Exercise in a pill: feasibility of energy expenditure targets.

The possibility of developing a pill to increase energy expenditure is explored by examining the metabolic processes involved. Such a pill should be targeted at organ systems involved in facultative thermogenesis. In rodents, these are brown adipose tissue (BAT) and skeletal muscle. Since BAT-mediated thermogenesis is not available in adult humans, emphasis here is on skeletal muscle. A hypothesis is presented based on three known facts: (1) plasticity of skeletal muscle, with interconversion of fiber types that differ in their fuel efficiency; (2) presence of thyroxine 5'-deiodinase type 2 (TD2) in human skeletal muscle; (3) gradual increase in thermogenesis that occurs during rehabilitation after starvation, probably in muscle. A low capacity thermogenic system, muscle efficiency thermogenesis (MET), is proposed to occur as adipose stores refill during the transition from famine to feasting to obesity. This system involves increased activity of TD2 and a T3-induced increase in proportion of type II fibers, less efficient at rest and during activity. The protective effect of this system is probably overwhelmed by long-term eating in excess of energy needs. Better understanding of the complex remodeling of differentiated muscle fibers in the conversions proposed and of the regulation of TD2 activity in human skeletal muscle may reveal targets for increasing energy expenditure in humans. In addition, the possibility of exploiting the plasticity of the adipose organ, with conversion of white adipocytes in white adipose tissue to atypical brown adipocytes and increasing thermogenesis in them is considered as another potential target for increasing energy expenditure in humans.

Physiological role of UCP3 may be export of fatty acids from mitochondria when fatty acid oxidation predominates: an hypothesis.

This hypothesis proposes a physiological role for uncoupling protein-3 (UCP3) in the export of fatty acid anions from muscle and brown adipose tissue (BAT) mitochondria when fatty acids are the predominant substrate being used. It proposes that excess acyl CoA within the mitochondria is hydrolyzed by a mitochondrial acyl CoA thioesterase, yielding fatty acid anion and CoASH. The fatty acid anion is exported to the cytosol by being carried across the inner mitochondrial membrane by UCP3. The CoASH is conserved within the mitochondrion to participate in other reactions for which it is needed during fatty acid oxidation in the beta-oxidation cycle and in the tricarboxylic acid cycle. The export of the fatty acid anion thus permits continued rapid fatty acid oxidation in the face of an oversupply. The hypothesis provides a logical explanation for the observed up-regulation of gene expression for UCP3 in muscle when there is a switch to fatty acid oxidation, as during fasting, and in BAT when fatty acid oxidation is stimulated, as during exposure to cold. It provides a plausible physiological role for UCP3 as a transporter protein, not as an uncoupling protein.

Sim1 haploinsufficiency causes hyperphagia, obesity and reduction of the paraventricular nucleus of the hypothalamus.

The bHLH-PAS transcription factor SIM1 is required for the development of the paraventricular nucleus (PVN) of the hypothalamus. Mice homozygous for a null allele of Sim1 (Sim1(-/-)) lack a PVN and die perinatally. In contrast, we show here that Sim1 heterozygous mice are viable but develop early-onset obesity, with increased linear growth, hyperinsulinemia and hyperleptinemia. Sim1(+/-) mice are hyperphagic but their energy expenditure is not decreased, distinguishing them from other mouse models of early-onset obesity such as deficiencies in leptin and melanocortin receptor 4. Quantitative histological comparison with normal littermates showed that the PVN of Sim1(+/-) mice contains on average 24% fewer cells without a selective loss of any identifiable major cell type. Since acquired lesions in the PVN also induce increased appetite without a decrease in energy expenditure, we propose that abnormalities of PVN development cause the obesity of Sim1(+/-) mice. Severe obesity was described recently in a patient with a balanced translocation disrupting SIM1. Pathways controlling the development of the PVN thus have the potential to cause obesity in both mice and humans.

