Differential regulation of leptin expression and function in A/J vs. C57BL/6J mice during diet-induced obesity

Adaptive thermogenesis is intact in B6 and A/J mice studied at thermoneutrality

To investigate mechanisms of resistance to obesity, the physiologic responses to short-term moderate fat feeding were studied at ambient temperature (T(a)) = 23 degrees C and thermonuetrality (T(a) = 30 degrees C) in mice susceptible (B6) or resistant (A/J) to obesity. We hypothesized that A/J mice would exhibit greater adaptive thermogenic responses to consumption of moderate-fat diets, and that this response would be attenuated in thermoneutral conditions due to reduced activity of brown adipose tissue (BAT). B6 and A/J mice were adapted to either T(a) = 23 degrees C or T(a) = 30 degrees C, implanted with telemetry devices, housed in metabolic chambers for measurement of food intake, oxygen consumption (Vo(2)), and heart rate (HR), and studied before and during 1 week of consuming a diet containing 32% of calories from fat. Access to 32% fat diet resulted in increased caloric intake in both strains, but caloric intake for A/J mice returned to baseline levels within 72 hours, while B6 mice remained hyperphagic. Both strains exhibited increased light-phase Vo(2) indicative of adaptive thermogenesis; however, there was no strain difference in light-phase Vo(2) during the 1-week feeding trial. Surprisingly, T(a) had no effect on diet-induced thermogenesis in either mouse strain. Moderate high-fat feeding produced mild tachycardia that was similar in B6 and A/J mice and more clearly evident at thermonuetrality. We conclude that adaptive thermogenic responses are intact in both mouse strains studied at thermoneutrality, suggesting a minimal role for BAT in the initial metabolic response to hyperphagia. Furthermore, the results suggest that differences in control of caloric intake, rather than capacity for adaptive thermogenesis, may contribute to the relative susceptibility to obesity in A/J and B6 mice.

Short-term resistance to diet-induced obesity in A/J mice is not associated with regulation of hypothalamic neuropeptides

To investigate the mechanisms underlying long-term resistance of the A/J mouse strain to diet-induced obesity, we studied, over a period of 4 wk, the expression of uncoupling proteins in brown adipose tissue and the expression of hypothalamic neuropeptides known to regulate energy homeostasis and then used microarray analysis to identify other potentially important hypothalamic peptides. Despite increased caloric intake after 2 days of high-fat feeding, body weights of A/J mice remained stable. On and after 1 wk of high-fat feeding, A/J mice adjusted their food intake to consume the same amount of calories as mice fed a low-fat diet; thus their body weight and insulin, corticosterone, free fatty acid, and glucose levels remained unchanged for 4 wk. We found no changes in hypothalamic expression of several orexigenic and/or anorexigenic neuropeptides known to play an important role in energy homeostasis for the duration of the study. Uncoupling protein-2 mRNA expression in brown adipose tissue, however, was significantly upregulated after 2 days of high-fat feeding and tended to remain elevated for the duration of the 4-wk study. Gene array analysis revealed that several genes are up- or downregulated in response to 2 days and 1 wk of high-fat feeding. Real-time PCR analysis confirmed that expression of the hypothalamic IL-1 pathway (IL-1beta, IL-1 type 1 and 2 receptors, and PPM1b/PP2C-beta, a molecule that has been implicated in the inhibition of transforming growth factor-beta-activated kinase-1-mediated IL-1 action) is altered after 2 days, but not 1 wk, of high-fat feeding. The role of additional molecules discovered by microarray analysis needs to be further explored in the future.

Frontiers in nutrigenomics, proteomics, metabolomics and cancer prevention

While dietary habits continue to surface as a significant factor that may influence cancer incidence and tumor behavior, there is considerable scientific uncertainty about who will benefit most. Adequate [corrected] knowledge about how the responses depend on an individual's genetic background (nutrigenetic effects), the cumulative effects of food components on genetic expression profiles (nutritional transcriptomics and nutritional epigenomics effects), the occurrence and activity of proteins (proteomic effects) and/or the dose and temporal changes in cellular small molecular weight compounds (metabolomics effects) will [corrected] assist in identifying responders and non-responders. Expanding the information about similarities and differences in the "omic" responses across tissues will not only provide clues about specificity in response to bioactive food components but assist in the identification of surrogate tissues and biomarkers that can be used for predicting a response. Deciphering the importance of each of these potential sites of regulation will be particularly challenging but does hold promise in explaining many of the inconsistencies in the literature.

