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

Stress, glucocorticoid signaling pathway, and metabolic disorders

BACKGROUND AND AIMS

Glucocorticoids and the GR serve as an essential molecular mediator of stress and different physiologic processes. This review summarizes main findings from studies on the role of the GC/GR signaling in the modulation of genes for nutrient processing by the different organs involved in metabolic diseases.

METHODS

Descriptive review of relevant papers known to the author was conducted.

RESULTS

Several high-throughput screenings in the past 15 years have identified potential GR DNA-binding regions in different cell types with genes that are annotated to be important for the control of metabolism. Transcriptional regulation of these GC-responsive genes provides links between the hypothalamic-pituitary-adrenal axis (HPA) and systemic energy homeostasis in both physiological and pathophysiological states. Future studies must reconsider the use of agonist, the utilization of animal models of stress and metabolic disorders, and validation in humans.

CONCLUSION

This review recapitulates the significant role of the GC/GR signaling in molecular metabolic control and metabolic disorders. Potential future research focus and optimizations have also been identified.

Dexamethasone in the era of COVID-19: friend or foe? An essay on the effects of dexamethasone and the potential risks of its inadvertent use in patients with diabetes

BACKGROUND

The disclosure in the media of a benefit with the use of dexamethasone in patients with COVID-19 infection sets precedents for self-medication and inappropriate use of corticosteroids.

METHODS

This is a critical interpretive synthesis of the data available in the literature on the effects of the use of corticosteroids and the impact that their indiscriminate use may have on patients with diabetes. Reviews and observational and experimental studies published until June 18, 2020 were selected.

RESULTS

Corticosteroids are substances derived from cholesterol metabolism that interfere with multiple aspects of glucose homeostasis. Interactions between corticoid receptors and target genes seem to be among the mechanisms responsible for the critical functions of glucocorticoids for survival and anti-inflammatory effects observed with these medications. Corticosteroids increase hepatic gluconeogenesis, reduce peripheral use of glucose and increase insulin levels. Previous studies have shown that glucocorticoids have a pro-adipogenic function, increasing deposition of abdominal fat, and lead to glucose intolerance and hypertriglyceridemia. In addition, these drugs play a role in controlling liver metabolism and can lead to the development of hepatic steatosis. Glucocorticoids reduce the recruitment of osteoblasts and increase the number of osteoclasts, which results in increased bone resorption and greater bone fragility. Moreover, these medications cause water and sodium retention and increase the response to circulating vasoconstrictors, which results in increased blood pressure levels. Chronic or high-dose use of corticosteroids can, by itself, lead to the onset of diabetes. For those who were already diagnosed with diabetes, studies show that chronic use of corticosteroids leads to a 94% higher risk of hospitalization due to diabetes complications. In addition to the direct effects on glycemic control, the effects on arterial pressure control, lipids and bone metabolism also have a potential for severe consequences in patients with diabetes.

CONCLUSION

Fear and uncertainty toward a potentially serious infection may lead people to self-medication and the inappropriate and abusive use of corticosteroids. More than ever, it is necessary for health professionals to be alert and able to predict damages related to the use of these drugs, which is the first step to minimize the potential damages to come.

From whole body to cellular models of hepatic triglyceride metabolism: man has got to know his limitations

The liver is a main metabolic organ in the human body and carries out a vital role in lipid metabolism. Nonalcoholic fatty liver disease (NAFLD) is one of the most common liver diseases, encompassing a spectrum of conditions from simple fatty liver (hepatic steatosis) through to cirrhosis. Although obesity is a known risk factor for hepatic steatosis, it remains unclear what factor(s) is/are responsible for the primary event leading to retention of intrahepatocellular fat. Studying hepatic processes and the etiology and progression of disease in vivo in humans is challenging, not least as NAFLD may take years to develop. We present here a review of experimental models and approaches that have been used to assess liver triglyceride metabolism and discuss their usefulness in helping to understand the aetiology and development of NAFLD.

How Do Glucocorticoids Regulate Lipid Metabolism?

Glucocorticoids (GCs) and their cognate, intracellular receptor, the glucocorticoid receptor (GR) have been characterized as critical checkpoints in the hormonal control of energy homeostasis in mammals. Whereas physiological levels of GCs are required for proper metabolic control, aberrant GC action has been linked to a variety of severe metabolic diseases, including type 2 diabetes and obesity. As a member of the nuclear receptor superfamily of transcription factors, the GR translocates into the cell nucleus upon GC binding where it serves as a transcriptional regulator of distinct GC-responsive target genes that are in many cases associated with lipid regulatory pathways and thereby intricately control both physiological and pathophysiological systemic lipid homeostasis. Thus, this chapter focuses on the current knowledge of GC/GR function in lipid handling and its implications for systemic metabolic dysfunction.

Development of hepatocytes from ES cells after transfection with the HNF-3beta gene

We have attempted to generate embryonic stem (ES) cell-derived hepatocytes expressing liver-specific functional properties by use of ES cell technology. It was found that ES cells are allowed to differentiate into hepatocytes possessing high metabolic activities when hepatocyte nuclear factor (HNF)-3beta-transfected ES cells are cultured in alpha-MEM medium supplemented with 10% fetal bovine serum (FBS) and fibroblast growth factor (FGF)-2 in the three-dimensional cell culture system at 5% CO2. The differentiated cells induced albumin, triacylglycerol, urea, and glycogen synthesis as well as further expression of metabolic proteins and serum factors as markers of hepatocytic differentiation for at least 4 months. The cells differentiated from HNF-3beta-transfected ES cells also had hepatocyte-like ultrastructural characteristics, including several endoplasmic reticula, mitochondrion, and glycogen. Our findings indicate that generation of hepatocytes maintaining high metabolic functions developed from mouse ES cells will facilitate the study of the basic mechanism for hepatogenesis and will certainly provide new opportunities for tissue transplantation.

