This Article Abstract Submit a response Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Parini, P. Articles by Rudling, M. Search for Related Content PubMed PubMed Citation Articles by Parini, P. Articles by Rudling, M.

(Arteriosclerosis, Thrombosis, and Vascular Biology. 1997;17:1800-1805.)
© 1997 American Heart Association, Inc.

Articles

Importance of Estrogen Receptors in Hepatic LDL Receptor Regulation

Paolo Parini; Bo Angelin; ; Mats Rudling

From the Metabolism Unit, Center for Metabolism and Endocrinology, Department of Medicine, and the Molecular Nutrition Unit, Center for Nutrition and Toxicology, Novum, Karolinska Institute, Huddinge University Hospital, Huddinge, Sweden.

Correspondence to Mats Rudling, MD, Molecular Nutrition Unit, Center for Nutrition and Toxicology, Novum, S-141 57 Huddinge, Sweden. E-mail mats.rudling{at}cnt.ki.se.

	Abstract

Top Abstract Introduction Methods Results Discussion References

 
Abstract 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.

Key Words: estrogen receptor • fasting • growth hormone • LDL receptor • mRNA

	Introduction

Top Abstract Introduction Methods Results Discussion References

 
The ability of pharmacological doses of estrogens to reduce plasma cholesterol has been known for almost half a century.¹ Although the mechanisms for this remain unclear, it was shown in 1967 that this effect is blunted in Hx rats.² In 1971 Hay et al³ showed that elimination of radiolabeled LDL from plasma is increased in estrogen-treated rats and that the liver is the major site of elimination. After the discovery of the LDL receptor, Kovanen et al^{4 5} demonstrated that the number of hepatic LDL receptors is increased 5- to 10-fold in estrogen-treated male rats, and later studies on rabbits showed that this stimulation also occurs at the gene level.⁶ The role of the estrogen-stimulated hepatic LDL receptor for the estrogen-induced increased clearance of plasma LDL has been clearly shown.⁷ The role of this receptor stimulation in reducing plasma cholesterol, however, has been somewhat difficult to evaluate, in part because animals that receive high doses of estrogen reduce their food intake.⁸ It is also unclear by what mechanisms estrogen-induced LDL receptor stimulation occurs, in part because it has been difficult to induce LDL receptor stimulation in vitro.^{9 10}

Estrogen is the most potent LDL-receptor-stimulating agent available,^{4 11 12 13 14} and high-dose estrogen treatment of male rats,^{4 7} rabbits,^{6 15} and humans¹⁶ stimulates expression of hepatic LDL receptors by 4- to 10-fold. The stimulation of hepatic LDL receptor expression is very specific, and only the adrenal glands appear to also respond to this treatment.¹⁷

We recently demonstrated that stimulation of hepatic LDL receptors by estrogen in the rat requires an intact pituitary gland¹² and that GH is important in maintaining this response. GH can also stimulate LDL receptors on hepatocytes in vitro,¹⁸ an effect that is specific for GH and not mediated by IGF-I.^{18 19} It is unclear how the presence of GH influences the stimulatory response of estrogen on hepatic LDL receptors. One suggested possibility¹² is that hepatic ERs mediate this response because hepatic ERs disappear after hypophysectomy but reappear if animals are given GH as a substitute.²⁰ Estrogen may also exert this effect on hepatic LDL receptors indirectly through stimulation of the pituitary secretion of GH. Pituitary GH has also been shown to be of critical importance in maintaining the rat's high resistance to cholesterol feeding.²¹ How GH exerts this latter effect is also unclear, but important features could well be shared with the permissive role GH has for estrogen effects.

Considering the high doses of estrogen used in all these studies, the effects obtained may be unspecific and unphysiological. Very high concentrations of steroids may have direct pharmacological effects not mediated by ERs. Therefore, in the current study, we aimed to evaluate whether ERs play a role in the stimulation of hepatic LDL receptors that occurs in male rats after high-dose ethynylestradiol treatment. Our data show that the stimulation of hepatic LDL receptors by high-dose estrogen is completely abolished by concomitant administration of the antiestrogens tamoxifen or clomiphene, whereas the antiestrogens alone had no effect on LDL receptor expression. The data strongly suggest that ERs are critically involved in this important hormonal effect.

	Methods

Top Abstract Introduction Methods Results Discussion References

 
Materials
Ethynylestradiol, tamoxifen, and clomiphene were from Sigma Chemical Company. Proteinase K was obtained from Boehringer Mannheim. All other reagents and chemicals were from previously described sources.^{12 13}

Animals and Experimental Procedure
Altogether, 60 male Sprague-Dawley rats (250 to 300 g, 7 to 8 weeks old; B&K Universal AB, Sollentuna, Sweden) were used in three separate experiments. In each experiment, every group consisted of five rats. Animals were kept under standardized conditions with free access to water and chow. The light-cycle hours were between 6:00 AM and 6:00 PM. The studies were approved by our institution's Animal Care and Use Committee.

