Antigenotoxic effect of Xanthohumol in rat liver slices

Xanthohumol (XN), the principal prenylated flavonoid in the hop plant, Humulus lupulus L., is suggested to have cancer chemo-preventive activities. Its mechanisms of protection have been proposed to be inhibition of metabolic activation, induction of detoxifying enzymes and antioxidant activity. Our previous study showed that XN efficiently protected human hepatoma HepG2 cells against the genotoxic effects of two pro-carcinogens (2-amino-3-methylimidazo[4,5-f]quinoline (IQ) and benzo(a)pyrene (BaP)) that are dependent on cytochrome P450 (CYP) mediated metabolic activation, and against genotoxic effects of the oxidative damage inducing tert-butyl hydroperoxide (tBOOH). In the present study, we investigated the antigenotoxic effects of XN in precision-cut rat liver slices. Using the comet assay, we detected that at non-cytotoxic concentrations (0.01-10 microM) XN completely prevented IQ and BaP-induced DNA damage. The protective effects of XN against tBOOH-induced DNA damage was less efficient; the maximal 50% reduction of DNA damage was observed at 0.1 microM XN. In rat microsomes, XN (0.001-10 microM) inhibited CYP1A activity (7-ethoxycoumarin (7EC) de-ethylation) in a concentration-dependent manner. Surprisingly, no inhibition of 7EC metabolism by XN was observed in rat liver slices. XN also did not have any influence on mRNA expression of the enzymes CYP1A2 and quinone reductase (QR). These results indicate that inhibition of metabolic activation of pro-carcinogens by CYP1A is not likely to be the mechanism of its antigenotoxic action. In conclusion, XN efficiently protects DNA against genotoxicity of IQ and BaP and against oxidative DNA damage. Although the mechanism of the protective effect of XN is unclear, our results indicate that XN exhibits antigenotoxic effects in fresh liver tissue and provide additional evidence for the cancer preventive potential of XN.

Detection of xenobiotic-induced DNA damage by the comet assay applied to human and rat precision-cut liver slices

The comet assay is a simple and sensitive method for measuring DNA damage at the level of individual cells and is extensively used in genotoxicity studies. It is commonly applied to cultured cells. The aim of this study was to apply the comet assay for use in fresh liver tissue, where metabolic activity, all cell types and tissue architecture are preserved. The response of liver slices to genotoxic agents was tested with the reactive oxygen species generating tert-butyl hydroperoxide (tBOOH, 0.1-2 mM), [corrected] and the pro-carcinogens 2-amino-3-methylimidazo[4,5-f]quinoline (IQ, 0.5-2 mM) and benzo(a)pyrene (BaP, 10-100 microM). Dose-dependent DNA damage was observed and compared to HepG2 cells. At non-cytotoxic concentrations of carcinogens, human liver slices were more sensitive to tBOOH than rat liver slices, while no significant difference was found for BaP and IQ. Human liver slices were more sensitive to IQ than HepG2 cells, equally sensitive to BaP and less to tBOOH. Control slices showed low levels of DNA damage, which did not increase during 24 h preservation (0 degrees C) or 48 h culturing (37 degrees C). In conclusion, the comet assay that we applied for measuring DNA damage in precision-cut liver slices is an useful tool to study genotoxic effects induced by various potential genotoxicants, allowing for detection of species differences in susceptibility to carcinogens.

Rottlerin inhibits stimulated enzymatic secretion and several intracellular signaling transduction pathways in pancreatic acinar cells by a non-PKC-delta-dependent mechanism

Protein kinase C-delta (PKC-delta) becomes activated in pancreatic acini in response to cholecystokinin (CCK) and plays a pivotal role in the exocrine pancreatic secretion. Rottlerin, a polyphenolic compound, has been widely used as a potent and specific PKC-delta inhibitor. However, some recent studies showed that rottlerin was not effective in inhibiting PKCdelta activity in vitro and that may display unspecific effects. The aims of this work were to investigate the specificity of rottlerin as an inhibitor of PKC-delta activity in intact cells and to elucidate the biochemical causes of its unspecificity. Preincubation of pancreatic acini with rottlerin (6 microM) inhibited CCK-stimulated translocation, tyrosine phosphorylation (TyrP) and activation of PKC-delta in pancreatic acini in a time-dependent manner. Rottlerin inhibited amylase secretion stimulated by both PKC-dependent pathways (CCK, bombesin, carbachol, TPA) and also by PKC-independent pathways (secretin, VIP, cAMP analogue). CCK-stimulation of MAPK activation and p125(FAK) TyrP which are mediated by PKC-dependent and -independent pathways were also inhibited by rottlerin. Moreover, rottlerin rapidly depleted ATP content in pancreatic acini in a similar way as the mitochondrial uncouplers CCCP and FCCP. All studied inhibitory effects of rottlerin in pancreatic acini were mimicked by FCCP (agonists-stimulated amylase secretion, p125(FAK) TyrP, MAPK activation and PKC-delta TyrP and translocation). Finally, rottlerin as well as FCCP display a potent inhibitory effect on the activation of other PKC isoforms present in pancreatic acini. Our results suggest that rottlerin effects in pancreatic acini are not due to a specific PKC-delta blockade, but likely due to its negative effect on acini energy resulting in ATP depletion. Therefore, to study the role of PKC-delta in cellular processes using rottlerin it is essential to keep in mind that may deplete ATP levels and inhibit different PKC isoforms. Our results give reasons for a more careful choice of rottlerin for PKC-delta investigation.