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Journal of Lipid Research, Vol. 41, 368-375, March 2000
Copyright © 2000 by Lipid Research, Inc.

Original Article

Effects of continuous conjugated estrogen and micronized progesterone therapy upon lipoprotein metabolism in postmenopausal women

Bernard M. Wolfea, P. Hugh R. Barrettb, Lisanne Lauriera, and Murray W. Huffa
a Department of Medicine, The University of Western Ontario, London, Ontario, Canada, N6A 5A5
b Department of Medicine, University of Western Australia, Perth, WA 6000, Australia

Correspondence to: Bernard M. Wolfe


  ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES

The effects of continuously administering both conjugated equine estrogens (CEE) and micronized progesterone (MP) on the concentration, composition, production and catabolism of very low density (VLDL) and low density lipoproteins (LDL) have not previously been reported. The mechanism of the hormonally induced reductions of plasma LDL cholesterol of Sf 0;–20 (mean 16%, P < 0.005) and LDL apoB (mean 6%, P < 0.025) were investigated by studying the kinetics of VLDL and LDL apolipoprotein (apo) B turnover after injecting autologous 131I-labeled VLDL and 125I-labeled LDL into each of the 6 moderately hypercholesterolemic postmenopausal subjects under control conditions and again in the fourth week of a 7-week course of therapy (0.625 mg/d of CEE + 200 mg/d of MP). The combined hormones significantly lowered plasma LDL apoB by increasing the mean fractional catabolic rate of LDL apoB by 20% (0.32 vs. 0.27 pools/d, P < 0.03). Treatment also induced a significant increase in IDL production (6.3 vs. 3.7 mg/kg/d, P = 0.028). However, this did not result in an increase in LDL production because of an increase in IDL apoB direct catabolism (mean 102%, P = 0.033). VLDL kinetic parameters were unchanged and the concentrations of plasma total triglycerides (TG), VLDL-TG, VLDL-apoB did not rise as often seen with estrogen alone. Plasma HDL-cholesterol rose significantly (P < 0.02).

Our major conclusion is that increased fractional catabolism of LDL underlies the LDL-lowering effect of the combined hormones.—Wolfe, B. M., P. H. R. Barrett, L. Laurier, and M. W. Huff. Effects of continuous conjugated estrogen and micronized progesterone therapy upon lipoprotein metabolism in postmenopausal women. J. Lipid Res. 2000. 41: 368;–375.

Supplementary key words: estrogen, progesterone, VLDL, LDL, cholesterol


  INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES

Ovarian hormone replacement therapy, defined as treatment with estrogen and progestin (HRT), provides effective relief of vasomotor symptoms (1), prevents osteoporosis (2) and may reduce coronary heart disease (CHD, 3). Although a recent randomized clinical trial in older postmenopausal (PMP) women with advanced CHD including coronary bypass surgery found no overall cardiovascular benefit of HRT (4), observational studies of HRT or replacement of estrogen alone (ERT) have reported reductions in both all-cause and CHD mortality in other PMP women (5) (6).

Elevations of plasma cholesterol (7) (8) and triglycerides (7) have been reported in a high proportion of PMP women and are thought to contribute to the associated increase in CHD (3) (9) (10) (11). Estrogen replacement by HRT or ERT has the potential to reduce CHD risk through favorable effects on both lipoproteins and the arterial wall (12). Increases in both plasma triglycerides and HDL cholesterol have been reported in response to ERT in PMP women (13). However, triglyceride responses to HRT involving continuous conjugated estrogen (CEE, 0.625 mg/d) and medroxyprogesterone acetate (MPA, 2.5 mg/d) have been inconsistent. Significant increases in triglycerides have been reported in some, but not all, studies of 12 months or more in duration (14) (15) (16) (17), but no significant increase in plasma triglycerides has been reported in shorter studies of 1.7 to 9 months in duration (18) (19) (20) (21) (22) (23). MPA has also been reported to significantly reduce the CEE-induced increase in plasma triglycerides (15). Furthermore, estrogen-induced increments in HDL cholesterol have tended to be blunted more by concomitant administration of MPA than by natural ovarian progestin administered as micronized progesterone (MP 14, 24). Because prolonged estrogen therapy that is unopposed by progestin is associated with risk of endometrial neoplasia, there is a need for progestin (14) (25) (26).

Despite clinical usage of MP together with CEE by PMP women (14) (24), there has been no information about the effects of continuously administering CEE and MP (CEEcont/MPcont) on the concentrations, production, catabolism and/or composition of the major apolipoprotein (apo) B-containing lipoproteins [very low density (VLDL), intermediate density (IDL) and low density lipoproteins (LDL)] in PMP women. The present study was undertaken to test the hypothesis that HRT with CEEcont/MPcont would lower LDL-cholesterol by enhancing its fractional catabolism and would blunt the increases in VLDL production.


