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Department of Nutrition and Food Science, Wayne State University, Detroit, MI 48202 and * Department of Molecular Biology and Immunology, University of North Texas Health Science Center, Fort Worth, TX 76107
3To whom correspondence should be addressed. E-mail: tfungwe{at}sun.science.wayne.edu
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ABSTRACT |
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 TOP ABSTRACT MATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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KEY WORDS: • fatty acids • high density lipoproteins • lecithin:cholesterol acyltransferase (LCAT) • scavenger receptor B1 • dietary fat • apoA-1
Elevated plasma levels of HDL protect vessel walls against the development of atherosclerosis (1 ,2 ). Current evidence suggests that HDL facilitates reverse cholesterol transport (RCT), a process by which free cholesterol from the peripheral tissues, including the artery, is transported to the liver for removal from the body (3 ,4 ). Lecithin:cholesterol acyltransferase (LCAT),4 which circulates in association with HDL, is believed to play a central role in this RCT process. LCAT, by transesterification of free cholesterol accepted by nascent HDL, maintains a free cholesterol gradient between peripheral cells and the HDL particle surface and thus promotes efflux of free cholesterol from the tissues (4 ). This transesterification occurs preferentially on the surface of HDL, where the reaction is facilitated by apolipoprotein A-1 (apoA-1), the major apolipoprotein of HDL (3 ,4 ). The newly formed cholesteryl esters accumulate within the HDL core, leading to the formation of mature HDL.
The important role that LCAT plays in HDL metabolism has been established in both patients and animals with LCAT deficiency, as well as in animals overexpressing human LCAT. LCAT deficiency is associated with severely reduced concentrations of HDL and apoA-1, whereas transgenic animals overexpressing LCAT show markedly higher plasma HDL and apoA-1 levels (5 –7 ). At present, the molecular mechanism(s) that regulates the circulating concentrations of LCAT is not well understood. A number of studies point to a relationship of LCAT expression with the expression of apoA-1. For example, incubation of hepatocytes with cytokines or injection of endotoxin in rats simultaneously reduced the concentration of apoA-1 and LCAT activity (8 ,9 ). On the other hand, even though plasma apoA-1 concentration is drastically reduced in LCAT knockout mice, there is no reduction in hepatic apoA-1 mRNA (10 ), suggesting that the two genes may not be coordinately regulated.
In humans and in experimental animals fatty acids modulate plasma HDL levels. Diets rich in polyunsaturated fatty acids (PUFA), including fish oils, reduce HDL (11 –13 ), whereas diets rich in saturated fatty acids (SFA, C12:0, C16:0) raise HDL cholesterol (13 ,14 ). Oleic acid-enriched diets were shown either to have no significant effect (15 ) or to elevate the levels of HDL-C (16 ,17 ).
The mechanism(s) involved in the modulation of plasma HDL-C levels by dietary fatty acids is also not well understood. In addition to regulation at the level of synthesis and secretion, HDL-C levels could also be modulated by 1) the transfer of cholesteryl esters (CE) to triacylglycerol-rich lipoproteins via cholesterol ester transfer protein [CETP] (2 ,18 ,19 ), 2) the transfer to tissues via the selective uptake mediated by scavenger receptor B-1 (SR-B1) or 3) the uptake of the whole particle (20 –22 ). In rats that lack CETP, although HDL is taken up by a number of tissues, the liver is the major organ for both the removal and catabolism of HDL (23 ). Like the LDL-receptor (LDL-R) in humans (24 ), the SR-B1 receptor is also important in cholesterol regulation, especially in rats (20 ). To date, few studies have focused on the role of fatty acids on the expression of SR-B1, which has been shown to be an HDL receptor (25 ). Spady et al. (26 ) demonstrated that PUFA increased the expression of SR-B1 compared to SFA in hamsters. More recently, Loison and colleagues (27 ) suggested that myristic acid increases HDL-C concentration and hepatic SR-B1 expression in hamsters.
We previously reported that fatty acids modulate the expression of LCAT and apolipoprotein A-1, two major components of HDL involved in RCT (28 ). The purpose of the present study was to examine whether the observations made in vitro in primary rat hepatocytes also occurred in vivo in rats fed dietary fats enriched with the same fatty acids. Because hepatic SR-B1 receptors also play a role in modulating plasma HDL levels, the effect of these diets on the expression of hepatic SR-B1 receptors was also determined.