Drugs were dissolved in propylene glycol and injected subcutaneously under light ether anesthesia at 9:00 AM for 7 days. The doses of the drugs used (in mg/kg · d^-1 were as follows: ethynylestradiol, 5; tamoxifen, 60; and clomiphene, 60. Control rats received only propylene glycol. In the experiment in which rats were fasted, all animals had free access to water and were allowed to walk on a metal grid to prevent coprophagia. Animals were killed at 11:00 AM, and blood and liver tissue were collected and stored as described previously.¹²

Size fractionation of lipoproteins, by FPLC, was performed using a previously described system.¹³ Equal volumes of plasma from each animal were pooled (5 mL), and the density was adjusted to 1.21 g/mL with KBr. After ultracentrifugation at 100x10³ g for 48 hours, the removed supernatant was adjusted to 2.5 mL with 0.15 mol/L NaCl, 0.01% EDTA, and 0.02% sodium azide, pH 7.3; 2 mL of this solution, corresponding to 4 mL of plasma, was then injected on a 54- x1.8-cm Superose 6B column after filtration through a Millipore 0.45-µm HA filter. (In the experiment illustrated in Fig 1, the column was 47 cm long.) FPLC fractions of 2 mL were collected at a flow rate of 1 mL/min.

View larger version (46K): [in this window] [in a new window]   Figure 1. Effect of fasting on hepatic LDL receptor expression and FPLC pattern of plasma lipoproteins. Rats (five per group) were fasted for 72 hours before killing; controls received chow ad libidum. Inset, LDL receptor expression in membranes from pooled livers of each group. Two lanes were loaded with 100 and 200 µg of membrane protein for each group.

Cholesterol Assays
Cholesterol in plasma and FPLC fractions was assayed with the Boehringer Mannheim cholesterol assay kit (MPR 2 1 442 350) using a 5.2-mmol/L cholesterol standard from Merck.

Preparation of Hepatic Membranes and Ligand Blot Assay of LDL Receptors
Liver membranes were prepared as described previously.¹² In brief, gels (6% SDS-polyacrylamide) were loaded with the indicated amount of membrane protein prepared from pooled samples of liver. Size markers were reduced with mercaptoethanol and boiled. Filters were incubated with ¹²⁵I-labeled rabbit ß-VLDL as described previously.¹² Filters were exposed on Dupont Cronex film. LDL receptor expression was measured using a Fujix bioimaging analyzer. The measured 120-kd bands were expressed in arbitrary units after subtraction of background.

TNA Preparation
Frozen liver specimens (0.4 g) from each animal were homogenized in 4 mL of SET buffer (1% [wt/vol] SDS, 10 mmol/L EDTA, and 20 mmol/L Tris-HCl, pH 7.5) with a Polytron (type PT 10/35). The samples were subsequently sonicated on ice by two 5-second pulses in a Branson B 15 Sonifier and digested with proteinase K (200 µg/mL) for 45 minutes at 45°C. TNA was precipitated with ethanol after phenol-chloroform extraction, and the pellet was suspended in 300 µL of 20% SET buffer. The concentration of TNA in the samples was measured by absorbance at 260 nm, assuming 1 optical density=40 µg TNA/mL.

LDL receptor mRNA was measured by a solution hybridization titration assay²² using a mouse [-³⁵S]UTP-cRNA probe (corresponding to nucleotides 1247 to1308 in the human LDL receptor cDNA¹⁸ ) and TNA extracts (5 to 40 µg of TNA). The slopes of the linear hybridization signals were calculated by the method of least squares and compared with the slope generated by a synthetic mouse LDL receptor mRNA standard. Data are expressed as amoles of mRNA per microgram of TNA. The assay is described in detail elsewhere.²²

Statistical Analyses
Data are presented as the mean±SEM. One-way ANOVA, fully randomized design, and two-way ANOVA, repeated-measurement design, were used to evaluate the presence of significant differences between groups, followed by post-hoc comparisons of the group means according to the method of Tukey (using Statistica software).

	Results

Top Abstract Introduction Methods Results Discussion References

 
When rats receive high-dose estrogen therapy, they reduce their food intake and lose or cease to gain weight.⁸ It has been shown, in pair-feeding experiments, that if rats are given the same low quantity of food as consumed by those treated with high-dose estrogen, plasma cholesterol is not altered, although triglycerides are reduced.²³ Whether fasting of rats per se alters the expression of hepatic LDL receptors has not been documented. To exclude this possibility as a possible confounding factor, we first determined the hepatic expression of LDL receptors and plasma lipoproteins in rats that had been fasted for 72 hours (Fig 1). Hepatic LDL receptor expression in fasted rats was unchanged as compared with those fed ad libitum, and total plasma cholesterol was not altered. The average size of HDL particles increased upon fasting, as judged from a slight shift in the elution pattern of the FPLC analysis of plasma lipoproteins. Thus, it was evident that the pronounced stimulation of hepatic LDL receptors known to occur during high-dose estrogen treatment is not caused by reduced intake of food.

