Meal fatty acid uptake in adipose tissue: gender effects in nonobese humans

We tested for gender differences in dietary fatty acid metabolism in 12 nonobese men and 12 nonobese women using the meal fatty acid tracer/adipose tissue biopsy study design. In addition to determining body composition, measurements of regional adipose tissue lipoprotein lipase activity, blood flow, and fat cell size were performed to place the meal fatty acid kinetic studies in perspective. Twenty-four hours after ingesting the test meal, the concentration of meal fatty acids was greater (P < 0.05) in abdominal subcutaneous than in thigh adipose tissue in both men (0. 61 +/- 0.12 vs. 0.45 +/- 0.09 mg/g) and women (0.59 +/- 0.10 vs. 0. 43 +/- 0.05) but was not different between men and women. A greater percentage of dietary fat was stored in subcutaneous adipose tissue in women than in men (38 +/- 3 vs. 24 +/- 3%, respectively, P < 0. 05), and a greater portion of meal fatty acid disposal was unaccounted for in men. Significant gender differences in regional adipose tissue blood flow after meal ingestion were noted; the differences were in the direction that could support greater nutrient storage in lower body fat in women.

Meal fatty acid uptake in human adipose tissue: technical and experimental design issues

The adipose tissue uptake of dietary fat has been studied using fatty acid radiotracers incorporated into a meal, followed by adipose tissue biopsies. A number of experimental design issues, including the use of isotopic tracers to measure meal fatty acid oxidation and plasma appearance of tracer, as well as the heterogeneity of adipose tissue fatty acid uptake, have been addressed. We examined these questions in a study of 24 volunteers (12 men and 12 women) who consumed a meal containing [(3)H]triolein and [(14)C]triolein. Slight differences in the purity of [(3)H]triolein vs. [(14)C]triolein were found, which could affect the apparent adipose tissue uptake of meal fatty acids. The adipose tissue triglyceride specific activity from bilateral biopsy sites agreed well, implying that a unilateral biopsy is satisfactory for measuring tracer uptake. Meal fatty acid oxidation measured using [(3)H]triolein and [(14)C]triolein was well correlated (r = 0.79, P < 0.0001). The peak tracer appearance in plasma chylomicrons occurred 1 h after the ingestion of a second, unlabeled meal. Our findings have implications for the experimental design of future meal fatty acid tracer/adipose tissue biopsy studies.

Prospective evaluation of Roux-en-Y gastric bypass as primary operation for medically complicated obesity

OBJECTIVE

To determine prospectively the results of Roux-en-Y gastric bypass (RYGB) used as the primary weight-reducing operation in patients with medically complicated ("morbid") obesity. The RYGB procedure combines the advantages of a restrictive physiology (pouch of 10 mL) and a "dumping physiology" for high-energy liquids without requiring an externally reinforced (banded) stoma.

PATIENTS AND METHODS

Between April 1987 and December 1998, a total of 191 consecutive patients with morbid obesity (median weight, 138 kg [range, 91-240 kg]; median body mass index, 49 kg/m2 [range, 36-74 kg/m2]), all of whom had directly weight-related morbidity, underwent RYGB and prospective follow-up.

RESULTS

Hospital mortality was 0.5% (1/191), and hospital morbidity occurred in 10.5% (20/191). Good long-term weight loss was achieved, and patients adapted well to the required new eating habits. The mean +/- SD weight loss at 1 year after operation (113 patients) was 52 +/- 1 kg or 68% +/- 2% of initial excess body weight. By 3 years postoperatively (74 patients), weight loss was still 66% +/- 2% of excess body weight. Overall, 53 (72%) of 74 patients had achieved and maintained a weight loss of 50% or more of their preoperative excess body weight 3 years after the operation. In addition, only 1 (1%) of 98 patients had persistent postoperative vomiting 1 or more times per week.

CONCLUSION

We believe that RYGB is a safe, effective procedure for most patients with morbid obesity and thus may be the current procedure of choice in patients requiring bariatrics++ surgery for morbid obesity.

