Regulation of guinea pig hepatic acyl-coa:cholesterol acyltransferase activity by dietary fat saturation and cholesterol

We measured the interactive effects of dietary cholesterol and fat on the regulation of hepatic acyl-CoA:cholesterol acyltransferase (ACAT) activity and its relationship to hepatic microsomal lipid composition in guinea pigs fed 15 g/100 g (w/w) fat diets (corn oil, olive oil, or lard) with 0.01, 0.08, 0.17, or 0.33 g/100 g (w/w) added cholesterol. Guinea pigs exhibited a dose dependent increase in hepatic microsomal ACAT activity, with increasing levels of cholesterol intake (P < 0.001) in all dietary fat groups. Animals fed monounsaturated olive oil had the highest hepatic ACAT activity with the exception of the 0.33 g/100 g cholesterol diet (P < 0.001). There were no differences in ACAT activity with intake of polyunsaturated corn oil or saturated lard. Dietary cholesterol resulted in increased microsomal free cholesterol (FC) concentrations in a dose dependent manner but had no effects on microsomal phosphatidylcholine (PC) concentrations. Guinea pigs fed olive oil generally had the highest microsomal FC/PC molar ratios, and hepatic ACAT activities correlated significantly with this parameter. After modification of the lipid compositions of the microsomes from guinea pigs fed the 12 test diets with FC/PC liposome treatment, microsomal ACAT activities remained significantly related to the microsomal FC/PC molar ratios, and dietary fat type did not affect this correlation. Our findings do not support the hypothesis that the stimulation of hepatic ACAT activity with cholesterol intake is enhanced by polyunsaturated fat intake. The data demonstrate that although dietary fat type and cholesterol amount have differential effects on hepatic ACAT activity, substrate availability, expressed as microsomal FC/PC molar ratio, is a major regulator of hepatic microsomal ACAT activity.     

Effects of saturated and unsaturated fats given with and without dietary cholesterol on hepatic cholesterol synthesis and hepatic lipid metabolism

Hepatic cholesterol synthesis was studied in rats after consuming diets of varying neutral lipid and cholesterol content. Cholesterol synthesis was evaluated by measuring 3-hydroxy-3-methylglutaryl-CoA reductase and by determining the rate of 3H-labeled sterol production from [3H]mevalonate. Results were correlated with sterol balance data and hepatic lipid content. Hepatic cholesterol synthesis was relatively great when cholesterol was excluded from the diet. The source of neutral dietary lipids, saturated vs. unsaturated, produced no change in hepatic sterol synthesis. Values for fecal sterol outputs and hepatic cholesterol levels were also similar in rats consuming either saturated or unsaturated fats. When 1% cholesterol was added to the diet, hepatic cholesterol synthesis was suppressed but the degree of suppression was greater in rats consuming unsaturated vs. saturated fats. This was associated with greater accumulation of cholesterol in livers from rats consuming unsaturates and a reduction in fecal neutral sterol output in this group as opposed to results from rats on saturated fats. Cholesterol consumption also altered the fatty acid composition of hepatic phospholipids producing decreases in the percentages of essential polyunsaturated fatty acids. It is concluded that dietary cholesterol alters cholesterol and fatty acid metabolism in the liver and that this effect is enhanced by dietary unsaturated fats.     

Whole body and tissue cholesterol turnover in the baboon

Cholesterol turnover was studied in four baboons by injecting [14C]cholesterol 186 days and [3H]cholesterol 4 days before necropsy, and fitting a two- or three-pool model to the resulting specific activity-time data. At necropsy, cholesterol mass and specific activity were determined for the total body (minus the central nervous system) and for many tissues. A pool model permits the estimation, from the plasma specific activity-time curve alone, of total body cholesterol within a limited range, depending upon the extent of side pool synthesis. The principal aim of this study was to estimate the extent of cholesterol synthesis in the side pools of the model, by computing the amount of side pool synthesis needed to equal the measured total body cholesterol. Central pool synthesis varied from 61 to 89% of the total cholesterol production rate. Thus, approximately 25% (11 to 39%) of the production rate arose from peripheral (pool 3 for the three-pool, and pool 2 for the two-pool model) cholesterol synthesis. Moreover, the finding that the measured total body cholesterol fell within the range obtained from the kinetic analysis by using reasonable assumptions (namely, that zero or that half the production rate occurred in the side pools), provides evidence for the physiological validity of the model. A second aim of this study was to explore cholesterol turnover in various tissues. A pool model predicts that rapidly turning over tissues will have higher specific activities at early times and lower specific activities at later times after injection of tracer relative to slowly turning over tissues, except where significant synthesis occurs. Tissues were ranked 1 to 17 for 3H and 17 to 1 for 14C cholesterol specific activity values. Except for the GI tract and testis, the tissues had similar ranks for both 3H and 14C, further validating model predictions. Results in all four baboons were similar. Turnover rates for the different tissues loosely fell into three groups which were turning over at fast, intermediate, and slow rates. Finally, the magnitude of variation of cholesterol specific activity was moderate for several distributed tissues (fat, muscle, arteries, and the alimentary tract), but was small for liver. Cholesterol turnover in serial biopsies of skin, muscle, and fat could, however, be fitted with a single pool to estimate tissue turnover rates.     

