Estrogens induce low-density lipoprotein receptor activity and decrease intracellular cholesterol in human hepatoma cell line Hep G2

[Hormone replacement therapy in peri- and postmenopause]

Peri- and postmenopausal disorders can have a significant impact on quality of life. Hormone replacement therapy (HRT) might be necessary in order to decrease women's symptoms. The German S3 guideline "Peri- and Postmenopause-Diagnostics and Therapy" (2020) provides recommendations that include the most recent evidence as well as the Women's Health Initiative (WHI) study results from 2002 and 2004. These results led to reduced prescription patterns due to a high risk of cardiovascular diseases as well as an increased risk for breast cancer if HRT had been administered. Both ongoing analyses of subgroups and other studies extenuated the WHI data, since the increased risks were neither generalizable to the typical postmenopausal patient (regarding age and risk profile) nor to the medication being used today. This article summarizes all aspects of HRT in peri- and postmenopausal women (indications, contraindications, practical approaches, risks, prevention) and provides recommendations with respect to the most recent S3 guideline.

Low-Density Lipoprotein Receptor Deficiency Attenuates Neuroinflammation through the Induction of Apolipoprotein E

Objective

We aimed to determine the role of the low-density lipoprotein receptor (LDLr) in neuroinflammation by inducing experimental autoimmune encephalomyelitis (EAE) in ldlr knock out mice.

Methods

MOG_35-55 induced EAE in male and female ldlr^-/- mice was assessed clinically and histopathologically. Expression of inflammatory mediators and apolipoprotein E (apoE) was investigated by qPCR. Changes in protein levels of apoE and tumor necrosis factor alpha (TNFα) were validated by western blot and ELISA, respectively.

Results

Ldlr^-/--attenuated EAE disease severity in female, but not in male, EAE mice marked by a reduced proinflammatory cytokine production in the central nervous system of female ldlr^-/- mice. Macrophages from female ldlr^-/- mice showed a similar decrease in proinflammatory mediators, an impaired capacity to phagocytose myelin and enhanced secretion of the anti-inflammatory apoE. Interestingly, apoE/ldlr double knock out abrogated the beneficial effect of ldlr depletion in EAE.

Conclusion

Collectively, we show that ldlr^-/- reduces EAE disease severity in female but not in male EAE mice, and that this can be explained by increased levels of apoE in female ldlr^-/- mice. Although the reason for the observed sexual dimorphism remains unclear, our findings show that LDLr and associated apoE levels are involved in neuroinflammatory processes.

Role of Estrogens in the Regulation of Liver Lipid Metabolism

Before menopause, women are protected from atherosclerotic heart disease associated with obesity relative to men. Sex hormones have been proposed as a mechanism that differentiates this risk. In this review, we discuss the literature around how the endogenous sex hormones and hormone treatment approaches after menopause regulate fatty acid, triglyceride, and cholesterol metabolism to influence cardiovascular risk.The important regulatory functions of estrogen signaling pathways with regard to lipid metabolism have been in part obscured by clinical trials with hormone treatment of women after menopause, due to different formulations, routes of delivery, and pairings with progestins. Oral hormone treatment with several estrogen preparations increases VLDL triglyceride production. Progestins oppose this effect by stimulating VLDL clearance in both humans and animals. Transdermal estradiol preparations do not increase VLDL production or serum triglycerides.Many aspects of sex differences in atherosclerotic heart disease risk are influenced by the distributed actions of estrogens in the muscle, adipose, and liver. In humans, 17β-estradiol (E2) is the predominant circulating estrogen and signals through estrogen receptor alpha (ERα), estrogen receptor beta (ERβ), and G-protein-coupled estrogen receptor (GPER). Over 1000 human liver genes display a sex bias in their expression, and the top biological pathways are in lipid metabolism and genes related to cardiovascular disease. Many of these genes display variation depending on estrus cycling in the mouse. Future directions will likely rely on targeting estrogens to specific tissues or specific aspects of the signaling pathways in order to recapitulate the protective physiology of premenopause therapeutically after menopause.

Sex Hormone Decline and Amyloid β Synthesis, Transport and Clearance in the Brain

Sex hormones (SH) are essential regulators of the central nervous system. The decline in SH levels along with ageing may contribute to compromised neuroprotection and set the grounds for neurodegeneration and cognitive impairments. In Alzheimer's disease, besides other pathological features, there is an imbalance between amyloid β (Aβ) production and clearance, leading to its accumulation in the brain of older subjects. Aβ accumulation is a primary cause for brain inflammation and degeneration, as well as concomitant cognitive decline. There is mounting evidence that SH modulate Aβ production, transport and clearance. Importantly, SH regulate most of the molecules involved in the amyloidogenic pathway, their transport across brain barriers for elimination, and their degradation in the brain interstitial fluid. This review brings together data on the regulation of Aβ production, metabolism, degradation and clearance by SH.

LDL receptor deficiency results in decreased cell proliferation and presynaptic bouton density in the murine hippocampus

An aberrant cholesterol metabolism in the brain may contribute to the pathogenesis of Alzheimer's disease (AD). The LDL receptor (LDLR) regulates plasma cholesterol levels and recently we and others obtained evidence that it is also involved in regulating brain cholesterol homeostasis. Moreover, we found that LDLR-deficient mice display impaired spatial memory. Because cholesterol, in part derived from cellular uptake via LDLR, is required for peripheral cell proliferation and growth, we examined the effect of absence of the LDLR on hippocampal proliferation and the density of synaptic connections. Mice deficient for the LDLR displayed a reduced number of proliferating (BrdU-labeled) cells in the hippocampus as compared to wild type control mice. In addition, the number of synaptophysin-immunoreactive presynaptic boutons in the hippocampal CA1 and the dentate gyrus (DG) areas, but not in cortical areas, was lower in the LDLR-knockout mice than in the control mice. In vitro experiments showed that LDLR activity is increased when cell growth is enhanced by the addition of N2 supplement. This further supports a role for the LDLR in the outgrowth of neurites. These findings support the notion that, similar to its role in the periphery, the LDLR is important for the cellular uptake of cholesterol in the brain and that disturbance of this process affects neuronal plasticity.

Identification of two herbal compounds with potential cholesterol-lowering activity

Low-density lipoprotein receptor (LDLR) plays a pivotal role in the control of plasma LDL-cholesterol level. This occurs predominantly at the transcriptional level through two gene regulation elements, named SRE: sterol-responsive element and SIRE: sterol-independent responsive element. We have developed a high-throughput screening using LDLR promoter activation-based assay to search for cholesterol-lowering compounds from a Chinese herb-based natural compound library. With this approach, we identified two compounds, named Daphnetoxin and Gniditrin, from Chinese herb Daphne giraldii Nitsche, which could activate LDLR promoter. Characterization of these compounds showed that they increased the level of LDLR mRNA and consequently up-regulate LDLR expression. The structures of these compounds are different from well-known LDLR promoter activating compounds such as GW707. The results suggested that these herbal compounds could represent good candidates for development of new classes of cholesterol-lowering drugs.

Regulation of Low-Density Lipoprotein Receptor Activity by Estrogens and Phytoestrogens in a HepG2 Cell Model

BACKGROUND/AIMS

Estrogen treatment is thought to lower low-density lipoprotein (LDL) cholesterol levels by increasing clearance through hepatic LDL receptors. This study aimed to determine the effect of estrogens and phytoestrogens on LDL receptor activity in a human hepatoma cell line, HepG2.

METHODS

HepG2 cells in culture were incubated for 24 h with estrogen or phytoestrogen and LDL receptor activity was measured by examining the cellular binding of colloidal gold-labelled LDL.

