Interactions of insulin, glucagon and dexamethasone in controlling the activity of glycerol phosphate acyltransferase and the activity and subcellular distribution of phosphatidate phosphohydrolase in cultured rat hepatocytes

Ischemic-Trained Monocytes Improve Arteriogenesis in a Mouse Model of Hindlimb Ischemia

OBJECTIVE

Monocytes, which play an important role in arteriogenesis, can build immunologic memory by a functional reprogramming that modifies their response to a second challenge. This process, called trained immunity, is evoked by insults that shift monocyte metabolism, increasing HIF (hypoxia-inducible factor)-1α levels. Since ischemia enhances HIF-1α, we evaluate whether ischemia can lead to a functional reprogramming of monocytes, which would contribute to arteriogenesis after hindlimb ischemia.

METHODS AND RESULTS

Mice exposed to ischemia by 24 hours (24h) of femoral artery occlusion (24h trained) or sham were subjected to hindlimb ischemia one week later; the 24h trained mice showed significant improvement in blood flow recovery and arteriogenesis after hindlimb ischemia. Adoptive transfer using bone marrow-derived monocytes (BM-Mono) from 24h trained or sham donor mice, demonstrated that recipients subjected to hindlimb ischemia who received 24h ischemic-trained monocytes had remarkable blood flow recovery and arteriogenesis. Further, ischemic-trained BM-Mono had increased HIF-1α and GLUT-1 (glucose transporter-1) gene expression during femoral artery occlusion. Circulating cytokines and GLUT-1 were also upregulated during femoral artery occlusion.Transcriptomic analysis and confirmatory qPCR performed in 24h trained and sham BM-Mono revealed that among the 15 top differentially expressed genes, 4 were involved in lipid metabolism in the ischemic-trained monocytes. Lipidomic analysis confirmed that ischemia training altered the cholesterol metabolism of these monocytes. Further, several histone-modifying epigenetic enzymes measured by qPCR were altered in mouse BM-Mono exposed to 24h hypoxia.

CONCLUSIONS

Ischemia training in BM-Mono leads to a unique gene profile and improves blood flow and arteriogenesis after hindlimb ischemia.

Surgical castration versus chemical castration in donkeys: response of stress, lipid profile and redox potential biomarkers

Background

Castration is a husbandry practice raising important questions on the welfare and physiological status of farm animals. Searching for effective castration methods that minimally compromise the body physiology is worthy of attention. Therefore, this study aimed to evaluate the differential response of biological systems in donkeys to surgical castration versus the chemical one by CaCl₂ with special emphasis on stress, lipid profile, and oxidative stress biomarkers. Donkeys were divided randomly and equally into two groups; the chemical (Ch) and surgical (S) groups (n = 6). The Ch group was chemically castrated by intratesticular injection of 20% CaCl₂ dissolved in absolute ethanol. Blood samples were collected prior to castration and at 15, 30, 45, and 60 days after the beginning of experiment.

Results

Surprisingly, the Ch group at the end of the experiment was characterized by significantly higher cortisol level compared to the S group. TC and LDL-C levels in the S group significantly decreased at day 45, while TG levels significantly increased at days 45 and 60 in comparison with day 0. HDL-C levels at days 30 and 60 in the Ch group significantly increased in comparison with day 0. At day 30 post-castration, HDL-C was significantly higher and LDL-C was significantly lower in the Ch group than the S group. A significant elevation in TC and LDL-C was observed at day 45 and in HDL-C at the end of experimental duration in the Ch group when compared with the S group. TPX level was significantly lower and TAC was significantly higher in the Ch group at day 45 than the S group.

Conclusion

Surgical castration evoked less stress and minor changes in lipid profile and oxidant/antioxidant balance relative to chemical castration by intratesticular 20% CaCl₂ dissolved in absolute ethanol.

Induction of Lipin1 by ROS-Dependent SREBP-2 Activation

Lipin1 was identified as a phosphatidate phosphatase enzyme, and it plays a key role in lipid metabolism. Since free radicals contribute to metabolic diseases in the liver, this study investigated the effects of free radicals on the regulation of Lipin1 expression in Huh7 and AML12 cells. Hydrogen peroxide induced mRNA and protein expression of Lipin1 in Huh7 cells, which was assayed by quantitative RT-PCR and immunoblotting, respectively. Induction of Lipin1 by hydrogen peroxide was confirmed in AML12 cells. Hydrogen peroxide treatment significantly increased expression of sterol regulatory element-binding protein (SREBP)-2, but not SREBP-1. Moreover, nuclear translocation of SREBP-2 was detected after hydrogen peroxide treatment. Hydrogen peroxide-induced Lipin1 or SREBP-2 expression was significantly reduced by N-acetyl-l-cysteine treatment, indicating that reactive oxygen species (ROS) were implicated in Lipin1 expression. Next, we investigated whether the hypoxic environments that cause endogenous ROS production in mitochondria in metabolic diseases affect the expression of Lipin1. Exposure to hypoxia also increased Lipin1 expression. In contrast, pretreatment with antioxidants attenuated hypoxia-induced Lipin1 expression. Collectively, our results show that ROS activate SREBP-2, which induces Lipin1 expression.

The role of lipin-1 in the pathogenesis of alcoholic fatty liver

AIMS

The aim of this review was to focus on the knowledge of the role of lipin-1 in the pathogenesis of alcoholic fatty liver.

METHODS

Systematic review of animal clinical and cell level studies related to the function of lipin-1 on alcoholic fatty liver, alcoholic hepatitis and alcoholic liver cirrhosis disease.

RESULT

Ethanol could increase the expression of lipin-1 through the AMPK-SREBP-1 signaling and dramatically increase the ratio of Lpin1β to Lpin1α by SIRT1-SFRS10-Lpin1β/α axis in the liver. Moreover, research has shown that over-expression of lipin-1 could also remarkably suppress very low density lipoprotein-triacylglyceride secretion. Last, lipin-1 has potent anti-inflammatory property.

CONCLUSION

In conclusion, lipin-1 has dual functions in lipid metabolism. In the cytoplasm, lipin-1β functions as a Mg(2+)-dependent phosphatidic acid phosphohydrolase (PAP) enzyme in triglyceride synthesis pathways. In the nucleus, lipin-1α acts as a transcriptional co-regulator to regulate the capacity of the liver for fatty acid oxidation and activity of the lipogenic enzyme. In hepatocytes of alcoholic fatty liver disease (AFLD), ethanol increases the expression of lipin-1 through the AMPK-SREBP-1 signaling and the Lpin1β/α ratio by SIRT1-SFRS10- Lpin1β/α axis. Of course, in addition to that, ethanol could also produce the PAP activity and interrupt the nucleus function of lipin-1. Furthermore, over-expression of lipin-1 could remarkably suppress very low-density lipoprotein-triacylglyceride (VLDL-TAG) secretion. In the end, endogenous lipin-1 has potent anti-inflammatory property. Increased synthesis of TAG, decreased fatty acid oxidation, impaired VLDL-TAG secretion and activated inflammatory factors act together to exacerbate the development of AFLD.

Regulation of triglyceride metabolism by glucocorticoid receptor

Glucocorticoids are steroid hormones that play critical and complex roles in the regulation of triglyceride (TG) homeostasis. Depending on physiological states, glucocorticoids can modulate both TG synthesis and hydrolysis. More intriguingly, glucocorticoids can concurrently affect these two processes in adipocytes. The metabolic effects of glucocorticoids are conferred by intracellular glucocorticoid receptors (GR). GR is a transcription factor that, upon binding to glucocorticoids, regulates the transcriptional rate of specific genes. These GR primary target genes further initiate the physiological and pathological responses of glucocorticoids. In this article, we overview glucocorticoid-regulated genes, especially those potential GR primary target genes, involved in glucocorticoid-regulated TG metabolism. We also discuss transcriptional regulators that could act with GR to participate in these processes. This knowledge is not only important for the fundamental understanding of steroid hormone actions, but also are essential for future therapeutic interventions against metabolic diseases associated with aberrant glucocorticoid signaling, such as insulin resistance, dyslipidemia, central obesity and hepatic steatosis.

