Very low density lipoprotein triglyceride kinetics during hepatic lipase suppression by estrogen. Studies on the physiological role of hepatic endothelial lipase

Physiological interindividual variability in endogenous estradiol concentration does not influence adipose tissue and hepatic lipid kinetics in women

Objective

Increased triglyceride (TG) and apolipoprotein B-100 (apoB-100) concentrations in plasma are important risk factors for cardiovascular disease in women. Administration of some estrogen preparations raises plasma TG and apoB-100 concentrations by increasing hepatic very low-density lipoprotein (VLDL) TG and apoB-100 secretion rates. However, the influence of physiological variation in endogenous estradiol on VLDL-TG and VLDL-apoB-100 metabolism and on free fatty acid (FFA) release into plasma (the major source of fatty acids for VLDL-TG production) is not known.

Design and methods

We measured basal VLDL-TG, VLDL-apoB-100, and plasma FFA kinetics by using stable isotopically labeled tracers in 36 eumenorrheic, premenopausal women (age: 33 ± 2 years, BMI: 31 ± 1 kg/m2; mean ± s.e.m.) during the follicular phase of the menstrual cycle; participants were divided into two groups based on low (n = 18) or high (n = 18) plasma estradiol concentrations (defined as below or above the median value of 140 pmol/L in the whole group).

Results

Mean plasma estradiol concentration was >3-fold higher in the high-estradiol than in the low-estradiol group (299 ± 37 and 96 ± 7 pmol/L, P < 0.001); there was no difference in plasma progesterone concentrations between the two groups (P = 0.976). There were no significant differences in plasma FFA concentration, FFA rate of appearance in plasma, VLDL-TG and VLDL-apoB-100 concentrations, hepatic VLDL-TG and VLDL-apoB-100 secretion rates, VLDL-TG and VLDL-apoB-100 plasma clearance rates, and mean residence times (all P ≥ 0.45). No significant associations were found between plasma estradiol concentration and FFA, VLDL-TG, and VLDL-apoB-100 concentrations and kinetics (all P > 0.19).

Conclusions

Plasma estradiol concentration is not an important correlate of basal plasma FFA, VLDL-TG, and VLDL-apoB-100 kinetics in premenopausal women.

Sex differences feed into nuclear receptor signaling along the digestive tract

Sex differences in physiology are noted in clinical and animal studies. However, mechanisms underlying these observed differences between males and females remain elusive. Nuclear receptors control a wide range of physiological pathways and are expressed in the gastrointestinal tract, including the mouth, stomach, liver and intestine. We investigated the literature pertaining to ER, AR, FXR, and PPAR regulation and highlight the sex differences in nutrient metabolism along the digestive system. We chose these nuclear receptors based on their metabolic functions, and hormonal actions. Intriguingly, we noted an overlap in target genes of ER and FXR that modulate mucosal integrity and GLP-1 secretion, whereas overlap in target genes of PPARα with ER and AR modulate lipid metabolism. Sex differences were seen not only in the basal expression of nuclear receptors, but also in activation as their endogenous ligand concentrations fluctuate depending on nutrient availability. Finally, in this review, we speculate that interactions between the nuclear receptors may influence overall metabolic decisions in the gastrointestinal tract in a sex-specific manner.

Influence of glucocorticoids and growth hormone on insulin sensitivity in humans

The seminal concept proposed by Sir Harold Himsworth more than 75 years ago that a large number of patients with diabetes were 'insulin insensitive', now termed insulin resistance, has now expanded to include several endocrine syndromes, namely those of glucocorticoid excess, and growth hormone excess and deficiency. Synthetic glucocorticoids are increasingly used to treat a wide variety of chronic diseases, whereas the beneficial effects of recombinant growth hormone replacement therapy in children and adults with growth hormone deficiency have now been well-recognized for over 25 years. However, clinical and experimental studies have established that increased circulating levels of glucocorticoids and growth hormone can also lead to worsening of insulin resistance, glucose intolerance, overt diabetes mellitus and cardiovascular disease. Improved understanding of the physiological 24-h rhythmicity of glucocorticoid and growth hormone secretion and its influence on the dawn phenomenon and the Staub-Trauggot effect has therefore led to renewed interest in studies on the mechanisms of insulin resistance induced by exogenous administration of glucocorticoids and growth hormone in humans. In this review, we describe the physiological events that result from the presence of resistance to insulin action at the level of skeletal muscle, adipose tissue, and liver, describe the known mechanisms of glucocorticoid- and growth hormone-mediated insulin resistance, and provide an update of the contributions of glucocorticoids and growth hormone to understanding the pathophysiology of insulin resistance and its effects on several endocrine syndromes.

