Atherosclerosis 197 (2008) 34–42 Regression of hypercholesterolemic atherosclerosis in rabbits by secoisolariciresinol diglucoside isolated from flaxseed Kailash Prasad ∗ Department of Physiology, College of Medicine, University of Saskatchewan, 107 Wiggins Road, Saskatoon, Saskatchewan, Canada S7N 5E5 Received 22 May 2007; received in revised form 10 July 2007; accepted 27 July 2007 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 (20mg·kg body wt -1 ·day -1 PO) for 2mo; 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. © 2007 Elsevier Ireland Ltd. All rights reserved. Keywords: Atherosclerosis; Regression; Hypercholesterolemia; Secoisolariciresinol diglucoside; Antioxidants; Oxygen radicals; Antioxidant reserve 1. Introduction Reactive oxygen species (ROS) have been implicated in the genesis and maintenance of hypercholesterolemic atherosclerosis [1–7]. Antioxidants and suppressants of sources of ROS are effective in the suppression of develop- ment of atherosclerosis [1,2,4–7]. Flaxseed contains 35% of its mass as oil, of which 55% is -linolenic acid (-3 fatty ∗ Tel.: +1 306 966 6539; fax: +1 306 966 6532. E-mail address: k.prasad@usask.ca. 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 [1,2,13–16]. SDG suppresses the development of atherosclerosis and this 0021-9150/$ – see front matter © 2007 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.atherosclerosis.2007.07.043