Toshikazu Noguchi

Anti-platelet action of isoliquiritigenin, an aldose reductase inhibitor in licorice

The mechanism was studied by which isoliquiritigenin, a new aldose reductase inhibitor purified from licorice (Glycyrrhizae radix), inhibits platelet aggregation. This new agent significantly inhibited the phosphorylation of 40,000- and 20,000-dalton proteins, and inhibited the formation of 12 (S)-hydroxy-5,8,10-heptadecatrienoic acid, 12-hydroxyeicosatetraenoic acid and thromboxane B2. The inhibitory effect of isoliquiritigenin on platelet aggregation in vitro was comparable to that of aspirin. Our findings may indicate that isoliquiritigenin elicits an anti-platelet action by inhibiting not only cyclooxygenase but also lipoxygenase or peroxidase activity in platelets. Isoliquiritigenin also showed an anti-platelet action in vivo. Isoliquiritigenin appears to be the only aldose reductase inhibitor with a significant anti-platelet action. Since the hyperaggregability of platelets has been implicated in the pathogenesis of diabetic complications, isoliquiritigenin may offer a unique benefit as an aldose reductase inhibitor.

Vitamin D3 stimulates the production of prostacyclin by vascular smooth muscle cells

The effects of vitamin D3 on the production of prostacyclin (PGI2) by cultured rabbit vascular smooth muscle cells (VSMCs) were investigated. PGI2 synthesis by VSMCs was significantly increased in the presence of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) and 1 alpha hydroxyvitamin D3 (1 alpha(OH)D3) at 48 hours [1,25(OH)2D3 greater than 1 alpha(OH)D3]. Physiological concentration of 1,25(OH)2D3 (10(-10) M) significantly increased the synthesis of PGI2. Further, we observed that treatment with 1,25(OH)2D3 significantly induced the activity of cyclooxygenase without changing the activity of phospholipase A2. These findings suggest that the mechanism of action of 1,25(OH)2D3 on the synthesis of PGI2 is mediated by the cyclooxygenase pathway. It seems possible that vitamin D3 is a vasoactive agent and may play a protective role in the development of atherosclerosis.

The effects of sex hormones on the synthesis of prostacyclin (PGI2) by vascular tissues

The effects of estradiol and testosterone on prostacyclin (PGI2) release (measured as 6-keto-PGF1 alpha) by vascular tissues using rat aortic rings and cultured rabbit aortic smooth muscle cells (SMC) were investigated. Aortic SMC were prepared from either explants of atherosclerotic intima or those of normal media. Aortic rings obtained from male and female rats which had been treated with estradiol resulted in increased PGI2 synthesis. Furthermore, PGI2 synthesis by cultured medial SMC was significantly increased in the presence of estradiol (10(-7), 10(-9) M). An increased tendency in PGI2 synthesis was also observed in intimal SMC. On the other hand, aortic rings obtained from female rats treated with testosterone resulted in a significant decrease in PGI2 synthesis. However, aortic rings from testosterone-treated male rats and cultured medial and intimal SMC treated with testosterone (10(-6), 10(-8) M) for 48 hr did not show any significant changes in PGI2 synthesis. We also found greater PGI2 synthesis by intimal SMC compared with that by medial SMC. These results suggest that estradiol and testosterone may have opposite functions in the development of atherosclerosis, that is, estradiol for anti-atherosclerotic and testosterone for atherogenic, by modulating PGI2 synthesis by vascular tissues.

Effects of aldose reductase inhibitors on prostacyclin (PGI2) synthesis by aortic rings from rats with streptozotocin-induced diabetes

The effects of aldose reductase inhibitors (ARIs) on the synthesis of prostacyclin (PGI2) by aortic rings from diabetic rats were examined. The ARIs studied were ONO-2235 and isoliquiritigenin, a new compound extracted from glycyrrhizae radix. The content of sorbitol in the sciatic nerve of diabetic rats induced by streptozotocin was significantly increased as compared with that of controls. This increase was significantly inhibited by the administration of an ARI. On the other hand, there was a marked decrease in the synthesis of PGI2 by the diabetic rats compared with the control rats. The decrease in PGI2 synthesis was significantly reversed by the administration of an ARI. Furthermore, the synthesis of PGI2 by the aortic rings was inversely correlated with the content of sorbitol in sciatic nerves. Those observations suggest that an ARI may have a beneficial effect on the vascular synthesis of PGI2 in diabetes mellitus.