Obesity is associated with a decreased leptin transport across the blood-brain barrier in rats

Leptin exerts important effects on the regulation of food intake and energy expenditure by acting in the brain. Leptin is secreted by adipocytes into the bloodstream and must gain access to specific regions in the brain involved in regulating energy balance. Its action is mediated by interaction with a receptor that is mainly expressed in the hypothalamus but is also present in other cerebral areas. To reach these target areas, leptin most likely needs to cross the blood-brain barrier (BBB). In this study, we compared the permeability of leptin at the BBB in homozygous lean (FA/FA), high-fat diet-induced (HFD) obese rats (FA/FA rats on a highfat diet), and genetically obese fa/fa Zucker rats by quantifying the permeability coefficient surface area (PS) product after correction for the residual plasma volume (Vp) occupied by leptin in the vessel bed of different brain regions. The intravenous bolus injection technique was used in the cannulated brachial vein and artery using leptin radioiodinated with 2 isotopes of iodine (125I and 131I) to separately determine the PS and Vp values. The PS for leptin at the BBB in lean FA/FA rats ranged from 11.0 +/- 1.6 at the cortex to 14.8 +/- 1.4 x 10(-6) ml x g(-1) x ml(-1) at the posterior hypothalamus. The PS for leptin in HFD obese FA/FA and obese fa/fa rats ranged from 3.0- to 4.0-fold lower than in lean FA/FA rats. The Vp values were not significantly different among the 3 groups studied. SDS-PAGE analysis of the radioiodinated leptin after 60 min of uptake revealed intact protein in the 8 different brain regions. Plasma leptin levels were significantly higher in both obese rat groups compared with those in lean FA/FA rats. Leptin levels in cerebrospinal fluid were not significantly different among the 3 groups of rats. These findings strongly suggest that the leptin receptor (OB-R) in the BBB can be easily saturated. Saturation of the BBB OB-R in obese individuals would explain the defect in leptin transport into the brain described in this study.

Genetic and environmental contributions

There is a remarkable variability in insulin action in humans. Depending upon the definition of the insulin resistance syndrome, different inheritability/environmental influences on insulin action are reported. The environmental contributions to insulin resistance appear to account for approximately 50% of this syndrome. Obese and sedentary insulin-resistant individuals can see dramatic improvement in insulin sensitivity with weight reduction and fitness training. The degree to which obesity is determined by genetic influences will have a substantial impact on insulin resistance in the Western populations. Familial components also appear to account for approximately 50% of the variation and insulin action (as commonly defined by glucose metabolic effects).

The long form of the leptin receptor (OB-Rb) is widely expressed in the human brain

Leptin exerts important effects on the regulation of food intake and energy expenditure by acting in the brain. Leptin action is mediated by the interaction with a receptor that is alternatively spliced, resulting in at least five different isoforms. The long form (OB-Rb) has a long intracellular domain that is essential for intracellular signal transduction. The specific aim of this study was to further investigate the role that the brain may play in the pathogenesis of obesity in humans. We studied the expression of OB-R mRNA (both short or common and long isoforms) in the brains of obese, lean and diabetic subjects, by in situ hybridization, semiquantitative RT-PCR and Northern blots analysis. We used two alternative probes: one that recognizes all known splice variants (OB-Ra) and a second that recognizes only the long form, OB-Rb. Several brain regions, including hypothalamus, cerebellum, neocortex, entorrhinal cortex, amygdala, and rostral medulla, were evaluated. In situ hybridization studies revealed that both OB-Ra and OB-Rb mRNAs are widely distributed in the human brain. The specific hybridization signal with both probes was detected exclusively in the cytoplasm of the cell body, dendrites and proximal axons of neurons. Hypothalamic nuclei, Purkinje cells and dentate nuclei of the cerebellum, inferior olivary and cranial nerves nuclei in the medulla, amygdala and neurons from both neocortex and entorrhinal cortex demonstrated positive signals. The hybridization signal obtained in ependyma was lower than that in neurons and no specific hybridization was detected in glial cells. No significant differences were identified among the regions or among the three groups studied. These results match those previously obtained by us [Couce et al.: Neuroendocrinology 1997;66:145] in which the distribution of the OB-R protein in the human brain was first described. RT-PCR indicated that the OB-Rb was highly expressed in the hypothalamus and cerebellum. No significant differences of OB-Ra or OB-Rb mRNA expression were identified in lean or obese individuals in these two cerebral regions. The levels of OB-Rb were significantly higher in cerebellum compared to hypothalamus in lean and obese individuals. The original hypothesis that OB-Rb is present only in the hypothalamus needs to be reconsidered. This OB-Rb isoform seems to be widely expressed in the human brain with highest levels in the cerebellum. Obesity and hyperleptinemia appears not to be associated with down-regulation of the OB-Rb in the human brain.

