Investigations into the direct effects of insulin on hepatic ketogenesis, lipoprotein secretion and pyruvate dehydrogenase activity

Augmentation of Ca2+-Induced Microsomal Triglyceride Transfer Protein Activity by Glucose Supply Enhances Hypertriglyceridemia in Vivo

We have investigated possible roles of intra-glucose supply on microsomal triglyceride (TG) transfer protein (MTP) in the secretion of TG-rich very low-density lipoprotein (VLDL) from the liver. Due to the activation of MTP, TG and apolipoprotein B (apoB) in the liver are assembled into VLDL and then the VLDL is transferred into the blood stream. High MTP activity can increase the release of VLDL into the blood stream, and this would lead high levels of TG and apoB in the blood. High MTP activity was found when the liver (or hepatocytes) contained a high level of total Ca2+ as a response of glucose administration. However, the MTP activity was reduced in response to the calmodulin antagonist N-(6-aminohexyl)-5-chloro-1-naphthalene sulfonamide (W-7, Ki=25 microM), the intracellular Ca2+ chelator BAPTA-AM, and the extracellular Ca2+ chelator EDTA. These suggested that there might be a very close relationship between high MTP activity and high Ca2+ level in the liver by glucose administration. Glucose-derived hyperglycemic condition resulted from those elevations of TG and total cholesterol in the liver. This hyperglycemic phenomenon may be associated with the increase of TG and apoB levels in blood. The possibility for the regulation of VLDL formation in the liver and, further, those related circulatory diseases due to the excess of VLDL in the blood stream by controlling MTP activity in association with Ca2+ was investigated.

The Possible Role of Ca2+ on the Activation of Microsomal Triglyceride Transfer Protein in Rat Hepatocytes

Microsomal triglyceride (TG) transfer protein (MTP) is involved in the secretion of TG-rich very low-density lipoprotein (VLDL), a process which leads to the generation of hypertriglyceridemia and atherosclerosis. We investigated the possible role of Ca(2+) on MTP activity in hepatocytes. Exogenous CaCl(2) and calmodulin increased MTP activity dose-dependently, and calcium ionophore A23187 (A23187) also increased total Ca(2+) level and MTP activity in hepatocytes. Moreover, MTP activity increased by CaCl(2) or A23187 was abrogated in the presence of EDTA, a Ca(2+) chelator. MTP activity was increased by the simultaneous addition of CaCl(2) and calmodulin. However, this increase was inhibited by N-(6-aminohexyl)-5-chloro-1-naphthalene sulfonamide (W-7), a Ca(2+) antagonist. A23187 increased the release of TG and cholesterol from hepatocytes, and these were inhibited by EDTA. A23187 also increased the ratio of TG to HDL-cholesterol in hepatocytes culture medium, which indicates the release of TG is higher than that of HDL-cholesterol from hepatocytes. Thus, our findings demonstrate that hepatocellular Ca(2+) contributes directly or indirectly to MTP activation. In conclusion, the inhibition of MTP activity via the suppression of hepatocellular Ca(2+) may result in the inhibition of hypertriglyceridemia.

Effect of meal sequence on postprandial lipid, glucose and insulin responses in young men

OBJECTIVE

To investigate whether the postprandial changes in plasma triacylglycerol (TAG), nonesterified fatty acids (NEFA), glucose and insulin concentrations in young men were the same if an identical meal was fed at breakfast and lunch, and if the response to lunch was modified by consumption of breakfast.

METHODS

In two trials (1 and 2) healthy subjects (age 22+/-1 y, body mass index 22+/-2 kg/m(2)) were fed the same mixed macronutrient meal at breakfast at 08:00 h and lunch at 14:00 h. In the third trial, no breakfast was fed and the overnight fast extended until lunch at 14:00 h. Addition of [1,1,1-(13)C]tripalmitin to one meal in each trial was used to distinguish between endogenous and meal-derived lipids.

RESULTS

The postprandial changes in TAG, NEFA and glucose concentrations were similar in trials 1 and 2. The change in plasma total TAG concentration was about two fold less (P<0.05) after lunch compared to breakfast. Postprandial NEFA suppression was the same after breakfast and lunch. Glucose and insulin responses were significantly greater following lunch suggesting decreasing insulin sensitivity during the day. Consumption of breakfast did not alter the postprandial total TAG or NEFA responses after lunch. Measurement of [(13)C]palmitic acid concentration showed that handling of TAG and NEFA from the meal was the same after breakfast and lunch, and was not altered by consumption of breakfast.

CONCLUSIONS

Overall, these data suggest that in young, healthy men regulation of plasma TAG from endogenous sources, principally VLDL, but not chylomicrons during the postprandial period leads to differences in the magnitude of lipaemic response when the same meal was consumed at breakfast or at lunch 6 h later.

