Biological response of human diploid keratinocytes to quinone-producing compounds: role of NAD(P)H:quinone oxidoreductase 1

Abstract

Reactive oxygen species (ROS) and quinones are known to determine redox balance alteration, oxidative stress and carcinogenicity. Keratinocytes of the human epidermis, a tissue particularly exposed to oxidant stimuli, possess a wide range of antioxidant and detoxifying mechanisms aimed to avoid oxidative damage of the tissue. In the present study, we evaluate the response of diploid and transformed human keratinocytes to exposure to l-dopa and tetrahydropapaveroline (THP), catechol compounds susceptible to undergo oxidation to form quinones with concomitant production of reactive oxygen species. We demonstrated that these compounds elicit up-regulation of intracellular antioxidant enzymes, in a different degree in normal cells with respect to transformed ones. Normal diploid keratinocytes adequately scavenge toxic substances through the activation of several, concurrent pathways. Conversely, in transformed cells, the whole oxidative burden must be detoxified by the limited set of conserved pathways that, accordingly, have to be highly activated. The biological response to catechol toxicity appears to rely on the pathway of NAD(P)H:quinone oxidoreductase 1 (NQO1). In conclusion, NAD(P)H:quinone oxidoreductase 1 confirms its antioxidant and detoxifying role contributing to the capacity of keratinocytes to protect epidermis against oxidative stress. Being retained in almost any cell, it represents a mechanism of general relevance in cell physiology.

Introduction

The importance of oxidative stress to the pathogenesis of many diseases as well as to degenerative processes and ageing is undisputed by now. The condition of oxidative stress sets up when the generation of oxidant species, primarily reactive oxygen species (ROS), exceeds the cell's antioxidant capacity. When present at high levels, ROS cause DNA damage, lipid peroxidation, membrane damage, cytotoxicity, mutagenicity and carcinogenicity (Floyd, 1999; Hensle, Robinson, Gabbita, Salsman, & Floyd, 2000; Shackelford, Kaufmann, & Paules, 2000).

A class of compounds that can cause oxidative stress are quinones, highly reactive molecules that readily undergo either one- or two-electron reduction. One-electron reduction generates unstable semiquinones, which undergo redox-cycling in the presence of molecular oxygen, leading to the formation of ROS, such as superoxide radicals and hydrogen peroxide (Rosen & Freeman, 1984).

Quinone species and ROS can be formed from the oxidation of catechol compounds, that for this reason become toxic. Thus, the inherent cytotoxicity of l-dopa and melanin precursors is well established; it was attributed to the generation of H₂O₂ (Urabe et al., 1994) and provided the basis for the design of potential chemotherapeutic agents to treat melanoma (Riley, 1991). On the other hand, dopa auto-oxidative breakdown lead to the formation of highly unstable dopa-quinones that are thought to induce neuronal damage (Asanuma, Miyazaki, & Ogawa, 2003). The same mechanism has been also proposed for tetrahydroisoquinolines (TIQs), a family of endogenous catechols detected in various human tissues and physiological fluids (Musshoff, Daldrup, Bonte, Leitner, & Lesch, 1997; Niwa, Takeda, Kaneda, Hashizume, & Nagatsu, 1987). TIQs are produced within the cell through the non-enzymatic condensation of dopamine with aldehydes (Dietrich & Erwin, 1980) (Fig. 1). Their catechol moiety is susceptible of oxidation to form quinone derivatives, responsible of oxidative damage. In previous works, we have shown that TIQs, which are toxic for melanoma cells (De Marco et al., 2002), are better tolerated by melanocytes; in these cells, tyrosinase activation provides an important protective mechanism to scavenge TIQs from cellular compartments through their incorporation into melanins (Perluigi et al., 2003).

The enzyme specifically committed to quinones detoxification is NAD(P)H:quinone oxidoreductase 1 (NQO1; EC 1.6.99.2, also known as quinone reductase 1 and DT-diaphorase), a cytosolic flavoenzyme that catalyzes the two-electron reduction of a broad range of substrates (Joseph, Long, Klein-Szanto, & Jaiswal, 2000; Lind, Cadenas, Hochstein, & Ernster, 1990). Through the action of this enzyme, deleterious quinones are turned into less toxic and more stable hydroquinones, susceptible to further inactivation and/or cellular excretion (Cadenas, 1995; Dinkova-Kostova & Talalay, 2000). In a variety of cell types, the NQO1 gene expression as well as enzyme activity has been shown to increase upon exposure to ROS produced during the redox cycling of quinones (Ahlgren-Beckendorf, Reising, Schander, Herdler, & Johnson, 1999; Prestera, Holtzclaw, Zhang, & Talalay, 1993; van Muiswinkel et al., 2000).

The epidermis is the tissue most heavily exposed to stimuli able to determine oxidative stress conditions and represents the first physical and biological defence of the organism. Keratinocytes, in addition to providing a barrier layer, are highly metabolically active cells, secreting several cytokines and growth factors (Stingl, Maurer, & Hauser, 1999) that regulate a wide number of responses involved in homeostasis maintenance and in injury reduction to the tissue. Moreover, epidermal cells bear a variety of antioxidant systems, including low molecular weight compounds and enzymes, all of them devoted to the protection from ROS accumulation, a constant threat in these cells (Kohen, 1999; Mates, Perez-Gomez, & Nunez de Castro, 1999; Podda & Grundmann-Kollmann, 2001; Schallreuter & Wood, 2001).

We designed the present work to evaluate the effects of two quinone and ROS-producing compounds, namely l-dopa and tetrahydropapaveroline (THP), on the growth and on the biochemical mechanisms of detoxication of primary keratinocytes as compared with continuous cell lines. In consideration of the role of NQO1 in quinones detoxification, the enzymatic activity, protein level and mRNA expression of NQO1 in cells exposed to the quinone-producing compounds were investigated. Reported data validate the role of NQO1 in protecting against catechol toxicity, although other mechanisms have also to be involved as indicated by the consistently observed higher resistance of primary cells respect to transformed cells. The subsequent implications on a physiological ground are also discussed.