Effect of dietary medium chain triacylglycerols on plasma triacylglycerol levels in horses

The hypothesis tested was that the feeding of medium chain triacylglycerols (MCT) to horses would raise the level of plasma triacylglycerols by increasing the availability of glucose as lipogenic substrate, implying that the MCT effect would be greater with glucose in the diet instead of cellulose. A Latin square experiment was carried out with 4 horses and 4 dietary treatments. The experimental periods lasted 21 d. Blood samples were taken 16 h after feeding. The diets consisted of hay and experimental concentrates, differing in fat source (MCT or soybean oil) and carbohydrate source (corn starch plus glucose or cellulose). The dietary variables, MCT or soybean oil, provided on average 27% of total dietary net energy, while glucose plus constarch or cellulose provided 33%. The feeding of MCT versus soybean oil raised the level of plasma triacylglycerols significantly from 196.7 +/- 30.2 to 427.3 +/- 85.7 mmol/l and that of VLDL cholesterol from 0.028 +/- 0.01 to 0.069 +/- 0.01 mmol/ml. As based on analysis of variance, for the four experimental diets there was no significant effect of carbohydrate source and no fat-carbohydrate interaction. Thus, the hypothesis was rejected. When the diets contained soybean oil, cellulose versus starch plus glucose produced significantly greater increase plasma triacylglycerols. This carbohydrate effect was not seen when horses were fed the MCT diets. The experimental concentrates did not differently influence the concentrations of plasma glucose, total serum cholesterol, phospholipids, insulin, free fatty acids and the activity of post-heparin lipoprotein lipase. We suggest that the MCT-induced increase in plasma triacylglycerols is related to an increase in hepatic VLDL secretion, with the extra substrate for increased synthesis of triacylglycerols being the acetyl-CoA derived from the hepatic oxidation of medium chain fatty acids.

Effect of heparin-stimulated plasma lipolytic activity on VLDL APO B subclass metabolism in normal subjects

Heparin given intravenously enhances lipolysis, although fasting lipids are not markedly altered in long-term administration. In the present study we investigated heparin-induced acute perturbation of VLDL subclass metabolism. Eight men were examined during a control study and during an 8.5 h infusion of heparin. 2H3-leucine was used as tracer and kinetic constants derived using a non-steady-state model. Heparin infusion increased both plasma lipoprotein and hepatic lipase activity and raised plasma FFAs two-fold (P < 0.001). The fractional catabolic rate (FCR) of VLDL1 apo B increased on heparin (25.7 +/- 4.2 and 10.8 +/- 1.7 pools/d, heparin vs. control, P < 0.02). The FCR of VLDL2 apo B increased to 12.6 +/- 1.9 pools/d on heparin vs. 8.8 +/- 1.1 pools/d during the control (NS). Total VLDL apo B production was not significantly changed (824 +/- 45 and 692 +/- 91 mg/d, heparin vs. control, NS). We conclude that during heparin infusion, the catabolism of especially large triglyceride-rich VLDL1 apo B is greatly increased. However, although the FFA levels were high during the heparin study, the production of total VLDL apo B did not rise. These findings are consistent with the known action of heparin on lipoprotein lipase but indicate that acute increase in plasma FFA levels does not lead to a rise in VLDL apo B production.

The intracellular triacylglycerol/fatty acid cycle: a comparison of its activity in hepatocytes which secrete exclusively apolipoprotein (apo) B100 very-low-density lipoprotein (VLDL) and in those which secrete predominantly apoB48 VLDL

Hamster hepatocytes, like human hepatocytes, secrete triacylglycerol (TAG) as very-low-density lipoprotein (VLDL) in association with apolipoprotein (apo) B100, whereas in the rat, TAG is secreted predominantly in association with apoB48. Nevertheless, in hepatocytes from both species, a minimum of between 60% and 70% [69. 1+/-1.4% (hamster), 60.6+/-2.5% (rat)] of the VLDL TAG was secreted following lipolysis and re-esterification of intracellular TAG. The fractional rates of hepatocellular TAG turnover (lipolysis and re-esterification) were similar in both species [1.83+/-0.28 pools/24 h (hamster), 1.39+/-0.23 pools/24 h (rat)]. Comparison of the relative changes in the 3H and 14C specific radioactivities of the VLDL and cellular TAG, pre-labelled with [3H]glycerol and [4C]oleate, suggested that fatty acids released by lipolysis either were recruited directly into a VLDL assembly pool or were recycled to the cellular pool following re-esterification. Recycling in the hamster was somewhat greater than in the rat (66.1+/-5.7% versus 53. 7+/-4.8% of TAG lipolysed respectively). Similarly, a larger proportion of newly synthesized TAG was retained within the cell, rather than secreted as VLDL, in the hamster compared with the rat (37.9+/-2.8% versus 20+/-3.8%, P<0.01). These factors may have contributed to the somewhat lower rate of VLDL TAG secretion in the hamster hepatocytes compared with those from the rat (43.3+/-4.2 versus 96.4+/-3.4 microg/24 h per mg of cell protein). Rat hepatocytes were more sensitive to inhibition of VLDL secretion by insulin than were those from hamster. In neither case did insulin affect total or fractional TAG turnover. The results suggest that assembly of both apoB100 VLDL and apoB48 VLDL is associated with efficient intracellular TAG lipolysis.

