Insulin increases the synthetic rate and messenger RNA level of lipoprotein lipase in isolated rat adipocytes

Lipoprotein lipase (LPL) is the enzyme responsible for hydrolysis of circulating triglyceride-rich lipoproteins and is important for storage of adipocyte lipid. To study the regulation of LPL synthetic rate in adipose tissue, primary cultures of isolated rat adipocytes were pulse-labeled with [35S]methionine, and LPL was immunoprecipitated with an LPL-specific antibody. A pulse-chase experiment identified the cellular and secreted forms of LPL as a 55-57-kDa protein. In the presence of heparin, there was a large increase in secretion of newly synthesized LPL from the cells, although heparin did not stimulate cellular LPL synthetic rate. When cells were exposed to insulin for 2 h, pulse-labeling revealed that insulin stimulated a maximal dose-related increase in LPL synthetic rate of 300% of control. This increase in LPL synthetic rate was observed after an exposure to insulin for as little as 60 min and was accompanied by only a 10-25% increase in total protein synthesis. In addition, insulin had no effect on the turnover of intracellular LPL. Using a cDNA probe for LPL, insulin induced a 2-fold increase in the LPL mRNA. Thus, insulin stimulated an increase in specific LPL mRNA in isolated rat adipocytes. This increase in LPL mRNA then leads to an increase in the synthetic rate of the LPL protein.     

Cachectin/tumor necrosis factor decreases human adipose tissue lipoprotein lipase mRNA levels, synthesis, and activity

The effects of the cytokine cachectin/tumor necrosis factor (TNF) on human adipose tissue lipoprotein lipase (LPL) were studied. TNF is produced by activated macrophages and is thought to play a role in mediating hypertriglyceridemia and wasting of adipose tissue triglyceride stores (cachexia) that often accompany infection and malignancy. TNF effects were studied in human adipose tissue fragments maintained in organ culture in the presence of insulin and dexamethasone to induce high LPL activity. Addition of TNF to the culture medium for 20 h caused a dose-dependent inhibition of LPL activity to an average of 37% of controls at 50 U/ml TNF. This inhibition of LPL activity was explained by specific decreases in levels of LPL mRNA (to 40% of controls) and rates of LPL synthesis determined by biosynthetic labeling and immunoprecipitation (to 32% of controls). The decline in LPL synthesis was specific, as it occurred despite a small increase in overall protein synthesis in the presence of TNF. Comparable decreases in LPL activity were observed when TNF was added to adipose tissue cultured solely in the presence of insulin. Thus, similar to results in rodent models, TNF is a potent inhibitor of LPL gene expression in human adipose tissue. TNF may therefore play a role in the disorders of triglyceride catabolism and the pathogenesis of cachexia that occur with stimulation of the immune system in humans.     

Lipoprotein lipase mRNA in white adipose tissue but not in skeletal muscle is increased by pioglitazone through PPAR-gamma

Lipoprotein lipase (LPL), a key enzyme for triglyceride hydrolysis, is an insulin-dependent enzyme and mainly synthesized in white adipose tissue (WAT) and skeletal muscles (SM). To explore how pioglitazone, an enhancer of insulin action, affects LPL synthesis, we examined the effect of pioglitazone on LPL mRNA levels in WAT or SM of brown adipose tissue (BAT)-deficient mice, which develop insulin resistance and hypertriglyceridemia. Both LPL mRNA of WAT and SM were halved in BAT-deficient mice. Pioglitazone increased LPL mRNA in WAT by 8-fold, which was substantially associated with a 4-fold increase of peroxisome proliferator activated receptor (PPAR)-gamma mRNA (r=0.97, p<0.0001), whereas pioglitazone did not affect LPL mRNA in SM. These results suggest that pioglitazone exclusively increases LPL production in WAT via stimulation of PPAR-gamma synthesis. Since pioglitazone does not affect LPL production in SM, this would contribute to prevent the development of insulin resistance due to lipotoxicity.     