  SUBJECTS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES

Subjects
Six participants, who had experienced typical menopausal symptoms and were amenorrheic for at least 6 months (serum follicle stimulating hormone >70 U/L) were recruited from London Health Sciences Centre University Campus out-patient clinics ( Table 1). Baseline fasting plasma cholesterol concentrations during Phase I American Heart Association diets prior to entry into the study exceeded 5.2 mmol/L, the cut-point of eligibility for dietary treatment recommended by the National Cholesterol Education Program (27). Two subjects had fasting levels of plasma HDL cholesterol levels which were below the recommended cut-point of 0.9 mmol/L (27), 2 had plasma triglycerides which exceeded the desirable upper limit of 2.3 mmol/L (28), and 2 had fasting levels of lipoprotein [a] (Lp[a]) which were slightly above the 95th percentile (29). One subject had primary hypothyroidism and was treated throughout the control and experimental periods with L-thyroxine 0.15 mg/d, along with 50 mg of captopril and 50 mg of hydrochlorothiazide daily for hypertension. Although hydrochlorothiazide can raise serum cholesterol (30), any potential effect on lipid metabolism was minimized by maintaining the same dosage level during both control and treatment periods of the experiment. Subjects received no other medications known to affect lipid metabolism. Intake of ethanol was less than 30 ml/d for the month before each study and was disallowed during the turnover studies. Subjects were instructed to maintain their low fat diets and their regular physical activity for the duration of the study, and body weight was monitored on eight occasions during each control and treatment period of their studies. Thus, mean values for body weight (Table 1) and body mass index (23.7 ± 1.3 vs. 23.6 ± 1.3) were similar during the control and treatment periods (P > 0.5). All subjects had normal fasting concentrations of serum glucose, thyroxine, creatinine, calcium, albumin, sodium, potassium, chloride, and bicarbonate. Hematological and urine analyses were normal. The experimental protocol approved by The Standing Committee on Human Research of The University of Western Ontario was explained to each subject and informed consent was obtained in writing.


 
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  		Table 1. Characteristics of postmenopausal subjects

Subjects were maintained on moderately low fat, low cholesterol diets containing approximately 29% of energy from fat (mean ratio of polyunsaturated:monounsaturated:saturated fat ca 0.8:1.3:1.0, 200 mg cholesterol per day), 19% of energy from protein, and 52% of energy from carbohydrate. During admissions to hospital for the first day only of each turnover study the diet was unchanged, except that the carbohydrate content of the diet was increased at the expense of fat to maintain caloric intake. Fat intake was reduced to <5 g per day to essentially eliminate the contribution of intestinal lipoproteins to plasma triglyceride-rich lipoproteins (31). Samples of venous blood for determination of plasma total cholesterol, LDL cholesterol (Sf 0;–20) and HDL cholesterol, total triglycerides, and Lp[a] concentrations were obtained from a forearm vein (after a 12-h fast) from each subject on four occasions at approximately weekly intervals a) during the initial 3-week control period before starting hormonal replacement and b) during weeks 3 to 6 of the 7-week course of treatment with CEE (0.625 mg/d) and MP (200 mg/d), together referred to as CEEcont/MPcont. Subjects underwent studies of the kinetics of VLDL and LDL apoB turnover during the control period and again during the last 16 days of hormone replacement therapy (HRT) with CEEcont/MPcont. All subjects received 300 mg/d potassium iodide for 3 days before and 13 days after the re-injection of their radiolabeled lipoproteins.

Preparation of labeled lipoproteins and kinetic studies
The procedures for the lipoprotein turnover studies, including the isolation and preparation of labeled lipoproteins and their reinjection, have previously been described, along with the sampling and fractionation of VLDL (Sf 60;–400), intermediate density lipoprotein (IDL, Sf 12;–60), and LDL (Sf 0;–12) and the isolation of apoB from each lipoprotein fraction by isopropanol precipitation and determination of its specfic activity (23) (32) (33) (34) (35). The bolus injection of 131I-labeled VLDL was immediately followed by the 125I-labeled LDL. Isotopic crossover was corrected throughout the studies. Lipids were measured in these lipoprotein fractions as described below.

Kinetic analysis
A multicompartmental model was used to describe VLDL, IDL, and LDL-apoB tracer data. In multicompartmental modeling, each compartment or pool represents a group of kinetically homogeneous particles. In this study the SAAM II program (SAAM Inst., Seattle, WA) was used to fit the model to the observed tracer data. ApoB metabolic parameters are subsequently derived from the model parameters giving the best fit. All tracer data was fit simultaneously using the compartmental model ( Fig 1).



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  		Figure 1. Multicompartmental model for apoB metabolism. See Methods: Kinetic Analysis for details.

