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Arteriosclerosis, Thrombosis, and Vascular Biology. 1996;16:431-440

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(Arteriosclerosis, Thrombosis, and Vascular Biology. 1996;16:431-440.)
© 1996 American Heart Association, Inc.

Articles

Oral Estrogen Replacement Therapy in Postmenopausal Women Selectively Raises Levels and Production Rates of Lipoprotein A-I and Lowers Hepatic Lipase Activity Without Lowering the Fractional Catabolic Rate

Eliot A. Brinton

From the Department of Internal Medicine, Bowman Gray School of Medicine, Winston-Salem, NC.

Correspondence to Eliot A. Brinton, MD, Department of Internal Medicine, Bowman Gray School of Medicine, Medical Center Blvd, Winston-Salem, NC 27157-1047. E-mail ebrinton@bgsm.edu.


*    Abstract
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 *Abstract
down arrowIntroduction
 down arrowMethods
down arrowResults
down arrowDiscussion
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Abstract To characterize the apolipoprotein (apo) subfraction specificity of the increase in high density lipoprotein (HDL) levels that is induced by oral estrogen and to explore the metabolic mechanisms thereof, six healthy, postmenopausal women were studied during each of two 5-week periods of a low-fat diet with and without ethinyl estradiol 0.05 mg/d. With estrogen, HDL cholesterol levels increased 36% (mean±SD, 47±15 versus 63±18 mg/dL, basal versus estrogen periods, respectively; P=.004), plasma levels of apo A-I increased by 27% (147±24 versus 186±34 mg/dL; P=.003), and apo A-II levels increased 17% (39±5 versus 46±4 mg/dL; P=.0003). Apo A-I mass in particles that contained apo A-I but no apo A-II (ie, Lp A-I) increased 66% (43±13 versus 71±17 mg/dL; P=.002) while the sum of apo A-I and apo A-II mass in particles with both apo A-I and apo A-II (ie, Lp A-I/A-II) showed only a nonsignificant 14% increase (144±23 versus 163±29 mg/dL; P=.12). In radioiodinated-protein turnover studies the production rate (PR) of each species changed in proportion to the change in its plasma concentration: Lp A-I PR increased 76% (4.7±1.8 versus 8.2±2.5 mg·kg-1·day-1; P=.001) while Lp A-I/A-II PR showed only a nonsignificant increase of 22% (14.1±1.8 versus 17.2±5.4 mg·kg-1·day-1; P=.2). Hepatic lipase (HL) activity in postheparin plasma decreased 66% with estrogen (10.4±4.3 versus 3.5±0.8 µmol·mL-1·h-1; P=.005) while lipoprotein lipase activity was unchanged (7.1±1.6 versus 7.6±1.6 µmol·mL-1·h-1; P>.2). Despite the large decrease in HL activity, the fractional catabolic rate of Lp A-I did not decrease (0.241±0.048 versus 0.258±0.066 pool/d; P=.2), nor was that of Lp A-I/A-II lessened (0.221±0.030 versus 0.233±0.053 pool/d; P>.2). Thus, oral estrogen replacement therapy has a novel and potentially antiatherogenic effect: a large and selective increase in Lp A-I levels. The metabolic mechanism of the estrogen-induced increase in Lp A-I concentration appears to be the increase in its PR. Although the ability of estrogen to suppress HL activity has been confirmed, surprisingly this decrease was not accompanied by any decrease in the fractional catabolic rate of Lp A-I or Lp A-I/A-II. This suggests that HL may not play a major role in regulating HDL protein catabolism during suppression of HL by estrogen.


Key Words: HDL • turnover kinetics • ethinyl estradiol • postmenopausal women • hepatic lipase


*    Introduction
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 up arrowAbstract
 *Introduction
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down arrowResults
down arrowDiscussion
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Plasma levels of HDL are strong inverse predictors of atherosclerosis risk,1 but the mechanisms regulating HDL levels in humans remain poorly understood. Owing to numerous differences in HDL metabolism between animals and humans, turnover studies of HDL in human subjects may be necessary to clearly delineate the factors that regulate variability in HDL levels in the clinical setting.

One promising means of raising HDL levels is estrogen replacement therapy in postmenopausal women. Oral estrogen therapy is among the most potent methods of raising levels of HDL-C and apo A-I.2 3 4 5 6 This effect is believed to contribute significantly to the evident antiatherogenic effects of estrogen replacement. Studies of estrogen replacement in postmenopausal women are of additional interest and importance for several other reasons, including the high rates of atherosclerosis in older women 7 and evidence that these rates increase at the menopause8 and may be blocked by oral estrogen replacement.9 Furthermore, because estrogen replacement therapy constitutes a potent method of raising HDL levels, it may serve as a useful probe for exploring the mechanisms that regulate HDL metabolism. Despite the clinical and physiological importance of postmenopausal estrogen in HDL metabolism, there are only three published studies of HDL turnover in postmenopausal subjects,3 6 10 one of which reports on only a single subject.3

