Elsevier

Atherosclerosis

Volume 197, Issue 1, March 2008, Pages 34-42
Atherosclerosis

Regression of hypercholesterolemic atherosclerosis in rabbits by secoisolariciresinol diglucoside isolated from flaxseed

https://doi.org/10.1016/j.atherosclerosis.2007.07.043Get rights and content

Abstract

Secoisolariciresinol diglucoside (SDG) isolated from flaxseed is a lipid-lowering and antioxidant agent. It suppresses the development of hypercholesterolemic atherosclerosis in rabbits. It is however not known if SDG would produce regression of atherosclerosis. The objectives of this study were to determine (i) if SDG produces regression of atherosclerosis; (ii) if regression is associated with reduction in serum lipids, oxidative stress or both; and (iii) if the duration of treatment has an effect on regression. Rabbits were assigned to five groups: Group I, regular diet (control); Group II, 0.5% cholesterol diet for 2 months (mo); Group III, same as Group II but followed by regular diet for 2 mo; Group IV, same as Group II and followed by regular diet with SDG (20 mg·kg body wt−1·day−1 PO) for 2 mo; and Group V, same as Group IV but SDG treatment for an additional 2 mo. Blood samples were collected from rabbits before and at monthly intervals thereafter on their respective diet regimen for measurement of triglycerides (TG), total cholesterol (TC), LDL-C, HDL-C and malondialdehyde (MDA), a lipid peroxidation product. At the end of the protocol, the aorta was removed for assessment of atherosclerotic lesions, aortic MDA and aortic chemiluminescence (Aortic-CL), a measure of antioxidant reserve. MDA and Aortic-CL provide an index of oxidative stress. Increases in serum TG, TC, LDL-C, HDL-C and the risk ratio TC/HDL-C in Group II were associated with an increase in oxidative stress and development of atherosclerosis (57% of aortic intimal surface covered with lesions). Serum lipids decreased to a similar extent in Groups III–V, however atherosclerotic lesions were 84%, 63% and 44%, respectively in Groups III–V. There were more atherosclerotic lesions in Group III (+48.9%) as compared to Group II. The atherosclerotic lesions decreased by 24% and 45%, respectively in Groups IV and V compared to Group III. The reduction in atherosclerotic lesions was associated with a reduction in oxidative stress. These results suggest that (i) regular diet following a high cholesterol diet accelerates atherosclerosis in spite of a decrease in serum lipids; (ii) SDG treatment prevents the progression of atherosclerosis on a regular diet following a high cholesterol diet; (iii) prevention of progression is associated with a reduction of aortic oxidative stress and not with reductions in serum lipids; (iv) a longer duration of treatment reduces the progression of atherosclerosis to a greater extent, and tends to regress the atherosclerosis.

Introduction

Reactive oxygen species (ROS) have been implicated in the genesis and maintenance of hypercholesterolemic atherosclerosis [1], [2], [3], [4], [5], [6], [7]. Antioxidants and suppressants of sources of ROS are effective in the suppression of development of atherosclerosis [1], [2], [4], [5], [6], [7]. Flaxseed contains 35% of its mass as oil, of which 55% is α-linolenic acid (ω-3 fatty acid) and 15–18% is linoleic acid [8], [9]. Secoisolariciresinol diglucoside (SDG), a lignan, has been isolated in pure form from flaxmeal devoid of oil by Agriculture and Agrifood Canada, a member of our flaxseed-Lignan consortium [10]. The level of SDG in flaxseed varies between 0.6 and 1.8 g/100 g. SDG and its metabolites (secoisolariciresinol, enterolactone and enterodiol) are potent antioxidants [11], [12]. SDG is also a hypolipidemic agent [4]. Antioxidants and hypolipidemic agents suppress and produce regression of hypercholesterolemic atherosclerosis [1], [2], [13], [14], [15], [16]. SDG suppresses the development of atherosclerosis and this effect was associated with antioxidant and lipid-lowering effects [4]. However, in that study, SDG was administered before the development of atherosclerosis. To be of potential clinical benefit in patients with atherosclerosis, the drug should produce regression and/or slow progression of atherosclerosis. Although SDG suppresses the development of atherosclerosis, it is not known if it would produce regression of atherosclerosis.

The main objectives of this investigation were to determine if (i) SDG produces regression of hypercholesterolemic atherosclerosis; (ii) regression is associated with reduction in serum lipids; and (iii) regression is associated with reduction in oxidative stress. An investigation was therefore made of the effects of SDG on the regression of pre-existing atherosclerosis in rabbits on a high cholesterol diet. Serum lipids [triglycerides (TG), total cholesterol (TC), HDL-cholesterol (HDL-C), LDL-cholesterol (LDL-C), and risk ratio (TC/HDL-C)] and oxidative stress parameters [serum and aortic malondialdehyde (MDA), a measure of levels of reactive oxygen species (ROS), and aortic chemiluminescence (Aortic-CL), a measure of aortic antioxidant reserve] were also measured.

Section snippets

Methods

New Zealand white rabbits weighing between 1.2 and 1.5 kg, after 1 week of adaptation on regular rabbit chow diet, were assigned to five groups (Table 1). The rabbits in Group I were fed regular rabbit chow diet. The other groups received 0.5% cholesterol diet followed by regular diet with or without SDG for various durations. The 0.5% cholesterol diet was prepared by the Department of Agricultural and Bioresource Engineering, University of Saskatchewan, using regular rabbit chow diet purchased

Serum lipids

Initial values for serum TG in Groups I–V were 0.73 ± 0.14, 0.97 ± 0.11, 0.68 ± 0.02, 0.65 ± 0.11 and 0.74 ± 0.05 mmol/l, respectively, and were similar in all groups except those in Group III which were lower than those in Group II. The changes in the serum TG levels of the five groups are summarized in Fig. 1. Serum TG increased in all the groups on the high cholesterol diet at months 1 and 2, however its levels decreased in Group I at month 2. Serum TG levels decreased significantly in groups on

Discussion

The baseline values of serum TG, TC, LDL-C and the ratio of TC/HDL-C were not similar for the different groups. These differences cannot be explained. The weight of the rabbits, duration of acclimatization, living conditions, and food intake were similar in all the groups. The fasting blood sampling procedures were similar for all the groups. The present study has shown that a high cholesterol diet increased the serum levels of TG, TC, LDL-C, HDL-C and the risk ratio of TC/HDL-C in rabbits.

Acknowledgements

This work was supported by a grant from the Heart and Stroke Foundation of Saskatchewan, Canada. The technical assistance of Ms. Barbara Raney and Mr. P.K. Chattopadhyay is highly appreciated.

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