Plasma lipoproteins and regulation of hepatic metabolism of fatty acids in altered thyroid states.

This article reviews our understanding of effects of thyroid hormone excess and deficiency on hepatic metabolism of FFA, and consequent effects on production, secretion, and metabolism of plasma lipoproteins. In the hyperthyroid state the following alterations are observed. Fatty acid oxidation and ketogenesis are stimulated simultaneously with a paradoxical stimulation of fatty acid synthesis, which may be linked by virtue of a blunted response of mitochondrial carnitine palmitoyltransferase I (CPT-I) to malonyl coenzyme A (CoA). Esterification of fatty acid to triglyceride (TG) is reduced, as is the secretion of the very low density lipoprotein (VLDL) (including VLDL TG, cholesterol, and apoprotein); this may be due, in part, to decreased concentrations of glycerol-3-phosphate (G3P) in the hepatic cell. In the intact animal or patient, however, serum TG concentration is variable, which may reflect increased adipose tissue lipolysis and elevated concentrations of plasma FFA, which would tend to drive VLDL secretion by the liver. Clearance of the VLDL and its metabolic product, the low density lipoprotein (LDL), is increased, resulting in decreased plasma total and LDL cholesterol. Although high density lipoprotein (HDL) cholesterol may also be reduced, the ratio of LDL/HDL cholesterol is further decreased. The regulatory role of the lipoprotein apoproteins is less clear, but hepatic apolipoprotein (apo) B secretion (required for VLDL) is diminished, while apo-AI secretion (required for HDL) is stimulated, perhaps both reflecting rates of synthesis. Plasma concentrations of apo-AI are variable, dependent on relative rates of secretion and clearance. In the hypothyroid, many of these effects are reversed, which results in hyperlipoproteinemias and greater risk for the development of atherosclerotic cardiovascular disease.

Effects of hyperthyroidism on synthesis, secretion and metabolism of the VLDL apoproteins by the perfused rat liver.

Livers from fed male Sprague-Dawley rats, made hyperthyroid by treatment with triiodothyronine (T3), were isolated and perfused in vitro. T3 (9.6 micrograms/day) was administered by osmotic minipump implanted intraperitoneally. Treatment with T3 for either 7 or 28 days reduced hepatic output of very-low-density lipoprotein (VLDL) and net synthesis of total associated apoproteins. After 7 days treatment, incorporation of [4,5-3H]leucine by livers from hyperthyroid rats into VLDL apo E was reduced while incorporation into apo B100, apo B48, and apo C's did not differ from euthyroid controls. The depressed incorporation of radioactivity into total VLDL protein was accounted for almost entirely on the basis of apo E. Incorporation of leucine into the total lipoprotein apo E isolated in the d less than 1.210 was also diminished by the hyperthyroid state, while that into apo B100, apo B48, and apo C in the total perfusate lipoprotein was similar to that of the euthyroid, as was found for the VLDL. Increased amounts of radioactive apo B100 and apo B48, however, were detected in the HDL fraction isolated from the medium perfusing livers from hyperthyroid rats. Hepatic uptake of VLDL protein and lipid was similar in euthyroid and hyperthyroid rats. Reduction of VLDL lipid and protein in the medium perfusing livers from T3-treated rats, therefore reflects hormonal action on synthesis and secretion, rather than uptake. Since the availability of apo B is thought to be required for secretion of VLDL, our observation suggests that synthesis of apo B is not depressed by treatment with T3 and that apoprotein synthesis is not a significant factor in the decreased output of VLDL by the liver, but that, as reported earlier, the lower output is a consequence of decreased synthesis of TG, the result of a diminished supply of hepatic glycero-3-phosphate in the hyperthyroid. The diminished amount of VLDL protein appears to be accounted for by the decreased quantity of apo E associated with a smaller VLDL particle secreted by livers from T3-treated rats.

Effects of high-dose ethinyl estradiol on serum concentrations and hepatic secretion of the very-low-density lipoprotein, triacylglycerol, cholesterol, and apolipoprotein A-I in the rat.