Compartments 1 through 5 are used to describe the kinetics of apoB in the VLDL fraction. It was assumed that all VLDL apoB enters plasma via compartment 1. Compartments 1 though 4 represent a delipidation chain or cascade as originally described by Phair et al. (36). It is assumed that the residence time of particles in each compartment of the chain is equal. In addition, the fraction of each compartment in the cascade converted to the slowly turning over VLDL compartment, compartment 5, is the same. VLDL particles in compartment 4 can be converted to IDL or can be removed directly from plasma. The IDL section of the model includes compartments 6 and 7, a rapidly and slowly turning over pool of IDL particles, respectively. Particles in compartment 6 can be converted to the slow IDL compartment, to LDL, or can be removed directly from plasma. Because labeled IDL was not injected, it was assumed that all IDL was derived from VLDL. The LDL section of the model is characterized by a plasma compartment, compartment 8, and an extravascular exchange compartment, compartment 9. This model assumes that LDL apoB is kinetically homogeneous; however, the isolation of LDL subfractions and/or the collection of urine radioactivity, after the injection of labeled LDL, may provide evidence of kinetic heterogeneity within the LDL fraction. Examination of the raw data revealed that significant input of "cold" or unlabeled LDL must occur, as seen by observing the relationship between the 131I-labeled IDL and LDL apoB specific radioactivity curves. The present model puts quantitative values on these observations. The model that was fit to the experimental data was the simplest model consistent with the data. Errors of parameters were low (<20%) and the residuals were randomly allocated around the model fits.

Analyses of metabolites
Plasma VLDL and IDL apoB concentrations were measured immunoturbidimetrically using Tina-quant apolipoprotein B Kit obtained from Boehringer Mannheim Canada, Laval, Quebec. The assay was standardized to LDL isolated by ultracentrifugation (d 1.040;–1.063) in which it was determined that apoB was the only protein present. The protein was determined by the modified Lowry procedure (37). Lp[a] was measured using Macra Lp[a] Kit obtained from Terumo Medical Corp., Elkton, MD. The intra-assay coefficients of variation for apoB and Lp[a] were 1.8 and 4.7%, respectively.

Fasting venous blood samples were also obtained at weekly intervals from weeks 3;–6 for measurement of lipids, including LDL cholesterol of Sf 0;–20, which was determined at these time points by subtraction of the cholesterol content of VLDL of Sf 20;–400 (38) plus HDL cholesterol from that of whole plasma. Concentrations of cholesterol and triglycerides in the chloroform;–methanol extract of whole plasma were determined as previously described (39), except that diagnostic kits from Boehringer Mannheim GmbH Diagnostica, Montreal, Quebec (C-system Kit for cholesterol and Test Combination Kit for triglycerides) were used to measure cholesterol content of fasting plasma VLDL, IDL, and LDL and triglyceride content of VLDL during the last 16 days the turnover studies because of the small amounts of material available for assay. HDL cholesterol was measured after precipitation with heparin;–manganese chloride (40).

Values from control and treatment periods were compared using Student's paired two-tailed t-test (41) or the Wilcoxon signed rank test (42), as specified. Variance was expressed as standard error of the mean.


  RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES

Concentrations of lipoprotein lipids and apoB
During weeks 3 to 6 of the administration of CEEcont/MPcont, there were significant decreases of 8 ± 2% in fasting plasma total cholesterol (6.05 ± 0.27 vs. 6.57 ± 0.19 mmol/L, P < 0.01, Table 1) and 16 ± 2% in the concentration of cholesterol of LDL of Sf 0;–20 (4.13 ± 0.21 vs. 4.86 ± 0.21 mmol/L, P < 0.005)33 LDL cholesterol of Sf 0;–20 was not determined during turnover studies when LDL of Sf 0;–12 was measured., whereas HDL cholesterol rose by 30 ± 9% (P < 0.025, Table 1). The hormones induced a 29 ± 3% decrease in the mean value for the ratio of total cholesterol to HDL cholesterol versus control (4.6 ± 0.5 vs. 6.6 ± 1.0, P < 0.02) and a 35 ± 2% decrease in the value of the ratio of LDL (Sf 0;–20) cholesterol to HDL cholesterol (3.1 ± 0.3 vs. 4.9 ± 0.7, P < 0.01).

Data obtained at time of the turnover studies comparing control versus treatment indicated that CEEcont/MPcont reduced the mean plasma concentration of cholesterol transported in LDL of Sf 0;–12 by 13 ± 3% (3.70 ± 0.24 vs. 4.27 ± 0.19 mmol/L, P < 0.005) and increased mean HDL cholesterol by 30 ± 8% (1.32 ± 0.10 vs. 1.01 ± 0.09 mmol/L, P < 0.01, Table 2). Simultaneously, the mean values tended to decline during treatment for each of plasma total triglycerides (1.72 ± 0.36 vs. 2.20 ± 0.34 mmol/L), VLDL triglycerides (0.75 ± 0.17 vs. 0.96 ± 0.20 mmol/L, Table 2), IDL triglycerides (0.52 ± 0.16 vs. 0.69 ± 0.10 mmol/L), and LDL triglycerides (0.28 ± 0.02 vs. 0.36 ± 0.056 mmol/L), however, the changes were not statistically significant. The changes in the mean values for concentrations of HDL triglycerides (0.17 ± 0.02 vs. 0.19 ± 0.02 mmol/L) and Lp[a] (17 ± 8 vs. 20 ± 10 mg/dl) also failed to be statistically significant. The mean value for concentration of apoB in plasma LDL was significantly lower during hormonal treatment versus control (82 ± 4 vs. 87 ± 3 mg/dl, P < 0.05); however, there was no significant change in mean VLDL apoB concentration (5.1 ± 0.9 vs. 4.3 ± 0.5 mg/dl, P > 0.2).


