Regulation of human apolipoprotein A-I gene expression by equine estrogens

Search for Natural Compounds That Increase Apolipoprotein A-I Transcription in HepG2 Cells: Specific Attention for BRD4 Inhibitors

Although increasing apolipoprotein A‐I (apoA‐I) might lower the cardiovascular disease risk, knowledge on natural compounds that elevate apoA‐I transcription is limited. Therefore, the aim of this study was to discover natural compounds that increase apoA‐I transcription in HepG2 cells. Since BRD4 inhibition is known to elevate apoA‐I transcription, we focused on natural BRD4 inhibitors. For this, the literature was screened for compounds that might increase apoA‐I and or inhibit BRD4. This resulted in list A, (apoA‐I increasers with unknown BRD4 inhibitor capacity), list B (known BRD4 inhibitors that increase apoA‐I), and list C (BRD4 inhibitors with unknown effect on apoA‐I). These compounds were compared with the compounds in two natural compound databases. This resulted in (1) a common substructure (ethyl‐benzene) in 60% of selected BRD4‐inhibitors, and (2) four compounds that increased ApoA‐I: hesperetin, equilenin, 9(S)‐HOTrE, and cymarin. Whether these increases are regulated via BRD4 inhibition and the ethyl‐benzene structure inhibits BRD4 requires further study.

Human versus non-human sex steroid use in hormone replacement therapies part 1: Preclinical data

Prior to 2002, hormone replacement therapy (HRT) was considered to be an important component of postmenopausal healthcare. This was based on a plethora of basic, epidemiological and clinical studies demonstrating the health benefits of supplementation with human sex steroids. However, adverse findings from the Women's Health Initiative (WHI) studies that examined the 2 major forms of HRT in use in the US at that time - Premarin (conjugated equine estrogens; CEE) and Prempro (CEE + medroxyprogesterone acetate; MPA), cast a shadow over the use of any form of HRT. Here we review the biochemical and physiological differences between the non-human WHI study hormones - CEE and MPA, and their respective human counterparts 17β-estradiol (E₂) and progesterone (P₄). Preclinical data from the last 30 years demonstrate clear differences between human and non-human sex steroids on numerous molecular, physiological and functional parameters in brain, heart and reproductive tissue. In contrast to CEE supplementation, which is not always detrimental although certainly not as optimal as E₂ supplementation, MPA is clearly not equivalent to P₄, having detrimental effects on cognitive, cardiac and reproductive function. Moreover, unlike P₄, MPA is clearly antagonistic of the positive effects of E₂ and CEE on tissue function. These data indicate that minor chemical changes to human sex steroids result in physiologically distinct actions that are not optimal for tissue health and functioning.

β3-Adrenergic receptor regulates hepatic apolipoprotein A-I gene expression

BACKGROUND

β₃-adrenergic receptor (β₃-AR) was shown to upregulate hepatic apolipoprotein A-I (apoA-I) expression and reverse atherosclerotic plaques in vivo experiments. However, the effect of β₃-AR on apoA-I expression in vitro is unknown. The specific mechanism underlying β₃-AR prevention of atherosclerosis is unclear.

OBJECTIVE

The present study was designed to investigate the molecular mechanism of β₃-AR-mediated regulation of hepatic apoA-I gene expression.

METHODS

HepG2 cells were preincubated with/without a selective protein kinase A inhibitor (H-89) and then treated with a selective β₃-AR agonist (BRL37344) or antagonist (SR59230A). The hepatic apoA-I expression was detected by reverse transcription real-time quantitative polymerase chain reaction and Western blot analysis. Enzyme-linked immunosorbent assay was used to evaluate the secretion of apoA-I. A recombinant plasmid containing the apoA-I promoter was constructed and transiently transfected into HepG2 cells, and dual-luciferase reporter assays were used to examine the activity of the apoA-I promoter. A chromatin immunoprecipitation polymerase chain reaction assay was used to evaluate binding activities of hepatocyte nuclear factor-4 (HNF-4), HNF-3, and early growth response protein-1.

RESULTS

β₃-AR activation significantly upregulated apoA-I expression, promoted apoA-I secretion, and enhanced the activities of the apoA-I promoter, HNF-4, and HNF-3 in hepatocytes, whereas early growth response protein-1 was not affected. Moreover, protein kinase A inhibition partially suppressed the activation of the apoA-I promoter, HNF-4, and HNF-3 and almost completely blocked the upregulation of apoA-I expression induced by β₃-AR.