Sex differences in lipid and lipoprotein metabolism: it's not just about sex hormones

It is commonly thought that sex hormones are important regulators of plasma lipid kinetics and are responsible for sexual dimorphism in the plasma lipid profile. Here we discuss the findings from studies evaluating lipid and lipoprotein kinetics in men and women in the context of what we know about the effects of exogenous sex hormone administration, and we conclude that it is more complicated than that. It has become clear that normal physiological alterations in the hormonal milieu (i.e. due to menopause or throughout the menstrual cycle) do not significantly affect plasma lipid homeostasis. Furthermore, parenterally administered estrogens have either no effect or only very small beneficial effects, whereas orally administered estrogens raise plasma triglyceride concentrations--a phenomenon that is not consistent with the observed sex differences and likely results from the hepatic "first-pass effect." The effects of progestogens and androgens mimic only in part the differences in plasma lipids between men and women. Thus, the underlying physiological modulators of plasma lipid metabolism responsible for the differences between men and women remain to be elucidated.

Acute estrogen exposure does not affect basal very low-density lipoprotein-triglyceride production or oxidation in postmenopausal women

CONTEXT

Long-term hormone replacement therapy (HRT) with estradiol (E(2)) is associated with an altered lipid profile including unfavorable increases in triglyceride (TG) concentrations and augmented hepatic very low-density lipoprotein (VLDL)-TG production. There are indications that this effect of estrogens may be immediate.

OBJECTIVE

To study the in vivo effect of a single dose of E(2) on VLDL-TG kinetics and oxidation in humans.

METHODS

Eight healthy, postmenopausal women were given a single dose of either placebo or E(2) (4 mg) orally. VLDL-TG kinetics was assessed by a 240-min primed-continuous infusion of ex vivo labeled [1-(14)C]triolein-labeled VLDL. Fractional and absolute VLDL-TG oxidation was determined by hyamin trapping of exhaled (14)C label. Indirect calorimetry provided measurements of lipid oxidation.

RESULTS

Administration of 4 mg of E(2) orally rapidly increased plasma E(2) concentrations from below detection threshold to premenopausal levels. Free fatty acids (FFA) and TG concentrations were unaltered. No immediate effect was observed on either VLDL-TG production (placebo versus E(2)): 20.0+/-12.4 vs 24.1+/-10.7 micromol/min, P=0.33; VLDL-TG oxidation: 12.3+/-10.9 vs 12.6+/-5.6 micromol/min, P=0.93); or VLDL-TG clearance rates: 51.4+/-16.8 vs 64.9+/-28.8 ml/min, P=0.34).

CONCLUSIONS

Short-term E(2) elevation does not affect VLDL-TG production, oxidation, or clearance in humans. We therefore propose that HRT-associated dyslipidemia has a gradual rather than immediate onset.

Estrogen deficiency after menopause does not result in male very-low-density lipoprotein metabolism phenotype

CONTEXT

Sex differences in lipid metabolism result in a less proatherogenic plasma lipid profile in premenopausal women than men. The mechanisms responsible for this are unclear but are thought to be related to differences in the sex hormone milieu in men and women.

OBJECTIVE

Our objective was to evaluate the effect of endogenous sex hormones on very-low-density lipoprotein (VLDL) triglyceride (TG) and apolipoprotein B-100 (apoB-100) metabolism. EXPERIMENTAL DESIGN AND MAIN OUTCOME MEASURES: We measured basal VLDL-TG and VLDL-apoB-100 concentrations and kinetics by using stable isotope-labeled tracers.

