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B B Lowell

Publications and source records attributed to B B Lowell.

At least 37 records · Page 2Linked to original sources
PubMed1999

Role of the beta(3)-adrenergic receptor and/or a putative beta(4)-adrenergic receptor on the expression of uncoupling proteins and peroxisome proliferator-activated receptor-gamma coactivator-1.

Administration of beta-adrenergic receptor (beta-AR) agonists, especially beta(3)-AR agonists, is well known to increase thermogenesis in rodents and humans. In this work we studied the role of the beta(3)-AR in regulating mRNA expression of genes involved in thermogenesis, i.e., mitochondrial uncoupling proteins UCP2 and UCP3, and peroxisome proliferator-activated receptor-gamma coactivator-1 (PGC-1), in mouse skeletal muscle. For this purpose, different beta(3)-AR agonists were administered acutely to both wild type mice and mice whose beta(3)-AR gene has been disrupted (beta(3)-AR KO mice). CL 316243 increased the expression of UCP2, UCP3 and PGC-1 in wild type mice only. By contrast, BRL 37344 and CGP 12177 increased the expression of UCP2 and UCP3 in both wild type and beta(3)-AR KO mice, whereas they increased the expression of PGC-1 in wild type mice only. Finally, acute (3 h) cold exposure increased the expression of UCP2 and UCP3, but not PGC-1, in skeletal muscle of both wild type and beta(3)-AR KO mice. These results show that selective stimulation of the beta(3)-AR affects the expression of UCP2, UCP3 and PGC-1 in skeletal muscle. This effect is probably indirect, as muscle does not seem to express beta(3)-AR. In addition, our data suggest that BRL 37344 and CGP 12177 act, in part, through an as yet unidentified receptor, possibly a beta(4)-AR.

Adrenergic beta-Agonists↗
PubMed1999

Assessment of uncoupling activity of the human uncoupling protein 3 short form and three mutants of the uncoupling protein gene using a yeast heterologous expression system.

The human uncoupling protein 3 gene generates two mRNA transcripts, uncoupling protein 3L and uncoupling protein 3S, which are predicted to encode long and short forms of the uncoupling protein 3 protein, respectively. While uncoupling protein 3L is similar in length to the other known uncoupling proteins, uncoupling protein 3S lacks the last 37 C-terminal residues. A splice site mutation in the human uncoupling protein 3 gene, resulting in the exclusive expression of uncoupling protein 3S, and a number of point mutations in the uncoupling protein 3 gene have been described. This study compares the biochemical activity of uncoupling protein 3S as well as three mutants of the uncoupling protein 3 gene (V9M, V102I, R282C) with that of uncoupling protein 3L utilizing a yeast expression system. All proteins were expressed at similar levels and had qualitatively similar effects on parameters related to the uncoupling function. Both uncoupling protein 3S and uncoupling protein 3L decreased the yeast growth rate by 35 and 52%, increased the whole yeast basal O2 consumption by 26 and 48%, respectively, and decreased the mitochondrial membrane potential as measured in whole yeast by uptake of the fluorescent potential-sensitive dye 3'3-dihexyloxacarbocyanine iodide. In isolated mitochondria, uncoupling protein 3S and uncoupling protein 3L caused a similar (33 and 35%, respectively) increase in state 4 respiration, which was relatively small compared to uncoupling protein 1 (102% increase). A truncated version of uncoupling protein 3S, lacking the last three C-terminal residues, Tyr, Lys and Gly, that are part of a carrier motif that is highly conserved among all mitochondrial carriers, had a greatly reduced uncoupling activity. The two naturally occurring uncoupling protein 3 mutants, V9M and V102I, were similar to uncoupling protein 3L with respect to effects on the yeast growth and whole yeast O2 consumption. The R282C mutant had a reduced effect compared to uncoupling protein 3L. In summary, uncoupling protein 3S and the three mutants of uncoupling protein 3 appear to be functional proteins with biochemical activities similar to uncoupling protein 3L, although uncoupling protein 3S and the R282C mutant have a modestly reduced function.

Carrier Proteins↗
PubMed1999

Assessment of uncoupling activity of uncoupling protein 3 using a yeast heterologous expression system.

