Molecular targets of calcium and vitamin D in mouse genetic models of intestinal cancer.

We have identified intracellular targets that are affected by vitamin D in intestinal epithelial cells in two ways: analyzing profiles of gene expression in a matrix of dietary-genetic interactions in mouse models, and exploring a novel mechanism of transcriptional attenuation. Genetic models of intestinal cancer in the mouse models included inheritance of a mutation in the Apc gene (Apc1638N /– mice) and the targeted inactivation in the mouse germ line of the Muc2 gene (Muc2 mice) that encodes the major gastrointestinal mucin. Each of these models is highly significant in terms of the etiology and mechanism of human intestinal cancer. Inheritance of an Apc mutation, with its subsequent reduction to homozygosity due to somatic alteration of the remaining wild-type allele, is the etiology of familial adenomatous polyposis. Moreover, targeting of Apc somatically, or of the Wnt pathway in which it has an important role, is associated with almost all colorectal cancers. Muc2 mutations have not yet been found in human colon tumors, but in the mouse Muc2 model, goblet cell differentiation is perturbed and the protective mucus layer in the small and large intestine is compromised. In this regard, goblet cells and the mucin they produce are often depleted in aberrant crypt foci (ACF), the earliest precursors to colon tumors. Thus, the mucosa of the Muc2 mouse may mimic focal changes that occur in development of human colon cancer (Velcich, unpublished observations). While the mechanism by which targeted inactivation of Muc2 in the mouse causes tumor formation throughout the intestinal tract is not entirely clear, it may involve chronic low-level inflammation in the mucosa and/or exposure of intestinal epithelial cells to genotoxic substances and cytokines. What is clear is that the mechanism of tumor development in Muc2 mice differs from that in Apc /– mice: there is no activation (e.g., nuclear accumulation) of -catenin in the mucosal epithelial cells of the Muc2 mice or the tumors that form, and the introduction of an Apc mutation into the Muc2 mice greatly increases the number of tumors, indicating that the two mutations are at least complementary, and perhaps synergistic (Velcich and Yang, unpublished observations). Mouse genetic models such as these have been fundamental for understanding the etiology and mechanisms of intestinal tumorigenesis. However, the vast majority of human colonic tumors are “sporadic,” indicating that there is no known genetic predisposition. Instead, a single tumor generally develops in about 25% to 50% of the population in the United States and other Western countries over six to seven decades of life— about two-thirds of the human life span. During this time, there are highly significant increases in the probability of individuals developing a single colonic tumor if they consume diets with characteristic amounts of a number of various macroand micronutrients. To study the effects of these dietary factors on tumorigenesis, Newmark designed a number of mouse “Western-style diets” that mimic the levels of consumption, on the basis of nutrient density, of major risk factors in the human Western diet. We have studied the original Western diet (WD) (high fat, low calcium and vitamin D), as well as a more recent new Western diet (NWD). The NWD is similar to the WD, but it is also low in providing methyl-donor groups for intracellular functions (choline, methionine, folate, and fiber). It is important to note that the levels of these risk factors in the diets are not abnormally high (fat), nor low (calcium, vitamin D, choline, methionine, folate, fiber), but reflect levels that are associated epidemiologically in the general Drs. Yang and Lipkin are with the Strang Cancer Prevention Center, New York, New York, USA; Dr. Newmark is with Rutgers, The State University of New Jersey, New Brunswick, New Jersey, USA; Dr. Rigas is with the State University of New York at Stonybrook, Stonybrook, New York, USA; Drs. Daroqui, Maier, and Augenlicht are with the Department of Oncology, Albert Einstein Cancer Center, Bronx, New York, USA. Please address all correspondence to: Dr. Leonard Augenlicht, Department of Medicine and Cell Biology, Albert Einstein Cancer Center, 111 East 210th Street, Bronx, NY 10467, USA; Phone: 718-920-4663; Fax: 718-882-4464; E-mail: [email protected]. doi: 10.1301/nr.2007.aug.S134–S137