Decreased secretion of very-low-density lipoprotein triacylglycerol and apolipoprotein B is associated with decreased intracellular triacylglycerol lipolysis in hepatocytes derived from rats fed orotic acid or n-3 fatty acids

Hepatocytes from rats fed a chow (control) diet or from rats fed a chow diet supplemented with either orotic acid (OA; 1%, w/w) or fish oil (FO; 20%, v/w) were maintained in culture for periods up to 48 h. during the first 24 h period, the low rates of output of very-low-density lipoprotein (VLDL)-associated triacylglycerol (TAG) and apolipoprotein B (apoB) in hepatocytes from the FO- and OA-fed animals were associated with significantly lower rates of intracellular TAG lipolysis and re-esterification. Most of the VLDL TAG secreted was mobilized via lipolysis of the intracellular TAG pool, but the proportion of VLDL TAG secreted via this route in cells from the FO-fed and OA-fed animals was decreased compared with that in the control-fed animals' cells. In the presence of exogenous oleate the inhibitory effect of OA feeding on VLDL apoB and TAG secretion persisted in the derived hepatocytes for up to 48 h following isolation. However, when oleate was absent no inhibitory effect on the secretion of TAG and apoB was observed between 24 and 48 h. Under these conditions the rate of intracellular TAG turnover returned to normal. The initial inhibitory effect of FO feeding on VLDL TAG and apoB secretion did not persist in the derived hepatocytes between 24 h and 48 h of culture in the presence of exogenous oleate. Although intracellular TAG lipolysis and VLDL TAG and apoB secretion rates appear to be positively correlated, a causal relationship has not been conclusively established.

Origin of hepatic very-low-density lipoprotein triacylglycerol: the contribution of cellular phospholipid

When rat hepatocytes were cultured for 24 h in the absence of exogenous fatty acid, the amount of very-low-density lipoprotein (VLDL) triacylglycerol (TAG) secreted (114 +/- 14 micrograms/mg of cell protein) could not be accounted for by the mass of TAG lost from the cells (29 +/- 6.1 micrograms/mg of cell protein) during this period (n = 12). Of the balance (85 +/- 14 micrograms/mg; 94 +/- 15 nmol/mg), a maximum of only 37 nmol/mg of cell protein of TAG could be accounted for by fatty acids synthesized de novo. When labelled exogenous oleate (initial concentration, 0.75 nM) was present in the culture medium, the net gain in cellular plus VLDL TAG (253 +/- 38 micrograms/mg of cell protein per 24 h) was greater than that contributed by the exogenous fatty acid (155 +/- 18.2 micrograms/mg of cell protein, n = 5). Again, the balance (98.8 +/- 18.2 micrograms/mg of cell protein per 24 h) was too great to be accounted for by fatty acid synthesis de novo. In experiments in which cellular glycerolipids were prelabelled with [9, 10(n)-3H]oleic acid, following removal of the labelled fatty acid, there was a net increase in labelled cellular plus VLDL TAG over the next 24 h. That cellular phospholipids are the source of a substantial part of the excess TAG synthesized is supported by the following evidence. (1) The loss of prelabelled cellular phospholipid during culture was greater than could be accounted for by secretion into the medium. (2) During culture of cells prelabelled with 1,2-di-[l-14C]palmitoyl phosphatidylcholine, a substantial amount of label was secreted as VLDL TAG. (3) In pulse-chase experiments, the kinetics of labelled phospholipid turnover were consistent with conversion into a non-phospholipid pool. The enzymology involved in the transfer of phospholipid fatty acids into TAG is probably complex, but the present results suggest that this pathway may represent an important route by which extracellular fatty acids are channelled into VLDL TAG.

Involvement of lipogenesis in the lower VLDL secretion induced by oligofructose in rats

Dietary supplementation with oligofructose (OFS; 100 g/kg), a non-digestible oligomer of beta-D-fructose, decreases serum triacylglycerols in serum and VLDL of rats. In order to investigate the role of hepatic metabolism in the hypolipidaemic effect of OFS, male Wistar rats were fed on a standard diet with or without 100 g Raftilose P95/kg as OFS source for 30 d. OFS feeding (1) significantly decreased triacylglycerol and phospholipid concentrations in both blood and liver, (2) increased the glycerol-3-phosphate liver content but decreased the hepatic activity of glycerol-3-phosphate acyltransferase (EC 2.3.1.15), suggesting a decrease in acylglycerol synthesis, (3) did not affect the blood non-esterified fatty acid concentrations, but (4) reduced by 54% the capacity of isolated hepatocytes to synthesize and secrete triacylglycerols from labelled acetate; the activity of fatty acid synthase, a key lipogenic enzyme was also significantly decreased. These findings suggest that OFS decreases serum triacylglycerols by reducing de novo fatty acid synthesis in the liver; the lower insulin level in the serum of OFS-fed rats could explain, at least partly, the metabolic effect induced by such non-digestible carbohydrates.

Contributions of de novo synthesis of fatty acids to total VLDL-triglyceride secretion during prolonged hyperglycemia/hyperinsulinemia in normal man