Glucose-6-phosphate isomerase deficiency results in mTOR activation, failed translocation of lipin 1α to the nucleus and hypersensitivity to glucose: Implications for the inherited glycolytic disease

Inherited glucose-6-phosphate isomerase (GPI) deficiency is the second most frequent glycolytic erythroenzymopathy in humans. Patients present with non-spherocytic anemia of variable severity and with neuromuscular dysfunction. We previously described Chinese hamster (CHO) cell lines with mutations in GPI and loss of GPI activity. This resulted in a temperature sensitivity and severe reduction in the synthesis of glycerolipids due to a reduction in phosphatidate phosphatase (PAP). In the current article we attempt to describe the nature of this pleiotropic effect. We cloned and sequenced the CHO lipin 1 cDNA, a gene that codes for PAP activity. Overexpression of lipin 1 in the GPI-deficient cell line, GroD1 resulted in increased PAP activity, however it failed to restore glycerolipid biosynthesis. Fluorescence microscopy showed a failure of GPI-deficient cells to localize lipin 1α to the nucleus. We also found that glucose-6-phosphate levels in GroD1 cells were 10-fold over normal. Lowering glucose levels in the growth medium partially restored glycerolipid biosynthesis and nuclear localization of lipin 1α. Western blot analysis of the elements within the mTOR pathway, which influences lipin 1 activity, was consistent with an abnormal activation of this system. Combined, these data suggest that GPI deficiency results in an accumulation of glucose-6-phosphate, and possibly other glucose-derived metabolites, leading to activation of mTOR and sequestration of lipin 1 to the cytosol, preventing its proper functioning. These results shed light on the mechanism underlying the pathologies associated with inherited GPI deficiency and the variability in the severity of the symptoms observed in these patients.

Shedding light on the enigma of myocardial lipotoxicity: the involvement of known and putative regulators of fatty acid storage and mobilization

Excessive fatty acid (FA) uptake by cardiac myocytes is often associated with adverse changes in cardiac function. This is especially evident in diabetic individuals, where increased intramyocardial triacylglycerol (TG) resulting from the exposure to high levels of circulating FA has been proposed to be a major contributor to diabetic cardiomyopathy. At present, our knowledge of how the heart regulates FA storage in TG and the hydrolysis of this TG is limited. This review concentrates on what is known about TG turnover within the heart and how this is likely to be regulated by extrapolating results from other tissues. We also assess the evidence as to whether increased TG accumulation protects against FA-induced lipotoxicity through limiting the accumulations of ceramides and diacylglycerols versus whether it is a maladaptive response that contributes to cardiac dysfunction.

Insulin-stimulated interaction with 14-3-3 promotes cytoplasmic localization of lipin-1 in adipocytes

Lipin-1 is a bifunctional protein involved in lipid metabolism and adipogenesis. Lipin-1 plays a role in the biosynthesis of triacylglycerol through its phosphatidate phosphatase activity and also acts as a transcriptional co-activator of genes involved in oxidative metabolism. Lipin-1 resides in the cytoplasm and translocates to the endoplasmic reticulum membrane to catalyze the phosphatidate phosphatase reaction. It also possesses a nuclear localization signal, which is required for its translocation to the nucleus and may therefore be important for lipin-1 co-activator function. Thus, subcellular localization may be an important factor in the regulation of this protein. Here, we show that the nuclear localization signal alone is not sufficient for lipin-1 nuclear localization, and identify lipin-1 interaction with 14-3-3 as a determinant of its subcellular localization. We demonstrate that lipin-1 interacts with 14-3-3 proteins and that overexpression of 14-3-3 promotes the cytoplasmic localization of lipin-1 in 3T3-L1 adipocytes. The effect of 14-3-3 is mediated through a serine-rich domain in lipin-1. Functional mapping of the 14-3-3-interacting region within the serine-rich domain indicates redundancy and cooperativity among several sites, including five phosphorylated serine and threonine residues. Insulin stimulation of 3T3-L1 adipocytes results in increased lipin-1 phosphorylation, enhanced interaction with 14-3-3, and predominantly cytoplasmic localization. In summary, our studies suggest that insulin may modulate the cellular function of lipin-1 by regulating its subcellular localization through interactions with 14-3-3 proteins.

Regulation of lipin-1 gene expression by glucocorticoids during adipogenesis

Lipin-1 deficiency in the mouse causes generalized lipodystrophy, characterized by impaired adipose tissue development and insulin resistance. Lipin-1 expression in differentiating preadipocytes is required for normal expression of adipogenic transcription factors, including peroxisome proliferator-activated receptor gamma and CCAAT enhancer binding protein alpha, and for the synthesis of triacylglycerol. The requirement of lipin-1 for adipocyte differentiation can be explained, in part, by its activity as the sole adipocyte phosphatidic acid phosphatase-1 enzyme, which converts phosphatidate to diacylglycerol, the immediate precursor of triacylglycerol. Here we identify glucocorticoids as the stimulus for the induction of lipin-1 expression in differentiating adipocytes, and characterize a glucocorticoid response element (GRE) in the Lpin1 promoter. The Lpin1 GRE binds to the glucocorticoid receptor and leads to transcriptional activation in adipocytes and hepatocytes, as demonstrated by reporter gene transcription, electrophoretic mobility shift, and chromatin immunoprecipitation assays. This represents the first gene regulatory element identified to directly influence lipin-1 expression levels, and may modulate lipin-1 mRNA levels in adipose tissue and liver in conditions associated with increased local glucocorticoid concentrations in vivo, such as obesity and fasting.

Glucocorticoids and cyclic AMP selectively increase hepatic lipin-1 expression, and insulin acts antagonistically

Glucocorticoids (GCs) increase hepatic phosphatidate phosphatase (PAP1) activity. This is important in enhancing the liver's capacity for storing fatty acids as triacylglycerols (TAGs) that can be used subsequently for beta-oxidation or VLDL secretion. PAP1 catalyzes the conversion of phosphatidate to diacylglycerol, a key substrate for TAG and phospholipid biosynthesis. PAP1 enzymes in liver include lipin-1A and -1B (alternatively spliced isoforms) and two distinct gene products, lipin-2 and lipin-3. We determined the mechanisms by which the composite PAP1 activity is regulated using rat and mouse hepatocytes. Levels of lipin-1A and -1B mRNA were increased by dexamethasone (dex; a synthetic GC), and this resulted in increased lipin-1 synthesis, protein levels, and PAP1 activity. The stimulatory effect of dex on lipin-1 expression was enhanced by glucagon or cAMP and antagonized by insulin. Lipin-2 and lipin-3 mRNA were not increased by dex/cAMP, indicating that increased PAP1 activity is attributable specifically to enhanced lipin-1 expression. This work provides the first evidence for the differential regulation of lipin activities. Selective lipin-1 expression explains the GC and cAMP effects on increased hepatic PAP1 activity, which occurs in hepatic steatosis during starvation, diabetes, stress, and ethanol consumption.

Insulin controls subcellular localization and multisite phosphorylation of the phosphatidic acid phosphatase, lipin 1

Brain, liver, kidney, heart, and skeletal muscle from fatty liver dystrophy (fld/fld) mice, which do not express lipin 1 (lipin), contained much less Mg(2+)-dependent phosphatidic acid phosphatase (PAP) activity than tissues from wild type mice. Lipin harboring the fld(2j) (Gly(84) --> Arg) mutation exhibited relatively little PAP activity. These results indicate that lipin is a major PAP in vivo and that the loss of PAP activity contributes to the fld phenotype. PAP activity was readily detected in immune complexes of lipin from 3T3-L1 adipocytes, where the protein was found both as a microsomal form and a soluble, more highly phosphorylated, form. Fifteen phosphorylation sites were identified by mass spectrometric analyses. Insulin increased the phosphorylation of multiple sites and promoted a gel shift that was due in part to phosphorylation of Ser(106). In contrast, epinephrine and oleic acid promoted dephosphorylation of lipin. The PAP-specific activity of lipin was not affected by the hormones or by dephosphorylation of lipin with protein phosphatase 1. However, the ratio of soluble to microsomal lipin was markedly increased in response to insulin and decreased in response to epinephrine and oleic acid. The results suggest that insulin and epinephrine control lipin primarily by changing localization rather than intrinsic PAP activity.