We then wanted to directly assess whether involvement of the ERs is important in the stimulation of hepatic LDL receptors by estrogen at high doses. Two antiestrogens, tamoxifen and clomiphene, were used in eightfold molar excess. Normal rats were treated with vehicle (controls), ethynylestradiol (5 mg/kg · d^-1), tamoxifen (60/mg/kg · d^-1), or clomiphene (60 mg/kg · d^-1). In addition, two groups of rats received ethynylestradiol in combination with tamoxifen or clomiphene. After 7 days of treatment, the animals were killed, and tissues were collected. During the course of the experiment, body weights of treated animals tended to decrease, but this was not statistically significant (data not shown).

Analysis of hepatic LDL receptor expression revealed that ethynylestradiol-treated rats had a fourfold increase, as expected (Fig 2, A). However, this effect could be reversed by the concomitant administration of tamoxifen or clomiphene, resulting in levels of hepatic LDL receptors similar to those observed in controls given vehicle only. Treatment with only tamoxifen or clomiphene had no major effect on LDL receptor expression; if anything, a slight reduction was observed in the clomiphene-treated group. Analysis of LDL receptor mRNA levels by solution hybridization revealed a similar response pattern (Fig 2, B). Thus, a threefold increase was observed in rats receiving ethynylestradiol only (P<.001), whereas LDL receptor mRNA levels were similar to those observed in controls after combined treatment with estrogen and either tamoxifen or clomiphene. Tamoxifen treatment in itself had no effect on the mRNA levels, whereas clomiphene tended to reduce LDL receptor mRNA levels, but this effect was not statistically significant.

View larger version (37K): [in this window] [in a new window]   Figure 2. Effect of antiestrogens on the estrogen-stimulated LDL receptor (LDLr) expression. Rats (five per group) were injected with ethynylestradiol (5 mg/kg), tamoxifen (60 mg/kg), or clomiphene (60 mg/kg) at 9:00 AM; controls received vehicle. In addition, two groups received tamoxifen or clomiphene in combination with ethynylestradiol. After 7 days of treatment, animals were killed at 11:00 AM. A, Ligand blot using rabbit 125I-ß-VLDL. Membranes from pooled livers were separated by SDS-PAGE (200 or 100 µg protein per lane) and subsequently transferred onto a nitrocellulose filter. LDLr expression was measured from the 120-kd band using an image analyzer as described in "Methods." B, LDLr mRNA of hepatic TNA samples from each individual measured by solution hybridization as described. Results shown are the mean±SEM. *P<.001 (versus control). P<.001 (versus estrogen-treated animals). C, Total plasma cholesterol in all animals. Results shown are the mean±SEM. *P<.05 (versus control). **P<.001 (versus control). P<.05 (versus estrogen-treated animals). D, Plasma lipoprotein patterns after separation by FPLC. Pooled plasma from each group was separated on a Superose column after ultracentrifugation. Two-milliliter fractions were collected and assayed for cholesterol. The solid line indicates the control group; the dashed line, 17-ethynylestradiol-treated animals; , tamoxifen-treated animals; , clomiphene-treated animals; , animals receiving 17-ethynylestradiol in combination with tamoxifen; and , animals receiving 17-ethynylestradiol in combination with clomiphene, respectively.

We then analyzed total plasma cholesterol in these animals (Fig 2, C). Ethynylestradiol treatment strongly reduced plasma cholesterol (P<.001), as expected. When animals received tamoxifen or clomiphene in addition to ethynylestradiol, plasma cholesterol levels increased fourfold so that the levels were not significantly different from those of controls. Treatment with only tamoxifen or clomiphene tended to reduce plasma cholesterol, but this reduction was not statistically significant when compared with the untreated controls. Separation of plasma lipoproteins by FPLC showed that estrogen-treated animals had reduced cholesterol within all lipoprotein fractions (Fig 2, D). When estrogen-treated animals were concomitantly treated with an antiestrogen, cholesterol was increased predominantly within the LDL and HDL fractions, and the pattern was similar to that observed in the fasted normal rats. Animals receiving only an antiestrogen showed a slight shift in lipoprotein cholesterol so that HDL particles increased in size, whereas LDL and IDL cholesterol levels increased somewhat. These results strongly suggest that the induction of hepatic LDL receptors by ethynylestradiol requires available ERs.