Triglycerides (TG) are synthesized in the liver principally from two sources of fatty acids (FA): FA synthesized de novo in the liver and preformed FA. We have measured the rate of secretion of de novo synthesized FA and total secretion of FA bound to VLDL-TG in healthy men (n = 5) in the basal state, and after 1 (day 1) and 4 d (day 4) of a hypercaloric carbohydrate diet (approximately 2.5 times energy expenditure) that generated a moderate endogenous hyperinsulinemia (plasma insulin approximately 60 microU/ml). Prolonged carbohydrate hyperalimentation/hyperinsulinemia increased plasma VLDL-TG approximately 10-fold in part due to a 3.4-fold increase in total VLDL-TG secretion rate (basal state = 72+/-23, day 4 = 242+/-78 micromol TG/kg/d). Although the secretion of de novo synthesized FA increased throughout the study (basal state = 1.1+/-0.4, day 1 = 15.9+/-7.9, day 4 = 50.0+/-18.8 micromol TG/ kg/d), the 2.7-fold increase in secretion rate of preformed FA (basal state = 70+/-23, day 4 = 191+/-57 micromol TG/kg/d) quantitatively contributed the most to total VLDL-TG secretion rate. Decreased catabolism of VLDL-TG also contributed to the hypertriglyceridemia as reflected by an approximately fourfold decrease in both fractional turnover rate (basal state = 9.2+/-3.8, day 1 = 2.1+/-0.2, day 4 = 2.1+/-0.3 pools/d) and rate of clearance (basal state = 0.35+/-0.08, day 1 = 0.11+/-0.01, day 4 = 0.09+/-0.01 liter/kg/d) of VLDL-TG. Thus, the primary difference between 1 and 4 d of hyperinsulinemia in conjunction with carbohydrate hyperalimentation is the increase in hepatic secretion of preformed FA into VLDL-TG.

Decreased sensitivity of very-low-density lipoprotein secretion to the inhibitory effect of insulin in cultured hepatocytes from lactating rats

Hepatocytes were prepared from 10-11-day lactating rat dams and from lactating dams which had been weaned for periods of either 1-2 days or 7 days. Hepatocytes from each group were cultured for periods of up to 48 h in a chemically defined medium. Compared with those from the 7-day weaned animals, hepatocytes from the lactating rats were resistant to the inhibitory effects of insulin on the secretion of very-low-density lipoprotein (VLDL) triacylglycerol (TAG). These differences persisted for up to 48 h in culture. Hepatocytes from the 1-2 day weaned animals remained relatively insulin-resistant in this respect. Similar differences in the response to insulin were not observed for the secretion of VLDL apolipoprotein B. TAG production increased and ketogenesis decreased in the hepatocytes from the lactating compared with those from the 7-day weaned rats. Insensitivity of the liver to the normal effects of insulin on the secretion of VLDL TAG may arise from a need to maintain an adequate flux of hepatic lipids to the lactating mammary gland in order to meet the large demand for milk-fat production.

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.

Chronic exogenous hyperinsulinaemia does not modify the acute inhibitory effect of insulin on the secretion of very-low-density lipoprotein triacylglycerol and apolipoprotein B in primary cultures of rat hepatocytes

Male Wistar rats were fitted with subcutaneous osmotic mini-pumps that delivered insulin at a constant rate of 0.20 i.u./h for 7 days. This treatment raised the plasma insulin concentration from 31 +/- 4 to 201 +/- 64 micro-i.u./ml. Hepatocytes prepared from the hyperinsulinaemic animals secreted very-low-density lipoprotein (VLDL) triacylglycerol (TAG) at a higher rate (172 +/- 21 microgram per 24 h per mg cell protein) than did those from sham-operated controls (109 +/- 12 microgram per 24 h per mg) (P<0.05). However, chronic exogenous hyperinsulinaemia had no stimulatory effect on the secretion of VLDL apolipoprotein B (apoB) in derived hepatocytes compared with those from the sham-operated controls (2.32 +/- 0.38 compared with 3.09 +/- 0.40 microgram per 24 h per mg). Hepatocytes from the hyperinsulinaemic rats thus secreted larger VLDL particles as evidenced by the increased TAG:apoB ratio (78.4 +/- 13.1 compared with 38.4 +/- 7.6; P<0.05). In hepatocytes from the hyperinsulinaemic rats a larger proportion of the newly synthesized TAG was secreted as VLDL. Hepatocytes from the hyperinsulinaemic and the sham-operated control animals were equally sensitive to the inhibitory effect of insulin added in vitro on the secretion of VLDL TAG. Insulin added in vitro to the culture medium of hepatocytes from hyperinsulinaemic animals significantly decreased the TAG:apoB ratio of the secreted VLDL. This change did not occur in hepatocytes from sham-operated rats. These results suggest that, in vivo, chronic hyperinsulinaemia is not in itself sufficient to desensitize the liver to the acute inhibitory effect of insulin on the secretion of VLDL.

Interaction between free fatty acids and insulin in the acute control of very low density lipoprotein production in humans

Changes in VLDL triglyceride and VLDL apo B production were determined semiquantitatively in healthy young men by examining the effect of altering plasma insulin and/or FFA levels on the change in the slopes of the specific activity of VLDL [3H]triglyceride glycerol or the 131I-VLDL apo B versus time curves. In one study (n = 8) insulin was infused for 5 h using the euglycemic hyperinsulinemic clamp technique. Plasma FFA levels declined by approximately 80% (0.52 +/- 0.01 to 0.11 +/- 0.02 mmol/liter), VLDL triglyceride production decreased by 66.7 +/- 4.2% (P = 0.0001) and VLDL apo B production decreased by 51.7 +/- 10.6% (P = 0.003). In a second study (n = 8) heparin and Intralipid (Baxter Corp., Toronto, Canada) were infused with insulin to prevent the insulin-mediated fall in plasma FFA levels. Plasma FFA increased approximately twofold (0.43 +/- 0.05 to 0.82 + 0.13 mmol/liter), VLDL triglyceride production decreased to a lesser extent than with insulin alone (P = 0.006) (-31.8 +/- 9.5%, decrease from baseline P = 0.03) and VLDL apo B production did not decrease significantly (-6.3 +/- 13.6%, P = NS). In a third study (n = 8) when heparin and Intralipid were infused without insulin, FFA levels rose approximately twofold (0.53 +/- 0.04 to 0.85 +/- 0.1 mmol/liter), VLDL triglyceride production increased by 180.1 +/- 45.7% (P = 0.008) and VLDL apo B production increased by 94.2 +/- 28.7% (P = 0.05). We confirm our previous observation that acute hyperinsulinemia suppresses VLDL triglyceride and VLDL apo B production in healthy humans. In addition, we have demonstrated that elevation of plasma FFA levels acutely stimulates VLDL production in vivo in healthy young males. Elevating plasma FFA during hyperinsulinemia attenuates but does not completely abolish the suppressive effect of insulin on VLDL production, at least with respect to VLDL triglycerides. Therefore, in normal individuals the acute inhibition of VLDL production by insulin in vivo is only partly due to the suppression of plasma FFA, and may also be due to an FFA-independent process.