Insulin stimulation of hepatic triacylglycerol secretion in the insulin-replete state: implications for the etiology of peripheral insulin resistance

Observations on humans, on rats in vivo, and on isolated perfused rat livers indicate that insulin stimulates hepatic very-low-density lipoprotein (VLDL)-TAG secretion when the liver is chronically exposed to the hormone. They suggest that frequent stimulation of insulin secretion throughout the diurnal cycle may result in a chronic stimulation of VLDL secretion and increased delivery of acyl moieties to the periphery, particularly to muscle, the most important site of insulin-sensitive glucose disposal. If acyl groups are provided in excess of the oxidative needs of the tissue, this may lead to induction of insulin resistance, irrespective of whether obesity is established concomitantly. Dietary factors that stimulate hepatic VLDL secretion may have the same effect and contribute to the induction of a vicious spiral leading to the development of the full-blown Metabolic Syndrome and its pathological consequences, including type-2 diabetes, stroke, and cardiovascular disease.

Insulin stimulation of hepatic triacylglycerol secretion and the etiology of insulin resistance

The recent observations that insulin can either stimulate or inhibit triacylglycerol secretion by the liver, depending on prior metabolic (possibly insulinemic) state, have rationalized the many apparently contradictory observations, obtained over the past three decades, on the effects of the hormone on this aspect of hepatic metabolism. Extrapolation to the situation in vivo suggests that frequent stimulation of insulin secretion may result in a chronic stimulation of VLDL secretion, and increased delivery of acyl moieties to muscle, where they induce insulin resistance if provided in excess of the oxidative needs (mostly due to exercise) of the tissue. High fructose/sucrose diets, which also stimulate hepatic VLDL secretion, will have the same effect, especially if consumed frequently during the diurnal cycle. Due to the quantitative importance of muscle as a site for insulin-sensitive glucose metabolism, these effects may initiate the metabolic vicious cycle that results in the development of the metabolic syndrome, well in advance of overt obesity or the diagnosis of type-2 diabetes.

Insulin stimulates triacylglycerol secretion by perfused livers from fed rats but inhibits it in livers from fasted or insulin-deficient rats implications for the relationship between hyperinsulinaemia and hypertriglyceridaemia

We determined whether the direction of the acute effect of insulin on hepatic triacylglycerol secretion is dependent on the prior physiological state or on the in vitro experimental system used. The effect of insulin on triacylglycerol secretion was studied using perfused livers isolated from rats under three metabolic conditions: fed normo-insulinaemic, 24-h fasted and fed, streptozotocin-diabetic (insulin-deficient). Insulin acutely activated triacylglycerol secretion (by 43%) in organs from fed, normo-insulinaemic animals, whereas it inhibited triacylglycerol secretion in livers isolated from fasted or insulin-deficient rats (by 30 and 33%, respectively). By contrast, in 24-h-cultured hepatocytes insulin invariably acutely inhibited triacylglycerol secretion irrespective of the metabolic state of the donor animals. It is concluded that the use of perfused livers enables the observation of a switch in the direction of insulin action on hepatic triacylglycerol secretion from stimulatory, in the normo-insulinaemic state, to inhibitory in the fasting or insulin-deficient state. The possible implications of this switch for the relationship between hyperinsulinaemia, increased hepatic very-low-density lipoprotein-triacylglycerol secretion and hypertriglyceridaemia observed in vivo are discussed.

Studies in hypertriglyceridaemia. VI: Serum lathosterol concentration is raised in hypertriglyceridaemic non-diabetic males with hyperinsulinaemia

OBJECTIVES

To assess the cholesterol synthesis rate in primary hypertriglyceridaemia using the serum unesterified lathosterol concentration as an indicator.

DESIGN

A cross-sectional, case-control study.

SETTING

The Karolinska Hospital, Stockholm.

SUBJECTS

Randomly selected hyper- (n = 53) and normotriglyceridaemic (n = 57) males, 40-50 years, with a fasting serum triglyceride concentration (mean +/- SD) of 3.81 +/- 1.65 and 1.28 +/- 0.53 mmol L-1, respectively. The exclusion criterion was diabetes mellitus, defined according to the World Health Organization (WHO) guidelines.

MAIN OUTCOME MEASURES

To compare the fasting serum concentration distributions of lathosterol, a cholesterol precursor, in hyper- and normotriglyceridaemic groups.

RESULTS

Thirty-six per cent of the hypertriglycerdaemic group had raised serum lathosterol concentrations, based on the 90th percentile of the lathosterol distribution of the normotriglyceridaemic group. In the hyper- but not in the normotriglyceridaemic group, lathosterol concentration was directly correlated with serum insulin responses to oral (r = 0.38; P = 0.007) and intravenous (r = 0.41; P = 0.005) glucose challenges.

CONCLUSIONS

One-third of a randomly selected non-diabetic hypertriglyceridaemic population had an increased serum lathosterol concentration and this might indicate an increased cholesterol synthesis rate compatible with increased production of VLDL particles, possibly as the result of chronic hyperinsulinaemia.