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MATERIALS AND METHODS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Semipurified diets (AIN 93M) were obtained from Dyets (Bethlehem, PA). Semisynthetic fats [triolein (TO) and tripalmitin (TP)] used to modify the diets were a generous donation by ABITEC Corporation (Columbus, OH). Purified menhaden fish oil (MO) was a kind donation by Zapata Protein (Reedville, VA). LCAT cDNA was generously provided by Dr. R. Taramelli of Milan, Italy. The antisense probe template for glyceraldehyde-3 phosphate dehydrogenase (GAPDH) was obtained from Ambion (Austin, TX) and was used to correct for RNA loading in addition to normalizing LCAT mRNA abundance. The procedure for labeling probes and measurement of LCAT mRNA abundance were previously reported (28 ). Antibody for rat apoA-1 was a gift provided by Dr. Roheim (LSU, New Orleans, LA). All other chemicals were obtained from Sigma Chemical Company (St. Louis, MO).
Animals and diets.
Male Sprague-Dawley (Harlan Sprague-Dawley, Indianapolis, IN) rats, weighing 
200 g, were used in this study. The rats were housed in suspended stainless steel cages (40 x 24 x 18 cm), in a temperature-controlled room (20–23°C) with a 12-h light/dark cycle. All rats were fed a pelleted commercial nonpurified diet (Purina Rodent Chow, #500; TMI Nutrition, St. Louis, MO) for 1 wk after arrival. They were then randomly assigned to AIN-93M (29
) diets (8/group), to which either triolein (92% oleic acid, maximum peroxide value 0.84 meq/kg), purified fish oil (menhaden, maximum peroxide value 10 meq/kg) or tripalmitin (98% palmitic acid, maximum peroxide value 0.48 meq/kg) was added (15 g/100 g diet). The compositions of the isocaloric, cholesterol-free experimental diets, all of which contained essential fatty acids (EFA) (5% soybean oil), were described previously (30
). Additions to the control diet were made at the expense of sucrose. Diets were prepared weekly by mixing in the respective fats and were stored refrigerated (4°C) while being used. The rats were given uninterrupted access to food and deionized water during the experimental period, and were weighed at weekly intervals. After 20 d, rats were anesthetized with Nembutal (50 mg/kg body wt) after overnight food deprivation and 3 h into the diurnal phase of the light cycle. Blood was collected via the inferior vena cava, and livers were harvested after perfusion through the portal vein with ice-cold saline. Serum was obtained by centrifugation (500 x g) at 4°C. Aliquots of serum were taken for the determination of serum lipids, LCAT activity and for the isolation of lipoproteins. Livers were weighed, rapidly frozen in liquid nitrogen and stored at -80°C before analyses. Protocols for animal use were approved by Institutional Animal Investigation Committees and the study was conducted in accordance with the Guide for the Care and Use of Laboratory Animals (31
).
Analyses of serum lipids and LCAT activity.
Serum lipoproteins were isolated within 24 h of obtaining the serum. Aliquots of serum were adjusted to a density of 1.3 kg/L using KBr and CaCl2 (1 mmol/L). Serum was then sequentially layered under NaCl-KBr salt solutions and the lipoprotein fractions were separated by density gradient ultracentrifugation, as described in detail previously (32 ,33 ). Total cholesterol (TC), triacylglycerol (TG) and phospholipids [pyridoxal (PL)] in serum in addition to individual lipoprotein fractions, including HDL (d = 1.059–1.23 kg/L) were determined using enzymatic reagents (TC and TG; DMA, Arlington, TX; PL, Wako Pure Chemicals, Richmond, VA) as described previously (30 ).
Because the serum LCAT activity as measured by using an exogenous substrate is highly correlated with LCAT mass determined by radioimmunoassay (34 ), the exogenous substrate method was used to determine the apparent levels of LCAT in serum. Substrate containing apoA-1, lecithin and cholesterol (including [3H]cholesterol) were prepared according to the method of Manabe et al. (35 ). To determine LCAT activity, 10 µL of the diluted (1:1) serum was added to 200 µL of the substrate and the mixture was incubated at 37°C for 1 h. The reaction was terminated by the addition of isopropanol. After separating the radiolabeled unesterified and esterified cholesterol by TLC, LCAT activity was calculated as previously described (28 ).
