Large Fat Cells Research Articles

The microvascular bed of fatty bone marrow in the adult beagle

The structure and ultrastructure of the microvascular bed of fatty bone marrow and the relationships of this vascular bed to endosteal bone surfaces was studied in adult beagles. The vascular volume of fatty bone marrow, as demonstrated by India ink-gelatin perfusion, appears less than the vascular volume of red bone marrow. The capillaries in fatty bone marrow are found between the large fat cells in a loose reticular connective tissue and have a continuous endothelial lining and basal lamina. Capillaries are frequently found adjacent to bone surfaces in these fatty marrow sites, yet are separated from these surfaces by very thin bone-lining cells. The nuclei of bone-lining cells are often found near these capillaries. The association of bone-lining cells to the microvasculature suggests that these cells may play a role in partitioning the extracellular fluids from the bone.

Diminished activities of fatty acid synthesis enzymes in insulin-resistant adipocytes from spontaneously obese rats.

Acetyl coenzyme A carboxylase and fatty acid synthetase activities were studied to determine the biochemical basis of the markedly impaired capacity of fat cells from spontaneously obese, old rats to convert glucose to fatty acids relative to cells from lean, young rats. Michaelis constants for the substrates of both enzymes were similar in large and small adipocyte homogenates. In contrast, Vmax values were over 80% less in homogenates from large relative to small cells on a per cell basis. Long-term dialysis or the presence of albumin during the assays failed to restore the activities of these enzymes in homogenates of large fat cells. The combination of equal volumes of homogenates from the two cell types resulted in carboxylase and synthetase activities intermediate between activities found in the two homogenates alone. Therefore, the presence of endogenous allosteric inhibitors does not appear to account for the markedly blunted fatty acid synthesis enzyme activities in large fat cells. These results suggest that the fatty acid synthesis impairment, which is a primary defect in the insulin resistance of the large cells, is at least partly due to diminished cellular contents of acetyl coenzyme A carboxylase and fatty acid synthetase.

Increased adipose tissue lipoprotein lipase activity during the development of the genetically obese rat (fa/fa)

Lipoprotein lipase (LPL) enzyme activity in epididymal adipose tissue from obese and lean Zucker rats was measured. At 5, 10, 13, and 20 wk of age obese rats have heavier fat pads, larger fat cells, and more LPL per epididymal fat pad and per fat cell than do their lean littermate controls. Although LPL per fat cell increased as fat cell size increased in lean rats, the increased LPL activity in the obese could not be attributed solely to increased fat cell size. When obese and lean rats had similar cell sizes, LPL per fat cell was still significantly increased in the obese compared to lean. Furthermore LPL activity was increased in "preobese" (fa/fa) rats compared to either lean genotype (Fa/fa or Fa/Fa) during the second postnatal week. The data suggest that early increments in LPL activity in adipose tissue of the "pre-obese" rat may significantly contribute to the early fat cell hypertrophy seen during the development of this genetic obesity. Furthermore, early increased LPL activity may prove useful as a predictor of the onset of obesity.

Adipose Cellularity and Body Composition in Polygenic Obese Mice as Influenced by Preweaning Nutrition

Male mice from a line selected for rapid postweaning growth (M16) and an unselected control (ICR) were reared from birth to 3 weeks either in litters of eight (N8) or 14 (N14). Body weight gain and feed intake of M16 mice were greater than ICR. These high rates of gain and feed intake also were extended to an older age in M16 (10 weeks) than in ICR (6 weeks). The M16 line exceeded the ICR line and N8 mice exceeded N14 for fat, lean, ash, and live body weights at 4, 6, 10, 16, and 30 weeks of age. Fat percentage was greater in N8 than N14 for both ICR and M16. The adipose cellularity of the epididymal fat pads of M16 indicated a hypertrophic-hyperplastic form of obesity at 10, 16, and 30 weeks. Within each line, the N14 mice had fewer and slightly smaller fat cells than N8. However, M16-N14 mice still had considerably more and larger fat cells than ICR-N8. Restriction of energy intake from birth to 3 weeks reduced subsequent feed intake and degree of obesity. After 4 weeks, the genetic effect exerted a greater influence on the development of obesity than the preweaning nutritional regimen.

