PubMed Health⌕ Search

Biomedical subjects

B B Lowell

Publications and source records attributed to B B Lowell.

At least 55 records · Page 3Linked to original sources
PubMed1997

The human uncoupling protein-3 gene. Genomic structure, chromosomal localization, and genetic basis for short and long form transcripts.

Uncoupling protein-3 (UCP3) is a recently identified candidate mediator of adaptive thermogenesis in humans. Unlike UCP1 and UCP2, UCP3 is expressed preferentially and at high levels in human skeletal muscle and exists as short and long form transcripts, UCP3S and UCP3L. UCP3S is predicted to encode a protein which lacks the last 37 C-terminal residues of UCP3L. In the present study, we have defined the intron-exon structure for the human UCP3 gene and determined that UCP3S is generated when a cleavage and polyadenylation signal (AATAAA) located in the last intron prematurely terminates message elongation. In addition we have mapped UCP3 to the distal segment of human chromosome 11q13 (between framework markers D11S916 and D11S911), adjacent to UCP2. Of note, UCP2 and UCP3 in both mice and humans colocalize in P1 and BAC genomic clones indicating that these two UCPs are located within 75-150 kilobases of each other and most likely resulted from a gene duplication event. Previous studies have noted that mouse UCP2 maps to a region of chromosome 7 which is coincident with three independently mapped quantitative trait loci for obesity. Our study shows that UCP3 is also coincident with these quantitative trait loci raising the possibility that abnormalities in UCP3 are responsible for obesity in these models.

Animals↗
PubMed1997

Beta3-adrenergic receptors on white and brown adipocytes mediate beta3-selective agonist-induced effects on energy expenditure, insulin secretion, and food intake. A study using transgenic and gene knockout mice.

beta3-Adrenergic receptors (beta3-ARs) are expressed predominantly on white and brown adipocytes, and acute treatment of mice with CL 316,243, a potent and highly selective beta3-AR agonist, produces a 2-fold increase in energy expenditure, a 50-100-fold increase in insulin levels, and a 40-50% reduction in food intake. Recently, we generated gene knockout mice lacking functional beta3-ARs and demonstrated that each of these responses were mediated exclusively by beta3-ARs. However, the tissue site responsible for producing these actions is unknown. In the present study, genetically engineered mice were created in which beta3-ARs are expressed exclusively in white and brown adipocytes (WAT+BAT-mice), or in brown adipocytes only (BAT-mice). This was accomplished by injecting tissue-specific beta3-AR transgenic constructs into mouse zygotes homozygous for the beta3-AR knockout allele. Control, knockout, WAT+BAT, and BAT-mice were then treated acutely with CL, and the effects on various parameters were assessed. As previously observed, all effects of CL were completely absent in gene knockout mice lacking beta3-ARs. The effects on O2 consumption, insulin secretion, and food intake were completely rescued with transgenic re-expression of beta3-ARs in white and brown adipocytes (WAT+BAT-mice), demonstrating that each of these responses is mediated exclusively by beta3-ARs in white and/or brown adipocytes, and that beta3-ARs in other tissue sites were not required. Importantly, transgenic re-expression of beta3-ARs in brown adipocytes only (BAT-mice) failed to rescue, in any way, CL-mediated effects on insulin levels and food intake and only minimally restored effects on oxygen consumption, indicating that any effect on insulin secretion and food intake, and a full stimulation of oxygen consumption required the presence of beta3-ARs in white adipocytes. The mechanisms by which beta3-AR agonist stimulation of white adipocytes produces these responses are unknown but may involve novel mediators not previously known to effect these processes.

Adipocytes↗
PubMed1997

UCP3: an uncoupling protein homologue expressed preferentially and abundantly in skeletal muscle and brown adipose tissue.

