Development of hyperlipidemia associated with increased lipolytic response of isolated adipose tissue cells following prolonged stimulation by an ectopic pituitary tumor.

Implantation of MtT-F4 tumor, a mammotropic tumor that secretes large quantities of ACTH, GH and prolactin, into male Fisher rats induced the development of hyperlipidemia. Free fatty acid, triglyceride and cholesterol levels in the plasma were significantly increased at 31 days after tumor implantation. Blood glucose and glycerol levels remained normal, while uric acid concentration in the blood was significantly decreased. The lipolytic response of isolated adipose tissue cells to ACTH was significantly higher in cells derived from rats bearing an MtT-F4 tumor for 31 days than from their corresponding controls. However, the activity of adenylate cyclase in fat cells stimulated with ACTH was not significantly higher in cells derived from tumor bearing rats than in cells from control rats.     

Cholesterol biosynthesis in transplantable hepatomas: evidence for impairment of uptake and storage of dietary cholesterol

Cholesterol feeding inhibits cholesterol biosynthesis in normal but not in malignant liver tissue. It has been postulated that hepatomas have suffered a specific intracellular deletion of the cholesterol feedback control mechanism, but there is little direct evidence to support this hypothesis. Rats bearing Morris transplantable hepatomas were fed high cholesterol diets for periods of up to 21 days. Cholesterol biosynthesis, as expected, was suppressed in the normal liver but not in hepatomas. The livers accumulated large amounts of cholesteryl ester but the hepatomas showed little or no increase in ester content. Cholesterol-1alpha-(3)H was administered intragastrically to other tumor-bearing rats. Uptake of radioactivity by the tumors was much slower than by normal liver. Comparison of the specific activities of liver and tumor cholesterol with that of the plasma suggested that the liver took up dietary cholesterol selectively from the blood, while the appearance of radioactivity in the tumors could be explained by slow equilibration with plasma cholesterol. Our results suggest that the insensitivity of cholesterol biosynthesis to dietary cholesterol in hepatomas could be explained by an impairment in the uptake and storage of dietary cholesterol and that the concept of an intracellular deletion of the feedback mechanism requires further evidence.     

Increased central serotonergic activity associated with nocturnal anorexia induced by Walker 256 carcinoma

The role of brain serotonin levels in Walker 256 tumor induced anorexia was investigated. Total and free plasma tryptophan, regional brain serotonin and 5-hydroxyindoleacetic acid were determined at night, and their relationship to nocturnal anorexia assessed by linear regression analysis. No significant difference in tryptophan, serotonin, or 5-hydroxyindoleacetic acid levels was detected between pair fed and tumor bearing rats exhibiting a 20% reduction of nighttime food intake. Tumor bearing rats with a 40% reduction in food intake had higher nighttime plasma free tryptophan and regional 5-hydroxyindoleacetic acid levels than their pair fed malnourished controls. These results indicate that increased plasma free tryptophan and elevated serotonin metabolism may not be the initial dysfunction responsible for nocturnal anorexia. However, it may contribute to the decreasing nocturnal food intake in severely anorexic tumor rats.     

Chronic intraperitoneal infusion of low doses of tumor necrosis factor alpha in rats induces a reduction in plasma triglyceride levels.

Single and repeated bolus injections of tumor necrosis factor alpha (TNF) in laboratory animals have been reported to result in hypertriglyceridaemia, suggesting that TNF is a mediator of hypertriglyceridaemia occurring during infection. However, as during infection production of TNF is probably chronically elevated, we determined the effects of continuous infusion of low doses of TNF on plasma levels of triglycerides and cholesterol. Male rats, bearing a venous catheter to allow repeated blood sampling, were intraperitoneally equipped with osmotic minipumps which continuously delivered TNF or saline for 7 days. Infusion of rats with doses of TNF as low as 4.0 and 8.0 micrograms/24 h resulted in significant decreases in plasma levels of triglycerides as compared with those after saline infusion. Although plasma triglyceride concentrations were persistently lower in TNF than in saline animals throughout the study period, the differences were most prominent during the first days and reached statistical significance at day 1, 3, 4 and 5 and of the 4.0 micrograms experiment and on day 1, 2 and 3 of the 8.0 micrograms experiment. This suppression of plasma triglyceride concentrations was not accompanied by changes in plasma cholesterol levels. No effects of chronic TNF treatment on food intake, body weight change and rectal temperature of the animals were observed. These findings indicate that chronic infusion of low doses of TNF induces hypotriglyceridaemia in rats. The role of TNF as a factor in mediating hypertriglyceridaemia during infectious diseases needs to be reconsidered.     

