Impact of metabolism on the bioactivity profile of soy isoflavones

Supervisor: Doris Marko, Department of Food Chemistry and Toxicology

Student: Anika Schröter

Isoflavones such as genistein or daidzein have been associated with a spectrum of biological effects among which their role as phytoestrogens is the most studied. As for many other polyphenols, an important role in the defense against oxidative stress has been postulated. One pivotal mechanism to guard against cancer, aging or atherosclerosis is the protection of cells against reactive oxygen species (ROS). In this regard, cells have developed defense mechanisms, such as the induction of the expression of phase II detoxifying enzymes. The expression of many phase II genes is regulated via the activation of antioxidant or electrophilic response elements (ARE/EpRE) which are located in the 5’-flanking region of their respective promoters. ARE, a cis-active element, is activated via binding of the transcription factor Nrf2 (Nuclear factor-erythroid 2 p45 subunit (NF-E2)-related factor 2). Activation by ROS induces the translocation of Nrf2 into the nucleus, its binding to ARE and the onset of the transcription of phase II enzymes. Recently, genistein has been reported to act as an activator of Nrf2/ARE-signaling in vitro. However, genistein has been repeatedly reported to possess clastogenic properties, which is discussed to be associated with topoisomerase targeting. Topoisomerases are essential enzymes crucial for DNA processing. Many of the topoisomerase targeting compounds stabilize the transiently formed covalent DNA–topoisomerase intermediate, the so-called “cleavable complex”, probably resulting in DNA damage. Whereas several flavonoids are known to act as catalytic topoisomerase inhibitors, genistein represents a topoisomerase II poison. So far, studies on the effects of polyphenols on topoisomerases have focused on in vitro models. It is unknown whether topoisomerase poisoning by polyphenols is also of relevance in vivo. In preliminary studies, we identified several oxidative metabolites of isoflavones as potent inhibitors of topoisomerase II, exceeding by far the effects of the parent compounds. These results raise the question whether sufficient in vivo tissue concentrations can be achieved to target topoisomerase and cause DNA damage. Thus, the contribution of these potential bioactive properties in respective target tissues is an important factor in understanding the physiological relevance of these polyphenols in food.

The PhD student will perform structure-activity studies with isoflavones and respective metabolites for selected potential targets on the enzyme level and subsequently in cell culture, also comprising impact on cell proliferation, topoisomerase poisoning and DNA integrity. Appropriate biomarkers will be analyzed in intestinal tissue in rats (external collaboration). These data will be correlated with the analysis of tissue concentrations of isoflavones and metabolites. The PhD student will furthermore be involved in the conduction and evaluation of the human intervention trial (project K.-H. Wagner).