Metabolic complications of obesity. Pathophysiologic considerations

Given a specific research interest in human fatty acid metabolism, this article focuses primarily on the evidence surrounding the hypothesis that dysregulation of the fuel release function of fat cells (lipolysis) is an important contributing factor to the health hazards of obesity.

Measuring leg muscle and fat mass in humans: comparison of CT and dual-energy X-ray absorptiometry

Dual-energy X-ray absorptiometry (DEXA) is reported to be inferior to computed tomography (CT) to measure changes in appendicular soft tissue composition. We compared CT- and DEXA-measured thigh muscle and fat mass to evaluate the random and systematic discrepancies between these two methods. Thigh skeletal muscle area (single-slice CT) was suboptimally (r(2) = 0.74, P < 0.0001) related to DEXA-measured thigh fat-free mass (FFM). In contrast, thigh muscle and adipose tissue volumes (multislice CT) were highly related to DEXA-measured thigh FFM and fat (both r(2) = 0.96, P < 0.0001). DEXA-measured leg fat was significantly less than multislice-CT-measured leg adipose tissue volume, whereas multislice-CT-measured leg muscle mass was less (P < 0.0001) than DEXA-measured leg FFM. The systematic discrepancies between the two approaches were consistent with the 10-15% nonfat components of adipose tissue. In conclusion, CT and DEXA measures of appendicular soft tissue are highly related. Systematic differences between DEXA and CT likely relate to the underlying principles of the techniques.

Effects of type 2 diabetes on the ability of insulin and glucose to regulate splanchnic and muscle glucose metabolism: evidence for a defect in hepatic glucokinase activity

Insulin-induced stimulation of muscle glucose uptake (MGU) is impaired in people with type 2 diabetes. To determine whether insulin-induced stimulation of splanchnic glucose uptake (SGU) is also impaired, we simultaneously measured leg glucose uptake (LGU) and SGU in 14 nondiabetic subjects and 16 subjects with type 2 diabetes using a combined organ catheterization-tracer infusion technique. Glucose was clamped at approximately 9.3 mmol/l, while insulin concentrations were maintained at approximately 72 pmol/l (low) and approximately 150 pmol/l (high) for 3 h each. Endogenous hormone secretion was inhibited with somatostatin. Total body glucose disappearance was lower (P < 0.01) and glucose production higher (P < 0.01) during both insulin infusions in the diabetic compared with the nondiabetic subjects, indicating insulin resistance. Splanchnic glucose production was higher (P < 0.05) in the diabetic subjects during the low but not the high insulin infusion. SGU was slightly lower in the diabetic than in the nondiabetic subjects during the low insulin infusion and 50-60% lower (P < 0.05) during the high insulin infusion. LGU (P < 0.001), but not SGU, was inversely correlated with the degree of visceral adiposity. The contribution of the indirect pathway to hepatic glycogen synthesis did not differ in the diabetic and nondiabetic subjects. In contrast, both flux through the UDP-glucose pool (P < 0.05) and the contribution of the direct pathway to glycogen synthesis (P < 0.01) were lower in the diabetic than in the nondiabetic subjects, indicating decreased uptake and/or phosphorylation of extracellular glucose. On the other hand, glycogenolysis was equally suppressed in both groups. In summary, type 2 diabetes impairs the ability of insulin to stimulate both MGU and SGU. The defect appears to reside at a proximal (e.g., glucokinase) metabolic step and is not related to the degree of visceral adiposity. These data suggest that impaired hepatic glucose uptake as well as MGU contribute to hyperglycemia in people with type 2 diabetes.