Immuno-analyses of serum apolipoprotein A-1.
Serum apoA-1 was determined by immunoprecipitation using antibodies specific to rat apoA-1 as described (28 ). Briefly, samples were incubated and immunoprecipitated overnight at 4°C in TBS buffer containing 1% Triton X-100, 5 g/L BSA, while also using 5 µL of polyclonal anti-rat apoA-1. Antibody-antigen complexes were recovered by adding 50 µL of protein A- Sepharose (50%, v/v) (Amersham-Pharmacia Biotech, Piscataway, NJ) and incubating while rotating for 2 h at 4°C. The protein A- Sepharose complex was pelleted using a tabletop refrigerated centrifuge and washed three times with TBS containing 1% Triton X-100. The pellets were boiled for 10 min in SDS- PAGE sample buffer and resolved in 10% SDS- PAGE using a Bio-Rad (Hercules, CA) PROTEAN II system. Known quantities of purified rat apoA-1 were also resolved on the gels and were used as standards to quantify the levels of apoA-1 based on the intensity of the dye absorption.
Immuno-analyses of hepatic scavenger receptor class B type 1 receptor protein.
Membranes for SR-B1 receptor protein mass determinations were carried out following published protocols (36 ,37 ). For this purpose, rat liver membranes were prepared by homogenizing 1 g of liver in 10 mL of 150 mmol/L NaCl, 1 mmol/L CaCl2, 10 mmol/L Tris- HCl (pH 7.5); and the protease inhibitors, 0.5 µmol/L leupeptin and 1 mmol/L phenylmethylsulfonyl fluoride, pH 7.5 (buffer A). A low speed supernatant was first prepared by centrifuging the homogenate at 500 x g for 5 min and then at 8000 x g for 15 min. The supernatant was centrifuged at 100,000 x g for 1 h and the newly formed supernatant was discarded. The pellet was suspended in 1 mL of buffer A by flushing 10 times through a 1-mL syringe with a 22-gauge needle. The dispersed pellet was transferred to a new ultracentrifuge tube, 4 mL of buffer A was added and the mixture was centrifuged at 100,000 x g for 60 min. The pellet was resuspended in 0.5 mL of 50 mmol/L NaCl, 1 mmol/L CaCl2, and 20 mmol/L Tris- HCl, pH 7.5. Protein concentrations were determined by the Bio-Rad dye-binding protein assay.
For SR-B1 receptor protein immunoblotting, 25 µg of solubilized liver membrane protein was subjected to protein electrophoresis using 4–12% precast polyacrylamide gradient gels under nonreducing conditions (Novex, San Diego, CA). Proteins were transferred onto nitrocellulose membranes and incubated overnight with a 1:2000 dilution of mouse SR-B1 antibody (Novus Biologicals, Littleton, CO) and visualized by chemiluminescence detection (Alpha Innotech, San Leandro, CA). Quantification of the specific luminescent protein bands was performed with Image-Quant software (Molecular Dynamics, Sunnyvale, CA).
Analyses of hepatic LCAT mRNA.
Poly (A+) RNA was isolated from the liver according to the Trizol reagent protocol (Invitrogen kit; Invitrogen, Carlsbad, CA). Northern blot analysis was performed to quantitate mRNA levels of LCAT. For each sample, 5 µg of RNA was electrophoresed through formaldehyde- 1.2% agarose gels and transferred to 20x SSC-equilibrated Hybond ECL nitrocellulose membranes. Membranes were hybridized after UV crosslinking and washed at a high stringency (65°C, 0.1x SSC). Northern blots were probed with labeled ([32P]dCTP, specific activity > 3000 Ci/mmol) rat LCAT cDNA, as previously described (28 ). The level of LCAT mRNA in liver samples was normalized to glyceraldehyde phosphate dehydrogenase (GADPH) mRNA levels for each sample.
Statistical analyses.
Values reported in the text are means ± SD. One-way ANOVA followed by Bonferroni/Dunn tests were performed to determine any differences (P < 0.05) among groups. Analyses were performed using the Stat View 4.5 statistical software (Abacus Concepts, Berkley, CA).