The effects of thyroxine and methimazole treatment on the synthesis of prostacyclin (PGI2) in the rat

The effects of thyroxine (T4) and methimazole administration on plasma prostacyclin (PGI2) levels in vivo and on PGI2 release by aortic rings incubated in vitro were investigated in rats. Male rats were given single injection of T4 (200 micrograms/100 g body wt) ip every 24 h for either 3, 7 or 14 days for hyperthyroid rats. For hypothyroid rats, a group of rats were given methimazole (0.01 % in drinking water) for 14 days. PGI2 concentrations were determined in plasma and also in the medium in which aortic rings were incubated. PGI2 was measured as 6-keto-PGF1 alpha by RIA. Plasma PGI2 levels in T4-treated groups were found to be significantly higher than those of control animals. Aortic rings obtained from rats given single injection of T4 for 7 and 14 days showed significant increases in release of PGI2 into the incubation medium. In contrast, rats given methimazole for 14 days showed a significant decrease in the production of PGI2 by aortic rings without any significant changes in plasma levels. Direct addition of T4 into the incubation medium did not cause any significant changes in PGI2 release by aortic rings obtained from control rats. These results suggest the regulatory role of thyroid hormone in PGI2 synthesis in vivo.

Reevaluation of circulating prostacyclin and thromboxane in diabetes.

Although several investigators have attempted to measure the plasma levels of prostacyclin (PGI2) and thromboxane A2 (TXA2) in diabetes and normal subjects, their results have been controversial. In this study, we measured plasma PGI2 and TXA2 levels in diabetic patients and normal subjects. The plasma PGI2 and TXA2 were determined by RIA as 6-keto-PGF1 alpha and TXB2, respectively. The plasma levels of 6-keto-PGF1 alpha were significantly reduced in diabetics with microangiopathy (52.5 +/- 18.9 pg/ml, mean +/- SE, p less than 0.05) compared with those of normal subjects. Diabetics as a whole also showed lower levels of 6-keto-PGF1 alpha than normal subjects (57.8 +/- 26.1 vs. 70.2 +/- 20.7 pg/ml), though this was not significant statistically. The plasma 6-keto-PGF1 alpha levels did not significantly correlate with either age of the patients or duration of diabetes in diabetics. Interestingly, however, hemoglobin A1c significantly correlated inversely with 6-keto-PGF1 alpha levels in diabetics without microangiopathy (r = -0.60, p less than 0.05). The plasma levels of TXB2 in diabetics were significantly higher than those of normal subjects (155.2 +/- 69.5 vs. 108.0 +/- 30.0 pg/ml, p less than 0.05). These data suggest that an imbalance of circulating PGI2 and TXA2 may contribute to the development of diabetic microangiopathy.

Effect of insulin on the production of prostacyclin and cell proliferation in cultured smooth muscle cells

We have investigated the effects of insulin on the synthesis of prostacyclin and cell proliferation in cultured vascular smooth muscle cells, which have been thought to play important roles in the development of atherosclerosis. Prostacyclin was measured as 6-keto-PGF1 alpha in the culture medium, and cell proliferation as incorporation of [3H]thymidine into DNA. Our studies showed that insulin reduced production of prostacyclin and stimulated cell proliferation in SMC. Like insulin, dibutyryl cAMP inhibited the production of prostacyclin, whereas it did not stimulate cell proliferation. No significant changes in cAMP levels were found on the addition of insulin into the culture medium. Therefore, cAMP does not appear to be involved in the mechanisms of these insulin effects. These results again suggest that hyperinsulinemia could be one of the important factors in atherosclerosis.

Increased Production of Prostacyclin (PGI 2 ) by Vascular Intimal Smooth Muscle Cells (SMC) from Atherosclerotic Rabbit Aorta

In vitro PGI2 synthesis by aortic strips obtained from thoracic aorta of rabbits fed a high cholesterol diet was examined and compared with that of control rabbits fed a normal diet. In this report, the amounts of PGI2 produced were shown as 6-keto-PGF1 alpha per microgram of aortic tissue DNA instead of per mg wet weight. We also investigated PGI2 synthesis by cultured smooth muscle cells (SMC) obtained from atherosclerotic intima. Basal PGI2 production by aortic strips from atherosclerotic rabbit aorta was significantly augmented compared with that of controls. Arachidonic acid (AA)-induced PGI2 production by atherosclerotic aorta was also significantly higher than that of controls. PGI2 producing capacities of intimal and medial layers, separated from atherosclerotic aorta, were examined and the intimal layer was found to elicit a significantly greater PGI2 production than the medial layer. Furthermore, cultured intimal SMC obtained from atherosclerotic rabbit aorta produced a greater amount of PGI2 than medial SMC from normal rabbit aorta at various cultured conditions. These results suggest that the possibility of enhanced PGI2 production by atherosclerotic aorta may well be considered as a defence mechanism of the vessel wall against damaging stimuli.

Prostacyclin (PGI2) Production in Hyperthyroid Rats and Patients with Graves’ Disease

Prostaglandins can stimulate thyroid hormone secretion from the thyroid gland (1). Conversely, T3 can stimulate prostacyclin synthesis by cultured vascular smooth muscle cells (2). These findings seem very interesting for understanding the mechanism of the anti-atherosclerotic action of thyroid hormone. Therefore, we have examined the effect of thyroid hormone administration in vivo on prostacyclin synthesis in rats, and the mechanisms of thyroid hormone action were also investigated.