Replacing dietary glucose with fructose increases ChREBP activity and SREBP-1 protein in rat liver nucleus.

Diets high in fructose cause hypertriglyceridemia and insulin resistance in part due to simultaneous induction of gluconeogenic and lipogenic genes in liver. We investigated the mechanism underlying the unique pattern of gene induction by dietary fructose. Male Sprague-Dawley rats (n=6 per group) were meal-fed (4h/d) either 63% (w/w) glucose or 63% fructose diet. After two weeks, animals were killed at the end of the last meal. Nuclear SREBP-1 was 2.2 times higher in fructose-fed rats than glucose-fed rats. Nuclear FoxO1 was elevated 1.7 times in fructose group, but did not reach significance (P=0.08). Unexpectedly, no difference was observed in nuclear ChREBP between two groups. However, ChREBP DNA binding was 3.9x higher in fructose-fed animals without an increase in xylulose-5-phospate, a proposed ChREBP activator. In conclusion, the gene induction by dietary fructose is likely to be mediated in part by simultaneously increased ChREBP activity, SREBP-1 and possibly FoxO1 protein in nucleus.

Dietary fructose induces a wide range of genes with distinct shift in carbohydrate and lipid metabolism in fed and fasted rat liver.

Dietary fructose has been suspected to contribute to development of metabolic syndrome. However, underlying mechanisms of fructose effects are not well characterized. We investigated metabolic outcomes and hepatic expression of key regulatory genes upon fructose feeding under well defined conditions. Rats were fed a 63% (w/w) glucose or fructose diet for 4 h/day for 2 weeks, and were killed after feeding or 24-hour fasting. Liver glycogen was higher in the fructose-fed rats, indicating robust conversion of fructose to glycogen through gluconeogenesis despite simultaneous induction of genes for de novo lipogenesis and increased liver triglycerides. Fructose feeding increased mRNA of previously unidentified genes involved in macronutrient metabolism including fructokinase, aldolase B, phosphofructokinase-1, fructose-1,6-bisphosphatase and carbohydrate response element binding protein (ChREBP). Activity of glucose-6-phosphate dehydrogenase, a key enzyme for ChREBP activation, remained elevated in both fed and fasted fructose groups. In the fasted liver, the fructose group showed lower non-esterified fatty acids, triglycerides and microsomal triglyceride transfer protein mRNA, suggesting low VLDL synthesis even though plasma VLDL triglycerides were higher. In conclusion, fructose feeding induced a broader range of genes than previously identified with simultaneous increase in glycogen and triglycerides in liver. The induction may be in part mediated by ChREBP.

Phosphatidylcholine-rich acceptors, but not native HDL or its apolipoproteins, mobilize cholesterol from cholesterol-rich insoluble components of human atherosclerotic plaques.

To examine the potential of high density lipoproteins (HDL) to ameliorate atherosclerotic plaques in vivo, we examined the ability of native HDL, lipid-free HDL apolipoproteins (apo HDL), cholesterol-free discoidal reconstituted HDL (R-HDL) comprised of apo HDL and phosphatidylcholine (PC) and PC liposomes to release cholesterol from cholesterol-rich insoluble components of plaques (ICP) isolated from atherosclerotic human aorta. Isolated ICP had a free cholesterol (FC) to phospholipid (PL) mass ratio (0.8-3.1) and a sphingomyelin (SPM) to PC mass ratio (1.2-4.2) that exceeded those of plasma membranes of cultured cells. Surprisingly, native HDL and its apolipoproteins were not able to release cholesterol from ICP. However, R-HDL and PC liposomes were effectively released cholesterol from ICP. The release of ICP cholesterol by R-HDL was dose-dependent and accompanied by the transfer of > 8 x more PC in the reverse direction (i.e., from R-HDL to ICP), resulting in a marked enrichment of ICP with PC. Compared to R-HDL, PC liposomes were significantly less effective in releasing cholesterol from ICP but were somewhat more effective in enriching ICP with PC. Native HDL was minimally effective in enriching ICP with PC, but became effective after prior in vitro enrichment of HDL with PC from multilamellar PC liposomes. The enrichment of ICP with PC resulted in the dissolution of cholesterol crystals on ICP and allowed the removal of ICP cholesterol by apo HDL and plasma. Our study revealed that the removal of cholesterol from ICP in vivo will be possible through a change in the level, composition, and physical state of ICP lipids mediated by PC-enriched HDL.