Brown adipose tissue: function and physiological significance

The function of brown adipose tissue is to transfer energy from food into heat; physiologically, both the heat produced and the resulting decrease in metabolic efficiency can be of significance. Both the acute activity of the tissue, i.e., the heat production, and the recruitment process in the tissue (that results in a higher thermogenic capacity) are under the control of norepinephrine released from sympathetic nerves. In thermoregulatory thermogenesis, brown adipose tissue is essential for classical nonshivering thermogenesis (this phenomenon does not exist in the absence of functional brown adipose tissue), as well as for the cold acclimation-recruited norepinephrine-induced thermogenesis. Heat production from brown adipose tissue is activated whenever the organism is in need of extra heat, e.g., postnatally, during entry into a febrile state, and during arousal from hibernation, and the rate of thermogenesis is centrally controlled via a pathway initiated in the hypothalamus. Feeding as such also results in activation of brown adipose tissue; a series of diets, apparently all characterized by being low in protein, result in a leptin-dependent recruitment of the tissue; this metaboloregulatory thermogenesis is also under hypothalamic control. When the tissue is active, high amounts of lipids and glucose are combusted in the tissue. The development of brown adipose tissue with its characteristic protein, uncoupling protein-1 (UCP1), was probably determinative for the evolutionary success of mammals, as its thermogenesis enhances neonatal survival and allows for active life even in cold surroundings.

Effects of dietary restriction on total body, femoral, and vertebral bone in SENCAR, C57BL/6, and DBA/2 mice

Dietary restriction (DR) increases the life span and retards aging, in part, by limiting free radical generation and oxidative damage. DR also reduces body mass, a major determinant of bone mass across the life span. We tested the hypothesis that DR has its most beneficial effects on bone in mouse strains with high free radical generation (sensitive to carcinogenesis [SENCAR] > C57 > DBA) versus the hypothesis that bone mass at weight-bearing sites is determined by body mass in DR and ad libitum (AL)-fed mice. Male mice of each strain were killed at 10 weeks of age (t(0)) or randomized to an AL-fed or 30% DR feeding regimen for 6 months. Food consumption by AL-fed mice was measured daily, and DR mice received 70% of the amount of food consumed by their respective AL-fed mice the previous day. Body fat (%) and bone mineral density (BMD) and content (BMC) were determined by PIXImus densitometry. There were strain-dependent effects on body mass, crown-to-rump length, percent body fat, and total body, femoral, and vertebral BMD and BMC under all conditions. SENCAR mice were heavier, longer, had larger bones, and generally exhibited higher total body, femoral, and vertebral BMC and BMD than C57 and DBA mice. DR had beneficial effects on BMD and BMC in the vertebrae of the SENCAR mouse model of high free radical generation and in the obese, diabetes-prone C57 mouse model of high end-stage protein glycation. DR DBA and SENCAR mice had lower femoral BMDs and BMCs than their respective AL-fed controls. Regression analysis confirmed linear relationships between total and lean body mass and total body and femoral BMDs and BMCs, suggesting that physiologic adaptation to a lower body mass accounts for the lower femoral bone mineral values observed in DR versus AL-fed mice. Thus, both hypotheses are, at least, partially valid. DR is beneficial in the trabeculae-rich vertebrae of animal models of high oxidant stress, and total/lean body mass determines BMD and BMC in the weight-bearing femur in DR and AL-fed mice.

Impact of life-long macronutrient choice on neuroendocrine and cognitive functioning in aged mice: differential effects in stressor-reactive and stressor-resilient mouse strains

Nutrient selection emerges as a result of both genetic and environmental factors and may be further modified by stressors. The impact of this complex interrelationship on pathological outcomes is poorly understood. In the present investigation the stressor-reactive BALB/cByJ and the relatively stressor resilient C57BL/6ByJ mice were maintained on a macronutrient selection protocol or given free access to chow for 20 months. The C57BL/6ByJ mice exhibited a marked preference for fat over carbohydrates, whereas BALB/cByJ mice preferred carbohydrates over fat. Cognitive testing in a Morris water maze indicated that while BALB/cByJ mice were clearly more impaired in this task relative to their C57BL/6ByJ counterparts, there was no substantial effect of the diet at either 13 or 19 months of age. Furthermore, despite their stressor resiliency, at 19 months of age, C57BL/6ByJ mice who invariably consumed fat, exhibited greater plasma corticosterone responses to a 20-min period of restraint than chow fed animals. Indeed, the corticosterone rise was as pronounced as in the more reactive BALB/cByJ strain. Furthermore, the C57BL/6ByJ diet-fed mice showed features of insulin insensitivity and increased adiposity. These data suggest that the adverse effects of fat consumption need to be considered in the context of genetically determined vulnerability/resilience factors.

