Hepatic genes altered in expression by food restriction are not influenced by the low plasma glucose level in young male GLUT4 transgenic mice

Short-term calorie restriction in male mice feminizes gene expression and alters key regulators of conserved aging regulatory pathways

Background

Calorie restriction (CR) is the only intervention known to extend lifespan in a wide range of organisms, including mammals. However, the mechanisms by which it regulates mammalian aging remain largely unknown, and the involvement of the TOR and sirtuin pathways (which regulate aging in simpler organisms) remain controversial. Additionally, females of most mammals appear to live longer than males within species; and, although it remains unclear whether this holds true for mice, the relationship between sex-biased and CR-induced gene expression remains largely unexplored.

Methodology/Principal Findings

We generated microarray gene expression data from livers of male mice fed high calorie or CR diets, and we find that CR significantly changes the expression of over 3,000 genes, many between 10- and 50-fold. We compare our data to the GenAge database of known aging-related genes and to prior microarray expression data of genes expressed differently between male and female mice. CR generally feminizes gene expression and many of the most significantly changed individual genes are involved in aging, hormone signaling, and p53-associated regulation of the cell cycle and apoptosis. Among the genes showing the largest and most statistically significant CR-induced expression differences are Ddit4, a key regulator of the TOR pathway, and Nnmt, a regulator of lifespan linked to the sirtuin pathway. Using western analysis we confirmed post-translational inhibition of the TOR pathway.

Conclusions

Our data show that CR induces widespread gene expression changes and acts through highly evolutionarily conserved pathways, from microorganisms to mammals, and that its life-extension effects might arise partly from a shift toward a gene expression profile more typical of females.

Aryl hydrocarbon receptor-dependent induction of flavin-containing monooxygenase mRNAs in mouse liver

Flavin-containing monooxygenases (FMOs) are important in detoxication but generally are considered not to be inducible by xenobiotics. Our recent microarray studies revealed induction of FMO2 and FMO3 mRNAs by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in liver of mice with wild-type aryl hydrocarbon receptor (AHR) but not in Ahr-null mice. The aim of the present study was to delineate mechanisms of FMO regulation. In adult male mice, basal FMO3 mRNA is low but was induced 6-fold at 4 h and 6000-fold at 24 h. The ED50 was approximately 1 microg/kg for FMO2 and FMO3, similar to that for the classic AHR-regulated gene, Cyp1a1. In adult female mice basal FMO3 mRNA is high and was not induced at 4 h but was elevated 8-fold at 24 h. FMO5 mRNA was significantly down-regulated by TCDD in both male and female adult mice. Juvenile mice show no sex difference in response to TCDD; FMO3 was induced 4 to 6-fold by TCDD in both sexes. Chromatin immunoprecipitation demonstrated recruitment of AHR and aryl hydrocarbon nuclear translocator proteins to Fmo3 regulatory regions, suggesting that induction by TCDD is a primary AHR-mediated event. Although FMO2 and FMO3 mRNAs were highly induced by TCDD in adult males, overall FMO catalytic activity increased only modestly. In contrast to the striking up-regulation of FMO2 and FMO3 in mouse liver, TCDD has little effect on FMO mRNA in rat liver. However, FMO2 and FMO3 mRNAs were highly induced in transgenic mice that express wild-type rat AHR, indicating that lack of induction in rat is not due to an incompetent AHR in this species.

The in vivo gene expression signature of oxidative stress

How higher organisms respond to elevated oxidative stress in vivo is poorly understood. Therefore, we measured oxidative stress parameters and gene expression alterations (Affymetrix arrays) in the liver caused by elevated reactive oxygen species induced in vivo by diquat or by genetic ablation of the major antioxidant enzymes CuZn-superoxide dismutase (Sod1) and glutathione peroxidase-1 (Gpx1). Diquat (50 mg/kg) treatment resulted in a significant increase in oxidative damage within 3-6 h in wild-type mice without any lethality. In contrast, treatment of Sod1(-/-) or Gpx1(-/-) mice with a similar concentration of diquat resulted in a significant increase in oxidative damage within an hour of treatment and was lethal, i.e., these mice are extremely sensitive to the oxidative stress generated by diquat. The expression response to elevated oxidative stress in vivo does not involve an upregulation of classic antioxidant genes, although long-term oxidative stress in Sod1(-/-) mice leads to a significant upregulation of thiol antioxidants (e.g., Mt1, Srxn1, Gclc, Txnrd1), which appears to be mediated by the redox-sensitive transcription factor Nrf2. The main finding of our study is that the common response to elevated oxidative stress with diquat treatment in wild-type, Gpx1(-/-), and Sod1(-/-) mice and in untreated Sod1(-/-) mice is an upregulation of p53 target genes (p21, Gdf15, Plk3, Atf3, Trp53inp1, Ddit4, Gadd45a, Btg2, Ndrg1). A retrospective comparison with previous studies shows that induction of these p53 target genes is a conserved expression response to oxidative stress, in vivo and in vitro, in different species and different cells/organs.

