Environmental Impact on Intestinal Stem Cell Functions in Mucosal Homeostasis and Tumorigenesis

Abstract

Multiple cell compartments at or near the base of the intestinal crypt have been identified as contributing intestinal stem cells for homeostasis of the rapidly turning over intestinal mucosa and cells that can initiate tumor development upon appropriate genetic changes. There is a strong literature establishing the importance of the frequently dividing Lgr5+ crypt base columnar cells as the fundamental cell in providing these stem cell-associated functions, but there are also clear data that more quiescent cells from other compartments can be mobilized to provide these stem cell functions upon compromise of Lgr5+ cells. We review the data that vitamin D, a pleiotropic hormone, is essential for Lgr5 stem cell functions by signaling through the vitamin D receptor. Moreover, we discuss the implications of this role of vitamin D and its impact on relatively long-lived stem cells in regards to the fact that virtually all the data on normal functioning of mouse Lgr5 stem cells is derived from mice exposed to vitamin D levels well above those that characterize the human population. Thus, there are still many questions regarding how dietary and environmental factors influence the complement of cells providing stem cell functions and the mechanisms by which this is determined, and the importance of this in human colorectal tumor development. J. Cell. Biochem. 118: 943-952, 2017. © 2016 Wiley Periodicals, Inc.

Keywords: ENVIRONMENT; HOMEOSTASIS; INTESTINE; PLASTICITY; STEM CELLS; TUMORIGENESIS.

Fig. 1

Architecture of the intestinal mucosa. (A and B) Hematoxylin/eosin stained sections of the mouse small and large intestine; (C) schematic of the small intestine showing the various functional zones of along the crypt-luminal axis.

Fig. 2

Stem cells and their functioning in the mouse small intestine. (A) Lgr5+ crypt base columnar cells from a mouse engineered so that these cells fluoresce green (Lgr5^{EGFP-cre:ERT2}, as reported in [Barker et al., 2007]); (B) Lgr5^{EGFP-cre:ERT2}, Rosa26^RFP mice in which Tamoxifen injection causes the Lgr5+ cells and their daughters to permanently fluoresce red so that their fate can be tracked over time. Mice were fed different diets for 3 months from weaning before tamoxifen injection: AIN76A control diet, i–iii; NWD1 diet, iv–vi; NWD2 diet, vii–ix (reprinted from [Peregrina et al., 2015]); (C) Lgr5^{EGFP-cre:ERT2}, Rosa26^RFP mice that are also either wild-type for the vitamin D receptor, or are homozygous for a conditional knockout (floxed) allele that encodes the vitamin D receptor. Tamoxifen injection in the latter not only marks the Lgr5 cells and their progeny red, but also simultaneously inactivates expression of a functional vitamin D receptor. All mice were fed AIN76A control diet for 3 months from weaning (reprinted from [Peregrina et al., 2015]).

Fig. 3

Tumorigenesis initiated from Lgr5 cells in mice fed different diets. (A) Number of tumors and their histopathology in individual Lgr5^{EGFP–cre:ERT2}, Apc^flox/+ mice in which tamoxifen injection inactivates the floxed allele of the tumor suppressor gene Apc. Mice were fed AIN76A, NWD1, or NWD2 for 3 months from weaning, injected with tamoxifen, then each mouse continued on its respective diet for an additional 6 months. The difference in tumor frequency between AIN76A and NWD1 mice was significant at P <0.03. (B) Examples of tumor histopathology (reprinted from [Peregrina et al., 2015]).

Fig. 4

Spectrum of vitamin D exposure across the US population. Data are plotted from those made available by the National Health and Nutrition Examination Survey (NHANES) conducted by the Centers for Disease Control (http://www.cdc.gov/nchs/nhanes_products.htm). The arrows show the levels reached in mice by feeding a standard chow diet, control AIN76A purified diet, or NWD1 diet.