Fetal Brown Adipose Tissue Research Articles

Ontogeny of thyroid hormone receptors and c-erbA expression during brown adipose tissue development: evidence of fetal acquisition of the mature thyroid status

We have determined the developmental changes in thyroid hormone receptor expression and thyroid hormone status in rat brown adipose tissue (BAT). The present study demonstrates that c-erbA alpha and -beta genes are expressed in the developing BAT. The 6.5-kilobase (kb) beta 1, 2.6-kb alpha 2, 5.5-kb alpha 1, and 6.6-kb alpha transcripts showed peak values on day 20 of fetal life. High affinity T3-binding sites were found in fetal BAT. Binding capacity was similar (18-day-old fetuses) or higher (20-day-old fetuses) than in postnatal or adult brown fat. In contrast, T3 receptors were scarce in fetal liver, and values in adult liver were similar to those in fetal BAT on day 20. The profiles of iodothyronine 5'-deiodinase, nuclear T3 content, and receptor occupancy also showed peak values in fetal BAT on day 20, but they were very low in fetal liver. Thus, BAT has already achieved complete maturation of its thyroid status on day 20 of gestational age. This highly tissue-specific developmental pattern is unique among other mammalian thyroid-sensitive tissues studied to date. Between days 18 and 20 of rat fetal development, there is a specific induction of the uncoupling protein gene expression, the main marker of differentiated adipocytes. The results suggest that thyroid hormones may be involved in the establishment of the differentiated phenotype of the brown fat cell in fetal life.

Postnatal changes in lambs of two pathways for thyroxine 5'-monodeiodination in brown adipose tissue

We have recently shown that ovine fetal brown adipose tissue (BAT) contains two distinct iodothyronine 5'-monodeiodinase (5'MDI) activities, one with a high Km (type I) and another with a low Km (type II). Both activities increased to maximum levels near term (150 days gestation). BAT plays a major role in neonatal temperature regulation in lambs, and available evidence suggests that BAT 5'MDI activity is closely linked to thermogenic capacity. To better characterize the changes in 5'MDI after birth, we studied both type I and type II 5'MDI in lamb BAT from the time of birth to 30 days of postnatal age. Type I 5'MDI activity [pmol 3,5,3'-triiodothyronine (T3).mg protein-1.h-1] showed no significant changes during the first 11 days after birth [newborn (NB), 95 +/- 16; 1 day, 83 +/- 20; 3-4 days, 80 +/- 11; 10-11 days, 92 +/- 28]. Activity decreased significantly at 30 days (24 +/- 8.9, P less than 0.05). On the other hand, the type II 5'MDI activity (fmol I- released.mg protein-1.h-1) increased significantly (P less than 0.01) during the first 4 days, (NB, 348 +/- 23; 1 day, 679 +/- 37; 3-4 days, 785 +/- 199), decreased toward NB values (401 +/- 87) at 10-11 days of age, and fell to 66 +/- 31 at 30 days (P less than 0.05 vs. NB). Kinetic analysis of BAT type II thyroxine 5'MDI revealed a rise in maximum velocity from NB to 1 and 3-4 days of age without a change in the enzymatic activity Km.(ABSTRACT TRUNCATED AT 250 WORDS)

Two Pathways for Thyroxine 5’-Monodeiodination in Brown Adipose Tissue in Fetal Sheep: Ontogenesis and Divergent Responses to Hypothyroidism and 3,5,3’- Triiodothyronine Replacement*

Thermogenesis in rat brown adipose tissue (BAT) is thyroid hormone responsive. Rat BAT expresses a type II 5'-iodothyronine monodeiodinase (5'MDI) which mediates local T3 production from T4. Earlier studies show that BAT from fetal and newborn sheep contains a high Km type I, instead of type II, 5'MDI. To better characterize the 5'MDI of ovine fetal BAT, we studied the in vitro monodeiodination of [125I]T4 at a low substrate concentration (2 nM) and in the presence of 1 mM propylthiouracil in BAT homogenates of control and thyroidectomized fetuses at different gestational ages as well as in newborn lambs. Thyroidectomies were performed at three gestational ages: 99-107 days (group 1), 129-132 days (group 2), and 115-117 days (group 3A). Animals were studied 8-13 days after surgery. A significant increase in the activity of a low Km T4 5'MDI was noted in BAT from hypothyroid fetuses at all three gestational ages. This low Km activity was similar to the type II enzyme in rat BAT and brain in that the activity was also T3 resistant. A gradual rise in BAT type II 5'MDI activity was measured between 99 days gestation and term (150 days). These results indicate that ovine BAT contains two distinct iodothyronine 5'-monodeiodinating activities, one with a high Km and another with a low Km. The latter, resembling the type II 5'MDI in rat brain and BAT, is increased in ovine hypothyroid BAT. The former predominates in euthyroid tissue and is similar to the type I 5'MDI characterized in rat liver, kidney, and thyroid. We speculate that BAT type II 5'MDI may be important for neonatal BAT thermogenesis, while the type I enzyme may play a significant role in the increase in serum T3 concentration that occurs at birth.