CONCLUSION

β₃-AR activation increased the activities of the apoA-I promoter, HNF-4, and HNF-3, which might account for the mechanism of β₃-AR-mediated upregulation of hepatic apoA-I expression. β₃-AR might exert an anti-atherosclerotic effect by upregulating hepatic apoA-I expression and promoting the cholesterol reverse transport process.

Dietary Strategies and Novel Pharmaceutical Approaches Targeting Serum ApoA-I Metabolism: A Systematic Overview

The incidence of CHD is still increasing, which underscores the need for new preventive and therapeutic approaches to decrease CHD risk. In this respect, increasing apoA-I concentrations may be a promising approach, especially through increasing apoA-I synthesis. This review first provides insight into current knowledge on apoA-I production, clearance, and degradation, followed by a systematic review of dietary and novel pharmacological approaches to target apoA-I metabolism. For this, a systematic search was performed to identify randomized controlled intervention studies that examined effects of whole foods and (non)nutrients on apoA-I metabolism. In addition, novel pharmacological approaches were searched for, which were specifically developed to target apoA-I metabolism. We conclude that both dietary components and pharmacological approaches can be used to increase apoA-I concentrations or functionality. For the dietary components in particular, more knowledge about the underlying mechanisms is necessary, as increasing apoA-I per se does not necessarily translate into a reduced CHD risk.

Cocoa flavanol metabolites activate HNF-3β, Sp1, and NFY-mediated transcription of apolipoprotein AI in human cells

SCOPE

To identify the mechanisms by which cocoa induces HDL levels and since apolipoprotein AI (ApoAI) is the major protein in HDLs, we analyzed, upon incubation with cocoa metabolites, ApoAI mRNA levels, its transcriptional regulation, and the levels of the transcription factors involved in this process.

METHODS AND RESULTS

Epicatechin and cocoa metabolites caused an increase in ApoAI expression in HepG2 cells. Electrophoretic mobility shift assays revealed the involvement of Sites A and B of the ApoAI promoter in the induction of ApoAI mRNA upon incubation with cocoa metabolites. Using supershift assays, we demonstrated the binding of HNF-3β, HNF-4, ER-α, and RXR-α to Site A and the binding of HNF-3β, NFY, and Sp1 to Site B. Luciferase assays performed with a construct containing Site B confirmed its role in the upregulation of ApoAI by cocoa metabolites. Incubation with 3-methyl-epicatechin led to an increase in HNF-3β mRNA, HNF-3β, ER-α, Sp1, and NFY protein levels and the activation of ApoAI transcription mediated by NFY, Sp1, and ER-α.

CONCLUSION

The activation of ApoAI transcription through Site B by cocoa flavanol metabolites is mainly mediated by an increase in HNF-3β, with a significant contribution of Sp1 and NFY, as a mechanism for the protective role of these compounds in cardiovascular diseases.

TLRs, macrophages, and NK cells: our understandings of their functions in uterus and ovary

Inflammation involves multiple changes in many aspects of immune system. Interactions between immune system and female reproductive system strongly impact fertility and reproductive health in general. Many normal events of female reproduction system including ovulation, menstruation, implantation and labor onset are considered as inflammatory process. Emerging evidence reveals that three components of immune system that are critical to initiate and resolve inflammation, Toll-like receptors (TLRs), macrophages, and natural killer (NK) cells, play important roles not only to provide protection against infections by exogenous pathogens but also to regulate essential functions of uterus and ovary. This review will briefly summarize our understanding of the functions of TLRs, macrophages and NK cells in uterus and ovary.

The soy isoflavone genistein induces estrogen synthesis in an extragonadal pathway

Genistein is a phytoestrogen isolated from soyabean, and is a potential nutraceutical gearing for women suffering from perimenopausal symptoms. Because of its differential binding affinity to estrogen receptor (ER) isoforms, genistein is described as a selective estrogen receptor modulator (SERM). The ligand-receptor interaction is established, but the potential confounding factors have not been fully addressed. Alteration in estrogen metabolism is an important issue when determining the downstream effect of ER. Aromatase or CYP19 catalyzes the rate-limiting reaction of estrogen synthesis, and is highly expressed in the ovary. This organ is the source of estrogen in females. After menopause the ovaries cease to produce the hormone, and localized estrogen synthesis in extragonadal tissues could become physiologically significant. In the present study, effect of genistein on CYP19 regulation was investigated in the hepatic cells HepG2. The phytoestrogen induced aromatase activity in the cells. Increased mRNA expression with concurrent elevation in the usage of promoters I.3/II was also demonstrated. Luciferase reporter gene assays verified the transcriptional control dictated by the specific promoters under genistein treatment. Several protein kinases were examined, and PKC?, P38, ERK-1/2 appeared to be activated. Subsequent inhibition and expression experiments demonstrated the involvement of these kinases. The transcriptional factor CREB was ultimately activated in the gene regulation. The present study illustrated an extragonadal pathway by which genistein might increase estrogen synthesis.