Insulin modulation of newly synthesized apolipoproteins B-100 and B-48 in human fetal intestine: gene expression and mRNA editing are not involved

We investigated insulin's effect on intestinal lipid, transport and, particularly, the biogenesis of apolipoproteins crucial to lipoprotein secretion. Adding insulin (3 mU) to the serum-free medium of cultured jejunal explants from human fetuses (17-20 weeks) reduced triglyceride and chylomicron production and inhibited apo B-48 and apo B-100 secretion. When apo B mRNA was assayed by RT-PCR and its editing by primer extension, no change was detectable following the addition of insulin. HDL lipid content, apo A-1 synthesis and RNA level were unaffected by insulin. Collectively, these results suggest that the insulin-stimulated decline in intestinal chylomicron output may involve apo B co- or post-translational modifications.

Effect of insulin on triacylglycerol synthesis and secretion by chicken hepatocytes in primary culture

A primary culture system of chicken hepatocytes was developed to study the effects of genetic, hormonal and nutritional factors on hepatic triglyceride (TG) secretion in the chicken, and the effect of insulin on TG synthesis and secretion examined. TG synthesis and secretion was measured using [3H]-glycerol incorporation into cellular and secreted TG. An additional step consisting of brief incubation of the monolayer with trypsin solution to improve harvesting the medium immediately adjacent to the cell monolayer, is also proposed. In our culture system, TG secretion occurred at least up to 75 hr of culture, showing a maximum between 40 and 60 hr of culture. No significant effect of insulin could be observed after 24 hr of culture. A clear stimulatory effect was observed with 10(-9) M insulin concentration after 48 hr. The mean ratio of the secretion rates in the presence or absence of insulin was 2.20 +/- 0.30 (SEM, n = 4). In contrast, the 10(-6) M concentration of insulin had no effect on TG secretion. The use of an additional trypsinization step enhanced the findings obtained by simple removal of the culture medium: a clear stimulatory effect of insulin on TG synthesis was observed after both 24 and 48 hr of culture. Analysis of TG fatty acid composition showed an imbalance of monoene versus saturated fatty acids between cellular and secreted TG, secreted TG being more monounsaturated than cellular TG. These results were obtained in the absence of exogenous fatty acid. An oleic acid supplement in the culture medium did not significantly influence TG secretion. In summary, a primary culture system for chicken hepatocyte was developed. A high level of TG secretion was seen in these cells with or without exogenous fatty acid and this secretion was stimulated by insulin. It was concluded that chicken hepatocytes in primary culture provide a useful model for studying regulation of TG secretion.

Insulin-independent and extremely rapid switch in the partitioning of hepatic fatty acids from oxidation to esterification in starved-refed diabetic rats. Possible roles for changes in cell pH and volume

The requirement for a normal insulin response in mediating the starved-to-refed transition, with respect to the partitioning of hepatic fatty acids between beta-oxidation and esterification to glycerol, was studied. Diabetic rats were starved for 24 h and refed ad libitum for various periods of time. There was no increase in plasma insulin in response to the meal. However, the fatty acid oxidation:esterification ratio was very rapidly decreased from the starved to the fed value, most of the transition being achieved within the first hour of refeeding. There was a 2 h lag in the response of hepatic malonyl-CoA concentration, such that this rapid switch from oxidation to esterification could not be explained on the basis of changes in the absolute concentration of this inhibitor of carnitine palmitoyltransferase I (CPT I). Hepatic pyruvate and lactate concentrations both increased by several-fold upon refeeding and peaked after 1 h and 3 h, respectively. The hepatic lactate:pyruvate ratio increased 3.2-fold during the first 3 h of refeeding, suggesting that the cytosolic NAD(+)-NADH couple became much more highly reduced during the lag-period between the onset of inhibition of flux of fatty acids towards oxidation and the rise in malonyl-CoA concentration. This may be indicative of a lowering of intracellular pH, which would amplify greatly the sensitivity of CPT I to the inhibitor. In view of the very rapid and high food intake by these diabetic rats, the possibility is also considered that portal concentrations of amino acids and other metabolites could give rise to an increase in liver cell-volume that would inhibit CPT I acutely by an as yet unknown mechanism [M. Guzman, G. Velasco, J. Castro and V. A. Zammit (1994) FEBS Lett. 344, 239-241].