Uncoupling protein 3L, uncoupling protein 1 and the mitochondrial oxoglutarate carrier were expressed in Saccharomyces cerevisae. Effects on different parameters related to the energy expenditure were studied. Both uncoupling protein 3L and uncoupling protein 1 reduced the growth rate by 49% and 32% and increased the whole yeast O2 consumption by 31% and 19%, respectively. In isolated mitochondria, uncoupling protein 1 increased the state 4 respiration by 1.8-fold, while uncoupling protein 3L increased the state 4 respiration by 1.2-fold. Interestingly, mutant uncoupling protein 1 carrying the H145Q and H147N mutations, previously shown to markedly decrease the H+ transport activity of uncoupling protein 1 when assessed using a proteoliposome system (Bienengraeber et al. (1998) Biochem. 37, 3-8), uncoupled the mitochondrial respiration to almost the same degree as wild-type uncoupling protein 1. Thus, absence of this histidine pair in uncoupling protein 2 and uncoupling protein 3 does not by itself rule out the possibility that these carriers have an uncoupling function. The oxoglutarate carrier had no effect on any of the studied parameters. In summary, a discordance exists between the magnitude of effects of uncoupling protein 3L and uncoupling protein 1 in whole yeast versus isolated mitochondria, with uncoupling protein 3L having greater effects in whole yeast and a smaller effect on the state 4 respiration in isolated mitochondria. These findings suggest that uncoupling protein 3L, like uncoupling protein 1, has an uncoupling activity. However, the mechanism of action and/or regulation of the activity of uncoupling protein 3L is likely to be different.

Animals↗
PubMed1999

Effects of obesity and stable weight reduction on UCP2 and UCP3 gene expression in humans.

OBJECTIVES: The molecular determinants of energy expenditure are presently unknown. Recently, two uncoupling protein homologues, UCP2 and UCP3, have been identified. UCP2 is expressed widely, and UCP3 is expressed abundantly in skeletal muscle. Both could be important regulators of energy balance. In this paper, we investigated whether altered UCP2 and UCP3 mRNA levels are associated with obesity or weight reduction. RESEARCH METHODS AND PROCEDURES: UCP2, UCP3 long and short mRNA levels were examined in skeletal muscle and in white adipose tissue of lean, obese, and weight-reduced individuals by RNase protection assay. RESULTS: Expression of UCP2, UCP3S, and UCP3L mRNA in skeletal muscle was similar in lean individuals and in individuals with obesity at stable weight. In contrast, UCP3L and UCP3S mRNAs were decreased by 38% (p<0.0059) and 48% (p<0.0047), respectively, in 20% weight-reduced patients with obesity at stable weight. In contrast, UCP2 mRNA levels were increased by 30% in skeletal muscle of 20% weight-reduced subjects with obesity. In a different set of patients, mostly lean, UCP3L mRNA in skeletal muscle was decreased by 28% (p = 0.0425) after 10% weight reduction at stable weight. Expression of UCP2 mRNA in subcutaneous adipose tissue was similar in lean individuals and in individuals with obesity, and was increased by 58% during active weight loss. DISCUSSION: Stabilization at reduced body weight in humans is associated with a decrease in UCP3 mRNA in muscle. It is possible that reduced UCP3 expression could contribute to decreased energy expenditure in weight-stable, weight-reduced individuals.

Adipose Tissue↗
PubMed1999

Uncoupling protein-3 (UCP3): a mitochondrial carrier in search of a function.

UCP3 is a mitochondrial protein with high homology to the established uncoupling protein, UCP1. Its high degree of homology to UCP1 suggests that UCP3 may be a true uncoupling protein. Preliminary biochemical studies are consistent with UCP3 having uncoupling activity. However, detailed functional studies are required to understand the true biochemical and physiological purpose of UCP3. These efforts should be aided by identification of humans with inactivating mutations and/or the generation of gene knockout mice lacking UCP3.

Animals↗
PubMed1999

The long isoform uncoupling protein-3 (UCP3L) in human energy homeostasis.

The biological role(s) proposed for UCP3 in energy homeostasis have been based primarily upon amino acid sequence homology to UCP1. Spontaneous mutations of UCP3> have been described in humans, but not in rodents. The functional consequences-or lack thereof-of these mutations in humans will be of great importance in elucidating the biology of this protein. The results of two such studies are summarized here.

