Age and gender effects on glucose utilization in skeletal muscle and brown adipose tissue of cold-exposed rats

Perspectives on Brown Adipose Tissue Imaging: Insights from Preclinical and Clinical Observations from the Last and Current Century

Brown adipose tissue (BAT) was first described in the 16th century, but until late last century had mainly been considered a tissue with the function of nonshivering thermogenesis, maintaining body temperature in key organs in newborns who have high body surface areas relative to their weight and thus marked radiative heat loss. BAT was believed to have substantially disappeared by adulthood. Molecular imaging with ¹⁸F-FDG PET and PET combined with CT, as well as imaging with ¹³¹I-metaiodobenzylguanidine (MIBG) beginning late last century have shown BAT to be present and active well into adulthood. This review highlights key aspects of BAT biology, early empiric observations misidentifying BAT, pitfalls in image interpretation, and methods to intentionally reduce BAT uptake, and outlines multiple imaging methods used to identify BAT in vivo. The therapeutic potential of increasing the amount or activity of BAT for weight loss and improvement of glucose and lipid profiles is highlighted as a major opportunity. Molecular imaging can help dissect the physiology of this complex dynamic tissue and offers the potential for addressing challenges separating "active BAT" from "total BAT." Research in BAT has grown extensively, and ¹⁸F-FDG PET is the key imaging procedure against which all other BAT imaging methods must be compared. Given the multiple functions of BAT, it is reasonable to consider it a previously unrecognized endocrine tissue and thus an appropriate topic for review in this supplement to The Journal of Nuclear Medicine.

Glucose metabolism in brown adipose tissue determined by deuterium metabolic imaging in rats

BACKGROUND/OBJECTIVES

Brown adipose tissue (BAT) has gained growing interest as a potential target for treatment of obesity. Currently, the most widely used technique/method for in vivo measurements of BAT activity in humans is ¹⁸FDG PET/CT. To supplement these investigations novel radiation-free methods are warranted. Deuterium metabolic imaging (DMI) is a novel modality that combines magnetic resonance spectroscopic (MRS) imaging with deuterium-labelled glucose (²H-glucose). This allows for spatio-temporal and metabolic imaging beyond glucose uptake. We aimed to evaluate if DMI could discriminate glucose metabolism in BAT of cold-acclimatised and thermoneutral rats.

SUBJECTS/METHODS

Male Sprague-Dawley rats were housed in a cold environment (9 °C, n = 10) or at thermoneutrality (30 °C, n = 11) for 1 week. For imaging rats were anaesthetized, received a ²H-glucose (1 M, 1.95 g/kg) bolus and DMI was acquired at baseline followed by 20 min time intervals up to 2 h. Furthermore, Dixon MRI was performed for anatomical determination of the interscapular BAT (iBAT) depot along with dynamic contrast enhanced (DCE) MRI to evaluate perfusion.

RESULTS

²H-glucose signal was higher in cold-acclimatised rats compared with thermoneutral rats (p ≤ 0.001) indicating an overall increase in glucose uptake and metabolism. This was in line with a lower fat/water threshold, higher perfusion and increased UCP1 mRNA expression in iBAT (ninefold increment) of cold-acclimatised rats compared with thermoneutral rats.

CONCLUSIONS

We find that DMI can discriminate cold-acclimatised and thermoneutral BAT in rats. This is the first study to evaluate BAT activity by DMI, which may open up for the use of the non-radioactive DMI method for BAT measurements in humans.

Circadian rhythms of body temperature and locomotor activity in aging BALB/c mice: early and late life span predictors

Impairment of one or more parameters of circadian rhythms (CR) of body temperature (BT) and locomotor activity (LMA) are considered among the hallmarks of mammalian aging. These alterations are frequently used as markers for imminent death in laboratory mice. However, there are still contradictory data for particular strains and it is also uncertain which changes might predict senescence changes later in life, including the force of mortality. In the present paper we use telemetry to study LMA and CR of BT during aging of BALB/c mice. At our knowledge this is the first time that CR of BT and LMA are investigated in this strain in a range of age covering the whole lifespan, from young adult up to very old age. CR of BT was analyzed with a cosine model using a cross sectional approach and follow-up measurements. The results show that BT, LMA, amplitude, goodness-of-fit (GoF) to circadian cycle of temperature decrease with different shapes during chronological age. Moreover, we found that the % change of amplitude and BT in early life (5-19 months) can predict the remaining lifespan of the mice. Later in life (22-32 months), best predictors are single measurements of LMA and GoF. The results of this study also offer potential measures to rapidly identifying freely unrestrained mice with the worst longitudinal outcome and against which existing or novel biomarkers and treatments may be assessed.

