Effect of the flavonoid quercetin on cadmium-induced hepatotoxicity

Abstract

The present study was designed to evaluate whether treatment with quercetin exerts any beneficial effect on cadmium (Cd)-induced hepatotoxicity in order to establish the possible protective mechanisms of quercetin. Wistar rats were distributed in four experimental groups: control, Cd, quercetin, and Cd + quercetin. Hepatic toxicity was evaluated by measuring plasma concentrations of markers of hepatic injury. The activity of antioxidant enzymes in liver was also measured. Hepatic expression of metallothioneins (MT), and endothelial nitric oxide synthase (eNOS) was assayed by Western and Northern blot. Our results demonstrated that Cd administration induced an increased marker enzyme activity in plasma. This effect was not inhibited by quercetin. However, the administration of quercetin softened Cd-induced oxidative damage. MT levels in liver were substantially increased when the animals received Cd and quercetin. Hepatic eNOS expression was significantly increased after treatment with Cd and quercetin, being this increase higher than in animals receiving Cd alone. In conclusion, in this experimental model, quercetin was not able to prevent the Cd-induced liver damage although the animals that received both, Cd and quercetin showed a marked improvement in oxidative stress and an increase in the MT and eNOS expression. These results suggest that other mechanisms different to oxidative stress could be involved in hepatic damage.

Introduction

Cadmium (Cd) is a common environmental pollutant and Cd emissions to the atmospheric, aquatic and terrestrial environment have increased dramatically during the twentieth century (Järup et al., 1998). The wide environment distribution of Cd has led to an increased interest in its toxicity and biological effects (Thevenod, 2003). Cd accumulates mainly in the liver and kidney and can cause severe tissue damage in these organs (Fukumoto et al., 2001). Cd induces production of metallothionein (MT), a low-molecular-weigh protein that has high affinity for the metal (Klaassen et al., 1999, Nordberg and Nordberg, 2000). This seems to provide a mechanism by which the metal can be sequestered in a relatively inert and thus nontoxic state (Masters et al., 1994, Liu et al., 1995, Liu et al., 1998a, Liu et al., 1998b). When the amount of Cd in the liver and kidney exceeds the binding capability of MT, the non-MT-bound Cd ions are believed to cause hepato- and nephrotoxicity (Nordberg and Nordberg, 1987). One mechanism by which Cd can produce tissue injury is the generation of reactive oxygen species (ROS) and lipid peroxidation, which in turn depress hepatic and renal functions (Nomiyama et al., 1998, Shaikh et al., 1999, Thevenod and Friedmann, 1999). If these ROS-mediated stress events are not balanced by repair processes, affected cells undergo apoptosis or necrosis (for review see Thevenod, 2003). In this sense, MT seems to act also as an antioxidant (Min et al., 2005). Exogenous metallothionein has been described to protect against acute renal and liver Cd toxicity in rats, probably based on its antioxidant properties (Kara et al., 2005).

Flavonoids are natural phenolic substances present in vegetables and in the red wine (Goldberg et al., 1995) that can act as antioxidants in biological systems. Quercetin, one of the most abundant flavonoids, is a potent oxygen free radicals scavenger and a metal chelator (Middleton, 1998). Recent work from our laboratory presented evidence that quercetin had a marked protective effect on cadmium-induced nephrotoxicity that seems to be based on its strong antioxidant activity (Morales et al., 2006b) and on its ability to increase MT and eNOS expression (Morales et al., 2006a).

The present study was designed to evaluate whether treatment with quercetin exerts any beneficial effect on liver function in an experimental model of rats chronically treated with Cd at relatively high levels as a model of chronic Cd-induced hepatotoxicity. In this model we have assessed the effect of the antioxidant quercetin on Cd-induced liver damage as well the liver expressions of MT and eNOS in rats exposed to Cd and treated or not with quercetin, in order to establish the possible protective mechanisms of quercetin in cadmium-induced hepatotoxicity.