Intracellular triacylglycerol lipase: its role in the assembly of hepatic very-low-density lipoprotein (VLDL)

Extracellular fatty acids entering the hepatocyte are either esterified to cytosolic TAG or oxidized to ketone bodies. Very little is esterified and secreted directly in association with VLDL. Thus, even when extracellular fatty acids are available, the major, direct source of VLDL TAG is the cytosolic pool. The recruitment of cytosolic TAG for VLDL assembly involves lipolysis followed by re-esterification. At least 70% of the secreted TAG is derived via this route. Fatty acids released at this lipolytic step are utilized exclusively for VLDL TAG synthesis and are not available for ketogenesis. Substantially more cytosolic TAG undergoes lipolysis than is required to meet the needs of VLDL assembly. The remaining fatty acids are re-esterified and re-cycled to the cell cytosol. From a physiological viewpoint, the presence of this indirect route for VLDL TAG recruitment would provide a means of regulation of VLDL secretion which is independent of the plasma fatty acid concentration. In this respect, several pathophysiological conditions are known in which there is a negative association between plasma fatty acid concentration and the rate of VLDL secretion. These are: (a) insulin-dependent diabetes, (b) starvation, (c) fat-feeding. Lipolysis of cytosolic TAG and transfer of fatty acids into the ER lumen may provide a regulatory focus for the control of hepatic VLDL output.

Effects of insulin treatment of diabetic rats on hepatic partitioning of fatty acids between oxidation and esterification, phospholipid and acylglycerol synthesis, and on the fractional rate of secretion of triacylglycerol in vivo

1. The hypothesis that insulin treatment of streptozotocin-diabetic rats does not alter acutely the ability of acylcarnitine synthesis to compete successfully for cytosolic long-chain acyl-CoA [Grantham and Zammit (1988) Biochem. J. 249, 409-414], was tested in vivo by using the technique of selective labelling of hepatic fatty acids in awake unrestrained rats. In the same animals, the partitioning of hepatic fatty acids between acylglycerol and phospholipid synthesis, and of newly labelled triacylglycerol between secretion into the plasma and retention in the liver, was also studied. 2. In untreated diabetic animals, the ratio of fatty acid oxidation to esterification was double that found in normal fed animals, whereas there were no differences in the values of the above-mentioned parameters of glycerolipid metabolism. Thus the insulin status of the rats only has chronic effects on specific aspects of fatty acid metabolism in the liver. 3. Treatment of diabetic rats with insulin resulted in no change in the oxidation/esterification ratio for the first 5 h after the start of insulin administration. Thereafter, there were reciprocal changes in the 14CO2 expired (an index of oxidation) and 14C label recovered in hepatic and plasma glycerolipids. However, the pattern of partitioning observed in normal fed rats was still not re-established after 8 h of insulin treatment. 4. There was a small and transient decrease in the fractional rate of triacylglycerol secretion by the liver at the start of insulin treatment and an increase in the proportion of labelled fatty acid that was utilized for phospholipid synthesis such that phospholipid labelling as a proportion of that of total glycerolipids was doubled after 8 h of insulin treatment. 5. The data are discussed in relation to the roles of insulin in mediating acute changes in hepatic fatty acid metabolism and very-low-density-lipoprotein triacylglycerol secretion by the liver.

Hormonal and substrate regulation of 3-thia fatty acid metabolism in Morris 7800 C1 hepatoma cells

The 3-thia fatty acid tetradecylthioacetic acid (TTA) has recently been shown to inhibit growth rate and increase peroxisomal acyl-CoA oxidase (ACO) (EC 1.3.99.3) activity in the Morris 7800 C1 hepatoma cells. Dexamethasone potentiates and insulin antagonizes these effects of TTA. We demonstrate here the metabolism of the 3-thia acids in these cells and the influence of insulin and dexamethasone on this. (1) The Morris 7800 C1 hepatoma cells exhibited a low omega-hydroxylation activity of the 3-thia acid (and lauric acid). The combination of TTA and dexamethasone induced the omega-hydroxylation and ACO activities in these cells. TTA alone induced ACO activity, but not omega-hydroxylation activity. Insulin counteracted the induction of both enzyme activities. These results indicate that these two enzyme activities are under similar but independent regulation. (2) Hepatoma cells grown with 80 microM TTA in the medium accumulated phospholipids containing the 3-thia fatty acid. After 7 days, TTA accounted for approx. 40% of the total fatty acids in the phospholipids. In addition, TTA affected the incorporation of endogenous fatty acids into phospholipids by decreasing the amounts of palmitic (C16:0) and vaccenic (C18:1(n-7)) acid and increasing the amounts of linoleic (C18:2(n-6)) and alpha-linolenic (C18:3(n-3)) acid in the phospholipids. (3) Dexamethasone increased the incorporation of labelled TTA into both phospholipids and triacylglycerol. Most of the labelled triacylglycerol formed was secreted into the medium. Insulin increased the incorporation of labelled TTA into triacylglycerol, but not into phospholipids. The labelled triacylglycerol formed was retained in the cells.