An important aspect of HDL metabolism is the fact that essentially all HDL particles have apo A-I, but many lack apo A-II.11 HDL that contains apo A-II is called Lp A-I/A-II and appears to be distinct in many ways from HDL without apo A-II, which is called Lp A-I. Lp A-I may be a better cholesterol acceptor in vitro12 13 and appears to be more antiatherogenic than Lp A-I/A-II in cross-sectional epidemiological studies.14 15 16 17 Further evidence for this difference comes from studies of transgenic mice, which show that overexpression of human apo A-I prevents atherogenesis, whereas this effect is blocked by overexpression of human18 or mouse19 apo A-II. The turnover of Lp A-I and Lp A-I/A-II has been studied in human subjects by exchange labeling with purified apo A-I,20 and the FCR of apo A-I on Lp A-I has been reported to be 18% faster than that on Lp A-I/A-II in young adult subjects. Although convenient, the exchange-label method has certain limitations, including rapid attainment of equilibrium among tracers20 and the potential for artifactual elevations of the FCR.21 22 These drawbacks may be at least partially circumvented by use of whole-particle (in situ) labeling; however, there are no published studies that have employed whole-particle–labeled HDL in the study of Lp A-I and Lp A-I/A-II turnover in human subjects.

Estrogen is known to cause greater elevations in apo A-I than in apo A-II levels.2 4 6 If one assumes that the molar ratio of apo A-I to apo A-II in Lp A-I/A-II remains constant,11 then an agent such as estrogen, which disproportionately increases apo A-I levels, should selectively raise levels of Lp A-I compared with Lp A-I/A-II. Because women appear to have higher levels of Lp A-I (but not of Lp A-I/A-II) than do men,23 estrogen may selectively raise Lp A-I levels. Studies of estrogen and HDL turnover have indeed revealed a selective increase in the PR of apo A-I while the PR of apo A-II remained unchanged.2 6 These data suggest that estrogen may selectively increase the PR of Lp A-I without altering the PR of Lp A-I/A-II. If, as recently reported, the PR of apo A-II is a key regulator of Lp A-I/A-II levels,24 then a lack of effect of estrogen on the apo A-II PR implies that estrogen may fail to alter either the levels or the turnover of Lp A-I/A-II.

The other key effect of estrogen on HDL turnover is its reported ability to reduce the FCR,3 10 which is believed to be related to its well-known downregulation of HL activity in postheparin plasma.3 4 5 10 Lipolytic activity has been reported to promote the loss of apo A-I from HDL,22 25 which may lead to rapid apo A-I catabolism.22 Because apo A-I appears to dissociate much more readily from Lp A-I than Lp A-I/A-II26 and because apo A-II may inhibit HL activity,27 profound reductions in HL activity during estrogen therapy might be expected to selectively reduce the FCR of Lp A-I by lessening the tendency of apo A-I to dissociate from Lp A-I. Thus, various lines of evidence suggest that estrogen should selectively raise Lp A-I levels and that it may do so by either or both mechanisms: a selective increase in the PR of Lp A-I and/or a selective reduction in the FCR of Lp A-I due to reductions in HL-related self-dissociation of Lp A-I. Despite the interesting and important nature of these possibilities, there appear to be no published studies exploring them in human subjects.

The current study was designed to explore the effects of postmenopausal oral estrogen replacement therapy on the levels and metabolism of the protein components of HDL subspecies Lp A-I and Lp A-I/A-II. Furthermore, it is the first turnover study to utilize whole-labeled Lp A-I and Lp A-I/A-II in human subjects. It was hypothesized that estrogen therapy would (1) selectively raise Lp A-I more than Lp A-I/A-II levels, (2) raise the PR of Lp A-I more than that of Lp A-I/A-II, and (3) decrease HL activity, thereby reducing the FCR of Lp A-I more than that of Lp A-I/A-II. The results of this study confirm the first two hypotheses but provide mixed results for the third, in that HL activity decreased dramatically without decreasing the FCR of either Lp A-I or Lp A-I/A-II.


*    Methods
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 *Methods
down arrowResults
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Subjects
Postmenopausal women were recruited for the study if at least 1 year had elapsed since their last menses or if they were surgically menopausal, at least 45 years old, and had an elevated follicle stimulating hormone level (>27.6 mIU/mL). Although subjects with preexisting lipid disorders were allowed to participate in the study, none were taking any medications known to alter lipid levels. Subjects were free from significant systemic illness, as judged by their medical history, physical examination, screening chemistry panel, complete blood count, and urinalysis.

Kinetic Studies
All six subjects participated in two study periods of 5 weeks each, during which time they took either ethinyl estradiol 0.05 mg/d or no medicine. Subjects No. 3 and 6 took the estrogen during the first study period and the others took it during the second. Subjects No. 3 through 6 consumed a constant metabolic diet throughout both study periods, which they received daily from the General Clinical Research Center kitchen. The diet consisted of typical whole foods that had been carefully chosen and weighed to provide the following composition: 17% of calories as protein, 20% as fat (6% saturated, 7% monounsaturated, and 7% polyunsaturated), and 63% as carbohydrate, with 150 mg cholesterol per 2000 calories. Instead of the fixed metabolic diet, subjects No. 1 and 2 consumed their usual diet; however, they were carefully instructed not to depart from their typical intake during the study periods and, as judged by analysis of a 7-day food diary kept during each phase of the study, their diets were determined to have differed significantly neither between the two paired studies nor from the metabolic intake of the other subjects. Caloric needs for each subject were initially estimated from the Harris-Benedict equation,28 and adjustments were allowed as needed for changes in body weight >2 kg during the first 3 weeks of each diet period. Body mass, measured daily during the final 10 days of each study period, was constant during each turnover study. Although body mass was slightly higher during the estrogen therapy compared with the basal period (1.1±0.8 kg higher; range, 0 to 2.2 kg; P=.023), this difference was probably due to fluid retention, and its variability did not appear to explain heterogeneity in any major study variable. Neither alcohol nor tobacco were allowed during the study periods. There was a 1- to 3-week interval between study periods. The study protocol was approved by the Clinical Research Practices Committee of Bowman Gray School of Medicine, and informed, written consent was received from each subject for each study.