Effect of dietary cholesterol on erythrocyte peroxidant stress in vitro and in vivo

The effect of dietary variation of plasma cholesterol concentrations on the susceptibility of erythrocytes to in vitro and in vivo peroxidant stress was studied in rats. Malonyldialdehyde, produced in vivo (endogenous malonyldialdehyde) or following in vitro exposure of cells to 10 mM H2O2 (H2O2 malonyldialdehyde), was used as a measure of peroxidant stress. After 5 weeks, the plasma cholesterol concentrations in rats receiving 1.2% cholesterol + 0.6% cholic acid in their diet rose to 6-times that of control rats receiving a diet without added cholesterol; at the same time, erythrocyte H2O2 malonyldialdehyde in the cholesterol-fed rats decreased significantly relative to the control rats. During subsequent exposure of both groups to in vivo peroxidant stress with phenylhydrazine in two separate dose trials, erythrocyte peroxidant stress remained significantly lower in the cholesterol-fed rats: at a dose of 100 mumol/100 g body weight, H2O2 malonyldialdehyde was lower; at a dose of 25 mumol/100 g body weight, both endogenous and H2O2 malonyldialdehyde were lower. Erythrocyte membrane cholesterol concentrations were 12% higher in the cholesterol-fed rats than in controls. The effects of in vivo peroxidant stress on plasma cholesterol were also studied. In vivo peroxidant stress at the higher dose of phenylhydrazine produced a decrease in plasma cholesterol concentrations of control rats. The lower dose had no effect on this group and the plasma cholesterol concentrations were unchanged in the cholesterol-fed rats during both treatments. The data suggest that elevated plasma cholesterol concentrations are protective against erythrocyte peroxidant stress. The mechanism of cholesterol's protective effect is probably mediated through elevated membrane cholesterol concentrations.     

gamma-Glutamyltranspeptidase in dimethylbenz[a]anthracene-induced mammary adenocarcinomas and in livers of tumor bearing rats

A single dose of dimethylbenz[a]anthracene (DMBA) at 20 mg/kg resulted in 100% incidence of intraductal mammary adenocarcinomas in Wistar rats, the large tumors averaging 1.87 +/- 0.45 g. gamma-Glutamyltranspeptidase activities were elevated in DMBA-induced mammary adenocarcinomas relative to lactating mammary tissue in all fractions examined: 18.8-fold in homogenates; 22.1-fold in particulate fractions; and 5.7-fold in supernatant fractions. In DMBA-induced mammary adenocarcinomas, gamma-glutamyltranspeptidase was 95% particulate, 5% supernatant, whereas in lactating mammary tissue, gamma-glutamyltranspeptidase was equally distributed between particulate and supernatant fractions. Particulate gamma-glutamyltranspeptidase from DMBA-induced mammary adenocarcinomas as well as lactating mammary tissue displayed classical Michaelis-Menten characteristics: for the adenocarcinoma enzyme Km was 2.5 nM and Vmax 200 nmol mg-1 min-1; for mammary tissue enzyme Km was 2.5 nM and Vmax 11.1 nmol X mg-1 X min-1. Both particulate enzymes were activated at 50 degrees C relative to 37 degrees C to the same extent: 1.37-fold. The activities of gamma-glutamyltranspeptidase were increased 1.8-fold in the livers of rats bearing DMBA-induced mammary adenocarcinomas relative to age-matched controls. Plasma levels of gamma-glutamyltranspeptidase were also increased 1.6-fold in tumor bearing rats. There was no observable sign of liver damage in tumor bearing rats; plasma glutamic pyruvic transaminase levels were normal in these animals. Blood glucose levels were elevated 17% in rats bearing DMBA-induced mammary adenocarcinomas compared to age-matched controls, although plasma insulin levels were the same in both groups: 35.4 +/- 3.5 microIU/ml for the former; 31.9 +/- 3.1 microIU/ml for the latter.     