Regulation of the enzymes of hepatic microsomal triacylglycerol lipolysis and re-esterification by the glucocorticoid dexamethasone

Hepatic VLDL (very-low-density lipoprotein) assembly is a complex process that is largely regulated by the provision of lipid for apolipoprotein B assembly. Intracellular stored TAG (triacylglycerol) undergoes an initial lipolysis followed by re-esterification of the lipolytic products to form TAG prior to their incorporation into a VLDL particle. TGH (TAG hydrolase) is a lipase that hydrolyses intracellular TAG within the hepatocyte. We have utilized both dexamethasone-injected mouse and primary hepatocyte models to address whether stimulation of TAG biosynthesis by the synthetic glucocorticoid, dexamethasone, altered hepatic lipolysis and re-esterification and the provision of stored TAG for lipoprotein secretion. Dexamethasone treatment resulted in decreased TGH expression, primarily due to a dexamethasone-induced decrease in TGH mRNA stability. The expression and activities of diacylglycerol acyltransferases 1 and 2 were stimulated by dexamethasone. The combination of reduced intracellular TAG lipolysis and increased TAG biosynthesis contributed to the accumulation of TAG within the livers of dexamethasone-injected mice. The rate of hepatic TAG secretion in dexamethasonetreated mice was maintained at similar levels as in control mice. Our data demonstrate that stimulation of de novo TAG synthesis by dexamethasone increased the proportion of secreted TAG that was derived from de novo sources, while the utilization of stored TAG for secretion was reduced. The results show that, during markedly increased TAG synthesis, some TAGs are diverted from the cytosolic storage pool and are utilized directly for VLDL assembly within the endoplasmic reticulum lumen.

Transforming growth factor beta increases the activity of phosphatidate phosphohydrolase-1 in rat hepatocytes

Phosphatidic acid (PA) is a potent second messenger arising from growth factor-induced stimulation of phospholipase D which hydrolyses phosphatidylcholine. PA is hydrolysed to diacylglycerol by PA phosphohydrolase (PAP) which exists in two forms: PAP-1 and PAP-2. In rat hepatocyte cultures, overnight (20h) incubation with transforming growth factor (TGF) beta (1 ng/ml) increased PAP-1 activity two-fold. This effect was concentration and time dependent and was greatest at low cell density. The TGFbeta effect on PAP-1 was additive to stimulation induced by dexamethasone but not by glucagon and it reversed the inhibition by insulin. Epidermal growth factor had no effect on PAP-1 activity. None of the above hormones or growth factors affected the subcellular distribution of PAP-1. Stimulation of PAP-1 by TGFbeta may be involved in mediating some of its biological effects.

Purification and characterization of N-ethylmaleimide-insensitive phosphatidic acid phosphohydrolase (PAP2) from rat liver

N-Ethylmaleimide-insensitive phosphatidic acid phosphohydrolase (PAP; EC 3.1.3.4) was purified 5900-fold from rat liver. The enzyme was solubilized from membranes with octylglucoside, fractionated with (NH4)2SO4, and purified in the presence of Triton X-100 by chromatography on Sephacryl S300, hydroxyapatite, heparin-Sepharose and Affi-Gel Blue. Silver-stained SDS/PAGE indicated that the enzyme was an 83 kDa polypeptide. Sephacryl S-300 gel filtration also produced a second peak of enzyme activity, which was eluted from all of the chromatography columns at a different position from the purified enzyme. SDS/PAGE indicated that it contained three polypeptides (83 kDa, 54 kDa and 34 kDa), and gel filtration suggested that it was not an aggregate of the purified enzyme. Both forms were sensitive to inhibition by amphiphilic amines, Mn2+ and Zn2+, but not by N-ethylmaleimide. Purified PAP required detergent for activity, but was not activated by Mg2+, fatty acids or phospholipids. The enzyme was able to dephosphorylate lysophosphatidic acid or phosphatidic acid, and was inhibited by diacylglycerol and monoacylglycerol. No evidence was obtained for regulation of PAP by reversible phosphorylation.

Regulatory effect of various steroid hormones on the incorporation and metabolism of [14C]stearate in rat hepatoma cells in culture

We have examined the incorporation and metabolism of [14C] stearic acid within the total lipids of HTC rat-hepatoma cells in suspension culture in presence and in absence of steroidal hormone stimulation. Both, glucocorticoids (dexamethasone, cortisol and corticosterone) and mineralocorticoids (deoxycorticosterone and aldosterone) as well as the estrogen beta-estradiol and the androgen testosterone enhanced the extent of delta 9 desaturation to oleic acid of the saturated precursors, whereas only the two mineralocorticoids affected the incorporation rate of the exogenous acid into total cellular lipids, thus promoting a little stimulation. Furthermore, all the hormones tested increased the radiolabelling of the total cellular phospholipids except deoxycorticosterone and testosterone, the former having no effect and the latter exerting a moderate inhibition. On the other hand, the incorporation of 14C into neutral lipids was stimulated by testosterone, in contrast to the inhibition of this parameter observed exclusively with either the mineralocorticoids or the estrogen. Within the phospholipid subclasses, the radiolabelling of phosphatidylcholine was augmented by means of all the steroids tested save deoxycorticosterone and testosterone, whereas phosphatidylethanolamine exhibited a decrease only in the presence of testosterone. In a similar fashion, within the neutral lipids, the predominating triglyceride fraction was preferentially labelled--at the expense of other subclasses of lesser abundance--upon treatment with the steroids except aldosterone, which exerted no effect. The results obtained were correlated with those changes observed in the mass distribution of the different lipid subclasses either with or without prior hormonal stimulation.

Subcellular distribution of N-ethylmaleimide-sensitive and -insensitive phosphatidic acid phosphohydrolase in rat brain

The dephosphorylation of phosphatidic acid by phosphatidic acid phosphohydrolase (PAP) is important in both cell-signalling and in glycerolipid metabolism. However, these roles are apparently performed by two different enzymes, which can be distinguished by their sensitivity in vitro to N-ethylmaleimide (NEM). Both of these enzymes are present in rat brain as well as a wide range of other rat tissues. However, the quantity and specific activity of each enzyme varies considerably between different tissues, as does the ratio of the two enzymes in each tissue. Tissues rich in glycerolipids are abundant in NEM-sensitive PAP, whereas there is no obvious pattern to the distribution of the NEM-insensitive enzyme in the different tissues tested. Studies on brain cortex, which is relatively rich in both forms of PAP, indicate that the NEM-insensitive PAP is located in the synaptosomes, and the NEM-sensitive enzyme present in the cytosol and microsomes. The NEM-sensitive PAP can also be translocated from the cytosol to the microsomes by oleate. When assayed against a range of phosphatidic acids, NEM-sensitive PAP showed a preference for phosphatidic acids with short acyl chains and for those containing arachidonate, whereas NEM-insensitive PAP had a preference for short and unsaturated acyl chains. The two isozymes also had different activity profiles against these substrates suggesting that they are in fact different enzymes. The implications for these results on the putative roles of the two forms of PAP are discussed.

