Abstract
Wnt/β-catenin signaling plays a pivotal role in regulating cell growth and differentiation by activation of the β-catenin/T-cell factor (TCF) complex and subsequent regulation of a set of target genes that have one or more TCF-binding elements (TBEs). Hyperactivation of this pathway has been implicated in numerous malignancies including human neuroendocrine tumors (NETs). Neurotensin (NT), an intestinal hormone, induces proliferation of several gastrointestinal (GI) cancers including cancers of the pancreas and colon. Here, we analyzed the human NT promoter in silico and found at least four consensus TBEs within the proximal promoter region. Using a combination of ChIP and luciferase reporter assays, we identified one TBE (located ∼900 bp proximal from the transcription start site) that was immunoprecipitated efficiently by TCF4-targeting antibody; mutation of this site attenuated the responsiveness to β-catenin. We also confirmed that the promoter activity and the mRNA and protein expression levels of NT were increased by various Wnt pathway activators and decreased by Wnt inhibitors in NET cell lines BON and QGP-1, which express and secrete NT. Similarly, the intracellular content and secretion of NT were induced by Wnt3a in these cells. Finally, inhibition of NT signaling suppressed cell proliferation and anchorage-independent growth and decreased expression levels of growth-related proteins in NET cells. Our results indicate that NT is a direct target of the Wnt/β-catenin pathway and may be a mediator for NET cell growth.