STAT1 Activation and HPP1 Tumor Suppression

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SUMMARY HPP1 is a recently identified gene that was initially discovered by virtue of its significant downregulation in colorectal neoplasms derived from patients with Hyperplastic Polyposis. Inactivation of HPP1, which is mediated by promoter hypermethylation, has subsequently been found to occur in tumors of the colon and rectum, stomach, small bowel, pancreas, gallbladder, prostate and lung. Hypermethylated HPP1 DNA can also be detected in both the serum and stools of patients with gastrointestinal malignancies. Despite growing interest in HPP1's role as an epigenetic target and as a tumor biomarker, knowledge of its biologic mechanisms of action has been limited. HPP1 is predicted to encode a transmembrane protein with an EGF-like domain, suggesting a role in erbB receptor signaling. We have demonstrated that overexpression of HPP1 in a colon cancer cell line results in reduced proliferation and increased apoptosis, as well as a marked reduction in in-vitro and in-vivo tumorigenicity. Subsequently, we have found that these phenotypic alterations are associated with a gene expression profile characterized by a dramatic upregulation of STAT1 and interferon-inducible genes. Interestingly, activating mutations of K-ras in colon cancer cell lines are associated with nearly an exact opposite profile with downregulation of STAT1 and interferon-regulatory elements. We have shown that attenuation of STAT1 abrogates the tumor suppressive effects of HPP1. Two very important mechanistic issues will be addressed in this grant proposal: 1) While HPP1 and K- ras have opposite effects on STAT1, and a c-myc repressive binding site is present in the HPP1 promoter, there is no evidence that the actions of HPP1, c-myc and/or K-Ras are mechanistically linked. Therefore, we propose the following hypothesis: K-ras downregulates STAT1 by a c-myc dependent suppression of HPP1. 2) HPP1 is postulated to be an EGF-like ligand and contains an EGF-like domain in its extracellular domain that has a His/Arg alteration that may bind to the ErbB receptor family. However, there is no direct evidence that this EGF-like domain is critical to HPP1-mediated STAT1 activation and tumor suppression. Therefore, we will test a second hypothesis: HPP1 suppresses tumor growth by EGF-like domain stimulation of erbB-mediated STAT1 activation. These hypotheses will be tested by the following specific aims: 1) To determine the mechanisms by which STAT1 mediates HPP1-associated tumor suppression. 2) To determine if the EGF-like domain of HPP1 activates erbB family receptors and subsequently, STAT1. 3) To determine whether K-ras- associated downregulation of HPP1 is mediated by c-myc.