HPP1‐mediated tumor suppression requires activation of STAT1 pathways

Abstract

HPP1 is a recently discovered gene that is epigenetically silenced in a number of tumor types, suggesting a potential role as a tumor suppressor. However, whether HPP1 has tumor suppressor activity is not clearly known. We have sought to investigate the effects of HPP1 on tumor growth and survival and to identify signaling pathways that mediate HPP1's mechanism of action. Forced expression of HPP1 into HCT116 colon cancer cell lines blocked the ability of HCT116 tumors to grown in vivo in nude mice. In cell culture, ectopic expression of HPP1 induces apoptosis and potently inhibits soft agar colony formation. HPP1 overexpression was also associated with a moderate reduction in in vitro proliferation characterized by an accumulation of cells in the G0/G1 phase of the cell cycle. Microarray analysis revealed that ectopic expression of HPP1 resulted in a dramatic upregulation of STAT1 as well as a large number of associated interferon-inducible genes. RNA interference-mediated abrogation of STAT1 reversed HPP1's antiproliferative effects. We conclude that HPP1 demonstrates tumor suppressive and pro-apoptotic activity, both in vitro and in vivo. Coupled with its inactivation in a number of tumor types, our data provides evidence to support the role of HPP1 as a tumor suppressor gene. Moreover, activation of the STAT1 pathway likely represents the principal mediator of HPP1's tumor suppressive properties. ' 2007 Wiley-Liss, Inc.Key words: HPP1; STAT1; colon cancer; tumor suppressorThe recently identified gene, HPP1 (also known as TMEFF2, TPEF) was discovered using a global methylation screening assay to isolate differentially methylated sequences in colorectal neoplasms derived from patients with Hyperplastic Polyposis. 1 HPP1 is predicted to encode a transmembrane protein with a short cytoplasmic tail and an extracellular component with 2 follistatin modules and an EGF-like domain. 1 HPP1 appears to be distributed not only in neurons and glial cells of the central nervous system but also in pericryptal myofibroblasts and the epithelium of the gastrointestinal tract. 2 The exact function of HPP1 has not been elucidated; however, its EGF-like domain appears to be a ligand for c-erbB-4, 2 while its follistatin domains may bind and regulate TGF-b family members. 1 It is possible that HPP1 may play multiple roles in cell growth, maturation and adhesion and its inactivation may serve as an early event in the initiation of gastrointestinal neoplastic progression. HPP1 has demonstrated in vitro anti-proliferative effects in a prostate cancer cell line. 3 Sequence analysis of the HPP1 promoter region reveals a suppressive E-box that is recognized by c-myc. 4 HPP1 gene silencing appears to occur exclusively by epigenetic alteration via methylation. A gene-wide search has revealed no evidence of pathogenic mutations in the HPP1 gene. 5 Hypermethylation with concomitant inactivation of HPP1 has been found to occur in the majority of colorectal adenomas (66%), hyperplastic polyps (63%) and colorectal cancers (84%) 1 .We have pre...

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