RESULTS

17Beta-estradiol significantly increased LDL receptor activity whereas estriol had negligible effects. Incubation with the isoflavonoids, formononetin, biochanin A and daidzein, caused significant elevations in receptor activity at concentrations above 40 microM. Coumestrol, a coumestan with a high level of estrogenic activity, caused a 3-fold increase in receptor activity at a concentration of 50 microM. Of the phytoestrogenic mammalian lignans enterolactone and enterodiol, only enterolactone displayed the ability to significantly upregulate LDL receptor activity at 50 microM.

CONCLUSION

This study suggests that the LDL receptor-stimulating effect of natural estrogens is mainly due to estradiol and that the cholesterol-lowering effect of diets high in phytoestrogens may be due in part to their ability to increase hepatic LDL receptor activity.

The low-density lipoprotein receptor is regulated by estrogen and forms a functional complex with the estrogen-regulated protein ezrin in pituitary GH3 somatolactotropes

Estrogen regulates the function, growth, and proliferation of lactotropes in the pituitary. We report here that low-density lipoprotein (LDL) receptor (LDLR) gene expression and LDL uptake are strongly up-regulated by estrogen in pituitary somatolactotropic GH(3) cells. The uptake of LDL was significantly inhibited by the F-actin-severing drug, swinholide A, indicating that LDL uptake is dependent on the integrity of the cortical actin cytoskeleton in GH(3) cells. We examined whether the estrogen-inducible cytoskeletal linker protein, ezrin, interacts with the LDLR. The LDLR coimmunoprecipitated with ezrin, and fluorescently labeled LDL bound to regions of the cell membrane that colocalized with the active, phosphorylated form of ezrin (phosphoezrin). Evidence for a functional interaction between ezrin and the LDLR was obtained by transient transfection experiments using ezrin-green fluorescent protein (GFP) expression constructs. We observed that transient transfection of GH(3) cells with an ezrin N terminus-GFP dominant-negative construct prevented the uptake of LDL particles, whereas expression of GFP alone or an ezrin C terminus-GFP construct had no effect on LDL uptake. Transfection with the ezrin N terminus dominant- negative construct had no effect on the endocytosis of transferrin. Thus, estrogen stimulates the expression of ezrin and the LDLR in GH(3) cells, which interact physically and functionally to facilitate the endocytosis of LDL. We propose that the up-regulation and interaction of ezrin and the LDLR serves to augment the delivery of cholesterol and other lipids in support of the hypertrophic and proliferative response of cells to estrogen.

The effect of estrogens and phytoestrogenic lignans on macrophage uptake of atherogenic lipoproteins

The present study examined the effect of estrogens and compounds with estrogenic activity on the uptake of atherogenic lipoproteins into macrophages, thought to be the initiating step in the development of atherosclerotic lesions. Isolated low density lipoprotein (LDL) and lipoprotein(a) (Lp(a)) were radiolabelled with (3)H-cholesterol linoleate, and incubated with J774 macrophages for 24 hours in the presence of pharmacological doses of estrogens and phytoestrogens. At a concentration of 0.1 microM, the estrogen 17beta-estradiol significantly reduced LDL uptake by macrophages by 14% (p < 0.05), but estrone did not have any effect. At 10 microM, both estrogens significantly reduced macrophage LDL uptake, but the phytoestrogenic-lignans enterodiol and enterolactone had no effect on LDL uptake. Lp(a) uptake into cells was significantly reduced by both estrone and estradiol, and by enterolactone and enterodiol at concentrations of 10 microM (p < 0.01), with enterodiol being most effective. The results of this study suggest that the uptake of these structurally similar lipoproteins is regulated differently. Macrophage Lp(a) uptake appears more phytoestrogen sensitive than does LDL uptake.

Synergistic activation of human LDL receptor expression by SCAP ligand and cytokine oncostatin M

OBJECTIVE

A recent study identified a new class of compounds designated as the sterol-regulatory element binding protein (SREBP) cleavage-activating protein (SCAP) ligands that putatively bind to SCAP, leading to increased LDL receptor (LDLR) expression. In this study, we examined the effects of SCAP ligand GW707 in comparison with lovastatin and cytokine oncostatin M (OM) on the regulation of LDLR expression in cultured HepG2 cells.

METHODS AND RESULTS

Our studies uncovered several new features that distinguish SCAP ligand from lovastatin, a classic 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor, and from OM, which utilize an SREBP-independent regulatory pathway. We show that the induction of LDLR mRNA expression by GW707 is not affected by intracellular cholesterol but is completely abolished by blocking de novo protein synthesis. Moreover, the effects of GW707 but not lovastatin on LDLR promoter activity, mRNA expression, and uptake of 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanin perchlorate-LDL are markedly enhanced by OM. We further demonstrate that the amounts of the mature form of SREBP-2 translocated to the nucleus under GW707 treatment are increased by costimulating cells with OM.

CONCLUSIONS

Our studies provide the first evidence that higher levels of LDLR expression and function can be achieved through simultaneous stimulation of the SREBP-dependent and SREBP-independent pathways, suggesting a strategy to develop an adjunct therapeutic intervention utilizing both pathways.

Anti-atherosclerotic effects of tamoxifen in cholesterol-fed ovariectomized rabbits

It has been indicated that the anti-estrogen agent, tamoxifen, developed for the treatment of breast cancer, may act on the vascular system as an estrogen agonist. However, to our knowledge few reports suggest that tamoxifen exerts anti-atherogenic actions. In the present study, we evaluated the anti-atherosclerotic effects of tamoxifen in ovariectomized cholesterol-fed rabbits. Ovariectomized rabbits were fed a 1% cholesterol diet and divided into 4 groups: control group (C, n=5); estrogen treatment (E, n=6); low-dose tamoxifen treatment (0.5 mg/kg) (LT, n=6); and high-dose tamoxifen (1.0 mg/kg) (HT, n=7). After 6 weeks, both Oil red O-positive areas on the intimal surfaces of aortae and the ratios of intimal to medial areas (I/M ratios) measured from cross-sections of aortae were significantly lower in groups E, LT and HT compared with group C. Although there were no significant differences in serum NOx (NO2 and NO3) levels among the 4 groups, NOx levels were slightly higher in groups E, LT and HT than group C. Acetylcholine (ACh) was administered to all animals, and the responses of ear arteriole diameters were compared among the 4 groups. While ear arteriole diameters were significantly decreased in group C, no significant changes were observed in groups E, LT or HT following ACh administration. Ratios of ear arteriole diameters after to before ACh administration were significantly greater in groups E, LT and HT compared to group C. These findings suggest that tamoxifen exerts anti-atherosclerotic effects, and that these effects are attributed to the maintenance of vascular endothelial function.

Influence of age and ovariectomy on the hypocholesterolemic effects of dietary taurine in rats fed a cholesterol-free diet

The effects of taurine feeding on plasma cholesterol concentrations and fecal bile acid excretion were examined in young and aged male and female rats (5 weeks and 10 months old respectively), the latter either ovariectomized (ovx) or sham-operated. The rats were fed a cholesterol-free diet (C diet) or a cholesterol-free taurine-supplemented diet (T diet; C + 5% taurine) for 28 days. In males, plasma cholesterol concentrations and fecal bile acid excretion were higher and lower, respectively, in aged rats than in young rats, but were not affected by feeding with the T diet. In female rats, plasma cholesterol concentrations were higher in aged rats than in young rats and higher in ovx-rats than in sham-operated rats. In contrast to male rats, plasma cholesterol concentrations were lower in female rats fed the T diet than in those fed the C diet. Plasma cholesterol concentrations were increased in aged ovx-rats, but decreased by feeding with the T diet. Fecal bile acid excretion was higher in rats fed the T diet than in those fed the C diet. Thus, these data indicate that the hypocholesterolemic effect of taurine is greater in aged rats than in young rats.

