Regulation of leptin by agouti

Obesity partially potentiates dimethylbenz[a]anthracene-exposed ovotoxicity by altering the DNA damage repair response in mice†

Obesity adversely affects reproduction, impairing oocyte quality, fecundity, conception, and implantation. The ovotoxicant, dimethylbenz[a]anthracene, is biotransformed into a genotoxic metabolite to which the ovary responds by activating the ataxia telangiectasia mutated DNA repair pathway. Basal ovarian DNA damage coupled with a blunted response to genotoxicant exposure occurs in obese females, leading to the hypothesis that obesity potentiates ovotoxicity through ineffective DNA damage repair. Female KK.Cg-a/a (lean) and KK.Cg-Ay/J (obese) mice received corn oil or dimethylbenz[a]anthracene (1 mg/kg) at 9 weeks of age for 7 days via intraperitoneal injection (n = 10/treatment). Obesity increased liver weight (P < 0.001) and reduced (P < 0.05) primary, preantral, and corpora lutea number. In lean mice, dimethylbenz[a]anthracene exposure tended (P < 0.1) to increase proestrus duration and reduced (P = 0.07) primordial follicle number. Dimethylbenz[a]anthracene exposure decreased (P < 0.05) uterine weight and increased (P < 0.05) primary follicle number in obese mice. Total ovarian abundance of BRCA1, γH2AX, H3K4me, H4K5ac, H4K12ac, and H4K16ac (P > 0.05) was unchanged by obesity or dimethylbenz[a]anthracene exposure. Immunofluorescence staining demonstrated decreased (P < 0.05) abundance of γH2AX foci in antral follicles of obese mice. In primary follicle oocytes, BRCA1 protein was reduced (P < 0.05) by dimethylbenz[a]anthracene exposure in lean mice. Obesity also decreased (P < 0.05) BRCA1 protein in primary follicle oocytes. These findings support both a follicle stage-specific ovarian response to dimethylbenz[a]anthracene exposure and an impact of obesity on this ovarian response.

Mutation yellow in agouti loci prevents age-related increase of skeletal muscle genes regulating free fatty acids oxidation

The lethal yellow mutation in agouti loci (Ay mutation) reduces the activity of melanocortin (MC) receptors and causes hyperphagia, obesity and type two diabetes mellitus in aging mice (Ay mice). It is unknown if changes in distinct elements of the metabolic system such as white adipose tissue (WAT) and brown adipose tissue (BAT), and skeletal muscle will manifest before the development of obesity. The aim of this work was to measure the relative gene expression of key proteins that regulate carbohydrate-lipid metabolism in WAT, BAT and skeletal muscle in Ay mice before the development of obesity. C57Bl/6J mice bearing a dominant autosomal mutation Ay (Ay /a mice) and mice of the standard genotype (a/a mice, control) have been studied in three age groups: 10, 15 and 30 weeks. The relative mRNA level of genes was measured by real-time PCR in skeletal muscles (uncoupling protein 3 (Ucp3) and carnitine palmitoyl transferase 1b (Cpt1b) (free fatty acids oxidation), solute carrier family 2 (facilitated glucose transporter), member 4 (Slc2a4) (glucose uptake)), in WAT lipoprotein lipase (Lpl) (triglyceride deposition), hormone-sensitive lipase (Lipe) (lipid mobilization), and Slc2a4 (glucose uptake)), and in BAT: uncoupling protein 1 (Ucp1) (energy expenditure). The expression of Cpt1b was reduced in young Ay mice (10 weeks), there was no transient peak of transcription of Cpt1b, Ucp3 in skeletal muscle tissue and Lipe, Slc2a4 in WAT in early adult Ay mice (15 weeks), which was noted in а/а mice. Reduction of the transcriptional activity of the studied genes in skeletal muscle and white adipose tissue can initiate the development of melanocortin obesity in Ay mice.