Contribution of postprandial lipemia to the dietary fat-mediated changes in endogenous lipoprotein-cholesterol concentrations in humans.

BACKGROUND: Dietary fats alter LDL and HDL cholesterol while serving as precursors of postprandial triacylglycerol-rich lipoproteins (TRLs). OBJECTIVE: We hypothesized that the saturated fatty acid (SFA)-mediated increase and the polyunsaturated fatty acid (PUFA)-mediated decrease in endogenous lipoprotein cholesterol are promoted by postprandial TRLs. DESIGN: We performed a 16-d crossover diet study to examine the effect of PUFA-rich [ratio of PUFAs to SFAs (P:S) = 2.0] and SFA-rich (P:S = 0.25) diets on fasting and postprandial plasma lipid and lipoprotein-cholesterol concentrations in 16 normolipidemic subjects. RESULTS: Fasting plasma cholesterol decreased significantly after a PUFA-rich diet because of a decrease in LDL (-12.3%; P < 0.05) and HDL (-3.8%; NS), but did not change after an SFA-rich diet. The appearance of postprandial TRLs in plasma at 4 h was linked to a significant lowering of both LDL (-7.4%) and HDL (-4.8%) after a PUFA-rich diet; no such effect was observed after the SFA-rich diet. At 7 h, LDL and HDL cholesterol returned to near fasting concentrations without postprandial TRL accumulation after a PUFA-rich diet but with a significant postprandial TRL accumulation after an SFA-rich diet. Thus, the in vivo postprandial clearance of cholesterol in LDL+HDL was greater after a PUFA-rich diet than after an SFA-rich diet. The appearance of postprandial TRLs in plasma increased the cholesteryl ester transfer protein-mediated transfer of cholesteryl ester from LDL+HDL to TRLs in vitro without a significant influence from dietary fat. CONCLUSION: Dietary fat-mediated alterations in the rate of hepatic removal of postprandial TRLs, which carry cholesterol accepted from LDL+HDL via cholesteryl ester transfer protein in vivo, may contribute to the dietary fat-mediated change in endogenous lipoprotein cholesterol.

Postprandial chylomicrons: potent vehicles for transporting cholesterol from endogenous LDL+HDL and cell membranes to the liver via LCAT and CETP.

We examined whether postprandial (PP) chylomicrons (CMs) can serve as vehicles for transporting cholesterol from endogenous cholesterol-rich lipoprotein (LDL+HDL) fractions and cell membranes to the liver via lecithin:cholesterol acyltransferase (LCAT) and cholesteryl ester transfer protein (CETP) activities. During incubation of fresh fasting and PP plasma containing [(3)H]cholesteryl ester (CE)-labeled LDL+HDL, both CMs and VLDL served as acceptors of [(3)H]CE or cholesterol from LDL+HDL. The presence of CMs in PP plasma suppressed the ability of VLDL to accept [(3)H]CE from LDL+HDL. In reconstituted plasma containing an equivalent amount of triglycerides from isolated VLDL or CMs, a CM particle was about 40 times more potent than a VLDL particle in accepting [(3)H]CE or cholesterol from LDL+HDLs. When incubated with red blood cells (RBCs) as a source for cell membrane cholesterol, the cholesterol content of CMs, VLDL, LDL, and HDL in PP plasma increased by 485%, 74%, 13%, and 30%, respectively, via LCAT and CETP activities. The presence of CMs in plasma suppressed the ability of endogenous lipoproteins to accept cholesterol from RBCs. Our data suggest that PP CMs may play an important role in promoting reverse cholesterol transport in vivo by serving as the preferred ultimate vehicle for transporting cholesterol released from cell membranes to the liver via LCAT and CETP.

Alcohol-mediated enhancement of postprandial lipemia: a contributing factor to an increase in plasma HDL and a decrease in risk of cardiovascular disease.