Overexpression of UCP-3 in skeletal muscle of mice results in increased expression of mitochondrial thioesterase mRNA.

Mice overexpressing human UCP-3 in skeletal muscle (UCP-3tg) are lean despite overeating, have increased metabolic rate, and their skeletal muscle mitochondria show increased proton conductance. The true function of UCP-3 however, has yet to be determined. It is assumed that UCP-3tg mice have increased fatty acid beta-oxidation to fuel their increased metabolic rate. In this study we have quantified skeletal muscle mRNA levels of a number of genes involved in fatty acid metabolism. mRNA levels of uncoupling protein-2, carnitine palmitoyl transferase-1beta and fatty acid binding proteins, and transporters were unchanged when compared to wild-type mice. Lipoprotein lipase mRNA was slightly, but significantly, increased by 50%. The most notable change in gene expression was a threefold increase in mitochondrial thioesterase (MTE-1) expression. In the face of a chronic increase in mitochondrial uncoupling these changes suggest that increased flux of fatty acids through the beta-oxidation pathway does not necessarily require marked changes in expression of genes involved in fatty acid metabolism. The large increase in MTE-1 both confirms the importance of this gene in situations where mitochondrial beta-oxidation is increased and supports the hypothesis that UCP-3 exports fatty acids generated by MTE-1 in the mitochondrion.

The adipose tissue phenotype of hormone-sensitive lipase deficiency in mice.

OBJECTIVE: To directly ascertain the physiological roles in adipocytes of hormone-sensitive lipase (HSL; E.C. 3.1.1.3), a multifunctional hydrolase that can mediate triacylglycerol cleavage in adipocytes. RESEARCH METHODS AND PROCEDURES: We performed constitutive gene targeting of the mouse HSL gene (Lipe), subsequently studied the adipose tissue phenotype clinically and histologically, and measured lipolysis in isolated adipocytes. RESULTS: Homozygous HSL-/- mice have no detectable HSL peptide or cholesteryl esterase activity in adipose tissue, and heterozygous mice have intermediate levels with respect to wild-type and deficient littermates. HSL-deficient mice have normal body weight but reduced abdominal fat mass compared with normal littermates. Histologically, both white and brown adipose tissues in HSL-/- mice show marked heterogeneity in cell size, with markedly enlarged adipocytes juxtaposed to cells of normal morphology. In isolated HSL-/- adipocytes, lipolysis is not significantly increased by beta3-adrenergic stimulation, but under basal conditions in the absence of added catecholamines, the lipolytic rate of isolated HSL-/- adipocytes is at least as high as that of cells from normal controls. Cold tolerance during a 48-hour period at 4 degrees C was similar in HSL-/- mice and controls. Overnight fasting was well-tolerated clinically by HSL-/- mice, but after fasting, liver triglyceride content was significantly lower in HSL-/- mice compared with wild-type controls. CONCLUSIONS: In isolated fat cells, the lipolytic rate after beta-adrenergic stimulation is mainly dependent on HSL. However, the observation of a normal rate of lipolysis in unstimulated HSL-/- adipocytes suggests that HSL-independent lipolytic pathway(s) exist in fat. Physiologically, HSL deficiency in mice has a modest effect under normal fed conditions and is compatible with normal maintenance of core body temperature during cold stress. However, the lipolytic response to overnight fasting is subnormal.

Mitochondrial efficiency: lessons learned from transgenic mice.

Metabolic research has, like most areas of research in the life sciences, been affected dramatically by the application of transgenic technologies. Within the specific area of bioenergetics it has been thought that transgenic approaches in mice would provide definitive proof for some longstanding metabolic theories and assumptions. Here we review a number of transgenic approaches that have been used in mice to address theories of mitochondrial efficiency. The focus is largely on genes that affect the coupling of energy substrate oxidation to ATP synthesis, and thus, mice in which the uncoupling protein (Ucp) genes are modified are discussed extensively. Transgenic approaches have indeed provided proof-of-concept in some instances, but in many other instances they have yielded results that are in contrast to initial hypotheses. Many studies have also shown that genetic background can affect phenotypic outcomes, and that the upregulated expression of genes that are related to the modified gene often complicates the interpretation of findings.