Inhibition of hepatic very-low-density lipoprotein secretion in obese Zucker rats adapted to a high-protein diet

The effect of a high-protein (HP) diet on hepatic very-low-density lipoprotein (VLDL) secretion was studied in obese and lean Zucker rats. With the control (C) diet, isolated hepatocytes from obese as compared with lean rats displayed higher uptake of [1-14C]oleate 0.7 mmol/L, 95% of which was esterified to glycerolipids; greater oleate incorporation into VLDL-triacylglycerol (TG); 2.6 times higher total VLDL-TG secretion; and 11-fold higher de novo fatty acid synthesis. Adaptation to HP feeding decreased weight gains in both phenotypes and hepatocyte TG content in obese rats. Oleate uptake by hepatocytes was appreciably reduced in the obese phenotype only. Despite esterification rates similar to those for the C diet, oleate incorporation into VLDL-TG decreased by 34% and 55% in obese and lean rats, respectively. Total (mass) VLDL-TG secretion was drastically decreased by 65% and 48% in obese and lean rat hepatocytes, respectively. HP feeding combined with overnight fasting accentuated the above decreases. Fatty acid synthesis was 50% lower in cells from HP-fed obese rats, but increased 1.7-fold in lean ones. Plasma glucagon increased in both phenotypes under HP feeding, whereas plasma insulin either increased (obese) or decreased (lean), with the insulin to glucagon ratio slightly decreasing. Thus, HP feeding drastically inhibited hepatic VLDL secretion in obese and lean Zucker rats by an undefined mechanism that was apparently related neither to de novo fatty acid synthesis nor to changes in oleate partitioning between esterification and oxidation.

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)

Triacylglycerol accumulation and secretion in hepatocyte cultures. Effects of insulin, albumin and Triton WR 1339

We have investigated the possibility that the apparent inhibition of very-low-density lipoprotein (VLDL)-triacylglycerol secretion by the addition of insulin to rat hepatocyte cultures may result from insulin-mediated enhancement of hepatic lipase secretion and, consequently, of extracellular triacylglycerol hydrolysis. We have, therefore, studied the effects of the inhibitor of lipase activity, Triton WR 1339, on the secretion of triacylglycerol by cultured rat hepatocytes. Incubation of hepatocyte cultures with increasing concentrations of Triton WR 1339 increased the accumulation of acylglycerol in the medium, suggesting that, in normal incubations, a substantial rate of degradation of secreted triacylglycerol does occur and that it results in an under-estimation of the rate of triacylglycerol secretion. However, Triton did not counteract the inhibitory effects of insulin, suggesting that the observed increased activity of hepatic lipase induced by the hormone cannot account for the inhibition of acylglycerol accumulation in the medium that occurred in the presence of insulin. BSA increased the accumulation of triacylglycerol in culture media by about 2-fold and also decreased the activity of hepatic lipase by 80%. A causative relationship between these two effects was supported by the further observation that Triton abolished the effects of BSA on triacylglycerol accumulation in the medium. The implications of these data for the validity of the use of Triton for the study of hepatic rates of triacylglycerol production in vivo and of secretion by hepatocytes in vitro are discussed.

Quantitative studies on fatty acid metabolism in isolated parenchymal cells from normal and cirrhotic livers in rats

Parenchymal cells isolated from normal and thioacetamide-induced micronodular cirrhotic rat livers were used to evaluate changes in hepatocellular fatty acid (FA) metabolism in cirrhosis. Exogenous free FA (FFA) are rapidly taken up by hepatocytes obtained either from normal or cirrhotic livers. They are predominantly esterified to triglycerides and accumulate intracytoplasmatically as lipid droplets with increasing cellular FFA uptake. In the parenchymal cells of cirrhotic livers, however, the following changes were observed when compared with controls: (i) decreased cellular output of esterified FA derived both from exogenous sources and from de novo FA synthesis; (ii) increased total ketone body production, mainly as beta-hydroxybutyrate; (iii) decreased cholesterol synthesis; and (iv) enhanced incorporation of newly synthesized FA into phospholipids in spite of an unaffected rate of FA synthesis. In summary, the data provide evidence for intrinsic alterations in the lipid metabolism in the parenchymal cells of cirrhotic livers which are preserved in the isolated hepatocytes under optimum incubation conditions for oxygen and substrate supply.

Therapy of diseases of ruminant intermediary metabolism

Diseases of intermediary metabolism include ketosis and fatty liver of dairy cattle and pregnancy toxemia of ewes. These conditions occur when there is a failure of the homeostatic mechanisms regulating the mobilization of fats and the conservation of carbohydrates. The therapeutic approach is to reestablish the normal homeostatic patterns of fuel utilization. Suppression of excessive ketogenesis is the most important factor in reestablishing homeostasis. Ketogenesis can be suppressed by a number of therapeutic agents that act either by suppressing the mobilization of fatty acids or by inhibiting the transport of fatty acids into the hepatic mitochondria, the site at which fatty acids are converted to ketone bodies. Useful therapies include bolus glucose infusions, glucose precursors, and glucocorticoids.