Bile acid synthesis and intracellular and extracellular cholesterol concentrations in isolated rat hepatocytes: the effect of dietary cholesterol

Bile acid synthesis in isolated hepatocytes prepared from rats given 1% cholesterol in the diet and incubated for 1 h in suspension was not increased compared to that in cells from control rats. When the hepatocytes were maintained in monolayer culture for 24 h, however, increased production of bile acid (X2.5) was observed in the cholesterol-fed group. The amount of bile acid synthesised during incubation in suspension was significantly correlated with intracellular unesterified cholesterol levels, but showed no correlation with intracellular esterified or medium cholesterol concentrations after 1 h. Bile acid production in hepatocytes maintained in monolayer culture was also significantly correlated with the intracellular unesterified, but not esterified, cholesterol content. In addition, in this case, there was a significant correlation with the levels of both unesterified and esterified cholesterol found in the medium after 24 h. These results suggest that the amount of cholesterol available to liver cells from extracellular sources has a role in the regulation of bile acid synthesis in cholesterol-fed rats, while the concentrations of esterified cholesterol stored within the cells are not important in this process.     

Effect of dietary fat on whole body fatty acid synthesis in weanling rats

The effect of dietary fat on body composition, whole body lipogenesis, and enzyme activity was measured in rats over the first 16 weeks post-weaning. Rats were fed either a low fat (5% w/w fat) or high fat (20% w/w fat) diet for the first 4 weeks. After this time all rats were fed the low fat diet. The results showed no significant effect of diet on the rate of fat synthesis over the first 8 weeks of the experiment. However, the activities of the enzymes of fatty acid synthesis [glucose 6-phosphate dehydrogenase, malic enzyme, adenosine triphosphate-citrate lyase, acetyl-coenzyme A carboxylase (ACCX), fatty acid synthetase] were dependent on the age and dietary status of the animals. The exact pattern depended on the specific enzyme and the tissue source. No significant differences in pyruvate dehydrogenase (PDH) activity were observed. Mathematical analysis of the enzyme activities suggested that ACCX and PDH were the most likely sites of fat synthesis regulation. In addition, an examination of body composition and overall weight retention showed that the "weight increasing" effect of a high fat diet could be completely reversed by subsequent feeding of a low fat diet. However, the reversal required an additional 12 weeks. Interestingly, at this time the rats switched from a high fat to a low fat diet had a lower body weight and lower body fat content than rats fed a low fat diet throughout the course of the experiment.     

Chronic dietary fat and cholesterol inhibit the normal postprandial stimulation of plasma cholesterol metabolism

The response of parameters of plasma cholesterol metabolism was studied in baboons adapted either to a low-fat, low-cholesterol diet or a high-fat, high-cholesterol diet. Animals adapted to the low-fat diet responded to a single low-fat or high-fat meal, as do normal humans, by a stimulation of cholesterol transport from blood cells to plasma, a stimulation of esterification of cholesterol, and a stimulation of cholesteryl ester transfer to very low and low density lipoproteins. While fasting rates of esterification and transfer increased as a result of diet-induced hypercholesterolemia, the postprandial response was reversed, so that postprandial metabolism was characterized by a movement of cholesterol from plasma to blood cells, an inhibition of cholesterol esterification, and a net transfer of cholesteryl esters from VLDL and LDL to HDL. These data indicate that the effects of postprandial lipemia on plasma cholesterol metabolism critically depend upon fasting plasma cholesterol levels.     