Requirement of Sp1 and estrogen receptor alpha interaction in 17beta-estradiol-mediated transcriptional activation of the low density lipoprotein receptor gene expression

Estrogen is one of the most important physiological regulators of low density lipoprotein receptor (LDLR) expression. Despite many studies conducted in animals and humans showing increased expressions of LDLR messenger RNA by hormone treatment, the molecular basis of the effect of estrogen on LDLR transcription has not been clearly elucidated. By using HepG2 cells that transiently express functional estrogen receptor alpha (ERalpha) and LDLR promoter constructs, we show that the specific interaction of ERalpha with the transcription factor Sp1 bound to the LDLR promoter is responsible for the activation of LDLR transcription by estrogen. We demonstrate that 1) mutations to abrogate the binding of Sp1 to its recognition sequences present in repeat 1 and repeat 3 elements of the LDLR promoter completely abolish the ERalpha-mediated activation of the LDLR promoter activity; 2) mutations that abolish the selective DNA-binding activity or inactivate the C-terminal transcription activation function (AF2) of ERalpha had no effect on the ability of ERalpha to activate LDLR transcription; however, transcriptional activation was completely lost by deletion of the N-terminal transcription activation region (AF1); 3) a subregion of AF1 (amino acids 67-139) was further identified to be important for ERalpha to activate the LDLR promoter; and 4) ERalpha enhanced the formation of Sp1-repeat 3 DNA complexes. We also show that mutation at the sterol-responsive element-1 site diminishes the activity of ERalpha on LDLR transcription, thereby suggesting that the sterol-responsive element-1-binding protein may interact with the Sp1-ERalpha complex to trans-activate LDLR gene transcription. This study for the first time provides a molecular basis for an understanding of the regulation of LDLR transcription by estrogens.

The effects of lifibrol (K12.148) on the cholesterol metabolism of cultured cells: evidence for sterol independent stimulation of the LDL receptor pathway

Lifibrol (4-(4'-tert. butylphenyl)-1-(4'-carboxyphenoxy)-2-butanol) is a new hypocholesterolemic compound; it effectively lowers low density lipoprotein (LDL) cholesterol. We studied the effects of lifibrol on the cholesterol metabolism of cultured cells. In the hepatoma cell line HepG2, Lifibrol decreased the formation of sterols from [14C]-acetic acid by approximately 25%. Similar to lovastatin, lifibrol had no effect on the synthesis of sterols from [14C]-mevalonic acid. Lifibrol did not inhibit 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase. Instead, cholesterol synthesis inhibition by lifibrol was entirely accounted for by competitive inhibition of HMG-CoA synthase. Lifibrol enhanced the cellular binding, uptake, and degradation of LDL in cultured cells in a dose dependent fashion. The stimulation of LDL receptors was significantly stronger than expected from the effect of lifibrol on sterol synthesis. In parallel, lifibrol increased the amount of immunologically detectable receptor protein. Stimulation of LDL receptor mediated endocytosis was observed both in the presence and in the absence of cholesterol-containing lipoproteins. In the absence of an extracellular source of cholesterol, both lifibrol and lovastatin induced microsomal HMG-CoA reductase. Co-incubation with LDL was sufficient to suppress the lifibrol mediated increase in reductase activity, indicating that lifibrol does not affect the production of the non-sterol derivative(s) which are thought to regulate HMG-CoA reductase activity at the post-transcriptional level. Considered together, the data suggest that the hypolipidemic action of lifibrol may, at least in part, be mediated by sterol-independent stimulation of the LDL receptor pathway. A potential advantage of lifibrol is that therapeutic concentrations do not interfere with the production of mevalonate which is required not only to synthesize sterols but also as a precursor of electron transport moieties, glycoproteins and farnesylated proteins.

Effects of menopause and hormone replacement therapy on plasma lipids, lipoproteins and LDL-receptor activity

A cross-sectional study of ninety six women was conducted to examine the effect of menopause and hormone replacement therapy (HRT) on plasma lipids, lipoproteins and oxidation of low density lipoproteins. The sample consisted of 26 premenopausal women, 26 postmenopausal women taking no replacement hormones and 43 postmenopausal women on hormone replacement therapy. Postmenopausal women not taking replacement hormones had significantly higher plasma cholesterol, low density lipoprotein (LDL) cholesterol and lipoprotein[a] (Lp[a]) levels compared to premenopausal women or postmenopausal women on HRT [6.00 +/- 0.15, 5.36 +/- 0.17 (P < 0.01), 5.63 +/- 0.13 (P < 0.05) mmol/l, respectively for total cholesterol; 4.13 +/- 0.15, 3.64 +/- 0.15 (P < 0.05), 3.82 +/- 0.12 (P < 0.05) mmol/l, respectively for LDL-cholesterol; 48.19 +/- 9.90, 26.59 +/- 5.53 (P < 0.03), 25.12 +/- 4.62 (P < 0.03) mg/dl, respectively for Lp[a]]. The differences in LDL cholesterol concentrations were inversely related to changes in LDL receptor activity (r = -0.27, P < 0.01). HRT use was found to be associated with a significantly smaller LDL particle size. Plasma triglyceride was significantly higher in women on HRT (1.16 +/- 0.07 mmol/l) than in the premenopausal group (0.96 +/- 0.07) or postmenopausal group not using HRT (0.87 +/- 0.06). There were no differences in LDL oxidation between the groups when LDL was oxidised in the presence of copper. Nor was there any difference in the uptake of copper-oxidised or macrophage-modified LDL into J774 macrophages. These results confirm the effect of menopause and exogenous hormones on plasma lipids and lipoproteins, and suggest that HRT modifies the activity of the LDL receptor. Hormone replacement did not appear to protect LDL from oxidation.

Comparison of the LDL-receptor binding of VLDL and LDL from apoE4 and apoE3 homozygotes

Compared with apolipoprotein E3 (apoE3), apoE2 is less effective in mediating the binding of lipoproteins to the low-density lipoprotein (LDL) receptor. The influence of the E4 isoform, which is associated with adverse effects on plasma lipids and coronary heart disease, is less clear. We compared the ability of very low density lipoprotein (VLDL) and LDL from paired E4/4 and E3/3 subjects to compete against 125I-labeled LDL for binding with the LDL receptor on cultured fibroblasts and Hep G2 cells. The concentrations of VLDL or LDL required to inhibit binding of 125I-LDL by 50% (IC50, microgram apoB/ml) were determined, and results were assessed in terms of an IC50 ratio, E4/4 IC50 relative to E3/3 IC50, to reduce the influence of interassay variability. In Hep G2 cells, E4/4 VLDL was more effective than E3/3 VLDL in competing for the LDL receptor, the IC50 ratio being lower than unity (0.73 +/- 0.31, P < 0.05, two-tailed t-test). IC50 values themselves were marginally lower in E4/4 than E3/3 subjects (3.7 +/- 1.3 vs. 6.1 +/- 3.7, P < 0.08). However, there was no difference between E4/4 and E3/3 VLDL in competing for the LDL receptor on fibroblasts or between E4/4 and E3/3 LDL in competing for the LDL receptor on either cell type. These results suggest that inheritance of apoE4 is associated with an increased affinity of VLDL particles for LDL receptors on hepatocytes and may partly explain the influence of the E4 isoform on lipid metabolism.