Relationship between the tissue level of cyclic AMP and the fat cell size of human adipose tissue.

The relationship between mean fat cell size, maximal tissue cyclic AMP concentration, and glycerol release was investigated in human subcutaneous adipose tissue incubated in vitro with or without isoprenaline or noradrenaline added at maximal effective concentrations. Basal and stimulated glycerol release and cyclic AMP concentration were each related to the fat cell size. Whether or not the phosphodiesterase inhibitor theophylline was present in the incubation system, basal and noradrenaline-induced cyclic AMP levels were significantly correlated with the fat cell size. The noradrenaline-induced cyclic AMP levels resulted in twice as rapid glycerol release as could be expected from the basal ratio between glycerol release and cyclic AMP. Furthermore, both basal and noradrenaline-induced glycerol release in relation to the cyclic AMP levels were more rapid in enlarge fat cells. It is concluded that basal and catecholamine-induced production of cyclic AMP is related to the fat cell size and that a quantitative relationship exists between rate of lipolysis and maximal tissue levels of cyclic AMP in human adipose tissue. Basal and noradrenaline-induced lipolysis are probably regulated by different mechanisms and the lipolytic sensitivity to cyclic AMP seems increased in large fat cells.

Studies on the morphologic and metabolic changes of human fat cells maintained in culture.

Specimens of human adipose tissue were maintained in vitro for one week and cell size as well as metabolic rates determined before and after the culture period. Large fat cells from obese donors decreased towards 'normal' cell size while this was not the case for cells from non-obese donors. This decrease was probably due to the increased lipolysis found while glucose incorporation was unchanged. Analogous to the findings with obese subjects placed on a weight-reducing regimen, the present data show that when large fat cells are removed from their anabolic, hyperinsulinic environment, 'normalization' in cell size occurs.

Primary role of decreased fatty acid synthesis in insulin resistance of large rat adipocytes.

The ability of large fat cells from spontaneously obese rats to synthesize fatty acids from D-[1-14C]glucose, D-[6-14C]glucose, or [2-14C]pyruvate was markedly diminished compared to small fat cells from lean animals. Furthermore, fatty acid synthetase and acetyl coenzyme A carboxylase activities in dialyzed homogenates of large fat cells were inhibited by 84 and 90%, respectively, compared to small cells. Pentose shunt activity, but not glycolytic flux, was also markedly inhibited in large fat cells incubated with or without insulin. However, the NADPH oxidant vitamin K5 completely restored pentose shunt activity in large cells to the elevated levels observed in small fat cells in the presence of this agent or insulin. Furthermore, inhibition of mitochondrial oxidation and fatty acid synthesis in small cells by rotenone led to a secondary inhibition of pentose shunt activity indicating a link between these two pathways. Direct measurements of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase activities in fat cell homogenates showed no difference between cell types. The data provide strong support for the hypothesis that the fatty acid synthetic pathway is the primary metabolic defect in large insulin-resistant rat adipocytes, a defect which secondarily leads to inhibited pentose shunt activity.

Spontaneous and experimental human obesity: Effects of diet and adipose cell size on lipolysis and lipogenesis