After the first 3 weeks of each study period, venous blood samples were drawn into standard vacuum tubes (Vacutainer, Becton Dickinson) containing EDTA and the plasma was separated by low-speed centrifugation for autologous tracer preparation. One milliliter of each plasma sample was passed over three immunoaffinity columns in series, ie, anti–apo B, anti–apo A-II, and anti–apo A-I, with a standard column buffer (0.15 mol/L NaCl; 0.01 mol/L EDTA; 0.05 mol/L Tris HCl, pH 7.4; and 0.05% NaN3). Retained fractions from the second and third columns were eluted promptly with 0.1 mol/L Gly HCl, pH 2.5, and the resulting protein peak was collected and immediately neutralized to pH 7.4 by addition of Tris base. The lipoproteins that eluted from the anti–apo A-II column were considered to be Lp A-I/A-II, whereas those from the anti–apo A-I column were considered to be Lp A-I (with <1% apo A-II by RIA). These two species were concentrated by using a stirred cell (Amicon), and total protein concentration was measured before and after concentration with a Bio-Rad protein assay kit.

The resulting immunoaffinity-prepared HDL subspecies were subjected to radioiodination with 125I or 131I using the modified ICl method of Bilheimer et al.29 This radiolabeled material was loaded onto a gel filtration column system (Superdex 200 and Superdex 75 column in series), and tracer approximately the size of HDL3b was pooled and tested for sterility and pyrogenicity before injection. More than 95% of the label was associated with protein (precipitable in 10% trichloroacetic acid). Thus, this study deals with the metabolism of only the protein component of HDL. Label distribution among proteins by size was assessed by SDS-PAGE. Of the Lp A-I tracer, 77±4% was in the 28 000-D band, corresponding to apo A-I; the C apolipoproteins (C-I, C-II, and C-III) had 2.6±1.1%; apo E had <1%; and the remainder was divided among other proteins both larger and smaller than apo A-I on the gel. Of the Lp A-I/A-II tracer, 45±2% was in apo A-II, 43%±3% in apo A-I, 7.3±2.5% on the C apolipoproteins, <1% on apo E, and the remainder in various larger and smaller proteins.

Four weeks into the study period, the subjects received a bolus injection of 25 µCi each of 125I–Lp A-I/A-II and 131I–Lp A-I (or vice versa) by vein. EDTA-plasma was prepared from blood drawn at 5, 10, 15, and 30 minutes; 1, 2, 3, 7, 24, and 31 hours; and 2, 3, 6, and 7 days after injection to measure the die-away of injected radioactivity. A curve was fitted to the plasma die-away data by using the SAAM II multicompartmental modeling program, from which the FCR was also derived. Urine samples were collected at the beginning and end of the blood collection times, and radioactivity was also measured. The ratio of urinary to plasma radioactivity, an independent measure of fractional catabolism, was calculated and generally found to corroborate the FCR that was calculated from the die-away results (data not shown). PR was calculated as the product of FCR and pool size (concentration of compound being traced times plasma volume, estimated as 4.5% of body weight) divided by body weight.

Because the radioiodine for each tracer was attached to several apolipoproteins, the possibility of selective catabolism of one or more of these apolipoproteins was assessed in representative subjects by the following method. One milliliter of plasma was taken at several times after tracer injection, denatured, reduced by adding urea and ß-mercaptoethanol to yield concentrations of 6 mol/L and 0.25% (vol/vol), respectively, and incubated overnight at 4°C. The sample was then loaded onto a preparatory scale Superdex 75 column and eluted with 6 mol/L urea, 0.025% (vol/vol) ß-mercaptoethanol, and 0.05 mol/L Tris, pH 8.2, at a flow rate of 5 mL/min. Column fractions were monitored for apo A-I concentration by RIA and for radioactivity by gamma counting. Both curves were subjected to gaussian deconvolution by the program of Verdery et al,30 and the percentage of counts of each tracer in apo A-I and apo A-II was estimated as the percentage of counts in the corresponding peaks. By this method the percentage of total counts of Lp A-I tracer in apo A-I and of Lp A-I/A-II in apo A-I and apo A-II were unaltered during the 7-day turnover period during both basal and estrogen studies. Thus, the die-away of Lp A-I tracer is assumed to be a reasonable approximation of the turnover of apo A-I on that particle. Similarly, it appears that the turnover of Lp A-I/A-II tracer approximates the turnover of apo A-I and apo A-II on that species. Furthermore, as discussed below, this suggests that apo A-I and apo A-II components of Lp A-I/A-II tracer are largely catabolized together.