Uncoupling proteins (UCPs) are inner mitochondrial membrane transporters which dissipate the proton gradient, releasing stored energy as heat. UCP1 is expressed exclusively in brown adipocytes while UCP2 is expressed widely. We now report the molecular cloning of a third uncoupling protein homologue, designated UCP3. At the amino acid level, hUCP3 is 71% identical to hUCP2 and 57% identical to hUCP1. UCP3 is distinguished from UCP1 and UCP2 by its abundant and preferential expression in skeletal muscle in humans, and brown adipose tissue and skeletal muscle in rodents. Since skeletal muscle and brown adipose tissue are believed to be important sites for regulated energy expenditure in humans and rodents, respectively, UCP3 may be an important mediator of adaptive thermogenesis. Since UCP3 is minimally expressed in human heart and other critical organs, it is a promising target for anti-obesity drug development aimed at increasing thermogenesis.

Adipose Tissue, Brown↗
PubMed1997

Functional antagonism between CCAAT/Enhancer binding protein-alpha and peroxisome proliferator-activated receptor-gamma on the leptin promoter.

The ob gene product, leptin, is a major hormonal regulator of appetite and fat cell mass. Recent work has suggested that the antidiabetic agents, the thiazolidinediones (TZ), which are also high affinity ligands of peroxisome proliferator-activated receptor-gamma (PPARgamma), inhibit leptin expression in rodents. To examine the effects of this class of drug on the leptin gene in adipocytes we performed Northern analysis on primary rat adipocytes cultured in the presence or absence of TZ. TZ reduced leptin mRNA levels by 75%. To determine whether this effect was mediated at the transcriptional level, we isolated 6510 base pairs of 5'-flanking sequence of the leptin promoter and studied reporter constructs in primary rat adipocytes and CV-1 cells. Sequence analysis demonstrated the presence of a consensus direct repeat with a 1-base-pair gap site between -3951 and -3939 as well as a consensus CCAAT/enhancer binding protein (C/EBP) site between -55 and -47. Our functional analysis in transfected primary rat adipocytes demonstrates that, despite the presence of a canonical direct repeat with a 1-base-pair gap site, TZ alone decreases reporter gene expression of leptin promoter constructs ranging from -6510 to +9 to -65 to +9. In CV-1 cells, which contain endogenous PPARgamma, TZ treatment alone had little effect on these constructs. However, TZ treatment did inhibit C/EBPalpha-mediated transactivation of the leptin promoter. This down-regulation of leptin reporter constructs mapped to a -65 to +9 promoter fragment which binds C/EBPalpha in gel-mobility shift assays but does not bind PPARgamma2 alone or as a heterodimer with 9-cis-retinoic acid receptor. Conversely, the promoter (-5400 to +24 base pairs) of the aP2 gene, another adipocyte-specific gene, was induced 7.3-fold by TZ. Co-transfection with C/EBPalpha minimally stimulated the aP2 promoter from basal levels but notably blocked activation by TZ. These data indicate that PPARgamma and C/EBPalpha can functionally antagonize each other on at least two separate promoters and that this mechanism may explain the down-regulation of leptin expression by thiazolidinediones.

Adipocytes↗
PubMed1997

Multiple cytokines and acute inflammation raise mouse leptin levels: potential role in inflammatory anorexia.

Several inflammatory cytokines, most notably tumor necrosis factor (TNF) and IL-1, induce anorexia and loss of lean body mass, common manifestations of acute and chronic inflammatory conditions. In C57BL/6 female mice, the administration of TNF, IL-1, and, to a lesser extent, leukemia inhibitory factor (LIF), produced a prompt and dose-dependent increase in serum leptin levels and leptin mRNA expression in fat. IL-10, IL-4, ciliary neurotrophic factor, and IL-2, cytokines not known to induce anorexia or decrease food intake, had no effect on leptin gene expression or serum leptin levels. After administration of Escherichia coli lipopolysaccharide (LPS), leptin gene expression and leptin levels were increased. These findings suggest that leptin levels may be one mechanism by which anorexia is induced during acute inflammatory conditions.

Adipose Tissue↗
PubMed1997

Brown adipose tissue, beta 3-adrenergic receptors, and obesity.

