Kinetics of estrogen-dependent modulation of apolipoprotein A-I synthesis in human hepatoma cells

Nonylphenol and Octylphenol Differently Affect Cell Redox Balance by Modulating the Nitric Oxide Signaling

Nonylphenol (NP) and octylphenol (OP) are pervasive environmental contaminants belonging to the broader class of compounds known as alkylphenols, with potential human toxic effects. Classified as “xenoestrogens,” NP and OP are able to interfere with the cell endocrine physiology via a direct interaction with the estrogen receptors. Here, using HepG2 cells in culture, the changes of the cell redox balance and mitochondrial activity induced by OP and NP have been investigated at μM concentrations, largely below those provoking acute toxicity, as those typical of environmental contaminants. Following 24 h cell exposure to both OP and NP, ROS production appeared significantly increased (p ≤ 0.01), together with the production of higher NO oxides (p = 0.003) and peroxynitrated protein-derivatives (NP versus CTR, p = 0.003). The mitochondrial proton electrochemical potential gradient instead was decreased (p ≤ 0.05), as the oxygen consumption by complex IV, particularly following incubation with NP (NP versus CTR, p = 0.017). Consistently, the RT-PCR and Western blot analyses proved that the OP and NP can modulate to a different extent the expression of the inducible NOS (NP versus CTR, p ≤ 0.01) and the endothelial NOS (OP versus CTR, p ≤ 0.05), with a significant variation of the coupling efficiency of the latter (NP versus CTR, p ≤ 0.05), a finding that may provide a novel clue to understand the specific xenoestrogenic properties of OP and NP.

Estrogen modulates transactivations of SXR-mediated liver X receptor response element and CAR-mediated phenobarbital response element in HepG2 cells

The nuclear receptors, steroid and xenobiotic receptor (SXR) and constitutive androstane receptor (CAR) play important functions in mediating lipid and drug metabolism in the liver. The present study demonstrates modulatory actions of estrogen in transactivations of SXR-mediated liver X receptor response element (LXRE) and CAR-mediated phenobarbital response element (PBRU). When human estrogen receptor (hERα) and SXR were exogenously expressed, treatment with either rifampicin or corticosterone promoted significantly the SXR-mediated transactivation of LXRE reporter gene in HepG2. However, combined treatment with estrogen plus either rifampicin or corticosterone resulted in less than 50% of the mean values of the transactivation by rifampicin or corticosterone alone. Thus, it is suggested that estrogen may repress the SXR-mediated transactivation of LXRE via functional cross-talk between ER and SXR. The CAR-mediated transactivation of PBRU was stimulated by hERa in the absence of estrogen. However, the potentiation by CAR agonist, TCPOBOP, was significantly repressed by moxestrol in the presence of ER. Thus, ER may play both stimulatory and inhibitory roles in modulating CAR-mediated transactivation of PBRU depending on the presence of their ligands. In summary, this study demonstrates that estrogen modulates transcriptional activity of SXR and CAR in mediating transactivation of LXRE and PBRU, respectively, of the nuclear receptor target genes through functional cross-talk between ER and the corresponding nuclear receptors.

Increased apolipoprotein AI production rate and redistribution of high-density lipoprotein size induced by estrogen plus progestin as oral contraceptive

CONTEXT

The impact of estrogen plus progestin as an oral contraceptive on high density lipoprotein (HDL) apolipoprotein (apo) AI metabolism in humans is poorly understood.

OBJECTIVES

This study was designed to measure the in vivo effect of Moneva (30 microg ethinylestradiol, 75 microg gestodene) on HDL apoAI production rate and fractional catabolic rate.

DESIGN

Using (13)C-leucine, we performed two kinetic studies in the fed state in 10 normolipidemic young women, before and 3 months after beginning Moneva.

RESULTS

On Moneva, serum triglycerides increased by 12% (P = 0.03) in the fed state, whereas low-density lipoprotein and HDL cholesterol remained unchanged. HDL apoAI pool size and production rate were increased by 9.2% (67.3 +/- 7.1 vs. 61.6 +/- 6.7 mg x kg(-1); P = 0.05) and 26.5% (14.3 +/- 2.7 vs. 11.3 +/- 2.2 mg x kg(-1) x d(-1); P = 0.02), respectively. HDL apoAI fractional catabolic rate was not significantly modified. Three-month treatment by Moneva induced a shift of HDL size distribution from HDL2 toward HDL3 (HDL3 = 51.5 +/- 8.1 vs. 46.5 +/- 9.2% of total HDL; P = 0.02) and an increase in the proportion of apoAI among HDL components (38.8 +/- 4.3 vs. 34.4 +/- 2.8%; P = 0.01).

CONCLUSION

Oral contraception by estrogen plus progestin induces changes in HDL apoAI metabolism characterized by an increase in production rate and pool size, with a higher proportion of HDL3 particles. Whether or not these changes are beneficial to prevent atherosclerosis has to be explored further.

Different Modulation of ER-Mediated Transactivation by Xenobiotic Nuclear Receptors Depending on the Estrogen Response Elements and Estrogen Target Cell Types

Recent studies demonstrated that constitutive androstane receptor (CAR) inhibits ER-mediated transactivation of both endogenous and synthetic estrogen responsive promotor in Hep G2. Whereas steroid and xenobiotic receptor (SXR) but not peroxisome proliferator-activated receptor-gamma (PPAR-gamma) was also reported to repress estrogen receptor (ER) transactivation of the synthetic 4ERE in Hep G2, the effects of these xenobiotic nuclear receptors (XNRs) on the endogenous estrogen responsive promotor remain to be determined. Effects of CAR, SXR, and PPAR-gamma on ER transactivation were also examined in three different kinds of breast cancer cell lines. However, except in MCF-7, studies were limited either in single dose response (MDA-MB-231) or with CAR only (MCF-7-K3). And there is presently no report on the effects of CAR, SXR, and PPAR-gamma on ER-mediated transactivation in ovarian-derived CHO-S cells. Accordingly, this article further examined the effects of the endogenous vitellogenin B1 estrogen responsive promotor on the SXR- and PPAR-gamma-modulated ER transactivation in Hep G2, and either dose-dependent or single dose effects of SXR, PPAR-gamma, and CAR in two different breast cancer cell lines and the ovarian-derived cell line respectively, on the ER-mediated transactivation of the synthetic (4ERE)-tk-luciferase reporter. Consistent with the previous report, CAR significantly repressed ER-mediated transactivation of the endogenous vitellogenin B1 promotor in Hep G2 cells. However, contrary to the effects on the synthetic promotor, PPAR-gamma potentiated whereas SXR did not have any effects on the ER transactivation of the vitellogenin promotor in Hep G2. In the breast cancer cell line of MDA-MB-231 in which endogenous ER is known not to be expressed, CAR modestly stimulated ER transactivation of the synthetic 4ERE in a low dose whereas both SXR and PPAR-gamma did not have any effects in all doses examined (20-500 ng). And in both CHO-S and estrogen-independent breast cancer cell line, MCF-7-K3, none of the three xenobiotic receptors significantly influenced the ER-mediated 4ERE transactivation in all doses examined. XNRs modulate ER-mediated transactivation depending on the estrogen response elements (EREs) and estrogen target cell types.