Lipid, Lipoprotein, and Apolipoprotein Levels
On the days of the heparin test and tracer injection and on postinjection days 2 or 3 and 6 or 7, fasting EDTA-plasma was collected for measurement of lipid, lipoprotein, and apolipoprotein levels, including TC, TGs, and HDL-C by Lipid Research Clinics methods31 32 ; calculation of LDL-C by the Friedewald equation33 ; and determination of Lp(a), apo A-I/A-II, and apo A-I contents of Lp A-I and Lp A-I/A-II. Lp(a) was measured turbidimetrically (Incstar); this method measures total Lp(a) mass by employing a polyclonal goat antibody to purified human apo(a). Plasma apo A-I levels were measured by rocket immunoelectrophoresis (Hydragel A-I B, Sebia). The apo A-I content of Lp A-I was measured by a two-stage rocket immunoelectrophoresis kit (Hydragel Lp A-I, Sebia). The precast gels have a high concentration of anti–apo A-II antibody that forms a "rocket" with Lp A-I/A-II near the origin. Lipoproteins lacking apo A-II are free to migrate farther into the gel, where apo A-I–containing particles (Lp A-I, by definition) form a rocket with the lower concentration of the anti–apo A-I antibody. The apo A-I content of Lp A-I/A-II was calculated by difference between total plasma apo A-I and apo A-I in Lp A-I, as previously published.23 Plasma apo A-II content was measured from the lower (apo A-II) rocket of the Lp A-I gel. The intra-assay CVs were 3.8% and 5.7% for Lp A-I and Lp A-I/A-II, respectively, whereas the respective interassay CVs were 12.7% and 8.6%. Lp A-I/A-II levels have traditionally been expressed in terms of apo A-I content alone.14 15 17 However, because the major focus of this work was the turnover study and an in situ labeling method was used, nearly half of the radioactivity on Lp A-I/A-II was found on apo A-II. Therefore, the sum of apo A-I and apo A-II mass was chosen to more accurately express the levels of Lp A-I/A-II. Apo A-II mass in Lp A-I/A-II was assumed to equal total plasma apo A-II, as essentially all of apo A-II is found on Lp A-I/A-II. The mean of three or four determinations of these parameters was used for statistical analyses.

The apo A-I content of urea column fractions (for specific activity determinations) was measured by a liquid-phase, double antibody–based RIA. The RIA uses 3% (vol/vol) polyethylene glycol 6000 to enhance the binding kinetics, 0.2% (vol/vol) Tween 20 to maximally expose the apolipoprotein epitopes, 125I-HDL (prepared by the ICl method) as the tracer, polyclonal goat anti–apo A-I as the primary antibody, and a high-titer donkey anti-goat antiserum (Chemicon) as the secondary, precipitating antibody.

Activities of HL and LPL in Postheparin Plasma
After 3 weeks into each study period, porcine heparin (60 U/kg body weight) was given by intravenous bolus injection. Fifteen minutes later postheparin blood was drawn into EDTA tubes and placed on ice, and plasma was promptly separated at 4°C and then frozen at -70°C until assay. The lipase assay was performed using [3H]triolein-labeled Triton X-100 emulsion as described previously,34 except that HL activity was measured in the presence of 1 mol/L NaCl. As noted previously, LPL activity was calculated by difference between total lipase activity (0.15 mol/L NaCl) and HL activity.

Statistical Analyses
Statistical significance of differences between basal and estrogen periods and between one value and another within a given study period was assessed by a two-tailed paired t test. Values for each study period are given as mean±SD for the six subjects. Single linear regression analysis was performed by the least-squares method, and the r value (Pearson's) was determined. Analyses were performed by Microsoft Excel software and were denoted statistically significant if P<.05. Actual values are given in text, figures, or tables if P<=0.2; otherwise it is reported as >.2.


*    Results
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*Results
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Ages and anthropometric data for the study subjects are in Table 1Down. The mean age was 53 years (range, 47 to 62) and they ranged from normal weight for height to moderately overweight (body mass index ranging from 21.8 to 34.7 kg/m2). Because body mass differed minimally between the estrogen and basal periods, mean body mass for both periods is presented and was used for body mass index calculation.


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  		Table 1. Age and Anthropometric Data

Non-HDL lipid and lipoprotein data were measured three or four times during each study period, and the mean value for each was calculated for each subject during each study (Table 2Down). Subjects were neither selected nor excluded for study on the basis of lipid abnormalities; in fact, some had abnormal lipid and/or lipoprotein levels during the basal and/or estrogen study. One subject (No. 1) had significantly elevated basal levels of TC, TGs, and Lp(a) and low levels of HDL-C (see Table 3Down). She also may have had an elevated LDL-C level, which could not be calculated because her TG level was >400 mg/dL. Another subject (No. 3) had elevated TGs and Lp(a) with a low HDL-C, while yet another subject (No. 4) had an isolated low HDL-C. Nevertheless, the key changes in HDL metabolic parameters that were induced by estrogen (see below) appeared unrelated to the presence or absence of these dyslipidemias, so the results for all subjects were used for statistical analyses.


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  		Table 2. Non-HDL Lipoprotein Data


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  		Table 3. HDL Levels and Related Parameters

Differences in non-HDL lipid and lipoprotein parameters between estrogen treatment and basal periods were tested by a two-tailed paired t test (Table 2Up). Mean TC was not significantly changed (230±52 mg/dL basal versus 202±19 mg/dL on estrogen; P=.2). Although the expected estrogen-induced increase in plasma TG levels was observed in three subjects, in three others TG levels decreased, and the mean level for the six subjects was therefore unchanged (246±170 mg/dL basal versus 239±143 mg/dL on estrogen, for a 3% decrease; P>.2). There was an expected (albeit nonsignificant) 30% decrease in LDL-C levels (127±22 mg/dL basal versus 102±20 mg/dL on estrogen; P=.09). Also as expected, estrogen caused a decrease in Lp(a) values in all five subjects with detectable Lp(a) levels, with a mean change from 56.1±53.5 mg/dL basal to 42.1±49.8 mg/dL on estrogen (P=.02).