The role of glucose and glycosylation in the regulation of lipoprotein lipase synthesis and secretion in rat adipocytes

Several studies have suggested that insulin and glucose increase adipose tissue lipoprotein lipase (LPL). To study the mechanism of the glucose-induced stimulation of LPL, the effects of glucose and glycosylation were examined in primary rat adipocyte cultures. In cells cultured in the presence of 1 mg/ml glucose, a 55-kDa LPL protein was synthesized and secreted into the medium, whereas cells cultured in glucose-free medium synthesized a 49-kDa form of LPL which was not secreted. The treatment of the mature 55-kDa form of LPL with peptide:N-glycosidase-F resulted in the formation of a 49-kDa form of LPL. When cells were cultured in the presence of tunicamycin, a 49-kDa form of LPL was synthesized by the cells but was not secreted. In addition, LPL activity was reduced by 90% when glycosylation was blocked by either tunicamycin or glucose deprivation. LPL synthetic rate was examined in cells cultured in a spectrum of glucose concentrations. LPL synthetic rate increased directly with medium glucose concentration and was decreased 80% in the absence of glucose compared to the synthetic rate in the presence of 1 mg/ml glucose. In addition, LPL synthetic rate in the presence of insulin was approximately 200% of the synthetic rate in untreated control cells at all glucose concentrations and even in the absence of glucose. In spite of the effect of glucose on LPL synthetic rate, glucose had no effect on the level of LPL mRNA. In contrast, the mRNA for the 78-kDa glucose-regulated protein (GRP78) was increased in adipocytes cultured in glucose-free medium. In summary, glucose was essential for glycosylation of LPL, and glycosylation was essential for LPL catalytic activity and secretion. In addition, glucose stimulated LPL synthetic rate and potentiated the stimulatory effects of insulin, but had no specific effect on LPL mRNA. Whereas insulin stimulates LPL by increasing the level of LPL mRNA, glucose stimulates LPL translation and post-translational processing.     

Studies on the role of insulin in the regulation of glyceride synthesis in rat epididymal adipose tissue.

1. When rat isolated fat-cells were incubated with fructose and palmitate, insulin significantly stimulated glyceride synthesis as measured by either [14C]fructose incorporation into the glycerol moiety or of [3H]palmitate incorporation into the acyl moiety of tissue glycerides. Under certain conditions the effect of insulin on glyceride synthesis was greater than the effect of insulin on fructose uptake. 2. In the presence of palmitate, insulin slightly stimulated (a) [14C]pyruvate incorporation into glyceride glycerol of fat-cells and (b) 3H2O incorporation into glyceride glycerol of incubated fat-pads. 3. At low extracellular total concentrations of fatty acids (in the presence of albumin), insulin stimulated [14C]fructose, [14C]pyruvate and 3H2O incorporation into fat-cell fatty acids. Increasing the extracellular fatty acid concentration greatly inhibited fatty acid synthesis from these precursors and also greatly decreased the extent of apparent stimulation of fatty acid synthesis by insulin. 4. These results are discussed in relation to the suggestion [A.P. Halestrap & R.M.Denton (1974) Biochem. J. 142, 365-377] that the tissue may contain a specific acyl-binding protein which is subject to regulation. It is suggested that an insulin-sensitive enzyme component of the glyceride-synthesis process may play such a role.     

Defective uptake of triglyceride-associated fatty acids in adipose tissue causes the SREBP-1c-mediated induction of lipogenesis

Lipoprotein lipase (LPL) is the only known enzyme in the capillary endothelium of peripheral tissues that hydrolizes plasma triglycerides and provides fatty acids (FAs) for their subsequent tissue uptake. Previously, we demonstrated that mice that express LPL exclusively in muscle develop essentially normal fat mass despite the absence of LPL and the deprivation of nutritionally derived FAs in adipose tissue (AT). Using this mouse model, we now investigated the metabolic response to LPL deficiency in AT that enables maintenance of normal AT mass. We show that the rate of FA production was 1.8-fold higher in LPL-deficient AT than in control AT. The levels of mRNA and enzymatic activities of important enzymes involved in FA and triglyceride biosynthesis were induced concomitantly. Increased plasma glucose clearing and (14)C-deoxyglucose uptake into LPL-deficient mouse fat pads indicated that glucose provided the carbon source for lipid synthesis. Leptin expression was decreased in LPL-deficient AT. Finally, the induction of de novo FA synthesis in LPL-deficient AT was associated with increased expression and processing of sterol regulatory element binding protein 1 (SREBP-1), together with an increase in INSIG-1 expression. These results suggest that in the absence of LPL in AT, lipogenesis is activated through increased SREBP-1 expression and processing triggered by decreased availability of nutrition-derived FAs, elevated insulin, and low leptin levels.     