Effect of physiological concentrations of insulin and glucagon on the relationship between nonesterified fatty acids availability and ketone body production in humans

To determine the effect of insulin and glucagon on the transformation of nonesterified fatty acids (NEFA) into ketone bodies (KB), we measured simultaneously in normal subjects NEFA and KB kinetics at different NEFA levels in the presence of basal (control test) or increasing insulin concentrations with glucagopenia (somatostatin + insulin infusion, insulin test) and without glucagopenia (somatostatin + insulin + glucagon infusion, glucagon test). NEFA levels were controlled during these tests by an intravenous (IV) infusion of a triglyceride emulsion. During the control test, a moderate increase of NEFA (464 +/- 30 to 715 +/- 56 mumol/L) increased the percentage of NEFA converted into KB (13.3% +/- 1.4% to 26.4% +/- 2.1%, P less than .05), and there was a linear relationship between this percentage and NEFA levels (r = .788, P less than .01). During the insulin and glucagon tests, the progressive increase in NEFA induced by the triglyceride emulsion infusion was associated, despite the increase of insulinemia, with an increase in KB production rate (P less than .05) and in the proportion of NEFA used for ketogenesis in the presence (8.1% +/- 1.2% to 14.2% +/- 6.3%, P less than .05) and absence (15.7% +/- 2.8% to 25.2% +/- 3.99%, P less than 0.05) of glucagopenia. In both tests, this percentage was always linearly related with NEFA levels (P less than .05) and the slopes of these relationships were comparable to that observed in the control test. However, the fraction of NEFA used for ketogenesis was always higher (P less than .05) during glucagon substitution than in its absence.(ABSTRACT TRUNCATED AT 250 WORDS)

Decreased serum lipids, serum insulin and triacylglycerol synthesis in adipose tissue of JCR:LA-corpulent rats treated with benfluorex

Rats of the JCR:LA-corpulent strain were treated with benfluorex daily at a dose of 25 mg/kg body weight. This strain of rat, if homozygous for the cp gene (cp/cp), is hyperphagous, obese, hypertriglyceridemic, insulin resistant and in the case of male rats, atherosclerosis prone. The benfluorex treatment produced a sharp reduction in food intake which remained suppressed despite recovery toward normal after 2 weeks of treatment. This was accompanied by sustained decreases in body weight and adipose tissue mass. The ability of adipose tissue from female rats to take up glucose and convert it to lactate, glyceride-glycerol and fatty acids was decreased. This decrease was largely due to decreased adipose tissue mass. The serum concentrations of glucose, lactate, triacylglycerol, cholesterol, phospholipids and insulin were decreased in both sexes. The treatment also improved glucose tolerance and decreased corticosterone concentrations in male rats only. While reduction of food consumption contributes to the effects seen, benfluorex clearly had significant direct metabolic effects. The effects are consistent with an improved insulin sensitivity leading to a decrease in circulating triacylglycerol. The changes produced by benfluorex are all in directions that should inhibit atherogenesis in this animal model for the human obesity/hypertriglyceridemia/insulin resistant syndrome.

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.

Assembly and secretion of hepatic very-low-density lipoprotein

In contrast to water-soluble fuels such as glucose or ketone bodies, the use of lipids as an energy source for tissues has required the development of complex structures for their transport through the aqueous plasma. In the case of endogenously synthesized triacylglycerol this is achieved by the assembly and secretion of hepatic VLDL which provides the necessary stability in an aqueous medium. An essential component of this assembly process is apo B. Dietary changes which require an increase in hepatic VLDL secretion appear to be accompanied by increases in the availability of functional apo B. Interesting questions relate to: (a) the intracellular site(s) of triacylglycerol association with apo B, and (b) the mechanism(s) by which the availability of functional apo B at this site responds to metabolic and hormonal signals which reflect dietary status and, thus, the need to secrete triacylglycerol. As regards the latter, although in some cases changes in apo B synthesis occur in response to VLDL secretion hepatic apo B mRNA levels appear to be quite stable in vitro. Intracellular switching of apo B between the secretory and degradative pathways may be important in controlling VLDL assembly and post-translational modifications of the apoprotein may also play a role by influencing its ability to bind to triacylglycerol. Transport is not the only problem associated with the utilization of a concentrated energy source such as triacylglycerol and the complex problems of waste product disposal and recycling have to be dealt with. In the case of triacylglycerol, potentially toxic waste products include atherogenic remnants and LDL. The overall problem, then, in the long-term, involves the development of a 'safe' means of utilizing triacylglycerol and this requirement accounts for much of the complexity of plasma lipoprotein metabolism. In this area, the rat could teach the human a few tricks. One of these appears to be the utilization of hepatic apo B48 rather than apo B100 for VLDL assembly in response to increases in the extrahepatic utilization of hepatically synthesized triacylglycerol. Under these conditions, the remnants of hepatic triacylglycerol utilization by peripheral tissues are cleared from the plasma much more readily via a process which seems to involve the cycling of more triacylglycerol back to the liver than that which occurs in humans. The means by which this is achieved, though, are obscure and may involve a chylomicron remnant receptor, the nature of which, itself, remains controversial.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of ketone body flux in septic patients