Sex steroids increase cholesterol 7alpha-hydroxylase mRNA in nonhuman primates

One mechanism that may account for our prior observation that oral contraceptives decrease the hepatic cholesterol concentration independently of the low-density lipoprotein (LDL) receptor in sexually intact nonhuman primates is that sex hormones increase biliary cholesterol secretion by increasing hepatic mRNA abundance for cholesterol 7alpha-hydroxylase, the rate-limiting enzyme in the conversion of cholesterol into bile acids. To examine the independent effect of estrogen, progestin, and combined estrogen and progestin on the hepatic cholesterol concentration and cholesterol 7alpha-hydroxylase mRNA abundance, 34 ovariectomized adult female cynomolgus monkeys were fed a moderately atherogenic diet for 12 weeks with either oral conjugated equine estrogen ([CEE] n = 8), medroxyprogesterone acetate ([MPA] n = 9), or combined CEE + MPA (n = 9) and compared with a control group (n = 8) that did not receive exogenous sex hormones. After 12 weeks, hepatic cholesterol was significantly lower in CEE-treated (6.2 +/- 1.2 mg/g liver) and CEE + MPA-treated (6.4 +/- 0.9 mg/g liver) animals compared with the control (12.6 +/- 1.9 mg/g liver) and MPA-treated (14.6 +/- 1.6 mg/g liver) groups. Hepatic cholesterol 7alpha-hydroxylase mRNA abundance was significantly increased in CEE-treated (0.553 +/- 0.08 pg/microg RNA), MPA-treated (0.734 +/- 0.12 pg/microg RNA), and CEE + MPA-treated (0.487 +/- 0.07 pg/microg RNA) animals compared with the controls (0.318 +/- 0.03 pg/microg RNA). There was no significant difference in the plasma LDL cholesterol concentration and hepatic LDL receptor mRNA abundance between the groups. These data support but do not prove the hypothesis that low-dose oral estrogen induces an increase in cholesterol 7alpha-hydroxylase mRNA abundance, which is correlated with biliary cholesterol secretion and may result in depletion of hepatic cholesterol.

Importance of estrogen receptors in hepatic LDL receptor regulation

Treatment with pharmacological doses of estrogen is the most potent way to stimulate hepatic LDL receptor expression in vivo. The mechanism for this effect is unclear, in part because of difficulties in inducing this stimulation in vitro. A fundamental question, whether estrogen receptors (ERs) mediate this stimulation, has not been addressed. The aim of the current study was to determine the involvement of ERs in the estrogen-induced stimulation of LDL receptors. Treatment of rats with high doses of ethynylestradiol for 7 days increased the hepatic LDL receptor protein and mRNA levels four- and threefold, respectively. LDL receptor stimulation in estrogen-treated rats was not due to their reduced food intake because hepatic LDL receptor expression did not increase in rats fasted for 72 hours. Treatment with antiestrogen (tamoxifen or clomiphene) abolished the LDL receptor stimulatory effect of ethynylestradiol at both the protein and mRNA levels. Antiestrogen alone had no effect on hepatic LDL receptor expression and did not influence the strong resistance to dietary cholesterol normally present in rats. It is concluded that ERs are critically involved in the induction of hepatic LDL receptor expression by ethynylestradiol. The known role of growth hormone for the expression of hepatic ERs may therefore play a role in the modulation of the effects of estrogen on cholesterol metabolism and hepatic LDL receptors in the rat.

The cardiovascular safety of triphasic contraceptive steroids

It was hypothesized that estrogen-induced cardioprotection is mediated by up-regulation and down-regulation of expression of nitric oxide (NO) and P-selectin, respectively. Published data on circulating levels of the vasodilator NO, atherogenic glycoprotein P-selectin, and lipoprotein-a [Lp(a)] in users of triphasic contraceptive steroids are lacking. A total of 30 healthy women (nonusers, controls) and 82 women using oral triphasic contraceptive steroids (ethinyl estradiol and levonorgestrel: Triovlar, Schering AG) for 18 to 24 cycles participated in this study. Fasting blood samples were obtained from users and nonusers for the determination of P-selectin and Lp(a) by enzyme immunoassay and NO by a colorimetric method. The serum Lp(a) levels in OC users were significantly higher than those of nonusers. On the other hand, the serum NO levels in OC users were significantly elevated when compared to nonusers. Plasma P-selectin was significantly lowered in OC users p < 0.005. These results demonstrate the beneficial effects of ethinyl estradiol in the triphasic contraceptive regimen. Ethinyl estradiol may afford a degree of anti-atherogenic-cardioprotective effect by up-regulation of the expression of the vasodilator NO and down-regulation of the expression of the atherogenic P-selectin. This may outweigh the cardiovascular risk of the increased atherogenic Lp(a). This study may explain the very low rate of mortality from venous thromboembolism in OC users, which compares favorably with the risks that many people accept in daily life.

Characterization of the ovariectomized rat model for the evaluation of estrogen effects on plasma cholesterol levels

Estrogens protect against cardiovascular disease in women through effects on the vascular wall and liver. Here we further characterize the rat as a model for the evaluation of estrogenic effects on plasma lipid levels vs. uterine wet weight. In adult ovariectomized female rats treated for 4 days s.c., 17alpha-ethinyl estradiol (EE) was the most potent agent to lower plasma total and high density lipoprotein cholesterol levels, followed by 17beta-estradiol and 17alpha-estradiol. However, 17alpha-estradiol had the greatest separation of uterotropic vs. cholesterol-lowering effects. EE had the same lipid-lowering potency whether administered s.c. or orally to adult rats. It had no effect on cholesterol levels in immature rats, even though the uterotropic response was dramatic. Testosterone propionate, dexamethasone, and progesterone did not significantly lower cholesterol levels. The antiestrogens tamoxifen and raloxifene lowered cholesterol levels, but with less efficacy and potency than the estrogens. ICI 182780 had no effect on cholesterol levels. When coadministered with EE, ICI 182780 inhibited the cholesterol-lowering and uterotropic activities of EE, suggesting that the estrogen receptor pathway is involved. In conclusion, although the information from the rat is limited as a model of the low density lipoprotein-lowering effects of estrogens in humans, it can be used to study the effects and mechanism of action of estrogen and antiestrogens on plasma cholesterol levels.

Familial defective apolipoprotein B-100 (FDB): effect of simvastatin therapy on LDL-receptor binding

Heterozygotes for familial defective apolipoprotein B-100 (FDB) have two populations of low density lipoprotein (LDL), one bearing normal apolipoprotein B-100 (apo B) and the other bearing defective apo B which exhibits a much lower affinity for the LDL-receptor. If HMGCoA reductase inhibitors such as simvastatin lowered LDL mainly by up-regulating LDL-receptor mediated clearance, they should decrease the overall binding affinity of LDL from an FDB heterozygote by selectively decreasing LDL bearing normal apo B. We compared how LDL from FDB heterozygotes competed with normal 125I-labelled LDL for binding to LDL-receptors while on and off therapy with simvastatin. The LDL of FDB heterozygotes had 40% (n = 10) the affinity of normal LDL (n = 12) for the LDL receptor on cultured fibroblasts, and 55% (n = 6) of normal LDL (n = 6) for that on HepG2 cells. Treatment of FDB subjects with simvastatin (n = 10) decreased serum LDL by 22% but had no effect on its binding affinity for LDL receptors, indicative of lowering of LDL containing both normal and defective apo B. This is consistent with the major LDL lowering effect being associated with decreased synthesis of LDL, rather than enhanced LDL-receptor clearance.