Effects of dietary cholesterol on the regulation of total body cholesterol in man

Studies on the interaction of cholesterol absorption, synthesis, and excretion were carried out in eight patients using sterol balance techniques. Absorption of dietary cholesterol was found to increase with intake; up to 1 g of cholesterol was absorbed in patients fed as much as 3 g per day. In most patients, increased absorption of cholesterol evoked two compensatory mechanisms: (a) increased reexcretion of cholesterol (but not of bile acids), and (b) decrease in total body synthesis. However, the amount of suppression in synthesis was extremely variable from one patient to another; one patient had no decrease in synthesis despite a large increment in absorption of dietary cholesterol, and two patients showed a complete suppression of synthesis. In the majority of cases the accumulation of cholesterol in body pools was small because of adequate compensation by reexcretion plus reduced synthesis, but in a few patients large accumulations occurred on high cholesterol diets when absorption exceeded the compensatory mechanisms. These accumulations were not necessarily reflected in plasma cholesterol levels; these increased only slightly or not at all.     

Reduced body fat and increased hepatic lipid synthesis in mice bearing interleukin-6-secreting tumor

Chronic secretion of interleukin-6 (IL-6) in mice causes metabolic alteration in the liver, leading to increased synthesis of hepatic cholesterol and fatty acids (FA). Mice were injected with allogeneic tumor cells transduced with the murine IL-6 gene. During the 3 wk after tumor inoculation, elevated serum IL-6 levels were associated with increased spleen and liver weight. Histological examination of sections from the liver showed increased hepatocyte proliferation, resulting in liver enlargement. Body composition analysis revealed that IL-6 caused a significant loss in fat tissue without affecting lean body mass and water content. Hepatic de novo synthesis of FA and cholesterol, as measured by (3)H(2)O incorporation, was three to five times as high in mice secreting IL-6 (IL-6 mice) as in pair-fed mice bearing nonsecreting tumors. This increase in FA and cholesterol synthesis is sufficient to maintain hepatic triglyceride secretion at levels comparable with those of pair-fed mice bearing nonsecreting tumors and, presumably, is the main source of cholesterol and FA-phospholipids necessary for hepatocyte proliferation.     

Perinatal development of hepatic cholesterol synthesis in the rat

Rates of cholesterol synthesis and HMG CoA reductase activity in rat liver, have been reported to be high before and low after birth. The timing of the decline in perinatal rates of cholesterol synthesis, however, is uncertain. These studies, therefore, determined in vivo rates of cholesterol synthesis using [3H]water and hepatic reductase activity in vitro in perinatal rats. The lipid composition of the plasma, liver and its microsomal subfraction were also determined. Reductase activity increased during late gestation, remained high immediately after birth, then decreased with the commencement of suckling. Rates of cholesterol synthesis increased from gestation day 18 to 20, but in contrast to reductase activity, decreased on the day before birth. Plasma cholesterol and triacylglycerol levels increased to gestation day 19, then decreased to term. By the 6th h after birth, plasma and liver cholesterol and triacylglycerol levels had increased markedly. By 48 h after birth, the high hepatic cholesterol content was associated with an increase in the cholesteryl ester fraction. The microsomal cholesterol/phospholipid molar ratio decreased from gestation day 16 until 12 h after birth, then increased markedly from 36 to 48 h. There was an apparent inverse relationship between the change in microsomal cholesterol/phospholipid molar ratio and the fatty acid unsaturation index from gestation day 16 to 36 h after birth. The results suggest that in late gestation and before suckling, the low in vivo rate of hepatic cholesterol synthesis may not be due to low activity of HMG CoA reductase.     

Fat and female fecundity: prospective study of effect of body fat distribution on conception rates.

OBJECTIVES--To study the effect of body fat distribution in women of reproductive age on fecundity. DESIGN--Prospective cohort study of all women who had entered a donor insemination programme. SETTING--One fertility clinic serving a large part of the midwest of the Netherlands. SUBJECTS--Of 542 women attending the clinic for artificial insemination for the first time, 500 women were eligible for study. MAIN OUTCOME MEASURES--Probability of conception per cycle and number of insemination cycles before pregnancy or stopping treatment. RESULTS--A 0.1 unit increase in waist-hip ratio led to a 30% decrease in probability of conception per cycle (hazard ratio 0.706; 95% confidence interval 0.562 to 0.887) after adjustment for age, fatness, reasons for artificial insemination, cycle length and regularity, smoking, and parity. Increasing age was significantly related to lower fecundity (p < 0.05); very lean and obese women were less likely to conceive (p < 0.10) as were women with subfertile partners (p < 0.10). All other exposure variables were not significantly related to fecundity. CONCLUSIONS--Increasing waist-hip ratio is negatively associated with the probability of conception per cycle, before and after adjustment for confounding factors. Body fat distribution in women of reproductive age seems to have more impact on fertility than age or obesity.