Structure activity relationships and differential interactions and functional activity of various equine estrogens mediated via estrogen receptors (ERs) ERalpha and ERbeta

The human estrogen receptors (ERs) alpha and beta interact with 17beta-estradiol (17beta-E2), estrone, 17alpha-estradiol, and the ring B unsaturated estrogens, equilin, 17beta-dihydroequilin, 17alpha-dihydroequilin, equilenin, 17beta-dihydroequilenin, 17alpha-dihydroequilenin, Delta8-estrone, and Delta8, 17beta-E2 with varying affinities. In comparison to 17beta-E2, the relative binding affinities of most ring B unsaturated estrogens were 2- to 8-fold lower for ERalpha and ERbeta, however, some of these unique estrogens had two to four times greater affinity for ERbeta than ERalpha. The transcriptional activity of these estrogens in HepG2 cells transfected with ERalpha or ERbeta, or both, and the secreted-alkaline phosphatase gene showed that all estrogens were functionally active. 17beta-E2 induced the activity of secreted-alkaline phosphatase by ERalpha to a level higher than any other estrogen. Activity of other estrogens was 12-17% that of 17beta-E2. In contrast, 17beta-E2 stimulated the activity of ERbeta to a 5-fold lower level than that with ERalpha, whereas the activity of other estrogens was 66-290% that of 17beta-E2, with equilenin being the most active. The presence of both ER subtypes did not alter the functional activity of 17beta-E2, although it further enhanced the activity of 17beta-dihydroequilin (200%), 17beta-dihydroequilenin (160%), and Delta8, 17beta-E2 (130%). Except for 17beta-E2, no correlation was observed between the functional activities and their binding affinities for ER. In conclusion, our results show that the effects of ring B unsaturated estrogens are mainly mediated via ERbeta and that the presence of both ER subtypes further enhances their activity. It is now possible to develop hormone replacement therapy using selective ring B unsaturated estrogens for target tissues where ERbeta is the predominant ER.

Genistein and daidzein induced apoA-1 transactivation in hepG2 cells expressing oestrogen receptor-α

Studies have shown that soya consumption has been associated with low incidence of CVD. Because the chemical structures of soya isoflavones are similar to oestrogen, the beneficial outcome may be attributed to the oestrogenicity of these compounds. In this study, effect of the soya isoflavone genistein on the mRNA expression of apoA-1 in the human hepatoma HepG2 cell was investigated. Without oestrogen receptor (ER) α transfection, soya isoflavones in the physiological range had no effect on the apoA-1 transcription. Once ERα was ectopically expressed in these cells, soya isoflavone dramatically increased the apoA-1 mRNA abundance quantified by real-time PCR.ApoA-1-reporter assays with plasmid constructed from the 5′-flanking segment upstream to the coding region revealed that the transactivation of theapoA-1promoter was induced by the soya isoflavone in HepG2 cells expressing ERα. This induction was reduced by the anti-oestrogen ICI 182780, but not the inhibitors of protein kinase (PK) C, PKA, or mitogen-activated PK. Based on the previously identified response elements on the promoter, a series of truncated promoter reporter plasmids were then constructed. An induction profile of genistein was built and insulin response core element at − 411 to − 404 appeared to be a potential site of interaction. This study illustrated that soya isoflavones at physiological concentrations could up regulate apoA-1 mRNA expression in ERα-transfected HepG2 cells.

Increasing apoA-I production as a target for CHD risk reduction

Dyslipidemia leading to coronary heart diseases (CHD) enables venues to prevent or treat CHD by other strategies than only lowering serum LDL cholesterol (LDL-C) concentrations, which is currently the most frequently targeted change. Unlike LDL-C, elevated high-density lipoprotein cholesterol (HDL-C) concentrations may protect against the development of CHD as demonstrated in numerous large-scale epidemiological studies. In this review we describe that besides elevating serum HDL-C concentrations by increasing alpha-HDL particles, approaches to elevate HDL-C concentrations by increasing pre-beta HDL particle concentrations seems more attractive. Besides infusion of apoA-I(Milano), using apoA-I mimetics, or delipidation of alpha-HDL particles, elevating de novo apoA-I production may be a suitable target to functionally increase pre-beta HDL particle concentrations. Therefore, a detailed description of the molecular pathways underlying apoA-I synthesis and secretion, completed with an overview of known effects of pharmacological and nutritional compounds on apoA-I synthesis will be presented. This knowledge may ultimately be applied in developing dietary intervention strategies to elevate apoA-I production and serum HDL-C concentrations and consequently lower CHD risk.

