Impaired fat storage capacity in adipocyte precursors of I versus C57BL mice

I mouse strain displays adipocyte hypoplasia responsible for smaller fat pad size compared with C57BL mice. We investigated possible alterations in the proliferation and/or differentiation capacity of preadipocytes from the stroma-vascular fraction of adipose tissue in the I mouse strain. Control C57BL and I mice were studied at 8 weeks of age, and both adipose and stromal cells were isolated from epididymal and inguinal adipose tissue localizations. Results showed that the lower epididymal adipose mass in I mice was accompanied by a decrease in stromal cell number compared with C57BL mice. In inguinal fat pads, total cell number in the stroma-vascular fraction was unmodified; lipoprotein lipase activity significantly increased in stromal cells from I mice compared with control mice. In this depot, further characterization of cells from the stroma-vascular fraction by separation of cells according to density showed an increased number of preadipocytes in the I mouse whole stromal cell population. These preadipocytes seemed unable to undergo terminal maturation, thus leading to a decrease in the number of mature adipocytes. These results indicated that resistance to fat accumulation in I mice is characterized by site-dependent impairment of both the proliferative rate and the differentiation capacity of adipocyte precursors.

Evidence of enhanced storage capacity in adipose tissue of genetically fat chickens

The storage capacity for plasma triglyceride in abdominal adipose tissue has been investigated in two lines of chickens selected for either high or low adiposity (fat line and lean line, respectively). Adipose tissue cellularity and lipoprotein lipase (LPL) activity were estimated in 2- and 5-wk-old birds. In 2-wk-old birds, cellularity and LPL and glycerophosphate dehydrogenase (GPDH) activity were evaluated in the stromavascular fraction. At both ages studied, the fat line exhibited a twofold increase in mature fat cell number and a marked hypertrophy. LPL activity per cell did not differ with genotype, regardless of the nutritional state; however, LPL activity per whole fat pad was higher in fat birds. In 2-wk-old fat chickens, the stromavascular fraction was characterized by an increase in cell number and a decrease in LPL activity, while GPDH was unchanged. Finally, adipocyte hyperplasia in the fat line appeared to reflect an excessive proliferation of precursor cells. The higher LPL activity in fat birds resulted mainly from cell hyperplasia, rather than from a greater intrinsic activity of adipocytes. Consequently, enhanced fatty acid uptake by adipose tissue represents a major factor in determination of adiposity in the chicken.

Adipsin mRNA amounts are not decreased in the genetically obese Zucker rat

Adipsin gene expression as assessed by mRNA amounts was examined in adipose tissue of genetically obese rats at the onset (16 days of age) or at later stages (30 and 60 days of age) of obesity. Amounts of mRNA were equivalent in obese and lean rats at 16 days of age. In adult rats, we observed a 2-fold decrease in adipsin mRNA in the obese rats compared with control lean rats, which was abolished by weaning the animals on a high-fat diet. Our data show that, in sharp contrast with genetically obese mice, adipsin mRNA is not suppressed in genetically obese Zucker rats.

Adipose-tissue-specific increase in glyceraldehyde-3-phosphate dehydrogenase activity and mRNA amounts in suckling pre-obese Zucker rats. Effect of weaning

The regulation of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) gene expression was studied during the onset of obesity in the genetically obese (fa/fa) rat by determination of GAPDH activity and hybridizable mRNA amounts in adipose tissue and liver from suckling and weanling rats. GADPH activity remained low throughout the suckling period, and a burst of activity occurred after weaning in both lean and obese pups. As early as 7 days of age, adipose tissue from pre-obese rats displayed a significant increase in enzyme activity, whereas no difference could be detected in the liver. In both suckling (16 days of age) and weanling (30 days of age) obese rats a proportionate increase in GAPDH activity and mRNA amounts was observed in adipose tissue, but not in liver. It is concluded that the obese genotype influences GAPDH gene expression at a pretranslational level and in a tissue-specific manner. This phenomenon could partly contribute to the hyperactive fat accretion in the obese rat, since glycolysis is the major metabolic pathway for lipogenic substrates in adipose tissue.

Differential effects of retinoic acid upon early and late events in adipose conversion of 3T3 preadipocytes

When confluent 3T3-F442A cell cultures (Day 0) were grown for 3 days in fetal calf serum-supplemented medium containing isobutyl methyl xanthine and dexamethasone (induction phase) and then shifted to serum-free hormone-defined medium (expression phase), they spontaneously exhibited a sharp rise in lipoprotein lipase activity (LPL); at the peak (Day 7) the LPL activity was about 25 times higher than in control cultures and was further enhanced by insulin. Although this expression of LPL activity was spontaneous, the emergence of glycerophosphate dehydrogenase (G3PDH) activity was completely dependent upon insulin as well as upon the expression of the differentiated phenotype. In committed cells, insulin elicited sustained DNA synthesis associated with limited cell proliferation. The addition of retinoic acid during the phase of expression inhibited insulin-dependent terminal differentiation (i.e., the emergence of G3PDH activity and acquisition of the differentiated phenotype). In addition, retinoic acid counteracted the stimulating effect of insulin upon LPL activity, but affected neither the mitotic process nor the spontaneous emergence of LPL activity. When added during the phase of induction, it prevented the overall process of adipogenic differentiation. Thus, the use of retinoic acid can indicate independent control of the mitogenic and lipogenic effects of insulin following commitment to adipogenic differentiation.

