Low density lipoprotein binding to monolayer cultures of hepatocytes isolated from hamsters fed different dietary fatty acids

Decreasing dietary fat saturation lowers HDL-cholesterol and increases hepatic HDL binding in hamsters

In order to study the mechanism by which increasing unsaturation of dietary fat lowers HDL-cholesterol levels, we studied various measures of HDL metabolism in hamsters fed with fats with different degrees of saturation. Hamsters were fed on a cholesterol-enriched (1 g/kg) semipurified diet containing 200 g/kg of maize oil, olive oil, or palm oil for 9 weeks. Increasing saturation of dietary fat resulted in increasing concentrations of total plasma cholesterol (4·29 (SD 0·51), 5·30 (sd 0·67) and 5·58 (sd 0·76) mmol/l respectively,n12) and HDL-cholesterol (3·31 (sd 0·50), 3·91 (sd 0·12) and 3·97 (sd 0·43) mmol/l) and these concentrations were significantly higher (P< 0·05) in the palm-oil and olive-oil-fed hamsters compared with the maize-oil group. Total plasma triacylglycerol levels also increased with increasing fat saturation (1·01 (sd 0·59), 1·56 (sd 0·65) and 2·75 (sd 1·03) mmol/l) and were significantly higher (P< 0·05) in the palm-oil group compared with the olive-oil and maize-oil-fed hamsters. The three diets did not have differential effects on plasma activity levels of lecithin: cholesterol acyltransferase (LCAT) and cholesteryl ester transfer protein (CETP). Levels of phospholipid transfer protein (PLTP) tended to be higher with increasing fat saturation but this effect was not significant. The capacity of liver membranes to bind human HDL3was significantly higher (P< 0·05) in the hamsters fed with maize oil (810 (sd 100) ng HDL3protein/mg membrane protein,n4) compared with those fed on palm oil (655 (sd 56) ng/mg), whereas the olive-oil group had intermediate values (674 (sd 26) ng/mg). The affinity of HDL3for the binding sites was not affected by the type of dietary fat. Hepatic lipase (EC3.1.1.3) activity, measured in liver homogenates, increased with increasing fat saturation. We conclude that dietary maize oil, when compared with either olive oil or palm oil, may lower HDL-cholesterol concentrations by enhancing HDL binding to liver membranes.

Dietary fatty acids regulate acyl-CoA:cholesterol acyltransferase and cytosolic cholesteryl ester hydrolase in hamsters

To investigate the effects of dietary fatty acids on acyl-CoA:cholesterol acyltransferase (ACAT) and cytosolic cholesteryl ester hydrolase (cCEH), male Syrian hamsters (F(1)B hybrid) were fed a modified version of the NIH-07 open formula, cereal-based rodent diet enriched with one of the following 4 dietary fatty acids: palmitic acid (16:0), trans fatty acids (18:1t), oleic acid (18:1c), or linoleic acid (18:2). Hamsters fed 16:0 and 18:1t had significantly higher plasma non-HDL cholesterol concentrations compared with those fed 18:1c and 18:2. However, differences in plasma apolipoprotein (apo)B(100) concentration, hepatic cCEH mRNA abundance, and hepatic ACAT activity between 16:0- and 18:1t-fed hamsters suggest that the hypercholesterolemic effects are achieved by different mechanisms. Specifically, an increase in ACAT activity by 16:0 may induce enrichment of cholesteryl esters in apoB(100)-containing particles, whereas 18:1t may increase the number of the particles. Hepatic cholesteryl esters accumulated in the 18:1c- and 18:2-fed groups with no differences in hepatic ACAT activity and cCEH mRNA abundance among hamsters fed unsaturated fatty acids (i.e., 18:1t, 18:1c, and 18:2). Considering the lack of change in free cholesterol concentration and increased cholesteryl esters in the liver, the hypocholesterolemic effect of 18:1c and 18:2 compared with 18:1t may be attributed to decreased production of apoB(100)-containing particles. ACAT-1 was expressed in all the tissues examined; in contrast, ACAT-2 was highly expressed in the liver and small intestine. Hepatic ACAT activity was disproportionate to the levels of ACAT-1 and ACAT-2 mRNA and protein, indicating post-transcriptional regulation of ACAT by dietary fatty acids. The data suggest that cholesterolemic effects of individual dietary fatty acids can be achieved through their independent modulation of pathways regulating assembly and secretion of apoB(100)-containing particles.