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.

Biphasic estradiol-induced AKT phosphorylation is modulated by PTEN via MAP kinase in HepG2 cells

We reported previously in HepG2 cells that estradiol induces cell cycle progression throughout the G1-S transition by the parallel stimulation of both PKC-alpha and ERK signaling molecules. The analysis of the cyclin D1 gene expression showed that only the MAP kinase pathway was involved. Here, the presence of rapid/nongenomic, estradiol-regulated, PI3K/AKT signal transduction pathway, its modulation by the levels of the tumor suppressor PTEN, its cross-talk with the ERK pathway, and its involvement in DNA synthesis and cyclin D1 gene promoter activity have all been studied in HepG2 cells. 17beta-Estradiol induced the rapid and biphasic phosphorylation of AKT. These phosphorylations were independent of each other, being the first wave of activation independent of the estrogen receptor (ER), whereas the second was dependent on ER. Both activations were dependent on PI3K activity; furthermore, the ERK pathway modulated AKT phosphorylation by acting on the PTEN levels. The results showed that the PI3K pathway, as well as ER, were strongly involved in both G1-S progression and cyclin D1 promoter activity by acting on its proximal region (-254 base pairs). These data indicate that in HepG2 cells, different rapid/nongenomic estradiol-induced signal transduction pathways modulate the multiple steps of G1-S phase transition.

Distinct nongenomic signal transduction pathways controlled by 17beta-estradiol regulate DNA synthesis and cyclin D(1) gene transcription in HepG2 cells

Estrogens induce cell proliferation in target tissues by stimulating progression through the G1 phase of the cell cycle. Activation of cyclin D(1) gene expression is a critical feature of this hormonal action. The existence of rapid/nongenomic estradiol-regulated protein kinase C (PKC-alpha) and extracellular signal-regulated kinase (ERK) signal transduction pathways, their cross talk, and role played in DNA synthesis and cyclin D(1) gene transcription have been studied herein in human hepatoma HepG2 cells. 17Beta-estradiol was found to rapidly activate PKC-alpha translocation and ERK-2/mitogen-activated protein kinase phosphorylation in this cell line. These actions were independent of each other, preceding the increase of thymidine incorporation into DNA and cyclin D(1) expression, and did not involve DNA binding by estrogen receptor. The results obtained with specific inhibitors indicated that PKC-alpha pathway is necessary to mediate the estradiol-induced G1-S progression of HepG2 cells, but it does not exert any effect(s) on cyclin D(1) gene expression. On the contrary, ERK-2 cascade was strongly involved in both G1-S progression and cyclin D(1) gene transcription. Deletion of its activating protein-1 responsive element motif resulted in attenuation of cyclin D(1) promoter responsiveness to estrogen. These results indicate that estrogen-induced cyclin D(1) transcription can occur in HepG2 cells independently of the transcriptional activity of estrogen receptor, sustaining the pivotal role played by nongenomic pathways of estrogen action in hormone-induced proliferation.

Inhibitory cross-talk between estrogen receptor (ER) and constitutively activated androstane receptor (CAR). CAR inhibits ER-mediated signaling pathway by squelching p160 coactivators

Estrogen receptor (ER) activity can be modulated by the action of other nuclear receptors. To study whether ER activity is altered by orphan nuclear receptors that mediate the cellular response to xenobiotics, cross-talk between ER and constitutive androstane receptor (CAR), steroid and xenobiotic receptor, or peroxisome proliferator-activated receptor gamma was examined in HepG2 cells. Of these receptors, CAR substantially inhibited ER-mediated transcriptional activity of the vitellogenin B1 promoter as well as a synthetic estrogen responsive element (ERE)-containing promoter. Treatment with an agonist of CAR, 1,4-bis-(2-(3,5-dichloropyridoxyl))benzene, potentiated CAR-mediated transcriptional repression. In contrast, an antagonist of CAR, androstenol, alleviated the repression effect. Although CAR interacted with the ER in solution, CAR did not interact with the ER bound to the ERE. CAR/retinoid X receptor bound to the ERE but with much lower affinity than ER. Incremental amounts of CAR elicited a progressive reduction of the ER activity induced by the p160 coactivator glucocorticoid receptor interacting protein 1 (GRIP-1). In turn, increasing amounts of GRIP-1 progressively reversed the depression of ER activity by CAR. An agonist or antagonist of CAR potentiated or alleviated, respectively, the CAR-mediated repression of the GRIP-1-enhanced ER activity, which is consistent with the ability of theses ligands to increase or decrease, respectively, the interaction of CAR with GRIP-1. A CAR mutant that did not interact with GRIP-1 did not inhibit ER-mediated transactivation. Our data demonstrate that xenobiotic nuclear receptor CAR antagonizes ER-mediated transcriptional activity by squelching limiting amounts of p160 coactivator and imply that xenobiotics may influence ER function of female reproductive physiology, cell differentiation, tumorigenesis, and lipid metabolism.