Lipogenic potential in liver of the preweanling rat: influence of dietary cholesterol

We examined the effect of a lower dietary cholesterol load on hepatic lipogenic capacity and plasma cholesterol concentrations during the normal suckling period in artificially reared preweanling rats. The artificially reared rats were fed a milk formula that contained low or normal concentrations of cholesterol during the period from the 5th to 17th day after birth. The activities of HMG-CoA synthase and HMG-CoA reductase in livers of 17-day-old rat pups reared on the low-cholesterol diet were enhanced three- to five-fold over those observed in the age-matched rats in the normal cholesterol and mother-reared control groups. The concentration of cholesterol in plasma of rats reared on the low-cholesterol milk was about 20% lower than that for mother-reared controls. In contrast, rats reared on milk with normal cholesterol content exhibited plasma cholesterol levels about 25 and 50% higher than the mother-reared and low cholesterol groups, respectively. The long-term metabolic consequences of rearing rats on milk formulations without adequate cholesterol remains to be determined.     

Effect of dietary lipids on plasma lipoproteins and fluidity of lymphoid cell membranes in normal and leukemic mice

Mice of the GR/A strain were fed four different isocaloric semipurified diets, enriched in either (1) saturated fatty acids (palm oil), or (2) polyunsaturated fatty acids (corn oil), or (3) palm oil plus cholesterol, or (4) a fat-poor diet containing only a minimal amount of essential fatty acids. We have studied the effects of these dietary lipids on the density profile and composition of the plasma lipoproteins and on the lipid composition and fluidity of (purified) lymphoid cell membranes in healthy mice and in mice bearing a transplanted lymphoid leukemia (GRSL). Tumor development in these mice occurred in the spleen and in ascites. While the fatty acid composition of the VLDL-triacylglycerols still strongly resembled the dietary lipids, the effects of the diets decreased in the order VLDL-triacylglycerols greater than HDL-phospholipids greater than plasma membrane phospholipids. Diet-induced differences in the latter fraction were virtually confined to the content of oleic acid and linoleic acid, and they were too small to affect the membrane fluidity, as measured by fluorescence polarization using the probe 1,6-diphenyl-1,3,5-hexatriene. Healthy mice were almost irresponsive to dietary cholesterol, but in the tumor bearers, where lipoprotein metabolism has been shown to be disturbed, the cholesterol diet caused a substantial increase in the low- and very-low density regions of both blood and ascites plasma lipoproteins. The cholesterol-rich diet also increased the cholesterol/phospholipid molar ratio and lipid structural order (decreased fluidity) in GRSL ascites cell membranes, but not in the splenic GRSL cell membranes. We conclude that the composition of plasma lipoproteins and cell membrane lipids in GR/A mice is subject to exquisite homeostatic control. However, in these low-responders to dietary lipids the development of an ascites tumor may lead to increased responsiveness to dietary cholesterol. The elevated level of membrane cholesterol thus obtained in GRSL ascites cells did not affect the expression of various cell surface antigens or tumor cell growth.     

Evidence that reverse cholesterol transport occurs in vivo and requires lecithin-cholesterol acyltransferase

The transport of cholesterol from extrahepatic tissues into plasma (reverse cholesterol transport) and the possible requirement for lecithin:cholesterol acyltransferase was examined in the rat. One hour after removal of the liver plasma cholesterol ester concentrations were significantly increased by 20%, whereas free cholesterol concentrations were unchanged. The lecithin:cholesterol acyltransferase inhibitor, 5,5′-dithiobis(2-nitrobenzoic acid) (DTNB) was administered to eviscerated rats. It inhibited plasma lecithin:cholesterol acyltransferase activity by 90% which in turn totally prevented the increase in plasma cholesterol ester concentrations. In addition, heat-inactivated plasma from DTNB-treated eviscerated rats was 50% more reactive toward a standard source of lecithin:cholesterol acyltransferase compared to plasma from control or untreated eviscerated rats. These data suggest that in the rat a reactive lecithin:cholesterol acyltransferase substrate is formed extrahepatically. Together with lecithin:cholesterol acyltransferase, this reactive substrate removes cholesterol from peripheral tissues.     