Insulin regulation of triacylglycerol-rich lipoprotein synthesis and secretion

This review has considered a number of observations obtained from studies of insulin in perfused liver, hepatocytes, transformed liver cells and in vivo and each of the experimental systems offers advantages. The evaluation of insulin effects on component lipid synthesis suggests that overall, lipid synthesis is positively influenced by insulin. Short-term high levels of insulin through stimulation of intracellular degradation of freshly translated apo B and effects on synthesis limit the ability of hepatocytes to form and secrete TRL. The intracellular site of apo B degradation may involve membrane-bound apo B, cytoplasmic apo B and apo B which has entered the ER lumen. How insulin favors intracellular apo B degradation is not known. An area of recent investigation is in insulin-stimulated phosphorylation of intracellular substrates such as IRS-1 which activates insulin specific cellular signaling molecules [245]. Candidate molecules to study insulin action on apo B include IRS-1 and SH2-containing signaling molecules. Insulin dysregulation in carbohydrate metabolism occurs in non-insulin-dependent diabetes mellitus due to an imbalance between insulin sensitivity of tissue and pancreatic insulin secretion (reviewed in Refs. [307,308]). Insulin resistance in the liver results in the inability to suppress hepatic glucose production; in muscle, in impaired glucose uptake and oxidation and in adipose tissue, in the inability to suppress release of free FA. This lack of appropriate sensitivity towards insulin action leads to hyperglycemia which in turn stimulates compensatory insulin secretion by the pancreas leading to hyperinsulinemia. Ultimately, there may be failure of the pancreas to fully compensate, hyperglycemia worsens and diabetes develops. The etiology of insulin resistance is being intensively studied for the primary defect may be over secretion of insulin by the pancreas or tissue insulin resistance and both of these defects may be genetically predetermined. We suggest that, in addition to effects in carbohydrate metabolism, insulin resistance in liver results in the inability of first phase insulin to suppress hepatic TRL production which results in hypertriglyceridemia leading to high levels of plasma FA which accentuate insulin resistance in other target organs. As recently reviewed [17,254] the role of insulin as a stimulator of hepatic lipogenesis and TRL production has been long established. Several lines of evidence support that insulin is stimulatory to the production of hepatic TRL in vivo. First, population based studies support a positive relationship between plasma insulin and total TG and VLDL [253]. Second, there is a strong association between chronic hyperinsulinemia and VLDL overproduction [309].(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of dietary fatty acid composition on glycerophosphate acyltransferase activity in rat adipose tissue and liver

1. How source of dietary fat influences glycerophosphate acyltransferase (GPAT) activity in homogenates of adipose tissue and liver was determined using 5 rats per diet that contained 12% fat from either corn- (CO), high-oleate-safflower (HOSO), palm- or high-stearate-oils. 2. Activity was greatest in adipose tissue homogenates of rats fed HOSO and CO whereas no treatment effects were observed in liver homogenates. 3. No differences in fat pad weights were observed; thus HOSO- and CO-diets likely depressed other lipid synthetic rates. 4. Data indicate that mono- and diunsaturated fatty acid diets supported greater adipose tissue GPAT activity than did saturated fat diets when fed to rats.

The activity of the metabolic form of hepatic phosphatidate phosphohydrolase correlates with the severity of alcoholic fatty liver in human beings

Increased esterification of fatty acids to triglyceride is common to most of the mechanisms proposed to explain the causation of alcoholic fatty liver. However, it is unclear whether this is caused by increased substrate supply or whether direct stimulation of the enzymes of the esterification pathway occurs after excessive alcohol intake. The rate-limiting step in triglyceride synthesis is catalyzed by the enzyme phosphatidate phosphohydrolase, which is present in the cytosol and microsomes and is sensitive to inhibition by N-ethylmaleimide. This enzyme is physically distinct from a second form of phosphatidate phosphohydrolase that is located predominantly in the plasma membrane, is insensitive to N-ethylmaleimide inhibition and has a putative role in cell-signaling. We have investigated whether the activity of the N-ethylmaleimide-sensitive ("metabolic") form of phosphatidate phosphohydrolase is increased in patients with alcoholic liver disease and whether any increased activity correlates with the severity of steatosis. N-ethylmaleimide-sensitive and -insensitive phosphatidate phosphohydrolase activities were measured in needle liver biopsy specimens from 42 alcoholic patients and 6 patients with primary biliary cirrhosis and in wedge biopsy specimens from 6 normal patients undergoing routine cholecystectomy. Steatosis was "scored" on coded slides from 0 to 3. N-ethylmaleimide-sensitive activity was higher in alcoholic biopsy specimens scoring 3 (3.25 +/- 0.4 units/mg protein, n = 10) than in those scoring either 0 (1.21 +/- 0.2, n = 14) or 1 to 2 (1.58 +/- 0.2, n = 18), and it was also higher than in biopsy specimens from normal and primary biliary cirrhosis patients (1.65 +/- 0.3, n = 12; p < 0.0001, analysis of variance).(ABSTRACT TRUNCATED AT 250 WORDS)

Hepatic monoacylglycerol acyltransferase activity in HA1 and HA7 hepatoma/hepatocyte hybrid cells: regulation by insulin and dexamethasone and by cell density

Hepatic monoacylglycerol acyltransferase (MGAT) (EC 2.3.1.22) is a developmentally-expressed enzyme that catalyzes the stereospecific synthesis of sn-1,2-diacylglycerol from sn-2-monoacylglycerol and long-chain fatty acyl-CoA. In order to study the regulation of MGAT, we developed a rapid assay that can be performed directly on permeabilized HA rat hepatocyte/hepatoma hybrid cells, a line that expresses levels of hepatic MGAT activity and a lipogenic program characteristic of fetal hepatocytes. In permeabilized HA cells, MGAT activity was proportional to the time of incubation and was highly dependent on added sn-2-monoacylglycerol and palmitoyl-CoA. The apparent Km values were 16.6 and 12.7 microM for palmitoyl-CoA and 2-monooleoylglycerol, respectively. Activity was low with the 1(3)- and sn-2-ether analogs of monooleoylglycerol, supporting the conclusion that the cells express the hepatic isoenzyme of MGAT. MGAT activity increased directly with cell density and was unrelated to the number of days in culture. Long-term incubation (2-4 days) of HA cells with various hormones (including triiodothyronine, human placental lactogen, epidermal growth factor, glucagon and growth hormone) showed that only a combination of dexamethasome and insulin resulted in significantly decreased MGAT activity. None of these hormones affected MGAT activity in short-term (0.5-4 h) incubations. These studies suggest that the developmental decline in rat hepatic MGAT activity may be regulated by glucocorticoids and insulin, hormones that increase during and after the second postnatal week.

Effects of sphingosine, albumin and unsaturated fatty acids on the activation and translocation of phosphatidate phosphohydrolases in rat hepatocytes

The activities of two phosphatidate phosphohydrolases were measured in cultured rat hepatocytes incubated with 0.1 mM albumin. The activity, which is inhibited by N-ethylmaleimide (PAP-1) is located in the cytosolic and membrane fractions. PAP-1 activity is stimulated by Mg2+ and it can be translocated from the cytosol to the membranes by relatively low (0.5-1 mM) concentrations of fatty acids. In addition, higher concentrations (1-3 mM) of fatty acids cause an increase in the total PAP-1 activity. Translocation of PAP-1 activity in the hepatocytes is preferentially promoted by unsaturated fatty acids (C18:1, C18:2, C18:3, C20:4 and C20:5), rather than by saturated acids (C14:0, C16:0, C18:0). Increasing the extracellular concentration of albumin from 30 microM to 1 mM displaces PAP-1 activity from the membrane fraction. Sphingosine, but not staurosporine, can inhibit the redistribution of PAP-1 activity induced by oleate. The amphiphilic amines, sphingosine, chlorpromazine and propranolol, also decrease membrane-bound PAP-1 activity in the absence of fatty acids, but they do not alter, significantly, the activity of the cytosolic PAP-1. In the presence of 1 mM oleate, sphingosine, chlorpromazine and propranolol decrease the translocation of PAP-1 from the cytosol to the membranes. The phosphohydrolase activity, which is insensitive to N-ethylmaleimide (PAP-2), is specifically located in the plasma membrane (Jamal, Z., Martin, A., Gomez-Muñoz, A. and Brindley, D.N. (1991) J. Biol. Chem. 266, 2988-2996) and it is not stimulated by Mg2+. Saturated fatty acids, albumin, sphingosine and propranolol have no significant effects on PAP-2 activity. However, chlorpromazine decreases PAP-2 activity by about 14%. Linolenate, arachidonate and eicosapentaenoate at 1 mM also produced small (7-10%) decreases in PAP-2 activity. It is proposed that both PAP-1 and PAP-2 activities may be involved in signal transduction, although the main function of PAP-1 seems to be involved in the synthesis of glycerolipids.