Effect of improved diabetes control on the expression of lipoprotein lipase in human adipose tissue.

Patients with diabetes commonly manifest hypertriglyceridemia along with decreased adipose tissue lipoprotein lipase (LPL) activity, and improved diabetes control tends to reverse these abnormalities. To better understand the mechanism of regulation of LPL in diabetes, 11 diabetic patients (3 type I, 8 type II) were brought under improved glycemic control, and adipose tissue LPL gene expression was assessed by performing paired fat biopsies. Six of the 11 patients attained improved control with insulin, with a decrease in glycohemoglobin (glyc Hgb) from 13.8 +/- 0.9 to 10.4 +/- 0.6%; 5 patients attained improved control with glyburide (glyc Hgb fell from 14.2 +/- 2.4 to 8.8 +/- 0.6%), and together they demonstrated a lowering of serum triglycerides and total cholesterol. No changes were observed in HDL cholesterol. Improved diabetes control resulted in a significant increase in LPL activity in both the heparin-releasable (HR) and extractable (EXT) fractions of adipose tissue, as well as in LPL immunoreactive mass. The change in LPL activity with improved control was variable, and showed a positive correlation with the HDL levels prior to treatment (r = 0.74, P less than 0.02). When adipose tissue was pulse-labeled with [35S]methionine, there was an increase in isotope incorporation into LPL after treatment, indicating an increase in LPL synthetic rate. However, improved diabetes control resulted in no significant change in LPL mRNA levels. Thus, improved glycemic control resulted in an increase in LPL activity which correlated with each patient's basal high density lipoprotein. This increase in LPL activity was accompanied by an increase in LPL immunoreactive mass, and an increase in LPL synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)     

Regulation of protein synthesis in lactating rat mammary tissue by cell volume

The effect of changing cell volume on rat mammary protein synthesis has been examined. Cell swelling, induced by a hyposmotic challenge, markedly increased the incorporation of radiolabelled amino acids (leucine and methionine) into trichloroacetic acid (TCA)-precipitable material: reducing the osmolality by 47% increased leucine and methionine incorporation into mammary protein by 147 and 126% respectively. Conversely, cell shrinking, induced by a hyperosmotic shock, almost abolished the incorporation of radiolabelled amino acids into mammary protein: increasing the osmolality by 70% reduced leucine and methionine incorporation into mammary protein by 86 and 93% respectively. The effects of cell swelling and shrinking were fully reversible. Volume-sensitive mammary tissue protein synthesis was dependent upon the extent of the osmotic challenge. Isosmotic swelling of mammary tissue, using a buffer containing urea (160 mM), increased the incorporation of radiolabelled leucine into TCA-precipitable material by 106%. Swelling-induced mammary protein synthesis was dependent upon calcium: removing extracellular calcium together with the addition of EGTA markedly reduced volume-activated protein synthesis. Cell swelling-induced protein synthesis was inhibited by the Ca(2+) ATPase blocker thapsigargin suggesting that volume-sensitive protein synthesis is dependent upon luminal calcium.     

Increase of total protein synthesis during mouse oocyte growth

Quantitative changes of mouse oocyte protein synthesis during oogenesis have been studied by determining the rate of leucine incorporation into total cell protein. The leucine intracellular poor was artificially expanded by exposing the isolated oocytes to different concentrations of radiolabelled leucine, and leucine incorporation was followed as a function of the time. Protein synthesis by mouse oocytes increases linearly with the increase of the cell volume, and such increase is constant throughout the entire period of oocyte growth.     