Oestrogen receptor α is required for biochanin A-induced apolipoprotein A-1 mRNA expression in HepG2 cells

Epidemiological studies have indicated that soya consumption may produce a better plasma lipid profile. The effect may be attributed to the phyto-oestrogens in soya. The red clover (Trifolium pratense) isoflavone biochanin A has a chemical structure similar to those phyto-oestrogens found in soya beans, and is marketed as a nutraceutical for alleviating postmenopausal symptoms. In the present study we investigated the effect of biochanin A on the mRNA expression of ApoA-1 in the hepatic cell line HepG2. Real-time PCR revealed that biochanin A increased ApoA-1 mRNA abundance in cells expressing oestrogen receptor (ER) α. Without ERα transfection, biochanin A had no effect on mRNA abundance. In order to study the transcriptional control, a fragment of the 5′-flanking region of theApoA-1gene was amplified and inserted in a firefly luciferase reporter plasmid. The reporter assay indicated that the transactivation of theApoA-1promoter was induced by biochanin A in HepG2 cells transfected with the ERα expression plasmid. This induction was reduced by the anti-oestrogen ICI 182,780, whereas the inhibitors of protein kinase (PK) C, PKA, or mitogen-activated kinase (ERK) had no suppressive effect. The present study illustrated that biochanin A might up regulate hepatic apoA-1 mRNA expression through an ER-dependent pathway.

Structure, function and amyloidogenic propensity of apolipoprotein A-I

Apolipoprotein A-I, the major structural apolipoprotein of high-density lipoproteins, efficiently protects humans from cholesterol accumulation in tissues; however, it can cause systemic amyloidosis in the presence of peculiar amino acid replacements. The wild-type molecule also has an intrinsic tendency to generate amyloid fibrils that localise within the atherosclerotic plaques. The structure, folding and metabolism of normal apolipoprotein A-I are extremely complex and as yet not completely clarified, but their understanding appears essential for the elucidation of the amyloid transition. We reviewed present knowledge on the structure, function and amyloidogenic propensity of apolipoprotein A-I with the aim of highlighting the possible molecular mechanisms that might contribute to the pathogenesis of this disease. Important clues on apolipoprotein A-I amyloidogenesis may be obtained from classical comparative studies of the properties of the wild-type versus the amyloidogenic counterpart. Additionally, in the case of apoA-I, further insights on the molecular mechanisms underlying its amyloidogenic propensity may derive from comparative studies between amyloidogenic variants and other mutations associated with hypoalphalipoproteinemia without amyloidosis.

Inhibition of apolipoprotein AI gene expression by 1, 25-dihydroxyvitamin D3

Members of the steroid receptor superfamily are known to alter the transcription of apolipoprotein AI (apo AI), the major apoprotein of high-density lipoprotein (HDL). To assess the role of vitamin D receptor (VDR) in apo AI gene expression, we investigated the effect of 1alpha, 25-dihydroxycholecalciferol (1, 25-(OH)2 D3) as well as the vitamin D antagonist ZK-191784 (ZK), on apo AI gene expression and promoter activity in the human hepatoma cell line HepG2. Apo AI secretion and mRNA levels were both suppressed in a dose-dependent manner in HepG2 cells treated 1, 25-(OH)2 D3. This was accompanied by a similar decrease in apo AI promoter activity. Mapping of the vitamin D response element showed that suppression required a region of the apo AI gene promoter identified previously to contain site A. However, vitamin D treatment had no effect on nuclear factor binding to site A of the apo AI promoter. Treatment with vitamin D receptor antagonist ZK inhibited the ability of 1, 25-(OH)2 D3 to repress apo AI promoter activity, while higher doses of ZK increased apo AI promoter activity. ZK did not alter estradiol stimulated apo AI promoter activity. The VDR antisense ODN had no effect on apo AI promoter activity in control cells, however, it reversed the repression normally seen in cells treated with 1, 25-(OH)2D3. It is concluded that 1, 25-(OH)2 D3 suppresses apo A1 gene expression at the transcriptional level, possibly by altering coactivators or corepressors. This effect requires the VDR as well as a vitamin D response element in the apo AI promoter.