Female and male rats were treated with ethinyl estradiol (5.0 mg/kg daily for 5 days). Control animals were pair fed to compensate for the reduction in food intake induced by the estrogen, or were fed ad libitum. Treatment with ethinyl estradiol reduced total cholesterol and apolipoprotein A-I concentrations in the serum of female and male animals. The concentrations of serum and hepatic triacylglycerol were depressed markedly in animals of both sexes in groups treated with ethinyl estradiol, compared to the control group fed ad libitum. Compared to the pair-fed controls, however, ethinyl estradiol had only a very minor further reduction on serum triacylglycerol concentration. In male and female rats, the synthesis and secretion of triacylglycerol by the liver was, in comparison to the pair-fed controls, stimulated by estrogen, whereas the secretion of unesterified cholesterol was unaffected by any of the treatment regimens. The synthesis and secretion of total cholesteryl esters by livers from male and female rats was increased by treatment with ethinyl estradiol. The hepatic synthesis and secretion of VLDL triacylglycerol and cholesteryl ester was stimulated by ethinyl estradiol in male and female rats, and the VLDL particle was enriched with cholesteryl ester. Treatment with the high-dose estrogen increased the secretion of apolipoprotein A-I by livers from female rats. It is suggested that the depression in the serum concentrations of cholesteryl esters and apolipoprotein A-I is the result of increased rates of hepatic and/or peripheral catabolism of these components and that the hepatic production rates were increased or unaffected in animals administered high doses of ethinyl estradiol. Since the secretion of apolipoprotein A-I by livers from male rats was unaffected by treatment with ethinyl estradiol, the response to estrogen may be sex related.

Alterations of plasma HDL lipids and apolipoproteins in female rats treated with ethynyl estradiol.

Female Sprague-Dawley rats (160-200 g body weight) were injected subcutaneously daily for 14 days with 1, 5, 15, 50 or 100 micrograms ethynyl estradiol in sesame oil/kg body weight, or with oil alone. Food consumption was restricted to 15 g/day, and animals were fasted overnight before exsanguination. Concentrations of plasma cholesteryl ester decreased with increasing dosage of ethynyl estradiol, as a result of decreases in HDL cholesteryl ester. Concentrations of plasma unesterified cholesterol were not altered by treatment; the tendency for a decrement in the HDL cholesterol was not statistically significant. Plasma and HDL phospholipid also tended to decrease with increasing dose of ethynyl estradiol. Plasma levels of apolipoprotein A-I increased on treatment with 5 micrograms ethynyl estradiol/kg but were diminished at a dose of 50 micrograms ethynyl estradiol/kg. The ultracentrifugally isolated high density lipoproteins were delipidated and the distribution of apolipoproteins was characterized by SDS-polyacrylamide gel electrophoresis and esoelectric focusing. The proportion of apolipoprotein E in HDL was depressed, while apolipoprotein A-I was increased and apolipoprotein C unchanged by treatment with ethynyl estradiol. Clearly, the alterations in plasma HDL lipid levels resulting from treatment with ethynyl estradiol were accompanied by distinct changes in composition of the apolipoproteins of HDL, and a biphasic response dependent on dose of the drug. A possible mechanism for the diminution in the proportion of HDL apolipoprotein E may be the enhanced removal of the apolipoprotein E-rich (HDL1) subfraction of the HDL from the circulation on treatment with ethynyl estradiol.

Cholesterol is required for the secretion of the very-low-density lipoprotein: in vivo studies.

Rats were fed for 1 week with a standard chow diet, a diet supplemented with lovastatin (0.1%), or a diet supplemented with both lovastatin and cholesterol (0.1/0.1%), to study effects of depletion of a putative hepatic metabolic pool of cholesterol on secretion of the very-low-density lipoprotein (VLDL) in the intact animal. Triton WR-1339 (50 mg/100 g body wt.) or the 0.9% NaCl vehicle alone was given intravenously via a sacral vein. Treatment with lovastatin decreased the secretion of all plasma VLDL lipids, the average decrease after 2 h for VLDL triacylglycerol, phospholipid, cholesterol and cholesteryl ester being 45%. When both lovastatin and cholesterol were included in the diet, the secretion of VLDL triacylglycerol and phospholipid was similar to that of control animals, while secretion of VLDL cholesterol and cholesteryl ester was increased. Treatment with lovastatin reduced the hepatic concentration of cholesteryl esters 42% without affecting free cholesterol. In separate experiments, in vivo synthesis of cholesterol was determined 1 h after intraperitoneal administration of 3H2O. Incorporation into hepatic and plasma free cholesterol and cholesteryl esters was greater in the rats fed lovastatin than in control animals, concurrent with decreased VLDL secretion. The metabolism of VLDL was determined in vivo by intravenous administration of 125I-VLDL. The fractional clearance rates of 125I-VLDL from the plasma were similar among the three experimental groups. Synthesis of hepatic triacylglycerol from [1-14C]oleate in vivo was similar in all treatment groups; incorporation into plasma triacylglycerol was reduced with lovastatin treatment and reversed partially by inclusion of 0.1% cholesterol in the lovastatin diet. Plasma concentrations of triacylglycerol followed patterns of incorporation of [1-14C]oleate. Triacylglycerol concentration in the liver increased when cholesterol was included in the diet. In companion experiments, incorporation of [1-14C]oleate into perfusate triacylglycerol in vitro was reduced with perfused livers from lovastatin-treated animals. In these experiments, oxidation of fatty acid into CO2 and perchloric acid-soluble counts was not affected by lovastatin, added either to the diet or to the perfusate in vitro. It appears, therefore, that lovastatin does not affect triacylglycerol synthesis or fatty acid oxidation, which per se might reduce formation and secretion of VLDL. These data, therefore, strengthen the hypothesis that reduced availability of cholesterol in a putative hepatic metabolic pool, required for secretion and transport of triacylglycerol in the VLDL, is a factor contributing to decreased secretion of the VLDL.