Restoration in vitro of normal rates of very-low-density lipoprotein triacylglycerol and apoprotein B secretion in hepatocyte cultures from diabetic rats

Hepatocytes derived from diabetic rats were cultured in serum-free Waymouth's medium containing various supplements, after an initial 4 h period during which the cells were allowed to attach to the culture dish in the presence of foetal-bovine serum (10%). After removal of serum, these cells secreted much less very-low-density lipoprotein (VLDL) apoprotein B (apoB) and triacylglycerol than those derived from normal rats when cultured for 24 h in the basal medium. Inclusion of oleate (0.75 mM) in the medium initially increased the output of apoB and triacylglycerol, but the rates remained lower than those observed in normal hepatocytes and declined to zero after 72 h. This time-dependent decline in VLDL output was prevented by addition of dexamethasone to the oleate-containing medium. Levels of apoB and triacylglycerol output characteristic of normal hepatocytes could only be completely restored, however, by further addition of a mixture of lipogenic substrates (lactate plus pyruvate) to the medium. Restoration of normal levels of VLDL secretion in diabetic hepatocytes in vitro by this means was accompanied by a normal inhibitory response of apoB and triacylglycerol output to short-term (24 h) treatment with insulin or glucagon. Exposure of the cells to insulin for 72 h enhanced the secretion of VLDL, whereas treatment with glucagon for the same period potentiated the original inhibitory effect. The defective secretion of VLDL apoB observed when diabetic hepatocytes were cultured in the basal medium for 24 h could also be rectified by inclusion of a mixture of oleate (0.75 mM), lactate (10 mM), pyruvate (1 mM), dexamethasone (1 microM) and insulin (78 nM) in the medium during the 4 h attachment period in the presence of serum. Under these conditions, the increase in the secretory response of triacylglycerol was not so pronounced.

Dependence of the plasma triacylglycerol-lowering effect of fish oil on insulin replacement in streptozotocin diabetic rats

Streptozotocin diabetic rats, with and without insulin replacement, and sham-injected controls were fed a high-fat (30% of energy) menhaden oil (MO) or corn oil (CO) diet for 2 weeks. After an overnight fast, plasma and livers were collected for analysis of insulin, glucose, triacylglycerol, cholesterol and glucose-6-phosphate dehydrogenase activity. Streptozotocin treatment resulted in decreased plasma insulin and elevated glucose. MO-feeding to insulin-replaced diabetic rats resulted in higher insulin and lower glucose levels compared to the respective CO-fed rats, suggesting decreased hepatic insulin extraction and greater peripheral utilization of glucose with MO. Plasma triacylglycerol and cholesterol, and hepatic glucose-6-phosphate dehydrogenase activity were reduced in MO-fed vs. CO-fed control rats. These effects of MO were prevented in the diabetic rats but were restored by insulin replacement. We conclude from our data that the presence of insulin is required to observe at least some of the effects of fish oil (FO). To explain our observations we propose that many of the effects of FO on hepatic metabolism are mediated by an inhibition of insulin action in the liver, thus providing a possible central mechanism for the regulation of hepatic lipid metabolism by dietary FO.

Monitoring of changes in hepatic fatty acid and glycerolipid metabolism during the starved-to-fed transition in vivo. Studies on awake, unrestrained rats

1. The technique of selective labelling of hepatic fatty acids in vivo [Moir and Zammit (1992) Biochem. J. 283, 145-149] has been used to monitor non-invasively the metabolism of fatty acids in the livers of awake unrestrained rats during the starved-to-refed transition. Values for the incorporation of labelled fatty acid into liver and plasma glycerolipids and into exhaled carbon dioxide after injection of labelled lipoprotein and Triton WR 1339 into rats with chronically cannulated jugular veins were obtained for successive 1 h periods from the start of refeeding of 24 h-starved rats. 2. Starvation for 24 h resulted in marked and reciprocal changes in the incorporation of label into glycerolipids and exhaled 14CO2, such that a 4-fold higher value was obtained for the oxidation/esterification ratio in livers of starved rats compared with fed animals. 3. Refeeding of starved rats did not return this ratio to the value observed for fed animals for at least 7 h; during the first 3 h of refeeding the ratio was at least as high as that for starved rats. Between 4 h and 6 h of refeeding the ratio was still approx. 70% of that in starved animals, and 2.5-fold higher than in fed rats. 4. These data support the hypothesis that the capacity of the liver to oxidize fatty acids is maintained at a high level during the initial stages of refeeding [Grantham and Zammit (1986) Biochem. J. 239, 485-488] and that control of the flux of hepatic fatty acids into the oxidative pathway is largely lost from the reaction catalysed by mitochondrial overt carnitine palmitoyltransferase (CPT I) during this phase of recovery from the starved state. 5. Refeeding also resulted in a rapid (< 1 h) increase in hepatic malonyl-CoA concentrations to values intermediate between those in livers of fed and starved animals. The sensitivity of CPT I to malonyl-CoA inhibition in isolated liver mitochondria was only partially reversed even after 5 h of refeeding. 6. Refeeding resulted in an acute 35% inhibition of the fraction of synthesized triacylglycerol that was secreted into the plasma; the maximal effect occurred 2-3 h after the start of refeeding. The inhibition of the fractional secretion rate was fully reversed after 5 h of refeeding. 7. The amount of 14C label that was incorporated into phospholipids as a fraction of total glycerolipid synthesis was doubled within 2 h of the start of refeeding.(ABSTRACT TRUNCATED AT 400 WORDS)