SETTING AND PARTICIPANTS

Eight premenopausal women [age, 43 + or - 8 yr; body mass index (BMI), 35 + or - 4 kg/m(2); mean + or - sd], eight postmenopausal women (age, 55 + or - 4 yr; BMI, 34 + or - 4 kg/m(2)), and eight men (age, 41 + or - 13 yr; BMI, 34 + or - 4 kg/m(2)) were studied at Washington University School of Medicine, St. Louis, MO.

RESULTS

VLDL-TG secretion rate was approximately double (P < 0.05) in postmenopausal women and men compared with premenopausal women but not different in postmenopausal women and men. The secretion rate of VLDL-apoB-100 was not different in pre- and postmenopausal women but was greater (P < 0.05) in men than in women.

CONCLUSIONS

Endogenous ovarian sex steroids are responsible for sexual dimorphism in VLDL-TG secretion, whereas VLDL-apoB-100 secretion is not regulated by female reproductive hormones.

Gender differences in lipid metabolism and the effect of obesity

There are many differences between men and women, and between lean and obese subjects, in fatty acid and very low-density lipoprotein triglyceride and apolipoprotein B-100 metabolism. Currently, observations in this area are predominantly descriptive. The mechanisms responsible for sexual dimorphism in lipid metabolism are largely unknown.

Effects of hormone replacement therapy on serum lipids and phospholipids in postmenopausal women

Hormone replacement therapy (HRT) has been shown to reduce the risk of cardiovascular disease and the beneficial effects may be mediated in part by favourable changes in plasma lipid levels. Evidence exists concerning the effect of combined oestrogen and progestogen on lipids, nevertheless no such evidence can be found on the phospholipid profile, which is important the lipid metabolic pathways. In the present study, involving the serum lipids and lipoproteins, we observed an increase in the concentration of total cholesterol (P < 0.001), HDL-C (P < 0.001), HDL-C (P < 0.001), 2 HDL-C (P < 0.001) and a decrease in the ratio LDL-C/ 3 HDL-C (P < 0.001) in the subjects of Group B (oestrogen plus progestogens) compared with controls (baseline). Also, we found an increased in triglycerides (P < 0.01) and ApoA-1 (P < 0.01) concentrations in the subjects of Group A (oestrogen alone) compared with controls (baseline). With regard to the phospholipids, the main changes observed in their concentrations were: an increase in phosphatidyl choline (P < 0.001) and a decrease in phosphatidyl serine (P < 0.01) for both groups compared with controls. Also, a decrease in phosphatidylinositol (P < 0.01) in Group B compared with controls (baseline). The significance of these results are discussed.

Women produce fewer but triglyceride-richer very low-density lipoproteins than men

CONTEXT

Very low-density lipoproteins (VLDL) are a major risk factor for cardiovascular disease. The concentrations of VLDL particles and VLDL-triglyceride (TG) in plasma are lower in women than men, but the mechanisms responsible for these differences are unclear.

OBJECTIVE

The objective of the study was to investigate the effects of sex on VLDL-TG and VLDL-apolipoprotein B-100 (apoB-100) metabolism. EXPERIMENTAL DESIGN AND MAIN OUTCOME MEASURES: We measured basal VLDL-TG and VLDL-apoB-100 kinetics by using stable isotope labeled tracers.

SETTING/PARTICIPANTS

Twenty-six healthy, lean subjects (13 men, aged 29+/-5 yr; 13 women, aged 28+/-6 yr) were studied in the General Clinical Research Center at Washington University School of Medicine.

RESULTS

VLDL-TG and VLDL-apoB-100 concentrations were less in women than men (P<0.05). The secretion rate of VLDL-TG was approximately 70% greater (P<0.05), whereas the secretion rate of VLDL-apoB-100 (i.e. VLDL particles) was approximately 20% less (P<0.05) in women than men. The molar ratio of VLDL-TG and VLDL-apoB-100 secretion rates was therefore more than double (P<0.05) in women than men. VLDL-TG plasma clearance rate was approximately 70% greater in women than men (P<0.05), whereas VLDL-apoB-100 plasma clearance rate was not different between sexes. However, VLDL-TG and VLDL-apoB-100 mean residence times in plasma were both shorter (by 45 and 25%, respectively; P<0.05) in women than men.