Brown adipose tissue is distinguished by its unique capacity for uncoupled mitochondrial respiration, which is highly regulated by sympathetic nerve activity. Because of this, energy expenditure in brown fat is capable of ranging over many orders of magnitude. The fact that the function of brown adipose tissue is impaired in obese rodents and that transgenic mice with decreased brown fat develop obesity demonstrates the importance of brown fat in maintaining nutritional homeostasis. However, the role of brown fat in humans is less clear. beta 3-Adrenergic receptors are found on brown adipocytes, and treatment with beta 3-selective agonists markedly increases energy expenditure and decreases obesity in rodents. Whether beta 3-selective agonists will be effective anti-obesity agents in humans is presently under investigation.

Adipose Tissue, Brown↗
PubMed1997

Immunohistochemical localization of leptin and uncoupling protein in white and brown adipose tissue.

Leptin is synthesized exclusively by adipocytes and acts on the hypothalamus to regulate energy balance. Previous messenger RNA expression studies demonstrated that leptin is expressed in white adipocytes and also in brown adipose tissue, however expression in brown fat is markedly lower than in white fat. This suggests the possibility that leptin expression in brown adipose tissue is due to the presence of white adipocytes that reside within brown adipose tissue, and that brown adipocytes actually do not express leptin. To address this point, we performed immunohistochemistry on paraffin sections and studied leptin protein expression in different depots of white and brown fat of lean and obese (db/db) mice. To establish the cell type expressing leptin, we also assessed the size and organization of lipid droplets, the ultrastructural features of mitochondria, and the presence or absence of uncoupling protein, a brown fat-specific marker. In white adipose tissue of lean and obese (db/db) mice, leptin protein was expressed in adipocytes of various sizes (range examined: 19.67-200 microns), including adipocytes at the multilocular stage of differentiation. Leptin staining was more intense in some depots (retroperitoneal), and appeared to decrease with fasting. In brown adipose tissue of lean animals, multilocular uncoupling protein (UCP)-positive brown adipocytes had typical brown mitochondria and were leptin-negative, both in fed and fasted conditions. At the periphery of the interscapular brown adipose tissue depot, unilocular, UCP-negative adipocytes (mean diameter: 41.55 microns) with white-type mitochondria were observed, and these cells were leptin-positive. In obese (db/db) animals, brown fat was composed mainly of small unilocular, UCP-positive. adipocytes (mean diameter: 40.08 microns), which were also leptin-positive. At the periphery of the organ, numerous large, unilocular, UCP-negative adipocytes (mean diameter: 73.65 microns) with white-like mitochondria were present. As expected, these cells were also leptin-positive. In summary, classical brown adipocytes differ from white adipocytes, not only by their morphology and UCP expression, but also by their apparent lack of detectable leptin expression. db/db brown adipocytes, however, were unilocular and leptin-positive. The molecular mechanisms mediating expression of leptin in white but not brown adipocytes of lean animals, and the significant expression of leptin in brown adipocytes of db/db mice will be the focus of future studies.

Adipocytes↗
PubMed1996

Regulation of PPAR gamma gene expression by nutrition and obesity in rodents.

The orphan nuclear receptor, peroxisome proliferator-activated receptor (PPAR) gamma, is implicated in mediating expression of fat-specific genes and in activating the program of adipocyte differentiation. The potential for regulation of PPAR gamma gene expression in vivo is unknown. We cloned a partial mouse PPAR gamma cDNA and developed an RNase protection assay that permits simultaneous quantitation of mRNAs for both gamma l and gamma 2 isoforms encoded by the PPAR gamma gene. Probes for detection of adipocyte P2, the obese gene product, leptin, and 18S mRNAs were also employed. Both gamma l and gamma 2 mRNAs were abundantly expressed in adipose tissue. PPAR gamma 1 expression was also detected at lower levels in liver, spleen, and heart; whereas, gamma l and gamma 2 mRNA were expressed at low levels in skeletal muscle. Adipose tissue levels of gamma l and gamma 2 were not altered in two murine models of obesity (gold thioglucose and ob/ob), but were modestly increased in mice with toxigene-induced brown fat ablation uncoupling protein diphtheria toxin A mice. Fasting (12-48 h) was associated with an 80% fall in PPAR gamma 2 and a 50% fall in PPAR gamma mRNA levels in adipose tissue. Western blot analysis demonstrated a marked effect of fasting to reduce PPAR gamma protein levels in adipose tissue. Similar effects of fasting on PPAR gamma mRNAs were noted in all three models of obesity. Insulin-deficient (streptozotocin) diabetes suppressed adipose tissue gamma l and gamma 2 expression by 75% in normal mice with partial restoration during insulin treatment. Levels of adipose tissue PPAR gamma 2 mRNA were increased by 50% in normal mice exposed to a high fat diet. In obese uncoupling protein diphtheria toxin A mice, high fat feeding resulted in de novo induction of PPAR gamma 2 expression in liver. We conclude (a) PPAR gamma 2 mRNA expression is most abundant in adipocytes in normal mice, but lower level expression is seen in skeletal muscle; (b) expression of adipose tissue gamma1 or gamma2 mRNAs is increased in only one of the three models of obesity; (c) PPAR gamma 1 and gamma 2 expression is downregulated by fasting and insulin-deficient diabetes; and (d) exposure of mice to a high fat diet increases adipose tissue expression of PPAR gamma (in normal mice) and induces PPAR gamma 2 mRNA expression in liver (in obese mice). These findings demonstrate in vivo modulation of PPAR gamma mRNA levels over a fourfold range and provide an additional level of regulation for the control of adipocyte development and function.

