Homeostatic responses of colonic LGR5 + stem cells following acute in vivo exposure to a genotoxic carcinogen

Abstract

Perturbations in DNA damage, DNA repair, apoptosis and cell proliferation in the base of the crypt where stem cells reside are associated with colorectal cancer (CRC) initiation and progression. Although the transformation of leucine-rich repeat-containing G protein-coupled receptor 5 (Lgr5) + cells is an extremely efficient route towards initiating small intestinal adenomas, the role of Lgr5 + cells in CRC pathogenesis has not been well investigated. Therefore, we further characterized the properties of colonic Lgr5 + cells compared to differentiated cells in Lgr5-EGFP-IRES-creER T2 knock-in mice at the initiation stage of carcinogen azoxymethane (AOM)-induced tumorigenesis using a quantitative immunofluorescence microscopy approach. At 12 and 24h post-AOM treatment, colonic Lgr5 + stem cells (GFP high ) were preferentially damaged by carcinogen, exhibiting a 4.7-fold induction of apoptosis compared to differentiated (GFP neg ) cells. Furthermore, with respect to DNA repair, O 6 -methylguanine DNA methyltransferase (MGMT) expression was preferentially induced (by 18.5-fold) in GFP high cells at 24h post-AOM treatment compared to GFP neg differentiated cells. This corresponded with a 4.3-fold increase in cell proliferation in GFP high cells. These data suggest that Lgr5 + stem cells uniquely respond to alkylation-induced DNA damage by upregulating DNA damage repair, apoptosis and cell proliferation compared to differentiated cells in order to maintain genomic integrity. These findings highlight the mechanisms by which colonic Lgr5 + stem cells respond to cancer-causing environmental factors.