Increased lipoprotein lipase content in the adipose tissue of suckling and weaning obese Zucker rats

The aim of this study was to determine whether the increase in lipoprotein lipase activity displayed by the adipose tissue of obese (fa/fa) rats as compared with that of lean (Fa/fa) rats could be ascribed to a change in the content or in the catalytic properties of the enzyme. The question was addressed in rats of two ages: in 7-day-old suckling and in 30-day-old post-weaning pups. Inguinal fat-pads were removed surgically (7 days of age) or after killing (30 days of age), and acetone-extract powders were prepared. The relative quantity of enzyme was assessed by immunotitration using an antiserum raised in goat against purified lipoprotein lipase from rat adipose tissue. The results indicate that increases in enzyme activity in obese animals were strictly paralleled by increases in the amount of enzyme in suckling as well as in post-weaning pups. Moreover, the apparent Km values of lipoprotein lipase for its substrate triacylglycerol were identical in the two genotypes. In conclusion, the genotype-mediated increase in lipoprotein lipase activity in adipose tissue of obese Zucker rats was fully accounted for by an increase in the content of the enzyme. In addition, this work documents the mechanism of the increase in lipoprotein lipase activity during weaning, which is mediated mainly through changes in the adipose-tissue enzyme content.

Growth hormone binding to cultured preadipocytes from obese fa/fa rats increases during cell differentiation

Preadipocytes from 7 day-old obese Zucker rats and from their lean littermates were grown in primary culture. Specific binding of human growth hormone (hGH) to the cells is demonstrated. There is no change in hGH binding to preadipocytes from lean animals before (day 5 of the culture) and after adipose conversion (day 12). Moreover, addition of insulin to the culture medium does not affect the specific binding of 125I-hGH to the cells. On the contrary, when 1 nM insulin is added to the culture medium of preadipocytes from obese rats, the specific binding of 125I-hGH is enhanced during the cell differentiation, with a 3-fold increase in the number of binding sites. The functional significance of these additional GH sites in differentiated preadipocytes from obese rats has to be examined.

A method for separating cultured preadipocytes according to their density: application to stromal cells from overfed suckling rats

The stroma vascular fraction of adipose tissue consists of a heterogeneous cell population; not all the cells in this compartment undergo adipose conversion in primary culture. A density gradient centrifugation procedure was used to separate cultured cells on the basis of their triglyceride content. This method was applied to both stroma vascular cells from rat adipose tissue and to a 3T3 F442A preadipose cell line as a reference. Comparison of the results obtained from these two cell types suggests that this separation procedure can lead to a quantification of adipose differentiation in the heterogeneous stromal cell population. Separation procedures were applied to cultured stromal cells derived from young rats during the onset of nutritional obesity induced by overfeeding in early life. Results show that early overfeeding induced an increase in the stromal cell differentiation capacity which is expressed in vitro.

Role of adipocyte precursors in the onset of obesity induced by overfeeding in suckling rats

Rats were overfed during the suckling period by litter size manipulation in order to investigate the possible contribution of preadipocytes from the stroma-vascular compartment of adipose tissue to the development of obesity. Rats raised in litters of four pups were overfed; for normal feeding we assigned eight pups per litter. As early as 10 d of age, overfed rats became fatter than controls, and showed an increase in both plasma insulin and triacylglycerol levels. At this age, adipose tissue overdevelopment arose only from adipocyte hypertrophy, since hyperplasia occurred only at 15 d of age. Concurrently, compared to normal feeding, overfeeding led to significantly higher activities of lipoprotein lipase (LPL), glycerophosphate dehydrogenase (GPDH) and glycerophosphate acyltransferase (GPAT) in mature fat cells; 10-d-old overfed pups exhibited a higher stromal cell number. Further separation of this heterogeneous fraction by density gradient centrifugation showed a higher preadipocyte number as compared to that of controls. In stromal cells, LPL, GPDH, GPAT and acyl CoA ligase activities were detected during the suckling period. As compared to controls, overfeeding induced an increase in both LPL and GPDH activities in 10-d-old pups. Results indicate that overfeeding in early life induced an excess of fat storage capacity through a simultaneous increase in proliferation and differentiation rates of adipocyte precursors.