Adipocytes↗
PubMed1996

Decreased brown fat markedly enhances susceptibility to diet-induced obesity, diabetes, and hyperlipidemia.

Previous studies have indicated that rodents are relatively resistant to diet-induced obesity and that this resistance may be mediated in part by the capacity for diet-induced thermogenesis in brown adipose tissue (BAT). To test this hypothesis, we fed UCP-DTA transgenic with toxigene-mediated ablation of BAT and their control littermates a "Western diet" [21% (wt/wt) fat] or normal mouse chow [6.5% (wt/wt) fat]. The diets were begun at weaning (19 days old). At the age of 12 weeks, transgenic mice receiving the Western diet were markedly obese. The increased body weight and total body lipid content were significantly greater in transgenic mice receiving the Western diet than were the additive individual effects of Western diet (in control mice) and decreased BAT (in chow-fed mice), suggesting a synergistic interaction between diminished BAT and diet. A synergistic effect of Western diet and BAT ablation was also observed for morbid metabolic complications, such as insulin resistance, hyperglycemia, and hyperlipidemia. These metabolic changes were accompanied by increased expression of tumor necrosis factor-alpha and decreased expression of GLUT4 and beta 3-adrenergic receptor messenger RNA levels in white adipose tissue of UCP-DTA transgenic mice receiving the Western diet compared to those in the other experimental groups. As previously described, transgenic mice with diminished brown fat are hyperphagic. Of note, the degree of hyperphagia in transgenics compared to controls was similar whether the animals were fed chow or a Western diet. Thus, the synergistic effect of Western diet on obesity in transgenic mice was not mediated by a further stimulation of food intake. Overall, this study demonstrates the existence of a synergistic interaction between decreased BAT and Western diet to cause marked obesity and its accompanying disorders, such as insulin resistance and hyperlipidemia, and gives further support for the view that an important function of BAT is protection from diet-induced obesity, diabetes, and insulin resistance.

Adipose Tissue, Brown↗
PubMed1996

Activation of beta(3) adrenergic receptors suppresses leptin expression and mediates a leptin-independent inhibition of food intake in mice.

To examine potential interactions between leptin and the beta3 adrenergic system in the regulation of food intake, we determined the effects of treatment with a selective beta3 adrenergic receptor (AR) agonist (CL 316,243 [1 mg/kg]) on body weight, food intake, and leptin expression. Studies were carried out in C57Bl/6J and FVB male control mice as well as in mice with targeted disruption of the beta3 AR gene. These findings were correlated with measurement of the expression in hypothalamus of neuropeptide Y (NPY) and melanin concentrating hormone (MCH), two neuropeptides that may be involved in the central regulation of food intake. Treatment with CL 316,243 (1 mg/kg) for 12 or 24 h decreased leptin mRNA abundance and circulating levels to 20% of baseline in normal animals. No effect of the CL 316,243 compound was seen in mice with targeted disruption of the beta3 AR gene. Despite the failing leptin levels, beta3 agonist administration acutely suppressed food intake. Finally, the induced suppression of food intake and leptin levels occurred despite unchanged or increased hypothalamic expression of the orexigenic neuropeptides NPY and MCH. Thus, beta3 AR agonists via beta3 ARs suppress leptin levels acutely and simultaneously suppress food intake via a mechanism that operates downstream of leptin and two of its putative central targets.