The contribution of intraabdominal fat to gender differences in hepatic lipase activity and low/high density lipoprotein heterogeneity

Hepatic lipase (HL) hydrolyzes triglyceride and phospholipid in low and high density lipoprotein cholesterol (LDL-C and HDL-C, respectively), and elevated HL activity is associated with small, dense atherogenic LDL particles and reduced HDL2-C. Elevated HL activity is associated with increasing age, male gender, high amounts of intraabdominal fat (IAF), and the HL gene (LIPC) promoter polymorphism (C nucleotide at -514). We investigated the mechanisms underlying the difference in HL activity between men (n = 44) and premenopausal women (n = 63). Men had significantly more IAF (144.5 +/- 80.9 vs. 66.5 +/- 43.2 cm(2), respectively; P < 0.001), higher HL activity (220.9 +/- 94.7 vs.129.9 +/- 53.5 nmol/mL.min; P < 0.001), more dense LDL (Rf, 0.277 +/- 0.032 vs. 0.300 +/- 0.024; P = 0.01), and less HDL2-C (0.19 +/- 0.10 vs. 0.32 +/- 0.16 mmol/L; P < 0.001) than women. After adjusting for IAF and the LIPC polymorphism, men continued to have higher (but attenuated) HL activity (194.5 +/- 80.4 vs.151.0 +/- 45.2, respectively; P = 0.007) and lower HDL2-C (0.23 +/- 0.11 vs. 0.29 +/- 0.14 mmol/L; P = 0.02) than women. Using multiple regression, HL activity remained independently related to IAF (P < 0.001), gender (P < 0.001), and the LIPC genotype (P < 0.001), with these factors accounting for 50% of the variance in HL activity. These data suggest that IAF is a major component of the gender difference in HL activity, but other gender-related differences, perhaps sex steroid hormones, also contribute to the higher HL activity seen in men compared with premenopausal women. The higher HL activity in men affects both LDL and HDL heterogeneity and may contribute to the gender difference in cardiovascular risk.

Estrogen increases apolipoprotein (apo) A-I secretion in hep G2 cells by modulating transcription of the apo A-I gene promoter

Estrogen administration to postmenopausal women has been shown to increase plasma levels of apolipoprotein (apo) A-I. A human hepatoma cell line, Hep G2, was used to test the hypothesis that estrogen increases the hepatic production of apo A-I by modulating gene expression. When Hep G2 cells were treated for 24 hours with E(2), the apo A-I content in the medium increased 4.3+/-1.0-fold at 10 micromol/L E(2) and 1.8+/-0.4-fold at 1 micromol/L E(2) compared with untreated cells. A time-course experiment indicated that there was no E(2)-dependent (10 micromol/L) increase in apo A-I medium content at 1 hour and 2 hours and that apo A-I was 165% of controls at 6 hours and 440% at 24 hours. Hep G2 cells were transfected, by the cationic lipid method, with constructs containing serial deletions of the 5' region of the apo A-I gene (-41/+397, -256/+397, and -2500/+397) cloned in front of the luciferase gene and with or without a 7-kb region spanning the apo C-III/A-IV intergenic region, which has been shown to contain regulatory elements for the expression of the apo A-I gene. With the exception of the construct containing only the basal promoter (-41/+397), the expression of all constructs was 2- to 3-fold greater in the presence of E(2). The smallest construct that maintained E(2) responsiveness, the -256/+397 construct, does not contain a typical estrogen-responsive element. In the same transfection experiments, the 4-fold increase in apo A-I in the culture medium was preserved. However, when the same set of transfections was performed by the calcium phosphate precipitation method, the E(2) effect on the apo A-I content in the culture medium and on transcription activation was nearly abolished. This effect was probably mediated by Ca(2+), because incubation of cells with 20 mmol/L CaCl(2) abolished the E(2) response. In conclusion, E(2) increases apo A-I production in hepatic cells by increasing the transcription of the apo A-I gene.

Apolipoprotein A-I in bile inhibits cholesterol crystallization and modifies transcellular lipid transfer through cultured human gall-bladder epithelial cells

BACKGROUND

Apolipoprotein A-I (Apo A-I), conventionally purified by several steps including organic solvent-delipidation from plasma, inhibits cholesterol crystallization in bile. To observe a significant effect in vitro, however, supraphysiological concentrations above 100 microg/mL are required. For this reason, this protein has not been considered to play a physiological role in vivo. In the present study, we examined the cholesterol crystal growth-inhibiting effect of biliary Apo A-I at its physiological concentration, the modification of transcellular transfer of biliary lipids through cultured human gall-bladder epithelial cells (GBEC) by Apo A-I at its physiological concentration and the binding and secretion of Apo A-I by GBEC.

METHODS AND RESULTS

We purified biliary Apo A-I to near homogeneity using immobilized artificial membrane chromatography. At 5 microg/mL, biliary Apo A-I reduced cholesterol crystal mass by 50%, whereas plasma-derived, solvent-delipidated Apo A-I had no effect. Using an antibody-capture enzyme-linked immunosorbent assay, we found reduced Apo A-I concentrations in bile samples from gallstone patients when compared with bile samples from gallstone-free controls (medians, 2.35 and 9.4 microg/mL, respectively). In a GBEC line, Apo A-I (5 microg/mL) enhanced transfer of phospholipid and cholesterol from the mucosal to the serosal side of cell monolayers by approximately 50%. These cells appear to bind Apo A-I reversibly in a dose- and time-dependent manner, compatible with receptor-type binding. Cultured human gall-bladder epithelial cells also showed basal secretion of Apo A-I, which was greatly increased by exposure to model bile solutions.

CONCLUSIONS

Apolipoprotein A-I in bile, thus, has both a direct effect on cholesterol crystal formation and enhances lipid removal from gall-bladder bile by GBEC. This effect may be specific and receptor mediated. These observations support two separate roles for human biliary Apo A-I and suggest that this protein may be important in preventing the formation of cholesterol crystals (the initial step in gallstone formation) in supersaturated bile.

Pre-beta-HDL stimulates placental lactogen release from human trophoblast cells

To examine whether pre-beta-high-density lipoprotein (HDL) may be involved in regulation of human placental lactogen (hPL) release, pre-beta-HDL was isolated from term pregnancy serum, and the effect of purified pre-beta-HDL on hPL release from trophoblast cells was examined after 1 h of exposure. Pre-beta-HDL stimulated a dose-dependent increase in hPL release with half-maximal stimulation at a dose of 300-400 microgram/ml, which is within the normal physiological range during pregnancy. Analysis of pre-beta-HDL and alpha-HDL in serum from pregnant women at different stages of gestation (determined by Western blot analysis) indicated that the pre-beta-HDL-to-alpha-HDL ratio increased linearly after the 10th week of gestation (r = 0.88, P < 0.001), reaching a maximum sixfold greater than that of nonpregnant women. The increase in serum pre-beta-HDL during pregnancy paralleled that of plasma hPL concentrations (r = 0.93, P < 0.001). Two-dimensional electrophoresis indicated that the increase in pre-beta-HDL was due primarily to an increase in pre-beta1-HDL and pre-beta2-HDL, two of the three forms of pre-beta-HDL present in blood. These results suggest a role for pre-beta-HDL in the regulation of hPL expression during pregnancy.