We examined the hypothesis that adipose tissue from lean and obese subjects might provide different internal signals in response to changes in stored calories. Adipose tissue was obtained before weight gain in nonobese subjects and after weight gain in five of the same individuals. Adipose tissue was removed before and after weight loss in seven obese patients. Two isocaloric diets were fed to both groups for 2–3 wk each; one diet was high in carbohydrate, and the other contained a low carbohydrate content. Incorporation of radioactivity from pyruvate into fatty acids in vitro was lower with the low-carbohydrate diet than with the high-carbohydrate diet. It was also significantly reduced after weight gain in the nonobese subjects but was not significantly altered in the obese. There were no significant effects of diet or weight gain on the enzymatic activities in the nonobese subjects. The large fat cells from both groups of subjects had an increased sensitivity to the lipolytic effects of isoproterenol as compared with the smaller fat cells. Variations in carbohydrate intake had no effect on the lipolytic response to isoproterenol. The dose response of fat cells to dibutyryl-cyclic-3′,5′-AMP did not change after weight gain in the nonobese males, but was significantly reduced on both levels of carbohydrate after weight loss in the obese (i.e., when studying the smaller fat cells). These studies suggest that differences in the metabolism of adipose tissue between obese and lean subjects persist when differences in the size of fat cells and caloric intake are controlled.

Cellular basis of insulin insensitivity in large rat adipocytes.

The marked stimulatory effect of insulin on the conversion of 20 mM D-[6-14C]glucose to CO2, glyceride-glycerol, and fatty acid observed in small rat adipocytes was greatly diminished in large cells from older rats. Similarly, total glucose utilization as estimated by summing the total metabolites accumulated intracellularly plus the release of labeled CO2 and lactate was substantially lower in large cells in the presence of insulin and 5 mM labeled glucose. However, under conditions of 0.2 mM medium glucose where transport of the hexose into adipocytes is relatively more rate-limiting for subsequent metabolism, large cells actually utilized slightly greater total amounts of glucose than small cells in the presence of insulin. Increments of total glucose utilization due to both submaximal and maximal doses of insulin were similar in large and small cells incubated with a low glucose concentration. Under these conditions, conversion of labeled glucose to CO2 and fatty acid in response to insulin was somewhat diminished in large cells, while conversion to glyceride-glycerol was enhanced. The activity of the D-glucose transport system in large and small cells was estimated by monitoring initial rates and small cells was estimated by monitoring initial rates of 3-O-[3H]methylglucose uptake by a rapid filtration method. Transport system activity on a per cell basis was actually severalfold higher in large adipocytes in the basal state as well as in the presence of submaximal and maximal concentrations of insulin compared to small cells. However, the percent stimulation by insulin was less in the large cells. Uptake of 2-deoxyglucose under basal conditions and in response to insulin was also higher in large cells compared to small cells. Analysis of the accumulated label in extracts from fat cells incubated with D-[14C]deoxyglucose revealed the presence of free deoxyglucose, deoxyglucose-6-phosphate, and 6-phosphodeoxygluconate. The levels of these metabolites were significantly higher in large cells compared to small cells indicating hexokinase activity appears not to account for the defective glucose utilization in large cells at high glucose concentrations. It is concluded that (a) possible defects in insulin receptor components, the D-glucose transport system, and the coupling mechanism which links these entities do not significantly contribute to the apparent insulin-insensitivity of large fat cells and (b) the principal cellular defect which confers this blunted insulin response to large rat adipocytes involves one or more intracellular enzymes involved in glucose metabolism.

Human adipose tissue in culture V. Studies on the metabolic effects of insulin

Specimens of human adipose tissue were cultured for one week with or without the addition of insulin. The basal as well as the noradenaline-stimulated lipolysis were enhanced in the explants cultured with insulin, showing that the long-term effect of the hormone is lipolytic. However, an acute antilipolytic effect of insulin could be demonstrated in these explants in the subsequent short-term incubations. The basal rate of glucose incorporation into the lipids was enhanced in the explants cultured with insulin. When insulin was added in the short-term incubations these explants did not further respond to the hormone while this was the case with the explants cultured without insulin. Thus, it seems that prolonged exposure to insulin leads to a diminished acute effect of the hormone on glucose metabolism. However, the same explants responded to the antilipolytic effect showing that insulin was able to bind itself to the membrane. The activities of hexokinase (HK), glucose-6-phosphage dehydrogenase (G6PDH), pyruvate kinase (PK) and lactate dehydrogenase (LDH) were increased in large fat cells both in freshly excised tissue and in cultured explants. However, the activity of phosphofructokinase (PFK) did not correlate with the cell size. The presence of insulin during the culture period enhanced the activities of G7PDH, PK, and LDH, while this was not found for HK or PFK. The data thus suggest that the metabolic capacity of human fat cells is enhanced by long-term exposure to insulin. Although enzyme induction could be shown for G6PDH, PK and LDH it seems unlikely that this is of importance for the increased rates of glucose metabolism in these explants since the rate-limiting enzymes, HK and PGK, were not increased. Most probably, then, this stimulating effect of insulin is exerted on the membrane and the rate of glucose transport.