An important question thus evolving was whether the level of expression of hepatic LDL receptors is also dependent on ERs in the normal situation. Particularly, we wanted to determine whether the resistance to dietary cholesterol in rats may be linked to the ER. The rationale for this was our previous findings²¹ that normal Sprague-Dawley rats, which are extremely resistant to cholesterol feeding, become very sensitive to dietary cholesterol after hypophysectomy and that substitution with GH is important to maintain this resistance^{19 21} Because hepatic ERs are reduced in Hx animals but reappear if GH is substituted,²⁰ we therefore investigated whether treatment with antiestrogen could increase the sensitivity to dietary cholesterol. For this purpose, normal rats received regular chow or chow supplemented with 2% cholesterol. Another two groups received dietary cholesterol plus tamoxifen or clomiphene. After 7 days of treatment, animals were killed, and tissues were collected. During the course of the experiment, body weights of the animals receiving antiestrogen were significantly reduced by 17% (P<.01, data not shown). When the hepatic LDL receptor expression was assayed (Fig 3, A), it was found that challenge with dietary cholesterol increased LDL receptor expression in normal rats by 40%, in agreement with previous reports.^{19 21} However, hepatic LDL receptor expression was not reduced in the cholesterol-fed animals when they were treated with antiestrogen. If anything, a slight induction was seen in the clomiphene-treated group. Measurement of LDL receptor mRNA levels (Fig 3, B) revealed that cholesterol-feeding reduced the levels somewhat, in agreement with previous results.¹⁹ However, additional treatment with antiestrogen did not reduce the LDL receptor mRNA levels. Cholesterol feeding increased plasma cholesterol by 30% (P<.01) (Fig 3, C). Antiestrogen treatment of cholesterol-fed animals did not further increase plasma cholesterol; instead, a 40% reduction was seen (P<.01). FPLC separation of lipoproteins showed that the reduction of plasma cholesterol following antiestrogen therapy was mainly due to reduced VLDL (Fig 3, D). Thus, high-dose treatment of rats with antiestrogen did not increase the sensitivity to dietary cholesterol, strongly disproving the hypothesis that basal ER expression is important in this situation.

View larger version (29K): [in this window] [in a new window]   Figure 3. Lack of effect of antiestrogen on resistance to dietary cholesterol. Rats were fed 2% dietary cholesterol for 2 weeks and were injected subcutaneously for 7 days with tamoxifen (60 mg/kg), clomiphene (60 mg/kg), or vehicle. Controls received regular chow and vehicle. Each group consisted of five animals. Animals were killed at 11:00 AM. A, Ligand blot using rabbit 125I-ß-VLDL. Membranes from pooled livers were separated by SDS-PAGE (200, 150, 100, or 50 µg protein per lane) and subsequently transferred onto a nitrocellulose filter. LDL receptor (LDLr) expression was measured from the 120-kd band using an image analyzer as described in "Methods." B, LDLr mRNA of hepatic TNA samples from each individual measured by solution hybridization as described. Results shown are the mean±SEM. *P<.05 (versus control). C, Total plasma cholesterol in all animal. Results shown are the mean±SEM. *P<.005 (versus control). P<.001 (versus estrogen-treated animals). D, Plasma lipoprotein patterns after separation by FPLC. Two-milliliter fractions were collected after separation on a Superose column, and cholesterol was determined. The solid line indicates the control group on standard chow; dashed line, 2% dietary cholesterol; , 2% dietary cholesterol plus tamoxifen; and , 2% dietary cholesterol plus clomiphene.

	Discussion

Top Abstract Introduction Methods Results Discussion References

 
From the current study, several new and important conclusions can be drawn. First, the stimulation of hepatic LDL receptors by high-dose estrogen involves ERs because concomitant treatment with antiestrogens that block ERs abolished LDL receptor stimulation, both at the protein and mRNA levels. It can be argued that this effect of the antiestrogens, at high doses, may be due to their interaction with the antiestrogen-binding sites and not to the interaction with ERs. However, the fact that the antiestrogens alone did not alter hepatic LDL receptor expression strongly argues against such an explanation. An important role of ERs in this process would explain previous difficulties in reproducing the stimulation of LDL receptors in vitro,^{9 10} since cellular ERs disappear rapidly after isolation of rat hepatocytes.²⁴ An obligate involvement of ERs in the stimulation of hepatic LDL receptors by estrogen would also explain why this hormone has no stimulatory effect on hepatic LDL receptors in Hx animals,¹² since hepatic ERs are diminished after hypophysectomy.²⁰ The current results also explain why estrogens do stimulate hepatic LDL receptors in Hx rats that receive GH as a substitute.¹² Previous experiments have clearly shown that hepatic ERs are normalized in Hx rats upon substitution with GH.²⁰ From the present data, it cannot be determined whether the ERs involved are localized within the liver or in some other tissue, such as the brain. For instance, ERs within the brain may modulate GH release. Thus, estrogen administration increases²⁵ and treatment with antiestrogens reduces GH release.²⁶ When GH is administered to humans¹³ or rats (M. Rudling, unpublished data), hepatic LDL receptors are stimulated. Therefore, at least part of the effect of estrogen could be secondary to altered GH release. However, this possibility is less likely because substitution in Hx rats with a constant infusion of GH at such a low rate that IGF-I levels are not normalized is sufficient to completely normalize the magnitude of LDL receptor stimulation by estrogen.^{12 19} Also, the present finding that administration of antiestrogen alone, which reduces GH secretion,²⁶ did not alter LDL receptor expression does not support such an explanation. In addition, after hypophysectomy, the basal expression of hepatic LDL receptors in the rat is reduced by only 18%.²⁷

