Silencing of GUCY2C through loss of guanylin expression is a necessary step in colorectal tumorigenesis and therapeutic target for chemoprevention
Colorectal cancer is the second leading cause of cancer death. Intestinal transformation begins ubiquitously with activation of Wnt signaling through mutations in APC (80%) or its degradation target β-catenin (15%). This produces a gain-of-function driving TCF-dependent nuclear transcription underlying epithelial dysfunction and tumorigenesis. While a role for APC and β-catenin in colorectal cancer is well-established, steps leading from gene mutation to tumorigenesis, and their reversibility, remain incompletely defined. Guanylin is the paracrine hormone for the receptor GUCY2C. This hormone is expressed in normal epithelium and is the most commonly lost gene product in colorectal cancer, silencing GUCY2C at the earliest step in neoplasia in a mechanism that is conserved across species. GUCY2C regulates homeostatic mechanisms organizing the intestinal crypt-surface axis, and its silencing through guanylin suppression drives hyperproliferation, DNA damage, metabolic reprogramming and desmoplasia, all contributing to tumorigenesis. Here, we demonstrate that inactivation of APC, or activation of β-catenin, induces TCF-dependent elimination of guanylin transcription and translation in human intestinal cells in vitro and in conditional genetic mouse models in vivo. Importantly, enforced genetic expression of guanylin in intestinal epithelial cells abrogated tumorigenesis in several orthogonal mouse models of colorectal tumorigenesis. These observations reveal for the first time a mechanism through which GUCY2C is silenced in colorectal tumorigenesis, and present a pathophysiologic model in which mutant APC-β-catenin signaling eliminates guanylin expression as an obligatory step in tumorigenesis. In addition to APC/β-catenin-associated tumorigenesis silencing GUCY2C, studies herein have also identified guanylin loss as a causative molecular event in obesity-associated tumorigenesis as well. In that context, caloric intake induces intestinal ER stress and silences guanylin expression, causing epithelial dysfunction and tumorigenesis. Guanylin loss was shown to be a function of ingested calories alone, and not body weight and adiposity. These findings have challenged the classical paradigm of obesity-associated tumorigenesis, which have implicated the adiposity-associated hormonal, adipokine and inflammatory milieu as causing tumorigenesis. Importantly, persistent expression of guanylin completely eliminated obesity-associated tumorigenesis. Taken together, the present studies suggest that GUCY2C silencing removes an essential block to transformation, creating a circuit which amplifies mutant APC-β-catenin signaling. These studies shift the prevailing paradigm for colorectal tumorigenesis from an irreversible oncogenomic mechanism to a reversible functional mechanism whose reconstitution abrogates those mutational defects. In that context, this disease-specific vulnerability can be leveraged to eliminate tumorigenesis by GUCY2C hormone replacement. In addition to validating our findings in exciting preclinical mouse and human in vivo and ex vivo enteroid models (respectively), we have translated these observations to an NCI-funded clinical program exploring the utility of oral GUCY2C ligands to prevent colorectal cancer.
Blomain, Erik S, "Silencing of GUCY2C through loss of guanylin expression is a necessary step in colorectal tumorigenesis and therapeutic target for chemoprevention" (2016). ETD Collection for Thomas Jefferson University. AAI10156229.