Extracellular fatty acids are not utilized directly for the synthesis of very-low-density lipoprotein in primary cultures of rat hepatocytes

In hepatocytes cultured in the presence of oleate (initial concn. 0.75 mM), the secretion of very-low-density lipoprotein (VLDL) triacylglycerol and, to a lesser extent, apoprotein B (apoB) increased with time, whereas there was a large decline in the extracellular concentration of fatty acid. There was thus no synchronous relationship between the extracellular fatty acid concentration and the secretion of VLDL. Rather, the appearance of VLDL in the medium was dependent on the intracellular triacylglycerol concentration. At a given concentration of extracellular fatty acid, cells depleted of triacylglycerol secreted less VLDL triacylglycerol and apoB than did control cells. A similar pattern was observed for triacylglycerol newly synthesized from extracellular [3H]oleate. By contrast, the synthesis and output of ketone bodies were directly dependent on the fatty acid concentration of the medium. These results suggest that, at least for oleic acid, extracellular fatty acids are not utilized directly for VLDL assembly, but first enter a temporary intracellular storage pool of triacylglycerol, which is the immediate precursor of secreted triacylglycerol. The size of this pool then determines the rate of secretion of VLDL triacylglycerol apoB. Ketogenesis, on the other hand, relies mainly on the direct utilization of extracellular fatty acids.

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.

The lipolysis/esterification cycle of hepatic triacylglycerol. Its role in the secretion of very-low-density lipoprotein and its response to hormones and sulphonylureas

In hepatocyte cultures maintained in the absence of extracellular fatty acids, at least 70% of the secreted very-low-density lipoprotein (VLDL) triacylglycerol was derived via lipolysis of intracellular triacylglycerol. This proportion was unchanged when the cells were exposed for 24 h to insulin or glucagon, hormones which decreased the overall secretion of intracellular triacylglycerol, or to chloroquine or tolbutamide, agents which inhibit lysosomal lipolysis. The rate of intracellular lipolysis was 2-3-fold greater than that required to maintain the observed rate of triacylglycerol secretion. Most of the fatty acids released were returned to the intracellular pool. Neither insulin nor glucagon had any significant effect on the overall lipolysis and re-esterification of intracellular triacylglycerol. In these cases a greater proportion of the released fatty acids re-entered the cellular pool, rather than being recruited for VLDL assembly. Tolbutamide inhibited intracellular lipolysis, but suppressed VLDL secretion to a greater extent. 3,5-Dimethylpyrazole did not affect lipolysis or VLDL secretion. The increased secretion of VLDL triacylglycerol observed after exposure of cells to insulin for 3 days was not accompanied by an increased rate of intracellular lipolysis. However, a larger proportion of the triacylglycerol secreted under these conditions may not have undergone prior lipolysis.

Characterization of 3,5,3'-triiodothyronine receptors in primary cultures of hepatocytes and neurons from chick embryo

We have detected the presence of nuclear 3,5,3'-triiodothyronine (T3) receptors in primary cultures of chick embryo hepatocytes and neurons. Hepatocytes were isolated from livers of embryos of 12, 16 and 19 days by treatment with 0.2% collagenase and hyaluronidase. They were plated at a density of 3-4 x 10(5)/35-mm petri dish in Ham's F-10 medium containing fetal calf serum, tryptose phosphate, and antibiotics. Cells were used for the binding assay at Day 3 of culture. Neurons from 8-day-old embryo brains were cultured in a serum-free medium at a density of 1.2 x 10(6) cells/35-mm petri dish and used for the binding assay after 7 days of culture. Biological activity of hepatocytes was determined by measuring insulin binding, inositol phosphate formation, and 5'-monodeiodinase activity. Neurons or glial cells in culture were identified by immunostaining with anti-neurofilaments and anti-glial fibrillary acidic protein antisera. Binding assay was performed with isolated nuclei and 0.4 M NaCl nuclear extracts. With the latter preparation, the Scatchard analysis showed, in both cells, a single, high-affinity, low-capacity T3 receptor. In the hepatocytes of 12-, 16-, and 19-day-old embryos association constants (Ka) were, respectively, 0.93 +/- 0.02, 0.74 +/- 0.03, and 0.56 +/- 0.04 nM-1, whereas the maximal binding capacities (MBC) were 2.26 +/- 0.2, 2.72 +/- 0.33, and 1.83 +/- 0.19 fmol/microgram DNA (mean +/- SE, n = 3). In neurons Ka was 1.25 +/- 0.53 nM-1 and MBC 0.59 +/- 0.14 fmol/microgram DNA (n = 3). The receptor had a sedimentation coefficient of 3.4 S, an estimated Mr of 59 kDa, and the following relative affinity for thyroid hormone analogues: TRIAC greater than L-T3 greater than L-T4. These data indicate that cultured hepatocytes and neurons of chick embryo contained T3 receptors with properties similar to those described in intact tissues from this and other species. Only the MBC of neurons was 50% lower than that observed in whole brain of embryo, but was comparable to values observed in cultured neurons from other species.

The role of pancreatic hormones in the regulation of lipid storage, oxidation and secretion in primary cultures of rat hepatocytes. Short- and long-term effects

Exposure of cultured rat hepatocytes to a high concentration of insulin (78 nM) for 24 h in the presence of extracellular oleate (0.75 mM) resulted in a decrease in the secretion of apoprotein B (apoB) and triacylglycerol associated with very-low-density lipoprotein (VLDL). However, continuous exposure of the cells to insulin for longer periods (72 h) stimulated the secretion of apoB and triacylglycerol. Treatment of hepatocytes with glucagon (0.1 microM) for 24 h also suppressed the secretion of VLDL apoB, cholesterol and triacylglycerol. The cells remained responsive to the inhibitory effect of glucagon for at least 3 days. In contrast with insulin, however, exposure of the cells to glucagon for a continuous period of 72 h did not lead to a reversal of the initial inhibition. Glucagon also stimulated ketogenesis, and in this regard the cells were responsive for at least 3 days in culture. These changes were accompanied by a transient increase in intracellular cyclic AMP (cAMP) concentration, which reached a peak 10 min after addition of glucagon. Between 12 h and 24 h after glucagon addition, cAMP levels had returned almost to normal, but the secretion of VLDL remained suppressed during this period.