Leg free fatty acid kinetics during exercise in men and women

We previously reported that epinephrine stimulates leg free fatty acid (FFA) release in men but not in women. The present studies were conducted to determine whether the same is true during exercise. Six men and six women bicycled for 90 min at 45% of peak O(2) consumption, during which time systemic and leg FFA kinetics ([9, 10-(3)H]palmitate) were measured. The catecholamine and hormonal responses to exercise were not different in men and women. The baseline systemic and leg palmitate release was 94 +/- 15 vs. 114 +/- 5 micromol/min and 16 +/- 2 and 20 +/- 3 micromol/min, respectively, in men and women [P = nonsignificant (NS)]. Systemic and leg palmitate release increased (both P < 0.001) to 251 +/- 18 vs. 212 +/- 16 micromol/min and 73 +/- 19 vs. 80 +/- 12 micromol/min in men and women, respectively, during the last 30 min of exercise (P = NS, men vs. women). We conclude that the systemic and leg adipose tissue lipolytic response to exercise is not different in nonobese men and women.

Androgen effect on body composition and fat metabolism

Recognizing the adverse metabolic effects of predominantly visceral body fat distribution associated with low testosterone levels, researchers have investigated the effects of androgen treatment on body fat distribution in men. This presentation reviews the results of research on acute and longer-term effects of treatment with testosterone and dihydrotestosterone. Methods for investigating these results, including measurement of lipoprotein lipase activity and of radiolabeled fat uptake and turnover, are described and discussed.

Echocardiographic improvement over time after cessation of use of fenfluramine and phentermine

OBJECTIVE

To determine the echocardiographic changes over time of valvular heart lesions in patients who took the weight loss drugs fenfluramine and phentermine.

SUBJECTS AND METHODS

This prospective cohort study began at the termination of a randomized, double-blind, placebo-controlled weight loss trial of 18 obese women and 13 obese men (mean age, 42 years; mean body mass index, 33.4 kg/m2) who had been assigned randomly to treatment with fenfluramine and phentermine or to placebo. Echocardiograms were obtained at termination of the trial when fenfluramine was withdrawn from the market and 6 months later. They were interpreted independently by 3 cardiologists blinded to treatment assignment and temporal sequence of the echocardiograms. The main outcome measure was the change in drug-related valvular disease over time.

RESULTS

One subject assigned to receive the drugs was lost to follow-up, and 3 subjects who did not meet a weight loss goal of 10 kg crossed over from placebo to drug treatment. Echocardiograms were obtained in 19 subjects who received the drugs and 11 subjects who received placebo, and 6-month follow-up echocardiograms were obtained in 15 subjects who received the drugs and 3 who received placebo. Subjects had taken fenfluramine and phentermine a mean of 41 weeks (range, 8-73 weeks). Five of 19 subjects who received the drugs (26%; 95% confidence interval, 7%-46%) and 1 of 11 who received placebo (9%) (odds ratio, 3.6; 95% confidence interval, 0.4-35.6) had findings that met criteria established for drug-related valvular disease. All 5 subjects (4 women and 1 man) receiving the drugs had mild aortic regurgitation, and 1 also had pulmonary hypertension (estimated pulmonary artery pressure, 59 mm Hg). Six months later, the echocardiographic findings had improved in all 5 subjects (P=.06), and 3 no longer met the criteria for drug-related valvular disease. Pulmonary artery pressures decreased to near normal in the subject with pulmonary hypertension (37 mm Hg). Overall, the echocardiographic valvular features improved in 8 of 15 subjects who received the drugs and had echocardiograms performed at both time periods (P=.008).

CONCLUSIONS

Valvular heart disease did not appear to progress after cessation of use of fenfluramine and phentermine, and echocardiographic valvular features appeared to improve over time.

Relationship between urinary albumin excretion, body composition, and hyperinsulinemia in normotensive glucose-tolerant adults

OBJECTIVE

Elevated urinary albumin excretion (UAE) has been associated with insulin resistance and is suggested to be elevated in prediabetic individuals. Upper body obesity, especially visceral obesity, predicts insulin resistance and development of type 2 diabetes. We examined whether UAE clusters with obesity-associated insulin resistance traits in healthy glucose-tolerant normotensive subjects.