Animals↗
PubMed1999

Regulation of skeletal muscle UCP-2 and UCP-3 gene expression by exercise and denervation.

The factors that regulate gene expression of uncoupling proteins 2 and 3 (UCP-2 and UCP-3) in skeletal muscle are poorly understood, but both genes are clearly responsive to the metabolic state of the organism. Therefore, we tested the hypothesis that denervation and acute and/or chronic exercise (factors that profoundly affect metabolism) would alter UCP-2 and UCP-3 gene expression. For the denervation studies, the sciatic nerve of rat and mouse hindlimb was sectioned in one leg while the contralateral limb served as control. Northern blot analysis revealed that denervation was associated with a 331% increase (P < 0.001) in UCP-3 mRNA and a 200% increase (P < 0. 01) in UCP-2 mRNA levels in rat mixed gastrocnemius (MG) muscle. In contrast, denervation caused a 53% decrease (P < 0.001) in UCP-3 and a 63% increase (P < 0.01) in UCP-2 mRNA levels in mouse MG. After acute exercise (2-h treadmill running), rat UCP-3 mRNA levels were elevated (vs. sedentary control) 252% (P < 0.0001) in white gastrocnemius and 63% (P < 0.05) in red gastrocnemius muscles, whereas UCP-2 levels were unaffected. To a lesser extent, elevations in UCP-3 mRNA (22%; P < 0.01) and UCP-2 mRNA (55%; P < 0.01) levels were observed after acute exercise in the mouse MG. There were no changes in either UCP-2 or UCP-3 mRNA levels after chronic exercise (9 wk of wheel running). These results indicate that acute exercise and denervation regulate gene expression of skeletal muscle UCPs.

Animals↗
PubMed1999

Cardiac hypertrophy with preserved contractile function after selective deletion of GLUT4 from the heart.

Glucose enters the heart via GLUT1 and GLUT4 glucose transporters. GLUT4-deficient mice develop striking cardiac hypertrophy and die prematurely. Whether their cardiac changes are caused primarily by GLUT4 deficiency in cardiomyocytes or by metabolic changes resulting from the absence of GLUT4 in skeletal muscle and adipose tissue is unclear. To determine the role of GLUT4 in the heart we used cre-loxP recombination to generate G4H(-/-) mice in which GLUT4 expression is abolished in the heart but is present in skeletal muscle and adipose tissue. Life span and serum concentrations of insulin, glucose, FFAs, lactate, and beta-hydroxybutyrate were normal. Basal cardiac glucose transport and GLUT1 expression were both increased approximately 3-fold in G4H(-/-) mice, but insulin-stimulated glucose uptake was abolished. G4H(-/-) mice develop modest cardiac hypertrophy associated with increased myocyte size and induction of atrial natriuretic and brain natriuretic peptide gene expression in the ventricles. Myocardial fibrosis did not occur. Basal and isoproterenol-stimulated isovolumic contractile performance was preserved. Thus, selective ablation of GLUT4 in the heart initiates a series of events that results in compensated cardiac hypertrophy.

Animals↗
PubMed1998

Mice lacking melanin-concentrating hormone are hypophagic and lean.

Feeding is influenced by hypothalamic neuropeptides that promote (orexigenic peptides) or inhibit feeding. Of these, neuropeptide Y (NPY) in the arcuate nucleus and melanin-concentrating hormone (MCH) and orexins/hypocretins in the lateral hypothalamus have received attention because their expression is increased during fasting and because they promote feeding when administered centrally. Surprisingly, absence of the orexigenic neuropeptide NPY fails to alter feeding or body weight in normal mice. As deficiency of a single component of the pathway that limits food intake (such as leptin or receptors for melanocortin-4) causes obesity, it has been suggested that orexigenic signals are more redundant than those limiting food intake. To define further the physiological role of MCH and to test the redundancy of orexigenic signals, we generated mice carrying a targeted deletion of the MCH gene. MCH-deficient mice have reduced body weight and leanness due to hypophagia (reduced feeding) and an inappropriately increased metabolic rate, despite their reduced amounts of both leptin and arcuate nucleus pro-opiomelanocortin messenger RNA. Our results show that MCH is a critical regulator of feeding and energy balance which acts downstream of leptin and the melanocortin system, and that deletion of a gene encoding a single orexigenic peptide can result in leanness.

