Research article
Effect of selenium-enriched broccoli diet on differential gene expression in min mouse liver1, 2

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Abstract

Multiple intestinal neoplasia (Min) mice are a good model for investigating the effects of dietary alterations in a genetic model for intestinal cancer. Previous studies have shown that selenium-enriched broccoli effectively reduces colon cancer susceptibility. Although colon cancer cells mainly metastasize to the liver, little is known about the effects of selenium-enriched broccoli on gene expression in mouse liver. To better understand the protective role for selenium-enriched broccoli in tumorigenesis, a gene profile of the mouse liver was analyzed. Mice were fed either 0.11 mg selenium/kg control diet or 2.1 mg selenium/kg selenobroccoli diets for 10 weeks. Use of mouse pathway finder-1 GEArrays revealed that selenium-enriched broccoli moderately increased ikBακB, hsp86, gadd45 gene transcripts. In addition, analysis of the binding of liver nuclear proteins to 32P-labeled probes demonstrated that selenium-enriched broccoli enhanced the binding of transcription factor p53, NFκB, AP-1 to their cis-acting elements. Collectively, these results suggest for the first time that selenium-enriched broccoli activates certain pro-apoptotic genes linked to p53, NFκB and stress signal pathways in response to “danger signals” such as tumorigenesis.

Introduction

Selenium is an essential trace element for humans and many other forms of life [1]. In addition to its known essentiality, a protective role for selenium has been observed in colon cancer risk [2], [3], [4], [5]. Grains and vegetables contain organic forms of the element such as selenomethionine or selenocystine. Broccoli contains primarily Se-methylselenocystine, which is more readily converted to methyl selenol than other organic forms of selenium [6]. It has been hypothesized that the production of methyl selenol is required for cancer prevention [7]. The Min mouse is highly susceptible to spontaneous formation of numerous tumors in both the small and large intestines [8], [9]. Recent studies suggest that selenium from selenium-enriched broccoli is more effective than other inorganic forms of selenium against aberrant crypts formation, a preneoplastic lesion for colon cancer [10]. Selenium-enriched broccoli also decreased intestinal tumor formation in the Min mouse [11]. In the case of colon cancer development, the genes associated with initiation and progression are well documented [12]. For example, the mutation of the adenomatous polyposis coli (APC) gene can result in a marked predisposition to colon cancer [12]. In contrast, although liver metastasis is usually responsible for the mortality of colon cancer patients, little is known about its mechanism. The Min mouse presents an opportunity to examine the early molecular events in mouse liver. The current study investigated whether selenium-enriched broccoli would affect the expression of genes linked to apoptosis.

Section snippets

Chemicals

T4 polynucleotide kinase was obtained from Promega (Madison, WI). Adenosine 5′-triphosphate (γ-32P) and cytidine 5′-triphosphate (α-32P) were purchased from Amersham Pharmacia Biotech (Piscataway, NJ). Oligonucleotides were synthesized by Gibco BRL (Rockville, MD).

Animals and diets

Twenty-eight heterozygotic male Min (C57Bl/6J - APCMin/+Apc) mice were obtained at 5 weeks of age from Jackson Laboratories (Bar Harbor, ME). All mice were housed individually in a room with controlled humidity, temperature and light.

Results

To determine the effect of selenium-enrich broccoli on the expression of genes linked to apoptosis, we applied gene array and DNA mobility shift assay to define the transcriptional response in Min mouse liver. Through a side-by-side sample preparation, we had total cellular RNA pools and nuclear protein pools. Compared with low selenium control diet, selenium-enriched broccoli up-regulated the mRNA levels of ikBα (91%), NFκB (65%), hsp86 (47%) and gadd45 (41%) (Table 1). Furthermore,

Discussion

Our previous studies showed that Min mice fed the selenium-enriched broccoli had fewer (p <0.02) small intestinal (46.4 ± 3.7 vs. 65.6 ± 6.1) and large intestinal (0.43 ± 0.17 vs. 1.93 ± 0.27) tumors than those fed the low selenium control diet [11]. Unlike other selenium deficient diet (0.01 mg Se/kg), our low selenium control diet (0.11 mg Se/kg) is closer to “real world” normal diet, and Min mice fed the selenium-enriched broccoli had only small increases in selenium concentration (72 ± 2

Acknowledgments

We thank Tom Johnson for critical review of the manuscript, and Phyllis Wilkes for preparation of the manuscript. The technical support of James Botnen is also greatly appreciated.

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    1

    1The U.S. Department of Agriculture. Agricultural Research Service. Northern Plains Area. is an equal opportunity/affirmative action employer and all agency services are available without discrimination.

    2

    2Mention of a trademark or proprietary product does not constitute a guarantee or warranty of the product by the U.S. Department of Agriculture and does not imply its approval to the exclusion of other products that may also be suitable.

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