TRIENNIAL GROWTH AND DEVELOPMENT SYMPOSIUM: Factors influencing bovine intramuscular adipose tissue development and cellularity

Appearance, distribution, and amount of intramuscular fat (IMF), often referred to as marbling, are highly variable and depend on environmental and genetic factors. On the molecular level, the concerted action of several drivers, including hormones, receptors, transcription factors, etc., determines where clusters of adipocytes arise. Therefore, the aim of future studies remains to identify such factors as biological markers of IMF to increase the ability to identify animals that deposit IMF early in age to increase efficiency of high-quality meat production. In an attempt to unravel the cellular development of marbling, we investigated the abundance of markers for adipogenic differentiation during fattening of cattle and the transcriptome of muscle and dissected IMF. Markers of different stages of adipogenic differentiation are well known from cell culture experiments. They are usually transiently expressed, such as delta-like homolog 1 (DLK1) that is abundant in preadipocytes and absent during differentiation to mature adipocytes. It is even a greater challenge to detect those markers in live animals. Within skeletal muscles, hyperplasia and hypertrophy of adipocytes can be observed throughout life. Therefore, development of marbling requires, on the cellular level, recruitment, proliferation, and differentiation of adipogenic cells to store excess energy in the form of lipids in new cells. In a recent study, we investigated the localization and abundance of early markers of adipogenic differentiation, such as DLK1, in bovine muscle tissue. An inverse relationship between IMF content and number of DLK1-positive cells in bovine muscle was demonstrated. Considering the cellular environment of differentiating adipocytes in muscle and the secretory action of adipocytes and myocytes, it becomes obvious that cross talk between cells via adipokines and myokines may be important for IMF development. Secreted proteins can act on other cells, inhibiting or stimulating their function via autocrine and paracrine actions. Such factors with potential influence on IMF, among them, agouti signaling protein and thrombospondin 4, were identified in transcriptome analyses and further investigated. Furthermore, results from transcriptome analysis indicate involvement of genes that are not directly related to adipogenesis and lipid metabolism, providing new candidates for future research.

The brain–adipose axis: A review of involvement of molecules

Many molecules are involved in the regulation of feeding behavior, and they and their receptors are located in the brain hypothalamus and adipocytes. On the basis of evidence suggesting an association between the brain and adipose tissue, we propose the concept of the brain-adipose axis. This model consists of (l) the expression of endogenous molecules and/or their receptors in the hypothalamus and peripheral adipose tissue, (2) the function of these molecules as appetite regulators in the brain, (3) their existence in the general circulation as secreted proteins and (4) the physiological affects of these molecules on fat cell size and number. These molecules can be divided into two anorexigenic and orexigenic classes. In adipose tissue, all orexigenic molecules possess adipogenic activity, and almost all anorexigenic molecules suppress fat cell proliferation. Although the manner, in which they present in the circulating blood connect the brain and peripheral adipocytes, remains to be well-organized, these observations suggest the positive feedback axis affecting molecules in the hypothalamus and adipose tissue. Analysis of the disturbance and dysregulation of this axis might promote the development of new anti-obesity drugs useful in treating the metabolic syndrome.

Agouti revisited: transcript quantification of the ASIP gene in bovine tissues related to protein expression and localization

Beside its role in melanogenesis, the agouti signaling protein (ASIP) has been related to obesity. The potentially crucial role in adipocyte development makes it a tempting candidate for economic relevant, fat related traits in farm animals. The objective of our study was to characterize the mRNA expression of different ASIP transcripts and of putative targets in different bovine tissues, as well as to study consequences on protein abundance and localization. ASIP mRNA abundance was determined by RT-qPCR in adipose and further tissues of cattle representing different breeds and crosses. ASIP mRNA was up-regulated more than 9-fold in intramuscular fat of Japanese Black cattle compared to Holstein (p<0.001). Further analyses revealed that a transposon-derived transcript was solely responsible for the increased ASIP mRNA abundance. This transcript was observed in single individuals of different breeds indicating a wide spread occurrence of this insertion at the ASIP locus in cattle. The protein was detected in different adipose tissues, skin, lung and liver, but not in skeletal muscle by Western blot with a bovine-specific ASIP antibody. However, the protein abundance was not related to the observed ASIP mRNA over-expression. Immuno-histochemical analyses revealed a putative nuclear localization of ASIP additionally to the expected cytosolic signal in different cell types. The expression of melanocortin receptors (MCR) 1 to 5 as potential targets for ASIP was analyzed by RT-PCR in subcutaneous fat. Only MC1R and MC4R were detected indicating a similar receptor expression like in human adipose tissue. Our results provide evidence for a widespread expression of ASIP in bovine tissues at mRNA and, for the first time, at protein level. ASIP protein is detectable in adipocytes as well as in further cells of adipose tissue. We generated a basis for a more detailed investigation of ASIP function in peripheral tissues of various mammalian species.