Estrogen increases low-density lipoprotein receptor-independent catabolism of apolipoprotein B in hyperlipidemic rabbits

Estrogen has been reported to increase the catabolism of low-density lipoprotein (LDL) apolipoprotein (apo) B by increasing LDL receptor activity. To determine the effect of estrogen on LDL receptor-independent pathways, paired turnover studies of native LDL and chemically modified LDL (methyl-LDL) were performed before and during estrogen administration in female New Zealand rabbits consuming a diet containing 0.5% (wt/wt) cholesterol. Rabbits were matched by plasma cholesterol concentration and assigned randomly to receive estrogen (estradiol cypionate 0.5 mg/kg/wk) or placebo. The residence time of both the native LDL apo B tracer and the methyl-LDL apo B tracer in plasma was decreased by estrogen but not by placebo. Multicompartmental modeling of the paired, double-labeled turnover studies indicated that an increase in fractional catabolic rate (FCR) of the fast-turnover pool, a kinetically distinct LDL subpopulation in plasma, accounted for the observed decrease in residence time in plasma for both tracers. These data support the hypothesis that, in addition to any effect on the LDL receptor, estrogen promotes the activity of LDL receptor-independent pathways.

Essential amino acids regulate fatty acid synthase expression through an uncharged transfer RNA-dependent mechanism

To better understand the regulation of gene expression by amino acids, we studied the effects of these macronutrients on fatty acid synthase (FAS), an enzyme crucial for energy storage. When HepG2 cells were fed serum-free media selectively deficient in each amino acid, the omission of any single classic essential amino acid as well as Arg or His (essential in some rapidly growing cells) resulted in FAS mRNA levels that were about half of those in complete medium. Control message levels were unaffected and omission of nonessential amino acids did not alter FAS expression. FAS mRNA levels peaked 12-16 h after feeding complete and Ser (nonessential)-deficient media but did not increase in cells fed Lys (essential)-deficient medium. With Lys, FAS mRNA increased over the physiologic concentration range of 15-150 microM, and low concentrations of lysine decreased FAS but not apoB protein mass. Transcription inhibitors mimicked treatment with Lys-deficient media, and nuclear run-off assays showed that Lys-deficient media abolished FAS but not apoB transcription. After treatment with Lys-deficient media, the intracellular Lys pool was rapidly depleted in association with an increase of uncharged (deacylated) tRNA Lys from < 1 to 64% of available tRNA Lys. Even in the presence of the essential amino acid His, increasing the level of uncharged tRNA His with histidinol, a competitive inhibitor of the histidinyl-tRNA synthetase, blocked FAS expression. Tyrosinol treatment did not alter FAS mRNA levels. These results suggest that essential amino acids regulate FAS expression by altering uncharged tRNA levels, a novel mechanism for nutrient control of gene expression in mammalian cells.

Effect of the plant compound indole-3-carbinol on hepatic cholesterol homoeostasis

The aim of this study was to elucidate the effects of the compound indole-3-carbinol (I3C), which is found in cruciferous vegetables, on hepatic cholesterol homoeostasis and metabolism in male CD-1 mice. Oral administration of 500 and 750 mg I3C/kg/day to mice for 1 wk resulted in increased liver mass and microsomal protein content. Hepatic microsomal cholesterol levels were not significantly altered following treatment with 100 and 250 mg I3C/kg/day, but were significantly decreased following treatment with 500 and 750 mg/kg/day. Conversely, the lower doses of I3C administered decreased serum cholesterol levels whereas the higher doses of I3C had no effect on this parameter. Alterations in cholesterol homoeostasis by I3C were not related to liver hypertrophy, since administration of phenobarbital to mice increased liver size, but had no significant effect on hepatic microsomal or serum cholesterol levels. Activities of the hepatic enzymes cholesterol ester hydrolase and cholesterol 7 alpha-hydroxylase were not altered by I3C. However, 500 and 750 mg I3C/kg/day elevated the activity of hepatic acyl-CoA:cholesterol acyltransferase (ACAT), the enzyme responsible for the formation of hepatic cholesteryl esters. These results demonstrate that (a) I3C lowers serum cholesterol levels at concentrations that have no discernible effect on hepatic cholesterol homoeostasis, and (b) at higher doses of I3C, hepatic microsomal cholesterol levels are significantly lowered and ACAT activity is significantly elevated. These latter effects are not accompanied by changes in serum cholesterol levels and may represent compensatory mechanisms to restore cholesterol homoeostasis in the body. Mechanisms responsible for the effects of I3C on cholesterol homoeostasis are proposed.

Update on the metabolic effects of steroidal contraceptives and their relationship to cardiovascular disease risk

Evaluation of metabolic disturbances has had an important role in the modification of oral contraceptive formulations toward estrogen-progestin combinations with reduced adverse metabolic impact. An increasing number of interrelationships between metabolic risk factors for cardiovascular disease are being recognized, and a metabolic syndrome of disturbances has been identified with insulin resistance as a potential underlying factor. The insulin resistance syndrome includes hyperinsulinemia and impaired glucose tolerance, hypertriglyceridemia, reduced high-density lipoprotein concentrations, and hypertension. Increased concentration of a small, dense, low-density lipoprotein subtype may also be important. Depending on steroid type and dose, combined oral contraceptives may induce all the features of the insulin resistance syndrome. Reduction in estrogen dose and modification of progestin content have resulted in formulations with no adverse effect on high-density lipoprotein and blood pressure, but insulin resistance and hypertriglyceridemia remain. These are caused primarily by the estrogen component. Therefore modification of the estrogen content of oral contraceptives might result in "metabolically transparent" formulations that could conceivably afford a degree of cardiovascular protection.

Oral contraceptives decrease hepatic cholesterol independent of the LDL receptor in nonhuman primates

Pharmacological doses of estrogens have been reported to increase hepatic catabolism of low-density lipoprotein (LDL) by the LDL receptor (LDL-R) pathway and to increase the concentration of mRNA for the LDL receptor. The induction of LDL-Rs by large doses of estrogen may not be relevant to the role of estrogens under physiological conditions. Furthermore, the mechanisms by which oral contraceptives, a combination of synthetic estrogen and progestin, may modulate LDL metabolism remain largely unexplored. Adult female cynomolgus monkeys were given combination ethinyl estradiol/norgestrel preparations (n = 16) for 16 weeks and were compared with a control group that did not receive exogenous sex hormones (n = 7). All animals consumed a diet containing 0.25 mg cholesterol/kcal with 40% of calories from saturated fats. After 16 weeks of treatment there was no significant difference in LDL cholesterol (LDL-C) and hepatic LDL-R mRNA concentration between oral contraceptive-treated animals (LDL-C, 242 +/- 113 mg/dL; LDL-R mRNA, 0.60 +/- 0.31 pg/microgram RNA) and control animals (LDL-C, 277 +/- 100 mg/dL; LDL-R mRNA, 0.51 +/- 0.21 pg/microgram RNA). In contrast, the hepatic cholesteryl ester concentration was significantly lower in the oral contraceptive-treated animals (7.28 +/- 3.59 mg/g liver) compared with the control animals (16.07 +/- 11.86 mg/g liver; P = .01) with no significant difference in hepatic free cholesterol concentration between the groups. Thus, oral contraceptives decrease hepatic cholesterol concentration independent of LDL-R expression.(ABSTRACT TRUNCATED AT 250 WORDS)