Multilocular fat cells in WAT of CL-316243-treated rats derive directly from white adipocytes.

Multilocular, mitochondria-rich adipocytes appear in white adipose tissue (WAT) of rats treated with the beta3-adrenoceptor agonist, CL-316243 (CL). Objectives were to determine whether these multilocular adipocytes derived from cells that already existed in the WAT or from proliferation of precursor cells and whether new mitochondria contained in them were typical brown adipocyte mitochondria. Use of 5-bromodeoxyuridine to identify cells that had undergone mitosis during the CL treatment showed that most multilocular cells derived from cells already present in the WAT. Morphological techniques showed that at least a subpopulation of unilocular adipocytes underwent conversion to multilocular mitochondria-rich adipocytes. A small proportion of multilocular adipocytes ( approximately 8%) was positive for UCP1 by immunohistochemistry. Biochemical techniques showed that mitochondrial protein recovered from WAT increased 10-fold and protein isolated from brown adipose tissue (BAT) doubled in CL-treated rats. Stained gels showed a different protein composition of new mitochondria isolated from WAT from that of mitochondria isolated from BAT. Western blotting showed new mitochondria in WAT to contain both UCP1, but at a much lower concentration than in BAT mitochondria, and UCP3, at a higher concentration than that in BAT mitochondria. We hypothesize that multilocular adipocytes present at 7 days of CL treatment have two origins. First, most come from convertible unilocular adipocytes that become multilocular and make many mitochondria that contain UCP3. Second, some come from a cell that gives rise to more typical brown adipocytes that express UCP1.

Biochemical aspects of the uncoupling proteins: view from the chair.

The discovery of nonshivering thermogenesis (NST) and its location in brown adipose tissue (BAT) in the 1950s to 1970s was soon followed by purification of the first uncoupling protein (UCP1) and later by cloning of the gene for UCP1 in 1985. The properties of UCP1 fully explained the long-known phenomenon of stimulated NST in BAT. An additional four 'uncoupling proteins' have been cloned in the last two years and are in search of phenomena they can explain. The four speakers in this first session of the symposium on uncoupling proteins reviewed biochemical properties of UCP1 and of three of the novel UCPs. Several suggested functions include mediation of the mitochondrial proton leak in tissues other than BAT, therefore a major role in energy expenditure, and protection against reactive oxygen species. Tools, techniques and information not yet available and for which further research is needed are reviewed.

Increased insulin sensitivity and obesity resistance in mice lacking the protein tyrosine phosphatase-1B gene.

Protein tyrosine phosphatase-1B (PTP-1B) has been implicated in the negative regulation of insulin signaling. Disruption of the mouse homolog of the gene encoding PTP-1B yielded healthy mice that, in the fed state, had blood glucose concentrations that were slightly lower and concentrations of circulating insulin that were one-half those of their PTP-1B+/+ littermates. The enhanced insulin sensitivity of the PTP-1B-/- mice was also evident in glucose and insulin tolerance tests. The PTP-1B-/- mice showed increased phosphorylation of the insulin receptor in liver and muscle tissue after insulin injection in comparison to PTP-1B+/+ mice. On a high-fat diet, the PTP-1B-/- and PTP-1B+/- mice were resistant to weight gain and remained insulin sensitive, whereas the PTP-1B+/+ mice rapidly gained weight and became insulin resistant. These results demonstrate that PTP-1B has a major role in modulating both insulin sensitivity and fuel metabolism, thereby establishing it as a potential therapeutic target in the treatment of type 2 diabetes and obesity.

Physiological roles of the leptin endocrine system: differences between mice and humans.