RESEARCH DESIGN AND METHODS

There were 49 volunteers with a wide range of body fat and body fat distribution studied. All had normal blood pressure and glucose tolerance and were maintained on a controlled diet for 2 weeks. UAE was assessed from three overnight urine collections, and body composition was assessed by whole-body dual-energy X-ray absorptiometry scanning and abdominal computed tomography scanning.

RESULTS

Fasting insulin and insulin responses to oral glucose were significantly increased in obese subjects, who also tended to have more dyslipidemia, greater blood pressure, and more visceral fat than lean subjects. These differences were more apparent in upper body obese subjects. UAE was normal in obese and upper body obese subjects and not different from that of lean subjects. UAE ranged from 0.3 to 8.3 micrograms/min in lean subjects and from 0.2 to 7.2 micrograms/min in obese subjects. UAE was not significantly correlated with body composition, plasma insulin, glucose, or lipids.

CONCLUSIONS

Obese subjects (even upper body obese subjects) with increased visceral and total body fat, high plasma insulin and triglycerides, and low HDL cholesterol concentrations do not have elevated UAE. This suggests that UAE is not closely associated with these characteristics and implies a later onset of abnormal albuminuria in the course of the insulin resistance syndrome.

Blood glycerol is an important precursor for intramuscular triacylglycerol synthesis

The utilization of blood glycerol and glucose as precursors for intramuscular triglyceride synthesis was examined in rats using an intravenous infusion of [2-(14)C]glycerol and [6-(3)H]glucose or [6-(14)C]glucose. In 24-h fasted rats, more glycerol than glucose was incorporated into intramuscular triglyceride glycerol in soleus (69 +/- 23 versus 4 +/- 1 nmol/micromol triglyceride/h, respectively, p = 0.02 glycerol versus glucose) and in gastrocnemius (25 +/- 5 versus 9 +/- 2 nmol/micromol triglyceride/h, respectively, p = 0.02). Blood glucose was utilized more than blood glycerol for triglyceride glycerol synthesis in quadriceps. In fed rats, the blood glycerol incorporation rates (4 +/- 2, 8 +/- 3, and 9 +/- 3 nmol/micromol triglyceride/h) were similar (p > 0.3) to those of glucose (5 +/- 2, 8 +/- 2, and 5 +/- 2 nmol/micromol triglyceride/h for quadriceps, gastrocnemius, and soleus muscle, respectively). Glucose incorporation into intramuscular triglycerides was less with [6-(3)H]glucose than with [6-(14)C]glucose, suggesting an indirect pathway for glucose carbon entry into muscle triglyceride. The isotopic equilibrium between plasma and intramuscular free glycerol ([U-(13)C]glycerol) was complete in quadriceps and gastrocnemius, but not soleus, within 2 h after beginning the tracer infusion. We conclude that blood glycerol is a direct and important precursor for muscle triglyceride synthesis in rats, confirming the presence of functionally important amounts of glycerol kinase in skeletal muscle.

Leptin responses to overfeeding: relationship with body fat and nonexercise activity thermogenesis

Administration of leptin to rodents results in weight loss through decreased food intake and increased energy expenditure that occurs in part through increased spontaneous activity. In humans, low levels of spontaneous physical activity and below normal plasma leptin concentrations predict subsequent excess weight gain. We recently found that failure to increase nonexercise activity thermogenesis (NEAT) with overfeeding results in greater fat gain in humans, and subsequently evaluated whether changes in leptin are related to NEAT activation. We measured plasma leptin concentrations and adipose tissue leptin messenger ribonucleic acid together with the components of energy expenditure in 16 nonobese humans before and after overfeeding to assess the relationship between leptin responses to overfeeding and the changes in NEAT. Adipocyte leptin expression was up-regulated with overfeeding, and leptin concentrations increased. Leptin concentrations correlated with body fat before and after overfeeding. Changes in leptin with overfeeding were strongly related to changes in body fat, but not to changes in NEAT. Changes in NEAT correlated inversely with fat gain. It is, therefore, unlikely that leptin mediates activation of NEAT with overfeeding in nonobese humans; rather, leptin directly reflects body fat mass and fat mass gain.