Role of hepatic lipase and scavenger receptor BI in clearing phospholipid/free cholesterol-rich lipoproteins in PLTP-deficient mice

Phospholipid transfer protein knock-out (PLTP0) mice have defective transfer of phospholipids (PL) from triglyceride-rich lipoproteins (TRL) into high-density lipoproteins (HDL). In this study, we examined the role of diet, hepatic lipase (HL) and scavenger receptor BI (SRBI) in determining the accumulation of excess PL and free cholesterol (FC, "surface remnants") in plasma of PLTP0 mice. PL and FC accumulated in the very low-density lipoprotein (VLDL)-LDL region of PLTP0 mice on a highly saturated, coconut oil-based diet, but not on chow or milk-fat based Western diets. Accumulation of PL and FC was dramatically increased in PLTP0/HL0 mice, compared to PLTP0 mice, but only on the coconut oil diet. Turnover studies indicated that the coconut oil diet was associated with delayed catabolism of PL of PL/FC-rich particles. Incubation of these particles with primary hepatocytes in the presence of SRBI neutralizing antibody indicated that SRBI was primarily responsible for removal of FC and PL on the Western diet. In hepatocytes of coconut oil-fed mice, removal of FC and PL from these particles by SRBI was markedly reduced, even though SRBI protein expression levels were unchanged. These studies indicate that HL and SRBI both have major role in the clearance of PL and FC of surface remnants in PLTP0 mice. SRBI appears to be dysfunctional in coconut oil diet-fed animals, possibly related to changes in hepatocyte membrane fatty acid composition.

Effect of ursodeoxycholic acid on hepatic LDL binding and uptake in dietary hypercholesterolemic hamsters

Administration of ursodeoxycholic acid (UDCA) has been shown to decrease serum total and low density lipoprotein (LDL) cholesterol in hypercholesterolemic patients with primary biliary cirrhosis. Results of previous studies prompted us to postulate that the cholesterol-lowering effect of UDCA may be due, at least in part, to a direct increment in hepatic LDL receptor binding [Bouscarel et al., Biochem J, 1991;280:589; Bouscarel et al., Lipids 1995;30:607]. The aim of the present investigation was to determine the ability of UDCA to enhance hepatocellular LDL receptor recruitment, as determined by its effect in vivo on LDL uptake, and its effect in vitro on LDL binding, under conditions of moderately elevated serum cholesterol. Study groups consisted of male golden Syrian hamsters fed either a standard chow diet (control), a 0.15% cholesterol-containing diet, or a 0.15% cholesterol-containing diet supplemented with either 0.1% UDCA, or 0.1% chenodeoxycholic acid (CDCA). Cholesterol feeding increased (P<0.01) total serum cholesterol by 44%, and was associated with a 10-fold accumulation of cholesteryl esters in the liver (P<0.01). In vivo, hepatic uptake of [U-(14)C]sucrose-labeled hamster LDL was increased (P<0.05) to a level of 454+/-101 microl in animals fed a cholesterol-containing diet supplemented with UDCA, compared to that either without UDCA (337+/-56 microl), or with CDCA (240+/-49 microl). The hepatic uptake of [U-(14)C]sucrose-labeled methylated human LDL, a marker of LDL receptor-independent LDL uptake, was unaffected by bile acid feeding. In vitro, specific binding of [125I]hamster LDL to isolated hepatocytes was determined at 4 degrees C, in presence and absence of 700 micromol/l UDCA. The K(D) ranged from 25 to 31 microg/ml, and was not affected by either cholesterol feeding or UDCA. In the presence of UDCA, the B(max) was increased by 19% (P<0.05) in cells isolated from control animals and by 29% (P<0.01) in cells isolated from hamsters fed a cholesterol-supplemented diet. In conclusion, in dietary hypercholesterolemic hamsters, both chronic in-vivo and acute in-vitro treatments with UDCA resulted in restoration of hepatic LDL binding and uptake to levels observed in control hamsters.

