Novel effects and mechanisms of action of heme and soy isoflavones

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Heme is widely used as a food chemical for iron supplementation. Heme is an essential molecule for us as prosthetic groups for hemoglobin and many other hemoproteins. Excess free heme is a pro-oxidant and toxic, and is degraded by heme oxygenase (HO) to iron, CO and biliverdin, which is rapidly converted to bilirubin, an antroxidant. There are two isozymes of HO, namely inducible HO-1 and constitutive 110-2. Induction of HO-1 is an important defense mechanism against oxidative stress, and some HO inhibitors have antitumor activity. In this project, I have been evalutated the effects and analyzed the mechanisms of action of food chemicals such as heme iron and soy isoflavones on heme catabolism, using various types of human cells.Prolonged hypoxia decreased the expression level of HO-2 as well as that of HO-1, and these down-regulations of HOs were caused at both transcriptional and post-transcriptional levels. Down-regulation of 110-2 by siRNA induced the HO-1 expression, and HO-2 knock-down 1 under some conditions increased the intracellular heme content, suggesting the significance of HO-2 in the maintenance of cellular heme levels and in the regulation of HO-1 expression. Genistein, a soy isoflavone, inhibited the induction of HO-1 by heme and other chemicals, while daidzein, another soy isoflavone, did not inhibited the induction of HO-1 in some cases. Together with results of our other experiments, these inhibitory effects of isoflavones on HO-1 induction were suggested to be mediated by estrogen receptor-independent mechanism.