Oestrogen replacement therapy and coronary heart disease

This review highlights recent progress in our understanding of the beneficial effects of hormone replacement therapy (HRT) in cardiovascular disease (CVD). The fact that HRT is increasingly advocated has raised concern about possible adverse effects weighed against the potential benefits of HRT regimens. Both favourable and unfavourable effects of oestrogens and HRT regimens on CVD risk factors are increasingly recognized. Consequently, the picture on cardiovascular effects of oestrogen and HRT has become more complicated, and research in this field has extended to novel areas.

Estradiol stimulates apolipoprotein A-I- but not A-II-containing particle synthesis and secretion by stimulating mRNA transcription rate in Hep G2 cells

Estrogen therapy increases plasma HDL levels, which may reduce cardiovascular risk in postmenopausal women. The mechanism of action of estrogen in influencing various steps in hepatic HDL and apolipoprotein (apo) A-I synthesis and secretion are not fully understood. In this study, we have used the human hepatoblastoma cell line (Hep G2) as an in vitro model system to delineate the effect of estradiol on multiple regulatory steps involved in hepatic HDL metabolism. Incubation of Hep G2 cells with estradiol resulted in the following statistically significant findings: (1) increased accumulation of apoA-I in the medium without affecting uptake/removal of radiolabeled HDL-protein; (2) accelerated incorporation of [3H]leucine into apoA-I; (3) selective increase in [3H]leucine incorporation into lipoprotein (LP) A-I but not LP A-I+A-II HDL particles (HDL particles without and with apoA-II, respectively); (4) increased ability of apoA-I-containing particles to efflux cholesterol from fibroblasts; (5) stimulated steady state apoA-I but not apoA-II mRNA expression; and (6) increased newly transcribed apoA-I mRNA message without effect on apoA-I mRNA half-life. The data indicate that estradiol stimulates newly transcribed hepatic apoA-I mRNA, resulting in a selective increase in LP A-I, a subfraction of HDL that is associated with decreased atherosclerotic cardiovascular disease, especially in premenopausal women.

The A/G polymorphism in the -78 position of the apolipoprotein A-I promoter does not have a direct effect on transcriptional efficiency

A promoter polymorphism A/G at position 78 bp upstream of the transcription initiation site characterizes the human apolipoprotein A-I gene. Some studies correlated the higher Apo A-I levels or increased Apo A-I transcription efficiency with the A allele, while other studies did not confirm these results. We have investigated the in vitro effects of this transition on the transcriptional efficiency of ApoAI gene by creating two sets of identical constructs with the whole Apo A-I promoter, carrying the A or the G, linked to the complete ApoAI gene. The relative activity of the two promoter alleles was determined through a quantitative RT-PCR system after transient tranfections of human HepG2 cell line in basal state and after stimulation with retinoic acid or 17beta-estradiol. Our results exclude differences in promoter activity linked to the A or G promoter alleles either in basal or in stimulated conditions. The data suggest that the A/G polymorphism does not directly affect the transcriptional efficiency of ApoAI gene, although it may be in linkage disequilibrium with other regulatory sequences and the combination of these elements may explain the contradictory results of the ApoAI gene expression.

The impact of GnRH agonist and menotropin therapy on lipoprotein metabolism

Two groups of women, 48 undergoing controlled ovarian hyperstimulation for in vitro fertilization (IVF) treatment with a gonadotropin-releasing hormone (GnRH) analog and human menopausal gonadotropin daily and a group of 15 treated with a GnRH analog using the 'luteal phase protocol' before the hyperstimulation for IVF, took part in a prospective randomized study at the IVF Unit of the Beilinson Medical Center, Israel. The aim was to evaluate the changes in lipid metabolism occurring in the course of a standard controlled ovarian hyperstimulation protocol during IVF treatment cycles by measuring serum lipid levels before and after treatment. In the former group, in parallel with the 17 beta-estradiol increase, the cholesterol and low-density lipoprotein-cholesterol levels dropped and the high-density lipoprotein-cholesterol levels increased significantly. The resultant atherogenic index demonstrated a statistically significant decrease. In the latter group the mean lipoprotein levels were slightly different after treatment, but no statistically significant difference was observed. Our results indicate that the GnRH analog alone, administered for 10-12 days, does not induce any significant change in serum lipid levels. When using a combination of a GnRH analog and menotropins, the change of lipoprotein levels is favorable.

Cladistic analysis of the apolipoprotein AI-CIII-AIV gene cluster using a healthy French Canadian sample. I. Haploid analysis

A cladistic analysis was carried out to identify haplotypes hypothesized to differ for functional DNA sequence variations within the apolipoprotein (apo) AI-CIII-AIV gene cluster that affect plasma lipid, lipoprotein and apolipoprotein levels. A sample of unrelated healthy French Canadians was studied. First, a cladogram of the observed apo AI-CIII-AIV haplotypes was estimated. Then this cladogram was used to define a statistical analysis of the association between haplotype variation and variation in plasma lipid, lipoprotein and apolipoprotein levels. Three haplotypes were identified which were associated with small (5-12% of the total sum of squares) pleiotropic effects on plasma lipid, lipoprotein and apolipoprotein traits and these effects were context, i.e. gender, dependent.