Regulation of enzyme turnover during tissue differentiation. Interactions of insulin, prolactin and cortisol in controlling the turnover of fatty acid synthetase in rabbit mammary gland in organ culture

1. Explants of mammary gland from mid-pregnant rabbits were cultured in Medium 199 containing combinations of insulin, prolactin and cortisol. With hormone combinations which included prolactin, a sustained increase in the apparent rate of synthesis and in the amount of fatty acid synthetase was measurable immunologically. Maximum increase was produced with insulin, prolactin and cortisol present together. 2. With prolactin present alone, synthetase activity in the explants decreased to undetectable values after 1 day in culture, whereas the incorporation of l-[U-(14)C]leucine into immunodetectable material increased. Prolactin may therefore direct the synthesis of immunologically cross-reactive precursors of fatty acid synthetase which are enzymically inactive. 3. Culture with dibutyryl cyclic AMP plus theophylline in the presence of insulin, prolactin and cortisol delayed the increase in the rate of synthesis and accumulation of the synthetase. These compounds may also prevent the apparent decrease in the rate of degradation of the synthetase which occurs on day 2 of culture. 4. A large decrease in the apparent rate of degradation of the synthetase on day 2 of culture occurs during culture with hormone combinations which include prolactin. The protein obtained by centrifugation of explant homogenates for 6min at 14000g(av.) is degraded continuously throughout the culture period. 5. This decrease in the apparent rate of degradation of the synthetase was measured by radio-immunological precipitation. It is probably part of a regulated programme of enzyme degradation and not a reflexion of the reutilization of radioactive amino acids for the following reasons. (a) The calculated increase in the amount of the synthetase in explants on day 2 of culture with insulin, prolactin and cortisol was approximately equal to the measured increase of the enzyme complex which accumulates in the explants. This suggests little or no enzyme degradation has occurred. (b) Explants were cultured for 24h with insulin, prolactin and cortisol. They were then incubated with l-[U-(14)C]leucine, washed and incubated again for up to 4(1/2)h. l-[U-(14)C]Leucine rapidly equilibrated with the intracellular amino acid pool. Within 10min of incubation after washing explants to remove endogenous l-[U-(14)C]leucine the previously linear incorporation of l-[U-(14)C]-leucine into total explant protein ceased. This suggests that protein is synthesized from an amino acid pool which rapidly equilibrates with amino acids in the culture medium. (c) Explants were cultured for 24h as described in (b) but after washing they were cultured with insulin, prolactin and cortisol for 24h. Approx. 90% of the radioactivity lost from the ;free' intracellular amino acid pool and from amino acids derived from the degradation of explant protein in this period was detected in the culture medium. This suggests that the ;free' intracellular amino acids and amino acids derived from protein degradation can equilibrate with amino acids in the medium. A residual ;free' radioactive amino acid pool was present in the tissue. (d) Casein represents approx. 20% of the protein synthesized after 1 day in culture with insulin, prolactin and cortisol. Histological evidence suggests that on day 2 of culture, casein is unlikely to be degraded in the tissue. No increase in the radioactivity incorporated into casein can be measured in the 23h after incubation of explants with l-[U-(14)C]leucine as described in (b). This suggests that the incorporation of radioactivity into proteins during culture after incubation with l-[U-(14)C]leucine is minimal. (e) Inhibition of protein synthesis in explants by cycloheximide after incubation with l-[U-(14)C]leucine does not reveal a latent continuous degradation of fatty acid synthetase on day 2 of culture which might have been masked by the high rates of protein synthesis and therefore the accumulation of the enzyme. 6. The conclusion is discussed that there is a real decrease (or even cessation) in the rate of degradation of fatty acid synthetase during the period when the enzyme accumulates in explants cultured with hormone combinations which contain prolactin.     

Insulin regulation of lipoprotein lipase (LPL) activity and expression in gilthead sea bream (Sparus aurata)

Lipoprotein lipase (LPL) is a key enzyme in lipoprotein metabolism by virtue of its capacity to hydrolyze triglycerides circulating in the form of lipoprotein particles. Here we analyzed the fasting effects of LPL in gilthead sea bream (Sparus aurata) and also present the first study in fish of the role of insulin as a potential modulator of both LPL activity and expression. Fasting for 2 weeks provoked a clear decrease in adipose tissue LPL activity, concomitant with lower levels of plasma insulin, while no effects were observed in red muscle. To elucidate the specific role of insulin, increases of plasma insulin were experimentally induced by arginine and insulin injections. However, arginine predominantly stimulated glucagon over insulin secretion in this fish species while LPL activity did not change significantly in adipose tissue. Instead, insulin administration induced an increase in adipose tissue LPL activity 3 h after the injection, whereas LPL activity in red muscle was not affected. Changes in LPL activity were accompanied by an increase in LPL mRNA levels in the adipose tissue of insulin-injected gilthead sea bream, although changes in LPL expression were delayed in time with respect to variations in LPL activity. Finally, LPL mRNA levels in red muscle were similar between control and insulin-injected gilthead sea bream, suggesting that insulin does not play a direct role in the regulation of LPL in this tissue. The current study shows that LPL activity is regulated by nutritional condition and underscores the importance of insulin as a modulator of LPL activity and expression in the adipose tissue of gilthead sea bream.     