Regional postprandial fatty acid metabolism in different obesity phenotypes

To examine if postprandial splanchnic/hepatic free fatty acid (FFA) delivery is increased in upper-body (UB) obesity, and to determine the adipose tissue depots responsible for the greater postprandial FFA availability, we measured systemic and regional uptake and release of FFAs ([1-(14)C]palmitate) before and during a 5-h frequent-feeding mixed meal in eight UB and eight lower-body (LB) obese women. Postabsorptive FFA flux and splanchnic FFA delivery were not different in UB and LB obese women; however, postprandial FFA concentrations (257 +/- 45 vs. 81 +/- 12 micromol/l, P < 0.0001), FFA flux (8.5 +/- 1.2 vs. 3.9 +/- 0.8 micromol x kg(-1) fat-free mass x min(-1), P < 0.0001), splanchnic FFA delivery (275 +/- 45 vs. 88 +/- 24 micromol/min, respectively, P < 0.005), and estimated hepatic FFA delivery were greater in UB than LB obese women. Nonsplanchnic UB adipose tissue FFA release was greater in UB than in LB obese women (276 +/- 71 vs. 97 +/- 37 micromol/min, respectively, P < 0.05) and accounted for the greater postprandial FFA availability in UB obesity. Postprandial leg glucose uptake was less in UB than in LB obese women (8.4 +/- 5.1 vs. 22.9 +/- 2.6 micromol x kg(-1) leg fat-free mass x min(-1), P < 0.05). We conclude that the elevated postprandial FFA release observed in UB obese women originates from the nonsplanchnic UB fat, not visceral fat. These results suggest that visceral fat may be a marker for, but not the source of, excess postprandial FFAs in obesity.

Collection and interpretation of plasma leptin concentration data in humans

OBJECTIVE

To reassess the relationship between body fat and plasma leptin concentrations when a) replicate measures of leptin are made; b) energy intake is controlled at isoenergetic levels before the study; and c) body fat and percent body fat are measured with dual energy X-ray absorptiometry (DXA).

RESEARCH METHODS AND PROCEDURES

Two separate studies were conducted. In the first study, four plasma samples were collected for measurement of leptin over 30 minutes on a single day in 43 lean and obese men and women. For the second study, plasma samples were collected on four consecutive days from a group of 50 lean and obese men and women. Percent body fat (and body fat mass) was related to log-transformed mean plasma leptin concentrations using linear regression analysis; multiple linear regression analysis was used to determine whether there was an effect of gender on this relationship, and the analysis of Choi was used to examine whether percent body fat or body fat mass better predicts plasma leptin concentrations.

RESULTS

For the first study, percent body fat was highly correlated (r=0.96, p<0.0001) with log-transformed mean leptin concentrations. No difference in the relationship between leptin and percent body fat in men and women was detected. The second study confirmed this observation; the relationship between ln leptin and percent body fat was virtually identical (r=0.93, p<0.001). Analysis of the pooled data suggests that percent body fat is a better predictor of plasma leptin concentration than body fat mass. The use of multiple (as opposed to a single) leptin measurements did not significantly improve the leptin/body fat relationship.

DISCUSSION

When robust body composition techniques and diet control measures are taken into consideration, the relationship between ln plasma leptin concentrations and percent body fat is not different in men and women.