Physical evidence for the presence of two forms of phosphatidate phosphohydrolase in rat liver

Phosphatidate phosphohydrolase (PAP)-catalysed dephosphorylation of phosphatidic acid to diacylglycerol is an important step in glycerolipid metabolism and cell-signalling. Gel filtration chromatography on Superose 6 and anion-exchange chromatography on Mono Q of rat liver subcellular fractions has provided physical evidence for the presence of two distinct forms of PAP activity. One form was sensitive to inhibition by N-ethylmaleimide (NEM), had an apparent M(r) of 540,000 and was eluted from the anion-exchange column by 0.35 M NaCl, while the other was insensitive to inhibition by NEM, had an apparent M(r) of 240,000 and was eluted from the anion-exchange column by 0.15 M NaCl. Studies on the subcellular distribution of these two enzymes, using 5'-nucleotidase as a plasma membrane marker, demonstrated that the NEM-sensitive form was predominantly cytosolic but translocated to the microsomal membranes in response to oleate. The NEM-insensitive form was predominantly located in the plasma membrane but a small proportion (approx. 10%) of total cell activity was present on the endoplasmic reticulum. The implications of these results for the likely roles of the two different forms of PAP in fatty acid esterification and cell-signalling are discussed.

Effects of okadaic acid on the activities of two distinct phosphatidate phosphohydrolases in rat hepatocytes

Incubation of hepatocytes with okadaic acid displaced the N-ethylmaleimide-sensitive phosphatidate phosphohydrolase from the membrane fraction into the cytosol and partially prevented the oleate-induced movement of phosphohydrolase from cytosol to membranes. However, higher concentrations of oleate still caused translocation and activation of the phosphohydrolase. This enzyme is stimulated by Mg2+, and is probably involved in glycerolipid synthesis. Okadaic acid also decreased the concentration of diacylglycerol within the hepatocytes. Okadiac acid had no observable effect on the activity of an N-ethylmaleimide-insensitive phosphatidate phosphohydrolase which remained firmly attached to membranes. This activity is not stimulated by Mg2+ and is probably involved in signal transduction by the phospholipase D pathway.

Biphasic effects of glucagon and cyclic AMP on the synthesis and secretion of lipids by rat hepatocytes

Cultured rat hepatocytes were preincubated with glucagon or a cyclic AMP analogue for up to 24 h and lipid synthesis and secretion were determined during the next 2 h. Glucagon or cyclic AMP did not change the incorporation of choline or glycerol into phosphatidylcholine, or choline into sphingomyelin, in the cells after 0-12 h of preincubation. After 12 h these incorporations were increased. Incorporations into hepatic lysophosphatidylcholine were decreased after preincubation with glucagon or cyclic AMP for 0-12 h, but by 24 h they increased. There was no change in the lysophosphatidylcholine in the medium after preincubation with glucagon or cyclic AMP for up to 6 h, but increases occurred after preincubation from 12 to 24 h. The secretion of triacylglycerol was decreased after preincubation for 0-1 h, but it returned to control values after 4 h. After preincubation for 18-24 h the incorporation of glycerol into secreted triacylglycerol was increased. The results are discussed in relation to the control of lipid metabolism in starvation and diabetes.

Activities of enzymes of glycerolipid synthesis in brown adipose tissue after treatment of rats with the adrenergic agonists BRL 26830A and phenylephrine, after exposure to cold and in streptozotocin-diabetes

1. Measurements were made, relative to tissue DNA, of the activities of enzymes of glycerolipid synthesis in homogenates of interscapular brown adipose tissue. These were: mitochondrial and microsomal forms of glycerolphosphate acyltransferase (GPAT), Mg(2+)-dependent phosphatidate phosphohydrolase (PPH) and fatty acyl-CoA synthetase (FAS). 2. In normal animals, 3 days of cold-exposure (4 degrees C) increased all activities. The increase in mitochondrial GPAT activity was particularly pronounced (5-fold). Administration of the beta-adrenergic agonist BRL 26830A mimicked the effect of cold on microsomal GPAT activity. Mitochondrial GPAT, PPH and FAS activities were unresponsive to BRL 26830A. The alpha-adrenergic agonist phenylephrine significantly decreased activities of GPAT and PPH. 3. Streptozotocin-diabetes decreased mitochondrial GPAT activity, but did not abolish the effect of cold to increase this activity or the activity of microsomal GPAT. Diabetes abolished the effect of cold on PPH and FAS activities. 4. The findings are relevant to signals that drive early events in mitochondriogenesis and cell proliferation in brown adipose tissue on exposure to cold.

Inhibition of arachidonate biosynthesis in hepatoma tissue culture cells by 11-deoxycorticosterone-induced factor

In this work it was demonstrated that the incubation of hepatoma cultured cells (HTC 7288 c) with 11-deoxycorticosterone (DOC) ranging from 0 to 10(-4) M concentration provoked a dose-dependent inhibition in the conversion of [1-14C] eicosatrienoic acid to arachidonic acid. This steroid also produced an increase in the uptake of exogenous 20:3 (n-6) acid. The depressive effect evoked by DOC on delta 5 desaturating activity was reflected on the fatty acid composition changes of the hepatoma cells. The delta 5 desaturase activity was inhibited by a soluble factor that would be induced by the hormone and that was present in the cytosol fraction from DOC-treated cells, corresponding to a low molecular mass below 25 kDa. Presently we report that an 11-beta-OH group on the steroid molecule is not an essential requirement for the production of a delta 5 desaturase inhibitory factor.

Stimulation of apolipoprotein secretion in very-low-density and high-density lipoproteins from cultured rat hepatocytes by dexamethasone

The effects of dexamethasone (a synthetic glucocorticoid) and insulin on the secretion of very-low-density lipoprotein (VLDL) and high-density lipoprotein (HDL) were investigated. Rat hepatocytes in monolayer culture were preincubated for 15 h in the presence or absence of combinations of 100 nM-dexamethasone and 2 nM-, 10 nM- or 50 nM-insulin. Dexamethasone increased [3H]oleate incorporation into secreted triacylglycerol by 2.7-fold and the mass of triacylglycerol secreted by 1.5-fold. Insulin alone decreased these parameters and antagonized the effect of dexamethasone. Dexamethasone increased the secretion of [3H]leucine in apolipoprotein (apo) E, and in the large (BH) and small (BI) forms of apo B in VLDL by about 7.1-, 3.6- and 4.0-fold respectively. Insulin alone decreased the secretion of these 3H-labelled apolipoproteins in VLDL. However, 2 nM-insulin with dexamethasone increased the secretion of 3H-labelled apo BH and apo BL by a further 0.8- and 3.2-fold respectively; 50 nM-insulin decreased the secretions of apo E, apo BH and apo BL in VLDL. Similar effects for dexamethasone or insulin alone were also obtained for the masses of apo E and apo BL + H secreted in VLDL. Albumin secretion was not significantly altered by either dexamethasone or insulin alone, but in combination they stimulated by 2.1-2.6-fold. Insulin or dexamethasone alone had little effect on the secretion of apolipoproteins in the HDL fraction. However, dexamethasone plus 2 nM-insulin increased the incorporation of [3H]leucine into apo AI, apo AH plus apo C, apo AIV and apo E of HDL by about 1.8-, 1.6-, 1.7- and 2.0-fold respectively. The apo E in the bottom fraction represented about 69% of the total 3H-labelled apo E secreted. The responses in the total secretion of apo E from the hepatocytes resembled those seen in HDL. The interactions of insulin and dexamethasone are discussed in relation to the general regulation of lipoprotein metabolism, the development of hyperlipidaemias and the predisposition to premature atherosclerosis.