Altered biodistribution on FDG-PET with emphasis on brown fat and insulin effect

(18)F-fluorodeoxyglucose (FDG) is the radiotracer used in the vast majority of positron emission tomography (PET) cancer studies. FDG is a powerful radiotracer that provides valuable data in many cancer types. Normal FDG biodistribution is easily identified. In the PET-only era, physiological uptake provided important anatomical landmarks. However, the normal biodistribution of FDG is often variable and can be altered by intrinsic or iatrogenic factors. Recognizing these patterns of altered biodistribution is important for optimal FDG-PET interpretation. Altered FDG uptake in muscles, brown adipose tissue, bone marrow, the urinary tract, and the bowel is demonstrated in a significant proportion of patients, which can hide underlying malignant foci or mimic malignant lesions. The introduction of PET/computed tomography revolutionized PET imaging, bringing much-needed anatomical information. This modality allowed better characterization of some types of uptake, particularly brown adipose tissue FDG uptake. Different approaches to minimize interference from altered FDG biodistribution should be considered when performing PET scans. Otherwise, careful review and correlation of metabolic (FDG-PET) and anatomical (computed tomography) data should be performed to accurately characterize the foci of increased FDG uptake.

Thermoregulation in mice exhibits genetic variability early in senescence

Aging leads to a loss of thermoregulation that can be readily monitored in laboratory mice. However, it is unclear from previous studies-we provide a tabular summary of 15 articles-whether significant loss occurs by midlife ( approximately 15 months of age). In this study, we examined 34 females from 22 LSXSS strains starting at 4 and 8 months of age (17 mice per age group). We used transponders inserted just under the loose skin of the pelt and calibrated against rectal body temperature to measure temperatures quickly without restraint. We found that the mean body temperatures measured 5 months later (9 and 13 months of age) had dropped significantly below normal in both groups: 0.6 masculineC lower in the younger cohort and 1.0 masculineC lower in the older cohort. These drops were not associated with weight loss or signs of pathology. Notably, the loss of thermoregulation between 8 and 13 months of age also exhibited genetic variation that was highly significant (P = 0.004). Such variation is potentially a powerful tool for determining the cause of thermoregulatory loss with age and whether this loss predicts senescence changes later in life, including the force of mortality.

Variants in neuropeptide Y receptor 1 and 5 are associated with nutrient-specific food intake and are under recent selection in Europeans

There is a large variation in caloric intake and macronutrient preference between individuals and between ethnic groups, and these food intake patterns show a strong heritability. The transition to new food sources during the agriculture revolution around 11,000 years ago probably created selective pressure and shaped the genome of modern humans. One major player in energy homeostasis is the appetite-stimulating hormone neuropeptide Y, in which the stimulatory capacity may be mediated by the neuropeptide Y receptors 1, 2 and 5 (NPY1R, NPY2R and NPY5R). We assess association between variants in the NPY1R, NPY2R and NPY5R genes and nutrient intake in a cross-sectional, single-center study of 400 men aged 40 to 80 years, and we examine whether genomic regions containing these genes show signatures of recent selection in 270 HapMap individuals (90 Africans, 90 Asians, and 90 Caucasians) and in 846 Dutch bloodbank controls. Our results show that derived alleles in NPY1R and NPY5R are associated with lower carbohydrate intake, mainly because of a lower consumption of mono- and disaccharides. We also show that carriers of these derived alleles, on average, consume meals with a lower glycemic index and glycemic load and have higher alcohol consumption. One of these variants shows the hallmark of recent selection in Europe. Our data suggest that lower carbohydrate intake, consuming meals with a low glycemic index and glycemic load, and/or higher alcohol consumption, gave a survival advantage in Europeans since the agricultural revolution. This advantage could lie in overall health benefits, because lower carbohydrate intake, consuming meals with a low GI and GL, and/or higher alcohol consumption, are known to be associated with a lower risk of chronic diseases.

Single nucleotide polymorphisms of thrifty genes for energy metabolism: evolutionary origins and prospects for intervention to prevent obesity-related diseases

The "thrifty" genotype and phenotype that save energy are detrimental to the health of people living in affluent societies. Individual differences in energy metabolism are caused primarily by single nucleotide polymorphisms (SNPs), some of which promote the development of obesity/type 2 diabetes mellitus. In this review, four major questions are addressed: (1) Why did regional differences in energy metabolism develop during evolution? (2) How do genes respond to starvation and affluence? (3) Which SNPs correspond to the hypothetical "thrifty genes"? (4) How can we cope with disease susceptibility caused by the "thrifty" SNPs? We examined mtDNA and genes for energy metabolism in people who live in several parts of Asia and the Pacific islands. We included 14 genes, and the SNP frequencies of PPAR gamma 2, LEPR, and UCP3-p and some other genes differ significantly between Mongoloids and Caucasoids. These differences in SNPs may have been caused by natural selection depending on the types of agriculture practiced in different regions. Interventions to counteract the adverse effects of "thrifty" SNPs have been partially effective.