Several parameters of plasma HDL concentration were measured (Table 3Up). As expected, there were large increases in HDL-C and apo A-I levels; HDL-C was 36% higher on estrogen (47±15 mg/dL basal versus 63±18 mg/dL on estrogen; P=.004), and plasma apo A-I rose by 27% (147±24 mg/dL basal versus 186±34 mg/dL on estrogen; P=.003). Somewhat unexpectedly, apo A-II levels also increased significantly, by 17% (39±5 mg/dL basal versus 46±6 mg/dL on estrogen; P=.0003), surprisingly close to the percent increase in apo A-I. The fact that the percent increase in total plasma apo A-II was roughly comparable to that of total plasma apo A-I suggested that a similar increase in protein concentration between Lp A-I and Lp A-I/A-II might also exist; however, the increase in apo A-I occurred largely in Lp A-I (28 of the 39-mg/dL increase) and the apo A-I content of Lp A-I increased by 66% (43±13 mg/dL basal versus 71±17 mg/dL on estrogen; P=.002). This increase was several times greater than the nonsignificant trends in Lp A-I/A-II concentration, which consisted of a 10% increase in apo A-I (105±16 mg/dL basal versus 115 mg/dL on estrogen; P>.2) and a 14% increase in apo A-I plus apo A-II (144±23 mg/dL basal versus 163±29 mg/dL on estrogen; P=.12).

During the final week of each study period, each subject underwent a turnover study that used radioiodinated Lp A-I and Lp A-I/A-II tracers prepared from autologous plasma. Because essentially all of the radiolabel was in the protein moiety, this study explored the turnover of the protein components only of HDL. The mean die-away curves for all subjects for each tracer during each study period are shown in Fig 1Down. The overall FCR and PR for each tracer were calculated for each subject and are given in Table 4Down. Despite the striking increase in Lp A-I level and the decrease in HL activity (see below), the Lp A-I FCR was unchanged on estrogen (0.241±0.048 pool/d basal versus 0.258±0.066 pool/d on estrogen, or 7% higher; P=.2). Similarly, the FCR of Lp A-I/A-II showed no significant change (0.221±0.030 pool/d basal versus 0.233±0.053 pool/d on estrogen, or 5% higher; P>.2). In contrast, the PR of Lp A-I increased dramatically by 76% (4.7±1.8 mg·kg-1·d-1 basal versus 8.2±2.5 mg·kg·-1d-1 on estrogen; P=.002), while the PR of Lp A-I/A-II increased nonsignificantly (22%: 14.1±1.5 mg·kg-1·d-1 basal versus 17.2±5.4 mg·kg-1·d-1 on estrogen; P=.2).



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  		Figure 1. Mean die-away curves of Lp A-I and Lp A-I/A-II tracers for the six subjects as modeled by SAAM II software. Data are expressed as a fraction of the 0-minute point, which was extrapolated backward from the first venipuncture at 5 minutes. Bars denote the SE of counts at each time. A, Lp A-I die-away with ({square}) and without ({circ}) estrogen. B, Lp A-I/A-II die-away with ({square}) and without ({circ}) estrogen.


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  		Table 4. Turnover Parameters

Because there are few published studies comparing the metabolism of Lp A-I with that of Lp A-I/A-II in humans and none of them have either used whole-labeled tracer or focused on postmenopausal women, this study provided an opportunity to more closely examine the relationships between the FCRs of Lp A-I and Lp A-I/A-II. In the basal state, the mean Lp A-I FCR was higher than that of Lp A-I/A-II in four of six subjects, the group difference being of borderline statistical significance (9% higher; P=.08). Because the FCR of neither Lp A-I nor Lp A-I/A-II changed significantly between basal and estrogen treatment phases, the mean Lp A-I FCR during the estrogen period persisted in being slightly higher than that of Lp A-I/A-II (11% higher; P=.02). Although the subjects' mean FCR was unchanged between basal and estrogen treatment studies, there was considerable variation among subjects during basal and estrogen periods for both Lp A-I (CV, 20% and 26%, respectively) and Lp A-I/A-II (CV, 14% and 23%, respectively). Thus, the relationship between the Lp A-I FCR and that of Lp A-I/A-II was explored further by linear regression analysis. There was a strong, positive correlation between Lp A-I FCR and that of Lp A-I/A-II in the basal state (r=.94, P=.004) and during estrogen treatment (r=.98, P=.001). In addition, there was a strong, positive correlation between the change in FCR (basal versus estrogen periods) for Lp A-I tracer and that of Lp A-I/A-II (r=.85, P=.03). For all of these correlations, the slope of the linear regression did not differ significantly from 1.0. Thus, variation among the subjects' Lp A-I FCR remarkably paralleled that of Lp A-I/A-II in the basal and estrogen replacement states. Also, the change in FCR with estrogen, although very small and nonsignificant for the group, was very similar for Lp A-I and Lp A-I/A-II within each subject.

