Studies on the Metabolism of Adipose Tissue. XVII. In vitro Effects of Insulin upon the Metabolism of the Carbohydrate and Triglyceride Stores of Adipose Tissue from Fasted-refed Rats*

A Receptor of the Immunoglobulin Superfamily Regulates Adaptive Thermogenesis

Exquisite regulation of energy homeostasis protects from nutrient deprivation but causes metabolic dysfunction upon nutrient excess. In human and murine adipose tissue, the accumulation of ligands of the receptor for advanced glycation end products (RAGE) accompanies obesity, implicating this receptor in energy metabolism. Here, we demonstrate that mice bearing global- or adipocyte-specific deletion of Ager, the gene encoding RAGE, display superior metabolic recovery after fasting, a cold challenge, or high-fat feeding. The RAGE-dependent mechanisms were traced to suppression of protein kinase A (PKA)-mediated phosphorylation of its key targets, hormone-sensitive lipase and p38 mitogen-activated protein kinase, upon β-adrenergic receptor stimulation—processes that dampen the expression and activity of uncoupling protein 1 (UCP1) and thermogenic programs. This work identifies the innate role of RAGE as a key node in the immunometabolic networks that control responses to nutrient supply and cold challenges, and it unveils opportunities to harness energy expenditure in environmental and metabolic stress.

Hurtado del Pozo et al. show that the deletion of adipocyte RAGE, whose ligands accumulate in metabolic stress, protects from obesity and cold challenges through the modulation of protein kinase A activities. This work adds RAGE to the immunometabolic networks that regulate energy expenditure in environmental and metabolic stress.

Utilization of dietary glucose in the metabolic syndrome

This review is focused on the fate of dietary glucose under conditions of chronically high energy (largely fat) intake, evolving into the metabolic syndrome. We are adapted to carbohydrate-rich diets similar to those of our ancestors. Glucose is the main energy staple, but fats are our main energy reserves. Starvation drastically reduces glucose availability, forcing the body to shift to fatty acids as main energy substrate, sparing glucose and amino acids. We are not prepared for excess dietary energy, our main defenses being decreased food intake and increased energy expenditure, largely enhanced metabolic activity and thermogenesis. High lipid availability is a powerful factor decreasing glucose and amino acid oxidation. Present-day diets are often hyperenergetic, high on lipids, with abundant protein and limited amounts of starchy carbohydrates. Dietary lipids favor their metabolic processing, saving glucose, which additionally spares amino acids. The glucose excess elicits hyperinsulinemia, which may derive, in the end, into insulin resistance. The available systems of energy disposal could not cope with the excess of substrates, since they are geared for saving not for spendthrift, which results in an unbearable overload of the storage mechanisms. Adipose tissue is the last energy sink, it has to store the energy that cannot be used otherwise. However, adipose tissue growth also has limits, and the excess of energy induces inflammation, helped by the ineffective intervention of the immune system. However, even under this acute situation, the excess of glucose remains, favoring its final conversion to fat. The sum of inflammatory signals and deranged substrate handling induce most of the metabolic syndrome traits: insulin resistance, obesity, diabetes, liver steatosis, hyperlipidemia and their compounded combined effects. Thus, a maintained excess of energy in the diet may result in difficulties in the disposal of glucose, eliciting inflammation and the development of the metabolic syndrome.

Experimental copper and chromium deficiency and additional molybdenum supplementation in goats. II. Concentrations of trace and minor elements in liver, kidneys and ribs: haematology and clinical chemistry