Regulation of apolipoprotein A-I gene expression by phenobarbital in the human hepatocarcinoma cell line, Hep3B

Apolipoprotein (apo) A-I is the major protein constituent of plasma high density lipoprotein (HDL), which has been suggested to play a protective role against the development of atherosclerosis. The effect of phenobarbital on apo A-I mRNA and protein levels was studied in the human hepatoma cell line, Hep3B. Exposure of Hep3B cells to the drug (200 micrograms/ml) for 16 h resulted in a 4-fold and 8-fold increase in apo A-I mRNA and secreted protein levels, respectively. The induction of apo A-I mRNA level caused by phenobarbital could be due to increased rates of transcription and/or alteration in mRNA stability. To test these possibilities, nuclear run-off transcription assays and pulse-chase deinduction experiments were performed. We have demonstrated that phenobarbital treatment is associated with a 2-fold induction in apo A-I transcriptional activity. The estimated half-lives for apo A-I mRNA are 2 h and 3.6 h in the absence or presence of phenobarbital, respectively. The combination of increase in apo A-I transcription rate and mRNA stabilization could explain the 4-fold induction in apo A-I mRNA levels caused by phenobarbital treatment. However, these events could not be solely responsible for the 8-fold increase in secreted apo A-I protein level observed. The results suggest that the mechanism(s) by which phenobarbital induces apo A-I production operate at both pre- and either co- or post-translational mechanisms. The induction of apo A-I is specific since no significant alteration in apo E mRNA and proteins was observed in drug-treated cells.

Retinoids stimulate ApoA-I synthesis by induction of gene transcription in primary hepatocyte cultures from cynomolgus monkey (Macaca fascicularis)

The influence of different retinoids on apolipoprotein A-I (apoA-I) synthesis and secretion was investigated in primary monolayer cultures of hepatocytes from cynomolgus monkeys. Addition of retinol (vitamin A) and retinoic acid to the culture medium resulted in a time- and dose-dependent increase in the secretion of apoA-I. No effect was observed during the first 24-hour incubation period; however, apoA-I secretion was enhanced 1.5-fold in the following 24-hour period in the presence of 10 mumol/L retinoic acid. Maximal stimulation (2.7-fold) was obtained at 10 mumol/L retinoic acid during a third 24-hour incubation. In these experiments apoB-100 secretion was unaffected. When [35S]methionine incorporation studies were performed de novo synthesis of apoA-I was increased, whereas total protein synthesis remained constant. These observations indicated that the induction of apoA-I synthesis is not part of a general effect of retinoic acid on hepatic protein synthesis. Among different natural and synthetic retinoids, retinoic acid and its 9-cis and 13-cis isomers were equally active and were the most potent inducers of apoA-I synthesis, whereas the maximal stimulation induced by retinol was lower (1.6-fold). ApoA-I mRNA abundance was increased threefold in hepatocytes exposed for 72 hours to 10 mumol/L retinoic acid, which was associated with a twofold increase in the transcriptional rate of the apoA-I gene. In contrast, no changes were found in the apoB-100 mRNA level and transcriptional activity of the apoB-100 gene. We conclude that retinoids enhance apoA-I synthesis in simian hepatocytes by transcriptional regulation.

In vivo regulation of low-density lipoprotein receptors by estrogen differs at the post-transcriptional level in rat and mouse

Rats and mice are frequently used in studies of the regulation of lipoprotein metabolism. Although the species are closely related, they differ dramatically in the responses of their lipoproteins to estrogen administration. In rats, estrogens produce profound decreases in the levels of all plasma lipoproteins and this is attributed largely to estrogen-induced increases of hepatic low-density lipoprotein receptor (LDL-receptor) activity. Estrogens affect mouse plasma lipoproteins to a much lesser extent. Therefore, one of our aims was to compare the regulation of LDL-receptor gene expression in rats and mice at several potential loci of regulation. To assess the specificity of the estrogen effect, we also compared the responses of apolipoprotein AI (apoAI), apolipoprotein B (apoB), and beta-actin to the response of the LDL-receptor. In male Sprague Dawley rats given 17 beta-estradiol or 17 alpha-ethinyl estradiol at supraphysiological doses of 5 micrograms/g body mass/day, plasma total cholesterol and triacylglycerols fell to approximately 5% and approximately 50%, and, plasma apoAI and apoB fell to approximately 12% and approximately 16% of controls, respectively. By contrast, in male C3H/HeJ mice the above parameters dropped only to approximately 65% of controls and apoB concentrations rose to approximately 200% of controls. In rats, relative rates of LDL-receptor mRNA transcription (nuclear 'run-off' assay) and total hepatic, nuclear and polysomal LDL-receptor mRNA levels (RNase protection assay) increased by 1.5-2-fold, while synthesis of LDL-receptor protein on hepatic polysomes (in a wheat-germ translation system) increased 8-fold and LDL-receptor protein mass in hepatic plasma membranes increased 10-fold (by immunoblotting). In mouse liver, too, LDL-receptor mRNA levels increased 1.5-fold and the LDL-receptor mRNA transcription start sites in rat and mouse were found to be the same, but mouse LDL-receptor protein mass did not change, i.e. LDL-receptors of mice were similar to rat with respect to transcriptional regulation, but differed in their post-transcriptional control mechanisms. In rats, estrogen administration increased apoAI mRNA transcription rates 1.6-fold and also apoAI mRNA levels in total liver homogenates, nuclei and polysomes, (2-fold for each) consistent with transcriptional regulation. However, apoAI synthesis on total RNA increased less than apoAI mRNA, indicating that apoAI translational control mechanisms, at least in part, also regulate hepatic rates of apoAI production. ApoB mRNA transcription rates and levels showed small increases following estrogen administration. Hepatic beta-actin mRNA transcription and levels did not change.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of ethanol on lipoprotein secretion in two human hepatoma cell lines, HepG2 and Hep3B

The two human hepatoma cell lines, HepG2 and Hep3B, have been demonstrated to metabolize ethanol efficiently even in the absence of alcohol dehydrogenase. By using specific metabolic inhibitors, it was found that the microsomal ethanol-oxidizing system (MEOS) plays a significant role in ethanol metabolism in these two cell lines. There is a strong positive correlation between the rates of ethanol metabolism and the total cytochrome P-450 levels in the hepatoma cells. The involvement of the cytochrome P-450 system was further supported by the induction of aniline p-hydroxylase activity after ethanol treatment. However, the 3- to 4-fold elevation in aniline p-hydroxylase activity was not accompanied by an increase in cytochrome P450IIE1 mRNA level. Exposure of HepG2 and Hep3B cells to ethanol resulted in an increase of accumulation of apoA-I (15%-45% over control) in a dose-dependent manner (from 5 to 50 mM) of ethanol over a 24-hr period. All other major apolipoproteins which included apo CII, apo CIII and apoE, with the exception of apoB, were not affected by these treatments. At a concentration of ethanol of 25 mM or greater, accumulation of apoB, VLDL and LDL triglyceride were increased by 20% to 25% over the control level. Elevation of HDL cholesterol (40%-70% over control) was observed when the cells were exposed to an ethanol concentration of > or = 10 mM. Metyrapone, which inhibited the MEOS, was capable of blocking the induction of apoAI caused by ethanol treatment.