 
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  		Table 2. Effects of CEEcont/MPcont on levels of cholesterol and triglycerides of plasma lipoproteins on first day of turnover studies

The mean value for the ratio of cholesterol to apoB in plasma VLDL (Sf 60;–400) was 27 ± 4% lower during hormonal treatment versus control (1.5 ± 0.6 vs. 2.1 ± 0.9, P < 0.02 by Wilcoxon signed rank test), but there was no change in the ratio of triglycerides to apoB (16 ± 6 vs. 17 ± 5, P > 0.5). The mean value for the ratio of cholesterol to apoB in plasma IDL (Sf 12;–60) was significantly lower (by 17 ± 2%) during CEEcont/MPcont versus control (1.8 ± 0.1 vs. 2.2 ± 0.2, P < 0.005), but there was no change in the ratio of triglycerides to apoB in IDL (4.0 ± 0.6 vs. 4.1 ± 0.5, respectively, P > 0.5). There was no significant change in the mean value for the ratio of cholesterol to apoB of plasma LDL (Sf 0;–12) during CEEcont/MPcont versus control (1.4 ± 0.1 vs. 1.5 ± 0.1, respectively, P > 0.1), nor in the ratio of triglyceride to apoB (0.29 ± 0.02 vs. 0.27 ± 0.03, P > 0.3).

Consistent with a steady-state, there were no systematic changes during the turnover studies in plasma concentrations of the most readily quantified indices, namely total protein content of LDL (mean coefficient of variation of 10.7%, corresponding to a mean SD of 9.2 mg/dl, n = 6 paired studies) and total protein content of VLDL (mean coefficient of variation of 23%, corresponding to a mean SD of 1.7 mg/dl, n = 6 paired studies).

Metabolism of lipoproteins
Values for the specific activities of 131I-labeled apoB in VLDL, IDL, and LDL over 72 h after injection of 131I-labeled VLDL during CEEcont/MPcont versus control for a representative subject are shown in Fig 2. The kinetic parameters of apoB were determined from the simultaneous analysis of all the specific activity data by using the model shown in Fig 1. The kinetic parameters of VLDL turnover are summarized in Table 3. There were nonsignificant trends during treatment with CEEcont/MPcont towards increases in production rate of VLDL apoB (mean values 11.4 ± 2.1 vs. 9.4 ± 1.6 mg/kg/d, P = 0.26), in fractional catabolic rate of VLDL apoB (mean values 5.2 ± 0.8 vs. 4.7 ± 0.5, P = 0.47) and in pool size of VLDL apoB (133 ± 20 vs. 113 ± 9 mg, P > 0.2). That the conversion of VLDL to LDL did not change (36 ± 6 vs. 33 ± 6%, P > 0.4) despite increased conversion of VLDL to IDL (62 ± 7 vs. 45 ± 11%, P = 0.05) is consistent with a greater proportion of the IDL pool being cleared directly and not converted to LDL. IDL to LDL conversion decreased during treatment (74 ± 5 vs. 87 ± 5, P = 0.033) indicating that a higher proportion of IDL was cleared directly (26 vs. 13%). Significantly increased conversion of VLDL to IDL during hormone administration partly explains the significant increase in IDL apoB production (6.3 ± 0.5 vs. 3.7 ± 0.8 mg/kg/d, P = 0.028, Table 4). The increased IDL production would more than offset the trend towards an increase in IDL FCR (1.55 ± 0.29 vs. 1.00 ± 0.11 pools/d, P = 0.086), such that there was no significant change in IDL apoB pool size (294 ± 59 vs. 229 ± 42 mg, P > 0.4).



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  		Figure 2. Specific activity;–time curves of apoB of VLDL, ILD, and LDL after intravenous injection of autologous 131I-labeled VLDL during control (open symbols) and conjugated equine estrogen/micronized progesterone treatment (closed symbols) for subject no. 1.


 
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  		Table 3. Kinetic parameters of human VLDL apoB turnover in CEEcont/MPcont-treated postmenopausal subjects


 
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  		Table 4. Kinetic parameters of human IDL apoB turnover in CEEcont/MPcont-treated postmenopausal subjects

The kinetic parameters of LDL turnover are summarized in Table 5. The FCR of LDL apoB increased significantly, by an average of 20 ± 7%, during hormonal administration versus control (0.320 ± 0.021 vs. 0.269 ± 0.016 pools/d, P = 0.03). This resulted in a significant 6 ± 2% decrease in LDL apoB pool size (2162 ± 173 vs. 2300 ± 149 mg, P < 0.05). Although production of LDL apoB tended to rise, with increases in 5 of the 6 subjects, the change was not significant (P = 0.10). LDL direct synthesis was not significantly altered by the combined hormones (8.3 ± 1.5 vs. 7.8 ± 0.9 mg/kg/d, P = 0.58) and direct LDL production as a percent of total LDL production was unchanged by hormonal treatment (67 ± 7 vs. 73 ± 4%, P = 0.35).