Second, it was found that short-term fasting of rats did not alter the hepatic expression of LDL receptors. Treatment of rats with high doses of estrogens induces a reduced food intake⁸ that will cause clear metabolic changes per se. However, the stimulation of hepatic LDL receptors in the rat, which appears within 24 hours of estrogen treatment,⁴ is clearly not even partially an effect of fasting. In humans, fasting reduces plasma LDL levels, suggesting that fasting may induce expression of hepatic LDL receptors in humans.²⁸ However, hormonal responses to fasting are obviously different in the rat.^{28 29 30}

Third, we found that treatment with high doses of antiestrogen did not reduce expression of hepatic LDL receptors. Because ERs are normally present in the liver of adult male rats,³¹ this suggests that the normal "estrogen tone" may not exert a major influence on hepatic LDL receptor expression in male rats. Whether antiestrogens suppress hepatic LDL receptors in adult female rats remains to be explored.

Finally, the resistance to dietary cholesterol in the male rat did not seem to involve the ERs. After hypophysectomy, Sprague-Dawley rats become very sensitive to dietary cholesterol and respond with pronounced hyperlipidemia and hepatic LDL receptor suppression.^{19 21} One important factor in maintaining this resistance in the male rat is pituitary GH,²¹ whereas IGF-I cannot be used as a substitute for this effect.¹⁹ The present results, which show that antiestrogens do not increase the sensitivity to dietary cholesterol, therefore strongly suggest that the role of GH in maintaining the male rat's resistance to dietary cholesterol is fundamentally different from its role in promoting the effects of estrogen. It is clear that estrogen and GH do not have identical effects on lipoprotein metabolism³¹ ; thus, in rats, they have opposite effects on apolipoprotein B editing,¹⁹ and in humans, on the concentration of lipoprotein (a).^{33 34 35} This concept is further strengthened by the fact that the reduced GH secretion during antiestrogen treatment does not result in changes in LDL receptor expression, as discussed above.

An important task now will be to identify the ER-dependent hepatic structures responsible for the stimulation of LDL receptor expression. In this respect, the hepatic pathways for excretion of biliary lipids and steroids must be considered. ER-dependent effects on the metabolism of ethynylestradiol may also be involved. Another important area for further study is the physiological regulation of ER in the liver because information on this is still very limited. The ER is subject to stimulation by estrogen itself, and the possible importance of such estrogen-induced ER expression for the stimulation of hepatic LDL receptors by high-dose estrogen remains to be studied. Several important questions regarding possible changes in hepatic ER expression during aging and the role of gender differences in the expression of ER in relation to lipoprotein metabolism should also be explored.

	Selected Abbreviations and Acronyms

 

ER = estrogen receptor FPLC = fast protein liquid chromatography GH = growth hormone Hx = hypophysectomized IGF-I = insulin like growth factor I TNA = total nucleic acids ß-VLDL = ß-migrating VLDL

	Acknowledgments

 
We thank Mrs Eva Ellis and Mrs Lilian Larsson for expert technical assistance.

This work was supported by grants from the Medical Research Council (03X-7137); the Swedish Society for Medical Research; the Nordic Insulin Fund; the Widengren, Thuring, Osterman, Jeansson, Axelsson Johnson, Ruth and Richard Julin, and Lundström Foundations, the Foundation of Old Female Servants, the Swedish Heart-Lung Foundation, "Förenade Liv" Mutual Group Life Insurance Company, and the Karolinska Institute.

Received September 24, 1996; accepted November 27, 1996.