Leptin is a 16-kDa cytokine secreted in humans primarily but not exclusively by adipose tissues. Its concentration in blood is usually proportional to body fat mass, but is higher in women than in men not only because of a different distribution of and greater fat mass in women, but also because testosterone reduces its level in men. Leptin features in different ways during the life span. It is synthesized in the ovary, transported in the oocyte, and made by both fetus and placenta, particularly during the last month of gestation. It is made by the lactating mammary gland and ingested by the newborn infant in its milk. The prime importance of leptin is realized at puberty when it is necessary for progression to a normal adult reproductive status in females. Fasting and chronic undernutrition result in a lower level of leptin in the blood. Lack of leptin results in hunger, ensuring that the individual eat to survive, and also inhibition of reproduction, until such time as food and fat stores are adequate to supply energy for pregnancy and lactation. Thus, leptin is important for survival of the individual and survival of the species. Although an extremely rare genetic absence of leptin induces hyperphagia and obesity in humans, as it does in mice, there appears to be little role for leptin in humans in ensuring that fat stores are not in excess of adequate, that is, in preventing obesity. The mouse differs from humans in many respects, in particular in the far more drastic ways it conserves energy when it very rapidly adapts to lack of food. These include not only suppression of reproduction but also lowering of its body temperature (torpor), suppressing its thyroid function, suppressing its growth, and increasing secretion of stress hormones (from the adrenal). This review concentrates on roles of leptin in human physiology and pathophysiology but also discusses why some observations on actions of leptin in mice are not applicable to humans.

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.

Temperature-dependent feeding: lack of role for leptin and defect in brown adipose tissue-ablated obese mice.

The objective was to characterize the ability of control and transgenic brown adipose tissue (BAT)-ablated uncoupling protein diphtheria toxin A chain (UCP-DTA) mice to adjust food intake in relation to changes in environmental temperature and to assess the involvement of leptin in this adjustment. We measured serum leptin in mice from a previous study of UCP-DTA mice raised at thermoneutrality (35 degrees C) or at the usual rearing temperature (24 degrees C) from weaning [Melnyk, A., M. -E. Harper, and J. Himms-Hagen. Am. J. Physiol, 272 (Regulatory Integrative Comp. Physiol. 41): R1088-R1093, 1997] and extended the study by acclimating control and obese UCP-DTA mice at 18 wk of age to cold (14 degrees C) for up to 14 days. Leptin levels did not change in control mice at 14 degrees C; however, food intake increased threefold within 1 day and remained at this level. Serum leptin level was elevated in UCP-DTA mice at 24 degrees C compared with control mice at 24 degrees C; this elevated level decreased within 1 day at 14 degrees C and was not different from the level in control mice by 14 days. Food intake of UCP-DTA mice that were hyperphagic at 24 degrees C did not change during 7 days at 14 degrees C, then increased slowly. Similar low leptin levels were present in control mice raised at 24 or 35 degrees C and in UCP-DTA mice raised at 35 degrees C. Food intake of control mice raised at 24 degrees C was two times that of control mice raised at 35 degrees C. UCP-DTA mice raised at 35 degrees C ate the same low amount as control mice raised at 35 degrees C. UCP-DTA mice at 24 degrees C were hyperphagic relative to control mice at 24 degrees C yet had elevated leptin levels in their serum. Two principal conclusions are drawn. First, adjustment of food intake over a fourfold range by control mice acclimated to temperatures from 35 down to 14 degrees C is independent of changes in serum leptin levels. Second, this adjustment of food intake in relation to temperature is defective in the UCP-DTA mouse; the defect leads to hyperphagia at 24 degrees C and a failure to increase food intake as rapidly as control mice when exposed to 14 degrees C. Because lack of UCP-1-mediated thermogenesis in BAT of knockout mice is known not to induce hyperphagia, we propose that deficiency of UCP-1-expressing brown adipocytes in BAT of UCP-DTA mice results in lack of a satiety factor, secreted by these cells in BAT of control mice in inverse relationship to sympathetic nervous system activity.

