Differentiation and Inflammation: 'Best Enemies' in Gastrointestinal Carcinogenesis

Abstract

While recent studies demonstrate that cancer can arise from mutant stem cells, this hypothesis does not explain why tissues without defined stem cell populations are susceptible to inflammation-driven tumorigenesis. We propose that chronic inflammatory diseases, such as colitis and pancreatitis, predispose to gastrointestinal (GI) adenocarcinoma by reprogramming differentiated cells. Focusing on colon and pancreas, we discuss recently discovered connections between inflammation and loss of cell differentiation, and propose that dysregulation of cell fate may be a novel rate-limiting step of tumorigenesis. We review studies identifying differentiation mechanisms that limit tumor initiation and that, upon reactivation, can prevent or revert the cancer cell transformed phenotype. Together, these findings suggest that differentiation-targeted treatments hold promise as a therapeutic strategy in GI cancer.

Keywords: colorectal cancer; differentiation; inflammation; pancreatic ductal adenocarcinoma; pancreatitis.

Figure 1

Top-Down Versus Bottom-Up Hypotheses of Colorectal Cancer (CRC) Initiation. During intestinal homeostasis, Lgr5⁺ or Bmi1⁺ basal crypt stem cells proliferate and give rise to transit-amplifying (TA) cells (black outline) and, subsequently, differentiated enterocytes and other mature cell types (green). Alterations in non-stem cells, such as simultaneous NF-κB activation and β-catenin stabilization, can produce adenomas, suggesting that CRC can initiate from the top of the villus and grow down into the crypt (top right). This ‘top-down’ hypothesis proposes that differentiated or committed cells acquire stem-like characteristics to drive tumor growth from the luminal side of the colon. According to the more-traditional ‘bottom up’ model, mutations occurring directly in Lgr5⁺ or Bmi1⁺ crypt stem cells, such as loss of Apc, initiate adenoma and CRC formation. The ‘bottom-up’ hypothesis suggests that mutations accumulate in crypt stem cells as they continually proliferate, and then can serve as the effective cell of origin in CRC; they propagate disease from the base of the crypt, growing toward the lumen (bottom right).

Figure 2

Mouse Models of Pancreatic Ductal Adenocarcinoma (PDAC) Replicate Human Pathology. To activate cancer-causing alleles specifically within the mouse pancreatic epithelium, Cre recombinase is expressed under the control of the Pdx1 or Ptf1a/p48 promoter, both of which are active in embryonic pancreatic progenitor cells. In the ‘KC’ (Kras/Cre) model, one copy of the endogenous Kras allele is mutated to change amino acid 12 from glycine to aspartic acid, locking the Kras protein in its GTP-bound (active) form. A loxP–STOP–loxP cassette (LSL) is placed upstream of this mutation and, before recombination, inhibits transcription of the mutant allele. When Cre is expressed it excises the STOP cassette, activating Kras^G12D in all epithelial cells of the early pancreas. This produces fully-penetrant pancreatic intraepithelial neoplasia (PanIN) formation, rare intraductal papillary mucinous neoplasia (IPMN) formation, and occasional PDAC (in ~50% of aged animals). In the ‘KPC’ (Kras/p53/Cre) model of invasive PDAC, the same oncogenic Kras allele is activated along with a Trp53 ‘gain-of-function’ (neomorphic) allele. The R172H point mutation in this Trp53 allele models a mutation frequently found in human PDAC. Expression of both Kras^G12D and p53^R172H using Pdx1–Cre results in fully-penetrant PanIN and PDAC development in young animals.

Figure 3

Outcome of Kras^G12D Activation in Different Adult Pancreatic Cell Types. Expression of Kras^G12D specifically in acinar cells (Ptf1a⁺ nuclei, top panel) using Ptf1a^CreERT, Mist1^CreERT, elastase (Ela)–tTA tetO–Cre, or Ela–CreERT, results in focal PanIN formation, which can be greatly accelerated and enhanced by pancreatitis. When compounded with mutations in either p53 (heterozygous) or p16-INK4a/p19-ARF (homozygous), PDAC develops within 2–6 months. Activating Kras^G12D using a duct-specific Sox9–CreERT or Hnf1b–CreER^T2 transgene (CK19⁺ duct cells, middle panel) only leads to rare PanIN formation even in the presence of pancreatitis. Combining duct-specific Kras^G12D expression and Brg1 deletion, however, induces lesions that are reminiscent of IPMNs, while combining ductal Kras^G12D with homozygous p53^R172H/R172H induces rapid PDAC development without intermediate PanIN precursors. Finally, one study has found that combining Kras^G12D expression in β-cells (insulin⁺ cells, bottom panel) with pancreatitis is sufficient to induce PanIN formation, while the combination of β cell-specific Kras^G12D, pancreatitis, and homozygous Trp53 deletion can produce PDAC.