	References

Top Abstract Introduction Methods Results Discussion References

 
1. Boyd GS, McGuire WD. The effect of hexoestrol on cholesterol metabolism in the rat. Biochem J. 1956;62:19P.

2. Steinberg M, Tolksdorf S, Gordon AS. Relation of the adrenal and pituitary to the hypocholesterolemic effect of estrogen. Endocrinology. 1967;81:340-344.[Abstract/Free Full Text]

3. Hay RV, Pottenger LA, Reingold AL, Getz GS, Wissler RW. Degradation of ¹²⁵I-labelled serum low density lipoprotein in normal and estrogen-treated male rats. Biochem Biophys Res Commun. 1971;44:1471-1477.[Medline] [Order article via Infotrieve]

4. Kovanen PT, Brown MS, Goldstein JL. Increased binding of low density lipoprotein to liver membranes from rats treated with 17 -ethinyl estradiol. J Biol Chem. 1979;254:11367-11373.[Free Full Text]

5. Windler EE, Kovanen PT, Chao YS, Brown MS, Havel RJ, Goldstein JL. The estradiol-stimulated lipoprotein receptor of rat liver: a binding site that mediates the uptake of rat lipoproteins containing apoproteins B and E. J Biol Chem. 1980;255:10464-10471.[Abstract/Free Full Text]

6. Ma PT, Yamamoto T, Goldstein JL, Brown MS. Increased mRNA for low density lipoprotein receptor in livers of rabbits treated with 17 -ethinyl estradiol. Proc Natl Acad Sci U S A. 1986;83:792-796.[Abstract/Free Full Text]

7. Chao YS, Windler EE, Chen GC, Havel RJ. Hepatic catabolism of rat and human lipoproteins in rats treated with 17 -ethinyl estradiol. J Biol Chem. 1979;254:11360-11366.[Free Full Text]

8. Ferreri LF, Naito HK. Effect of estrogens on rat serum cholesterol concentration: consideration of dose, type of estrogen, and treatment duration. Endocrinology. 1978;102:1621-1627.[Abstract/Free Full Text]

9. Semenkovich CF, Ostlund RE Jr. Estrogens induce low-density lipoprotein receptor activity and decrease intracellular cholesterol in human hepatoma cell line Hep G2. Biochemistry. 1987;26:4987-4992.[Medline] [Order article via Infotrieve]

10. Wade DP, Knight BL, Soutar AK. Hormonal regulation of low-density lipoprotein (LDL) receptor activity in human hepatoma Hep G2 cells: insulin increases LDL receptor activity and diminishes its suppression by exogenous cholesterol. Eur J Biochem. 1988;174:213-218.[Medline] [Order article via Infotrieve]

11. Brindley DN, Salter AM. Hormonal regulation of the hepatic low density lipoprotein receptor and the catabolism of low density lipoproteins: relationship with the secretion of very low density lipoproteins. Prog Lipid Res. 1991;30:349-360.[Medline] [Order article via Infotrieve]

12. Rudling M, Norstedt G, Olivecrona H, Reihnér E, Gustafsson J-Å, Angelin B. Importance of growth hormone for the induction of hepatic low density lipoprotein receptors. Proc Natl Acad Sci U S A. 1992;89:6983-6987.[Abstract/Free Full Text]

13. Rudling M, Angelin B. Stimulation of rat hepatic low density lipoprotein receptors by glucagon: evidence of a novel regulatory mechanism in vivo. J Clin Invest. 1993;91:2796-2805.

14. Angelin B. Studies on the regulation of hepatic cholesterol metabolism in humans. Eur J Clin Invest. 1995;25:215-224.[Medline] [Order article via Infotrieve]

15. Henriksson P, Stamberger M, Eriksson M, et al. Oestrogen-induced changes in lipoprotein metabolism: role in prevention of atherosclerosis in the cholesterol-fed rabbit. Eur J Clin Invest. 1989;19:395-403.[Medline] [Order article via Infotrieve]

16. Angelin B, Olivecrona H, Reihnér E, et al. Hepatic cholesterol metabolism in estrogen-treated men. Gastroenterology. 1992;103:1657-1663.[Medline] [Order article via Infotrieve]

17. Rudling MJ. Role of the liver for the receptor-mediated catabolism of low-density lipoprotein in the 17 -ethinylestradiol-treated rat. Biochim Biophys Acta. 1987;919:175-180.[Medline] [Order article via Infotrieve]

18. Parini P, Angelin B, Lobie PE, Norstedt G, Rudling M. Growth hormone specifically stimulates the expression of low density lipoprotein receptors in human hepatoma cells. Endocrinology. 1995;136:3767-3773.[Abstract]

19. Rudling M, Olivecrona H, Eggertsen G, Angelin B. Regulation of rat hepatic low density lipoprotein receptors—in vivo stimulation by growth hormone is not mediated by insulin-like growth factor I. J Clin Invest. 1996;97:292-299.[Medline] [Order article via Infotrieve]

20. Norstedt G, Wrange O, Gustafsson JÅ. Multihormonal regulation of the estrogen receptor in rat liver. Endocrinology. 1981;108:1190-1196.[Abstract/Free Full Text]

