Dual properties of hispidulin: antiproliferative effects on HepG2 cancer cells and selective inhibition of ABCG2 transport activity.

Hepatocellular carcinoma is the third most common cause of cancer-related deaths worldwide. Furthermore, the existing pharmacological-based treatments are insufficiently effective and generate many side effects. Hispidulin (6-methoxy-5,7,4'-trihydroxyflavone) is a flavonoid found in various medicinal herbs that present antineoplastic properties. Here we evaluated how modulation of reactive oxygen species (ROS) and alterations of antioxidant defenses could be associated to the antiproliferative effects of hispidulin in HepG2 cells. In addition, we studied the inhibitory activity of hispidulin on the efflux of drugs mediated by ABC transporters involved in multidrug resistance. In order to understand the increase of intracellular ROS promoted by hispidulin, we investigated the mRNA expression levels and activities of antioxidant enzymes, and the GSH/GSSG ratio. We showed that hispidulin significantly down-regulated the transcription levels of catalase, leading to reduction of enzyme activity and decrease of the GSH content. We also observed that, in the presence of N-acetylcysteine or exogenous catalase, the proliferation was lowered back to the control levels. These data clearly indicate a strong involvement of intracellular ROS levels for triggering the antiproliferative effects. We also demonstrated that the inhibition produced by hispidulin on drug efflux was specific for ABCG2, since no effects were observed with ABCB1 and ABCC1. Furthermore, HepG2 cells were more sensitive to hispidulin-mediated cell death than immortalized L929 fibroblasts, suggesting a differential toxicity of this compound between tumor and non-tumor cell lines. Our results suggest that hispidulin constitutes a promising candidate to sensitize chemoresistant cancer cells overexpressing ABCG2.

Flavone induces cell death in human hepatoma HepG2 cells.

Flavones have received considerable attention because of their antiproliferative properties and selective effects on cancer cells, making them good candidates for use in cancer therapy. In contrast to other flavones, little is known about the effects of the flavone core structure (2-phenyl-4H-1-benzopyran-4one) on cancer cells. Here, we report that flavone induces cell death in human hepatoma HepG2 cells. Furthermore, annexin-V+/PI- and SubG1 populations of HepG2 cells increased after flavone treatment. Exposure of HepG2 to flavone did not result in either cytochrome c release into the cytosol or changes in the mitochondrial membrane potential. Treatment of HepG2 cells with flavone for 24 h reduced the accumulation of intracellular ROS, which correlated with upregulation of Gred, CuZnSOD and MnSOD mRNA levels. Taken together, our results provided useful insights into the mechanism of cell death caused by flavones, in order to evaluate their future application in hepatocarcinoma therapy.

Novel rhythms of N1-acetyl-N2-formyl-5-methoxykynuramine and its precursor melatonin in water hyacinth: importance for phytoremediation.

N1-acetyl-N2-formyl-5-methoxykynuramine (AMFK) is a major metabolite of melatonin in mammals. To investigate whether AFMK exists in plants, an aquatic plant, water hyacinth, was used. To achieve this, LC/MS/MS with a deuterated standard was employed. AFMK was identified in any plant for the first time. Both it and its precursor, melatonin, were rhythmic with peaks during the late light phase. These novel rhythms indicate that these molecules do not serve as the chemical signal of darkness as in animals but may relate to processes of photosynthesis or photoprotection. These possibilities are supported by higher production of melatonin and AFMK in plants grown in sunlight (10,000-15,000 microW/cm2) compared to those grown under artificial light (400-450 microW/cm2). Melatonin and AFMK, as potent free radical scavengers, may assist plants in coping with harsh environmental insults, including soil and water pollutants. High levels of melatonin and AFMK in water hyacinth may explain why this plant more easily tolerates environmental pollutants, including toxic chemicals and heavy metals and is successfully used in phytoremediation. These novel findings could lead to improvements in the phytoremediative capacity of plants by either stimulating endogenous melatonin synthesis or by adding melatonin to water/soil in which they are grown.