Altered regulation of cholesterol metabolism in type I diabetic women during the menstrual cycle

This study examines the relationship of cellular cholesterol metabolism to oestrogen and progesterone during the menstrual cycle in diabetic and non-diabetic subjects. Nine premenopausal diabetic women were compared to nine non-diabetic women of the same age. Oestrogen, progesterone, lipoproteins, including lipoprotein (a) (Lp(a)) and cholesteryl ester transfer protein (CETP) were determined in serum. Cellular cholesterol content and cellular cholesterol synthesis were measured in mononuclear leucocytes. There was no significant change in serum lipoproteins including Lp(a) during the cycle in either group. CETP activity was significantly higher over the 4 weeks in the diabetic patients compared with non-diabetic subjects (mean 463 +/- 30 mumol l-1 h-1 vs 405 +/- 28 mumol l-1 h-1, p < 0.01). Serum high density lipoprotein (HDL) cholesterol was significantly lower during the 4 weeks in the diabetic patients (1.7 +/- 0.1 mmol l-1 vs 1.8 +/- 0.1 mmol-1, p < 0.05). Cellular cholesterol synthesis decreased steadily up to the third week in cells from the control subjects whereas there was no significant change in cells from diabetic patients whose cellular cholesterol synthesis was higher at week 3 compared with non-diabetic subjects (663 +/- 54 nmol mg-1 cell protein vs 432 +/- 43 nmol mg-1 cell protein, two-way interaction p < 0.05). There was a significant negative correlation between cellular cholesterol synthesis and serum oestrogen in the non-diabetic subjects (p < 0.05) but not in the diabetic patients.(ABSTRACT TRUNCATED AT 250 WORDS)

Re-uptake of nascent low-density lipoproteins by HepG2 cells

The kinetics and specificity of the interaction of nascent HepG2 LDL with the HepG2 LDL receptor were examined. 125I-Labeled HepG2 LDL and plasma LDL were bound by HepG2 cells in a specific and saturable manner at 4 degrees C. Competition studies with HepG2 LDL and plasma LDL indicated that both ligands bound to the same receptor. Scatchard analyses of the specific 4 degrees C-binding data revealed a Kd of 75 nM for HepG2 LDL and a Kd of 30 nM for plasma LDL suggesting that HepG2 LDL bind less efficiently to the HepG2 LDL receptor than plasma LDL. Binding, internalization and degradation studies carried out at 37 degrees C indicated that HepG2 cells are capable of catabolizing their own nascent LDL; however, under normal experimental conditions re-uptake of nascent LDL is quantitatively insignificant.

Plasma triglycerides determine low density lipoprotein composition, physical properties, and cell-specific binding in cultured cells

The relationship between the plasma triglycerides and the LDL triglycerides of 30 normal and 48 hypertriglyceridemic subjects has been quantified; the data fit a simple adsorption isotherm, LDL triglyceride/(LDL triglyceride+LDL cholesterol ester) = 0.65 plasma triglyceride/(464 + plasma triglyceride). In vitro transfer of triglyceride from concentrated VLDL to VLDL-depleted plasma produced triglyceride-rich LDL that had similar properties. LDL uptake by HepG2 cells increased with LDL triglyceride content whereas the reverse was found with skin fibroblasts. At 37 degrees C, the cores of both normal and hypertriglyceridemic LDL were isotropic liquids. Circular dichroic spectra revealed no difference in the secondary structure of normal and triglyceride-rich LDL. The affinity of monoclonal antibody MB47, which binds to the receptor ligand of apo B-100 was independent of LDL triglyceride content. MB3, which binds near residue 1022 of apo B-100, showed a triglyceride-dependent decrease in affinity for LDL from hypertriglyceridemic subjects and from in vitro incubations. LDL with an elevated triglyceride content formed in vitro had reduced proteolytic cleavage of apo B-100 by Staphylococcus aureus V8 protease. From these data, we infer that (a) LDL triglyceride is a predictable function of plasma triglyceride, (b) triglyceride induces subtle changes in apo B-100 structure at a site that is remote from the putative receptor binding ligand, and (c) the triglyceride-dependent receptor-binding determinants of apo B-100 are recognized differently by fibroblasts and HepG2 cells.

Hepatic cholesterol metabolism in estrogen-treated men

Operative liver biopsies were obtained from two male patients who developed gallstone disease during estrogen treatment of metastatic prostatic carcinoma. The heparin-sensitive binding of 125I-low-density lipoprotein (LDL) to liver homogenates (reflecting the expression of the LDL receptor) was determined, together with the activities of the rate-limiting enzymes in cholesterol synthesis [3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase], bile acid production (cholesterol 7 alpha-hydroxylase), and cholesterol esterification (acyl CoA:cholesterol acyl transferase). The results were related to data available in 18 patients (5 male, 13 female) who underwent cholecystectomy because of gallstone disease. The hepatic 125I-LDL-binding activity was increased threefold compared with five controls, and the activity of HMG-CoA reductase was increased twofold. There was no major difference in the activities of cholesterol 7 alpha-hydroxylase or acyl CoA:cholesterol acyl transferase. The concentration of free and total cholesterol in liver microsomes was approximately 30% lower in the estrogen-treated men than in 11 controls. The results indicate that estrogen at pharmacological doses stimulates hepatic LDL-receptor expression and HMG-CoA reductase activity in men. The increased LDL-receptor expression could in part explain the enhanced plasma clearance of injected 125I-LDL and hence the reduction in plasma LDL cholesterol previously shown to occur in estrogen-treated men.

Transforming growth factor-beta 1 and interleukin-1 beta stimulate LDL receptor activity in Hep G2 cells

The effect of transforming growth factor-beta 1 (TGF-beta 1) and interleukin-1 beta (IL-1 beta) on LDL receptor in Hep G2 cells was investigated. A greater than two-fold stimulation of the binding and internalisation of [125I]-labelled LDL at 37 degrees C was observed after an 18-h incubation of the cells with TGF-beta 1 at 50 ng/ml and IL-1 beta at 11,700 units/ml compared with control cells. Scatchard analysis of the binding of [125I]-labelled LDL at 4 degrees C after an 18-h incubation of the cells with 1170 units/ml IL-1 beta and 5 ng/ml TGF-beta 1 showed that they were both acting primarily by increasing LDL receptor number. The increase in LDL receptor activity could not be attributed to an increase in cell proliferation as TGF-beta 1 at concentrations from 0.05 ng/ml to 50 ng/ml had no significant effect on either cell number or [3H]thymidine incorporation into DNA whilst IL-1 beta inhibited DNA synthesis by more than 80% at a concentration of 11,700 units/ml but had significant effect on cell number. Cholesterol biosynthesis from [14C]acetate, in contrast to the stimulation of LDL receptor activity, was inhibited by approximately two-fold by incubation with TGF-beta 1 at 50 ng/ml and IL-1 beta at 11,700 units/ml.