Adipose Tissue↗
PubMed1995

Targeted disruption of the beta 3-adrenergic receptor gene.

beta 3-Adrenergic receptors (beta 3-ARs) are expressed predominantly in white and brown adipose tissue, and beta 3-selective agonists are potential anti-obesity drugs. However, the role of beta 3-ARs in normal physiology is unknown. To address this issue, homologous recombination was used to generate mice that lack beta 3-ARs. This was accomplished by direct injection of a DNA-targeting construct into mouse zygotes. Twenty-three transgenic mice were generated, of which two had targeted disruption of the beta 3-AR gene. Mice that were homozygous for the disrupted allele had undetectable levels of intact beta 3-AR mRNA, as assessed by RNase protection assay and Northern blotting, and lacked functional beta 3-ARs, as demonstrated by complete loss of beta 3-agonist (CL 316,243)-induced stimulation of adenylate cyclase activity and lipolysis. beta 3-AR-deficient mice had modestly increased fat stores (females more than males), indicating that beta 3-ARs play a role in regulating energy balance. Importantly, beta 1 but not beta 2-AR mRNA levels up-regulated in white and brown adipose tissue of beta 3-AR-deficient mice (brown more than white), strongly implying that beta 3-ARs mediate physiologically relevant signaling under normal conditions and that "cross-talk" exists between beta 3-ARs and beta 1-AR gene expression. Finally, acute treatment of normal mice with CL 316,243 increased serum levels of free fatty acids (FFAs) (3.2-fold) and insulin (140-fold), increased energy expenditure (2-fold), and reduced food intake (by 45%). These effects were completely absent in beta 3-AR-deficient mice, proving that the actions of CL are mediated exclusively by beta 3-ARs. beta 3-AR-deficient mice should be useful as a means to a better understanding of the physiology and pharmacology of beta 3-ARs.

Adenylyl Cyclases↗
PubMed1995

Leptin levels reflect body lipid content in mice: evidence for diet-induced resistance to leptin action.

The regulation of body weight and composition involves input from genes and the environment, demonstrated, for example, by the variable susceptibility of inbred strains of mice to obesity when offered a high-fat diet. The identification of the gene responsible for obesity in the ob/ob mouse provides a new approach to defining links between diet and genetics in the regulation of body weight. The ob gene protein product, leptin, is an adipocyte-derived circulating protein. Administration of recombinant leptin reduces food intake and increases energy expenditure in ob/ob mice, suggesting that it signals to the brain the magnitude of fat stores. Information on the regulation of this protein is limited. In several rodent models of obesity including db/db, fa/fa, yellow (Ay/a) VMH-lesioned, and those induced by gold thioglucose, monosodium glutamate, and transgenic ablation of brown adipose tissue, leptin mRNA expression and the level of circulating leptin are increased, suggesting resistance to one or more of its actions. We have assessed the impact of increased dietary fat on circulating leptin levels in normal FVB mice and FVB mice with transgene-induced ablation of brown adipose tissue. We find that high-fat diet evokes a sustained increase in circulating leptin in both normal and transgenic mice, with leptin levels accurately reflecting the amount of body lipid across a broad range of body fat. However, despite increased leptin levels, animals fed a high-fat diet became obese without decreasing their caloric intake, suggesting that a high content of dietary fat changes the 'set point' for body weight, at least in part by limiting the action of leptin.

Amino Acid Sequence↗
PubMed1995

Expression of ob mRNA and its encoded protein in rodents. Impact of nutrition and obesity.