21. Rudling M, Angelin B. Loss of resistance to dietary cholesterol in the rat following hypophysectomy: importance of growth hormone for the expression of hepatic low density lipoprotein receptors. Proc Natl Acad Sci U S A. 1993;90:8851-8855.[Abstract/Free Full Text]

22. Rudling M. Hepatic mRNA levels for the LDL receptor and HMG-CoA reductase show coordinate regulation in vivo. J Lipid Res. 1992;33:493-501.[Abstract]

23. Weinstein I, Wilcox HG, Heimberg M. Effects of high-dose ethinyl estradiol on serum concentrations and hepatic secretion of the very-low-density lipoprotein, triacylglycerol, cholesterol, and apolipoprotein A-I in the rat. Biochim Biophys Acta. 1986;876:450-459.[Medline] [Order article via Infotrieve]

24. Roepke JE, Crabb DW. Loss of sexual differentiation of rat hepatocytes in short-term culture. Alcohol & Alcoholism. 1987;Suppl 1:263-264.

25. Wiedemann E, Schwartz E, Frantz AG. Acute and chronic estrogen effects upon serum somatomedin activity, growth hormone, and prolactin in man. J Clin Endocrinol Metab. 1976;42:942-952.[Abstract/Free Full Text]

26. Tannenbaum GS, Gurd W, Lapointe M, Pollak M. Tamoxifen attenuates pulsatile growth hormone secretion: mediation in part by somatostatin. Endocrinology. 1992;130:3395-3401.[Abstract/Free Full Text]

27. Rudling M, Parini P, Angelin B. Growth hormone and bile acid synthesis: Key role for the activity of hepatic microsomal cholesterol 7-hydroxylase in the rat. J. Clin Invest.. 1997;99:2239-2245.[Medline] [Order article via Infotrieve]

28. Jungmann H, Stiehm M, Harm K. Influence of starvation and kinesitherapy on blood lipids and their fractions. Klin Wochenschr. 1981;59:1061-1064.[Medline] [Order article via Infotrieve]

29. Ho KY, Veldhuis JD, Johnson ML, et al. Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man. J Clin Invest. 1988;81:968-975.

30. Edén S, Albertsson-Wikland K, Isaksson O. Plasma levels of growth hormone in female rats of different ages. Acta Endocrinol. 1978;88:676-690.

31. Eisenfeld AJ, Aten R, Weinberger M, Haselbacher G, Halpern K, Krakoff L. Estrogen receptor in the mammalian liver. Science. 1976;191:862-865.[Abstract/Free Full Text]

32. Angelin B, Rudling M. Growth hormone and hepatic lipoprotein metabolism. Curr Opin Lipidol. 1994;5:160-165.[Medline] [Order article via Infotrieve]

33. Edén S, Wiklund O, Oscarsson J, Rosén T, Bengtsson BÅ. Growth hormone treatment of growth hormone-deficient adults results in a marked increase in Lp(a) and HDL cholesterol concentrations. Arterioscler Thromb. 1993;13:296-301.[Abstract/Free Full Text]

34. Henriksson P, Angelin B, Berglund L. Hormonal regulation of serum Lp (a) levels: opposite effects after estrogen treatment and orchidectomy in males with prostatic carcinoma. J Clin Invest. 1992;89:1166-1171.

35. Olivecrona H, Ericsson S, Berglund L, Angelin B. Increased concentrations of serum lipoprotein (a) in response to growth hormone treatment. Br Med J. 1993;306:1726-1727.

This article has been cited by other articles:

				L. A. Henriquez-Hernandez, A. Flores-Morales, R. Santana-Farre, M. Axelson, P. Nilsson, G. Norstedt, and L. Fernandez-Perez Role of Pituitary Hormones on 17{alpha}-Ethinylestradiol-Induced Cholestasis in Rat J. Pharmacol. Exp. Ther., February 1, 2007; 320(2): 695 - 705. [Abstract] [Full Text] [PDF]

				H. Yura, M. Ishihara, Y. Kanatani, B. Takase, H. Hattori, S. Suzuki, M. Kawakami, and T. Matsui Interaction Study between Synthetic Glycoconjugate Ligands and Endocytic Receptors Using Flow Cytometry. J. Biochem., April 1, 2006; 139(4): 637 - 643. [Abstract] [Full Text] [PDF]

				L. Johansson, M. Rudling, T. S. Scanlan, T. Lundasen, P. Webb, J. Baxter, B. Angelin, and P. Parini Selective thyroid receptor modulation by GC-1 reduces serum lipids and stimulates steps of reverse cholesterol transport in euthyroid mice PNAS, July 19, 2005; 102(29): 10297 - 10302. [Abstract] [Full Text] [PDF]