Peripheral, but not central, administration of adiponectin reduces visceral adiposity and upregulates the expression of uncoupling protein in agouti yellow (Ay/a) obese mice

To examine the peripheral and central roles of adiponectin in energy intake and expenditure, we investigated the effects of adiponectin on food intake, adiposity, sympathetic nerve activity (SNA), and mRNA expressions of uncoupling protein (UCP) in the brown adipose tissue (BAT), white adipose tissue (WAT) and skeletal muscle in agouti yellow (A(y)/a) obese mice. Intraperitoneal administration of adiponectin (1.5 mg/kg for 7 days) attenuated body weight gain and reduced visceral adiposity in A(y)/a obese mice compared with PBS-treated controls. In addition, adiponectin treatment increased the expression of UCP1 mRNA in BAT, UCP2 mRNA in WAT, and UCP3 mRNA in skeletal muscle compared with PBS-treated A(y)/a controls. Acute peripheral administration of adiponectin (1.5 mg/kg, one injection) also increased SNA in the BAT accompanied by an increase in rectal temperature. Finally, these above responses as well as expression of c-Fos-like immunohistochemistry in the hypothalamus were not induced by central application of adiponectin (0-15 micro g/kg). Taken together, adiponectin effectively regulated visceral adiposity, SNA, and UCP mRNA expression peripherally, suggesting that this substance can be used as a therapeutic tool, administered peripherally, in the treatment of visceral obesity and related metabolic disorders.

Dietary fat interacts with QTLs controlling induction of Pgc-1 alpha and Ucp1 during conversion of white to brown fat

To identify novel regulatory factors controlling induction of the brown adipocyte-specific mitochondrial uncoupling protein (Ucp1) mRNA in the retroperitoneal white fat depot, we previously mapped quantitative trait loci (QTLs) that control this trait to chromosomes 2, 3, 8, and 19. Since the peroxisome proliferator activator receptor-gamma coactivator-1alpha (PGC-1alpha) regulates Ucp1 and other genes of energy metabolism, we have evaluated whether the QTLs controlling Ucp1 mRNA levels also modulate Pgc-1alpha mRNA levels by analysis of backcross progeny from the A/J and C57BL/6J strains of mice. The results indicate that a locus on chromosome 3 orchestrates expression of Pgc-1alpha and Ucp1 in retroperitoneal fat of mice fed a low-fat diet; however, the effect of this locus on Pgc-1alpha is lost, and a significant correlation between Ucp1 and Pgc-1alpha is severely reduced in mice fed a high-fat diet. An additional QTL located on chromosome 5 has also been identified for the selective regulation of Ucp1 mRNA levels. Similar to the effects of a high-fat diet on the chromosome 3 QTL, linkage of the chromosome 5 QTL is also lost in mice on a high-fat diet. Thus dietary fat has a profound influence on PGC-1alpha-regulated pathways controlling energy metabolism in white fat. The allelic variation observed in the regulation of Ucp1 and Pgc-1alpha expression in brown adipocytes of white fat but not interscapular brown fat suggests that fundamentally different regulatory mechanisms exist to control the thermogenic capacities of these tissues.

Genetic contributions to body weight in mice: relationship of exploratory behavior to weight

OBJECTIVE

The A/J and C57BL/6J mouse strains differ markedly in their exploratory behavior and their weight gain on a high-fat diet. We examined the genetic contributions of exploratory behavior to body weight and tested for shared, pleiotropic loci influencing energy homeostasis.

RESEARCH METHODS AND PROCEDURES

Segregating (AxB6)F2 intercross (n = 514) and (B6AF1xA/J)N2 backcross (N = 223) populations were studied, phenotyping for weight and exploratory behaviors. Relationships among traits were analyzed by correlations. Weight traits were dissected with a genome-wide scan.

RESULTS

Modest correlations were found between exploratory behaviors and weight, explaining 2% to 14% of the variance. Quantitative trait loci (QTL) for body weight at 8 weeks (wgt8), 10 weeks (wgt10), and 2-week weight gain (difference between weeks 8 and 10) on a 6% fat diet were mapped. Two QTL on chromosome 1 (peaks at 66 cM and 100 cM; Bw8q1) affected wgt8 [likelihood of the odds ratio (Lod), 3.0 and 4.4] and wgt10 (Lod, 2.2 and 3.4), respectively. In the backcross, a significant QTL on chromosome 4 (peak at 66 cM; Bw8q2) affected wgt 8 (Lod, 3.3) and wgt10 (Lod, 3.1). For 2-week weight gain, suggestive QTL were mapped on chromosomes 4 and 6. The chromosome 6 QTL region overlaps a human 7q locus for obesity. A search for between-strain sequence polymorphisms in the leptin and NPY genes was unrevealing.