Hyperlipoproteinemia type I in a patient with active lipoprotein lipase in adipose tissue and indications of defective transport of the enzyme

This paper presents a case of typical hyperlipoproteinemia type I in a young woman. Her serum triglycerides varied between 2 and 90 mmol/l and she had substantial amounts of apolipoprotein B-48 in fasting plasma. She had no detectable lipoprotein lipase (LPL) activity in post-heparin plasma (less than 0.2 percent of normal). Southern blot analysis suggested no major defect in her LPL gene and Northern blot analysis of adipose tissue RNA showed normal-sized LPL-mRNA. A 2-h [35S]methionine incorporation experiment with adipose tissue pieces in vitro showed that she produced normal-sized LPL and had LPL catalytic activity in the tissue. The amounts were, however, only 5-10% of control. No detectable LPL radioactivity or catalytic activity was released from patient tissue even in the presence of heparin in the incubations. Immunofluorescent staining of adipose tissue biopsies from the patient showed LPL immunoreactivity only in adipocytes and little or none within the capillaries. Treatment of immunoprecipitated labeled LPL with endoglycosidase H showed that the oligosaccharide chains on her enzyme were of the high-mannose type and not processed as in controls. Taken together the data suggest that the patient synthesizes a relatively normal LPL protein which is core-glycosylated and folded into active enzyme as in normal subjects, but is not effectively transported via the Golgi to the cell surface.     

Insulin-stimulated protein synthesis in submandibular acinar cells: interactions with adrenergic and cholinergic agonists

The effects of insulin and secretory agonists on amino acid incorporation into submandibular gland proteins were studied using isolated acinar cell aggregates. Insulin stimulated the incorporation of 3H-leucine into TCA-precipitable proteins in a rapid, dose-dependent manner (half-maximal response at 1 nM). Isoproterenol, a beta-adrenergic agonist, also stimulated amino acid incorporation, and this effect was mimicked by both dibutyryl cAMP and IBMX, a phosphodiesterase inhibitor. Although insulin further stimulated incorporation in the presence of isoproterenol and IBMX, no additional increase in the rate of synthesis was observed after stimulation by dibutyryl cAMP. High concentrations of carbamylcholine, a cholinergic agonist, inhibited both basal and insulin-stimulated incorporation. At low concentrations, however, carbamylcholine stimulated synthesis, and the effects of insulin and carbamylcholine were additive. A23187, a calcium ionophore, also inhibited 3H-leucine incorporation and insulin stimulation, but in contrast to carbamylcholine, low concentrations of A23187 neither inhibited nor enhanced the rate of synthesis. Thus, protein synthesis in the rat submandibular gland is regulated by both insulin and neurotransmitters. Whereas beta-adrenergic stimulation appears to be mediated through cAMP, the intracellular signals mediating the actions of insulin and cholinergic agonists remain to be elucidated.     

Glycoprotein synthesis in explants of developing rabbit mammary gland in culture.