Genome-wide linkage analyses and candidate gene fine mapping for HDL3 cholesterol: the Framingham Study

High density lipoprotein cholesterol (HDL-C) is inversely associated with coronary heart disease and has a genetic component; however, linkage to HDL-C is not conclusive. Subfractions of HDL, such as HDL(3)-C, may be better phenotypes for linkage studies. Using HDL(3)-C levels measured on 907 Framingham Heart Study subjects from 330 families around 1987, we conducted a genome-wide variance components linkage analysis with 401 microsatellite markers spaced approximately 10 centimorgan (cM) apart. Nine candidate genes were identified and annotated using a bioinformatics approach in the region of the highest linkage peak. Twenty-eight single nucleotide polymorphisms (SNPs) were selected from these candidate genes, and linkage and family-based association fine mapping were conducted using these SNPs. The highest multipoint log-of-the-odds (LOD) score from the initial linkage analysis was 3.7 at 133 cM on chromosome 6. Linkage analyses with additional SNPs yielded the highest LOD score of 4.0 at 129 cM on chromosome 6. Family-based association analysis revealed that SNP rs2257104 in PLAGL1 at approximately 143 cM was associated with multivariable adjusted HDL(3) (P = 0.03). Further study of the linkage region and exploration of other variants in PLAGL1 are warranted to define the potential functional variants of HDL-C metabolism.

Dietary chemopreventive compounds and ARE/EpRE signaling

Chemoprevention comprises multiple intervention methods using either pharmacological or dietary agents to impede, arrest, or reverse carcinogenesis at various stages. Development of dietary compounds as potential cancer chemopreventive agents is highly desirable, due to their safety, low toxicity, and general acceptance as dietary supplements. In this review, potential application of the dietary detoxifying enzyme inducers for chemoprevention and their relevant signaling events are discussed. Overall, the detoxifying enzyme system plays an important role in determining the final fate of carcinogens/procarcinogens and their subsequent impact on carcinogenesis. Among those positive regulators, phenolic and sulfur-containing compounds are two major classes of dietary detoxifying enzyme inducers. Regulation of many detoxifying enzymes by dietary chemopreventive compounds is mediated by the antioxidant response element (ARE)/electrophile response element (EpRE), which is located in the promoter region of related genes. Transcription factor nuclear factor E2-related factor 2 (Nrf2) binds to the ARE sequence to initiate gene expression. In response to treatments of various detoxifying enzyme inducers, several signal transduction pathways, including the oxidative stress, mitogen-active protein kinase, protein kinase C, and phosphatidylinositol 3-kinase pathways, are activated. The consequences of the activation of these signaling cascades, whether directly or indirectly, lead to the dissociation of Nrf2 from its cytosolic sequester Kelch-like ECH associating protein 1, nuclear translocation, and accumulation of Nrf2 protein in the nucleus, and ultimately increase the expression level of detoxifying enzymes through transcriptional activation of ARE/EpRE in those responsible genes.

The Cardiovascular Effects of Chronic Hypoestrogenism in Amenorrhoeic Athletes

In premenopausal women, the most severe menstrual dysfunction is amenorrhoea, which is associated with chronic hypoestrogenism. In postmenopausal women, hypoestrogenism is associated with a number of clinical sequelae related to cardiovascular health. A cardioprotective effect of endogenous oestrogen is widely supported, yet recent studies demonstrate a deleterious effect of hormone replacement therapy for cardiovascular health. What remain less clear are the implications of persistently low oestrogen levels in much younger amenorrhoeic athletes. The incidence of amenorrhoea among athletes is much greater than that observed among sedentary women. Recent data in amenorrhoeic athletes demonstrate impaired endothelial function, elevated low- and high-density lipoprotein levels, reduced circulating nitrates and nitrites, and increased susceptibility to lipid peroxidation. Predictive serum markers of cardiovascular health, such as homocysteine and C-reactive protein, have not yet been assessed in amenorrhoeic athletes, but are reportedly elevated in postmenopausal women. The independent and combined effects of chronic hypoestrogenism and exercise, together with subclinical dietary behaviours typically observed in amenorrhoeic athletes, warrants closer examination. Although no longitudinal studies exist, the altered vascular health outcomes reported in amenorrhoeic athletes are suggestive of increased risk for premature cardiovascular disease. Future research should focus on the presentation and progression of these adverse cardiovascular parameters in physically active women and athletes with hypoestrogenism to determine their effects on long-term health.