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RESULTS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Data on body weights and serum and lipoprotein lipids in rats fed the AIN 93M diet enriched with triolein (TO), tripalmitin (TP) or menhaden oil (MO) were presented in detail in our earlier study (30 ). Relevant data on HDL-cholesterol, phospholipids and additional new data on serum apoA-1 are presented in Table 1 . Serum HDL-cholesterol and phospholipid (PL) concentrations were significantly (P < 0.05) higher in rats fed TO and lower in rats fed MO than in rats fed TP. HDL-C as a percentage of total cholesterol was significantly higher (P < 0.05) in rats fed MO compared to those fed TO and TP, groups that did not differ. However, the percentage of cholesterol in the HDL subfraction with VHDL (d > 1.15 kg/L) was significantly (P < 0.05) lower in rats fed MO compared to those fed TO.
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17% higher (P < 0.05) in rats fed the TO diet compared to MO- and TP-fed rats. Apolipoprotein A-1 concentrations of rats fed MO and TP did not differ. Serum LCAT activity.
Serum LCAT activity [µmol CE/(L·h)] was significantly higher (P < 0.05) in rats fed TO compared with TP- or MO-fed rats (Fig. 1 ). Serum LCAT activity in rats fed MO tended to be lower (P > 0.05) than in rats fed TP. Although serum LCAT correlated with lipids associated with VLDL (r = 0.58, P < 0.05, n = 23) and HDL (r = 0.67, P < 0.05, n = 24), multiple linear regression analysis showed a significant relationship (r = 0.55, P < 0.05) of serum LCAT activity only with the phospholipids of VHDL (d > 1.15 kg/L).
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In rats fed the TO diet, hepatic LCAT mRNA levels were 1.5- to 2-fold higher (P < 0.05) compared to rats fed either the TP or the MO diet (Fig. 2 ). The latter two groups did not differ.
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The hepatic scavenger receptor B1 protein concentration was increased nearly 2.5-fold in rats fed the TO diet compared to rats fed either the TP or the MO diet (P < 0.05) (Fig. 3 ). Rats fed TP and MO did not differ.
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DISCUSSION |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Menhaden oil is an efficient triacylglycerol (VLDL)-lowering agent both in humans and in animal models (38 ,39 ). Its effect on HDL-C and apoA-1 levels, however, are not so clear or consistent. In our study, MO decreased HDL-C; however, HDL-C as a percentage of total cholesterol was higher in rats fed MO, suggesting that the observed effects of MO were secondary to an overall lowering of serum total cholesterol. Moreover, MO did not affect the serum apoA-1 concentration. Serum HDL originate from synthesis and secretion by the liver and also via the catabolism of circulating VLDL (4 ,40 ). Decreased levels of HDL lipids in rats fed MO may be related to the decreased secretion of VLDL, the precursor of plasma HDL, and not to the decreased synthesis and secretion of apoA-1 by the liver. Our data are consistent with published observations that have shown fish oil to reduce serum TG and HDL-C levels but without any effect on apoA-1 in both rats and humans (41 –43 ).
Despite an important role for LCAT in RCT, and in the regulation of serum HDL-C, such factors as diet, modulating its expression and transport, are not well understood. In the current study we observed that dietary oleic acid increased both serum LCAT and apoA-1 levels. Although a number of previous studies hypothesized that the expression of LCAT and apoA-1 may be coordinately regulated, more recent studies do not agree with these observations. In mice transgenic for the human apoA-1 gene, the high expression of human apoA-1 coincided with an increase in transcription of the LCAT gene with a parallel increase in levels of plasma LCAT (44 ). Conversely, in LCAT knockout mice in which the plasma apoA-1 concentration is drastically reduced, there is no reduction in hepatic apoA-1 mRNA (10 ), suggesting that both genes may not be coordinately regulated, and that if there is coordinate regulation it may occur only at a posttranscriptional level.
In accord with previous studies (45 ,46 ), strong correlations between serum LCAT and lipids of the various lipoprotein fractions were observed. Multiple regression analyses, however, suggested that when all lipoprotein lipids were taken into account, only the phospholipids of VHDL correspond with serum LCAT. Earlier studies showed that LCAT is associated predominantly with VHDL (47 ), a subfraction of HDL with high density (d > 1.15 kg/L). Therefore, an elevation in LCAT production, along with increased availability of apoA-1 and phospholipid containing VHDL, as previously shown (30 ), may be responsible for the increased levels of LCAT in plasma of rats fed TO. In this regard, VHDL cholesterol was significantly lower in rats fed MO, even when expressed as a percentage of total cholesterol.