Pregnancy and lactation have anti-obesity and anti-diabetic effects in A(y)/a mice

AIM

Dominant 'yellow' mutation at the mouse agouti locus (A(y)) results in obesity. Pregnancy and lactation are characterized by large energy demand. The aim of this study was to investigate whether obesity would develop in pregnant and suckling A(y) mice.

METHODS

Body weight and food intake in pregnancy, lactation, and after weaning, plasma leptin, insulin, corticosterone and blood glucose concentrations on days 7, 13 and 18 of pregnancy, days 1, 10, 21 and 80 postpartum, glucose and insulin tolerance on pregnancy days 7 and 18 were measured in C57Bl/6J mice of a/a (normal metabolism) and A(y)/a genotypes. The same parameters were also measured in age-matched virgin females.

RESULTS

Virgin A(y)/a females exhibited hyperphagia, enhanced body weight, glucose intolerance and normal blood parameters at the mating age. With age, they developed obesity, hyperleptinaemia, hyperinsulinaemia and hyperglycaemia. Obesity did not develop in mated A(y)/a mice; during suckling, they had equal food intake and body weight as a/a mice. During pregnancy, glucose tolerance was enhanced in A(y)/a mice and became equal in both genotypes. In both genotypes, concentrations of hormones increased, and glucose decreased from pregnancy day 7 to day 18 and returned to normal values after parturition. A(y)/a mice did not differ from a/a in corticosterone, insulin and glucose levels during pregnancy and lactation, in leptin levels during suckling; however, A(y)/a mice had two times higher leptin levels than a/a during pregnancy. After weaning, A(y)/a mice began to eat and weigh more than a/a exhibiting normal metabolic parameters for 50 days.

CONCLUSION

Pregnancy and lactation retard obesity and diabetes development in A(y) mice.

The role of the Agouti-Related Protein in energy balance regulation

The Agouti-Related Protein (AgRP) is a powerful orexigenic peptide that increases food intake when ubiquitously overexpressed or when administered centrally. AgRP-deficiency, on the other hand, leads to increased metabolic rate and a longer lifespan when mice consume a high fat diet. In humans, AgRP polymorphisms have been consistently associated with resistance to fatness in Blacks and Whites and resistance to the development of type-2 diabetes in African Blacks. Systemically administered AgRP accumulates in the liver, the adrenal gland and fat tissue while recent findings suggest that AgRP may also have inverse agonist effects, both centrally and peripherally. AgRP could thus modulate energy balance via different actions. Its absence or reduced functionality may offer a benefit both in terms of bringing about negative energy balance in obesigenic environments, as well as leading to an increased lifespan.

Chronic consumption of a low-fat diet leads to increased hypothalamic agouti-related protein and reduced leptin

OBJECTIVE

This study examined the hypothesis that dietary fat under ad libitum feeding conditions influences expression levels (mRNA) of the mouse agouti-related protein (AgRP), leptin, leptin receptor (OBRb), and neuropeptide Y (NPY) at early stages of development.

METHODS

C57Bl/6J male mice were placed on a high-fat diet (HFD) or a low-fat diet (LFD) shortly after weaning. Groups of mice were euthanized at various ages and real-time one-step reverse transcriptase polymerase chain reaction was used to analyze gene expression in the hypothalamus (AgRP, NPY, OBRb), the adrenal gland (AgRP), the testis (AgRP), and epididymal fat (leptin).