Thyroid hormones and 5'-deiodinase in rat brown adipose tissue during fetal life.

Brown adipose tissue (BAT) iodothyronine 5'-deiodinase (5'D) activities are very high during fetal life but decrease 10-fold a few hours before birth. Accordingly, BAT 3,5,3'-triiodothyronine (T3) concentrations are also very high. The temporal patterns of changes in BAT 5'-D and fetal plasma insulin are similar (and differ from the pattern for catecholamines) but are not superimposable. A causal role for insulin in the activation of fetal BAT 5'-D is therefore not supported by the data. Maternal thyroidectomy leads to a decrease in the total and relative weight of fetal BAT and to a 30-50% increase in BAT 5'-D activities; BAT thyroid hormone concentrations are essentially unchanged. Fetal hypothyroidism was induced by giving methimazole and resulted in a marked decrease of BAT thyroxine (T4) and T3 concentrations. This treatment increased BAT 5'-D activity only on day 21 of gestation, but no effect was observed on day 20. The fetal 5'-D response to thyroid hormones infused into the methimazole-treated dams was studied at 21 days of gestation. The increase in BAT 5'-D induced by methimazole treatment was prevented by T4 infused into control dams but not by T3. In fetuses from thyroidectomized dams, the pattern of 5'-D regulation by thyroid hormones was impaired. It is suggested that the high concentrations of thyroid hormones present in fetal BAT might participate in the general maturation and development of fetal BAT.

Oxygen consumption and guanosine diphosphate binding by fetal brown adipose tissue in diabetic pregnancy

Oxygen consumption and 3H-guanosine diphosphate (GDP) binding were determined in brown adipocytes and mitochondria from 28-day gestation fetuses of alloxan-diabetic rabbit does and saline-injected controls. Maternal diabetes was classified as severe or mild determined by whether maternal blood glucose values were greater or less than 200 mg/dL, respectively, at death. Basal oxygen consumption and adipocyte diameters did not vary among groups. A significant reduction in maximal norepinephrine (NE) stimulated O 2 consumption by fetal brown adipose tissue (BAT) cells was seen in offspring of severely diabetic pregnancies when compared with control values ( 248 ± 53 ± v 482 ± 32 μ L O 2 10 6 cells/h ; P < .005 ). In contrast, a significant increase in maximal NE-stimulated O 2 consumption by fetal BAT cells occurred in offspring of mild diabetic pregnacies (807 ± 60, P < .001 v controls). A highly significant inverse correlation between serum glucose levels and maximal O 2 consumption by fetal BAT was observed in fetuses from mild and severe diabetic pregnancies ( r = −.98, P < .005), and there was no correlation between these two parameters in offspring of normal pregnancies. A significant inverse correlation was observed between maximal O 2 consumption by fetal BAT cells and serum insulin levels in offspring of both control and diabetic pregnancy ( r = −.74; P < .02). Tissue cytochrome oxidase activity was lower in offspring of severely affected diabetic does, indicating a reduction in BAT mitochondrial content compared with controls. BAT mitochondria from fetuses of severely diabetic does exhibited reduced 3H-GDP capacity, which was 2.5-fold lower than controls. These studies show that brown adipocytes obtained from offspring of severely diabetic pregnancies exhibit reduced NE-stimulated oxygen consumption. Reduced thermogenin content, indicated by lower GDP binding, is the most likely cause of the impaired oxygen consumption offspring of severely affected diabetic pregnancies.

Fetal sheep temperatures in utero during cooling and application of triiodothyronine, norepinephrine, propranolol and suxamethonium.

