Hoxc6 Is Overexpressed in Gastrointestinal Carcinoids and Interacts With JunD to Regulate Tumor Growth

Background & Aims: The molecular alterations that underlie carcinoid tumor pathogenesis remain poorly defined. The homeobox gene HOXC6 was highly up-regulated in human gastrointestinal carcinoid tumors, and we sought to define its pathogenic role. Methods: The functional and physical properties of Hoxc6 were investigated by establishing carcinoid cells that stably overexpressed Hoxc6 or were deficient in Hoxc6. Cellular proliferation assays, luciferase reporter assays, Western blotting, immunoprecipitation, DNA affinity precipitation, and DNA microarray studies were performed. Results: Expression of Hoxc6 in cultured human BON1 carcinoid cells enhanced their proliferation, and knock-down of Hoxc6 inhibited their growth. Hoxc6 activated the oncogenic activator protein-1 signaling pathway through a physical interaction with JunD. Mutations in the homeodomain of Hoxc6 blocked this interaction and inhibited proliferation of carcinoid cells. Of note, Hoxc6 induced the expression of genes that characteristically are up-regulated in carcinoid tumors, including neurotensin and connective tissue growth factor. Conclusions: A novel molecular pathway has been identified that links Hoxc6 with oncogenic signaling through the activator protein-1 pathway in carcinoid tumorigenesis.

Abbreviations used in this paper: AP-1, activator protein-1, CT, threshold cycle, CTGF, connective tissue growth factor, Hoxc6-V1, homeobox c6 variant 1, Hoxc6-V2, homeobox c6 variant 2, MEN1, multiple endocrine neoplasia 1, mHoxc6, mutant homeobox c6, NT, neurotensin, RT-PCR, reverse-transcription polymerase chain reaction, siRNA, small interfering RNA