Insulin increases the synthesis of phospholipid and diacylglycerol and protein kinase C activity in rat hepatocytes

The effects of insulin on phospholipid metabolism and generation of diacylglycerol (DAG) and on activation of protein kinase C in rat hepatocytes were compared to those of vasopressin and angiotension II. Insulin provoked increases in [3H]glycerol labeling of phosphatidic acid (PA), diacylglycerol (DAG), and other glycerolipids within 30 s of stimulation. Similar increases were also noted for vasopressin and angiotensin II. Corresponding rapid increases in DAG mass also occurred with all three hormones. As increases in [3H]DAG (and DAG mass) occurred within 30-60 s of the simultaneous addition of [3H]glycerol and hormone, it appeared that DAG was increased, at least partly, through the de novo synthesis of PA. That de novo synthesis of PA was increased is supported by the fact that [3H]glycerol labeling of total glycerolipids was increased by all three agents. Increases in [3H]glycerol labeling of lipids by insulin were not due to increased labeling of glycerol 3-phosphate, and were therefore probably due to activation of glycerol-3-phosphate acyltransferase. Unlike vasopressin, insulin did not increase the hydrolysis of inositol phospholipids. Insulin- and vasopressin-induced increases in DAG were accompanied by increases in cytosolic and membrane-associated protein kinase C activity. These findings suggest that insulin-induced increases in DAG may lead to increases in protein kinase C activity, and may explain some of the insulin-like effects of phorbol esters and vasopressin on hepatocyte metabolism.

Induction of ketogenesis and fatty acid oxidation by glucagon and cyclic AMP in cultured hepatocytes from rabbit fetuses. Evidence for a decreased sensitivity of carnitine palmitoyltransferase I to malonyl-CoA inhibition after glucagon or cyclic AMP treatment

The effects of pancreatic hormones and cyclic AMP on the induction of ketogenesis and long-chain fatty acid oxidation were studied in primary cultures of hepatocytes from fetal and newborn rabbits. Hepatocytes were cultivated during 4 days in the presence of glucagon (10(-6) M), forskolin (2 x 10(-5) M), dibutyryl cyclic AMP (10(-4) M), 8-bromo cyclic AMP (10(-4) M) or insulin (10(-7) M). Ketogenesis and fatty acid metabolism were measured using [1-14C]oleate (0.5 mM). In hepatocytes from fetuses at term, the rate of ketogenesis remained very low during the 4 days of culture. In hepatocytes from 24-h-old newborn, the rate of ketogenesis was high during the first 48 h of culture and then rapidly decreased to reach a low value similar to that measured in cultured hepatocytes from term fetuses. A 48 h exposure to glucagon, forskolin or cyclic AMP derivatives is necessary to induce ketone body production in cultured fetal hepatocytes at a rate similar to that found in cultured hepatocytes from newborn rabbits. In fetal liver cells, the induction of ketogenesis by glucagon or cyclic AMP results from changes in the partitioning of long-chain fatty acid from esterification towards oxidation. Indeed, glucagon, forskolin and cyclic AMP enhance oleate oxidation (basal, 12.7 +/- 1.6; glucagon, 50.0 +/- 5.5; forskolin, 70.6 +/- 5.4; cyclic AMP, 77.5 +/- 3.4% of oleate metabolized) at the expense of oleate esterification. In cultured fetal hepatocytes, the rate of fatty acid oxidation in the presence of cyclic AMP is similar to the rate of oleate oxidation present at the time of plating (85.1 +/- 2.6% of oleate metabolized) in newborn rabbit hepatocytes. In hepatocytes from term fetuses, the presence of insulin antagonizes in a dose-dependent fashion the glucagon-induced oleate oxidation. Neither glucagon nor cyclic AMP affect the activity of carnitine palmitoyltransferase I (CPT I). The malonyl-CoA concentration inducing 50% inhibition of CPT I (IC50) is 14-fold higher in mitochondria isolated from cultured newborn hepatocytes (0.95 microM) compared with fetal hepatocytes (0.07 microM), indicating that the sensitivity of CPT I decreases markedly in the first 24 h after birth. The addition of glucagon or cyclic AMP into cultured fetal hepatocytes decreased by 80% and 90% respectively the sensitivity of CPT I to malonyl-CoA inhibition. In the presence of cyclic AMP, the sensitivity of CPT I to malonyl-CoA inhibition in cultured fetal hepatocytes is very similar to that measured in cultured hepatocytes from 24-h-old newborns.

Translocation to rat liver mitochondria of phosphatidate phosphohydrolase

When a particle-free supernatant fraction from rat liver was incubated at 37 degrees C with mitochondria and oleate, some of the enzyme phosphatidate phosphohydrolase (PAP), initially present in the particle-free supernatant, was recovered, after the incubation, bound to mitochondria. This translocation of PAP from cytosol to mitochondria was stimulated by oleate or palmitate in a similar fashion to the stimulation of translocation of PAP to endoplasmic reticulum [Martin-Sanz, Hopewell & Brindley (1984) FEBS Lett. 175, 284-288]. Translocation of PAP from particle-free supernatant to a partially purified mitochondrial-outer-membrane preparation was also stimulated by oleate. More PAP was bound to a mitochondrial-outer-membrane fraction washed in 0.5 M-NaCl before resuspension in sucrose than to a sucrose-washed mitochondrial-outer-membrane preparation. In contrast, washing of microsomal membranes in 0.5 M-NaCl did not enhance the binding of PAP to these membranes. PAP also binds to phosphatidate-loaded mitochondria or microsomes (microsomal fractions). In the experimental system employed, more PAP bound to mitochondria loaded with phosphatidate than to microsomes loaded with phosphatidate. The results are discussed in relation to the role of mitochondrial phosphatidate in liver lipid metabolism.

Intracytoplasmic triglyceride accumulation produced by dexamethasone in adult rat hepatocytes cultivated on 3T3 cells

Glucocorticoids, such as hydrocortisone (HC) and dexamethasone (DEX), when administered to rats, induce lipid accumulation within hepatocytes (fatty liver). To investigate whether glucocorticoids can produce triglyceride (TG) accumulation as they do in vivo and the involved mechanisms, we have used primary cultures of rat hepatocytes which synthesized and secrete triglycerides into the culture medium. Hepatocytes cultivated on a feeder layer of lethally treated 3T3 cells were exposed for 2 weeks to micromolar concentrations of DEX. This glucocorticoid caused morphological alterations and cells accumulated lipid droplets in their cytoplasm; the TG content increased up to 6-fold in a concentration- and time-dependent manner. The removal of [14C]acetic or [14C]oleic acid from the culture medium was not altered in the cultures treated with 50 micrograms/ml DEX but the incorporation of [14C]acetic and [14C]oleic acid into TG in these cultures was about 13-fold and 60% higher than in non-treated cells, respectively. On the other hand, hepatocytes treated with 50 micrograms/ml DEX for 2 weeks showed a 16-fold decrease in TG release and 40% inhibition in protein export, whereas synthesis of total cellular proteins was not altered. Our results show that corticosteroids, such as DEX, caused lipid accumulation in liver cells through an increased synthesis and/or esterification of fatty acids, but mostly through a decrease in the secretion of TG.