Composition and properties of very low density lipoproteins secreted by the perfused rat liver and subfractionated by affinity chromatography.

Very low density lipoproteins (VLDL) were isolated from the perfusate of rat livers infused with a complex of oleic acid bound to bovine serum albumin. Very low density lipoprotein (VLDL) secretion, bile flow, histopathology, and transmission electron microscopy indicated that secretory functions but not morphologic integrity of the livers were maintained during the procedure. Plasma VLDL and liver perfusate VLDL did not have similar size distribution. VLDL isolated from recycling perfusate and single pass perfusate were also subfractionated with concanavalin A-Sepharose 4B affinity chromatography. Three subfractions were eluted sequentially from the perfusate VLDL: a non-adherent fraction A and two adherent fractions B and C. The size of these VLDL, determined after negative staining and examination by transmission electron microscopy, was significantly decreased by affinity chromatography. VLDL in fractions A, B and C were spherical and had diameters of 935 +/- 17, 881 +/- 34 and 415 +/- 30 A respectively. Fraction A, which did not adhere to the column, contained 65% of the lipid applied to the column. The carbohydrate composition of fraction A VLDL was 11.2 +/- 0.6% fucose, 14.7 +/- 1.2% galactose, 43.7 +/- 2.3% N-acetylglucosamine, and 30.5 +/- 1.9% sialic acid. Sugars such as glucose and mannose, which bind to concanavalin A, were not detected. In contrast, VLDL fractions B and C, which adhered to the column, contained both glucose (17.7 and 2.5%) and mannose (5.8 and 8.3%) as well as the other sugars present in VLDL fraction A. Sodium dodecyl sulfate gradient gel electrophoresis revealed that the affinity column procedure clearly altered the apolipoprotein patterns of the applied VLDL, thereby producing abnormal fractions B and C. Fractions B and C also differed from unfractionated VLDL and fraction A VLDL in lipid composition, in surface/interior core lipid ratio, and in fatty acid composition of the interior core lipids, primarily triacylglycerols. The steady-state anisotropy, the limiting anisotropy and the lipid order parameter of fluorescence probe molecules 1,6-diphenyl-1,3,5-hexatriene and trans-parinaric acid incorporated into the VLDL were of the following order: fraction B greater than fraction A greater than fraction C. These results are consistent with the interpretation that concanavalin A-Sepharose 4B affinity chromatography may artificially produce a series of VLDL subfractions whose composition and structural properties do not resemble those of native VLDL.

In vivo growth hormone treatment stimulates secretion of very low density lipoprotein by the isolated perfused rat liver.

We have previously demonstrated in hepatocyte suspensions prepared after in vivo GH deprivation [hypophysectomy (hypox)] that rates of esterification of [1-14C]oleic acid into triglyceride (TG) and phospholipid (PL) were diminished, and that these esterification rates were correspondingly restored by repletion with recombinant GH. The current studies were designed to determine if GH exerts a similar effect on the secretion of very low density lipoprotein (VLDL), the primary plasma carrier of TG. We assessed rates of secretion of VLDL lipid and apoprotein by perfused livers prepared from cortisol/T3-replaced hypox female rats in the presence and absence of recombinant human (h) GH infusion. We also determined rates of synthesis and secretion of VLDL TG from infused [1-14C]oleic acid. After hypox, rates of secretion of VLDL lipid (TG, PL, and cholesterol) and apoprotein (total) were significantly decreased. In addition, VLDL secreted under these conditions was depleted of PL, relative to the other lipid components. Secretion of newly synthesized VLDL TG from [1-14C]oleic acid was also decreased; however, neither intracellular accumulation of labeled TG nor absolute tissue levels of TG were significantly changed. Conversely, GH treatment of hypox rats effectively restored rates of secretion of VLDL TG, PL, cholesterol (C) and apoprotein to control levels. These findings support the putative role of GH in regulating VLDL secretion in vivo by demonstrating that alterations in plasma GH are accompanied by changes in VLDL secretion. The findings further suggest that GH may regulate VLDL secretion by altering the amount of PL and/or apoprotein available for formation of the VLDL particle.

