Gene expression is modulated by both physiological signals (hormones, cytokines, etc.) and environmental stimuli (physical parameters, xenobiotics, etc.). Oxidative stress appears to be a key pleiotropic modulator which may be involved in either pathway. Indeed, reactive oxygen species (ROS) have been described as second messengers for several growth factors and cytokines, but have also been shown to rise following cellular insults such as xenobiotic metabolism or enzymic deficiency. Extensive studies on the induction of stress-response genes by oxidative stress have been reported. In contrast, owing to the historical focus on gene induction, less attention has been paid to gene repression by ROS. However, a growing number of studies have shown that moderate (i.e. non-cytotoxic) oxidative stress specifically down-regulates the expression of various genes. In this review, we describe the alteration of several physiological functions resulting from oxidative-stress-mediated inhibition of gene transcription. We will then focus on the repressive oxidative modulation of various transcription factors elicited by ROS.

Human paraoxonase 1 (PON-1) is a serum high-density lipoprotein-associated enzyme mainly secreted by the liver. It has endogenous and exogenous substrates and displays protective properties with respect to cardiovascular disease and organophosphate intoxication. In the HuH7 human hepatoma cell line, PON-1 activity and mRNA levels were increased by dietary polyphenolic compounds such as quercetin but also by toxic ligands of the aryl hydrocarbon receptor (AhR) such as 3-methylcholanthrene (3-MC). However, the 2,3,7,8-tetrachlorobenzo(p)dioxin (TCDD) was a poor inducer. Transient and stable transfection assays indicated that these compounds increased the PON-1 gene promoter activity in an AhR-dependent manner, since their effect was inhibited by 7-keto-cholesterol and AhR-directed short interfering RNA. Deletions and mutations studies showed that a xenobiotic responsive element (XRE)-like sequence within the PON-1 promoter mediated the effect of 3-MC and quercetin. In contrast with consensus XREs from the cytochrome P450 1A1 gene, the PON-1 XRE-like element mediated preferentially the effect of quercetin compared to the results seen with TCDD.

Oxidative stress interferes with several cellular functions, in particular transcriptional regulation. We show here that the human cytochrome P450 1A1 (CYP1A1) is down-regulated at the transcriptional level by oxidative stress. Basal as well as 2,3,7,8-tetrachloro-p-dioxin-induced promoter activities are strongly impaired by H 2 O 2 treatment or glutathione depletion with L-buthionine-(S,R)-sulfoximine. Tumor necrosis factor ␣ inhibits CYP1A1 expression, and this inhibition is prevented by the antioxidant pyrrolidine dithiocarbamate. We show that these regulations depend on the integrity of the nuclear factor 1 (NFI) site located in the proximal promoter. We therefore examined the redox regulation of this transcription factor. Treatment of human HepG2 or rat H4 hepatoma cells with H 2 O 2 or L-buthionine-(S,R)-sulfoximine inactivates the binding of the NFI transcription factor to its DNA consensus sequence. Furthermore, H 2 O 2 treatment leads to a dose-dependent decrease of reporter gene expressions driven by promoters containing NFI binding sites. Glutathione depletion and catalase inhibition also repress a NFI-driven promoter. Under the same conditions, the CP-1 transcription factor activity is not affected by oxidative stress. Thus, NFI seems particularly sensitive to oxidative stress. This accounts, at least partially, for the regulation of cyp1A1 gene expression.

The human paraoxonase-1 (PON-1) is a serum high-density lipoprotein-associated phosphotriesterase secreted mainly by the liver. This enzyme is able to hydrolyze toxic organophosphate xenobiotics, endogenous oxidized phospholipids, and homocysteine thiolactone. Physiologically, it is thought to protect against cardiovascular diseases. The level of PON-1 gene expression is a major determinant of paraoxonase-1 status but little is known regarding the regulation of this gene. We identified several transcription start sites and characterized the regulation of its promoter by fibrates and statins. In HuH7 human hepatoma cells, the PON-1 secreted enzymatic activity and mRNA levels were increased by fenofibric acid (approximately 70%) and decreased by several statins (approximately 50%). Transient and stable transfection assays in HuH7 cells indicated that the modulation of the mRNA and enzymatic activity levels could be accounted for by the regulation of the PON-1 gene promoter activity by these drugs. These effects are probably not mediated by the PPAR alpha because over-expression of this receptor decreased the fibrate effect and did not modify statins activity. The repressive effect of statins is reversed by mevalonate and 22(R)-hydroxycholesterol, suggesting the involvement of the liver X receptor in the mechanism. The opposite effects of fenofibrate and statins could be consistent with clinical data on homocysteine levels after hypolipidemic drug treatment. Regarding the toxicological aspects, the induction achieved with fenofibric acid, although limited, could increase organophosphate metabolism and may be relevant in certain conditions for protective treatments.