Regional glycerol and free fatty acid metabolism before and after meal ingestion

We measured splanchnic and leg glycerol [and free fatty acid (FFA)] uptake and release in 11 healthy volunteers before and after meal ingestion to assess whether regional FFA-to-glycerol release ratios mirror systemic release ratios. Basal splanchnic triglyceride release was also assessed. Although basal splanchnic glycerol uptake (111 +/- 18 micromol/min) accounted for most of systemic glycerol rate of appearance (156 +/- 20 micromol/min), leg glycerol uptake was also noted. The basal, systemic FFA-to-glycerol release ratio was less (2.6 +/- 0.2, P < 0.05) than the splanchnic ratio of 6.1 +/- 1.3, and the leg FFA-to-glycerol release ratio under fed conditions was less than the systemic ratio (0.9 +/- 0.1 vs. 1.6 +/- 0.2, respectively, P < 0.05). Basal splanchnic triglyceride production rates were 74 +/- 20 micromol/min, which could produce equimolar amounts of glycerol in the peripheral circulation via lipoprotein lipase action. In summary, 1) regional FFA-to-glycerol release ratios do not mirror systemic ratios, 2) leg glycerol uptake occurs in humans, and 3) splanchnic triglyceride production rates are substantial relative to systemic glycerol appearance. Glycerol appearance rates may not be a quantitative index of whole body lipolysis.

Leptin acts on human marrow stromal cells to enhance differentiation to osteoblasts and to inhibit differentiation to adipocytes

Both bone mass and serum leptin levels are increased in obesity. Because osteoblasts and adipocytes arise from a common precursor in bone marrow, we assessed the effects of human recombinant leptin on a conditionally immortalized human marrow stromal cell line, hMS2-12, with the potential to differentiate to either the osteoblast or adipocyte phenotypes. By RT-PCR and Western immunoblot analysis, the hMS2-12 cells expressed messenger RNA (mRNA) and protein for the leptin receptor. Leptin did not affect hMS2-12 cell proliferation, but resulted in dose- and time-dependent increases in mRNA and protein levels of alkaline phosphatase, type I collagen, and osteocalcin, and in a 59% increase in mineralized matrix. Leptin increased mRNA levels of lipoprotein lipase at 3 days, but decreased mRNA levels of adipsin and leptin at 9 days and decreased lipid droplet formation by 50%. Leptin did not affect the expression of Cbfa1 or peroxisome proliferator-activated receptor-gamma2, transcription factors involved in commitment to the osteoblast and adipocyte pathways, respectively. Thus, leptin acts on human marrow stromal cells to enhance osteoblast differentiation and to inhibit adipocyte differentiation. Our data support the hypothesis that leptin is a previously unrecognized, physiological regulator of these two differentiation pathways, acting primarily on maturation of stromal cells into both lineages.

Leptin acts on human marrow stromal cells to enhance differentiation to osteoblasts and to inhibit differentiation to adipocytes

Both bone mass and serum leptin levels are increased in obesity. Because osteoblasts and adipocytes arise from a common precursor in bone marrow, we assessed the effects of human recombinant leptin on a conditionally immortalized human marrow stromal cell line, hMS2-12, with the potential to differentiate to either the osteoblast or adipocyte phenotypes. By RT-PCR and Western immunoblot analysis, the hMS2-12 cells expressed messenger RNA (mRNA) and protein for the leptin receptor. Leptin did not affect hMS2-12 cell proliferation, but resulted in dose- and time-dependent increases in mRNA and protein levels of alkaline phosphatase, type I collagen, and osteocalcin, and in a 59% increase in mineralized matrix. Leptin increased mRNA levels of lipoprotein lipase at 3 days, but decreased mRNA levels of adipsin and leptin at 9 days and decreased lipid droplet formation by 50%. Leptin did not affect the expression of Cbfa1 or peroxisome proliferator-activated receptor-gamma2, transcription factors involved in commitment to the osteoblast and adipocyte pathways, respectively. Thus, leptin acts on human marrow stromal cells to enhance osteoblast differentiation and to inhibit adipocyte differentiation. Our data support the hypothesis that leptin is a previously unrecognized, physiological regulator of these two differentiation pathways, acting primarily on maturation of stromal cells into both lineages.

Role of nonexercise activity thermogenesis in resistance to fat gain in humans

Humans show considerable interindividual variation in susceptibility to weight gain in response to overeating. The physiological basis of this variation was investigated by measuring changes in energy storage and expenditure in 16 nonobese volunteers who were fed 1000 kilocalories per day in excess of weight-maintenance requirements for 8 weeks. Two-thirds of the increases in total daily energy expenditure was due to increased nonexercise activity thermogenesis (NEAT), which is associated with fidgeting, maintenance of posture, and other physical activities of daily life. Changes in NEAT accounted for the 10-fold differences in fat storage that occurred and directly predicted resistance to fat gain with overfeeding (correlation coefficient = 0.77, probability < 0.001). These results suggest that as humans overeat, activation of NEAT dissipates excess energy to preserve leanness and that failure to activate NEAT may result in ready fat gain.