DISCUSSION

In mice, loci influencing exploratory activity play a modest role in body-weight regulation. Some forms of obesity may emerge from loci regulating normal body weight.

Identification of interactive loci linked to insulin and leptin in mice with genetic insulin resistance

Mice double heterozygous (DH) for deletion of insulin receptor and insulin receptor substrate-1 are lean, insulin resistant, and have a phenotype that strongly depends on the genetic background of the mouse. On the C57BL/6 (B6) background, DH mice develop marked hyperinsulinemia and diabetes, whereas on the 129S6 background, DH mice exhibit only mild elevations of insulin and remain free of diabetes. F2 male mice created by an intercross between these two strains exhibit a 60% incidence of diabetes and a bell-shaped distribution of insulin levels as related to glucose, reminiscent of that in humans with type 2 diabetes. These mice also exhibit a wide range of leptin levels as related to body weight. A genome-wide scan of F2 mice reveals a quantitative trait locus (QTL) related to hyperinsulinemia on chromosome 14 (D14Mit55) with a peak logarithm of odds (LOD) score of 5.6, accounting for up to 69% of this trait. A QTL with a LOD score of 3.7 related to hyperleptinemia is present on chromosome 7 at D12Mit38 (a marker previously assigned to chromosome 12) in the area of the uncoupling protein 2/3 gene cluster. This locus also interacts synergistically with D14Mit55 in development of hyperinsulinemia and with a QTL on chromosome 12 (D12Mit231) related to hyperglycemia. These data demonstrate how multiple genetic modifiers can interact and influence the development of diabetes and the phenotype of animals with genetically programmed insulin resistance and provide evidence as to the location and nature of these genes.

Dietary conjugated linoleic acid reduces body fat mass and affects gene expression of proteins regulating energy metabolism in mice

ICR and C57BL/6J mice were fed experimental diets containing either a 2% fatty acid preparation rich in conjugated linoleic acid (CLA) or a preparation rich in linoleic acid and free of CLA for 21 days. CLA greatly decreased weights of white adipose tissue and interscapular brown adipose tissue in the two strains. CLA reduced mRNA levels of glucose transporter 4 (Glut 4) in white and brown adipose tissue of both strains. A CLA-dependent decrease in mRNA levels of peroxisome proliferator activated receptor (PPAR) gamma was seen in interscapular brown adipose tissue of both strains and in white adipose tissue of C57BL/6J but not ICR mice. Dietary CLA was found to cause a decrease in the mRNA levels of uncoupling protein (UCP) 1 in brown adipose tissue when the value was corrected for the expression of a house-keeping gene (beta-actin) in the two strains. Uncorrected values were, however, indistinguishable between the animals fed the CLA diet and CLA-free diet. UCP 3 expression in brown adipose tissue was much lower in mice fed the CLA diet than in those fed the control diet in both strains. In contrast, CLA greatly up-regulated the gene expression of UCP 2 in brown adipose tissue. Dietary CLA also increased UCP 2 mRNA level in skeletal muscle. It is apparent that dietary CLA decreases white and brown adipose tissue mass, accompanying changes in the gene expression of proteins regulating energy metabolism in white and brown adipose tissues, and skeletal muscle of mice.

Leptin selectively reduces white adipose tissue in mice via a UCP1-dependent mechanism in brown adipose tissue

We tested the hypothesis that leptin, in addition to reducing body fat by restraining food intake, reduces body fat through a peripheral mechanism requiring uncoupling protein 1 (UCP1). Leptin was administered to wild-type (WT) mice and mice with a targeted disruption of the UCP1 gene (UCP1 deficient), while vehicle-injected control animals of each genotype were pair-fed to each leptin-treated group. Leptin reduced the size of white adipose tissue (WAT) depots in WT mice but not in UCP1-deficient animals. This was accompanied by a threefold increase in the amount of UCP1 protein and mRNA in the brown adipose tissue (BAT) of WT mice. Leptin also increased UCP2 mRNA in WAT of both WT and UCP1-deficient mice but increased UCP2 and UCP3 mRNA only in BAT from UCP1-deficient mice. These results indicate that leptin reduces WAT through a peripheral mechanism requiring the presence of UCP1, with little or no involvement of UCP2 or UCP3.