Furthermore, some of the resveratrol- mediated antitumor activity seems to be AG 556 attached to the modulation of SIRT1. Therefore, it is rather expected that resveratrol induces a deacetylation instead of an acetylation of histone proteins. But sirtuins differ drastically from classical HDAC enzymes, not only by depending on the co-factor NAD +, but also by an altered intracellular localization and substrate specificity. Sirtuins display a highly conserved catalytic and NAD + -binding domain, which is often described as the sirtuin core domain and not found in HDACs of classes I, II or IV. The exact mechanism how resveratrol interacts with sirtuins has been under intense investigation during the last years. Besides a direct SIRT1 activation, evidence exists that resveratrol is also able to enhance the association between SIRT1 and other cellular factors that directly influence the activity of SIRT1. Nevertheless, due to the pleiotropic molecular mechanisms of resveratrol it is reasonable that it may activate sirtuins as well as inhibit HDACs of classes I, II and IV. Interestingly, an elegant recent study that investigated a model of wound repair revealed that resveratrol induced a downregulation of the DNA binding capacity and of the activity of HDAC2 via activation of sirtuins. Inhibiting SIRT1 in our HepG2 tumor cell system prior to resveratrol treatment resulted in a slightly decreased hyperacetylation of cellular proteins, which did not reach significance. These data suggest that the crosstalk between SIRT1 and classical HDACs is rather weak in our test system and a direct HDAC inhibition by resveratrol occurs in HepG2 cells. Possible mechnisms how resveratrol could modulate HDAC activity are schematically summarized in Figure S5B in File S1. To investigate the clinically interesting newly found pan- HDACi activity of resveratrol into a cancer treatment setting, we exemplarily used a hepatoma tumor model. As a first approach we analyzed HepG2, Hep3B and HuH7 cells after treatment with concentrations ranging from 5 mM up to 100 mM of resveratrol. 5-BDBD Particularly the upper concentrations of 50 mM and 100 mM were chosen to ensure substantial HDACi activity. In addition to that, studies with rats show, that these values can be achieved as peak concentrations after high dose intravenous administration despite a rapid decrease and subsequent elimination. Real-time cell monitoring experiments and SRB assays affirmed the anticancer properties of resveratrol when used in this dosage. The calculated IC50 concentrations of our real-time cell monitoring of HepG2 and HuH7 cells were in line with data which have been published previously. Interestingly, treatment of HepG2 cells with resveratrol triggered a delayed antiproliferative response within the first 24 h indicating different modes of action in the hepatoma cell lines. Taken together, these experiments underline the therapeutically attractive antiproliferative effects of resveratrol on cancer cells described for various tumor entities.