To assess the effect of sepsis on ketone body (KB) kinetics in humans, we measured in normal and septic subjects KB appearance rate (Ra) before (initial state) and during a rise of free fatty acids (FFA) level (intravenous infusion of a triglycerides emulsion). We studied normal subjects in postabsorptive state and septic patients when receiving an hypocaloric intravenous infusion of glucose and amino acids or 12 h after its interruption. When receiving glucose and amino acids infusion, septic patients had higher glucose and insulin levels than normal subjects, and despite lower FFA concentrations (255 +/- 44 vs. 480 +/- 51 mumol/l, P less than 0.05) comparable initial KB Ra (2.50 +/- 0.10 vs. 2.48 +/- 0.30 mumol.kg-1.min-1). Triglyceride infusion increased FFA to comparable values (septic 780 +/- 130, normal 730 +/- 45 mumol/l), but KB Ra rose in septic patients only to 3.7 +/- 1.1 instead of 7.7 +/- 1.1 mumol.kg-1.min-1 as in normal subjects (P less than 0.05). Somatostatin infusion decreased the hyperinsulinemia of septic patients but did not restore a normal ketogenesis. After interruption of nutriment infusion, septic patients had normal FFA levels and only mild hyperglycemia and hyperinsulinemia. Their initial KB Ra was not modified. However, their response of KB Ra (increase to 6.27 +/- 2.0 mumol.kg-1.min-1) to raised FFA levels (842 +/- 170 mumol/l) was comparable to the response of normal subjects. In conclusion, although septic patients receiving an hypocaloric parenteral nutrition had a depressed ketogenesis they were able to restore a normal ketogenic capacity after a short-time caloric deprivation. Glucose and/or insulin appears to have a major role in this modulation of hepatic ketogenesis.

Long-term maintenance of high rates of very-low-density-lipoprotein secretion in hepatocyte cultures. A model for studying the direct effects of insulin and insulin deficiency in vitro

High rates of hepatic cellular triacylglycerol synthesis and very-low-density-lipoprotein (VLDL) triacylglycerol output were maintained in vitro for at least 3 days when hepatocytes were cultured in a medium lacking insulin but supplemented with 1 microM-dexamethasone, 10 mM-lactate, 1 mM-pyruvate and 0.75 mM-oleate (supplemented medium). Under these conditions VLDL output remained constant, whereas cell triacyglycerol content increased 10-fold over 3 days, suggesting that the secretory process was saturated. Insulin, present during the first 24 h period, enhanced the storage of cellular triacylglycerol by inhibiting the secretion of VLDL. This stored triacyglycerol was subsequently released into the medium as VLDL if insulin was removed. With the supplemented medium the increased rate of VLDL secretion after insulin removal exceeded that observed under 'saturating' conditions, suggesting that pre-treatment with insulin enhanced the capacity for VLDL secretion. In contrast with the short-term (24 h) effects of insulin, longer-term exposure (greater than 48 h) to insulin enhanced the secretion of VLDL compared with insulin-untreated cultures. Under these conditions, insulin increased the net rates of triacylglycerol synthesis. The results suggest that insulin affects the secretion of VLDL triacylglycerol by two distinct and opposing mechanisms: first, by direct inhibition of secretion; second by increasing triacylglycerol synthesis, which stimulates secretion. The net effect at any time depends upon the relative importance of each of these processes.

Human ketone body production and utilization studied using tracer techniques: regulation by free fatty acids, insulin, catecholamines, and thyroid hormones

Ketone body concentrations fluctuate markedly during physiological and pathological conditions. Tracer techniques have been developed in recent years to study production, utilization, and the metabolic clearance rate of ketone bodies. This review describes data on the roles of insulin, catecholamines, and thyroid hormones in the regulation of ketone body kinetics. The data indicate that insulin lowers ketone body concentrations by three independent mechanisms: first, it inhibits lipolysis, and thus lowers free fatty acid availability for ketogenesis; second, it restrains ketone body production within the liver; third, it enhances peripheral ketone body utilization. To assess these effects in humans in vivo, experimental models were developed to study insulin effects with controlled concentrations of free fatty acids, insulin, glucagon, and ketone bodies. Presently available data also support an important role of catecholamines in increasing ketone body concentrations. Evidence was presented that norepinephrine increases ketogenesis not only by stimulating lipolysis, and thus releasing free fatty acids, but also by increasing intrahepatic ketogenesis. Thyroid hormone availability was associated with lipolysis and ketogenesis. Ketone body concentrations after an overnight fast were only modestly elevated in hyperthyroidism resulting from increased peripheral ketone body clearance. There was a significant correlation between serum triiodothyronine levels and the ketone body metabolic clearance rate. Thus, ketone body homeostasis in human subjects resulted from the interaction of hormones such as insulin, catecholamines, and thyroid hormones regulating lipolysis, intrahepatic ketogenesis, and peripheral ketone body utilization.

