Reactivating the GUCY2C Signaling Axis for Colorectal Cancer Chemoprevention
Colorectal cancer (CRC) is a major cause of cancer-related mortality worldwide, with approximately 50,000 deaths per year in the United States. To date, surgical resection remains the recommended treatment for CRC, followed by adjuvant chemotherapy. While the standard of care is highly effective in eliminating traces of active disease, approximately 30-40% of CRC patients will experience recurrence, many of whom will ultimately succumb to their disease. The high risk of recurrence, due to both local and metastatic lesions, highlights the importance of developing preventative CRC therapies targeted at the earliest phases of the disease. CRC originates from near-universal activation of the Wnt pathway through mutations in APC (85%) and β-catenin (5%) which give rise to the expression of oncogenic transcriptional programs that underlie epithelial transformation. Although APC and β-catenin have been studied in great detail for their role in the development of CRC, the molecular mechanisms that lead from gene mutation to colorectal tumorigenesis remain poorly understood. Here, we define a novel pathophysiological paradigm for CRC initiation through silencing of the guanylyl cyclase C (GUCY2C) signaling axis, an emerging target for cancer chemoprevention. GUCY2C is a receptor primarily expressed on the apical surface of intestinal epithelial cells, which persists in nearly all CRCs. GUCY2C is activated by its two endogenous paracrine hormones guanylin and uroguanylin, as well as the exogenous diarrheagenic bacterial heat-stable enterotoxins (STs), to synthesize the second messenger cyclic GMP (cGMP). The GUCY2C signaling axis is known to regulate a number of hallmark pathways that maintain the crypt-to-villus architecture of the intestine and prevent the transformation of normal epithelial cells to cancer. Importantly, the GUCY2C hormone guanylin is the most commonly lost gene product in CRC. Loss of the hormone, which is associated with silencing of the GUCY2C signaling axis, occurs at the earliest step in colorectal tumorigenesis, and guanylin mRNA and protein are lost in >85% of human colorectal adenomas and tumors. Our work suggests a potential role for hormone loss and subsequent GUCY2C silencing as critical mediators of neoplastic transformation. The acquisition of an inactivating mutation in APC or an activating mutation in β-catenin induces TCF-dependent transcriptional repression of guanylin, in turn silencing the GUCY2C signaling axis. Further, this inactivating mutation in APC requires loss of heterozygosity (LOH) in order to mediate GUCY2C hormone suppression. These data support the hypothesis that APC/β-catenin/TCF-mediated loss of guanylin is a necessary step in colorectal tumorigenesis as it occurs after LOH initiating tumorigenesis. Accordingly, reactivating the GUCY2C signaling axis downstream of mutations in APC/β-catenin has the potential to reciprocally regulate the Wnt pathway to block oncogenic signaling. Therefore, we investigated the role of GUCY2C reactivation on β-catenin levels as it is the key regulator of aberrant cellular proliferation. Restoration of the GUCY2C signaling axis did not downregulate β-catenin expression or its direct downstream transcriptional programs. The effects of GUCY2C signaling on β-catenin dynamics were confirmed in vitro and in vivo using several genetically engineering mouse models in order to manipulate various aspects of the guanylin-GUCY2C axis. Further, targeting GUCY2C using the FDA-approved GUCY2C agonist linaclotide in Apcmin/+ mice did not lead to a significant reduction in tumorigenesis measured by tumor multiplicity, incidence, size and burden. Nevertheless, guanylin likely plays an important role in tumorigenesis as it is invariably lost in CRC. Additionally, preservation of the GUCY2C signaling axis is known to prevent tumor formation in mice. Therefore, the findings presented here reveal CRC as a disease of paracrine hormone insufficiency that can be translated into preventative strategies, including oral GUCY2C ligand replacement, to prevent tumorigenesis in patients at high risk for CRC.
Pattison, Amanda, "Reactivating the GUCY2C Signaling Axis for Colorectal Cancer Chemoprevention" (2020). ETD Collection for Thomas Jefferson University. AAI28088743.