Glucose availability is a decisive factor for Nrf2-mediated gene expression

Activation of the transcription factor Nrf2 (nuclear factor-erythroid 2-related factor 2) is one of the major cellular defense lines against oxidative and xenobiotic stress, but also influences genes involved in lipid and glucose metabolism. It is unresolved whether the cytoprotective and metabolic responses mediated by Nrf2 are connected or separable events in non-malignant cells. In this study we show that activation of Nrf2, either by the small molecule sulforaphane or knockout of the Nrf2 inhibitor Keap1, leads to increased cellular glucose uptake and increased glucose addiction in fibroblasts. Upon Nrf2 activation glucose is preferentially metabolized through the pentose phosphate pathway with increased production of NADPH. Interference with the supply of glucose or the pentose phosphate pathway and NADPH generation not only hampers Nrf2-mediated detoxification of reactive oxygen species on the enzyme level but also Nrf2-initiated expression of antioxidant defense proteins, such as glutathione reductase and heme-oxygenase1. We conclude that the Nrf2-dependent protection against oxidative stress relies on an intact pentose phosphate pathway and that there is crosstalk between metabolism and detoxification already at the level of gene expression in mammalian cells

Plumericin inhibits proliferation of vascular smooth muscle cells by blocking STAT3 signaling via S-glutathionylation

The etiology of atherosclerosis and restenosis involves aberrant inflammation and proliferation, rendering compounds with both anti-inflammatory and anti-mitogenic properties as promising candidates for combatting vascular diseases. A recent study identified the iridoid plumericin as a new scaffold inhibitor of the pro-inflammatory NF-κB pathway in endothelial cells. We here examined the impact of plumericin on the proliferation of primary vascular smooth muscle cells (VSMC). Plumericin inhibited serum-stimulated proliferation of rat VSMC. It arrested VSMC in the G1/G0-phase of the cell cycle accompanied by abrogated cyclin D1 expression and hindered Ser 807/811-phosphorylation of retinoblastoma protein. Transient depletion of glutathione by the electrophilic plumericin led to S-glutathionylation as well as hampered Tyr705-phosphorylation and activation of the transcription factor signal transducer and activator of transcription 3 (Stat3). Exogenous addition of glutathione markedly prevented this inhibitory effect of plumericin on Stat3. It also overcame downregulation of cyclin D1 expression and the reduction of biomass increase upon serum exposure. This study revealed an anti-proliferative property of plumericin towards VSMC which depends on plumericin’s thiol reactivity and S-glutathionylation of Stat3. Hence, plumericin, by targeting at least two culprits of vascular dysfunction –inflammation and smooth muscle cell proliferation -might become a promising electrophilic lead compound for vascular disease therapy

Activation of the Nrf2-ARE pathway by the Alternaria alternata mycotoxins altertoxin I and II

The mycotoxins altertoxin I and II (ATX I and II) are secondary metabolites produced by Alternaria alternata fungi and may occur as food and feed contaminants, especially after long storage periods. Although the toxic potential of altertoxins has been previously investigated, little is known about the pathways that play a role in their intracellular metabolism. In order to identify potential targets of ATX I and ATX II, the two toxins were tested for interaction with the nuclear factor erythroid-derived 2-like 2/antioxidant response element (Nrf2/ARE) pathway in mammalian cells. This pathway can be activated by various stressors resulting in the expression of enzymes important for metabolism and detoxification. In the present study, only ATX II triggered a concentration-dependent increase in Nrf2-ARE-dependent luciferase expression. Consistently, confocal microscopy revealed an ATX II-induced increase in Nrf2 signal in HT29 intestinal cells. In agreement with these data, ATX II induced the transcription of γ-glutamate cysteine ligase, the key enzyme in catalyzing GSH synthesis of the cells and which is regulated by Nrf2. Further investigations demonstrated that ATX II induced a concentration-dependent depletion of the cellular GSH levels after short incubation time (3 h) and an increase after longer incubation time (24 h). In conclusion, it was demonstrated that ATX II can interact at several levels of the Nrf2-ARE pathway in mammalian cells and that ATX I does not share the same mechanism of action