 
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  		Table 5. Kinetic parameters of human LDL apoB turnover in CEEcont/MPcont-treated postmenopausal subjects

Fig 3 illustrates the effect of CEEcont/MPcont on the decrease of the 125I-labeled LDL apoB specific activity from the plasma for a representative subject. Data in Fig 3 are presented as percent of peak specific activity because absolute values for peak specific activity were different between the treatment and control. The steeper slope during treatment indicates that the fractional catabolic rate of LDL is more rapid during hormonal treatment than control.



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  		Figure 3. Specific radioactivity of LDL apoB expressed as percent of the peak specific activity after intravenous injection of autologous 125I-labeled LDL for subject no 1.


  DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES

The present studies are the first to determine the effects of the continuously administered CEE and MP on the production, catabolism, and composition of VLDL, IDL, and LDL. The combined hormones reduced plasma concentrations of LDL-cholesterol and LDL apoB significantly by significantly increasing the FCR of LDL (Table 5). An increase in the FCR of LDL-apoB has also been observed during continuous administration of CEE + MPA (23). However, cyclical estradiol + continuous norgestrel reduce LDL pool size by inhibiting LDL production (32). Whereas a previous study involving the daily administration of 2 mg of estradiol alone to nine PMP women (23) found a significant 21% increase in LDL apoB (13) production, the present study found only a trend towards a small increase (mean 10%, Table 5) suggesting the possibility that MP and/or other progestins could modulate the effects of estrogens on LDL kinetics. Pharmacological doses of ethinyl estradiol in the rabbit have been shown to increase LDL receptor expression (43) and the increase in LDL fractional catabolism with the present hormones could be explained by estrogen-enhancement of LDL receptor activity.

Parameters of LDL metabolism in the present study were determined by multicompartmental analysis of radioiodinated LDL, which remains a reference method for determining LDL-FCR (13). The mean control value for LDL-FCR in the present PMP women of 0.269 pools/d was in the same range as that obtained using similar technology in other groups of estrogen-deficient PMP women (23) (32) (44) (45). Comparisons between the present findings involving exogenous labeling versus studies involving endogenous labeling of LDL should take into account the different experimental methods and different subject populations. Values for LDL-FCR derived from endogenous labeling studies can be overestimated if the experimental protocol is too short, resulting in insufficient information on the LDL tracer data to support the existence of an LDL exchange pool. Similarly, overestimation results if the studies rely upon LDL tracer data derived from injected VLDL. Thus, a somewhat higher mean value for LDL-FCR (i.e., 0.36, range 0.21;–0.60) has been obtained during endogenous labeling of LDL apoB in healthy young men who had mean levels of LDL cholesterol which were 13% lower than the present PMP women (46). Conversely, estrogen replacement increases values for LDL-FCR in PMP women, tending to raise them into the same range as those of healthy men with normal to borderline high plasma cholesterol (47) (48). One of the potential drawbacks of the exogenous approach is that VLDL isolated for labeling and reinjection may under-represent that portion of the VLDL population that is converted to LDL via the delipidation pathway. In contrast, endogenous labeling may allow the experimentalist to see the kinetics of all VLDL particles, leading to the conclusion that a larger proportion of LDL is derived from the VLDL fraction that is seen with exogenous labeling. However, the mean control value for direct production of LDL in the present PMP woman (8.3 mg/kg/d) is similar to that previously reported in other estrogen-deficient PMP women (23) (44). The 16% reduction in the fasting plasma concentration of LDL-cholesterol (Sf 0;–20) with the CEEcont/MPcont accords with reductions of 13;–15% observed during cyclical administration of MP together with continuous or cyclical CEE (14) (24).

The present findings indicate that CEEcont/MPcont significantly increases IDL production (Table 4) and that this, in turn, is largely explained by increased conversion of VLDL to IDL (Table 3). However, the increase in production of IDL did not result in an increase in LDL production because of the increase in direct removal of IDL, which is consistent with up-regulation of LDL receptors by the CEE. Treatment also significantly altered the composition of IDL (Sf 12;–60), as reflected in 17% lower value for the ratio of cholesterol to apoB in plasma IDL (Sf 12;–60) and this explains why IDL cholesterol concentration was unchanged despite the tendency for IDL apoB pool size to increase (Table 2 and Table 4).

There have been no previous reports of the effects on HDL-cholesterol of continuously adminstering both CEE and MP. However, the relatively high proportional increase in HDL-cholesterol with the present HRT could be explained, at least in part, by the observation that subjects with low HDL experience relatively larger than average increases in HDL-cholesterol during ovarian hormone replacement (49). Furthermore, it has been reported that administration of continuous CEE and cyclical MP tended to increase HDL more than CEE alone (24). However, other studies found that MP, like MPA, blunts estrogen-induced increases in HDL-cholesterol, but the reductions in increment are smaller than with MPA (14) (50).