Glucose Metabolism in Isolated Fat Cells: Enhanced Response of Larger Adipocytes from Older Rats to Epinephrine and Adrenocorticotropin

The effects of insulin and of two lipolytic hormones (epinephrine and ACTH1) on the rate and pattern of glucose metabolism were compared during incubation of isolated fat cells, obtained from epididymal fat pads of rats of varying age and degrees of adiposity. Glucose metabolism and the intracellular free fatty acid levels were expressed on a per cell basis and in relation to adipocyte size. The data for total glucose metabolism show that, in contrast to the declining insulin effect observed with adipocyte enlargement, the stimulation of glucose uptake and metabolism by these lipolytic hormones was significantly greater in the larger fat cells from the older fatter rats than in the smaller ones from the younger leaner rats. Lipolytic hormones suppressed, whereas insulin enhanced, fatty acid synthesis; moreover the lipolytic hormones stiumlated glucose ce effect of epinephrine on the intracellular free fatty acid levels was greater in the small fat cells than in the large ones; this effect of epinephrine was markedly curtained by the presence of glucose in the incubation medium, making it unlikely that acceleration of glucose metabolism by the lipolytic stimulus was mediated by an elevation of the intracellular free fatty acid level. The present results show a markedly enhanced capacity of the large adipocytes to accelerate glucose metabolism in response to these liplytic hormones. Thus, in contrast to prevailing notions of declining hormonal responsiveness with expanding fat cell size in older and more obese animals, this study documents an instance of increased hormonal response in enlarged adipocytes and points to the need for a more comprehensive reevaluation of the various hormonal effects in adipocytes of different size.

Effects of age and cell size on rat adipose tissue metabolism.

In order to analyze separately the effects of cell size and age on the metabolism of rat adipose tissue, fat cells of different sizes were obtained from the same animals. The rats were 4 or 15 wk old. The results show that age as well as cell size influences the metabolic rates. At a given cell size, the basal lipolysis, the lipolytic effects of glucagon and noradrenaline, the rate of glucose incorporation into the triglycerides, and the effect of insulin on glucose metabolism were considerably increased in the young animals. Furthermore, irrespective of fat cell size the lipolytic action of glucagon was reduced in old animals. The data thus show that experiments with large fat cells from old rats and with small cells from young animals cannot be directly compared because both variables may influence metabolic reactions.

Regional adipose tissue cellularity in relation to metabolism in young and middle-aged women

Body composition and total number of fat cells were investigated in 31 randomly selected women 52 yr of age and in 13 young women (mean age 22 yr) whose body weights were within ±10% of the ideal weight. Two of the 52-yr-old women were obviously obese and excluded. Regional determinations of adipose tissue thickness, fat cell size, and number were also determined. Middleaged women had more body fat (BF) but a lower body cell mass (BCM) than the younger group. The increased BF in the middle-aged women was exclusively explained by larger fat cells, since the younger women had a significantly higher total number of fat cells. This increase was also found when differences in height, body weight, and body fat were matched out. Local fat cell number was also increased in the younger group. Local fat cell size was increased in all regions investigated in the middle-aged women, but the increase was particularly pronounced in the abdominal region. The highest degrees of correlation between fat cell sizes of different regions were found between the epigastric and hypogastric regions and between femoral and gluteal regions. Furthermore, the plasma insulin levels correlated with the fat cell sizes of the abdominal region but not with those of the femoral or gluteal regions. In conclusion, the data might indicate that the fat cells of the abdominal region are more sensitive to nutritional and/or hormonal factors than those of other regions. This may in turn indicate the existence of different fat cell populations.