HL and LPL activities were measured in postheparin plasma during each study period (Table 5Down). There was a highly significant 66% decrease in HL activity (10.4±4.3 µmol·mL-1·h-1 basal versus 3.5±0.8 µmol · mL-1 · h-1 on estrogen; P=.006). In contrast, LPL activity was virtually unchanged (7.1±1.6 µmol·mL-1·h-1 basal versus 7.6±1.6 µmol·mL-1·h-1 on estrogen, or 8% higher; P>.2).


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  		Table 5. Postheparin Endothelial Lipase Activities


*    Discussion
up arrowTop
 up arrowAbstract
 up arrowIntroduction
 up arrowMethods
up arrowResults
*Discussion
down arrowReferences
 
This study was designed to explore the effects of postmenopausal oral estrogen replacement therapy on HDL protein metabolism. It was hypothesized that oral estrogen therapy in postmenopausal subjects would increase HDL levels primarily in the Lp A-I subspecies (1) and that this increase would be due to a selective increase in Lp A-I PR (with no increase in Lp A-I/A-II PR) (2) and a decrease in HL activity with a resulting decrease in FCR, also selective for Lp A-I (3).

To test the first hypothesis, the effects of estrogen on Lp A-I and Lp A-I/A-II levels (expressed as the mass of their major apolipoprotein constituents) were measured. The modest 14% rise in Lp A-I/A-II levels (apo A-I plus apo A-II) was not statistically significant (P=.12) and appeared to be even less than the 17% rise in plasma apo A-II (P=.0003). In contrast, the 66% increase in Lp A-I levels (apo A-I content) was more than fourfold greater. Even when measured as apo A-I content alone, the increase in Lp A-I/A-II (105±16 mg/dL basal versus 116±26 mg/dL on estrogen, or an 11% increase; P>.2) was much smaller than that of Lp A-I. Although the higher basal level of Lp A-I/A-II may bias one toward seeing a higher percent increase in Lp A-I, even in absolute terms the increase in Lp A-I was greater. A comparable difference was found in absolute levels: 70% of the total increase in plasma apo A-I occurred in Lp A-I (28 mg/dL for Lp A-I versus 11 mg/dL for Lp A-I/A-II). Thus, the first hypothesis has been validated and oral estrogen replacement has been shown to selectively raise plasma levels of Lp A-I in postmenopausal women. Because Lp A-I appears to be the primary antiatherogenic fraction of HDL in cell culture12 13 and in human clinical14 15 16 17 and transgenic mouse18 19 studies, the data suggest a novel antiatherogenic effect of estrogen, that of a relatively selective increase in Lp A-I.

Because the second and third hypotheses centered on metabolic distinctions between protein components of Lp A-I and Lp A-I/A-II, the choice of method for separate study of their turnover was important. Earlier turnover studies of Lp A-I and Lp A-I/A-II were performed by adding purified radiolabeled apo A-I to each subspecies.20 Such exchange labeling provides label homogeneity and simplifies data interpretation; however, these same studies have also demonstrated that tracer equilibrium between Lp A-I and Lp A-I/A-II is attained within 2 days after injection. Furthermore, exchange labeling has been found by some21 22 but not all35 ) workers to produce artifactually high rates of tracer dissociation from the parent HDL particle and to high FCR values. To maximize the ability to see metabolic differences between Lp A-I and Lp A-I/A-II particles, the whole-label method was used, even though it leads to heterogeneously labeled protein. Fortunately, most of the label in both HDL subfractions was on the primary protein(s) of interest, and no significant shift in the percentage of label to or from apo A-I or apo A-II during these studies was observed. The tracer heterogeneity in these studies should have made them extra sensitive to alterations in catabolic mechanisms. Thus, lack of a change in the FCR between study periods and between HDL species appears valid. The strong correlation between FCR values for the two tracers within subjects also tends to validate the labeling method.

An important finding is the validation of hypothesis 2, ie, that estrogen would increase the Lp A-I PR with little or no effect on the Lp A-I/A-II PR. No previous reports have explored the effects of postmenopausal estrogen therapy on Lp A-I and Lp A-I/A-II turnover. There was a striking 76% increase in the Lp A-I PR(P=.001), comparable to the 66% increase in Lp A-I levels. In contrast, there was only a (nonsignificant) 22% rise in the Lp A-I/A-II PR (P=.2), comparable to the nonsignificant 14% rise in its level. These data provide an interesting parallel to the cross-sectional turnover studies of Ikewaki et al,24 in that they found that the apo A-II PR appeared to regulate Lp A-I/A-II levels, whereas this study suggests that the Lp A-I PR may regulate Lp A-I levels.

Another key finding is the mixed result for the third hypothesis: an unexpected discordance between HL activity and the FCR of HDL apolipoproteins. Despite a large and remarkably consistent decline in HL activity in all subjects during oral estrogen treatment (range, 55% to 75% decrease), there was no apparent decrease in the FCR of either Lp A-I (7% mean increase; range, 3% decrease to 22% increase) or Lp A-I/A-II (5% mean increase; range, 4% decrease to 28% increase). Surprisingly, there was even a tendency toward an inverse relationship between the decrease in HL activity and the change in Lp A-I and Lp A-I/A-II FCRs (r=-.58, P=.2 and r=-.89, P=.02, respectively).