Treatment with CL 316,243, a beta 3-adrenoceptor agonist, reduces serum leptin in rats with diet- or aging-associated obesity, but not in Zucker rats with genetic (fa/fa) obesity.

OBJECTIVE: To assess the effect of chronic treatment with a beta 3-adrenoceptor agonist, CL 316,243 (CL) on serum leptin concentration in rats with diet-induced obesity (DIO) or with genetic obesity (fa/fa Zucker). DESIGN: Leptin concentration was measured in serum of young control rats, young rats with DIO and old control or genetically obese fa/fa Zucker rats, that were treated chronically with CL for 2-4 weeks in our previous studies. RESULTS: Treatment with CL reduced elevated leptin concentrations in young rats with DIO and in old mildly obese control rats to the low concentration of young lean rats. It did not alter the grossly elevated concentration in fa/fa rats. This effect of CL correlated well with its effect to reduce white adipocyte size, except in fa/fa rats. In CL-treated fa/fa rats, despite reductions in body fat mass and in white adipocyte size, and despite normalization of both hyperglycemia and hyperinsulinemia, the leptin concentration did not change. DISCUSSION: The reason for lack of change in leptin concentrations in fa/fa rats, despite shrinking of white adipocytes and partial reversal of the obesity, may be due to another defect. The large increase in white adipocyte number in these animals was not reversed by the treatment and might have contributed to elevated leptin production. In addition, all forms of leptin receptor are known to be defective in fa/fa rats. Since leptin is rapidly excreted in urine and leptin receptors (including a form known to be involved in leptin transport) are expressed in the kidney, we suggest that leptin excretion is impaired in the fa/fa rat. This impairment contributes to maintenance of an elevated concentration of leptin in its blood and prevents treatment with a beta 3-adrenoceptor agonist from reducing this elevated concentration despite reversal of both obesity and diabetes. In addition, we suggest that CL-induced suppression of hyperphagia in fa/fa rats is leptin-independent and due to the large increase in thermogenesis.

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.

Hypertrophy of brown adipocytes in brown and white adipose tissues and reversal of diet-induced obesity in rats treated with a beta3-adrenoceptor agonist.

In a previous study, we demonstrated that chronic treatment with a new beta3-adrenoceptor agonist, CL 316,243 [disodium (R,R)-5-[2-[[2-(3-chlorophenyl)-2-hydroxyethyl]-amino]propyl]-1,3-ben zodioxazole-2,2-dicarboxylate], promoted thermogenesis, caused the appearance of multilocular adipocytes in white adipose tissue (WAT), and retarded development of obesity in young rats eating a high-fat diet (Himms-Hagen et al., Am J Physiol 266: R1371-R1382, 1994). Objectives of the present study were to find out whether CL 316,243 could reverse established diet-induced obesity in rats and to identify the multilocular adipocytes that appeared in WAT. Infusion of CL 316,243 (1 mg/kg/day) reduced abdominal fat, with a decrease in enlarged adipocyte size but no loss of white adipocytes. The resting metabolic rate increased by 40-45%, but food intake was not altered. Abundant densely stained multilocular brown adipocytes expressing uncoupling protein (UCP) appeared in retroperitoneal WAT, in which a marked increase in protein content occurred. UCP content of interscapular brown adipose tissue (BAT) was also increased markedly. We suggest that the substantial increase in the resting metabolic rate induced by CL 316,243 occurs in brown adipocytes in both BAT and WAT. The origin of the brown adipocytes that appeared in WAT is uncertain. They may have been small brown preadipocytes, expressing beta3-adrenoceptors but with few mitochondria and little or no UCP, that were induced to hypertrophy by the beta3-agonist.

Appearance of brown adipocytes in white adipose tissue during CL 316,243-induced reversal of obesity and diabetes in Zucker fa/fa rats.