Effect of an energy-reduced dietary regimen in relation to adipose tissue cellularity in obese women

Twenty-eight obese women were divided after arbitrary statistical guidelines obtained from control studies into hyperplastic (increase in fat cell number) (n equal to 10), hypertrophic obesity (increase in average fat cell size) (n equal to 11), and a remaining group (n equal to 7). All these subjects were treated on an outpatient basis with an energy-reduced diet (1,100 kcal/day) until weight decrease failure occurred. The fat cells of the femoral and gluteal regions were larger than in the abdominal region in hypertrophic obese subjects. This regional fat cell size profile was found also in middle-aged and young controls. The hyperplastic obese subjects on the other hand had larger fat cells in the abdominal site. At failure of therapy enlarged fat cells in either of the two obesity groups had decreased to the size of fat cells of controls. Fat cell number remained unchanged. Thus the hypertrophic obese patients ended up with a normal body fat while hyperplastic obese subjects had a pronounced remaining obesity. The results suggest that when the fat cell size in different regions of an individual are known, as well as the total fat cell number, the success of an energy-reduced dietary regimen might be approximately predicted both in terms of remaining total body fat and in regional fat depot decrease.

Insulin Receptor: Role in the Resistance of Human Obesity to Insulin

Large adipocytes from obese subjects have similar receptor numbers and affinities for insulin as small adipocytes from subjects of normal weight. These results indicate that the insulin insensitivity of large fat cells from obese humans occurs after the insulin-receptor interaction and might be explained by either a dilution of receptors over the cell surface or by alterations in intracellular metabolism.

Cell size and the antilipolytic effect of insulin in human subcutaneous adipose tissue.

Lipolysis was studied in subcutaneous adipose tissue removed under local or general anaesthesia from subjects with marked difference in body weight and from obese subjects before and after intestinal shunt operations. The release of glycerol was measured in sections of adipose tissue incubated for two hours in bicarbonate buffer containing 4 per cent bovine albumin with no glucose added. The larger fat cells were more insulin sensitive than the smaller. A positive relationship was observed between the fat cell size and the antilipolytic effect of insulin (100 muU/ml). Insulin had no significant effect on the lipolytic effect of the catecholamines, when this is calculated as the increment in the glycerol release above the basal.

Suppression and regeneration of rat bone marrow under the influence of methotrexate. An EM study.

Adult male rats were given daily injections of methotrexate for 12 days, during which time specimens of femoral bone marrow were taken for correlative light- and electron-microscopic study. After 12 days methotrexate was discontinued and specimens were taken the marrow recovered from the effect of methotrexate. Normal rat bone marrow was used as a basis of comparison. As methotrexate was administered, cell division ceased and the immature erythroblasts and myelocytes proceeded in the maturational process and a rapid hypocellularity of the marrow developed. Eosinophilic leukocytes, lymphocytes, large fat cells, erythrocytes, and phagocytic reticuloendothelial cells were the only formed elements remaining after 12 days of methotrexate treatment. Two days after methotrexate was discontinued, myeloblasts, myelocytes and proerythroblasts were plentiful, and on the third day the marrow exhibited an erythroid hyperplasia. By the eighth day all cell types were present though the marrow still was hypocellular. 14 days post methotrexate the marrow returned to normal.