Reduced feeding response to neuropeptide Y in senescent Fischer 344 rats

The anorexia of aging syndrome in humans is characterized by spontaneous body weight loss reflecting diminished food intake. We reported previously that old rats undergoing a similar phenomenon of progressive weight loss (i.e., senescent rats) also display altered feeding behavior, including reduced meal size and duration. Here, we tested the hypothesis that blunted responsiveness to neuropeptide Y (NPY), a feeding stimulant, occurs concurrently with senescence-associated anorexia/hypophagia. Young (8 mo old, n = 9) and old (24-30 mo old, n = 11) male Fischer 344 rats received intracerebroventricular NPY or artificial cerbrospinal fluid injections. In response to a maximum effective NPY dose (10 microg), the net increase in size of the first meal after injection was similar in old weight-stable (presenescent) and young rats (10.85 +/- 1.73 and 12.63 +/- 2.52 g/kg body wt (0.67), respectively). In contrast, senescent rats that had spontaneously lost approximately 10% of body weight had significantly lower net increases at their first post-NPY meal (1.33 +/- 0.33 g/kg body wt (0.67)) than before they began losing weight. Thus altered feeding responses to NPY occur in aging rats concomitantly with spontaneous decrements in food intake and body weight near the end of life.

Cold-induced thermoregulation and biological aging

Aging is associated with diminished cold-induced thermoregulation (CIT). The mechanisms accounting for this phenomenon have yet to be clearly elucidated but most likely reflect a combination of increased heat loss and decreased metabolic heat production. The inability of the aged subject to reduce heat loss during cold exposure is associated with diminished reactive tone of the cutaneous vasculature and, to a lesser degree, alterations in the insulative properties of body fat. Cold-induced metabolic heat production via skeletal muscle shivering thermogenesis and brown adipose tissue nonshivering thermogenesis appears to decline with age. Few investigations have directly linked diminished skeletal muscle shivering thermogenesis with the age-related reduction in cold-induced thermoregulatory capacity. Rather, age-related declines in skeletal muscle mass and metabolic activity are cited as evidence for decreased heat production via shivering. Reduced mass, GDP binding to brown fat mitochondria, and uncoupling protein (UCP) levels are cited as evidence for attenuated brown adipose tissue cold-induced nonshivering thermogenic capacity during aging. The age-related reduction in brown fat nonshivering thermogenic capacity most likely reflects altered cellular signal transduction rather than changes in neural and hormonal signaling. The discussion in this review focuses on how alterations in CIT during the life span may offer insight into possible mechanisms of biological aging. Although the preponderance of evidence presented here demonstrates that CIT declines with chronological time, the mechanism reflecting this attenuated function remains to be elucidated. The inability to draw definitive conclusions regarding biological aging and CIT reflects the lack of a clear definition of aging. It is unlikely that the mechanisms accounting for the decline in cold-induced thermoregulation during aging will be determined until biological aging is more precisely defined.

Cellular proliferation and UCP content in brown adipose tissue of cold-exposed aging Fischer 344 rats

Previous investigations have demonstrated that older vs. younger rats respond to cold exposure with blunted cold-induced nonshivering thermogenesis of brown adipose tissue (BAT). This reduction in nonshivering thermogenesis is associated with reduced mass and blunted nonshivering thermogenic capacity of BAT. The purpose of this study was to test the hypothesis that brown fat in 26-mo-old Fischer 344 (F344) male rats has an impaired capacity to respond to the trophic stimulus of chronic cold exposure with increases in cell number, mass, and uncoupling protein (UCP) content. To test this hypothesis, the response of BAT to chronic cold exposure was evaluated in young and old rats. We exposed 6-, 12-, and 26-mo-old F344 male rats to 10 degrees C for 5 days and measured interscapular BAT (IBAT) mass, cell size and proliferation, and mitochondrial UCP1 content. Plasma concentrations of insulin-like growth factor I (IGF-I) and norepinephrine IBAT mass, cell proliferation, or UCP1 content in response to chronic cold, whereas the 6-mo-old rats had a nearly 2-fold cold-induced increase in IBAT mass, a 26-fold increase in cell proliferation, and a 4-fold increase in UCP1 content. Cold exposure also produced an increase of 29, 19, and 20% in mature brown adipocyte cell size of the 6-, 12-, and 26-mo-old animals, respectively. Plasma levels of IGF-I were unaffected by cold at all ages, whereas NE levels were increased by the cold exposure and by increasing age. These data support the hypothesis that brown fat in old F344 rats does not respond to the trophic stimulus of chronic cold exposure to the same degree as younger animals. Moreover, these data indicate that the observed cold- or age-induced changes in levels of growth factors evaluated in this study were not associated with the lack of cold-induced preadipocyte proliferation or increased UCP1 in brown fat of the 26-mo-old rats.