Dopamine receptors in rat pituitary and estradiol-induced pituitary tumor: effect of chronic treatment with bromocriptine

We have investigated dopamine (DA) receptors in estradiol-induced PRL-secreting pituitary tumors and intact pituitary tissue. Female rats were injected at 3-week intervals with 2 mg estradiol valerate (EV) or with diluent. After 21 weeks, adenomatous changes in the pituitary gland of EV-treated rats were seen and plasma PRL concentrations reached 2 micrograms/ml. Bromocriptine (2.5 mg/kg) was then administered for 1 month to half of the control rats and half of the rats bearing tumors. Anterior pituitary weight was increased in EV-treated rats compared to controls while the affinity and the density of DA receptors as assessed by [3H]spiperone binding remained unchanged. Bromocriptine (CB-154) induced a 70% decrease in the density of DA receptors without any change in affinity both in normal pituitaries and in tumors. Concurrently, the elevated plasma concentrations of PRL in the tumor bearing rats were decreased to control values following the CB-154 treatment. Our data suggest that rats with primary estrogen-induced PRL secreting tumors have normal pituitary DA receptors.     

Streptozotocin treatment of streptozotocin-induced islet cell adenomas in rats.

Hypoglycemic rats bearing insulin-secreting islet-cell adenomas produced by the combined action of streptozotocin and nicotinamide were treated with streptozotocin. Antitumor response was demonstrated by elevation of blood glucose, reduction in plasma and tumor IRI, and histopathologic changes in the beta-cell neoplasm. The rodent tumor model may serve as a predictive system for selection and investigation of mechanisms of action of future antitumor agents to be used in the treatment of malignant insulinoma in man.     

Production of platelet thromboxane A2 and arterial prostacyclin I2 from hypercholesterolemic rats.

The plasma cholesterol, plasma malonaldehyde (MDA), platelet thromboxane A2 (TXA2) and vascular prostacyclin (PGI2) were measured in male Sprague-Dawley rats fed diets supplemented with cholesterol (1%) and cholic acid (0.5%). For comparisons, measurements were made in rats fed normal diets. The concentration of cholesterol in the plasma of rats had reached a maximum in 1 week of feeding experimental diets. TXA2 production from collagen and thrombin stimulated platelets was significantly decreased in animals fed experimental diets for 1 week. The production of MDA in the plasma of animals fed experimental diets for 8 weeks was significantly lower compared to the animals fed normal diets. There was a small but significant reduction in the formation of PGI2 in rats fed experimental diets for 8 weeks. These data suggest that feeding cholesterol rich diets to rats alters the platelet membrane properties differently from human and rabbit. Furthermore, cholesterol feeding to rats had some damaging effect on the arterial PGI2 synthesis.     

Reduction in plasma cholesterol and increase in biliary cholesterol by a diet rich in n-3 fatty acids in the rat

Cholesterol and lipoprotein metabolism were investigated in a group of rats fed a fish oil-supplemented diet, a rich source of n-3 fatty acids. For comparison purposes, other groups of rats were fed either safflower oil (n-6 fatty acids) or coconut oil (saturated fatty acids). Diets were isocaloric and contained identical amounts of cholesterol. Rats fed fish oils for 2 weeks showed a 35% lower plasma cholesterol level than rats fed safflower oil, who in turn showed a 14% lower plasma cholesterol level than those fed coconut oil. The fall in plasma cholesterol level with fish oils was associated with significant falls in low density and high density lipoprotein cholesterol levels, but with no significant change in the ratio of low density to high density lipoprotein cholesterol. The fatty acid compositions of plasma, hepatic, and biliary lipids showed relative enrichment with n-3 fatty acids, reflecting the composition of the diet. The fish oil diet increased the basal secretion rate of cholesterol into bile, but the bile acid secretion rate remained unchanged. It is suggested that n-3 fatty acids reduce the plasma cholesterol level in rats by increasing the transfer of cholesterol into bile.     