Effects of thyroid status on the structure of the very low density lipoprotein secreted by the perfused liver.

To determine whether hyperthyroidism or hypothyroidism affected the structure of the very low density lipoprotein (VLDL) secreted by the perfused rat liver, rats were pretreated with 0.9% NaCl, T3, or propylthiouracil. The livers were perfused with 0-332 mumol bovine serum albumin-oleate/h for 4 h. The structure of the secreted VLDL was determined by compositional analysis and the use of the fluorescence probe molecules diphenylhexatriene, trans-parinaric acid, and cis-parinaric acid. Compositional analysis indicated that the VLDL secreted by livers from rats pretreated with T3 had lower unsaturated to saturated fatty acid ratios than did controls, regardless of the amount of oleate infused. Fluorescence polarization of diphenylhexatriene indicated that the interior core lipids of VLDL secreted by livers from T3-treated rats were more rigid than those secreted by livers of euthyroid animals. Arrhenius plots of polarization and corrected fluorescence of diphenylhexatriene and trans-parinaric acid, but not cis-parinaric acid, revealed that characteristic temperatures were shifted 8 to 10 C higher in VLDL secreted by livers from rats pretreated with T3. Treatment of rats for 7 days with propylthiouracil under these mild conditions did not alter the structure or composition of the VLDL secreted by the perfused liver. In summary, the lipid changes in the VLDL secreted by perfused rat livers from hyperthyroid animals were consistent with these VLDL being more rigid particles than those secreted by livers from euthyroid rats, independent of the infusion rate of the FFA.

Stimulation of in vitro triglyceride synthesis in the rat hepatocyte by growth hormone treatment in vivo.

Hepatic fatty acid metabolism in the rat is sexually differentiated. Rates of esterification by the liver of fatty acid into triglyceride and other esterification products (phospholipid, diglyceride, cholesteryl esters) are higher in the female than in the male. There is evidence to suggest that GH feminizes other hepatic systems that exhibit sexual dimorphism, including hepatic steroid metabolism, PRL receptors, and estrogen binding. To investigate the role of GH in maintenance of the high rates of fatty acid esterification observed in the female, we assessed rates of [1-14C]oleic acid utilization by hepatocytes prepared from hypophysectomized (hypox) cortisol/T3-replaced female rats with an without continuous in vivo infusion of human (h) GH (5 micrograms/h). In addition, we assessed the effect of in vivo hGH treatment (5 micrograms/h) on [1-14C]oleic acid utilization in the normal male rat. Hypophysectomy was accompanied by a reduction in incorporation of [1-14C]oleic acid into products of esterification (triglyceride, phospholipid, diglyceride) and oxidation (CO2, ketone bodies). Continuous infusion of hGH (5 micrograms/h; 14 days) restored rates of fatty acid esterification in the hypox-cortisol/T3-replaced female rat, with the exception of cholesteryl esters. hGH infusion partially restored rates of fatty acid oxidation in the hypox cortisol/T3-replaced female rat. Treatment of the adult male rat with continuous infusion of hGH (5 micrograms/h; 7 days) resulted in increased rates of incorporation of [1-14C] oleic acid into triglyceride. In contrast, incorporation of oleic acid into phospholipid, diglyceride, and cholesteryl esters was unaltered. These results suggest that GH may be an important regulator of hepatic fatty acid metabolism.

Modulation of organ ICAM-1 expression during IV-TPN with glutamine and bombesin.