Animals↗
PubMed1998

Adaptive thermogenesis: turning on the heat.

Brown adipocytes play important roles in the regulation of fat storage and body temperature, by virtue of their ability to uncouple mitochondrial fuel oxidation and ATP synthesis. The discovery of a tissue-specific transcriptional coactivator provides new insights into the regulation of thermogenesis by brown adipocytes.

Adaptation, Physiological↗
PubMed1998

Nutritional and insulin regulation of fatty acid synthetase and leptin gene expression through ADD1/SREBP1.

The ability to regulate specific genes of energy metabolism in response to fasting and feeding is an important adaptation allowing survival of intermittent food supplies. However, little is known about transcription factors involved in such responses in higher organisms. We show here that gene expression in adipose tissue for adipocyte determination differentiation dependent factor (ADD) 1/sterol regulatory element binding protein (SREBP) 1, a basic-helix-loop-helix protein that has a dual DNA-binding specificity, is reduced dramatically upon fasting and elevated upon refeeding; this parallels closely the regulation of two adipose cell genes that are crucial in energy homeostasis, fatty acid synthetase (FAS) and leptin. This elevation of ADD1/SREBP1, leptin, and FAS that is induced by feeding in vivo is mimicked by exposure of cultured adipocytes to insulin, the classic hormone of the fed state. We also show that the promoters for both leptin and FAS are transactivated by ADD1/SREBP1. A mutation in the basic domain of ADD1/SREBP1 that allows E-box binding but destroys sterol regulatory element-1 binding prevents leptin gene transactivation but has no effect on the increase in FAS promoter function. Molecular dissection of the FAS promoter shows that most if not all of this action of ADD1/SREBP1 is through an E-box motif at -64 to -59, contained with a sequence identified previously as the major insulin response element of this gene. These results indicate that ADD1/SREBP1 is a key transcription factor linking changes in nutritional status and insulin levels to the expression of certain genes that regulate systemic energy metabolism.

3T3 Cells↗
PubMed1998

Association between uncoupling protein polymorphisms (UCP2-UCP3) and energy metabolism/obesity in Pima indians.

The UCP2-UCP3 gene cluster maps to chromosome 11q13 in humans, and polymorphisms in these genes may contribute to obesity through effects on energy metabolism. DNA sequencing of UCP2 and UCP3 revealed three polymorphisms informative for association studies: an Ala-->Val substitution in exon 4 of UCP2, a 45 bp insertion/deletion in the 3'-untranslated region of exon 8 of UCP2 and a C-->T silent polymorphism in exon 3 of UCP3. Initially, 82 young (mean age = 30 +/- 7 years), unrelated, full-blooded, non-diabetic Pima Indians were typed for these polymorphisms by direct sequencing. The three sites were in linkage disequilibrium ( P < 0.00001). The UCP2 variants were associated with metabolic rate during sleep (exon 4, P = 0.007; exon 8, P = 0.016) and over 24 h (exon 8, P = 0.038). Heterozygotes for UCP2 variants had higher metabolic rates than homozygotes. The UCP3 variant was not significantly associated with metabolic rate or obesity. In a further 790 full-blooded Pima Indians, there was no significant association between the insertion/deletion polymorphism and body mass index (BMI). However, when only individuals >45 years of age were considered, heterozygotes (subjects with the highest sleeping metabolic rate) had the lowest BMI (P = 0.04). The location of the insertion/deletion polymorphism suggested a role in mRNA stability; however, it appeared to have no effect on skeletal muscle UCP2 mRNA levels in a subset of 23 randomly chosen Pima Indians. In conclusion, these results suggest a contribution from UCP2 (or UCP3) to variation in metabolic rate in young Pima Indians which may contribute to overall body fat content later in life.

Adolescent↗
PubMed1998

Differential regulation of uncoupling protein gene homologues in multiple tissues of hibernating ground squirrels.

