Hedgehog signaling in esophageal development and Barrett's esophagus

Barrett's esophagus, a condition in which the distal esophageal epithelium undergoes a metaplastic change from stratified squamous to columnar, is thought to occur in up to 1% of the Western world's population. A complication of chronic gastroesophageal reflux disease, Barrett's esophagus increases the risk of developing esophageal adenocarcinoma 30--125 fold. Elucidating the underlying molecular mechanism for Barrett's metaplasia could lead to an effective chemoprevention strategy for the prevalent form of esophageal cancer. We used mouse embryonic esophageal development, in which columnar epithelium becomes stratified squamous, to identify potential molecular mediators of epithelial transdifferentiation. We found active epithelial-mesenchymal Hedgehog signaling while the epithelium retains a columnar phenotype but inactive signaling in the adult esophagus. Sox9, a known Hedgehog pathway target, was downregulated as the epithelium changes from columnar to stratified squamous. We then demonstrated that Hedgehog pathway activation characterizes reflux-induced esophagitis, Barrett's esophagus, and esophageal adenocarcinoma. In vitro acid experiments in human esophageal epithelial cells and surgically induced bile reflux in mice stimulate Hedgehog pathway signaling, indicating that the physiologic causes of Barrett's esophagus activate the Hedgehog pathway. Epithelial-mesenchymal Hedgehog signaling, which recapitulates normal esophageal development, is seen in Barrett's esophagus with epithelial SOX9 expression induced by Hedgehog pathway mediated activation of mesenchymal BMP4. Both BMP4 signaling and SOX9 overexpression cause human esophageal squamous epithelial cells to express columnar genes. Transgenic overexpression of Sonic hedgehog in mouse esophageal epithelial cells causes stromal Bmp4 expression, epithelial Sox9 expression, and an epithelial columnar phenotype in a three-dimensional in vivo culture system. These results demonstrate that active Hedgehog signaling is sufficient to cause esophageal squamous epithelial cells to acquire columnar cell features and establish a rationale for using Hedgehog pathway inhibitors as a chemopreventative therapy for esophageal adenocarcinoma.