RESULTS

Leptin expression increased linearly with age but only under the HFD despite body weight gain under both diets. This pattern of expression coincided with reduced expression of hypothalamic AgRP under an HFD, whereas OBRb and NPY did not fluctuate in response to diet. By contrast, consumption of an LFD (i.e., high carbohydrate) increased hypothalamic AgRP and suppressed adipose leptin, which is consistent with the notion that leptin could regulate AgRP centrally. In contrast, AgRP expression in the adrenal gland initially decreased and then increased with age under both diets.

CONCLUSIONS

Dietary fat can have a tissue-dependent effect on AgRP that may be unfettered by leptin under an HFD.

Large-scale preparation of biologically active mouse and rat leptins and their L39A/D40A/F41A muteins which act as potent antagonists

Expression plasmids encoding mouse and rat leptins and their L39A/D40A/F41A muteins were prepared. The proteins were expressed in Escherichia coli, refolded and purified to homogeneity, yielding electrophoretically pure, over 98% monomeric protein. Circular dichroism (CD) analysis revealed that the mutations hardly affect the leptins' secondary structure, and they were similar to previously reported CD spectra for human leptin. Both mouse and rat leptins were biologically active in promoting proliferation in BAF/3 cells stably transfected with the long form of human leptin receptor. The mutations did not change the binding properties to BAF/3 cells as compared, respectively, to non-mutated mouse, rat or human leptins, or their ability to form 1:1 complexes with the leptin-binding domain of chicken leptin receptor. In contrast, their biological activity, tested in a BAF/3 proliferation assay, was abolished and both became potent antagonists. As the LDF (amino acids 39-41) sequence is preserved in all known leptins, the present results substantiate the hypothesis that this sequence plays a pivotal role in leptins' site III and that interaction of leptin with its receptors resembles the corresponding interactions of interleukin-6 and granulocyte colony-stimulating factor their receptors.

The agouti-related protein and its role in energy homeostasis

The melanocortin system plays an important role in the regulation of energy homeostasis. The Agouti-related protein (AGRP) is a natural antagonist of the action of alpha-melanocyte stimulating hormone (alpha-MSH) at the melanocortin receptors (MCR). AGRP is upregulated by fasting while intracerebroventricular injections of synthetic AGRP lead to increased appetite and food intake. Transgenic mice overexpressing AGRP are also hyperphagic and eventually become obese. AGRP is, therefore, a significant regulator of energy balance and a candidate gene for human fatness. Indeed, humans with common single nucleotide polymorphisms (SNPs) in the promoter or the coding region are leaner and resistant to late-onset obesity than wild-type individuals. AGRP is also expressed in the periphery. Recent studies show that AGRP in the adrenal gland is upregulated by fasting as much as it is in the hypothalamus. These data open up the possibility for a wider role by AGRP not only in food intake but also in the regulation of energy balance through its actions on peripheral tissues. This review summarizes recent advances in the biochemical and physiological properties of AGRP in an effort to enhance our understanding of the role this powerful neuropeptide plays in mammalian energy homeostasis.

Expression of interleukin-6 is greater in preadipocytes than in adipocytes of 3T3-L1 cells and C57BL/6J and ob/ob mice

Inflammation plays a major role in the development of chronic diseases such as cardiovascular disease and Type 2 diabetes. Further, it was demonstrated that obese animals and humans have significantly higher levels of circulating proinflammatory cytokines, such as interleukin-6 (IL-6). The aim of this study was to determine whether adipose tissue could be a major source of circulating IL-6 in leptin-deficient obese (ob/ob) mice by comparing the expression of IL-6 in different tissues of ob/ob mice. Our secondary goal was to determine whether preadipocytes are the source of adipose tissue IL-6. The ob/ob mice had higher levels of plasma IL-6 (P < 0.05) and adipose tissue IL-6 mRNA (P < 0.05) compared with lean mice. Interestingly, IL-6 mRNA levels of liver and spleen were not different between ob/ob and lean mice, whereas adipose tissue IL-6 mRNA levels were higher in the ob/ob mice compared with lean mice (P < 0.05). In addition, we showed that IL-6 secretion from the adipose tissue stromal vascular fraction cells was higher than that from fully differentiated adipocytes (P < 0.001). We further demonstrated that 3T3-L1 preadipocytes had significantly higher levels of lipopolysaccharide (LPS)-stimulated IL-6 mRNA and IL-6 secretion than differentiated 3T3-L1 adipocytes. Taken together, these data suggest that adipose tissue and preadipocytes from the adipose tissue stromal vascular fraction may contribute significantly to the increased plasma IL-6 levels in ob/ob mice.