Nonshivering thermogenesis in brown adipose tissue (BAT) provides heat through activation of a mitochondrial uncoupling protein (UCP1), which causes futile electron transport cycles without the production of ATP. Recent discovery of two molecular homologues, UCP2, expressed in multiple tissues, and UCP3, expressed in muscle, has resulted in investigation of their roles in thermoregulatory physiology and energy balance. To determine the expression pattern of Ucp homologues in hibernating mammals, we compared relative mRNA levels of Ucp1, -2, and -3 in BAT, white adipose tissue (WAT), and skeletal muscle of arctic ground squirrels (Spermophilus parryii) hibernating at different ambient and body temperatures, with levels determined in tissues from ground squirrels not in hibernation. Here we report significant increases in mRNA levels for Ucp2 in WAT (1. 6-fold) and Ucp3 in skeletal muscle (3-fold) during hibernation. These results indicate the potential for a role of UCP2 and UCP3 in thermal homeostasis during hibernation and indicate that parallel mechanisms and multiple tissues could be important for nonshivering thermoregulation in mammals.

Adipose Tissue↗
PubMed1998

Severe leptin resistance in brown fat-deficient uncoupling protein promoter-driven diphtheria toxin A mice despite suppression of hypothalamic neuropeptide Y and circulating corticosterone concentrations.

Brown adipose tissue (BAT) has the capacity for uncoupled mitochondrial respiration and is proposed to be a key site for regulating energy expenditure in rodents. To better define the role of BAT in energy homeostasis, we previously created a line of transgenic mice with deficiency of BAT (UCP promoter-driven diphtheria toxin A transgenic mice [UCP-DTA]) mice. These mice develop obesity that initially is due to decreased energy expenditure and later accompanied by hyperphagia despite increased levels of circulating leptin. In addition, the obesity of these mice is accompanied by severe insulin-resistant diabetes and hyperlipidemia. To better define the basis for leptin resistance in this model, we treated UCP-DTA mice with leptin (300 microg i.p., b.i.d.) and compared their response with that of leptin-treated ob/ob and FVB control mice (30 microg i.p., b.i.d.). Leptin treatment of FVB and ob/ob mice decreased their body weight and food intake and improved their glucose homeostasis. In contrast, tenfold higher dosages of leptin had no effect on body weight, food intake, or circulating insulin or glucose concentrations of UCP-DTA mice. Hypothalamic neuropeptide Y (NPY) mRNA expression was lower in UCP-DTA mice than in littermate control FVB mice in the fed state, and increased progressively in response to food restriction as leptin levels fell. In parallel to the levels of hypothalamic NPY, corticosterone levels were initially suppressed and rose with food restriction. Thus food intake, body weight, and insulin and glucose homeostasis of UCP-DTA mice are all extraordinarily resistant to leptin, whereas hypothalamic NPY and the hypothalamopituitary adrenal (HPA) axis may remain under leptin control. Further elucidation of the mechanisms underlying leptin resistance in UCP-DTA mice may provide valuable insights into the basis for leptin resistance in human obesity.

Adipose Tissue, Brown↗
PubMed1998

Mice expressing human but not murine beta3-adrenergic receptors under the control of human gene regulatory elements.

Beta-adrenergic receptors (ARs) are expressed predominantly in adipose tissue, and beta3-selective agonists are effective anti-obesity drugs in rodents. Rodent and human beta3-ARs differ with respect to expression in white versus brown adipocytes as well as their ability to be stimulated by beta3-AR-selective agonists. Humans express beta3-AR mRNA abundantly in brown but not white adipocytes, while rodents express beta3-AR mRNA abundantly in both sites. To determine the basis for this difference, we have transgenically introduced 74 kilobases (kb) of human beta3-AR genomic sequence into gene knockout mice lacking beta3-ARs. Importantly, human beta3-AR mRNA was expressed only in brown adipose tissue (BAT) of transgenic mice, with little or no expression being detected in white adipose tissue (WAT), liver, stomach, small intestine, skeletal muscle, and heart. This pattern of expression differed from that observed in mice bearing a murine beta3-AR genomic transgene in which beta3-AR mRNA was expressed in both WAT and BAT, but not in other sites. Furthermore, we have transgenically introduced smaller human constructs containing -14.5 and -0.6 kb of upstream sequence into beta3-AR gene knockout mice. Both -14.5 and -0.6 kb constructs were expressed in BAT but not WAT. Thus, human but not murine cis-regulatory elements direct beta3-AR gene expression preferentially to brown adipocytes. Identification of responsible cis-regulatory element(s) and relevant trans-acting factor(s) should provide insight into mechanisms controlling human beta3-AR gene expression. In addition, the beta3-AR agonist, CGP-12177, stimulated oxygen consumption in mice expressing human but not murine beta3-ARs by 91% compared with only 49% in control beta3-AR gene knockout mice, demonstrating that the human beta3-AR can functionally couple with energy expenditure. These "humanized" mice should assist us in the development of drugs that may become effective anti-obesity agents in humans.