Age-related changes in the translocation of phosphatidate phosphohydrolase from the cytosol to microsomal membranes in rat liver

The effects of oleate, spermine and chlorpromazine were assayed in the presence or absence of 0.15 M KCl on the translocation of phosphatidate phosphohydrolase activity from cytosol to endoplasmic reticulum membranes in liver homogenates obtained from rats aged 1, 30, 60, 180 and 360 days. Marked age-associated decreases in phosphatidate phosphohydrolase distribution onto the membranes were demonstrated under nearly all conditions. In liver homogenates taken from 1-day-old rats and incubated with 0.15 M KCl, most of the enzyme was active (associated with the membranes). Physiological salt concentration (0.15 M KCl) produced a 2-fold increase of oleate-induced translocation of phosphatidate phosphohydrolase activity in liver homogenates from 1-day-old rats; it had no effect on those from 60-day-old rats, and produced a notable decline in liver homogenates obtained from 180- and 360-day-old rats. The promoting effect of spermine on oleate-induced translocation of this enzyme activity was higher in younger rats when incubated in the absence of 0.15 M KCl. Chlorpromazine did not show its usual antagonizing effect on oleate-induced translocation of phosphatidate phosphohydrolase when added to homogenates taken from 1-day-old rats. The antagonizing effect was slightly apparent in liver homogenates from 30-day-old rats and was more pronounced in those from 60-day-old rats in which the values diminished to one-half and to one-third either in the presence or absence of 0.15 M KCl.

Fatty acid uptake and metabolism to ketone bodies and triacyglycerol in rat and human hepatocyte cultures is dependent on chain-length and degree of saturation. Effects of carnitine and glucagon

Rat and human hepatocyte cultures were incubated with 5 common plasma longchain fatty acids (C16-C18). Rates of fatty acid uptake were similar in rat and human hepatocytes and were of the order: 16:1 greater than 16:0; 18:2 greater than 18:1 greater than 18:0. Rates of ketogenesis were lower in human compared to rat hepatocytes. In rat hepatocytes glucagon stimulated ketogenesis only in the presence of exogenous carnitine and rates of ketogenesis were higher from unsaturated compared to corresponding saturated fatty acids. Glucagon decreased triacylglycerol secretion irrespective of the fatty acid substrate and it increased intracellular triacylglycerol accumulation. The latter effect of glucagon was more marked in the absence of carnitine supplementation.

Changes in activities of some enzymes of glycerolipid synthesis in brown adipose tissue of cold-acclimated rats

1. Measurements were made of the activities of the following enzymes of glycerolipid synthesis in homogenates of interscapsular brown adipose tissue obtained from rats subjected to a 4 degrees C environment for time periods of 6 h up to 12 days: fatty acyl-CoA synthetase (FAS), mitochondrial and microsomal forms of glycerolphosphate acyltransferase (GPAT), monoacylglycerolphosphate acyltransferase (MGPAT) and Mg2+-dependent phosphatidate phosphohydrolase (PPH). 2. Relative to tissue DNA content, the activities of mitochondrial GPAT, MGPAT and Mg2+-dependent PPH were significantly increased after 1 day of exposure to cold, and continued to increase thereafter. By contrast, FAS and microsomal GPAT activities were unchanged relative to tissue DNA. 3. The time profile of the increase in MGPAT activity correlated well with a concomitant increase in the microsomal marker NADP+-cytochrome c reductase. Changes in mitochondrial GPAT and in Mg2+-dependent PPH activities were larger in amplitude than that of MGPAT. 4. It is proposed that these selective changes in enzyme activity may be associated with the onset of brown-adipose-tissue hyperplasia or possibly with an increase in triacylglycerol synthesis during cold-acclimation.

Effects of chronic administration of benfluorex to rats on the metabolism of corticosterone, glucose, triacylglycerols, glycerol and fatty acid

(1) Rats were fed on diets enriched with sucrose, beef tallow or corn oil and treated for 11-16 days with 50 mg of benfluorex per kg of body weight. By these times the growth rate and food intake were not significantly different from those of control rats. (2) Benfluorex approximately halved the concentration of circulating triacylglycerol in rats fed the beef tallow or sucrose diets. (3) It did not significantly alter the total lipoprotein lipase activity in diaphragm, heart and adipose tissue. (4) The clearance of triacylglycerols from chylomicrons exhibited two t 1/2 values of about 0.6 and 6.9 min in rats fed the beef tallow diet. Benfluorex did not significantly alter these values. (5) Benfluorex did not significantly alter the rate of appearance of triacylglycerol in the blood of rats injected with Triton WR 1339 to block triacylglycerol uptake. It did, however, decrease the rise in circulating glucose which presumably resulted from the stress of the procedure. (6) Benfluorex decreased the extent and duration of the rise in serum corticosterone when rats maintained on the corn oil diet were fed acutely with fructose. It also decreased the circulating concentrations of glycerol, triacylglycerol and glucose after fructose feeding. (7) Rats fed on the corn oil diet and then treated with benfluorex had lower concentrations of circulating glucose, triacylglycerol, glycerol and fatty acids after being injected with 2-deoxyglucose. (8) It is proposed that some of the long-term hypoglycaemic and hypotriglyceridaemic effects of benfluorex could be mediated indirectly through changes in endocrine balance, perhaps via the serotonergic system and in particular, by decreasing the effects of stress hormones relative to insulin. The implications of these findings are discussed in relation to controlling metabolism in stress conditions and for the management of obesity, diabetes and atherosclerosis.

Binding of low-density lipoprotein to monolayer cultures of rat hepatocytes is increased by insulin and decreased by dexamethasone

Rat hepatocytes were maintained in monolayer culture for 20 h in the presence of 10% (v/v) newborn calf serum and then for a further 1-24 h in serum-free medium containing 2 g bovine serum albumin/l. The specific binding of human 125I-LDL to two distinct sites was then measured at 4 degrees C. Binding to site 1 was displaced by dextran sulphate while that to site 2 was not. The presence of 1-100 nM insulin for 24 h in the second incubation significantly increased binding to site 1. Significant increases were also seen when cells were incubated with 10 nM insulin for 1 h. No significant effects of insulin on binding to site 2 were observed. In contrast, 10 nM-1 microM dexamethasone decreased binding to both sites. The effects of these hormones were mutually antagonistic.

A rapid assay for measuring the activity and the Mg2+ and Ca2+ requirements of phosphatidate phosphohydrolase in cytosolic and microsomal fractions of rat liver

1. A rapid extraction and purification scheme was designed for the recovery of [3H]diacylglycerol formed during the assay of phosphatidate phosphohydrolase. 2. The importance of removing polyvalent cations, particularly Ca2+, from the phosphatidate and other reagents used in the assay of the phosphohydrolase activity was demonstrated. This was achieved mainly by treating the phosphatidate with a chelating resin and by adding 1 mM-EGTA and 1 mM-EDTA to the assays. 3. The activity of the phosphohydrolase in dialysed samples of the soluble and microsomal fractions of rat liver was very low. 4. Addition of optimum concentrations of MgCl2 resulted in a 110-167-fold stimulation in activity. 5. CaCl2 was also able to stimulate phosphohydrolase activity, but to a much smaller extent than MgCl2. 6. Chlorpromazine, an amphiphilic cation, inhibited the reaction when it was measured in these experiments by using a mixed emulsion of phosphatidylcholine and phosphatidate at pH 7.4. 7. Microsomal fractions that were preincubated with albumin contained very low activities of the Mg2+-dependent phosphohydrolase. When these were then incubated with the soluble fraction in the presence of oleate, the soluble phosphohydrolase attached to the microsomal membranes, and it retained its high dependency on Mg2+.