The effect of 25-hydroxycholesterol on the regulation of apolipoprotein E mRNA levels and secretion in the human hepatoma HepG2

The human hepatoma cell line, HepG2, was cultured with 25 OH cholesterol, a potent inhibitor of HMG-CoA reductase, in order to examine the effect of the oxysterol on apo E synthesis and secretion. Treatment of cells with oxysterol (2.5 microM) resulted in a greater than 90% inhibition of HMG-CoA reductase activity and a 3-fold reduction in its cognate mRNA level. However, apo E mRNA level and secretion were not affected after 24 h of drug treatment. This drug treatment was associated with a reduction in both cellular free and esterified cholesterol levels by 50% and 40%, respectively. Exposure of HepG2 cells to an ACAT inhibitor, the Sandoz compound (58-035) for 24 h, at a concentration of 5 micrograms/ml, resulted in a 30% increase and 70% decrease in the intracellular levels of free and esterified cholesterol, respectively. Under this regimen of drug treatment, the level of apo E mRNA was increased by approximately 70%, while HMG-CoA reductase mRNA level was decreased by 35%. When the cells were exposed to the combination of the ACAT inhibitor and 25 OH cholesterol, the cellular levels of free and esterified cholesterol were reduced by 30% and 80%, respectively. This combination of drugs had no effect on apo E mRNA; however, the level of HMG-CoA reductase mRNA was decreased by 3.5-fold. Taken together, the data suggested that reduction in the intracellular levels of either free or esterified cholesterol had no effect on apo E mRNA level. By contrast, a small increment in cellular free cholesterol content was associated with a significant induction in apo E mRNA level. Furthermore, 25 OH cholesterol caused a significant redistribution (50%) of apo E from the HDL fraction to the d greater than 1.21 g/ml infranatant. By using high performance liquid chromatography and molecular sieve columns, it was found that the appearance of a lipid-poor apo E particle was not an artifact of ultracentrifugation. This particle contained 85 wt% protein and 15 wt% of free cholesterol and phospholipid. The results suggested that a lipid-poor apo E particle was secreted by the HepG2 cells under certain circumstances.

Liver, lipoproteins and disease: I. Biochemistry of lipoprotein metabolism

Cholesterol is a structural component of biological membranes and an immediate precursor for steroid hormones and bile acids. The liver is central to the production and removal of cholesterol-rich lipoproteins and bile acids. The basic biochemical aspects of hepatic lipoprotein and cholesterol metabolism and how abnormalities in liver function impair these metabolic pathways are reviewed.

Transcription analysis of the EcoRI D region of the baculovirus Autographa californica nuclear polyhedrosis virus identifies an early 4-kilobase RNA encoding the essential p143 gene

We have investigated the transcriptional activity of the 60.1- to 68.3-map-unit region of the baculovirus Autographa californica nuclear polyhedrosis virus (AcMNPV). Twelve transcripts mapping to this region were expressed at various times during infection. An early 4.0-kb transcript, potentially coding for a 143-kDa peptide essential for viral DNA replication, was maximally abundant at 6 h postinfection (p.i.). Transcripts of 0.5, 1.1, 1.4, 2.1, and 3.1 kb were most abundant at 12 h p.i., while two large transcripts of 5.2 and 6.8 kb were expressed maximally at 24 h p.i. In the presence of cycloheximide, and in ts8-infected cells at the nonpermissive temperature, only the 4.0-kb RNA was expressed. Northern (RNA) blot analysis using DNA subfragments from the EcoRI D fragment as probes suggested that many of the transcripts overlapped. Strand-specific cRNA probes revealed that the majority of the RNAs were transcribed in the counterclockwise direction. S1 nuclease and primer extension analysis were used to map the 5' ends of transcripts coded within the 60.1- to 64.8-map-unit region. Mapping of the 3' ends of the 1.1-, 4.0-, 5.2-, and 6.8-kb transcripts suggested that these RNAs were all coterminal at their 3' ends. A minicistron was found between the early 4.0-kb transcription start site and the predicted ATG start codon of the p143 gene. Several similar sequence motifs were identified in the promoter regions of the p143 gene and the AcMNPV DNA polymerase gene.

Cholesterol fractions and apolipoproteins during endometriosis treatment by a gonadotrophin releasing hormone (GnRH) agonist implant or by danazol

OBJECTIVE

The evaluation of cholesterol fractions and apoproteins during ovarian suppression by a GnRH agonist implant vs danazol in the treatment of endometriosis.

DESIGN

A randomized study in 33 patients comparing goserelin (3.6 mg/4 weeks s.c., n = 20) with danazol (2 x 400 mg/day p.o., n = 13) in patients with a laparoscopic diagnosis of endometriosis and treated for 6 months.

MEASUREMENTS

Triglycerides, cholesterol (C), LDL-C, HDL-C subfractions and apoproteins A-1 and B were measured at admission, at months 2, 4 and 6 of treatment and at month 2 post-treatment.

RESULTS

After 1 month of therapy, serum oestradiol levels were maintained in the menopausal range with goserelin and in the early follicular phase range with danazol. Goserelin induced a significant elevation in HDL-C (by 31.4%), in HDL2-C (24.6%) and in HDL3-C (45.7%) but no significant change in LDL-C or in ApoA-1 and ApoB. By contrast, danazol caused significant diminutions in HDL-C (23.9%), HDL2-C (56.6%) and ApoA-1 (35.6%). Moreover, danazol increased LDL-C (10.5%) and ApoB (29.0%, P less than 0.05). The lipoprotein changes during goserelin had a favourable effect on the atherogenic index (cholesterol/HDL-C) and ApoA-1/ApoB ratio whereas those of danazol had opposite effects. These changes reverted 2 months after danazol while HDL was still elevated after goserelin.

CONCLUSIONS

In relation to cholesterol, goserelin is a safe medication. The significance of temporary adverse changes in cholesterol fractions due to danazol is still unknown.