				C. Lemieux, Y. Gelinas, J. Lalonde, F. Labrie, K. Cianflone, and Y. Deshaies Hypolipidemic action of the SERM acolbifene is associated with decreased liver MTP and increased SR-BI and LDL receptors J. Lipid Res., June 1, 2005; 46(6): 1285 - 1294. [Abstract] [Full Text] [PDF]

				Y. Suarez, C. Fernandez, D. Gomez-Coronado, A. J. Ferruelo, A. Davalos, J. Martinez-Botas, and M. A. Lasuncion Synergistic upregulation of low-density lipoprotein receptor activity by tamoxifen and lovastatin Cardiovasc Res, November 1, 2004; 64(2): 346 - 355. [Abstract] [Full Text] [PDF]

				K. N. Hewitt, W. C. Boon, Y. Murata, M. E. E. Jones, and E. R. Simpson The Aromatase Knockout Mouse Presents with a Sexually Dimorphic Disruption to Cholesterol Homeostasis Endocrinology, September 1, 2003; 144(9): 3895 - 3903. [Abstract] [Full Text] [PDF]

				J. Liu, F. Zhang, C. Li, M. Lin, and M. R. Briggs Synergistic Activation of Human LDL Receptor Expression by SCAP Ligand and Cytokine Oncostatin M Arterioscler Thromb Vasc Biol, January 1, 2003; 23(1): 90 - 96. [Abstract] [Full Text] [PDF]

				J. E Roeters van Lennep, H.T. Westerveld, D.W. Erkelens, and E. E van der Wall Risk factors for coronary heart disease: implications of gender Cardiovasc Res, February 15, 2002; 53(3): 538 - 549. [Abstract] [Full Text] [PDF]

				L. J. Wilcox, N. M. Borradaile, L. E. de Dreu, and M. W. Huff Secretion of hepatocyte apoB is inhibited by the flavonoids, naringenin and hesperetin, via reduced activity and expression of ACAT2 and MTP J. Lipid Res., May 1, 2001; 42(5): 725 - 734. [Abstract] [Full Text]

				C. Li, M. R. Briggs, T. E. Ahlborn, F. B. Kraemer, and J. Liu Requirement of Sp1 and Estrogen Receptor {{alpha}} Interaction in 17{beta}-Estradiol-Mediated Transcriptional Activation of the Low Density Lipoprotein Receptor Gene Expression Endocrinology, April 1, 2001; 142(4): 1546 - 1553. [Abstract] [Full Text]

				R. K. Dubey and E. K. Jackson Estrogen-induced cardiorenal protection: potential cellular, biochemical, and molecular mechanisms Am J Physiol Renal Physiol, March 1, 2001; 280(3): F365 - F388. [Abstract] [Full Text] [PDF]

				S. R Holmer, C. Hengstenberg, B. Mayer, A. Doring, H. Lowel, S. Engel, H.-W. Hense, M. Wolf, G. Klein, G. A.J Riegger, et al. Lipoprotein lipase gene polymorphism, cholesterol subfractions and myocardial infarction in large samples of the general population Cardiovasc Res, September 1, 2000; 47(4): 806 - 812. [Abstract] [Full Text] [PDF]

				P. Parini, B. Angelin, A. Stavreus-Evers, B. Freyschuss, H. Eriksson, and M. Rudling Biphasic Effects of the Natural Estrogen 17{beta}-Estradiol on Hepatic Cholesterol Metabolism in Intact Female Rats Arterioscler Thromb Vasc Biol, July 1, 2000; 20(7): 1817 - 1823. [Abstract] [Full Text] [PDF]

				A. Karjalainen, J. Heikkinen, M. J. Savolainen, A.-C. Backstrom, and Y. A. Kesaniemi Mechanisms Regulating LDL Metabolism in Subjects on Peroral and Transdermal Estrogen Replacement Therapy Arterioscler Thromb Vasc Biol, April 1, 2000; 20(4): 1101 - 1106. [Abstract] [Full Text] [PDF]

				P. Parini, B. Angelin, and M. Rudling Cholesterol and Lipoprotein Metabolism in Aging : Reversal of Hypercholesterolemia by Growth Hormone Treatment in Old Rats Arterioscler Thromb Vasc Biol, April 1, 1999; 19(4): 832 - 839. [Abstract] [Full Text] [PDF]

				N. Santanam, R. Shern-Brewer, R. McClatchey, P. Z. Castellano, A. A. Murphy, S. Voelkel, and S. Parthasarathy Estradiol as an antioxidant: incompatible with its physiological concentrations and function J. Lipid Res., November 1, 1998; 39(11): 2111 - 2118. [Abstract] [Full Text]

This Article Abstract Submit a response Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Parini, P. Articles by Rudling, M. Search for Related Content PubMed PubMed Citation Articles by Parini, P. Articles by Rudling, M.