CEE alone increases production of VLDL TG (51) and estradiol administered alone increases production of VLDL apoB (13). However, when CEE was combined with MP in the present study (Table 4) or combined with the C-19 progestin MPA (23), no significant increase in VLDL production or TG concentration was observed. This suggests that these progestins blunt the triglyceride-elevating effect of CEE, consistent with a previous report that the C-19 progestin norethindrone acetate inhibits hepatic triglyceride secretion in a swine model (50). As mentioned above, numerous short-term and some long-term studies are consistent with the view that co-administration of MPA with CEE blunts the expected estrogen-induced increments in plasma TG (15) (16) (17) (18) (19) (20).

In summary, the CEEcont/MPcont lowered LDL cholesterol and apoB by increasing the fractional catabolism of LDL of Sf 0;–12 and thereby improved the plasma lipoprotein vascular risk profile (52) (53).


  ACKNOWLEDGMENTS

This work is supported by grants from the Medical Research Council of Canada (MT4248) and the Heart and Stroke Foundation of Ontario (T2973), and the National Institutes of Health (NHLBI HL49110 and NCRR RR02176). Murray W. Huff is a Career Investigator of the Heart and Stroke Foundation of Ontario. We thank Brian Sutherland for expert technical assistance. We thank Mary Cann for co-ordinating the studies. We are grateful to Jeanette Mikulic and Elene Wolfe for typing the manuscript, and Elene Wolfe for the graphic art.

Manuscript received April 26, 1999; and in revised form November 23, 1999

Abbreviations: CHD, coronary heart disease; CEE, conjugated equine estrogen; MP, micronized progesterone; FCR, fractional catabolic rate; HRT, hormonal replacement therapy; IDL, intermediate density lipoproteins; MPA, medroxyprogesterone acetate; Lp[a], lipoprotein [a]


  REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES

  1. Greendale, G. A. 1998. Symptom relief and side effects of postmenopausal hormones: results from the postmenopausal estrogen/progestin interventions trial. Obstet. & Gynecol. 92:982-988[Abstract].

  2. Effects of hormone therapy on bone mineral density. (1996) J. Am. Med. Assoc. 276:1389-1396[Abstract].

  3. Goldman, L., Tosteson, A. N. 1991. Uncertainty about postmenopausal estrogen. N. Engl. J. Med. 325:800-802[Medline].

  4. Hulley, S., Grady, D., Bush, T., Furberg, C., Herrington, D., Riggs, B., Vittinghoff, E. 1998. Randomized trial of estrogen plus progestin for secondary prevention of coronary heart disease in postmenopausal women. J. Am. Med. Assoc. 280:605-613[Abstract/Free Full Text].

  5. Stampfer, M. J., Colditz, G. A., Willett, W. C., Manson, J. E., Rosner, B., Speizer, F. E., Hennekens, C. H. 1991. Postmenopausal estrogen therapy and cardiovascular disease. Ten-year follow-up from the Nurses' Health Study. N. Engl. J. Med. 325:756-762[Abstract].

  6. Grady, D., Rubin, S. M., Petitti, D. B., Fox, C. S., Black, D., Ettinger, B., Ernster, V. L., Cummings, S. R. 1992. Hormone therapy to prevent disease and prolong life in postmenopausal women. Ann. Int. Med. 117:1016-1037.

  7. Connelly, P. W., MacLean, D. R., Horlick, L., O'Connor, B., Petrasovits, A., Little, A. 1992. Plasma lipids and lipoproteins and the prevalence of risk for coronary heart disease in Canadian adults. Canadian heart health surveys: a profile of cardiovascular risk. Can. Med. Assoc. J. 146(Special Supplement):1977-1987[Abstract].

  8. Bengtsson, C., and O. Lindquist. 1978. Coronary heart disease during the menopause. In Coronary Heart Disease in Young Women. M. F. Oliver, editor. Churchill-Livingstone, New York, NY. 234;–242.

  9. Luciano, A. A. 1992. Hormone replacement therapy in postmenopausal women. Infert. Reprod. Clin. N. Am. 3:109-127.

  10. Mishell, D. R., Jr. 1992. Steroids from menarche to menopause. Introduction. Am. J. Obstet. Gynecol. 166:1949.

  11. Barrett-Connor, E., Bush, T. L. 1991. Estrogen and coronary heart disease in women. J. Am. Med. Assoc. 265:1861-1867[Abstract].

  12. Lobo, R. A. 1991. Clinical review 27: Effects of hormonal replacement on lipids and lipoproteins in postmenopausal women. J. Clin. Endocrinol. Metab. 73:925-930[Medline].

  13. Walsh, B. W., Schiff, I., Rosner, B. 1991. Effects of postmenopausal estrogen replacement on the concentrations and metabolism of plasma lipoproteins. N. Engl. J. Med. 325:1196-1204[Abstract].