Central and peripheral interactions between the agouti-related protein and leptin

The agouti-related protein (AgRP) is a powerful appetite modulator expressed in the hypothalamus and the adrenal gland and regulated by leptin. Here we report the robust expression of AgRP in epididymal fat and its upregulation in this tissue by feeding rather than by fasting. This was observed in both the obesity-susceptible C57BL/6J and the obesity-resistant CAST/Ei mouse strains. Surprisingly, AgRP expression was higher in the hypothalamus and the adrenal gland in the leaner and obesity-resistant CAST/Ei strain. In vitro leptin treatment upregulated endogenous AgRP in mouse hypothalamus and adrenal cells, after an acute 6-h exposure, but it downregulated AgRP after a long-term 60-h exposure. AgRP, on the other hand, upregulated its own endogenous expression in the hypothalamus and the adrenal cells and also upregulated endogenous leptin in the adrenal cells. These results reveal a novel feedback loop and reciprocal transcriptional regulation between AgRP and leptin centrally and peripherally.

Mouse models of obesity

Insights into the etiology of human obesity have arisen from the study of animal models. Animal models of obesity are also important for the development of future treatments of obesity. An agouti mouse mutation resulting in obese, yellow mice was described over a century ago and in 1992 agouti was cloned, making it the first obesity gene characterized at the molecular level. The lethal yellow mouse mutation is one of five dominant agouti mutations and is an excellent model for human obesity. The molecular categorization of agouti was responsible for the elucidation of the melanocortin system's involvement in hypothalamic weight regulation. As genetic knowledge increases many transgenic mice have been created with genes either over-expressed or deleted, models which further enhance the understanding of obesity.

Fecal corticosteroids in agouti and non-agouti deer mice (Peromyscus maniculatus)

Total and per gram fecal corticosteroid concentrations were determined for agouti and non-agouti deer mice (Peromyscus maniculatus gracilis) over 24 h under normal caging conditions and after exposure to the stress of novel caging. Per gram corticosteroid concentrations, fecal output, and 24-h corticosteroid production were greater in stressed compared with unstressed deer mice of both color morphs, whereas stressed agoutis had a greater increase in per gram corticosteroid concentrations when compared with non-agoutis. However, due to increased fecal output, stressed non-agouti deer mice had greater 24-h corticosteroid production. Thus, agouti and non-agouti deer mice differ in their hormonal reaction to stress. This is the first demonstration of corticosteroid differences associated with the agouti locus.

Body mass index-related human adipocyte agouti expression is sex-specific but not depot-specific

OBJECTIVE

To determine if human adipocyte agouti signal protein (ASIP) mRNA expression is associated with obesity and is gender and/or depot specific.

RESEARCH METHODS AND PROCEDURES

Subjects included 8 men (64 +/- 3 years) and 14 women (56 +/- 15 years) undergoing elective abdominal surgery. ASIP mRNA levels in isolated omental and subcutaneous abdominal adipocytes were measured by quantitative reverse transcription polymerase chain reaction.

RESULTS

No significant depot difference was observed between genders; ASIP mRNA levels of omental and subcutaneous abdominal adipocytes were pooled for this analysis. BMI and ASIP gene expression were negatively correlated in men (rho = -0.70; p < 0.05), whereas a positive relationship was observed in women (rho = 0.48; p < 0.05). No significant difference was observed in age, body weight, body mass index (BMI), and waist circumference between groups. Hip circumference was significantly higher in women than in men (p < 0.05). Also, no significant difference in ASIP mRNA expression was observed between men and women, regardless of the fat depot.

DISCUSSION

These results show that men and women of similar age and BMI present similar ASIP mRNA levels in omental and subcutaneous abdominal adipocytes. However, a sexual dimorphism exists in the relationship between ASIP expression and BMI. If ASIP is involved in appetite regulation or energy homeostasis in humans, this observation may contribute to the recognized differences in these parameters between men and women.