Rapid effects of insulin and glucose on the hepatic incorporation of gluconeogenic substrates into glyceride glycerol and glycogen

1. The hepatic utilization of gluconeogenic substrates was investigated shortly after portal infusion of either insulin or glucose in fasted rats. 2. After 20 min of insulin infusion blood glucose concentration decreased. However, neither glucose generation from precursors such as alanine or pyruvate nor their incorporation into fatty acids was modified. Under these conditions, insulin rapidly increased the incorporation of gluconeogenic substrates into the hepatic glyceride glycerol fraction. Insulin treatment led to a decrease in substrate incorporation into liver glycogen. 3. After 20 min of portal glucose infusion both plasma insulin and glucose concentrations increased and the incorporation of pyruvate into hepatic glyceride glycerol and into glycogen was also stimulated. 4. A close relationship was observed between blood glucose concentrations and the level of incorporation of gluconeogenic substrates into liver glycogen. 5. In conclusion, during fasting insulin stimulates the incorporation of gluconeogenic substrates into the glycerol moiety of hepatic glycerides, which may be the preferential mechanism through which fatty acid esterification is accomplished during refeeding. This effect of insulin is rapid and detected even before other classical modifications induced by the hormone such as gluconeogenesis inhibition or lipogenesis activation. Furthermore, the effect is not related to insulin-induced hypoglycemia since glucose infusion mimics insulin action on glyceride glycerol synthesis.

Lipid and lipoprotein metabolism in Hep G2 cells

Lipid composition, lipid synthesis and lipoprotein secretion by the Hep G2 cell line have been studied with substrate and insulin supplied under different conditions. The lipid composition of Hep G2 cells was close to that of normal human liver, except for a higher content in sphingomyelin (P less than 0.005) and a lower phosphatidylcholine/sphingomyelin ratio. Most of the [14C]triacylglycerols secreted into the medium were recovered by ultracentrifugation at densities of 1.006 to 1.020 g/ml. The main apolipoproteins secreted were apo B-100 and apo A-I. Hep G2 mRNA synthesized in vitro the pro-apolipoproteins A-I and E. Triacylglycerol secretion was 7.38 +/- 1.04 micrograms/mg cell protein per 20 h with 5.5 mM glucose in the medium and increased linearly with glucose concentration. Oleic acid (1 mM) increased the incorporation of [3H]glycerol into the medium and cell triacylglycerols by 251 and 899%, with a concomitant increment in cell triacylglycerols and cholesterol ester. Insulin (1 mU or 7 pmol/ml) inhibited triacylglycerol secretion and [35S]methionine incorporation into secreted protein by 47 and 28%, respectively, with a corresponding increase in the cells. Preincubation of cells with 2.5-10 mM mevalonolactone decreased the incorporation of [14C]acetate into cholesterol 6.2-fold, indicating an inhibitory effect on HMG-CoA reductase. It is concluded that in spite of some differences between Hep G2 and normal human hepatocytes, this line offers an alternative and reliable model for studies on liver lipid metabolism.

Fatty acid-independent inhibition of hepatic ketone body production by insulin in humans

To investigate whether elevated plasma insulin or glucagon concentrations are capable of modifying hepatic ketogenesis independently of plasma free fatty acid (FFA) concentrations, ketone body production was determined by [3-14C]acetoacetate infusions in overnight-fasted normal subjects during exogenous supply of FFA (Intralipid and heparin infusion). When plasma FFA concentrations were elevated from 0.73 +/- 0.07 to 1.53 +/- 0.16 mmol/l during low insulin concentrations (approximately equal to 13 microU/ml) in group A (n = 7), total ketone body production increased from 3.6 +/- 0.6 to 8.2 +/- 1.0 mumol.kg-1.min-1 (P less than 0.001). When plasma FFA were similarly elevated during raised plasma insulin concentrations (approximately equal to 110 microU/ml) in group B (n = 5), total ketone body production was only 3.8 +/- 0.8 mumol.kg-1.min-1 (P less than 0.01 vs. group A). Low plasma FFA and low insulin concentrations resulted in total ketone body production of 0.70 +/- 0.18 mumol.kg-1.min-1 in group C (n = 7; P less than 0.01 vs. groups A and B). Elevation of plasma glucagon during Intralipid infusion in group D (n = 7) failed to affect ketogenesis, but the beta-hydroxybutyrate-to-acetoacetate concentration ratio decreased significantly (P less than 0.01). The data indicate that elevation of plasma insulin to high physiological concentrations restrains FFA-induced ketogenesis.