The gut primes neutrophils (PMNs) during injury, which can then induce distant organ damage after a second insult. ICAM-1 is an important adhesion molecule in PMN attachment to the vascular endothelium. Parenteral nutrition (TPN) decreases gut levels of interleukin (IL)-4 and IL-10, two cytokines that are normal inhibitors of ICAM-1 expression. TPN also increases gut ICAM-1 expression and PMN accumulation. Since glutamine (GLN) and bombesin (BBS) prevent TPN-associated impairment of mucosal immunity, we hypothesized that GLN and BBS would modulate organ ICAM-1 expression in association with normalization of IL-4 and IL-10 levels. Forty-four mice were fed chow, TPN, or GLN-TPN (isonitrogenous 2% GLN-enriched TPN). After 5 days of diets, ICAM-1 expression was quantified in organs using the dual radiolabeled monoclonal antibody technique. In the next experiment, 29 mice were fed chow, TPN, or BBS-TPN (BBS 15 microg/kg TID) for 5 days to measure organ ICAM-1 expression. Total IL-4 and IL-10 levels were measured with ELISA from intestinal homogenates of another set of 52 mice fed chow, TPN, GLN-TPN, or BBS-TPN. TPN significantly increased ICAM-1 expression in the lung, kidney, and intestine compared with chow mice. GLN-TPN decreased intestinal, but not lung, ICAM-1 expression, while BBS-TPN reduced pulmonary, but not gut, ICAM-1 levels. GLN- and BBS-TPN returned gut IL-4 levels to normal, but failed to increase IL-10 levels. GLN and BBS had different effects on organ ICAM-1 expression induced by lack of enteral nutrition. Mechanisms other than recovery of IL-4 alone may be responsible for gut ICAM-1 expression.

Differential effects of alanine on ketogenesis and triacylglycerol formation by isolated perfused livers from euthyroid and hyperthyroid rats.

We examined the effects of infusion of alanine on hepatic concentration of glycero-3-phosphate (glycero-3-P) and output of triacylglycerol (TG) by isolated perfused livers from triiodothyronine (T3)-treated rats. It was expected that because of its gluconeogenic and antiketogenic properties, alanine might stimulate accumulation of glycero-3-P, which in turn might result in enhanced TG production by the hyperthyroid livers. The hepatic concentration of glycero-3-P is lower in livers from T3-treated rats than in euthyroid rats. Infusion of 1.83 and 4.23 mmol alanine/4 h did not alter the hepatic concentration of glycero-3-P and output of triacylglycerol by livers from T3-treated rats. However in these livers, the two concentrations of infused alanine increased the output of glucose and decreased the output of ketone bodies. In livers from euthyroid animals, the infusion of 1.83 mmol alanine/4 h had no effect, whereas 4.23 mmol/4 h decreased hepatic concentration of glycero-3-P. These concentrations of alanine did not alter the output of ketone bodies or TG, but progressively decreased the output of glucose by the euthyroid livers. Our data suggested that the decreased availability of glycero-3-P in livers from T3-treated rats is rate-limiting for TG production and correlated with the diminished output of TG, whereas in livers from euthyroid rats, the glycero-3-P concentration is sufficient to maintain maximal synthesis and secretion of TG. Furthermore, the effects of alanine on the output of ketone bodies or glucose appear to be independent of its effects on hepatic glycero-3-P.

Insulin increases expression of apobec-1, the catalytic subunit of the apolipoprotein B mRNA editing complex in rat hepatocytes.

We have previously shown that chronic insulin treatment of rat hepatocytes increases the fraction of edited apolipoprotein B (apoB) mRNA from approximately 50% to as much as 90%. We have now examined the effect of insulin on apobec-1 mRNA abundance and demonstrate that increased editing of apoB mRNA following insulin treatment is accompanied by elevated apobec-1 mRNA levels in primary rat hepatocytes. Time-course measurements of the effects of insulin on apoB mRNA editing and apobec-1 mRNA abundance showed that both were elevated almost maximally within 48 hours and sustained for at least 5 days of insulin treatment.

Increased expression of intestinal P-selectin and pulmonary E-selectin during intravenous total parenteral nutrition.

HYPOTHESIS: Intravenous total parenteral nutrition (TPN) induces intestinal polymorphonuclear neutrophil recruitment with increased intestinal intercellular adhesion molecule-1 expression. While intercellular adhesion molecule-1 causes firm adhesion of leukocytes to the endothelial cells, P- and E-selectin mediate leukocyte recruitment via rolling. Therefore, manipulation of nutrition may also affect P- and E-selectin expression in organs. DESIGN: Prospective randomized experimental trials. SETTING: Laboratory. MATERIALS: Male mice. INTERVENTIONS: Fifty-three mice were randomized to chow, intravenous TPN, or intragastric TPN. MAIN OUTCOME MEASURES: After 5 days of diet, mice were administered iodine 125-labeled anti-P-selectin antibody (or iodine 125-labeled anti-E-selectin antibody) and iodine 131-labeled nonbinding antibody to quantify P-selectin (or E-selectin) expression in organs (lung, liver, kidney, small intestine, colon, stomach, pancreas, mesentery, heart, and skeletal muscle). RESULTS: P-selectin in small intestine, colon, stomach, and pancreas in the intravenous TPN group increased significantly as compared with the chow and the intragastric TPN groups. E-selectin expression was up-regulated after intravenous TPN in the lung but not in other sites. CONCLUSIONS: In a time frame (5 days) when intercellular adhesion molecule-1 expression and neutrophil recruitment are increased, intestinal expression of P-selectin remains up-regulated. Early lung inflammatory changes are reflected by increases in E-selectin. This change may reflect early pulmonary dysfunction with intravenous TPN, but its significance requires further study.