CONCLUSIONS

Increased VLDL-TG plasma clearance is responsible for lower VLDL-TG concentration, whereas decreased VLDL-apoB-100 secretion rate, combined with shorter VLDL-apoB-100 residence time in plasma, is responsible for lower VLDL-apoB-100 concentration in women than men. The greater molar ratio of VLDL-TG and VLDL-apoB-100 secretion rates suggests that the liver in women secretes fewer but TG-richer VLDL particles than the liver in men.

No effect of menstrual cycle phase on basal very-low-density lipoprotein triglyceride and apolipoprotein B-100 kinetics

Dyslipidemia, manifested by increased plasma triglyceride (TG), increased total and LDL-cholesterol concentrations and decreased HDL-cholesterol concentration, is an important risk factor for cardiovascular disease. Premenopausal women have a less atherogenic plasma lipid profile and a lower risk of cardiovascular disease than men, but this female advantage disappears after menopause. This suggests that female sex steroids affect lipoprotein metabolism. The impact of variations in the availability of ovarian hormones during the menstrual cycle on lipoprotein metabolism is not known. We therefore investigated whether very-low-density lipoprotein (VLDL)-TG and VLDL-apolipoprotein B-100 (apoB-100) kinetics are different during the follicular (FP) and luteal phases (LP) of the menstrual cycle. We studied seven healthy, premenopausal women (age 27 +/- 2 yr, BMI 25 +/- 2 kg/m(2)) once during FP and once during LP. We measured VLDL-TG, VLDL-apoB-100, and plasma free fatty acid (FFA) kinetics by using stable isotope-labeled tracers, VLDL subclass profile by nuclear magnetic resonance spectroscopy, whole body fat oxidation by indirect calorimetry, and the plasma concentrations of lipoprotein lipase (LPL) and hepatic lipase (HL) by ELISA. VLDL-TG and VLDL-apoB-100 concentrations in plasma, VLDL-TG and VLDL-apoB-100 secretion rates and mean residence times, VLDL subclass distribution, FFA concentration and rate of appearance in plasma, whole body substrate oxidation, and LPL and HL concentrations in plasma were not different during the FP and the LP. We conclude that VLDL-TG and VLDL-apoB-100 metabolism is not affected by menstrual cycle phase.

Sex steroids and plasma lipoprotein levels in healthy women: The importance of androgens in the estrogen-deficient state

The role of endogenous estrogens and androgens and their potential interaction in atherosclerosis is not well understood. Therefore, we investigated the effects of natural menopause and endogenous sex steroids on triglycerides (TG), a major risk factor for cardiovascular disease in women. Fasting lipid and lipoprotein concentrations, postheparin lipase activities, kinetic indicators of triglyceride lipolysis, and various hormone levels, including dehydroepiandrostenedione-sulfate (DHEA-S), (bioavailable) testosterone, and androstenedione, were determined in 18 premenopausal and 18 postmenopausal women, matched for age and body composition. Fasting plasma TG were 0.69 +/- 0.29 mmol/L in postmenopausal women and 0.73 +/- 0.33 mmol/L in premenopausal women (difference not significant [NS]). Approximately 30% of all plasma TG were present in the very-low-density lipoproteins (VLDLs) in both groups. No differences were found between groups in plasma lipolytic potential of TG-rich lipoproteins. Univariate analysis revealed that VLDL-TG concentrations were strongly related to insulin (r = 0.84, P =.0001) and androstenedione (r = 0.65, P =.004) in postmenopausal women. Multivariate analysis of potential determinants of VLDL-TG showed that insulin, androstenedione, and bioavailable testosterone were independent variables, explaining 87% of the variability (r = 0.93, P =.0001) in postmenopausal women. In contrast, in premenopausal women, the only identified predictor of fasting VLDL-TG in univariate and multivariate analysis was insulin (r = 0.72, P =.001). Our results show that the association of androgens with TG varied depending on androgen concentrations, the relative androgenic potential, and most importantly on hormonal milieu. Endogenous androgens were only related to plasma VLDL-TG in the estrogen-deficient state.

