* Division of Pathology, National Institute of Health Sciences, Tokyo 158-8501, and Showa Women's University, Tokyo 154-8533, Japan
1 To whom correspondence should be addressed at Division of Pathology, National Institute of Health Sciences, 1181 Kamiyoga, Setagaya-ku, Tokyo 158-8501, Japan. E-mail: nishikaw{at}nihs.go.jp.
Received March 21, 2005; accepted May 11, 2005
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ABSTRACT |
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Key Words: soybean; iodine deficiency; thyroid tumor-promotion; synergism.
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INTRODUCTION |
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There are several different mechanisms underlying chemical promotion of thyroid tumor development in rodents. For example, sulfadimethoxine (SDM) inhibits thyroid hormone synthesis by inhibiting thyroid peroxidase (TPO) (Hill et al., 1989), and phenobarbital (PB) induces hepatic microsomal enzymes such as thyroxine (T4)-uridine disphosphate glucuronosyl transferase (T4-UDP-GT) responsible for T4 clearance in the liver (Hill et al., 1989
; McClain et al., 1989
). In both cases, subsequent increase in serum TSH stimulates thyroid follicular cell proliferation. We have reported that a combination of excess soybean and iodine deficiency synergistically results in remarkable induction of thyroid proliferative lesions in rats (Ikeda et al., 2000
, 2001
). In the present study, the modifying effects of multiple dose levels of soybean were first investigated in a two-stage thyroid carcinogenesis model in rats. Combined effects of soybean and iodine deficiency were then examined with reference to induction of thyroid tumors in rats.
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MATERIALS AND METHODS |
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Experimental designs.
In Experiment 1, rats were divided into four groups, each consisting of 10 males and 10 females. They were subcutaneously injected with DHPN at a dose of 2800 mg/kg body weight and then fed a diet containing 0.8%, 4%, or 20% defatted soybean or the basal diet alone for 12 weeks, the soybean flour proportionally replacing gluten. In Experiment 2, rats were also divided into four groups, each consisting of 10 males and 10 females. They were subcutaneously injected with DHPN at a dose of 2800 mg/kg body weight and then fed an iodine-normal or iodine-deficient diet supplemented with or without 20% soybean for 12 weeks.
Autopsy and measurements.
At week 12, all rats were killed under ether anesthesia after blood was collected from the aorta. At autopsy, the thyroid and pituitary glands were carefully examined macroscopically. After weighing, the thyroids were fixed in 10% phosphate-buffered formalin, and routinely processed for production of sections stained with hematoxylin and eosin (H-E). Thyroid proliferative lesions were histopathologically classified according to the criteria of Botts et al. (1991). Serum triiodothyronine (T3) and T4 were measured with radioimmunoassay Riabead kits (Dainabot, Tokyo), and serum TSH was measured with a rat TSH kit (Amersham Life Science, Buckinghamshire, UK).
Statistical analysis.
Variance in data for body and organ weights, serum hormones, and tumor multiplicity were analyzed by Tukey's test for multiple comparisons. Tumor incidence data were evaluated by Fisher's exact probability test.
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RESULTS |
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DISCUSSION |
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There are two basic mechanisms whereby chemicals induce thyroid gland tumors in rodents. One, classified as genotoxic, involves direct carcinogenic effects at the DNA level in follicular cells. The other, termed non-genotoxic, involves the so-called negative feedback system of the thyroidpituitaryhypothalamus axis, in which decreased circulating T3 or T4 levels due to disruption of thyroid function or accelerated turnover lead to increased TSH and its prolonged stimulation of follicular epithelium (Capen, 1997; Hill et al., 1989
; McClain, 1992
). A number of non-genotoxic factors may contribute to decreases in circulating thyroid hormone levels, through inhibition of iodine transport into the thyroid, iodine oxidation or organification defects resulting from inhibition of TPO, inhibition of thyroid hormone secretion, or increased hormone inactivation by hepatic microsomal enzymes such as T4-UDP-GT (Capen, 1997
; Hill et al., 1989
). Thus, it has been concluded that thyroid tumor promotion is consistently mediated by chronic increase of TSH secretion from the pituitary (Ikeda et al., 2001
).
Iodine-deficient diets produce enlargement of the thyroid, an increase in monoiodotyrosine/diiodotyrosine, and a progressive increase in the T3/T4 molar ratio (Fukuda et al., 1975; Martin et al., 1985
). Sustained decrease in intracellular iodine concentration (Wolff, 1969
) resulting from dietary iodine deficiency inhibits thyroglobulin hydrolysis (Bagchi et al., 1985
), tyrosine iodination by thyroid peroxidase (Nunez and Pommier, 1982
), or accumulation of cAMP in the thyroid (Van Sande et al., 1975
). In our previous study (Ikeda et al., 2000
), iodine deficiency alone reduced serum T4 levels and inversely increased TSH levels, whereas excess soybean increased both serum T4 and TSH, suggesting primary stimulation of TSH release from the pituitary of rats under certain conditions. In the present study using DHPN as an initiator, however, 20% soybean feeding only showed a tendency to increase serum TSH levels, with no effects on T4. One explanation might be that the pretreatment with DHPN affected hormone metabolism (Onodera et al., 1994
). In our previous study, either SDM or PB treatment reduced both serum T3 and T4 levels and increased TSH levels, but co-treatment with soybean rather attenuated changes in these parameters, suggesting that excess soybean may induce thyroid proliferation by acting on the pituitary and/or hypothalamus (Ikeda et al., 2001
). Our previous and present findings regarding specific interactions with iodine deficiency are also suggestive of different modes of action of the various thyroid tumor-promoters (Ikeda et al., 2001
).
It has been reported that soy isoflavones like genistein inactivate rat as well as human thyroid peroxidase in vitro, but measures of thyroid function in vivo such as serum levels of T3, T4, and TSH; thyroid weights; and thyroid histopathology may all be normal (Doerge and Chang, 2002; Doerge and Sheehan, 2002
). This is in line with our previous data demonstrating that both genistein and soy isoflavone lack modulating effects on thyroid tumorigenesis in rats initiated with DHPN (Son et al., 2000a
, 2000b
, 2001
). A recent epidemiological study conducted in the San Francisco Bay Area suggested that consumption of traditional and non-traditional soy-based foods and alfalfa sprouts may actually be associated with a reduced risk of thyroid cancer (Horn-Ross et al., 2002
). It has also been suggested that in post-menopausal women soy protein may have only minor effects on thyroid hormone values that are unlikely to be clinically important (Persky et al., 2002
). Therefore, additional factors such as iodine deficiency could be necessary for soy to cause overt thyroid effects (Doerge and Chang, 2002
; Doerge and Sheehan, 2002
). Although safety assessment of soy products may not be required, the possibility that widely consumed soy products may cause harm in the human population via goitrogenic activity might be a safety concern, especially in iodine-deficient areas.
In conclusion, our results clearly indicate that co-treatment with excess soybean and a diet deficient in iodine synergistically promotes DHPN-induced thyroid tumorigenesis in rats, with influence on hormone levels as a likely mechanism.
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ACKNOWLEDGMENTS |
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