Fetal sheep (n = 13) were chronically instrumented to measure temperatures in the maternal femoral artery (MAT), the amniotic fluid (AFT), the fetal brown adipose tissue (BFT) and the fetal arterial blood (DAT). Cooling loops were inserted into the amniotic cavity. In 4 fetuses osmotic minipumps delivering triiodothyronine (T3) were implanted subcutaneously. One to seven days after surgery the following results were obtained: 1) During control DAT was 0.59 +/- 0.2 degrees C (SD), BFT 0.60 +/- 0.24 degrees C and AFT 0.38 +/- 0.31 degrees C higher than MAT. T3 levels in treated fetuses were 3.4 +/- 1.5 micrograms/l. 2) Infusion of norepinephrine (NE) (5.2 +/- 0.9 micrograms/min per kg fetal body weight) with phentolamine (26.1 +/- 4.3 micrograms/min per kg) into a fetal vein did not change temperatures. 3) During cooling (-53 +/- 15 W) MAT decreased 0.45 +/- 0.3 degrees C, DAT 1.9 +/- 0.39 degrees C, BFT 1.61 +/- 0.52 degrees C and AFT 4.2 +/- 1.8 degrees C. 4) The amniotic fluid was cooled until steady state temperatures were achieved. Then propranolol (26.1 +/- 4.3 micrograms/min per kg) or suxamethonium (3 +/- 1 mg/kg) were introduced into the fetal vein. No consistent and significant changes of temperatures could be detected. It is concluded that 1) lowering the fetal core temperature by 1.6 - 1.9 degrees C and its ambient temperature (AFT) by 4.2 degrees C does not induce shivering or non-shivering thermogenesis suppressible by pharmacologic agents, 2) thermogenesis in fetal brown adipose tissue cannot be induced by NE (with or without supplemention of T3).(ABSTRACT TRUNCATED AT 250 WORDS)

Biochemical and ontogenic characterization of thyroxine 5'-monodeiodinase in brown adipose tissue from fetal and newborn lambs.

We characterized T4 5'-monodeiodinase (5'-MDI) and its ontogenic changes in brown adipose tissue (BAT) of fetal lambs from a gestational age of 99 days to the newborn period. BAT 5'-MDI activity in fetal or newborn lamb was markedly enhanced by dithiothreitol (DTT); a near-maximum stimulation was reached at a DTT concentration of 1 mM. The enzyme is heat-labile (inactivated when heated to 56 C for 5 min) and has a pH optimum at 6.5. In the presence of 1 mM DTT, BAT 5'-MDI was extremely sensitive to propylthiouracil (PTU): 0.04 mM PTU resulted in 94% inhibition of the T4 to T3-converting activity. An increase of DTT concentration to 20 mM in the incubation media reduced the inhibition by 0.04 mM PTU to 59%. Kinetic analysis, in the presence of 4 mM DTT, revealed a Michaelis-Menten constant (Km) of 1.43 microM for T4 and an inhibitory constant (Ki) of 22 microM for PTU. A marked and progressive rise of BAT 5'-MDI was noted from the 99-day fetus to the newborn period [6.0 vs. 94 +/- (SE) 12 pmol T3/mg protein X h]. The total sulfhydryl content in BAT varied from 3.8 to 5.4 mmol/kg wet weight equivalent, whereas nonprotein sulfhydryl content varied from 0.5 to 0.9 mmol/kg in groups of lambs between 99 days and the newborn period, without a significant change with age. These results indicate that 5'-MDI in fetal lamb BAT increases progressively between 99 days gestation and the neonatal period. Ovine fetal BAT 5'-MDI is highly sensitive to the inhibitory effect of PTU at low DTT concentration (1 mM) and is relatively insensitive to PTU only at high DTT concentration (20 mM). Finally, 5'-MDI in BAT from the fetal lamb reaches maximal activity near term and seems similar in its biochemical characteristics to the 5'-MDI of liver, kidney, or thyroid.