Spady and colleagues (26
) suggested that the HDL-CE uptake pathway in the livers of mice and hamsters is saturated at normal HDL-C levels and, therefore, increasing plasma HDL-CE concentration to supernormal levels cannot increase HDL-CE delivery to the liver in these species unless SR-B1 expression is increased. Liver-specific overexpression of SR-B1 (10-fold) was shown to decrease the fatty streak development in heterozygous LDL receptor-deficient mice but not in human apoB transgenic mice fed atherogenic diets (48
). On the other hand, a 2-fold increase in the expression of SR-B1 inhibited fatty streak development without significantly reducing non-HDL-C levels, suggesting that SR-B1 can exert an anti-atherogenic effect (49
) at a moderate level of expression. Presently, the role of dietary fat in modulating the expression of SR-B1 is not clear. Although others (20
,26
) have shown that some (n-6) polyunsaturated fats increased the hepatic expression of SR-B1, in the current study, hepatic expression of SR-B1 tended to be lower in rats fed the MO diet than in those fed the TP diet (P = 0.05) and was stimulated nearly 2.5-fold in those fed the TO diet.
Besides the selective uptake of HDL-CE, SR-B1 also binds apoB-containing lipoproteins (50
,51
), and contributes to the lowering of non-HDL cholesterol (apoB-containing lipoprotein cholesterol) when overexpressed (48
,52
,53
). In contrast, a modest overexpression (2-fold) of SR-B1 had no major effect on the levels of non-HDL cholesterol (49
,54
). Our results agree with those of these earlier studies, in which there were no significant differences in the LDL cholesterol fraction in rats fed either the TP or the TO diet. Furthermore, non-HDL phospholipid levels were higher in rats fed the TO diet than in rats fed the TP diet (see Table 3 in ref. 30
).
Alam et al. (55 ) recently showed that upregulation of the individual steps in the RCT pathway by overexpressing LCAT, SR-B1 or apoA-1 had no major impact on the efflux of cholesterol from the extra-hepatic tissues. These investigators noted, however, that the efflux of cholesterol from the extra-hepatic tissues was stimulated upon infusion of the apoA-1-phospholipid complex (rHDL). The essential role of phospholipids, especially the apoA-1-phospholipid complex, in mobilizing tissue cholesterol was demonstrated earlier (56 ,57 ). We showed previously (30 ) that TO substantially increases the phospholipid levels of lipoproteins, including that of VHDL, the subfraction of HDL, which is the most efficient in removing cholesterol from tissues (58 ).
In summary, our data suggest that dietary triolein exerted a positive impact on the metabolism of plasma HDL compared to tripalmitin or menhaden oil. Furthermore, triolein significantly increased the expression of components essential to metabolism of HDL and RCT (phospholipids, apoA-1, LCAT and SR-B1). These data, along with our previous observation that triolein increases the activity of serum paraoxonase 1 (30 ), an antioxidant enzyme associated with HDL, suggest that triolein may be beneficial in reducing the risk of atherosclerosis.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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2 This work was previously published in part [Kudchodkar, B. J., Lacko, A. G., Dory, L. & Fungwe, T. V. (2000) Dietary fat modulates serum paraoxonase 1 activity in rats. J. Nutr. 130: 2427–2433].
4 Abbreviations used: apo, apolipoproteins; apo-1, apolipoprotein A-1; CE, cholesteryl ester; FC, free cholesterol; GADPH, glyceraldehyde phosphate dehydrogenase; LCAT, lecithin:cholesterol acyltransferase; MO, menhaden (fish) oil; PC, phosphatidylcholine; PL, phospholipids; PUFA, polyunsaturated fatty acids; RCT, reverse cholesterol transport; SFA, saturated fatty acids; SR-B1, scavenger receptor B1; TC, total cholesterol; TG, triacylglycerol; TO, triolein; TP, tripalmitin.
Manuscript received 16 August 2002. Initial review completed 16 September 2002. Revision accepted 22 November 2002.
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