  14. Effects of estrogen or estrogen/progestin regimens on heart disease risk factors in postmenopausal women. The postmenopausal estrogen/progestin interventions (PEPI) Trial. (1995) J. Am. Med. Assoc. 273:199-218[Abstract].

  15. Lobo, R. A., Pickar, J. H., Wild, R. A., Walsh, B., Hirvonen, E. 1994. Metabolic impact of adding medroxyprogesterone acetate to conjugated estrogen therapy in postmenopausal women. Obstet. Gynecol. 84:987-995[Abstract/Free Full Text].

  16. Luciano, A. A., DeSouza, M. J., Roy, M. P., Schoenfeld, M. J., Nulsen, J. C., Halvorson, C. V. 1993. Evaluation of low-dose estrogen and progestin therapy in postmenopausal women. J. Rep. Med. 38:207-214.

  17. Weinstein, L., Bewtra, C., Gallagher, J. C. 1990. Evaluation of a continuous combined low-dose regimen of estrogen-progestin for treatment of the menopausal patient. Am. J. Obstet. Gynecol. 162:1534-1542[Medline].

  18. Luciano, A. A., Turksoy, R. N., Carleo, J., Hendrix, J. W. 1988. Clinical and metabolic responses of menopausal women to sequential versus continuous estrogen and progestin replacement therapy. Obstet. Gynecol. 71:39-43[Abstract/Free Full Text].

  19. Prough, S. G., Aksel, S., Wiebe, R. H., Shepherd, J. 1987. Continuous estrogen/progestin therapy in menopause. Am. J. Obstet. Gynecol. 157:1449-1453[Medline].

  20. Weinstein, L. 1987. Efficacy of a continuous estrogen-progestin regimen in the menopausal patient. Obstet. Gynecol. 69:929-932[Medline].

  21. Kable, W. T., Gallagher, J. C., Nachtigall, L., Goldgar, D. 1990. Lipid changes after hormone replacement therapy for menopause. J. Rep. Med. 35:512-518.

  22. MacLennan, A. H., MacLennan, A., Wenzel, S., Chambers, H. M., Eckert, K. 1993. Continuous low-dose oestrogen and progestogen hormone replacement therapy: a randomised trial. Med. J. Aust. 159:102-106[Medline].

  23. Wolfe, B. M., Huff, M. W. 1995. Effects of continuous low-dosage hormonal replacement therapy on lipoprotein metabolism in postmenopausal women. Metabolism. 44:410-417[Medline].

  24. Moorjani, S., Dupont, A., Labrie, F., DeLignieres, B., Cusan, L., Dupont, P., Mailloux, J., Lupien, P-J. 1991. Changes in plasma lipoprotein and apolipoprotein composition in relation to oral versus percutaneous administration of estrogen alone or in cyclic association with Urogestan in menopausal women. J. Clin. Endocrinol. Metab. 73:373-379[Abstract].

  25. Lane, G., Whitehead, M. I., King, R. J. B. 1982. Effects of oestrogens and progestogens on the histology, ultrastructure, and biochemistry of the postmenopausal endometrium. Int. Med. 2:13-18.

  26. Gambrell, R. D., Masse, F. M., Castaneda, T. A., Ugenas, A. J., Ricci, C. A., Wright, J. M. 1980. Use of the progestogen challenge test to reduce the risk of endometrial cancer. Obstet. Gynecol. 55:732-738[Abstract/Free Full Text].

  27. Summary of the Second Report of the National Cholesterol Education Program (NCEP) Expert Panel on Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults (Adult Treatment Panel II). (1993) J. Am. Med. Assoc. 269:3015-3023[Medline].

  28. Canadian Consensus Conference on Cholesterol. 1988. Final Report. Can. Med. Assoc. J. 139 (suppl): 1;–8.

  29. Jenner, J. L., Ordovas, J. M., Lamon-Fava, S., Schaefer, M. M., Wilson, P. W., Castelli, W. P., Schaefer, E. J. 1993. Effects of age, sex, and menopausal status on plasma lipoprotein [a] levels. The Framingham Offspring Study. Circulation. 87:1135-1141[Abstract/Free Full Text].

  30. Elliott, W. J. 1994. Dose-response of serum cholesterol during long-term therapy with thiazides. Clin. Pharmacol. Ther. 55:206. (Abstr.

  31. Havel, R. J., Kane, J. P., Balasse, E. O., Segel, N., Basso, L. V. 1970. Splanchnic metabolism of free fatty acids and production of triglycerides of very low density lipoproteins in normotriglyceridemic and hypertriglyceridemic humans. J. Clin. Invest. 49:2017-2035.

  32. Wolfe, B. M., Huff, M. W. 1989. Effects of combined estrogen and progestin administration on plasma lipoprotein metabolism in postmenopausal women. J. Clin. Invest. 83:40-45.

  33. Fidge, N. H., Poulis, P. 1974. Studies on the radio-iodination of very low density lipoproteins obtained from different mammalian species. Clin. Chim. Acta. 52:15-26[Medline].