Interaction of non-esterified fatty acid and insulin in control of triacylglycerol secretion by Hep G2 cells

The role of insulin in the regulation of plasma triacylglycerol is poorly understood. Conflicting actions of insulin on rat liver cells have been reported, insulin inhibiting triacylglycerol secretion in short incubations (less than 24 h) and stimulating triacylglycerol secretion in longer incubations (48-72 h). The present study was undertaken to examine regulation of triacylglycerol secretion by insulin and investigate the interaction between insulin and non-esterified fatty acid over 72 h in human hepatoblastoma (Hep G2) cells. Insulin inhibited triacylglycerol secretion throughout the 72 h period. The inhibition increased from 66% in the first 24 h to 88% in the final 24 h. Increasing the initial concentration of oleic acid from 200 microM to 1000 microM resulted in a 358% increase in triacylglycerol secretion and a 712% increase in accumulation over 24 h. Oleic acid uptake by the cells was rapid, with only 2.4% of the initial concentration (500 microM) remaining after 24 h. Supplementation of the medium with oleic acid to maintain the concentration between 750 microM and 1000 microM throughout a 5 h period resulted in a 350% increase in triacylglycerol secretion. Supplementation also decreased the insulin-induced inhibition of triacylglycerol secretion (18.2 to 7.8%; P less than 0.001). These results demonstrate that there is not a biphasic action of insulin on triacylglycerol secretion by Hep G2 cells. Experiments of this nature have not previously taken into account the rapid uptake of non-esterified fatty acid by hepatocytes and have consequently underestimated the effect of a sustained concentration on triacylglycerol metabolism. Oleic acid is therefore an even more potent stimulus to triacylglycerol synthesis and secretion than has previously been recognized. In addition, in the presence of a sustained increase in oleic acid concentration, there is a decrease in the action of insulin to inhibit triacylglycerol secretion.

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.

Apparent inhibition of hepatic triacylglycerol secretion, independent of synthesis, in high-fat fish oil-fed rats: role for insulin

Rats were fed Chow (C; low-fat control) or a purified high-fat (30% of calories) low cholesterol diet containing menhaden oil (MO), corn oil (CO) or lard (L) for 2, 4 or 6 weeks. Rats were killed after an overnight fast. MO-fed rats had a larger weight/body weight that was accompanied by a lower mg liver DNA/g liver but unchanged liver DNA/body weight, indicating that hepatomegaly in the MO-fed rats was due to cellular hypertrophy. MO-feeding prevented the rise in plasma triacylglycerol and cholesterol observed with the other high-fat diets. There was a marked progressive accumulation of total liver triacylglycerol in the MO- and CO-fed rats. Plasma insulin was reduced in the MO-fed rats relative to all other groups. There were strong positive relationships between plasma insulin and triacylglycerol and between insulin and cholesterol in the high-fat-fed rats. Total liver glucose-6-phosphate dehydrogenase and malic enzyme activities were reduced by MO-feeding and were directly correlated with plasma cholesterol and insulin. These data are consistent with an apparent inhibition of hepatic triacylglycerol secretion by high-fat fish oil-feeding that is independent of the inhibitory effects on triacylglycerol synthesis. These data suggest a role for insulin in regulating the plasma triacylglycerol and cholesterol concentrations in MO-fed rats.

Storage, mobilization and secretion of cytosolic triacylglycerol in hepatocyte cultures. The role of insulin

Cytosolic triacylglycerol labelled from [3H]oleate accounted for almost 50% (57 +/- 22 nmol/mg of protein) of the total cellular triacylglycerol which was newly synthesized by cultured hepatocytes during a 24 h incubation. Insulin decreased the export of triacylglycerol as very-low-density lipoprotein (VLDL) during this period. This resulted in a sequestration of newly synthesized triacylglycerol in the cytosol, rather than in the particulate fraction of the cell. Longer periods of incubation with [3H]oleate resulted in increased concentrations of newly synthesized triacylglycerol within the cell, most of which (78 +/- 3% after 48 h; 80 +/- 3% after 72 h) was located within the cytosolic fraction. The quantity of newly synthesized triacylglycerol in the cell cytosol was further increased by insulin. During these periods there were decreases in the amounts of triacylglycerol associated with the particulate fraction of the cell, irrespective of the presence or absence of insulin. In no case was a decrease in VLDL triacylglycerol secretion in response to insulin accompanied by an increased triacylglycerol content in the particulate fraction of the cell. In some experiments, the fate of the cytosolic triacylglycerol was studied by pulse labelling with [3H]oleate. In these cases, when insulin was removed from the medium of cells to which they had previously been exposed, more newly synthesized triacylglycerol was secreted compared with cells which had not been exposed to insulin. This extra triacylglycerol was mobilized from the cytosolic rather than from the particulate fraction of the cell. Subsequent addition of insulin to the medium prevented the mobilization of cytosolic triacylglycerol. These results suggest that insulin enhances the storage of hepatocellular triacylglycerol in a cytosolic pool. Deficiency of insulin in the medium stimulates the mobilization of this pool which is channelled into the secretory pathway, entering the extracellular medium as VLDL.

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)