The biochemistry of lipoproteins

Lipids are transported in the blood in four major classes of lipoproteins. The triacylglycerol-rich lipoproteins are chylomicrons and very-low-density lipoproteins (VLDL) which are produced by the small intestine and liver, respectively. These lipoproteins mainly carry fatty acids to adipose tissue and muscle where the triacylglycerol is hydrolysed by lipoprotein lipase. The resulting particles that remain in the blood are chylomicron remnants and low-density lipoprotein (LDL), respectively. The remnant is taken up by the liver via endocytosis which is mediated by a specific receptor for apolipoprotein E (apoE). LDL, which are rich in cholesterol, can also be taken up by the liver or extrahepatic tissues by a receptor-mediated endocytosis that specifically recognises apoB or apoE. 'Nascent' high-density lipoprotein (HDL) particles are secreted by the liver and intestine and then undergo modification to become HDL3 and then HDL2 as they acquire cholesterol ester. They facilitate the reverse transport of cholesterol back to the liver. Little is known of the hormonal regulation of lipoprotein uptake by the liver. Recently, we have shown that insulin and tri-iodothyronine (T3) increase the specific binding of LDL to cultured hepatocytes whereas dexamethasone (a synthetic glucocorticoid) has the opposite effect. The changes in binding produced by insulin and dexamethasone are paralleled by alterations in the rate of degradation of apoB. These findings may in part explain the hypercholesterolaemia and increased risk of premature atherosclerosis that can be associated with poorly controlled diabetes or hypothyroidism.

Insulin reverses effects of starvation on the activity of pyruvate dehydrogenase kinase in cultured hepatocytes

In tissue culture of hepatocytes, insulin (0.1-1 munits/ml for 4 h) reversed completely the effects of starvation of rats to decrease the activity of pyruvate dehydrogenase (PDH) complex and to increase the activities of PDH kinase and PDH kinase activator protein. It had no effect in hepatocytes from fed rats. Significant effects of insulin were detected with 0.01 munit/ml after 4 h, and in 1-2 h with 1 munit/ml.

Insulin effects on apolipoprotein B lipoprotein synthesis and secretion by primary cultures of rat hepatocytes

Lipoprotein synthesis and secretion were examined in primary cultures of rat hepatocytes cultured on collagen-coated plates and incubated with pharmacologic and physiologic concentrations of insulin. Media insulin concentration declined rapidly over the course of incubation indicating that hepatocytes rapidly degrade insulin. When insulin was present in the media, cellular triglyceride accumulated while lipid secretion declined. Insulin inhibited the incorporation of labeled amino acids into total secretory lipoprotein apoproteins and apolipoprotein B (apo B) as well as apo B mass as measured by monoclonal radioimmunoassay. The effect of insulin on apo B secretion occurred as early as three hours after the addition of insulin to the culture media and both apo B of higher molecular weight (apo BH) and apo B of lower molecular weight (apo BL) were affected. Cellular apo B did not accumulate within cells. The majority of secretory lipid radioactivity synthesized from acetate was in VLDL density lipoproteins. The composition of newly synthesized lipids as assessed by thin layer chromatography was not significantly altered with insulin. These studies support the finding that insulin inhibits VLDL secretion by hepatocytes while at the same time stimulating overall triglyceride synthesis. A suggested mechanism is that insulin uncouples triglyceride and apo B synthesis, which influences subsequent lipoprotein assembly and secretory pathways. These results are consistent with the concept that postprandial insulin release inhibits hepatic lipoprotein secretion while intestinal lipoprotein metabolic pathways are most active.

Effects of dexamethasone and insulin on the synthesis of triacylglycerols and phosphatidylcholine and the secretion of very-low-density lipoproteins and lysophosphatidylcholine by monolayer cultures of rat hepatocytes

Rat hepatocytes in monolayer culture were preincubated for 19 h with 1 microM-dexamethasone, and the incubation was continued for a further 23 h with [14C]oleate, [3H]glycerol and 1 microM-dexamethasone. Dexamethasone increased the secretion of triacylglycerol into the medium in particles that had the properties of very-low-density lipoproteins. The increased secretion was matched by a decrease in the triacylglycerol and phosphatidylcholine that remained in the hepatocytes. Preincubating the hepatocytes for the total 42 h period with 36 nM-insulin decreased the amount of triacylglycerol in the medium and in the cells after the final incubation for 23 h with radioactive substrates. However, insulin had no significant effect on the triacylglycerol content of the cell and medium when it was present only in the final 23 h incubation. Insulin antagonized the effects of dexamethasone in stimulating the secretion of triacylglycerol from the hepatocytes, especially when it was present throughout the total 42 h period. The labelling of lysophosphatidylcholine in the medium when hepatocytes were incubated with [14C]oleate and [3H]glycerol was greater than that of phosphatidylcholine. The appearance of this lipid in the medium, unlike that of triacylglycerol and phosphatidylcholine, was not stimulated by dexamethasone, or inhibited by colchicine. However, the presence of lysophosphatidylcholine in the medium was decreased when the hepatocytes were incubated with both dexamethasone and insulin. These findings are discussed in relation to the control of the synthesis of glycerolipids and the secretion of very-low-density lipoproteins and lysophosphatidylcholine by the liver, particularly in relation to the interactions of glucocorticoids and insulin.