The effect of environmental temperature during pregnancy on thermoregulation in the newborn lamb

AbstractTwenty-four pregnant Scottish Blackface ewes were divided into three temperature-treatment groups 14 days before expected lambing: closely shorn and kept at 6°C (cold treatment, CD); in full fleece at 26°C (warm treatment, WM); and in full fleece at 6°C (controls, CL). Food allocation and intakes were similar for each group. Their lambs were tested for cold-induced summit metabolic rate capability (SMR) on the day of birth at a mean age of 12 h using water immersion to provide cooling. On the following day, the calorigenic response to subcutaneous injections of noradrenaline (NA) was measured to assess non-shivering thermogenesis capability. The ewes were blood-sampled during pregnancy and the lambs before and after the SMR test.In the ewes, blood glucose and free fatty acid levels were higher during cold treatment, but not significantly so. Blood glucose was lower in lambs from CD ewes (CD lambs) before SMR tests; other differences were not significant.During the cold test, SMR was highest in CD lambs, but not significantly so. Rectal temperature declined least during test in the CD lambs (P &lt; 0·05).The peak metabolic response (PMR) following NA injection was about 1·5 times greater in CD lambs than in the CL and WM lambs (P &lt; 0·05). The mean elevation of PMR over thermoneutral metabolic rate was respectively: 2·8, 1·8 and 1·7 times in the CD, CL and WM lambs (P &lt; 0·05), and this elevation was sustained for longer in the CD lambs (P &lt; 0·01). Thus, the total metabolic response to NA was markedly greater in CD lambs.It was concluded that cold exposure during late pregnancy favoured the deposition of (or checked the normal decline in) foetal brown adipose tissue, so raising the neonatal capacity for non-shivering thermogenesis.

Preparation of hormone-sensitive adipocytes from foetal rat brown adipose tissue

A collagenase-dispersion method involving Percoll gradient separation is described for the production of adipocytes from foetal rat brown adipose tissue. The cells produced are hormone-sensitive as judged by cAMP accumulation and lipolysis.

Hibernoma: Distinctive light and electron microscopic features and relationship to brown adipose tissue

Four hibernomas and samples of developing human brown and white adipose tissue were observed. Distinctive features of hibernomas were 1) lobules of closely apposed large polygonal cells and capillaries; 2) three principal cell types (granular eosinophilic, multivacuolated, and univacuolated) varying in prominence from case to case; 3) investment of each tumor cell by basal lamina; 4) an inverse relationship between lipid droplet size and the number of mitochondria per unit of cytoplasm; 5) pleomorphic mitochondria with dense matrixes or large, round mitochondria with transverse lamellar cristae; 6) undulating plasmalemmal invaginations; 7) micropinocytotic vesicles; 8) periodic short plasmalemmal densities; and 9) a conspicuous lack of cytoplasmic membrane systems. The frequent association of micropinocytotic vesicles and undulating plasmalemmal invaginations in proximity to capillaries strongly suggests that the invaginations represent a localized cell membrane adaptation for efficient endocytosis. In human fetal brown adipose tissue, which is first recognizable in fetuses of 21 weeks' gestational age, the most characteristic cell was the polygonal multivacuolated cell. Univacuolated cells were present in brown adipose tissue of older fetuses, and in infants and adults entire lobules containing univacuolated cells coexisted with lobules of multivacuolated cells and granular eosinophilic cells. The ultrastructure of human brown adipose tissue resembled that of hibernomas and was similar to previously described features of this tissue in animals. Developing white adipose tissue differed from brown adipose tissue by its loose plexiform arrangement of capillaries and spindle-shaped cells in less circumscribed lobules and by the absence of polygonal multivacuolated cells. The authors did not identify centripetal lobular maturation in white adipose tissue, but peripheral growth "caps" were a common finding in maturing brown adipose tissue. They consider brown adipose tissue to be a special form of adipose tissue, the variable cytologic composition of which is reflected in the histologic spectrum of hibernomas.

Perinatal lipogenesis in the liver and brown adipose tissue of the rat

1. 1. Hepatic lipogenesis falls during late foetal life, reaching low levels soon after birth. 2. 2. Of the lipogenic enzymes studied only changes in foetal liver acetyl-CoA carboxylase activities correlate well with the changes in lipogenic flux. 3. 3. In -contrast to liver, brown adipose tissue lipogenesis increases during late foetal life. 4. 4. A similar developmental pattern in foetal brown adipose tissue was observed for the activities of a number of enzymes normally associated with lipogenesis. 5. 5. The studies suggest the existence of different controls over the development of lipogenesis in the two tissues investigated during the perinatal period.