Regulation of albumin synthesis by hormones and amino acids in primary cultures of rat hepatocytes

Culture conditions necessary for optimizing albumin secretion were studied in rat hepatocytes maintained in a chemically defined, serum-free medium. Amino acid analysis of the culture medium, which was based on a 1:1 mixture of Ham's F12:Dulbecco's modified Eagle's medium (unsupplemented medium), revealed that certain essential amino acids were depleted from this medium over a 24-h incubation. Rates of albumin secretion were significantly higher and better maintained when the medium was supplemented with additional amino acids (supplemented medium). Moreover, selective removal of an essential amino acid resulted in an immediate decrease in total protein and albumin synthesis and after 48 h a further selective decrease in albumin synthesis. Linear rates of albumin secretion were observed over a wide variety of experimental conditions, but secretion was not strictly proportional to cell number. Maximal rates of secretion were obtained at plating densities of 2-3 X 10(6) cells/60 mm culture dish. Albumin secretion also increased with time in culture reaching a maximum on days 3 and 4. When added singly, either insulin or dexamethasone increased rates of albumin secretion in a dose-dependent manner, but both hormones and an adequate supply of amino acids were necessary for maximal rates of secretion as well as long-term maintenance of the hepatocytes (greater than 3-4 days). In the presence of dexamethasone the dose-response curve for insulin was shifted toward physiological insulin concentrations. Changes in rates of albumin secretion in response to added hormones in supplemented media were found to parallel changes in albumin synthesis and relative amounts of albumin mRNA. Changes in gene transcription were probably involved.

Effects of insulin, glucagon, dexamethasone, cyclic GMP and spermine on the stability of phosphatidate phosphohydrolase activity in cultured rat hepatocytes

Hepatocytes were preincubated with 10mM-glucagon and 100 microM-corticosterone to increase phosphatidate phosphohydrolase activity. Addition of 10 nM-glucagon or 100 microM-8-bromo cyclic GMP to a second incubation mixture that contained cycloheximide increased the half-life of the phosphohydrolase activity. Dexamethasone (100 nM) had no significant effect, but insulin (500 pM) or spermine (1 mM) decreased the half-life. None of these compounds altered the general rate of degradation of proteins labelled with [3H]leucine. There appears to be a specific control of the half-life of phosphatidate phosphohydrolase activity, which could contribute to its long-term regulation in the liver.

Regulation of ketogenesis, gluconeogenesis and the mitochondrial redox state by dexamethasone in hepatocyte monolayer cultures

The effects of the glucocorticoid dexamethasone on fatty acid and pyruvate metabolism were studied in rat hepatocyte cultures. Parenchymal hepatocytes were cultured for 24 h with nanomolar concentrations of dexamethasone in either the absence or the presence of insulin (10 nM) or dibutyryl cyclic AMP (1 microM BcAMP). Dexamethasone (1-100 nM) increased the rate of formation of ketone bodies from 0.5 mM-palmitate in both the absence and the presence of BcAMP, but inhibited ketogenesis in the presence of insulin. Dexamethasone increased the proportion of the palmitate metabolized that was partitioned towards oxidation to ketone bodies, and decreased the cellular [glycerol 3-phosphate]. The latter suggests that the increased partitioning of palmitate to ketone bodies may be associated with decreased esterification to glycerolipid. The Vmax. of carnitine palmitoyltransferase (CPT) and the affinity of CPT for palmitoyl-CoA were not affected by dexamethasone, indicating that the increased ketogenesis was not due to an increase in enzymic capacity for long-chain acylcarnitine formation. Dexamethasone and BcAMP, separately and in combination, increased gluconeogenesis. In the presence of insulin, however, dexamethasone inhibited gluconeogenesis. Changes in gluconeogenesis thus paralleled changes in ketogenesis. Dexamethasone decreased the [3-hydroxybutyrate]/[acetoacetate] ratio, despite increasing the rate of ketogenesis and presumably the mitochondrial production of reducing equivalents. The more oxidized mitochondrial NADH/NAD+ redox couple with dexamethasone is probably due either to an increased rate of electron transport or to increased transfer of mitochondrial reducing equivalents to the cytoplasm.

Spermine antagonises the effects of dexamethasone, glucagon and cyclic AMP in increasing the activity of phosphatidate phosphohydrolase in isolated rat hepatocytes

Rat hepatocytes were incubated in monolayer culture, under serum free conditions, for 8 h. Glucagon (10 nM), 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate (100 microM) and dexamethasone (100 nM) increased the activity of phosphatidate phosphohydrolase by approx. 2-, 3.6- and 3.3-fold, respectively. Spermine alone had no significant effect. Spermine (2.5 mM) almost completely inhibited the glucagon induced increase in phosphohydrolase activity. It only partially inhibited the dexamethasone and cyclic AMP mediated inductions. Spermidine had no significant effect in this respect. The results are discussed in relation to the known effects of polyamines on glycerolipid synthesis, in particular, and on intermediary metabolism.

Adipose-tissue Mg2+-dependent phosphatidate phosphohydrolase. Control of activity and subcellular distribution in vitro and in vivo

The subcellular distribution of Mg2+-dependent phosphatidate phosphohydrolase in rat adipocytes between a soluble and a membrane-bound fraction was measured by using both centrifugal fractionation and a novel Millipore-filtration method. The relative proportion of the phosphohydrolase associated with the particulate fraction was increased on incubation of cells with noradrenaline or palmitate. Insulin on its own decreased the proportion of the phosphohydrolase that was particulate and abolished the effect of noradrenaline, but not that of palmitate. The effect of noradrenaline on phosphohydrolase distribution was rapid, the effect being maximal within 10 min. Noradrenaline exerted this effect with a similar concentration-dependence to its lipolytic effect. Inclusion of albumin in homogenization buffers decreased the proportion of the phosphohydrolase that was particulate, but did not abolish the effect of noradrenaline. There was limited correlation between the proportion of the phosphohydrolase that was particulate and the measured rate of triacylglycerol synthesis in adipocytes incubated under a variety of conditions. Starvation, streptozotocin-diabetes and hypothyroidism decreased the specific activities of the phosphohydrolase and glycerolphosphate acyltransferase in homogenates from epididymal fat-pads. Restoration of these activities in the diabetic state was seen after administration of insulin over 2 days or, in the short term, within 2 h after a single administration of insulin. Administration of thyroxine over 3 days caused restoration of these activities in the hypothyroid state. Starvation and diabetes increased the proportion of the phosphohydrolase found in the microsomal fraction. This change was not seen when albumin was present in homogenization buffers. The possible role of fatty acids as regulators of the intracellular translocation of the phosphohydrolase, together with the role of this enzyme in the regulation of triacylglycerol synthesis in adipose tissue, is discussed.

Effects of an antitumoural rhodium complex on thioacetamide-induced liver tumor in rats. Changes in the activities of ornithine decarboxylase, tyrosine aminotransferase and of enzymes involved in fatty acid and glycerolipid synthesis

Rats were injected daily for 8 weeks with 50 mg of thioacetamide per kg to produce liver tumours. Some of these rats were given three doses of 50 mg of an antitumoural Rh(III) complex/kg at 14, 9 and 5 days before the end of the thioacetamide treatment. Thioacetamide decreased the rate of weight gain of the rats and the Rh(III) complex partly restored it. The activities of ATP citrate lyase, acetyl-CoA carboxylase and fatty acid synthetase in the livers were decreased by thioacetamide treatment and the Rh(III) complex partly reversed this effect. By contrast the activity of malic enzyme was increased by both thioacetamide and the Rh(III) complex and this effect probably relates to NADPH production for detoxification rather than for lipogenesis. Treatment with thioacetamide increased the rate of synthesis of di- and triacylglycerols from glycerol phosphate by liver homogenates, the activity of phosphatidate phosphohydrolase and the incorporation of [3H]glycerol into liver triacylglycerol in vivo. The Rh(III) complex did not produce a significant reversal of these effects of thioacetamide on glycerolipid synthesis. The total uptake of intraportally injected [3H]glycerol by the livers of thioacetamide treated rats was decreased and this was associated with a lowered activity of glycerol kinase. Thioacetamide increased the activity of hepatic ornithine decarboxylase by about 40-fold, but the Rh(III) complex did not reverse this effect. However, the decrease in tyrosine aminotransferase activity that was produced by thioacetamide was partly reversed by the Rh(III) complex. These results are discussed in relation to the tumour-promoting effects of thioacetamide and the antitumoural action of the Rh(III) complex.