Route of nutrition influences intercellular adhesion molecule-1 expression and neutrophil accumulation in intestine.

HYPOTHESIS: The levels of intestinal interleukin 10 and interleukin 4, inhibitors of intercellular adhesion molecule-1 (ICAM-1) expression, decline with total parenteral nutrition (TPN). These cytokine changes induced by lack of enteral nutrition may increase ICAM-1 expression, resulting in polymorphonuclear neutrophil accumulation in intestine. DESIGN: Prospective randomized experimental trials. SETTING: Laboratory. MATERIALS: Male mice. INTERVENTIONS: Sixty-three mice were randomized to chow, intravenous TPN, or intragastric TPN. MAIN OUTCOME MEASURES: Experiment 1: After diet manipulation, iodine 125-labeled anti-ICAM-1 antibody and iodine 131-labeled nonbinding antibody were injected to quantify ICAM-1 expression on endothelial cells in the lung, liver, kidney, and small intestine. Measurement of myeloperoxidase was used to quantify polymorphonuclear neutrophil accumulation in the organs. Experiment 2: Intestine was harvested for both ICAM-1 and myeloperoxidase levels after chow refeeding of mice in the intravenous TPN group. RESULTS: In experiment 1, uninjured mice fed intravenous TPN showed significantly increased intestinal ICAM-1 expression and polymorphonuclear neutrophil accumulation with no significant changes in the lung, liver, or kidney. No changes occurred in mice fed chow or intragastric TPN. In experiment 2, reinstitution of enteral feeding returned intestinal ICAM-1 and myeloperoxidase levels to normal. CONCLUSION: Gut changes associated with lack of enteral feeding induce endothelial changes and an immunologic response, which may influence subsequent responses to injury.

Differences in ICAM-1 and TNF-alpha expression between large single fraction and fractionated irradiation in mouse brain.

PURPOSE: To elucidate the brain molecular response to irradiation. The expression of the intercellular adhesion molecule (ICAM-1) and tumour necrosis factor-alpha (TNF-alpha) in the mouse brain was compared after single-dose and fractionated whole-brain irradiation. MATERIALS AND METHODS: Mice received a single dose of 2, 10 or 20 Gy or a fractionated dose (2 Gy day(-1)) of 10, 20 or 40 Gy. ICAM-1, and TNF-alpha mRNA expression were quantified by the highly sensitive real-time polymerase chain reaction technique. Expression of ICAM-1 protein was quantified by dual-labelled monoclonal antibody assay. RESULTS: After a 20-Gy single dose, there was an increase in ICAM-1 and TNF-alpha mRNA levels (14- and 11-fold, respectively) as well as a significant increase in the level of ICAM-1 protein (p=0.0243). The expression of ICAM-1 and TNF-alpha mRNA increased at the end of the 40-Gy fractionated regimen (3.55- and 2.30-fold, respectively). CONCLUSIONS: The molecular response of the brain to single-dose irradiation was rapid, while its response to fractionated irradiation was slow. This finding is consistent with clinical observations and could be of use when designing strategies to mitigate radiation sequelae.

Effects of dietary triglyceride on the properties and lipid composition of plasma lipoproteins: acute experiments in rats fed safflower oil.

