Basic nutritional investigationDietary supplementation with fermented soybeans suppresses intimal thickening
Introduction
Many studies, over several decades, have been carried out to analyze the influence of diet habit on both generation and reduction of atherosclerosis.1, 2 Since the incidence of cardiovascular disease in Asia is very low, traditional foods from Asia have been the subject of much attention.3, 4 For more than 1000 y, people throughout Asia have been consuming soybeans in a variety of traditional soy food products.5 Westerners have wholeheartedly adopted some of these products, whereas others will undoubtedly take more time to accept. The most popular soy foods in Western countries are tofu, soy milk (tonyu), soy burger, soy sauce, and miso. Natto, a traditional fermented vegetable cheeselike food, is another soy product. Natto extracts are known to include nattokinase (NK), a potent fibrinolytic enzyme having an approximately four-times stronger activity than plasmin on clot lysis assay.6 It is composed of 275 amino acid residues (molecular weight = 27 728 Da) and exhibits a high homology with subtilisins.7 It has been reported that NK detected by its antibody was absorbed across rat intestinal tract after intraduodenal administration and then cleaved fibrinogen in plasma.8 Its remarkable thrombolytic efficacy also was shown in a chemically induced thrombosis model.9
Intravascular thrombus formation causes a variety of cardiovascular diseases,10, 11 in which abnormalities of platelet aggregation, coagulation system, and/or dysfunction of vascular endothelial cells play an important role. In recent years, intimal thickening and restenosis of arteries have become a major drawback in arterial reconstruction for atherosclerotic and thrombotic diseases. An important basic process for intimal thickening is the migration of smooth muscle cells from the media to the intima and their subsequent proliferation. In the process, platelet-derived growth factor released from the adherent platelets and cytokines from accumulated leukocytes are reported to play key roles by initiating the migration and proliferation of smooth muscle cells.12
We have recently developed a model of intimal thickening following vascular endothelial injury. In this model, we damage a small segment of vascular endothelium by a focally produced reactive oxygen species through photochemical reaction between a green light and a dye, rose bengal (RB). This photochemical reaction induces a thrombogenic condition at the site of endothelial injury without medial injury.13, 14 Using this model, we have investigated the effects of extracts obtained from natto by using a powdered product adjusted to be 50 times greater in NK activity than those of commercial products on intimal thickening and thrombogenesis in the rat femoral artery.
Section snippets
Preparation of natto extracts
One gram of crude natto extract needed 8.4 g of commercial natto. The commercial natto (17.5 kg; Kinnotsubu-fukumaro, Freshia Company Ltd., Ibaragi, Japan) was suspended in iron-exchanged water (87.5 L) at 4°C. After 21 h, the suspension was filtered, and the water-soluble extract was lyophilized to make a powder.
Animals and diets
Twenty-four male Sprague-Dawley rats (5 wk old; SLC, Hamamatsu, Japan) were assigned to one of two groups: control or treatment with a dietary supplementation of natto extracts (Table I
Inhibitory effect of natto extracts on intimal thickening
Twenty-one days after intimal denudation, significant intimal thickening was observed in the subendothelial layer in control animals, as previously reported.17 Dietary natto extract supplementation suppressed intimal thickening (Fig. 1). The results of the morphometric analysis are shown in Fig. 2. The cross-sectional area of the intima in the injured femoral arteries in the control group was 1.21 ± 0.16 (× 0.01 mm2). Dietary natto extract supplementation significantly reduced the area to
Discussion
Arteriosclerosis is a chronic process and frequently induces lethal vascular diseases including myocardial and cerebral infarctions. Much effort has been expended to establish strategies to prevent its progression. Some strategies, such as low-fat intakes or periodic physical exercise, are effective in lowering the risk for myocardial infarction. In this study, we investigated the effect of dietary natto extract supplementation on arterial intimal thickening after endothelial injury. We found
Acknowledgements
The authors thank Mr. H. Osugi for technical assistance.
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