Background and Aims: The neoplastic progression of Barrett's esophagus (BE) may involve genomic instability, inactivation of tumor suppressor genes, or activation of oncogenes. Because activation of Src tyrosine kinase occurs in malignant and premalignant epithelia of the colon, the aim of this study was to determine whether BE is associated with changes in Src expression and activity. Methods: Src expression and in vitro protein-tyrosine kinase activity in endoscopic tissue samples of BE and esophageal adenocarcinoma were measured and compared with expression and activity in normal esophagus and duodenum from the same patient. Src phosphorylation was assessed by immunoblotting using antiphosphotyrosine antibodies and two-dimensional tryptic phosphopeptide mapping. Results: Src-specific activity was 3-4-fold higher in BE and 6-fold higher in esophageal adenocarcinoma than in control tissues. Different regions of BE from the same patient showed heterogeneity in Src activity compared with the uniform Src activity observed in different regions of normal esophagus and duodenum. In all tissues, Src kinase activity and protein were associated preferentially with the Triton X-100-soluble rather than -insoluble fraction. Immunoblotting and two-dimensional tryptic phosphopeptide mapping showed dephosphorylation of Src at Tyr527 in BE. Conclusions: Src is activated in BE, in part, because of dephosphorylation of Tyr527. Src activation and its heterogeneous expression occur before development of dysplasia or carcinoma in BE.
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