Regulation of apoA-I gene expression: mechanism of action of estrogen and genistein

We have previously shown that 17-beta-estradiol (E2) and genistein increase the expression of apolipoprotein A-I (apoA-I), the major protein component of HDL, in Hep G2 cells. To elucidate the mechanism mediating the increase in apoA-I gene expression by these compounds, plasmid constructs containing serial deletions of the apoA-I promoter region were generated. The smallest region maintaining response to E2 and genistein spanned the -220 to -148 sequence, and the estrogen antagonist ICI182,780 completely inhibited the E2 and genistein effect. Nuclear extracts from cells treated with E2 and genistein showed increased binding to site B oligonucleotide (-169 to -146), and nuclear extracts from genistein-treated cells showed increased binding to an early growth response factor 1 (Egr-1) oligonucleotide compared to control cells. An increase in the concentrations of Egr-1 and hepatocyte nuclear factor-3beta was observed in nuclear extracts of cells treated with both compounds compared to control cells. Treatment with a specific inhibitor of mitogen-activated protein (MAP) kinase, but not with other inhibitors, abolished the stimulation of apoA-I gene expression by E2 and genistein. These results indicate that the MAP kinase pathway is involved in the regulation of apoA-I gene expression by genistein and E2, possibly through downstream regulation of transcription factors binding to the promoter region.

Nrf2, not the estrogen receptor, mediates catechol estrogen-induced activation of the antioxidant responsive element

The antioxidant responsive element (ARE) plays an important role in the gene expression of phase II detoxification enzymes, such as NAD(P)H:quinone oxidoreductase 1 (NQO1), and NF-E2-related factor2 (Nrf2) is the transcription factor for the ARE-driven genes. Interestingly, estrogen receptor (ER) was reported to increase NQO1 gene expression through the ARE. In this study, we investigated the role of ER and Nrf2 in ARE activation using IMR-32 cells and mouse primary astrocytes. Among tested estrogen-related compounds, only catechol estrogens (i.e. 4-hydroxyestradiol) activated the ARE. Since 4-hydroxyestradiol-induced ARE activation was not inhibited by either 17beta-estradiol or tamoxifen, and overexpression of ER-alpha decreased 4-hydroxyestradiol-induced ARE activation, ARE activation by catechol estrogen was independent of ER. Nrf2, however, was very important in the 4-hydroxyestradiol-induced ARE activation. 4-Hydroxyestradiol did not activate the ARE in Nrf2 knockout (-/-) primary astrocytes, but did activate the ARE when Nrf2 was transfected into Nrf2-/- astrocytes. In addition, dominant negative Nrf2 completely blocked 4-hydroxyestradiol-induced ARE activation in Nrf2+/+ astrocytes, and only 4-hydroxyestradiol induced Nrf2 nuclear translocation in IMR-32 cells. A selective phosphatidylinositol 3-kinase (PI3-kinase) inhibitor (LY294002) blocked 4-hydroxyestradiol-induced Nrf2 nuclear translocation and NQO1 activity induction in IMR-32 cells. Taken together, these observations suggest that 4-hydroxyestradiol activates the ARE by a PI3-kinase-Nrf2 dependent mechanism, not involving ER.

Regulation by estrogen of synthesis and secretion of apolipoprotein A-I in the chicken hepatoma cell line, LMH-2A

The synthesis and secretion of apolipoprotein A-I (apoA-I) in response to the treatment with estrogen were investigated in the chicken hepatoma cell line, LMH-2A. Exposure of these cells to exogenous estrogen for up to 48 h results in a decrease of apoA-I production, as evident from Western blotting, immunoprecipitation, and immunofluorescence experiments. Likewise, the secretion of apoA-I is also decreased in estrogen-treated cells when compared to controls. However, under both conditions, the disappearance of the apoprotein from the cells occurs very rapidly and with similar kinetics. The bulk of apoA-I secreted from LMH-2A cells is recovered on lipoprotein particles with a buoyant density of > or =1.10 g/ml, corresponding to HDL and heavy LDL. Interestingly, apoA-I is detectable on apoB-containing lipoproteins by sequential immunoprecipitation, suggesting that the two apoproteins co-reside at least on a subfraction of the secreted particles, or that apoB- and apoA-I-containing particles interact. These interactions are more pronounced in estrogen-treated cells, most likely due to the dramatic estrogen-mediated induction of apoB synthesis and secretion.