Adipose Tissue↗
PubMed1998

Obesity after genetic ablation of brown adipose tissue.

Brown adipose tissue (BAT) has been proposed to play an important role in the regulation of energy balance. The unique presence of uncoupling protein (UCP) permits BAT to expend calories unrelated to the performance of work with the net result being the generation of heat. The role of BAT in mediating diet-induced thermogenesis had led to the suggestion that BAT activity contributes to metabolic inefficiency and, as such, might provide a cellular and molecular explanation for protection from obesity. In order to directly test this hypothesis, we recently created mice with isolated BAT deficiency by using a suicide DNA transgenic vector in which regulatory elements of the UCP gene were used to drive brown fat specific expression of diptheria toxin A-chain (DTA). Transgenic mice are characterized by reduced energy expenditure and marked obesity, associated with insulin resistance and NIDDM with both receptor and post-receptor components. Feeding of a "Western diet" which derives 41% of its calories from fat leads to a synergistic effect on the development of obesity and its accompanying disorders in transgenics. The results of our studies support a critical role for BAT in the nutritional homeostasis of mice and suggest that the intact thermogenic function of BAT is required for protection from diet induced obesity. Obese UCP-DTA mice have many features in common with obesity as it appears in most humans, and should therefore be a useful model that may aid studies of the pathogenesis and treatment of human obesity, NIDDM and their complications.

Adipose Tissue, Brown↗
PubMed1998

Beta-3 adrenergic receptor agonists cause an increase in gastrointestinal transit time in wild-type mice, but not in mice lacking the beta-3 adrenergic receptor.

The effects of beta-3 adrenergic receptor (beta3-AR) agonists on gastrointestinal (GI) motility, as reported by stomach retention and intestinal transit of radiolabelled charcoal, were compared in wild-type (WT) mice and in transgenic mice lacking beta3-AR (beta3-AR[KO]) or having beta3-AR in white and brown adipose tissue only (beta3-AR[WAT+BAT]). After s.c. administration of 3 mg/kg of the selective, rodent specific beta3-AR agonists BRL 35135, CL 316, 243 or ICI 198,157, WT mice exhibited a significant decrease in the extent of movement of radiotracer through the stomach and intestines, indicative of decreased GI motility. These compounds also caused an increase in plasma glycerol levels in the WT mice, suggesting that increased lipolysis in adipose tissue had been evoked. None of these compounds had an effect on GI motility or evoked lipolysis in the beta3-AR[KO] mice. Treatment of WT mice with SR 56811A, a beta3-AR agonist that exhibited a relatively lower affinity for rodent beta3-AR in vitro, did not affect GI motility or plasma glycerol levels in WT or beta3[KO] mice when administered s.c. at 3 mg/kg. Clonidine, an alpha-2 adrenergic receptor agonist, used as a positive control in these GI studies, caused a decrease in GI motility in both WT and beta3-AR[KO] mice. These results are consistent with a postulated role for beta3-AR in regulation of GI motility in the mouse. However, treatment of beta3-AR[WAT+BAT] mice with 3 mg/kg BRL 35135 resulted in elevated plasma glycerol levels, as well as increased stomach retention and decreased intestinal transit of radiotracer. These results suggest that this beta3-AR agonist may exert its effects on the GI tract indirectly, through an unknown signaling mechanism activated by agonism of beta3-AR in adipose tissue.

Adrenergic beta-Agonists↗