Regulation of the apolipoprotein AI gene by ARP-1, a novel member of the steroid receptor superfamily

Apolipoprotein AI (apoAI) is a lipid-binding protein that participates in the transport of cholesterol and other lipids in the plasma. A complementary DNA clone for a protein that bound to regulatory elements of the apoAI gene was isolated. This protein, designated apoAI regulatory protein-1 (ARP-1), is a novel member of the steroid hormone receptor superfamily. ARP-1 bound to DNA as a dimer, and its dimerization domain was localized to the COOH-terminal region. ARP-1 also bound to a thyroid hormone-responsive element and to regulatory regions of the apoB, apoCIII, insulin, and ovalbumin genes. In cotransfection experiments, ARP-1 downregulated the apoAI gene. The involvement of ARP-1 in the regulation of apoAI gene expression suggests that it may participate in lipid metabolism and cholesterol homeostasis.

Characterization of three distinct size classes of human 3-hydroxy-3-methylglutaryl coenzyme A reductase mRNA: expression of the transcripts in hepatic and nonhepatic cells

3-Hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase mRNA is expressed in two highly differentiated human hepatoma cell lines, HepG2 and Hep3B, at exceptionally high levels relative to human fetal liver and fibroblasts. Blotting experiments revealed that the mRNA consists of three major size classes of approximately 4.7, 4.5, and 4.2 kb that responded coordinately to agents that alter HMG-CoA reductase activity. In view of the markedly elevated levels of reductase mRNA in the hepatoma cell lines, we compared the pattern of transcriptional initiation in these cells with those in normal liver and fibroblasts. These analyses revealed a complex pattern of initiation sites, all of which were suppressed by oxysterols, extending over approximately 300 nucleotides. However, all of the major sites detected in the hepatomas could also be found in human liver and fibroblasts. Heterogeneity of transcriptional initiation does not account for the three major size classes of mRNA detected by RNA blotting. RNase H mapping demonstrates that these are produced by use of three polyadenylation sites. To determine the extent to which these sites have been conserved between the human gene and the previously characterized Chinese hamster gene, we cloned and sequenced the 3' untranslated region of the longest form of the human mRNA. These studies revealed that, despite a high overall degree of sequence conservation, the spectrum of polyadenylation sites used differs qualitatively between the two species. Features of the mRNA sequence that may contribute to these differences are described.

Rapid regulation of apolipoprotein A-I secretion in HepG2 cells by a factor associated with bovine high-density lipoproteins

The effect of fetal bovine serum (FBS) on the secretion of apolipoprotein A-I (apo A-I) by HepG2 cells was studied. The cells incubated with FBS always secreted more apo A-I than the cells incubated with serum-free medium. The changes in the rate of apo A-I secretion were observed within 1 h after addition or depletion of serum. The high-density lipoproteins (HDL) or the lipoprotein-deficient serum (LPDS) obtained from FBS also stimulated apo A-I secretion rapidly to the same level as obtained with FBS. Addition of low-density lipoproteins did not have any effect. The rate of general protein synthesis was not affected by short-term incubations with or without serum or HDL. The rate of apolipoprotein E secretion by these cells did not change significantly, parallel to the changes in apo A-I secretion in the presence or absence of FBS. It is concluded that serum may have a factor that plays a specific role in the regulation of apo A-I secretion by the liver cells and this factor is associated with the HDL fraction.

Transcriptional and posttranscriptional regulation of apolipoprotein E, A-I, and A-II gene expression in normal rat liver and during several pathophysiologic states

Assessment of the relative transcription rates and mRNA steady-state levels for apolipoprotein genes E, A-I, and A-II has been performed in normal rat liver, during liver regeneration and following induction of cirrhosis, as well as in rats with inherited analbuminemia associated with hyperlipidemia. Apo E exhibits primarily transcriptional control with an additional component of posttranscriptional control, whereas Apo A-I is controlled primarily at the posttranscriptional level, thus indicating that these genes are regulated independently. The level of control for Apo A-II has not been determined, because of difficulty experienced in measuring the transcription rate of this gene. During liver regeneration, cirrhosis, and analbuminemia, there is a marked increase in the ratio of Apo A-I to Apo E mRNA, resulting from an increase in the Apo A-I mRNA steady-state level and a decrease in Apo E mRNA. These changes are similar in the three pathophysiologic states and seem to occur through a combination of transcriptional and posttranscriptional mechanisms.

Studies on the mechanism of action of oral contraceptives with regard to fibrinolytic variables

Evidence is provided that the fibrinolytic capacity in plasma is strongly dependent on circulating concentrations of tissue plasminogen activator rather than on concentrations of plasminogen activator inhibitor. Thus a decrease in plasma tissue plasminogen activator concentrations, as is the case in oral contraceptive users, may result in a decrease in plasma fibrinolytic capacity despite a parallel decrease in plasminogen activator inhibitor levels. It is now clear that the presence of specific intracellular receptors and a given motif in the genome are essential to mediate hormone-dependent regulation of gene expression. A computer search revealed potential estrogen and glucocorticoid-progesterone-responsive elements in the genes coding for tissue plasminogen activator, plasminogen activator inhibitor, and some other fibrinolytic variables. No convincing evidence for the presence of sex steroid receptors in endothelial cells was found, but liver cells clearly contain estrogen and androgen receptors. However, neither endothelial cells nor hepatocytes cultured in vitro showed a change in tissue plasminogen activator or plasminogen activator inhibitor synthesis on incubation with sex steroids (10(-9) to 10(-6) mol/L) for 3 days. An alternative explanation for the observed decreases in tissue plasminogen activator and plasminogen activator inhibitor concentrations in the plasma of oral contraceptive users is discussed.