Male rats were administered 1.5 ml safflower oil by gastric intubation 0, 4, and 8 hr after a 16 hr fast. Plasma, liver, and adipose tissue were collected 16 hr after the last fatty meal. Rats fasted for 16 hr served as controls. Following fat feeding, the fatty acid composition of the very low density lipoprotein, triglyceride, and hepatic triglyceride were similar, as were the percentages of 18:2 in the very low density lipoprotein and hepatic cholesteryl esters. The phospholipids of liver and plasma lipoproteins were similar in the control groups, except that more 16:0 was present in the plasma lipoproteins. After fat feeding, the plasma lipoproein phospholipids were enriched with 18:2 more than were the hepatic phospholipids. Furthermore, the percentage of 18:2 in phospholipid was much less than in triglyceride or cholesteryl esters. Clearly, esterified lipids of liver and plasma lipoproteins (very low density lipoprotein, low density lipoprotein, and high density lipoprotein), and to a lesser extent, adipose tissue, were enriched with 18:2 derived from dietary triglyceride fatty acid even 16 hr after the terminal meal. A major proportion of the very low density lipoprotein isolated by ultracentrifugation in zonal rotors from plasma of fat fed animals had a faster rate-zonal mobility than did the very low density lipoprotein isolated from plasma of control animals. The very low density lipoprotein isolated from plasma of fat fed rats contained fewer moles of phospholipids, cholesterol, and cholesteryl esters, relative to triglyceride than did the very low density lipoprotein from plasma of animals not receiving safflower oil. The molar ratio triglyceride:phospholipid:cholesterol:cholesterol esters in the very low denity lipoprotein was 100:42.0:22.1:44.5 in the control group and 100:35.4:17.8:19.5 in the fat fed animals. It is postulated that an important biochemical mechanism by which dietary triglyceride fatty acids consumed by the animal over a long period of time alter plasma concentrations of triglyceride, phospholipids, and cholesterol esters is the directive influence of plasma free fatty acid, derived from dietary triglyceride, on the secretion of very low density lipoprotein lipids by the liver.

Effects of sex on formation and properties of plasma very low density lipoprotein in vivo.

The concentration and composition of the very low density lipoprotein (VLDL) lipids and the behavior of the VLDL in a density gradient in the zonal ultracentrifuge were examined in plasma obtained from normal fed male and female rats before and after intravenous injection of Triton WR-1339. Concentration of lipids in plasma VLDL of female rats was about half that of male animals. Following injection with Triton WR-1339, the concentration of VLDL lipids was higher in female rats (triacylglycerol) or similar (phospholipid, cholesterol, and cholesteryl esters) in both sexes. Female rats secreted much more VLDL traicylglycerol into the plasma compartment than did the male animals under the same experimental conditions. No differences were observed in lipid composition of the VLDL or in the position of the VLDL in the zonal rotor after ultracentrifugation in a density gradient of the lipoprotein from plasma of normal male and female rats before treatment with the detergent. However, after treatment with Triton, a higher proportion of the VLDL particles isolated from plasma of female rats displayed a more rapid rate-zonal flotation in the ultracentrifuge than did the VLDL produced by the male. The VLDL secreted by female rats contained fewer moles of phospholipid and free sterol per mol triacylglycerol than did the VLDL secreted by male animals under identical experimental conditions. The molar ratio of free cholesterol:cholesteryl ester in the VLDL secreted after treatment with Trition increased in both male and female rats. Simultaneously, the content of arachidonic acid in phospholipid of VLDL increased with a concomitant decrease in cholesteryl ester. These changes in fatty acid composition suggest that the inhibitory effect of Triton on lecithin-cholesterol acyl transferase activity affects the exchange of lipids between VLDL and high density lipoprotein. It can be concluded from the data reported here that sex influences the concentration of plasma lipids in vivo and the output and properties of the VLDL.

Metabolism of n-3 polyunsaturated fatty acids by the isolated perfused rat liver.

The hepatic metabolism of oleic acid and n-3 fatty acids (eicosapentaenoic acid, EPA and docosahexaenoic acid, DHA), and secretion of very low density lipoprotein (VLDL) were studied in isolated perfused rat livers from normal chow fed male rats. The basal perfusion medium contained 30% bovine erythrocytes, 6% bovine serum albumin (BSA), and 100 mg/dL glucose, in Krebs-Henseleit bicarbonate buffer (pH 7.4), which was recycled through the liver for 2 hr. Individual fatty acids (EPA, DHA or oleic acid), as complexes with 6% BSA, or albumin alone, were infused at a rate of 70 mumol/hr. When any of these fatty acids was infused at this rate, the ambient concentration in the medium was maintained at 0.3-0.4 mumol/mL, indicative of similar hepatic rates of uptake for each fatty acid (i.e., approximately 6 mumol/g liver/hr). When fatty acid was not infused, the ambient free fatty acid level was 0.16 mumol/mL. The concentrations of infused free fatty acids increased appropriately in the perfusion medium; however, with infusion of EPA, DHA, or oleate, the concentrations of perfusate palmitate and linoleate were the same as when fatty acid was not infused. Additionally, the perfusate concentration of oleate in the free fatty acid fraction was not affected by infusion of EPA and DHA. These data indicate a constant outflow of endogenous fatty acid unaffected by the presence of the exogenously supplied fatty acid. The net secretion rate of VLDL lipids and protein was stimulated by infusion of oleate, whereas when EPA was infused, secretion rates were lower and similar [except for VLDL cholesterol (C), which was greater] to those occurring when fatty acid was not provided.(ABSTRACT TRUNCATED AT 250 WORDS)

Concentration-dependent effects of eicosapentaenoic acid on very low density lipoprotein secretion by the isolated perfused rat liver.