Growth requirements and expression of LDL receptor and HMG-CoA reductase in Hep G2 hepatoblastoma cells cultured in a chemically defined medium

A serum-free chemically defined medium (CDM) has been developed which sustains the growth in culture of the highly differentiated human hepatoma cell line Hep G2. Unlike rodent hepatoma lines, Hep G2 cells in serum-free medium have an absolute requirement for lipoprotein lipids (either low density lipoprotein (LDL) or high density lipoprotein (HDL)) for growth. In the presence of LDL (or HDL) growth was further enhanced by insulin, triiodo-L-thyronine, 17 alpha-ethinylestradiol but not by epidermal growth factor (EGF). On type I collagen gels cells cultured in CDM were contact inhibited and formed monolayers. This contrasted with the pattern of growth of cells cultured in the presence of serum on type I collagen gels and cells cultured on tissue-culture plastic in either CDM or medium containing serum which formed foci of multilayered cells. Expression of the LDL receptor and HMG-CoA (3-hydroxy-3-methylglutaryl coenzyme A) reductase genes was comparable in Hep G2 cells cultured in CDM and serum-containing medium. Furthermore, the binding and internalisation of 125I-LDL at 37 degrees C was modulated by hormones that have previously been shown to affect LDL receptor levels in liver in vivo or in hepatocytes cultured in serum-containing medium in vitro. The culture system described provides a basis for studying the regulation of hepatocyte-specific functions by soluble factors (either plasma- or cell-derived) and cell-substratum interactions in a human liver cell line.

Porcine smooth muscle cell-conditioned medium stimulates LDL receptor activity in Hep G2 cells

Paracrine factors may modulate low density lipoprotein (LDL) receptor activity in hepatocytes. To study this the effect of conditioned medium prepared from a range of cell types on the binding and internalisation of 125I-LDL in Hep G2 cells was studied. Seven of the fourteen conditioned media tested, including those from P388D1, U937, porcine smooth muscle (Pc SMC) Swiss 3T3, STO, = 48 and MDCK cells, were found to increase the binding and internalisation of 125I-LDL at 37 degrees C by Hep G2 cells (P &lt; 0.01). The largest increase in LDL receptor activity was produced by conditioned medium from Pc SMC cells and was, therefore, selected for further analysis. The Pc SMC-conditioned medium increased LDL receptor number in Hep G2 cells by three-fold but had no effect on LDL receptor activity in human skin fibroblasts. DNA synthesis and cholesterol synthesis by Hep G2 cells were inhibited by Pc SMC-conditioned medium. Preliminary characterisation of the Pc SMC-derived factor(s) suggests that it is a protein(s) of low relative molecular mass.

Importance of growth hormone for the induction of hepatic low density lipoprotein receptors

This investigation was undertaken to determine the possible role of growth hormone (GH) in the hormonal regulation of hepatic low density lipoprotein (LDL) receptor expression. Treatment of normal rats with estrogen (ethynylestradiol, 5 mg/kg per day) increased the number of hepatic LDL receptors, and the LDL receptor mRNA levels were increased 2.4-fold. However, when hypophysectomized rats were treated with estrogen, the hepatic LDL receptor number and the mRNA levels only increased slightly. Treatment with GH was important to restore the induction of hepatic LDL receptors in hypophysectomized estrogen-treated rats. Further, the hypocholesterolemic effect of estrogen was abolished in hypophysectomized rats, and GH reversed this effect. To assess the effect of GH in humans, hepatic LDL receptor binding activity was determined in liver biopsy specimens from gallstone patients pretreated with GH (12 international units/day) prior to operation. GH administration induced hepatic LDL receptors approximately 2-fold, and this was accompanied by a 25% decrease in serum cholesterol. The LDL receptor stimulation caused by GH treatment was of similar magnitude as that observed upon 3 weeks of treatment with an established hypolipidemic drug (pravastatin or simvastatin). The data show that GH has an important role in the regulation of hepatic LDL receptors and suggest that GH secretion may be important for the control of plasma LDL levels in humans.

Complete down-regulation of low-density-lipoprotein-receptor activity in the human hepatoma cell line Hep G2 by beta-migrating very-low-density lipoprotein and non-lipoprotein cholesterol. Different cellular regulatory pools of cholesterol

Regulation of low-density-lipoprotein-receptor activity by low-density lipoprotein (LDL), cholesteryl-ester-rich beta-migrating very-low-density lipoprotein (beta-VLDL) and non-lipoprotein cholesterol was investigated in the human hepatoma cell line Hep G2. Competition studies indicate that LDL and beta-VLDL are bound to the same recognition site, tentatively the LDL receptor. The regulatory response of the LDL receptor upon prolonged incubation with LDL or beta-VLDL was, however, markedly different. 22 h preincubation of Hep G2 cells with excess LDL caused a partial down regulation to 31% of the initial level of the high-affinity association of LDL and 26% of the high-affinity degradation of LDL, while with beta-VLDL a complete down regulation of the LDL-receptor activity is observed. Preincubation of Hep G2 cells with beta-VLDL for 22 h led to a fourfold increase in intracellular cholesterol esters and a twofold increase in acyl-coA:cholesterol acyltransferase activity. With LDL, the amount of intracellular cholesterol esters is increased 1.6-fold. The more effective down regulation of LDL receptors by beta-VLDL as compared to LDL can be explained by the more effective intracellular cholesterol delivery with beta-VLDL than with LDL. Preincubation of Hep G2 cells for 22 h with acetylated LDL hardly influenced the LDL-receptor activity. Non-lipoprotein cholesterol, however, caused a complete down regulation of LDL-receptor activity at even lower extracellular cholesterol concentrations than with beta-VLDL. The complete down regulation of LDL receptors by non-lipoprotein cholesterol is not accompanied by a significant increase in acyl-coA:cholesterol acyltransferase activity, while the intracellular cholesterol ester concentration is only increased 1.6-fold. It is suggested that the effectiveness of non-lipoprotein cholesterol to regulate LDL receptors is caused by its efficiency to reach the sterol regulatory site. The inability of LDL to down regulate its receptor completely can thus be explained by the inability of LDL to deliver cholesterol adequately at the intracellular regulatory site of the LDL receptor. The observed complete down regulation of the LDL receptor by beta-VLDL may be responsible for the cholesterol-rich-diet induced, complete down regulation of LDL-receptor-mediated clearance of LDL in vivo.

Regulation of LDL receptor, apoB, and apoE protein and mRNA in Hep G2 cells

The regulation of low-density lipoprotein (LDL) receptor activity, protein synthesis, and cellular mRNA content was evaluated in the human hepatoma cell line Hep G2. Incubation of the cells with LDL led to a complete downregulation of LDL receptor mRNA and LDL receptor protein synthesis. This LDL regulation of the LDL receptor and its mRNA was both time- and concentration-dependent. In contrast to protein synthesis and cellular mRNA concentrations of the LDL receptor, which were reduced to undetectable levels by prolonged incubation in the presence of LDL, LDL receptor activity was reduced to only 44% of preincubation levels. These findings support the presence of a second metabolic pathway for LDL uptake in human hepatocytic cells. The effect of LDL on cellular LDL receptor expression was specific for LDL because incubation in the presence of HDL did not affect any of these study end points. The potential coordinate regulation of the expression of the LDL receptor with its principal ligands, apolipoproteins (apo) B and E, was also investigated. In contrast to the LDL receptor mRNA downregulation with LDL incubation, cellular apoB and apoE mRNA concentrations were not affected by either LDL or HDL. Secretion of apoB, however, was significantly increased by incubating Hep G2 cells with LDL. These findings indicate that, in contrast to LDL receptor which is regulated at the mRNA level, the ligands for the LDL receptor are regulated either co- or post-translationally.