1. Explants of mammary glands of pregnant rabbits cultured in the absence of insulin, prolactin and cortisol incorporated [2-3H]mannose into lipid-linked mono- and oligo-saccharide and protein. 2. Inclusion of the hormones in the culture medium stimulated the incorporation of [2-3H]mannose into lipid-linked monosaccharide 4-fold, into lipid-linked oligosaccharide 4-fold and into protein 13-fold after 24 h in culture. 3. Addition of tunicamycin to the incubation medium completely inhibited the incorporation of [2-3H]mannose into lipid-linked oligosaccharide and protein after an initial lag period of about 2h. Incorporation of this radiolabel into lipid-linked monosaccharide was increased 4-fold under these conditions. 4. Incorporation of [4,5-3H]leucine into protein was unaffected by the presence of tunicamycin. 5. Analysis of mannose-labelled protein by polyacrylamide-gel electrophoresis indicated that a major radiolabelled protein of apparent mol.wt. 65,000-70,000 was synthesized and approx. 70% of this protein appeared in the soluble fraction. 6. Glycosylation of the protein but not synthesis of its peptide backbone was sensitive to tunicamycin. 7. Possible origins of this glycoprotein synthetized when the tissue is stimulated to differentiate in culture are discussed.     

Insulin stimulates synthesis of soluble proteins in isolated rat hepatocytes.

The incorporation of [3H]leucine into soluble cellular protein was measured in isolated hepatocytes at extracellular leucine concentrations ranging from 0.15 to 20.0 mM. Insulin caused a 12--15% stimulation of [3H]leucine incorporation in the presence of high extracellular leucine concentrations. It is concluded that insulin causes a small but significant increase in the rate of hepatic protein synthesis.     

Effects of putrescine on synthesis and degradation of ornithine decarboxylase in primary cultured hepatocytes

Changes in both synthesis rate and degradation rate of ornithine decarboxylase (ODC) were pursued in primary cultures of adult rat hepatocytes during the process of ODC induction caused by asparagine and glucagon and also during the process of rapid ODC decay caused by putrescine. The synthesis rate of ODC was determined by [35S]methionine incorporation into the enzyme, which was separated afterwards by immunoprecipitation and sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. The degradation rate of ODC was determined by following the decay of prelabeled ODC. The enzyme induction caused by asparagine (10 mM) and glucagon (1 microM) was due both to an increase in the synthesis rate and to a decrease in the degradation rate. Addition of 10 mM putrescine caused a rapid decay of ODC activity, which was faster than ODC decay in the presence of cycloheximide. This rapid decay in ODC activity was accompanied by slightly slower decay in ODC protein, which was due both to partial suppression of ODC synthesis and to several fold acceleration of ODC degradation.     

Amphibolic role of the Krebs cycle in the insulin-stimulated protein synthesis.

It has been a generally held view that insulin does not significantly affect the incorporation of amino acids into liver protein. This interpretation was based on data obtained from studies using the branched chain amino acids, which are poorly metabolized by the hepatic tissue. The effect of insulin on 14CO2 formation and protein incorporation of several 1-14C-labeled or U-14C-labeled amino acids was studied in isolated rat hepatocytes and diaphragm pieces. It was shown that insulin enhanced 14CO2 formation and protein incorporation primarily of those carbons of amino acids which are metabolized through the mitochondrial Krebs cycle. Using aminooxyacetic acid (0.5 mM), a potent inhibitor of the transamination reaction, it was shown that there exists an "insulin-sensitive" pool of glutamate which is preferentially utilized for protein synthesis in the presence of insulin. The insulin effect on protein incorporation of 14C-labeled glutamate generated in the Krebs cycle was abolished in the presence of aminooxyacetic acid. We interpret these results to signify that mitochondrial transamination of alpha-ketoglutarate to glutamate is essential for insulin stimulation of 14C incorporation into hepatocyte protein.     

Effects of adrenaline on the turnover of lipoprotein lipase in rat adipose tissue

The mechanisms by which adrenaline brings about a reduction in the lipoprotein lipase activity of adipose tissue in vitro were investigated. The incorporation of [3H]leucine into lipoprotein lipase was measured during 1-h pulse incubations of rat epididymal fat bodies that had been preincubated for 4 h in the presence of glucose, insulin and dexamethasone. When adrenaline was added to the incubation medium at the start of the pulse, the incorporation of [3H]leucine was markedly reduced, suggesting that the rate of the enzyme's synthesis had decreased. On the other hand, the degradation of lipoprotein lipase, as measured by the loss of 3H-labelled enzyme protein during pulse-chase incubations of the epididymal fat bodies, was found to be significantly increased by the addition of adrenaline to the incubation medium at the start of the chase period. It is concluded that adrenaline is able both to inhibit the synthesis of lipoprotein lipase and to stimulate its degradation.