Effects of Continuous and Cyclic Hormone Replacement Therapy on the Lipoprotein Profile in Postmenopausal Women

Objective:

To provide an overview of the information concerning cyclic and continuous hormone replacement therapy (HRT) and the effects of the various regimens on the lipid profile.

Data Sources:

MEDLINE (January 1966–December 1996) and Index Medicus (January 1995-December 1996) searches were conducted to identify relevant studies and review articles. Bibliographies of selected articles also were reviewed.

Study Selection:

Studies addressing continuous and cyclic use of a progesterone formulation along with daily conjugated equine estrogen (CEE) and their effects on the lipid profiles of postmenopausal women were selected for review.

Data Extraction:

Applicable data were selected and used in a review format.

Data Synthesis:

CEE has been shown to increase high-density-lipoprotein cholesterol, which may decrease the risk of cardiovascular heart disease. Medroxyprogesterone acetate (MPA) has been added to CEE for postmenopausal HRT to prevent endometrial hyperplasia and cancer. However, MPA negates the beneficial effects of CEE on the lipid profile. Many different HRT regimens, both continuous and cyclic, are being used to increase compliance and to decrease adverse effects. Which regimen offers the best lipid profile results is evaluated in this article.

Conclusions:

The lipid profile effects of HRT using continuous MPA are not significantly different from those obtained with cyclic MPA. Use of a continuous MPA regimen may also offer the long-term advantages of less vaginal bleeding and increased compliance.

Estrogens, progestins and lipid metabolism

OBJECTIVES

To review some aspects in the recent literature related to the effects of postmenopausal estrogen and progestin use on major plasma lipoprotein risk factors for coronary heart disease (CHD).

METHODS

Collection of relevant information from medical journals, and by the use of Medline and Current Contents.

RESULTS

The beneficial effects of estrogen (LDL cholesterol reduction and HDL cholesterol elevation) are well established. The effects on HDL are modified to different degrees by progestins, depending on the androgenic properties of the latter: the 'sex steroid sensitive' HDL2 subfraction is decreased by nortestosterone derived progestins with androgenic activity. Recently developed methodology employing stable isotopes has helped to clarify underlying mechanisms. Progestins alone, as well as estrogen-progestin combinations have been shown to reduce the plasma levels of Lp(a), another lipoprotein risk factor for CHD. According to one study, estrogen administered alone had a similar effect.

CONCLUSIONS

The effects of hormone replacement therapies on lipid metabolism have been partly established and investigations on the underlying mechanisms are being published. This information will be useful for developing new replacement regimens with more protection against CHD and less adverse effects.

Effect of hepatic lipase on LDL in normal men and those with coronary artery disease

Hepatic triglyceride lipase (HL) is thought to play a role in the formation of low density lipoproteins (LDLs) from small very low density lipoproteins (VLDLs) and intermediate density lipoproteins (IDLs). To analyze the possible physiological role of HL in determining LDL buoyancy, size, and chemical composition, HL activity and LDL were studied in 21 patients with coronary artery disease (CAD) and 23 normolipidemic subjects. In both groups, LDL buoyancy and size were inversely associated with HL activity levels. The effect of HL on LDL size was comparable in CAD patients and in normolipidemic subjects. HL appeared to influence LDL lipid composition primarily by affecting the surface lipid components. The free cholesterol content of LDL particles was highly correlated with HL activity in both CAD and normolipidemic individuals. The free cholesterol to phospholipid ratio in LDL particles correlated with HL in both CAD and normolipidemic subjects. When the individuals were separated according to their LDL subclass patterns, pattern B subjects had significantly higher HL than pattern A subjects in both CAD and normolipidemic groups. The analysis of the cholesterol distribution profiles across the lipoprotein density gradient confirmed that LDL buoyancy is affected by HL. These data support the hypothesis that HL modulates the physical and compositional properties of LDL and contributes to the expression of the LDL subclass phenotype, suggesting a physiological role for HL in LDL metabolism.

Effects of postmenopausal estrogen replacement on the concentrations and metabolism of plasma lipoproteins

BACKGROUND

Postmenopausal estrogen-replacement therapy may reduce the risk of cardiovascular disease, and this beneficial effect may be mediated in part by favorable changes in plasma lipid levels. However, the effects on plasma lipoprotein levels of postmenopausal estrogens in the low doses currently used have not been precisely quantified, and the mechanism of these effects is unknown.