Evidence for sterol-independent regulation of low-density lipoprotein receptor activity in Hep-G2 cells

The relationship between the serum factor(s)-mediated induction of low-density lipoprotein (LDL) receptor activity and changes in cellular cholesterol metabolism was examined in the human hepatoma cell line Hep-G2. Relative to incubation with serum-free media [Eagle's minimal essential medium (MEM) control], short-term (less than 8 h) incubation with medium containing 15% of either calf serum (MEM + serum) or the d greater than 1.25 fraction of calf serum (MEM + d greater than 1.25) produced a time- and concentration-dependent increase in the uptake of 125I-LDL. Immunoblotting with anti-(LDL receptor) antibodies demonstrated that this was correlated with a 2-fold increase in the amount of the mature 136,000 Da LDL receptor protein in detergent-solubilized Hep-G2 cell membranes. Incubation with MEM + serum, but not MEM + d greater than 1.25, increased the efflux of radiolabelled cholesterol from Hep-G2 cells. However, the induction of 125I-LDL uptake by MEM + d greater than 1.25 (2.3-fold) and MEM + serum (2.2-fold) was virtually identical. Addition of the d less than 1.063 lipoproteins of calf serum to MEM + d greater than 1.25 at their original or three times their serum concentration decreased the induction of 125I-LDL uptake by MEM + d greater than 1.25 by only 20-30%. Together, these results suggest that the stimulation of 125I-LDL uptake was not due to the presence of high-density lipoprotein, the absence of LDL or the stimulation of cholesterol efflux. MEM + serum stimulated 125I-LDL uptake in cells cholesterol-loaded by incubation with rat very-low-density lipoprotein with beta electrophoretic mobility (beta-VLDL). Compared to incubation with the MEM control, either MEM + serum or MEM + d greater than 1.25 produced time-dependent increases in the activity of 3-hydroxy-3-methylglutaryl-CoA reductase which also occurred in cholesterol-loaded cells. However, cholesterol biosynthesis, whether measured from 3H2O, [14C]acetate or [3H]mevalonic acid, was not increased. Incubation with MEM + serum or MEM + d greater than 1.25 did not affect [3H]oleate incorporation into cellular cholesteryl esters, hydrolysis of intracellular [3H]cholesteryl esters or the cellular mass of unesterified or esterified cholesterol. Incubation with MEM + serum or MEM + d greater than 1.25 produced a transient increase in the level of LDL receptor mRNA, reaching a maximum of 5-10-fold by 2 h and decreasing to near baseline levels by 4 h. Actinomycin D blocked the serum-factor-mediated induction of LDL receptor mRNA.(ABSTRACT TRUNCATED AT 400 WORDS)

The human asialoglycoprotein receptor is a possible binding site for low-density lipoproteins and chylomicron remnants

Binding and internalization of chylomicron remnants from rat mesenteric lymph by HepG2 cells was inhibited by both excess remnants and low-density lipoprotein (LDL) to the same extent. Ligand blots revealed binding of remnants and LDL to the LDL receptor. Measures regulating LDL receptor activity greatly influenced the binding of remnants: ethinyloestradiol, the hydroxymethylglutaryl-CoA reductase inhibitor pravastatin and the absence of LDL all increased binding, whereas high cell density or the presence of LDL decreased binding. Also, asialofetuin, asialomucin, the neoglycoprotein galactosyl-albumin and an antibody against the asialoglycoprotein receptor all decreased substantially the binding of remnants. At high cell density, binding internalization and degradation of chylomicron remnants was inhibited by up to 70-80%, yet binding of LDL was inhibited by no more than 20-30%. In cross-competition studies, the binding of 125I-asialofetuin was efficiently competed for by asialofetuin itself or by the antibody, and also by LDL and remnants, yet remnants displayed an approx. 100-fold higher affinity than LDL. Likewise, remnants of human triacylglycerol-rich lipoproteins and asialofetuin interfered with each others' binding to HepG2 cells or human liver membranes. It is concluded that the LDL receptor mediates the internalization of chylomicron remnants into hepatocytes depending on its activity, according to demand for cholesterol. Additionally, the asialoglycoprotein receptor may contribute to the endocytosis of LDL, but predominantly of chylomicron remnants.

Endocrine control of plasma lipoprotein metabolism: effects of gonadal steroids

Gonadal steroids are powerful modulators of plasma lipoprotein metabolism. In general, steroids with oestrogenic activity increase plasma levels of HDL, especially HDL2, and reduce levels of LDL. Steroids with androgenic activity have opposite effects, consistent with the sex difference in HDL and LDL levels. Triglyceride levels are lowered by exogenous administration of androgens and are raised by oral oestrogens, contrary to the observed sex difference in this lipid. The impact of administered gonadal steroids is modified by factors such as dosage and chemical structure, with synthetic steroids having a more pronounced effect than natural hormones. The effects of these steroids may depend on the pre-treatment lipoprotein pattern and endocrine status, and are modified by the route of administration, with oral hormones often having greater metabolic effects than those given parenterally. The net effects of oestrogen and progestagen combined preparations on lipoprotein metabolism depends on the balance between oestrogenic and androgenic activity. In contrast, endogenous changes in sex hormone levels, such as those accompanying puberty, the menstrual cycle and the menopause, have relatively little effect on plasma lipoproteins. Data concerning puberty and castration in males indicate that testosterone is a key factor in the sex difference in HDL levels. There is evidence that loss of ovarian function induces significant increases in LDL level, but endogenous changes in oestrogen levels have little effect on HDL metabolism in women. Changes in triglyceride levels are due mainly to alteration in VLDL secretion and catabolism. LDL levels are controlled by the activity of B100, E receptors and, to a lesser extent, changes in LCAT activity. Gonadal steroids affect HDL levels by altering apoAI synthesis and by controlling the activity of hepatic lipase. Lp(a) levels are increased during early pregnancy but may be decreased by anabolic steroids. The mechanisms behind such actions are unknown. Gonadal hormones influence all areas of plasma lipoprotein metabolism and therefore may affect cardiovascular risk by favourably affecting the plasma lipoprotein profile. In postmenopausal women, use of oestrogens has led to a 60% reduction in cardiovascular disease (Bush et al, 1987). Androgens and progestagens with androgenic properties induce changes in plasma lipoproteins which may increase risk. Further study of the mechanisms involved in these changes is obligatory given the widespread use of these hormones.

Hormonal determinants of apolipoprotein B,E receptor expression in human liver. Positive association of receptor expression with plasma estrone concentration in middle-aged/elderly women

The relationships of the expression of hepatic low-density lipoprotein (LDL) receptors (apo B,E receptors) to several plasma hormone concentrations were examined in 15 fasted women aged 37-75 years (mean, 57 years), who were undergoing laparotomy for non-neoplastic disease. No subject had clinical or biochemical evidence of familial hypercholesterolemia, renal disease, hepatic disease, or endocrine disease. Hepatic apo B,E receptor expression was quantified in vitro as the EDTA-suppressible binding of 125I-labeled human LDL (15 micrograms protein/ml) by liver homogenate at 37 degrees C; values were 23-75 ng LDL protein/mg cell protein (mean, 47 ng/mg). Receptor expression was strongly correlated with plasma estrone concentration (rs = +0.70, P = 0.035), but was unrelated to the concentrations of testosterone, thyroxine, free triiodothyronine, cortisol, sex hormone-binding globulin (SHBG) or cortisol-binding globulin. Insulin and estradiol concentrations were mostly very low. The correlation of receptor expression with plasma total estrone concentration reflected associations with both the albumin-bound (rs = +0.78, P = 0.014) and unbound (rs = +0.80, P = 0.009) fractions, but not with the SHBG-bound fraction (rs = -0.22, P = 0.574), of this hormone. As the non-SHBG-bound fractions of gonadal steroids are considered to be the biologically active components, these results are consistent with experimental evidence that the synthesis of apo B,E receptors in hepatocytes is stimulated by estrogens, and suggest that circulating estrone may be the major hormonal determinant of receptor expression in fasted middle-aged/elderly women.