Effects of epinephrine on plasma cholesterol levels in rats

The present study was undertaken to evaluate whether epinephrine increases plasma cholesterol in rats. Epinephrine suspended in sesame oil was subcutaneously administered at 21:00 hr (9 PM). Blood was drawn 12 hr later, and plasma cholesterol was shown to be increased by epinephrine in a dose-dependent manner (0.5-2.0 mg/kg). This epinephrine-induced hypercholesterolemia was enhanced by phentolamine (25 mg/kg) and inhibited by propranolol (25 mg/kg). Although the effect of epinephrine in normal rats was abolished by adrenalectomy, corticosterone (10 mg/kg) increased plasma cholesterol in both normal and adrenalectomized rats. These results demonstrate that epinephrine increases plasma cholesterol levels in rats, and that the effect of epinephrine appears to be mediated by the beta-adrenergic receptors, depending upon adequate amounts of corticosteroids.     

Role of the intestinal acyl-CoA:cholesterol acyltransferase activity in the hyperresponse of diabetic rats to dietary cholesterol.

Contrary to normal rats, diabetic rats are known to develop marked hypercholesterolemia when fed a cholesterol-enriched diet. The triggering factor involved in this hyperresponse has not been identified. With the aim of clarifying the role of the intestinal acyl-CoA:cholesterol acyltransferase (ACAT), we studied the effects of a high fat diet and the changes of intestinal ACAT activity during the early development of streptozotocin-diabetes in rats. Feeding diabetic rats with a diet enriched in cholesterol and saturated fat produced an increase in plasma and in tissue cholesterol as early as 3 days after streptozotocin injection in the absence of hyperphagia. Under these experimental conditions, treatment with insulin or with the ACAT inhibitor CL-277082 significantly reduced the plasma cholesterol to levels measured in nondiabetic rats fed the same high fat diet. An increase in [14C]cholesterol in plasma very low density lipoprotein was observed after oral administration of labeled cholesterol to 3-day diabetic rats. In parallel experiments, the direct measurement of small intestine microsomal ACAT activity revealed an increase, averaging 288% in diabetic rats 3 days after diabetes induction. This change in ACAT activity occurred simultaneously with an increase in plasma glucagon and was normalized by insulin treatment. The induction of intestinal ACAT activity in diabetic rats, its modulation by insulin, and the hypocholesterolemic effects of insulin or CL-277082 treatment clearly indicate that ACAT activity plays a major role in the initiation of diabetes-associated hypercholesterolemia.     

Opposite metabolic response to fenofibrate treatment in pregnant and virgin rats.

The level of maternal circulating triglycerides during late pregnancy has been correlated to newborns' weight in humans. To investigate the response to fenofibrate, a hypotriglyceridemic agent, in pregnant rats, 0, 100, or 200 mg of fenofibrate/kg body weight as oral doses were given twice a day from day 16 of gestation and studied at day 20. Virgin rats were studied in parallel. Liver weight was higher in pregnant than in virgin rats, and either dose of fenofibrate increased this variable in both groups. The highest dose of fenofibrate decreased fetal weight. Although plasma triglycerides decreased during the first 2 days of fenofibrate treatment in pregnant rats, the effect disappeared on day 3, and plasma triglycerides were even enhanced at day 4. In virgin rats, fenofibrate decreased plasma triglycerides throughout the experiment. Plasma cholesterol levels in pregnant rats decreased during the first 3 days of treatment, and the effect disappeared on day 4, whereas in virgin rats, values remained decreased. Changes in plasma triglycerides paralleled those of VLDL triglycerides. In pregnant rats, VLDL cholesterol levels increased while LDL cholesterol decreased with the treatment, whereas in virgin rats, cholesterol levels decreased in all lipoprotein fractions. Only in virgin rats did liver triglyceride concentration increase with fenofibrate treatment. Lumbar adipose tissue LPL was lower in pregnant than in virgin rats, and fenofibrate treatment decreased this variable in both groups. Maternal fenofibrate treatment increased fetal plasma and liver triglyceride and cholesterol concentrations.It is proposed that the opposite effects of fenofibrate treatment in virgin and pregnant rats are a consequence of both the enhanced liver capability for VLDL triglyceride production and a rebound response to the drug in the latter.     