The effects of increasing concentrations of eicosapentaenoic acid (20:5n-3; EPA) and oleic acid (18:1n-9; OA) on esterification to triacylglycerols (TG) and phospholipids (PL), and the relationship to formation and secretion of the very low density lipoproteins (VLDL) were compared in the isolated perfused rat liver. Mixtures of EPA and OA were also studied to determine whether substrate levels of one fatty acid might influence the metabolism of the other. The basal perfusion medium, which contained 30% (vol/vol) washed bovine erythrocytes, 6% (wt/vol) bovine serum albumin (BSA), and 100 mg glucose/dL in Krebs-Henseleit bicarbonate buffer (pH 7.4) was recycled through the liver for 2 h. EPA or OA, as a complex with 6% BSA, was infused at rates of 70, 105, 140 and 210 mumol/h. In other experiments, mixtures of EPA and oleic acid (70 mumol total), with molar percentages of 100, 75, 50, 25 and 0% of each fatty acid were infused per hour. BSA (6%) in the buffer was infused alone and served as the control. At an infusion rate of 70 mumol EPA per hour, hepatic VLDL lipid output was not different from that when fatty acid was not infused (approximately half that when 70 mumol OA/h was infused). However, when larger amounts of EPA and OA were infused individually, rates of VLDL secretion were stimulated to a similar extent with either fatty acid. The apparent inhibitory influence of EPA on TG synthesis and VLDL lipid output when 70 mumol EPA were infused per hour could also be overcome by the presence of as little as 25 mol% OA in a mixture.(ABSTRACT TRUNCATED AT 250 WORDS)

Apolipoprotein B mRNA editing and apolipoprotein gene expression in the liver of hyperinsulinemic fatty Zucker rats: relationship to very low density lipoprotein composition.

We previously demonstrated increased apolipoprotein B (apoB) mRNA editing, elevated levels of mRNA for the catalytic component of the apoB mRNA editing complex, apobec-1, and increased secretion of the product of the edited mRNA, apoB48, in very low density lipoproteins (VLDL) in primary cultures of Sprague-Dawley rat hepatocytes following insulin treatment. In order to determine the effect of in vivo hyperinsulinemia on these processes, we determined apoB mRNA editing, apobec-1 expression, hepatic expression of mRNA for apoB and other VLDL apoproteins, and the quantity and composition of plasma VLDL in the hyperinsulinemic fatty Zucker rat. Total apoB mRNA content of the livers of the fatty rats and lean littermates did not differ; however, edited apoB message coding for hepatic apo B48, and abundance of mRNA for the catalytic subunit of the apoB mRNA editing complex, apobec-1, was increased by 1.7- and 3.3-fold, respectively, in fatty rats. ApoCIII mRNA abundance was increased in livers of fatty rats as well, but the abundance of hepatic apoE mRNA in the fatty animal was not different from that of the lean rat. Hepatic apoAI mRNA abundance was also increased in the fatty rats. Associated with increased apoB mRNA editing, was the 1.7-fold increase in the fraction of apoB in plasma as apoB48 in fatty rats. VLDL-triglyceride and -apoB in plasma were 15- and 3-fold higher, respectively, in fatty Zucker rats compared to lean littermates, indicating both enrichment of VLDL with triglycerides and increased accumulation of VLDL particles. Increased hepatic expression of mRNA for apoCIII and apoAI was associated with increased content of apoC (and relative depletion of apoE) in VLDL of fatty rats, and plasma apoAI was increased in fatty Zucker rats, primarily in the HDL fraction. The current study provides further evidence that chronic exposure to high levels of insulin influences both the quantity of and lipid/apoprotein composition of VLDL in plasma. The increased apoC and decreased apoE (as well as increased triglyceride) content of VLDL in the fatty Zucker rat observed in the current study may affect VLDL clearance and therefore may be a factor in the observed accumulation of VLDL in the plasma of the fatty hyperinsulinemic Zucker rats.