of cyclin D1, and trigger an increase in cell proliferation and a decrease in apoptosis in normal esophageal squamous epithelium and in the metaplastic epithelium of BE. Based on our preliminary data, this basic hypothesis has clinical implications for both the development and neoplastic progression of BE. In the esophageal squamous epithelium from normal patients and patients who do not develop BE, acid activation of MAPK-dependent pathways may promote repair of the acid damaged esophageal mucosa; in patients who develop BE, failure of acid to activate MAPK pathways may prevent regeneration of the acid damaged squamous mucosa and predispose to repair through metaplasia. However, once BE develops, acid stimulation of MAPK-dependent pathways may predispose to the development of esophageal adenocarcinoma. The aims of our study are to evaluate the acid induced effects on the ERK, p38, and JNK MAP kinase pathways, the AP-1 family of transcription factors, cyclin D1 expression, and its effect on cell proliferation and apoptosis in Barrett's metaplasia and esophageal squamous mucosa using in vivo and in vitro systems. Our long term goals are to identify molecular markers of BE and molecular targets at which to direct chemopreventive and chemotheapeutic agents for patients with BE and esophageal adenocarcinoma.