BACKGROUND: Moderate alcohol consumption increases plasma HDL and lowers cardiovascular disease risk while transiently enhancing postprandial lipemia. OBJECTIVE: We hypothesized that the alcohol-mediated increase in postprandial triacylglycerol-rich lipoproteins (TRLs) and their clearance elevate HDL cholesterol and reverse cholesterol transport. DESIGN: We determined the effect in normolipidemic humans (n = 14) of postprandial lipemia produced 4 h after a test meal (M) or a test meal + 0.5 g alcohol/kg body wt (M+A) on postprandial changes in plasma lipids and on the balance of cholesterol between TRL and the cholesterol-rich LDL and HDL fractions (CRL) or red blood cells (RBCs) in fresh and incubated plasma or blood. RESULTS: Postprandial lipemia after the M and M+A test meals caused a 56% and 89% increase in plasma triacylglycerol, a 30% and 74% increase in TRL cholesterol, and a 3.8% and 6.6% decrease in CRL cholesterol, respectively. In vitro reaction of endogenous lecithin:cholesterol acyltransferase (EC 2.3.1.43) and cholesteryl ester transfer proteins via incubation of fasting plasma samples and postprandial M and M+A plasma samples for 16 h increased TRL cholesterol by 22.8% (0.08 mmol/L), 32.6% (0.16 mmol/L), and 45.8% (0.28 mmol/L) in plasma and by 71.1% (0.27 mmol/L), 89.4% (0.45 mmol/L), and 112.5% (0.70 mmol/L) in RBC-enriched blood, respectively. After the in vitro lipolysis of TRL, the elevation of HDL cholesterol in postprandial M+A plasma, but not in postprandial M plasma, was significantly greater than in fasting plasma. CONCLUSION: The alcohol-mediated increase in postprandial TRL flux and the hepatic removal of postprandial TRL after the acceptance of cholesterol from CRL and cell membranes contribute to increased HDL cholesterol and enhancement of reverse cholesterol transport in humans.

Oleic, linoleic and linolenic acids enhance receptor-mediated uptake of low density lipoproteins in Hep-G2 cells.

In the present study, the binding, internalization and degradation of low-density lipoprotein (LDL) was investigated in Hep-G2 cells treated with 18:0, 18:1, 18:2 and 18:3. In non-treated control cells, the surface binding (heparin-releasable) of 125I-LDL progressed in a saturable manner reaching equilibrium within 2 h, amounting 24.0 +/- 1.1, 29.5 +/- 1.3 and 31.4 +/- 2.8 (ng/mg cell protein) at 1, 2 and 4 h, respectively. The cells rapidly internalized 125I-LDL reaching a plateau at 2 h (72.4 +/- 6.3/1 h, 96.7 +/- 4.3/2 h and 100.8 +/- 4.6 ng/mg protein/4 h, respectively). The degradation of internalized LDL progressed slowly during the first hour of incubation reflecting the time required to an uptake and delivery of LDL to the cellular lysosomes. The levels of degraded LDL discharged into the medium then increased rapidly in a linear manner after the initial lag period, amounting 16.8 +/- 1.2, 51.8 +/- 7.0 and 118.2 +/- 5.7 ng/mg protein at 1, 2 and 4 h, respectively. The treatment of cells with of 1.0 mM of fatty acids for 4 h resulted in a significant increase in the surface binding of 125I-LDL compared to the control (34.9 +/- 3.0), but it was significantly lower in cells exposed to 18:0 (48.2 +/- 2.0) than to 18:1 (56.8 +/- 5.1), 18:2 (56.0 +/- 3.5) and 18:3 (57.8 +/- 6.0 ng/mg protein/4 h) (P < 0.05). The levels of degraded LDL in cells remained nearly the same regardless of fatty acid treatments, but degraded LDL levels in the medium were much higher in cells exposed to 18:1 (167.6 +/- 10.1), 18:2 (159.8 +/- 7.7) and 18:3 (165.1 +/- 14.7) than to 18:0 (142.1 +/- 8.4) and the control (121.2 +/- 3.4 ng/mg protein/4 h) (P < 0.05). The present finding that 18:1 is equally effective in enhancing the receptor-mediated LDL uptake and its degradation as those of 18:2 and 18:3 suggests that the major action of 18:1 in lowering LDL-cholesterol levels also involves an increased clearance of LDL via hepatic LDL-receptors.