Secretory Paneth-like cells as the origin of intestinal cancer
The cell of origin of cancer is often thought to have stem- and/or progenitor-like features which satisfy the need for active proliferation, self-renewal and differentiation capacity. This was elegantly demonstrated in the intestine where loss-of-function mutations in the Apc tumor suppressor gene successfully initiate adenoma formation only when they occur in the Lgr5+ stem cells. When the same Apc mutation is introduced in the more committed and shorter-lived transit-amplifying cells, tumor formation is considerably less efficient with growth stalling at early micro-adenoma stages. However, next to this “bottom-up” scenario, additional “top-down” models of intestinal tumorigenesis have also been proposed where more committed intestinal cells located at higher positions along the crypt-villus axis are likely to initiate colon cancer especially in the context of tissue injury and inflammation. In the specific case of colon cancer, western style dietary habits and inflammatory bowel disease (IBD; Crohn’s disease and ulcerative colitis), i.e. two among the major etiologic factors associated with increased risk of malignant disease in the digestive tract, have been shown to induce specific cellular and molecular alterations of the intestinal stem cell niche. In these conditions, activation of specific signaling pathways (e.g. NF-κB, Wnt, YAP/TAZ) may induce de-differentiation and tumor-initiating capacity in non-stem cells.
Paneth cells (PCs) are specialized secretory cells and the main source of antimicrobial peptides in the small intestine where they are primarily located. Moreover, they provide essential physical support and secreted signals to ensure Lgr5+ stem cell function. In the colon, where Paneth cells are not present, Paneth-like cells (PLCs) earmarked by cKit+ expression play secretory and niche-like functional roles analogous to those of PCs in the small intestine. As shown by our laboratory, upon tissue damage and inflammation PCs behave as quiescent stem-like cells capable of re-entering the cell cycle and de-differentiate thus contributing to the tissue regenerative response. Notably, Paneth cell metaplasia, i.e. the appearance of cells morphologically resembling secretory Paneth cells in the colon where they normally do not belong, has been observed in IBD and in mouse models of sporadic colon cancer driven by western dietary factors. PC metaplasia may represent an early colon cancer precursor in the context of inflammation and western-style diet, as suggested by the increased incidence of KRAS mutations in PCs in pre-neoplastic colon.
Here, we propose that two specific colon cancer risk factors, namely western-style dietary habits and chronic inflammation, affect the homeostatic equilibrium of the intestinal stem cell niche and determine micro- and macro-environmental conditions which favor tumor-initiation from more committed cells and in particular secretory Paneth-like cells both in the small and large intestine. Our preliminary results clearly show that, upon tissue injury and specific dietary nutrients, Paneth cells and their cKit+ equivalent (Paneth-like) in the colon, are able to re-enter the cell cycle, de-differentiate and acquire stem-like features. As such, they are also likely candidate to initiate sporadic colon cancer.
We will model sporadic colon cancer in the mouse by inducing chronic inflammation (multiple DSS cycles) and by feeding synthetic western-style and control diets (NWD1, NWD2, and AIN76A). These protocols were previously shown to mimic lag, incidence, frequency, histopathology, and ratio of carcinoma to adenoma of sporadic and IBD-associated colon cancer in man. Different readily available Paneth-specific Cre-Lox mouse models based on expression of the lysozyme (Lyz), defensin 6 (D6), and c-Kit genes will be employed to monitor and lineage-trace Paneth(-like) cells during tumor onset and progression towards malignancy. In a parallel approach, single and combined mutations in the Apc, Kras, and Tp53 genes will be introduced in Paneth cells to study their de-differentiating and tumor-initiating capacities both in vitro and in vivo. The cellular and molecular consequences of the environmental modifications and of the targeted genetic hits in Paneth cells will be studied by global (transcriptional, epigenetic, microbiome, and metabolome profiling), and more functional (stem cell dynamics, crypt fission, ‘mini-gut’ organoids) analyses.
The identification of the cell(s) of origin of colon cancer is critical to the development of preventive and therapeutic strategies. Their detailed analysis and characterization is likely to reveal molecular or cellular features that could be exploited to prevent cancer onset or block its progression. Paneth cells are likely to play a central role in diet- and inflammation-associated sporadic bowel cancer and as such represent a source of targets for future prevention and treatment programs.