Agouti: from mouse to man, from skin to fat

The agouti protein regulates pigmentation in the mouse hair follicle producing a black hair with a subapical yellow band. Its effect on pigmentation is achieved by antagonizing the binding of alpha-melanocyte stimulating hormone (alpha-MSH) to melanocortin 1 receptor (Mc1r), switching melanin synthesis from eumelanin (black/brown) to phaeomelanin (red/yellow). Dominant mutations in the non-coding region of mouse agouti cause yellow coat colour and ectopic expression also results in obesity, type 11 diabetes, increased somatic growth and tumourigenesis. At least some of these pleiotropic effects can be explained by antagonism of other members of the melanocortin receptor family by agouti protein. The yellow coat colour is the result of agouti chronically antagonizing the binding of alpha-MSH to Mc1r and the obese phenotype results from agouti protein antagonizing the binding of alpha-MSH to Mc3r and/or Mc4r. Despite the existence of a highly homologous agouti protein in humans, agouti signal protein (ASIP), its role has yet to be defined. However it is known that human ASIP is expressed at highest levels in adipose tissue where it may antagonize one of the melanocortin receptors. The conserved nature of the agouti protein combined with the diverse phenotypic effects of agouti mutations in mouse and the different expression patterns of human and mouse agouti, suggest ASIP may play a role in human energy homeostasis and possibly human pigmentation.

Thyroxine levels in agouti and non-agouti deer mice (Peromyscus maniculatus)

Total thyroxine was assessed by radioimmunoassay for 58 female deer mice of two subspecies (Peromyscus maniculatus bairdii and P. m. gracilis) and two color morphs (agouti and non-agouti). P. m. bairdii of both color-morphs had significantly higher mean thyroxine levels than P. m. gracilis. Non-agouti deer mice of both subspecies had significantly higher mean thyroxine levels than agouti deer mice. This is the first report of thyroid hormone differences associated with the non-agouti allele.

Secretory, endocrine and autocrine/paracrine function of the adipocyte

Obesity is a major public health problem in Western countries, and >55% of adult Americans are overweight or obese. A major contributor to the epidemic of obesity is the current environment, which is characterized by increased availability of high energy foods and decreased physical activity. Several studies also demonstrated that genetic susceptibility contributes to obesity in some populations. Obesity research has focused primarily on the role of the hypothalamus in neuroendocrine regulation of food intake. However, a growing number of studies support a potential contribution of adipose tissue, via its newly discovered secretory function, to the pathogenesis of obesity and co-morbid conditions including cardiovascular disease, diabetes and hypertension. This paper will review the role of four factors secreted by adipose tissue (leptin, agouti, angiotensin II and prostaglandins) and their functions in the regulation of energy balance and whole-body homeostasis. Several other peptide and nonpeptide substances are secreted from adipose tissue; their function and regulation have been documented extensively.

Relationship between human adipose tissue agouti and fatty acid synthase (FAS)

The human homologue of the murine obesity gene, agouti, is expressed in adipose tissue. We have shown that recombinant agouti protein regulates adipocyte lipogenesis and lipolysis coordinately and promotes lipid storage via a Ca(2+)-dependent mechanism in vitro, which may contribute to agouti-induced obesity. However, little is known about agouti's physiologic function in humans. We first studied the agouti content in human mature adipocytes vs. preadipocytes. The agouti content of human mature adipocytes was five times as abundant as in preadipocytes (19.18 +/- 2.46 vs. 4.07 +/- 0.51 pg/microg protein, P: < 0.005), suggesting that agouti is up-regulated during adipocyte differentiation. We next studied the relationship of agouti mRNA and protein to fatty acid synthase (FAS) mRNA and activity in adipose tissue obtained from nonobese and mildly obese patients (body mass index range, 21-31 kg/m(2)). Agouti protein was correlated with FAS activity (r = 0.782, P: < 0.005). Similarly, human adipose tissue agouti mRNA level was also correlated with FAS mRNA level (r = 0.846, P: < 0.001). These data suggest that agouti may be another adipocyte-produced factor that modulates adipocyte lipid metabolism via a paracrine/autocrine mechanism.