Metabolic patterns and insulin responsiveness of enlarging fat cells

The rate and pattern of glucose metabolism, basal lipolysis, and intracellular concentration of free fatty acids were determined in isolated epididymal fat cell preparations (mean volume 30-800 pl) from rats on the basis of fat cell number and in relation to the cell volume. The effects of increasing glucose concentrations in the medium and of insulin on the cellular metabolic activities were compared. Expanding fat cell volume correlated positively and significantly (P < 0.001) with the synthesis of glyceride glycerol from glucose (correlation coefficient, r = 0.919), with rates of basal lipolysis (r = 0.663), and with intracellular free fatty acid accumulation (r = 0.796); it correlated negatively and significantly with glucose conversion to glyceride fatty acids (r = -0.814, P < 0.01). The differences in patterns of glucose metabolism and basal lipolysis between small (<100 pl) and large (>400 pl) fat cells were not modified by insulin or by increments in glucose concentration. The results indicate that the reduced capacity of the large fat cells to respond to insulin cannot be attributed solely to a limited capacity of the cells to take up and metabolize increasing amounts of glucose. The acquired unresponsiveness of the large cells to insulin may result from an alteration in the mechanism of action of insulin and may be related to an intracellular metabolic derangement with increased basal lipolysis, free fatty acid accumulation, and accelerated glyceride synthesis resulting from the accumulation of triglyceride.

Body composition and adipose cellularity in human obesity.

Abstract. Body composition, age at onset of obesity, adipose tissue cellularity and metabolic variables have been determined in 137 obese subjects and controls. Weight stability in 34 of the patients could also be judged by a repeated examination after about 10 months. Fat cell number correlated strongly with body fat, while fat cell size increased only within the control range of body fat. Above approximately 30 kg body fat, the fat cell enlargement was equally pronounced at all degrees of obesity. A cross‐sectional analysis of the obese men and women indicated that fat cell size was larger at 30–40 years of age than before and after this age. Cell number was elevated but did not increase with age in the obese group. Groups of younger and older obese women did not differ in adipose tissue cellularity factors. This had the consequence that, in comparisons with age‐ and sex‐matched control groups, obesity depending primarily on enlargement of fat cells seemed to be more frequent in the younger age groups, while in older obese subjects cell number seemed to be a more important factor for contribution to obesity. Fat cell number correlated positively with body cell mass. The earlier the onset of obesity the more fat cells, particularly when obesity could be traced to infancy. Obesity starting at adult age was characterized by larger fat cells. Two types of obesity can be distinguished. One is a hypercellular severe obesity with an early onset and an elevated body cell mass. The fat cells may be enlarged or not. The other type of obesity is characterized by fat cell hypertrophy, moderately increased body fat and a later onset. There is no increase in body cell mass. Plasma insulin was dependent on weight stability. Hypertriglyceridemia in obesity was associated with a decreased glucose tolerance, somewhat elevated insulin values, and elevated serum uric acid.

Studies of glucagon resistance in large rat adipocytes: 125I-labeled glucagon binding and lipolytic capacity

This study is concerned with potential modifications of large fat cells from adult rats (400-450 g) that make them resistant to stimulation by glucagon. The lipolytic capacity and (125)I-labeled glucagon-binding capability of these cells were compared with these properties of small glucagon-sensitive cells from young rats (130-160 g). As determined by maximal stimulation with theophylline, dibutyryl cAMP, or epinephrine, the lipolytic capacity of large cells was not markedly different from small cells, which suggests that an alteration contributing to glucagon insensitivity is not present in the enzymes involved with hormone-mediated lipolysis. Glucagon-binding studies did indicate a difference between the two cell types. Both large cells and particulate fractions from large cells bound less (125)I-labeled glucagon than small cells or small-cell particles. That diminished binding is not a consequence of glucagon degradation is indicated by the similar amounts of (125)I-labeled glucagon degraded by both cell types. The decrease in (125)I-labeled glucagon binding was not as marked as the decrease in lipolytic response to glucagon stimulation. This lack of correlation and the relationship between elevated phosphodiesterase levels and glucagon insensitivity described in the accompanying report suggest that diminished binding explains only in part the marked resistance to glucagon found in large cells.