METHODS

We conducted two randomized, double-blind crossover studies in healthy postmenopausal women who had normal lipid values at base line. In study 1, 31 women received placebo and conjugated estrogens at two doses (0.625 mg and 1.25 mg per day), each treatment for three months. In study 2, nine women received placebo, oral micronized estradiol (2 mg per day), and transdermal estradiol (0.1 mg twice a week), each treatment for six weeks. The metabolism of very-low-density lipoprotein (VLDL) and low-density lipoprotein (LDL) was measured by endogenously labeling their protein component, apolipoprotein B.

RESULTS

In study 1, the conjugated estrogens at doses of 0.625 mg per day and 1.25 mg per day decreased the mean LDL cholesterol level by 15 percent (95 percent confidence interval, 11 to 19 percent; P less than 0.0001) and 19 percent (95 percent confidence interval, 15 to 23 percent; P less than 0.0001), respectively; increased the HDL cholesterol level by 16 percent (95 percent confidence interval, 12 to 20 percent; P less than 0.0001) and 18 percent (95 percent confidence interval, 14 to 22 percent; P less than 0.0001), respectively; and increased VLDL triglyceride levels by 24 percent (95 percent confidence interval, 8 to 40 percent; P less than 0.003) and 42 percent (95 percent confidence interval, 26 to 58 percent; P less than 0.0001), respectively. In study 2, oral estradiol increased the mean concentration of large VLDL apolipoprotein B by 30 +/- 10 percent (P = 0.05) by increasing its production rate by 82 +/- 18 percent (P less than 0.01). Most of this additional large VLDL was cleared directly from the circulation and was not converted to small VLDL or LDL. Oral estradiol reduced LDL cholesterol concentrations by 14 +/- 3 percent (P less than 0.005), because LDL catabolism increased by 36 +/- 7 percent (P less than 0.005). The oral estradiol increased the HDL cholesterol level by 15 +/- 2 percent (P less than 0.0001). Transdermal estradiol had no effect.

CONCLUSIONS

The postmenopausal use of oral estrogens in low doses favorably alters LDL and HDL levels that may protect women against atherosclerosis, while minimizing potentially adverse effects on triglyceride levels. The decrease in LDL levels results from accelerated LDL catabolism; the increase in triglyceride levels results from increased production of large, triglyceride-rich VLDL.

Estradiol increases the secretion of hepatic lipase by rat hepatocyte cultures

Hepatic lipase (EC 3.1.1.3) is synthesized and secreted by parenchymal hepatocytes and binds to endothelial cells of liver sinusoids. The present study shows that the activity of hepatic lipase secreted by hepatocyte cultures from male rats in increased approx. 6-fold after 10 h culture with 10 microM 17 beta-estradiol. The stimulatory effect of 17 beta-estradiol is biphasic and declines at higher concentrations. In hepatocytes from male rats: progesterone, unlike 17 beta-estradiol, had only a small stimulatory effect when present as the sole hormone and a small inhibitory effect in the presence of 17 beta-estradiol, while testosterone and dexamethasone had no effect. Hepatocyte cultures from female rats had a higher basal rate of hepatic lipase secretion than cells from male rats and showed a smaller stimulation by 17 beta-estradiol. These results suggest that 17 beta-estradiol might regulate the secretion of hepatic lipase by hepatocytes, and presumably the activity of the enzyme at either the endothelial surface of the liver sinusoids or at extrahepatic sites.

Endocrine control of plasma lipoprotein metabolism: effects of gonadal steroids

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

Role of plasma lipoproteins in the pathogenesis of atherosclerotic disease, with special reference to sex hormone effects

Plasma lipoproteins constitute a complex lipid transport system. Very low-density lipoproteins transport triglycerides to peripheral tissues, whereas low-density lipoproteins are the main carriers of cholesterol. Cholesterol transport in low-density lipoprotein enters body cells by way of "low-density lipoprotein receptor pathway" but may also be taken up by macrophages by way of the "scavenger pathway." Excessive influx of cholesterol by way of the "scavenger pathway" may result in deposition of cholesterol in arterial walls and atheroma formation. In a yet incompletely known process of "reverse cholesterol transport," cholesterol is carried away from the tissues to the liver by high-density lipoproteins. The above-mentioned transport processes are regulated by a well-synchronized system that involves several enzymes and lipid transport proteins. Under normal conditions, the lipoprotein system is able to balance the flow of cholesterol and other lipids in both directions between the liver and peripheral tissues. This delicate balance may be disturbed by many factors, including contraceptive steroids. The metabolic steps influenced by administration of contraceptive steroids are summarized.