Effects of phenotype, sex, and diet on plasma lipids in LA/N-cp rats

The LA/N-corpulent (cp) rat is a recently developed congenic strain which exhibits obesity. The effects of phenotype and sex on serum and lipoprotein lipid content were examined in LA/N-cp rats fed either a control or an atherogenic diet high in saturated fat and protein. Obese rats were pair-fed to equivalent lean animals. Results from this study indicate that sex, phenotype, and diet exert significant effects on plasma and lipoprotein cholesterol content. Plasma cholesterol levels were higher in obese compared with lean rats, females than in males, and rats consuming the atherogenic diet compared with the control diet. Plasma and lipoprotein triglyceride levels were significantly increased only in obese compared with lean animals. The increased plasma cholesterol and triglyceride was observed primarily in the chylomicron and very low density lipoprotein fractions. Increased levels of plasma cholesterol were not a result of increased dietary cholesterol absorption or increased liver cholesterol biosynthesis. These data suggest that LA/N-cp rats can serve as a unique rodent model for the study of the interrelationships between hyperlipidemia, obesity, and coronary heart disease.     

Cholesterol-rich diets have different effects on lipid peroxidation, cholesterol oxides, and antioxidant enzymes in rats and rabbits

The objective of this study was to compare the effect of cholesterol feeding of rats and rabbits. The levels of lipid peroxidation products and oxysterols in the plasma of the two species plus the antioxidant enzyme activities in the liver and erythrocytes were measured to explain their different susceptibilities to atherosclerosis. Our study showed that rats are less susceptible than are rabbits to the atherogenic effect of a cholesterol-rich diet because of differences in lipid peroxidation products as well as antioxidant enzymes activities in their livers. In rabbits, cholesterol feeding produced severe hypercholesterolemia (43-fold increase) and increased plasma and liver lipid peroxidation. Total as well as the individual oxysterol contents of 7alpha-, 7beta-hydroxycholesterol, alpha-epoxy, beta-epoxycholesterol, cholestanetriol, 7-keto, and 27-hydroxycholesterol significantly increased in the plasma of hypercholesterolemic (HC) rabbits. Erythrocyte glutathione peroxidase (GSH-Px) activity significantly decreased whereas catalase activity significantly increased in HC rabbits. In rats cholesterol feeding increased the plasma cholesterol only twofold and had no effect on plasma or liver lipid peroxidation. Only 7alpha- and 7beta-hydroxycholesterol increased and no change was observed in any of the antioxidant enzymes activity in the erythrocytes. Although cholesterol feeding caused a 10-fold increase of liver cholesterol as ester in both rats and rabbits, the antioxidant enzyme GSH-Px and catalase activities in the liver significantly increased in rats but significantly decreased in rabbits. The increase of GSH-Px and catalase activities in the liver of cholesterol fed rats could have a protective role against oxidation, thus preventing the formation of lipid peroxidation and oxysterols.     

Cholesterol metabolism in copper deficient rats

The influence of copper deficiency on the appearance of newly synthesized cholesterol in the plasma lipids of rats was examined following ³H mevalonate injection. At 181 days copper deficient rats exhibited a highly significant increase in plasma cholesterol concentration. Copper deficiency was associated with a greatly enhanced appearance of ³H in newly synthesized cholesterol and cholesteryl esters in the plasma lipids. A concomitant decrease in ³H incorporation into liver lipids was also observed. The results suggest that copper deficiency markedly influences the clearance of hepatic cholesterol to the plasma pool, and a highly significant correlation was observed between plasma copper concentrations and ³H incorporation into plasma cholesterol. The results are discussed in terms of a possible role for copper in lipoprotein metabolism, bile acid metabolism, and the uptake of cholesterol by extra-hepatic tissues.     

Lipoprotein profile during perchlorate toxicity

Perchlorate administration to rats for 45 days alters the lipoprotein profile in plasma. The levels of cholesterol, phospholipids and triglycerides in HDL, LDL and VLDL fractions are significantly increased in perchlorate-treated rats. Post-heparin lipolytic activity of plasma of sodium perchlorate-treated rats is decreased. The risk factor, i.e. the total cholesterol/HDL cholesterol, increases in the experimental animals, indicating that the treatment of rats with perchlorate may develop the susceptibility of the animals to cardiac heart disease.