The discrepancy between the reduction in HL activity and lack of a decrease in FCR in the current study was not due to lack of a consistently large decrease in HL activity among subjects, which was similar to that of other studies in postmenopausal women who were taking various oral estrogens, including 17ß-estradiol5 and ethinyl estradiol.3 4 Thus, lack of an estrogen-related decline in the FCR in this study raises two questions. First, how can the present data be reconciled with other reports showing significant reductions in the apo HDL FCR during estrogen therapy? Second, how should an estrogen-induced decrease in HL activity without a concurrent estrogen-induced decrease in apo HDL FCR be interpreted?

Regarding the first question, the effects of estrogen on HDL catabolism are controversial. There are four published studies exploring this topic.2 3 6 10 One of them2 was performed against a background of endogenous estrogen in premenopausal women, whereas the other three3 6 10 were conducted in postmenopausal subjects. The disagreement among the four studies is as follows. The studies of Schaefer et al2 and Walsh et al6 reported increases in the apo HDL and apo A-I PRs, respectively. An increased PR appeared to cause an increase in HDL levels, as there was no significant change in the FCR. Hazzard et al3 and Quintao et al10 reported the opposite result, ie, large decreases in the FCR, which were the apparent metabolic mechanism for increases in HDL levels and evidently overcame the effects of moderate decreases in PR. The data in the study reported herein clearly concur with the first two studies, in that there were large increases in the Lp A-I PR (with smaller, nonsignificant increases in the Lp A-I/A-II PR), proportional to and presumably causative of the increased levels, while there was no tendency toward any decrease in FCR.

The large discrepancies among studies may be resolved by consideration of the labeling technique used. The study by Quintao and coworkers10 was the only one that used the Iodo-Gen method of radioiodination. These workers reported a mean basal apo HDL FCR of 0.58 pool/d, more than twice the value found in most other human HDL turnover studies, including the current one and the other estrogen studies. The Iodo-Gen method36 and its "parent" chloramine T method may increase the labeling of lipoprotein lipid (Reference 37 and M.N. Nanjee, unpublished data, 1992) with respect to the ICl method used in other studies. The degree of lipid labeling was not reported by Quintao et al,10 but it may have been substantial. Catabolism of HDL lipid is known to occur faster than that of its accompanying apolipoproteins38 and should be decreased by estrogen, in parallel with HL activity. Thus, partial labeling of HDL in its lipid moiety might have accounted for both the much higher basal FCR and its decrease with estrogen in that study.10 The other study with results discordant with the present one3 consisted of only a single subject, whose high basal HDL-C and dramatic further elevation while she was taking estrogen (77 to 122 mg/dL) offer the possibility that her HDL metabolism might have been atypical in either or both states. Thus, it seems reasonable to argue that estrogen does not ordinarily lower the apo HDL FCR but will raise HDL levels by increasing the PR in most subjects.

The other question regarding hypothesis 3 is the relationship between HL activity and the FCR of HDL protein. Cross-sectional studies have shown that intersubject differences in HL activity correlate with differences in the apo HDL FCR.39 40 41 To test whether this relationship might be causal, the subjects in this study were treated with estrogen to lower HL activity and tested for the expected drop in the apo HDL FCR. Despite the dramatic decrease in HL activity in all six subjects, there was no apparent decrease in the protein FCR of either Lp A-I or Lp A-I/A-II. Of the two published studies that tested for a relationship between estrogen-induced changes in HL activity and the apo HDL FCR, Hazzard et al3 found concordant reductions of 63% and 53% in HL activity and the apo HDL2 FCR, respectively, in a single postmenopausal subject. In contrast, Schaefer and colleagues2 reported a 44% reduction in HL activity but only a nonsignificant 3% decrease in the apo HDL FCR in four premenopausal women who were undergoing estrogen therapy. Paradoxically, the present data agree with those for premenopausal women, suggesting that similar effects of exogenous estrogen can be observed with or without baseline endogenous estrogen. The lack of concordance with the results of Hazzard et al3 may simply reflect the difference between one postmenopausal subject and a larger postmenopausal group.

Despite the fact that several cross-sectional studies 39 40 41 have suggested a positive relationship between HL activity and HDL FCR, there is precedence for a divergence between HL and FCR in response to intervention. The study by Brinton et al42 on the effects of a low-fat diet on HDL turnover revealed modest but statistically significant increases in the apo A-I and apo A-II FCRs (11%, P=.005 and 6%, P=.03, respectively), concurrent with a 17% decrease in HL activity (P=.02), even though the subjects were part of cross-sectional studies40 41 that showed a positive relationship between FCR and HL activity.

Although the current data appear to support the aforementioned suggestion that HL activity may not always regulate the apo HDL FCR, the present findings may be peculiar to estrogen and/or the study methods in at least four ways. First, there may have been counterbalancing changes in distinct component pathways of overall HDL catabolism. This might be tested by subjecting the data to detailed kinetic modeling, but such is beyond the scope of this study. Second, there could have been opposing changes in the catabolism of individual HDL subfractions. The FCRs of the two major HDL subfractions, Lp A-I and Lp A-I/A-II, did not respond to estrogen. Minor HDL subpopulations, such as those with apo E and/or the C apolipoproteins, were not separated in these studies, so any changes in their metabolism would have gone undetected. Estrogen lowers plasma apo E levels by accelerating its removal from VLDL43 44 while apparently not altering the catabolism of apo E–containing HDL.44 Furthermore, because <1% of the injected label was on apo E, it is very unlikely to have played a significant role in the overall FCR of either Lp A-I or Lp A-I/A-II. The effects of estrogen on the C apolipoprotein are little known, but because they constitute only 3% to 7% on average of the turnover label, they probably had little impact on the results. Nevertheless, the effects of estrogen on the metabolism of E or C apolipoproteins within HDL particles were not addressed in these studies. A third possibility is that the lack of significant changes in catabolism may have been peculiar to the size of the injected HDL protein tracers. Because only HDL3-sized tracers were used, the method is insensitive to changes in catabolism of other HDL species, such as HDL2, wherein the major effects of HL may reside. Finally, estrogen may so alter the dissociation or exchange characteristics of HDL proteins that a true change in HDL metabolism could be obscured. Clearly, additional work is required to further explore such possibilities.