Time-dependent alterations in lipid metabolism during treatment with low-dose oral contraceptives

The effect of sex steroids on lipid metabolism depends on the type and dose of the compounds, the route of administration, and the duration of treatment. Therefore the composition of an oral contraceptive determines the resultant effect on lipids and lipoproteins. During 12 months of treatment, the effects of two oral contraceptives containing 30 micrograms of ethinyl estradiol and 150 micrograms of desogestrel (EE/DG) or 75 micrograms of gestodene (EE/GSD) on 19 serum parameters of lipid metabolism were followed in 11 women each. There was no change in total cholesterol and phospholipids. Total triglyceride levels were significantly elevated only by EE/GSD. After 3 and 6 months of intake of both preparations, a transitory increase in the triglyceride content of very low-density lipoprotein and low-density lipoprotein and a decrease in low-density lipoprotein-phospholipids was observed. After 12 months, very low-density lipoprotein cholesterol, very low-density lipoprotein phospholipids, and apolipoprotein B were significantly elevated, whereas very low-density lipoprotein triglycerides and all components of low-density lipoprotein were unchanged. Most of the components of high-density lipoprotein (HDL) were increased as a result of a rise in HDL3 and apolipoprotein A2, whereas HDL2 and apolipoprotein A1 were not altered. There was no significant difference between the effects of the two preparations, although those of EE/GSD were mostly more pronounced. The increase in high-density lipoprotein, very low-density lipoprotein, and total triglycerides reflects a slight preponderance of the effect of the estrogen component. Because low-density lipoprotein cholesterol and total cholesterol were not changed, treatment with both formulations is in all probability not associated with an elevated risk of atherosclerosis.

Endocrine, metabolic, and clinical effects of gestrinone in women with endometriosis

The effect of oral gestrinone, 2.5 mg twice weekly for 6 months, was studied in 11 women with mild or moderate endometriosis laparoscopically confirmed. The mean laparoscopic score decreased from 17.18 to 9.09 (P greater than 0.005). Painful symptoms were relieved in all patients within 2 months from start of therapy. Gonadotropins, prolactin (PRL) 17 beta-estradiol (17 beta-E2), estrone (E1), progesterone (P), androstenedione (A), and dehydroepiandrosterone sulfate (DHEA-S) remained in the follicular phase range. Total testosterone (TT) and sex hormone-binding globulin (SHBG) decreased, whereas free testosterone (FT) slightly increased. Metabolic studies showed a decrease of total triglycerides, very low-density lipoprotein (VLDL) triglycerides, and high-density lipoprotein (HDL) and VLDL cholesterol, parallel to the decrease of associated apoproteins. Low-density lipoprotein cholesterol and apoprotein B increased during therapy. The results suggest that gestrinone possesses antiestrogenic, androgenic, and progestigenic effects at therapeutic dosages both by acting on central and peripheral steroid receptors. For its efficacy and good tolerance, gestrinone may be considered an option for treating endometriosis.

Relationship between lipoprotein lipase activity and plasma sex steroid level in obese women

In obese women (n = 16) at their weight, fasting adipose tissue lipoprotein lipase (LPL) activity, obtained by elution with serum and heparin at 4 degrees and 37 degrees C, was inversely correlated to plasma estradiol levels (r = -0.724; P = 0.002) and (r = -0.641; P = 0.010), respectively. Furthermore, fasting postheparin plasma LPL activity during a heparin infusion, showed an even stronger inverse correlation to plasma estradiol when measured at 60 min (r = -0.815; P less than 0.001). None of the above parameters was correlated to the body mass index. Postprandial LPL activity in postheparin plasma, measured 10 min after a heparin injection, showed a strong positive correlation with plasma free testosterone (r = 0.780; P = 0.001). Neither of these parameters was correlated with the body mass index. The origin of this LPL activity is presently unknown but could conceivably represent a pool of LPL from skeletal muscle. Since it has been shown convincingly that estrogen decreases adipose tissue LPL activity in the rat, the present studies strongly suggest that estradiol is a major negative regulator of fasting adipose tissue LPL activity in women.