Serum leptin and weight gain over 8 years in African American and Caucasian young adults

OBJECTIVE

There is considerable interest in how to prevent weight gain in adulthood. Leptin, a peptide hormone expressed in adipose tissue, is believed to signal the central nervous system about the level of body fat stores, and thereby may control appetite. Little information exists on whether the serum leptin concentration influences long-term weight changes in the free-living population.

RESEARCH METHODS AND PROCEDURES

From an ongoing cohort study of young African American and white adults, we selected a sample of participants (n=492), stratified on sex, race, and weight changes over 8 years. Serum leptin was measured on stored specimens using a radioimmunoassay. Weight change was modeled in relation to baseline leptin concentrations.

RESULTS

Cross-sectionally, leptin concentration was associated positively with body mass index, negatively with physical activity level, and was higher in women than men. These variables explained 72% of the variance in serum leptin. Over the 8 years, the sample gained an average of 7.8 kg (standard deviation = 10.8). There was no evidence that 8-year weight change was associated with initial leptin concentration: 8-year weight change was only 0.5 kg less (95% confidence interval =-1.8 to 0.8, p = 0.47) per each 10 ng/ mL increment (approximately one standard deviation) of baseline leptin. In contrast, leptin change correlated highly (r=0.62) with weight change.

DISCUSSION

Our data corroborate evidence that adiposity determines leptin levels but do not support the hypothesis that leptin deficiency plays an important role in obesity in the general population.

Insulin regulation of regional free fatty acid metabolism

Studies were conducted to determine whether regional free fatty acid (FFA) release is differentially regulated by insulin. Systemic, leg, and splanchnic palmitate rate of appearance ([9,10-(3)H]palmitate) was measured in 26 healthy adults using the euglycemic-hyperinsulinemic clamp technique to achieve a physiological range of plasma insulin concentrations. We found that insulin inhibited systemic, leg, and splanchnic palmitate release in a dose-dependent fashion over the range of insulin infused (0-1.0 mU x kg(-1) x min(-1)). Progressive hyperinsulinemia changed the leg from a net producer to a net FFA consumer, whereas the splanchnic bed converted from a net FFA consumer to a net producer. At the 0.5 mU x kg(-1) x min(-1) insulin infusion rate, leg FFA release was almost completely suppressed, whereas even with the 1.0 mU x kg(-1) x min(-1) insulin infusion rate, splanchnic FFA release decreased by only approximately 65% (P < 0.05 leg vs. splanchnic). These results demonstrate the regional heterogeneity of insulin-regulated FFA release in vivo, and indicate that visceral adipose tissue lipolysis is more resistant to insulin suppression than is leg lipolysis in humans.

Regional leptin kinetics in humans

BACKGROUND

Leptin is known to be cleared by the kidney, a tissue with substantial leptin receptor mRNA expression; however, lung, liver, and muscle tissues also express leptin receptor messenger RNA and it is not known whether these tissues also clear leptin from the circulation.

OBJECTIVE

This study was conducted to determine whether net leptin clearance takes place in the pulmonary, splanchnic, and leg tissue beds to a similar extent as in the kidney.

DESIGN

Plasma leptin concentrations were measured in blood entering and exiting the renal bed, pulmonary bed, splanchnic bed, and leg in 4 groups of subjects. Regional plasma flow was measured in 3 of the 4 groups.

RESULTS

Renal leptin uptake was substantial, whereas no net uptake of leptin by the splanchnic or pulmonary vascular beds was detected; leg tissue was a net leptin producer. Net leptin release by leg tissue, relative to leg adipose tissue mass, was comparable with that reported previously for abdominal subcutaneous adipose tissue.

CONCLUSION

These results confirm that the kidney is a significant site of leptin clearance in humans, whereas pulmonary and splanchnic beds are not.