Perinatal lipogenesis in the liver and brown adipose tissue of the rat

1. Hepatic lipogenesis falls during late foetal life, reaching low levels soon after birth. 2. Of the lipogenic enzymes studied only changes in foetal liver acetyl-CoA carboxylase activities correlate well with the changes in lipogenic flux. 3. In contrast to liver, brown adipose tissue lipogenesis increases during late foetal life. 4. A similar developmental pattern in foetal brown adipose tissue was observed for the activities of a number of enzymes normally associated with lipogenesis. 5. The studies suggest the existence of different controls over the development of lipogenesis in the two tissues investigated during the perinatal period.

Distribution of label in maternal plasma, placenta, fetal plasma and tissues after injection of 14C-palmitate into the circulation of 21- and 28-day-pregnant rabbits.

Tissue and plasma samples were obtained from fetuses removed 1--20 min after injection of 14C-palmitate into the doe at day 21 or day 28 of gestation. Specific activity of maternal plasma free fatty acids (FFA) was higher, but that of fetal plasma FFA lower in 21- than 28-day-pregnant rabbits. In 21-day fetuses, total radioactivity in the liver never exceeded that in the placenta; in 28-day fetuses, the liver radioactivity exceeded that in the placenta after 3 min. Total radioactivity in the fetal brown adipose tissue (BAT) never exceeded that in the placental. Most of the placental radioactivity was initially in the FFA, but at later time intervals the majority of the label was in the phospholipids. The fetal liver and BAT rapidly esterified label into triglycerides. It is concluded that transport of FFA across the placenta is greater at 28 days than at 21 days gestation. The evidence suggests that a pool of placental FFA, which equilibrates rapidly with the maternal plasma FFA, is a possible source of fetal plasma FFA. The rapidity and quantity of label appearing in the fetal liver suggest that fatty acids may first be processed by the liver before transport to extrahepatic tissues.

Uptake and metabolism of 14C-palmitate by fetal rabbit tissues.

The uptake and esterification of 14C-palmitate into lipid classes in placenta, fetal brown adipose tissue (BAT) and liver of rabbits were investigated in vitro. Fetal BAT showed a high rate of fatty acid uptake, 8.5 mumol-a-1 tissue-h-1. From 5 min onwards, the majority of incorporated label was in the triglyceride fraction. The placenta and fetal liver also incorporated I-[14C]-palmitate into both FFA and esterified lipid fractions, although at much lower rates than observed for BAT. In the liver, triglycerides, but in the placenta phospholipids, contained the majority of the label after 1 h incubation. BAT from both fetal and newborn rabbits released 14CO2 and the production of 14 CO2 was greater in the presence of noradrenaline. The specific activity of the CO2 was the same in stimulated and unstimulated tissue. It is concluded that BAT as well as the liver are important sites of free fatty acid removal from the fetal circulation. The potential for fatty acid oxidation is present in BAT of the 28-day rabbit fetus.

Rat Fetal Brown Adipose Tissue &lt;i&gt;in vitro&lt;/i&gt;: Effects of Hormones and Ambient Temperature

Interscapular brown adipose tissue (BAT) from fetal rats was maintained in Eagle’s minimal essential medium for 6–24 h at 37 or 25 °C. Pyruvate kinase (PK), cytoplasmic and mitochondrial α-glycerophosphate dehydrogenase and phosphoenolpyruvate carboxykinase (PEPK) activities were determined at the start and end of incubation. During prenatal development, the activities of all enzymes, except PK, increased significantly. During incubation at 37 °C, PEPK activity decreased by about 60% in 24 h. The other enzyme activities also fell to a lesser extent. Only the mitochondrial enzyme remained unchanged. Both glucagon and insulin significantly slowed down the fall in PEPK activity. Norepinephrine added every hour for 6 h increased PEPK activity, but only in BAT from fetal rats weighing less than 3.5 g. Incubation at 25 °C largely prevented the decrease in enzyme activities seen at 37 °C. Fetal liver reacted in the opposite way. At 37 °C, PEPK activity increased while at 25 °C it decreased. However, postnatal liver responded to incubation temperature like fetal BAT. It is postulated that the initial level of PEPK (high in fetal BAT and postnatal liver and low in prenatal liver) plays a role in the &lt;i&gt;in vitro&lt;/i&gt; response.