Since the mid-1980s a previously undescribed disease has affected moose in south-western Sweden. Investigations made to reveal evidence of a viral aetiology have proved unsuccessful. Trace element studies in apparently healthy moose shot during regular hunting suggested a trace element imbalance, with excessive molybdenum uptake causing secondary copper deficiency. The results also indicated a possible chromium deficiency. To verify this hypothesis, an experimental study was performed in male goats fed a semi-synthetic diet for 1.5 years. The animals were kept and treated in four groups: Controls, Cu-deficient group (group 1), Cr-deficient group (group 2), and combined Cu- and Cr-deficient group with additional supplementation of tetrathiomolybdate for 10 weeks at the end of the study (group 3). The present paper presents tissue contents of trace and minor elements, haematology and clinical chemical parameters. Feed consumption and weight development, as well as pathological and histopathological investigations, were also performed in this study, but these results are presented elsewhere. Changes in trace element concentrations were determined by comparing groups 1, 2 and 3 with the control group. Increased concentrations were observed for Al, Ca, Co, Fe, Mo, Pb, Se in the liver; for Al, Cd, Co, Cr, Mo in the kidneys; and for Mn and Mo in the ribs. Considerable accumulation of Mn in ribs seems to be a useful way to determine oxidative stress. Decreases in Mg and P in all organs and blood serum is characteristic of Cu deficiency and molybdenosis. Also the ratio of Ca/Mg was increased as the result of tissue lesions causing an intracellular increase in Ca and decrease in Mg. The trace element changes observed in group 1 were enhanced by the Mo supplementation in group 3, resulting in characteristic patterns, 'spectra' of changes. The alterations were not as remarkable in group 2 as in the two other groups. However, Cr deficiency considerably influenced Al, Co, V and to a smaller extent also Mn in ribs. In groups 1 and 2, only a few minor changes were detected in the haematological parameters, probably caused by increased adrenal activity after transportation of the animals. In group 3, severe anaemia was present but also a leukopenia. For the different clinical chemical parameters measured, all three groups showed changes, explained mainly by the altered activity of enzymes induced by trace element deficiencies and imbalance. Impaired carbohydrate and lipid metabolism was seen in groups 1 and 3, with increased concentrations of glucose, lactate and triglycerides in serum. Increased concentrations of total bilirubin were measured in all three groups (bile stasis was also seen post mortem). A considerably increased concentration of serum urea was found in group 3, although there were no indications of renal insufficiency or dehydration. Regarding hormones, a substantial decrease was seen in thyroxine (T4) in group 3 as a result of the molybdenosis, but a minor decrease was also seen in group 1. Insulin on the other hand showed increased levels in group 3--and especially in group 2 due to the Cr deficiency but also affected by the molybdenosis. As could be expected, Cu deficiency (groups 1 and 3) caused low levels of caeruloplasmin, secondarily affecting the Fe metabolism in these animals. Protein abnormalities, detected as altered electrophoretic patterns of serum proteins, were also seen mainly in group 3. The findings were also confirmed by multivariate data analysis, where PCA revealed the overall impact of the deficiencies, and PLS regression coefficients indicated the influence on the various analytes.

Experimental copper and chromium deficiency and additional molybdenum supplementation in goats. I. Feed consumption and weight development

Secondary Cu deficiency, Cr deficiency and molybdenosis were suggested causes of the 'mysterious' disease afflicting moose (Alces alces L.) in a region in south-west Sweden affected by acid rain. A model experiment with goats was performed to study the clinical chemical parameters, determine the tissue contents of trace and minor elements, to perform pathological and histopathological investigations and to compare the findings with those in moose disease. Twenty 3-month-old male goats were assigned to four dietary treatments (five animals each) in an experiment lasting for 20 months. The four groups in the study were: control group, Cu-deficient group (group 1), Cr-deficient group (group 2), and Cu- and Cr-deficient group (group 3). The animals were fed a basic semi-synthetic diet. At the end of the study the three surviving animals of group 3 were supplemented with additional tetrathiomolybdate (TTM) during the last 2 months. Feed consumption and weight development of the animals were monitored and are presented. The feed consumption of the two Cu-deficient groups of goats (group 1 and group 3) supported the previously described observations in copper deficiency in ruminants, e.g. decreased appetite and feed intake. A previously unreported effect of Cr deficiency in ruminants is now described in goats. Chromium deficiency at adequate Cu supplementation (group 2), caused increased lipid synthesis and a weight gain of 32 kg compared with that of the control group (20 kg). A possible explanation for this unexpected weight increase in only Cr deficiency is discussed. It is concluded that the feeding experiment does not support the hypothesis concerning the relation of Cr deficiency to the moose disease.