Sex, plasma lipoproteins, and atherosclerosis: prevailing assumptions and outstanding questions

We review the hypothesis that the incidence of coronary heart disease (CHD) is higher in men than in women due to differences in plasma lipoprotein risk factors between the sexes. Men and women appear to be equally susceptible to the effects of lipoprotein risk factors for CHD, and the difference between the sexes in lipoprotein risk factors for CHD appears to be consistent with their being, at least in part, responsible for the sex difference in CHD. This is apparent both when men and women of equal age are compared, and when age-related variations in the sex differences in plasma lipoproteins and CHD are considered. Differences between the sexes in lipoprotein concentrations are still present when sex differences in adiposity, cigarette smoking, physical activity, and diet are taken into account. Evidence relating these sex differences in CHD and lipoproteins to the effects of sex hormones is critically examined. It is commonly accepted that androgens induce changes in lipoprotein concentrations that would predispose towards CHD, whereas estrogens are held to have opposite effects. However, much of the evidence for this comes from studies of changes associated with administration of synthetic gonadal steroids or with changes in gonadal function. Studies of differences in lipoprotein metabolism in normal men and women are extremely limited. In males high-density lipoprotein (HDL) cholesterol levels fall at puberty, correlating with the rise in plasma testosterone concentrations. In females, HDL levels do not change at puberty, despite the rise in estrogen concentrations. Evidence for lipoprotein changes during the menopause, when estrogen levels decline, is equivocal. Similarly, the evidence for an increase in CHD incidence at the menopause is inconclusive. National mortality data indicate that the decreasing sex difference in CHD after 50 years of age is due to a declining rate of increase in men rather than to an acceleration in CHD incidence in women. In men the age-related increase in low-density lipoprotein (LDL) concentrations diminishes beyond 50 years of age, whereas in women the rate of increase remains unchanged. Studies of the effects of gonadectomy are of doubtful relevance in assessing the roles of sex hormones in CHD, and have not been performed with sufficient rigor to provide definitive conclusions.(ABSTRACT TRUNCATED AT 400 WORDS)

Enzymes involved in triglyceride hydrolysis

The lipolytic enzymes LPL and HL play important roles in the metabolism of lipoproteins and participate in lipoprotein interconversions. LPL was originally recognized to be the key enzyme in the hydrolysis of chylomicrons and triglyceride, but it also turned out to be one determinant of HDL concentration in plasma. When LPL activity is high, chylomicrons and VLDL are rapidly removed from circulation and a concomitant rise of the HDL2 occurs. In contrast, low LPL activity impedes the removal of triglyceride-rich particles, resulting in the elevation of serum triglycerides and a decrease of HDL (HDL2). Concordant changes of this kind in LPL and HDL2 are induced by many physiological and pathological perturbations. Finally, the operation of LPL is also essential for the conversion of VLDL to LDL. This apparently clear-cut role of LPL in lipoprotein interconversions is contrasted with the enigmatic actions of HL. The enzyme was originally thought to participate in the catalyses of chylomicron and VLDL remnants generated in the LPL reaction. However, substantial in vitro and in vivo data indicate that HL is a key enzyme in the degradation of plasma HDL (HDL2) in a manner which opposes LPL. A scheme is presented for the complementary actions of the two enzymes in plasma HDL metabolism. In addition, recent studies have attributed a role to HL in the catabolism of triglyceride-rich lipoproteins, particularly those containing apo E. However, this function becomes clinically important only under conditions where the capacity of the LPL-mediated removal system is exceeded. Such a situation may arise when the input of triglyceride-rich particles (chylomicrons and/or VLDL) is excessive or LPL activity is decreased or absent.

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.