In vivo and in vitro estimations of the direct effect of estrogen on rat hepatocytes tested by the changes in the unusual estrogen-binding protein content

The direct effect of estradiol (E2) on the hepatocytes of mature male rats has been examined by measuring the changes in the unusual estrogen-binding protein (UEBP) content and parallel measuring the level of liver estrogen receptors (ER). The content of UEBP (NUEBP) and ER (NER) in the liver were determined using the quantitative methods for differential specific determination of the E2-binding sites of these proteins. It has been shown that the administration of E2 in vivo induced a considerable decrease in hepatic NUEBP not only in intact males, but also in hypophysectomized males during the initial period after the operation (when the content of hepatic ER was still high) and produced no effect in hypophysectomized males during the later period (when liver ER were depleted). Repeated administration of human growth hormone (hGH) (twice a day) resulted in a considerable increase in NER in hypophysectomized males and restored the sensitivity to the subsequent inhibitory effect of E2 on UEBP. We also used rat hepatocytes after a 4-day primary culturing. These cells had a stable morpho-functional status, high ER level, and sex-differentiated UEBP content. Culturing of mature male rat hepatocytes in the medium containing E2 at concentrations close to physiological levels (10(-10)-10(-7) M) decreased NUEBP in a dose-dependent manner. Hexestrol (10(-7) M) but not cholesterol (10(-5) M) also exhibited a direct effect on NUEBP in cultured rat hepatocytes. The effect of E2 was reversible: statistically significant increase in NUEBP was observed 3 days after 10(-9) M E2 had been removed from the culturing medium. It was concluded that hepatocytes may be a primary target for E2 under physiological conditions and that GH may modulate the direct effect of E2 at the hepatic level by modifying the content of liver ER.

Regulation of apolipoprotein gene expression and plasma high-density lipoprotein composition in experimental nephrosis

Hepatic and intestinal RNA levels were measured in rats made nephrotic by injection of puromycin aminonucleoside (PAN). The following increases in hepatic RNA levels, relative to controls, were measured: poly A+ (1.2), ribosomal (1.2), mRNA levels for transferrin (1.8), albumin (3.8) apolipoprotein (apo)E (2.3), apoB (2.5), apoA-II (1.9) and apoA-I (6.1). Increases of 1.5- to 2.2-fold in hepatic mRNA levels for albumin, apoA-II, apoB and apoE were measured in pre-nephrotic animals killed before the onset of proteinuria. Intestinal RNA levels in pre-nephrotic and nephrotic animals were not significantly different from control values. Transcription of the hepatic apoA-I gene increased 1.8-fold in nephrotic animals compared to controls. Immunological detection of apolipoproteins in high-density lipoproteins (HDL) separated by gradient gel electrophoresis indicated an increase in apoA-I and a decrease in apoA-IV and apoE containing HDL particles in nephrosis. To simulate the effects of increased apoA-I gene expression, human apoA-I was added to rat plasma in vivo and in vitro. ApoE was displaced from HDL by increased concentration of apoA-I. The results indicate that relatively small changes in apoA-I levels in the serum lead to significant changes in the apolipoprotein composition of HDL.

Homozygous hypobetalipoproteinemia: transcriptional regulation and 5'-flanking sequence analysis in an apolipoprotein B deficiency state

Apolipoprotein (apo) B is the principal apolipoprotein of chylomicrons, very-low-density lipoproteins (VLDL) and low-density lipoproteins (LDL). Patients with homozygous hypobetalipoproteinemia (HBL), characterized by apoB deficiency, have markedly decreased levels of hepatocyte mRNA as well as intracellular B apolipoprotein, and a virtual absence of plasma apoB. We have cloned, sequenced and analyzed the 5' regulatory region of the human apoB gene from -899 to +121 bp in normal and hypobetalipoproteinemic subjects. TATA and CAAT boxes were located at -30 and -61, respectively, and two GC-like boxes were identified at positions +56 and +108. The analysis of the HBL sequence revealed two substitutions at positions -838 and -517, when compared to the normal sequence. These substitutions were not present in any known apoB regulatory elements. The transcriptional activities of the homozygous hypobetalipoproteinemic and normal regulatory regions were compared by chloramphenicol acetyltransferase (CAT) assays in Hep G2 cells, and were found to be the same. Therefore, we conclude that the 5' regulatory region of the HBL apoB gene in this kindred is normal, and the two base substitutions do not affect promoter activity of the apoB gene. These studies suggest that a coding region abnormality in the apoB gene may lead to HBL.

Effect of low density lipoproteins, high density lipoproteins, and cholesterol on apolipoprotein A-I mRNA in Hep G2 cells

We have utilized the human hepatocellular carcinoma cell line, Hep G2, to study the effects of low density lipoproteins (LDL), high density lipoproteins (HDL), and free cholesterol on apolipoprotein (apo) A-I mRNA levels. Incubation of the Hep G2 cells with LDL and free cholesterol led to a significant increase in the cellular content of cholesterol without any effect on the yield of total RNA or in the cellular protein content. Our studies established that incubation with LDL or free cholesterol increased the relative levels of apoA-I mRNA in the Hep G2 cells. In contrast with cholesterol loading, HDL had the effect of lowering the levels of apoA-I mRNA. These results indicate the LDL and HDL pathways as well as intracellular cholesterol may be important in apoA-I gene expression and regulation.

Regulation of hepatic lipase and serum lipoproteins by sex steroids

The influence of sex steroids on the serum lipoprotein pattern was recognized more than 30 years ago, and it still remains among the areas of major interest. This is because of the compatible sex difference in plasma lipoprotein pattern and in coronary heart disease risk. Recent discoveries of the role of hepatic lipase in lipoprotein metabolism have elucidated mechanisms behind sex steroid-induced changes in lipoproteins. These steroids regulate the activity of hepatic lipase, an enzyme bound to the endothelial cells of liver sinusoids. Hepatic lipase has a central role in the removal of phospholipids and triglycerides from subfractions of high-density lipoprotein (HDL2) particles, but it may also function in the lipolysis of triglyceride-rich particles. Some older and more recent developments in this area will be reviewed.

Estrogen memory effect in human hepatocytes during repeated cell division without hormone

Transient stimulation of target tissues by sex steroids can cause long-lasting changes that may facilitate or alter responses to subsequent hormonal treatment. How these altered characteristics are propagated during cell division in the absence of the stimulating hormone is unknown. The human hepatocarcinoma cell line HepG2 was used as a model to examine the effects of estrogen on the synthesis of serum apolipoproteins in vitro. Treatment with low concentrations of estrogen for 24 to 48 hours resulted in long-lasting alterations in the kinetics with which the cells responded to subsequent stimulation with estrogen. Manifestation of this memory effect was correlated quantitatively with the induction and propagation of a moderate-affinity, nuclear, estrogen-binding protein with the characteristics of a type II estrogen receptor. The data indicate that transient exposure of these cells to estrogen can induce changes in their response characteristics and composition of nuclear proteins that are inherited by daughter cells grown in the absence of hormone for more than ten generations.