In accord with the modest differences in FCR between Lp A-I and Lp A-I/A-II in the basal state, the lack of effect of estrogen on FCR was similar between Lp A-I and Lp A-I/A-II (nonsignificant 7% and 5% increases, respectively), and the subjects' mean Lp A-I FCR was only slightly higher than that of Lp A-I/A-II (11% higher, P=.02) during estrogen treatment. Furthermore, the intrasubject correlation between the FCRs of Lp A-I and Lp A-I/A-II was very high in the basal state (r=.94, P=.004), during estrogen therapy (r=.98, P=.001), and even for the change between basal and estrogen periods (r=.85, P=.03); the slopes of these regressions did not differ significantly from 1.0. These similarities in FCR between Lp A-I and Lp A-I/A-II are remarkable, given the protein heterogeneity of the tracers and the divergent responses in Lp A-I and Lp A-I/A-II levels and of PR to estrogen replacement. Thus, the current data suggest that the protein components of Lp A-I and Lp A-I/A-II may be catabolized by a major common mechanism(s) or pathway(s), although this may or may not be true for particular subspecies of HDL, such as those with apo E.

In light of the strongly divergent responses of the Lp A-I PR and HL activity during estrogen use in this study, relationships between these two parameters were sought. Surprisingly, there was a strong, positive correlation between HL activity and the Lp A-I PR in the basal state (r=.95, P=.004; Fig 2Down) but not with the Lp A-I/A-II PR(r=.37, P>.2) or the FCR of either Lp A-I or Lp A-I/A-II (r=.65 and .52, respectively, P>.2). This positive relationship between HL activity and Lp A-I PR was not seen during the estrogen period (r=.40, P>.2) as might have been expected, given the opposite changes in Lp A-I PR and HL activity with estrogen use. One previous study in four postmenopausal women and two men of comparable age failed to find any significant relationship between the turnover of apo A-I or apo A-II and HL activity.45 Nevertheless, the finding of a positive correlation between HL activity and Lp A-I PR in the postmenopausal basal state suggests the possibility of a novel physiological relationship between them. Such a relationship might be allied with the opposite changes in these factors during estrogen use. These two factors might share a trans-acting genetic factor or synthetic signal or compete for a common packaging or transport pathway. Alternatively, HL activity on larger lipoproteins might facilitate survival of nascent HDL, which would be measured as an increased PR.



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  		Figure 2. Scatterplot showing the correlation between Lp A-I PR and HL activity in the six subjects in the basal state.

In summary, oral estrogen replacement selectively raises Lp A-I levels (possibly a more antiatherogenic change than are general increases in HDL) apparently due to selective elevation of the Lp A-I PR. Estrogen taken orally strongly downregulates HL activity but does not reduce the overall FCR of Lp A-I protein (nor that of Lp A-I/A-II). This suggests that there may be circumstances in which HL does not determine HDL protein catabolism, although this dissociation may be peculiar to estrogen therapy or the study method. The proteins of Lp A-I and Lp A-I/A-II have remarkably concordant FCRs between and within study conditions, suggesting that these HDL subspecies may share a common catabolic mechanism. Finally, the data suggest that regulation of HL activity and Lp A-I PR might be interrelated. Oral estrogen replacement therapy appears to have very favorable effects on HDL metabolism in postmenopausal women and constitutes an interesting and informative intervention in the study of the regulation of HDL levels.


*    Selected Abbreviations and Acronyms
 
CV(s) = coefficient(s) of variation
FCR = fractional catabolic rate
HDL-C = HDL cholesterol
HL = hepatic lipase
LDL-C = LDL cholesterol
LPL = lipoprotein lipase
PR = production rate
RIA = radioimmunoassay
SDS-PAGE = SDS–polyacrylamide gel electrophoresis
TC = total cholesterol
TG(s) = triglycerides


*    Acknowledgments
 
This work was supported in part by an unrestricted grant from Pfizer Pharmaceuticals, and a General Clinical Research Center grant (M01 RR07122) to Bowman Gray School of Medicine and and a Clinical Investigator Award (HL-02034) to Dr Brinton, both from the National Institutes of Health, Bethesda, Md. The author is indebted to Dr P. Hugh R. Barrett of the Resource for Kinetic Analysis, University of Washington (Seattle), for calculating the FCR for all studies and for assistance with the design of the turnover protocol; to Dr M. Nazeem Nanjee (currently at St Bartholomew's Hospital, London) for establishing the apo A-I RIA; to Dr David B. MacLean (of Pfizer Central Research) and Drs William Hazzard and Lawrence Rudel for numerous helpful discussions; to Ellen Burleson for expert technical assistance; and to the nurses and dietitians of the Bowman Gray General Clinical Research Center.

Received June 23, 1995; accepted December 20, 1995.


*    References
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*References
 

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