Interaction between insulin and thyroid hormones on the control of carbohydrate and lipid metabolism in rat adipose tissue

Adipose tissue segments excised from normal rats and from rats rendered experimentally hypothyroid and hyperthyroid retained insulin responsiveness when studied in vitro. Basal rates of glucose oxidation to CO2, conversion into glyceride-glycerol, fatty acids, and total lipids, and the activities of pyruvate dehydrogenase and fatty acid synthetase were enhanced in fat pads from hyperthyroid rats when compared with values seen with tissue from euthyroid animals. The response of each of these parameters was further enhanced by treating tissue from hyperthyroid rats in vitro with insulin. Basal rates of glucose oxidation and the activities of pyruvate dehydrogenase and fatty acid synthetase were depressed as a result of hypothyroidism. However, all of these values could be restored to levels approaching the values seen in the basal state for euthyroid rats when tissue segments from hypothyroid rats were incubated in vitro with insulin. The basal rates for glucose conversion into glyceride-glycerol, fatty acids, and total lipids were not changed by hypothyroidism but retained insulin responsiveness. These data suggest that the insulin-effector system in adipose tissue is not altered by thyroid status and that thyroid hormones may act independently with insulin to regulate glucose and lipid metabolism in this tissue at multiple intracellular metabolic sites.

Structure-function relationships in the adipose cell. II. Pinocytosis and factors influencing its activity in the isolated adipose cell

Pinocytic activity in the adipose cell has been examined by measuring the uptake of colloidal gold. Pinocytic activity occurs in the isolated adipose cell under all experimental conditions; a portion of the vesicular elements of the cell can be identified by electron microscopy as pinocytic in origin. The isolated adipose cell appears to take up serum albumin by pinocytosis. Pinocytic activity in the isolated adipose cell is enhanced by epinephrine, but not by insulin. The relationship between pinocytosis and the metabolic activity of the adipose cell has been studied by measuring simultaneously the uptake of radioactive colloidal gold, the incorporation of (14)C-counts from U-glucose-(14)C into CO(2), total lipid, triglyceride glycerol and triglyceride fatty acids, and the release of nonesterified fatty acids in the absence of hormones and in the presence of insulin or epinephrine. Correlations between hormone-produced alterations in lipid metabolism and in pinocytic activity suggest that intracellular nonesterified fatty acid levels are a factor in the regulation of both the cell's pinocytic activity and its metabolism and that pinocytosis in the adipose cell functions in the extracellular-intracellular transport of nonesterified fatty acids.

The role of the cytoplasmic redox potential in the control of fatty acid synthesis from glucose, pyruvate and lactate in white adipose tissue

The metabolism of lactate, pyruvate and glucose was studied in epididymal adipose tissue of starved, normally fed and starved-re-fed rats. Lactate conversion into fatty acid occurred at an appreciable rate only in the adipocyte of starved-re-fed animals. NNN'N'-Tetramethyl-p-phenylenediamine, an agent that transports reducing power from the cytoplasm to the mitochondria, caused large increments of fatty acid synthesis from lactate and a smaller one from glucose but a decrease in that from pyruvate. Glucose (1.0mm) increased fatty acid synthesis from lactate 4.3-fold but only 1.67-fold from pyruvate in adipocytes from normally fed animals. 2-Deoxyglucose decreased fatty acid synthesis from lactate to a greater degree (threefold) compared to that from pyruvate in adipocytes from starved-re-fed animals. l-Glycerol 3-phosphate contents were approximately equal in epididymal fat-pads, incubated in the presence of lactate or pyruvate, from normally fed animals, whereas the addition of 1mm-glucose resulted in a tenfold increase in l-glycerol 3-phosphate content only in the presence of lactate. The l-glycerol 3-phosphate content was tenfold higher in adipose tissue from starved-re-fed animals incubated in the presence of lactate than in the presence of pyruvate. 2-Deoxyglucose caused these values to be slightly lowered in the presence of lactate. We suggest that lactate metabolism is limited by the rate of NADH removal from the cytoplasm. In the starved-re-fed state, this occurs by reduction of dihydroxyacetone phosphate formed from glycogen to produce l-glycerol 3-phosphate, thus permitting lactate conversion into fatty acid. When glucose is the substrate, and rates of transport are not limiting, the rate of removal of cytoplasmic NADH limits glucose conversion into fatty acid.