Tissue specific and non-specific changes in gene expression by aging and by early stage CR

Aging alters the expression of a variety of genes. Calorie restriction (CR), which extends life span in laboratory rodents, also changes gene expression. This study investigated changes in gene expression across three different tissues from the same mouse to examine how aging and early stage CR influence gene expression in different tissues of an organism. Expression profiling of heart, liver, and hypothalamus tissues was done in young (4-6 months) ad libitum fed (AL), young CR (2.5-4.5 months of CR), and old (26-28 months) AL male C57BL/6 mice. Aging significantly altered the expressions of 309, 1819, and 1085 genes in heart, liver, and hypothalamus tissues, respectively. In nine genes, aging altered expression across all three tissues although the regulation directions did not agree across all three tissues for some genes. Early stage CR in young mice significantly changed the expressions of 192, 839, and 100 genes in heart, liver, and hypothalamus tissues, respectively, and seven genes altered expression across all three tissues; three were up regulated and four were down regulated. The results of Gene Ontology (GO) Biological Process analysis indicated up regulation of antigen processing/presentation genes by aging and down regulation of stress response genes by early stage CR in all three tissues. The comparison of the results of aging and short term CR studies showed there were 389 genes, 18 GO biological processes, and 20 GO molecular functions in common.

Early hypothalamic response to age-dependent gene expression by calorie restriction

Molecular events linking the initial detection of calorie restriction (CR) to changes in gene expression throughout the organism that ultimately retard aging in CR animals are unknown. This study measured changes in gene expression induced by CR and by aging in the hypothalamus, which likely plays a central role in the initial perception of and response to CR. Hypothalamic expression profiling was done in young (4-6 months) ad libitum fed (AL), young CR (2.5-4.5 months of CR), and old (26-28 months) AL male C57BL/6 mice. CR altered the expression of 137 genes and aging altered 1222. Only 8 age-related genes were oppositely regulated by CR. To test whether reduced plasma glucose is a signal in altering hypothalamic gene expression, we examined GLUT4 transgenic mice (C57BL/6 background; 4-6 months), which have reduced plasma glucose similar to that of CR mice. Twenty-seven genes differed between transgenic and non-transgenic mice; nine of these were only altered by CR. The decreased plasma glucose had a limited role in CR mediated hypothalamic gene expression.

Identification of potential caloric restriction mimetics by microarray profiling

To facilitate the development of assays for the discovery of pharmaceuticals capable of mimicking the effects of caloric restriction (CR) on life- and healthspan (CR mimetics), we evaluated the effectiveness of glucoregulatory and putative cancer chemopreventatives in reproducing the hepatic gene expression profile produced by long-term CR (LTCR), using Affymetrix microarrays. We have shown that CR initiated late in life begins to extend lifespan, reduce cancer as a cause of death, and reproduce approximately three-quarters of the genomic effects of LTCR in 8 wk (CR8). Eight weeks of metformin treatment was superior to CR8 at reproducing LTCR-like gene expression changes, maintaining a superior number of such changes over a broad range of statistical stringencies, and producing more Gene Ontology terms overlapping those produced by LTCR. Consistent with these results, metformin has been shown to reduce cancer incidence in mice and humans. Phenformin, a chemical cousin of metformin, extends lifespan and reduces tumor incidence in mice. Taken together, these results indicate that gene expression biomarkers can be used to identify promising candidate CR mimetics.

Microarray evaluation of dietary restriction

Dietary restriction (DR) extends the life span and retards many age-related cellular and molecular changes in laboratory rodents. However, neither its underlying mechanism nor the limits of its action are fully understood. In this review, we assessed the effect of DR on gene expression in vertebrate and invertebrate animals using data generated by microarrays. Altered genes in DR mice reported in 15 articles published since 1999 were compared. A comparison of altered genes by DR in mice, rats, pigs, monkeys, yeast, and flies showed no common gene altered by DR among different species. It seems that individual genes altered in the expression by DR were constrained within species. When we compared the functions of altered genes across all species, we found that certain functions such as metabolism, energy metabolism, stress and immune response, cell growth, and transcription regulation were shared among species. Although individual genes seem to be affected by DR differently among species, the overall physiologic influence of DR may be similar.