BACKGROUND: In our previous studies, chronic treatment of rats with a new beta 3-adrenoceptor agonist, CL 316,243, retarded diet-induced obesity and promoted thermogenesis in young animals and reversed established diet-induced obesity in older animals that continued to eat a high fat diet. Reversal of obesity was associated with shrinking of enlarged white adipocytes but the number of mature white adipocytes, which had not been increased by the diet, was not reduced. Drug-treatment induced appearance of abundant brown adipocytes in white adipose tissue (WAT) depots as well as hypertrophy of brown adipose tissue (BAT) in both lean and diet-induced obese rats. OBJECTIVES: To find out whether the known hyperplasia of white adipocytes in the obese fa/fa rat could be reversed by CL 316,243-treatment and whether the grossly enlarged WAT depots of the obese fa/fa rat contain precursors to brown adipocytes. RESULTS: CL 316,243 infusion (1 mg/kg/d) reduced abdominal fat. The loss of fat was due to a decrease in white adipocyte size, with no loss of the markedly elevated number of adipocytes in the fa/fa rats. Resting metabolic rate increased by 40% in lean rats, by 70% in fa/fa rats. Food intake decreased in the hyperphagic fa/fa rats but did not change in lean rats, in both lean and fa/fa rats, a marked increase in protein content of retroperitoneal WAT was associated with appearance of abundant densely-stained brown adipocytes expressing uncoupling protein (UCP) but total number of cells (from DNA content) actually decreased. Hyperinsulinemia and hyperglycemia of fa/fa rats were reduced by treatment, indicating improved sensitivity to insulin. CONCLUSIONS: Abundant precursors to brown adipocytes are present in WAT depots of fa/fa rats and much of the exaggerated increase in resting metabolic rate induced by CL 316,243 occurs in these cells. This beta 3-adrenoceptor agonist is an effective anti-obesity and anti-diabetic agent in fa/fa rats. It does not bring about disappearance of mature white adipocytes but does bring about a remodelling of WAT, with a marked change in cell composition.

Raising at thermoneutrality prevents obesity and hyperphagia in BAT-ablated transgenic mice.

Transgenic mice with ablation of brown adipocytes induced by brown adipocyte-specific expression of diphtheria toxin A chain (DTA) driven by the uncoupling protein (UCP) promoter (UCP-DTA mice) become obese and hyperphagic (Lowell, B. B., V. S. Susulic, A. Hamann, J. A. Lawitts, J. Himms-Hagen, B. B. Boyer, L. P. Kozak, and J. S. Flier. Nature 366: 740-742, 1993). A deficit in energy expenditure for brown adipose tissue (BAT) thermogenesis in these mice is presumed to contribute to the development of obesity. The objective of the present study was to obviate any deficit in BAT thermogenesis by raising transgenic and control mice at thermoneutrality (35 degrees C), where both would have equally inactive BAT, to see whether this would prevent the obesity and the hyperphagia. Transgenic and control mice were raised from weaning (3 wk of age) to 8 wk of age at either 24 or 35 degrees C. Raising at 35 degrees C completely prevented development of obesity of UCP-DTA mice, as indicated by their normal carcass fat, normal weights of four major white adipose tissue depots, and normal size of white adipocytes. As seen before, transgenic mice raised at 24 degrees C had excess weight gain by 6 wk of age and by 8 wk had doubled carcass fat, an obesity characterized by increased white adipocyte size with no increase in number of adipocytes. The treatment also prevented hyperphagia of UCP-DTA mice, consistent with the hypothesized role of BAT thermogenesis in control of thermoregulatory feeding (Himms-Hagen, J. Proc. Soc. Exp. Biol. Med. 208: 159-169, 1995). UCP-DTA mice thus differ from genetically obese mice (ob/ob, db/db) for which raising at thermoneutrality is known not to prevent either the obesity or the hyperphagia. Both the obesity and the hyperphagia of UCP-DTA mice appear to be due to their deficit in BAT thermogenesis.