Dietary corn oil versus olive oil enhances HDL protein turnover and lowers HDL cholesterol levels in hamsters

We studied the effect of dietary olive and corn oil on high-density lipoprotein (HDL) metabolism in golden Syrian hamsters. The animals were fed a semipurified diet containing 0.1% cholesterol and 40 energy % in the form of either olive or corn oil for a period of nine weeks. Hamsters fed corn oil had significantly lower very-low density and low-density lipoprotein (VLDL+LDL) cholesterol concentrations than those fed olive oil (0.98+/-0.24 vs. 1.40+/-0.34 mmol/l, means+/-S.D., n = 12), as well as significantly lower HDL cholesterol concentrations (3.31+/-0.50 vs. 3.91+/-0.12 mmol/l). The binding capacity of 125I-labelled HDL to liver membranes was 33% higher in the hamsters fed corn oil instead of olive oil (571+/-29 vs. 429+/-24 ng HDL protein/mg membrane protein, P<0.05, n = 4). HDL protein kinetics were studied with 125I-HDL using a constant infusion technique. Both HDL fractional catabolic rate (0.255+/-0. 058 vs. 0.121+/-0.023 /h, P<0.01, n = 5) and transport rate (2.386+/-0. 753 vs. 1.218+/-0.101 mg/h, P<0.01, n = 5) were about 2-fold higher in the hamsters fed corn oil. The rate of plasma cholesterol esterification by lecithin: cholesterol acyltransferase (LCAT) was essentially the same for the two diets. It is concluded that the low HDL level in the hamsters fed corn oil diets is linked with increased HDL binding and degradation in the liver and possibly other tissues. Due to increased HDL protein turnover, the capacity for reverse cholesterol transport is increased in hamsters fed corn oil despite the relative low HDL concentrations

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.

Effects of dietary fatty acid composition on plasma cholesterol

It should be clear from the preceding sections that the effects of dietary fatty acids on plasma lipids get more complicated the more we try to simplify them! We have presented one argument as to how different fatty acids may interact to impact human plasma lipids. This is by no means an endorsement that ours is the only argument. Nevertheless, a strong case can be made for 14:0 and 18:2 as being the key players in this scenario. The role of palmitic acid seems to be the most controversial. While clearly certain studies do indeed reveal 16:0 to be hypercholesterolemic relative to 18:1, the data from studies suggesting that it behaves similarly to 18:1 are equally compelling. What is certain is that it is erroneous to assume that 16:0 is the major cholesterol-raising SFA simply because it is the most abundant SFA in the diet. Clearly, 18:0 cannot be considered cholesterol-elevating. One is therefore left with the 12-16C SFA. However, 12:0 and 14:0 are only of concern if diets contain palm-kernel, coconut oil or dairy products as major dietary constituents. Accordingly one is left with 16:0 and its response is highly dependent on the metabolic status as well as the age of the subjects being used. While "elderly" hypercholesterolemic humans clearly benefit from decreased 16:0 (and all SFA) consumption, "younger" normocholesterolemic subjects fail to show such clear-cut effects. Additionally, the concomitant levels of dietary cholesterol and 18:2 also have a major bearing on the cholesterolemic response of 16:0 As far as guidelines for the general public are concerned, clearly for people with TC > 225 and LDL-C > 130 mg/dl and/or those who are overweight (i.e. those percieved to be at high risk), the primary emphasis should clearly be on reducing total fat consumption. Decreasing saturated fat consumption will invariably also lower dietary cholesterol consumption. The latter manouver will generally lower TC and LDL-C. Whether the reduction occurs because of the removal of 14:0, or 16:0 and/or dietary cholesterol is a mute point, since most dietary guidelines advocate curtailing intake of animal and dairy products, which will result in reductions of all the SFA. It remains to be established whether life-long adherence to the above dietary guidelines in those subjects with normal cholesterol levels and an absence of the other conventional risk factors for CHD, will result in a subsequent decrease in CHD risk. In the latest NCEP report 39 million Americans were targeted as those who would benefit from reductions in LDL-C, principally by dietary means. This is indeed a very high number. But that leaves almost 220 million Americans! For them the age old recommendation to consume a moderate fat load, maintain ideal body weight and eat a varied and balanced diet would still appear to be the most powerful advice.