  34. Reardon, M. F., Fidge, N. H., Nestel, P. J. 1978. Catabolism of very low density lipoprotein B apoprotein in man. J. Clin. Invest. 61:850-860.

  35. Huff, M. W., Telford, D. E., Woodcroft, K., Strong, W. L. 1985. Mevinolin and cholestyramine inhibit the direct synthesis of low density lipoprotein apolipoprotein B in miniature pigs. J. Lipid Res. 26:1175-1186[Abstract].

  36. Phair, R. D., Hammond, M. G., Bowden, J. A., Fried, M., Fisher, W. R., Berman, M. A. 1975. Preliminary model for human lipoprotein metabolism in hyperlipoproteinemia. Fed. Proc. 34:2263-2270[Medline].

  37. Lowry, O. H., Rosebrough, N. J., Farr, A. L., Randall, R. J. 1951. Protein measurement with the Folin phenol reagent. J. Biol. Chem. 193:265-275[Free Full Text].

  38. Havel, R. J., Eder, H. A., Bragdon, J. H. 1955. The distribution and chemical composition of ultracentrifugally separated lipoproteins in human serum. J. Clin. Invest. 34:1345-1353.

  39. Huff, M. W., Giovannetti, P. M., Wolfe, B. M. 1984. Turnover of very low-density lipoprotein-apoprotein B is increased by substitution of soybean protein for meat and dairy protein in the diets of hypercholesterolemic men. Am. J. Clin. Nutr. 39:888-897[Abstract/Free Full Text].

  40. Warnick, G. R., Albers, J. J. 1978. A comprehensive evaluation of the heparin-manganese precipitation procedure for estimating high density lipoprotein cholesterol. J. Lipid Res. 19:65-76[Abstract].

  41. Snedecor, G. W., and W. G. Cochran. 1967. The comparison of two samples. In Statistical Methods, 6th edition. Iowa State University Press, Ames, IA. 94;–97; 114.

  42. Kirkwood, B. R. 1988. Non-parametric methods. In Essentials of Medical Statistics. Blackwell Scientific Publications, Oxford. 147;–149.

  43. Ma, P. T., Yamamoto, T., Goldstein, J. L., Brown, M. S. 1986. Increased mRNA for low density lipoprotein receptor in livers of rabbits treated with 17{alpha}-ethinyl estradiol. Proc. Natl. Acad. Sci. USA. 83:792-796[Abstract/Free Full Text].

  44. Wolfe, B. M., Huff, M. W. 1993. Effects of low dosage progestin-only administration upon plasma triglycerides and lipoprotein metabolism in postmenopausal women. J. Clin. Invest. 92:456-461.

  45. Reardon, M. F., Fidge, N. H., Nestel, P. J. 1978. Catabolism of very low density lipoprotein B apoprotein in man. J. Clin. Invest. 61:850-860.

  46. Demant, T., Packard, C. J., Demmelmair, H., Stewart, P., Bedynek, A., Bedford, D., Seidel, D., Shepherd, J. 1996. Sensitive methods to study human apoliprotein B metabolism using stable isotope-labeled amino acids. Am. J. Physiol. 270:E1022-E1036.

  47. Simons, L. A., Balasubramaniam, S., Holland, J. 1983. Low density lipoprotein metabolism in the normal to moderately elevated range of plasma cholesterol: comparisons with familial hypercholesterolemia. J. Lipid Res. 24:192-199[Abstract].

  48. Grundy, S. M. 1991. Multifactorial etiology of hypercholesterolemia. Arterioscler. Thromb. 11:1619-1635[Abstract/Free Full Text].

  49. Gibbons, W. E., Judd, H. L., Luciano, A. A., Nachtigall, L. E., Dawood, M. Y., Burry, K., Ziegl, H., LeBoff, M. S., El-Haij, G., Ogrinc, F., Schoenfield, M. J., Buyalos, R. P., Adashi, E., Schnatz, P. 1991. Comparison of sequential versus continuous estrogen/progestin replacement therapy on serum lipid patterns. Am. Soc. Gynecol. Invest. 344. 491 (Abstr.)No.

  50. Barrett-Connor, E., Sloane, S., Greendale, G., Kritz-Silverstein, D., Espeland, M., Johnson, S. R., Maclawiw, M., Fineberg, S. E. 1997. The postmenopausal estrogen/progestin interventions study: primary outcomes in adherent women. Maturitas. 27:261-274[Medline].

  51. Wolfe, B. M., Grace, D. M. 1979. Norethindrone acetate inhibition of splanchnic triglyeride secretion in conscious glucose-fed swine. J. Lipid Res. 20:175-182[Abstract].

  52. Mishell, D. R. 1989. Estrogen replacement therapy: an overview. Am. J. Obstet. Gynecol. 161:1825-1827[Medline].

  53. Wolf, P. H., Madans, J. H., Finucane, F. F., Higgins, M., Kleinman, J. C. 1991. Reduction of cardiovascular disease-related mortality among postmenopausal women who use hormones: evidence from a national cohort. Am. J. Obstet. Gynecol. 164:489-494[Medline].


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