The mutant gene responsible for obesity in the ob/ob mouse was recently identified by positional cloning (Zhang Y., R. Proenca, M. Maffel, M. Barone, L. Leopold, and J.M. Friedman. 1994. Nature (Lond.) 372:425). The encoded protein and to represent and "adipostat" signal reflecting the state of energy stores. We confirm that the adipocyte is the source of ob mRNA and that the predicted 16-kD ob protein is present in rodent serum as detected by Western blot. To evaluate the hypothesis that it might represent an adipostat, we assessed serum levels of ob protein and expression of ob mRNA in adipose cells and tissue of rodents in response to a variety of perturbations which effect body fat mass. Both ob protein and ob mRNA expression are markedly increased in obesity. The levels of ob protein are approximately 5-10-fold elevated in serum of db/db mice, in mice with hypothalamic lesions caused by neonatal administration of monosodium glutamate (MSG), and in mice with toxigene induced brown fat ablation, (UCP-DTA). Very parallel changes are observed in adipocyte ob mRNA expression in these models and in ob/ob mice. As predicted however, no serum ob protein could be detected in the ob/ob mice. By contrast to obesity, starvation of normal rats and mice for 1-3 d markedly suppresses ob mRNA abundance, and this is reversed with refeeding. Similarly, ob protein concentration in normal mice falls to undetectable levels with starvation. In the ob/ob, UCP-DTA and MSG models, overexpression of ob mRNA is reversed by caloric restriction. These data support the hypothesis that expression of ob mRNA and protein are regulated as a function of energy stores, and that ob serves as a circulating feedback signal to sites involved in regulation of energy homeostasis.

Adipocytes↗
PubMed1995

Characterization of insulin resistance and NIDDM in transgenic mice with reduced brown fat.

We recently created a new model of murine obesity through transgenic ablation of brown adipose tissue (BAT) using a tissue-specific toxigene (6). The goal of the present study was to further define the altered glucose homeostasis and insulin resistance in these transgenic animals. Despite an approximately 30% increase in total body lipid, no abnormalities were observed in 6-week-old transgenic animals. At the age of 22-26 weeks, marked obesity in transgenic mice was associated with significant increases in blood glucose and plasma insulin levels and an abnormal response to both intraperitoneal glucose and insulin tolerance tests. Glucose transport in soleus muscle was reduced, with the response to insulin stimulation blunted by up to 85% in males and 55% in females. The total number of insulin receptors was decreased by 36% in muscle and 59% in adipose tissue of transgenic animals. Insulin receptor tyrosine kinase activity, which was assessed following maximal insulin stimulation in vivo, was reduced in transgenic animals by 59% in muscle and 56% in fat. GLUT4 mRNA and protein was unchanged in muscle of transgenic animals compared with in that of controls but was significantly reduced in adipose tissue. In conclusion, primary BAT deficiency results in the development of glucose intolerance or diabetes and severe insulin resistance with both receptor and postreceptor components. These animals should be a useful model for studies of obesity-linked diabetes and insulin resistance and related complications.

Adipose Tissue↗
PubMed1994

Concentrations of adipsin in blood and rates of adipsin secretion by adipose tissue in humans with normal, elevated and diminished adipose tissue mass.

Adipsin, which is identical to complement factor D, is synthesized by fat cells, circulates in the bloodstream and is profoundly deficient in mice with genetic and hypothalamic obesity. With the recent cloning of human adipsin, a quantitative human immunoassay has been developed. In the present study, we measured adipsin blood concentrations in humans with increased and decreased adipose stores as well as adipsin secretion by adipose tissue obtained from lean and obese individuals. The results demonstrate that adipsin is released by human adipose tissue fragments as has previously been shown in mice, and that, in contrast to obese mice, blood adipsin concentrations were not reduced in the obese humans tested in this study. We also observed that blood adipsin concentrations can vary as a function of feeding or adiposity, in that they tend to be mildly elevated in obese individuals or mildly reduced in individuals with total lipo-atrophy, cachexia related to AIDS and anorexia nervosa. Thus, the circulating concentration of adipsin tends to correlate positively with degree of adiposity. Clearly, no deficiency in blood adipsin concentrations or adipsin secretion by adipose tissue was observed in the obese individuals studied.

Adipose Tissue↗