J Steroid Biochem Mol Biol. 2011 Nov;127(3-5):345-50. Epub 2011 Jul 29.
Protective behavior of tamoxifen against Hg2+-induced toxicity on kidney mitochondria: in vitro and in vivo experiments.
Hernández-Esquivel L, Zazueta C, Buelna-Chontal M, Hernández-Reséndiz S, Pavón N, Chávez E.
Abstract
Heavy metals are known to induce functional alterations in kidney mitochondria, this damage plays a central role in the mercury-induced acute renal failure. In fact, mercury causes rapid and dramatic changes in the membrane's ionic permeability in such a way that a supra load of mitochondrial Ca(2+) occurs. As a consequence, the phenomenon of permeability transition takes place. In this work we studied in vitro and in vivo the protective effect of the selective estrogen receptor modulator tamoxifen on the deleterious action of mercury-induced nonselective permeability in kidney mitochondria. Added in vitro tamoxifen inhibited membrane nonspecific pore opening, brought about by Hg(2+), as well as the oxidative damage of the enzyme cis-aconitase. In vivo the administration of tamoxifen prevented Hg(2+)-induced poisoning on mitochondrial energy-dependent functions. Permeability transition was analyzed by measuring matrix Ca(2+) retention, mitochondrial swelling, and the build up and maintenance of a transmembrane electric gradient. The pharmacologic action of tamoxifen on mercury poisoning could be ascribed to its cyclosporin-like action.
PMID: 21821123
INTRODUCTION:
Mercury brings about acute renal failure through its interaction with sulfhydryl groups of membrane proteins responsible for the regulation of ion permeability [1]. In this way, mercury causes a fast and dramatic change on membrane permeability.
The latter implies an increasing cytosolic Ca2+ accumulation [2] and, therefore, a massive load of this cation into mitochondria [3].
Besides the action of Hg2+ on thiol groups to induce tissue damage, an additional mechanism is involved in mercury poisoning, i.e., oxidative stress [4–6].
As a consequence of Ca2+ accumulation and oxidative stress, kidney mitochondria undergo an increased nonselective permeability [7–9]. The membrane permeability transition, from selective to nonselective, to ions and metabolites, causes mitochondrial dysfunction.
This dysfunction is characterized by loss of the transmembrane electric gradient and, therefore, of the oxidative phosphorylation process [10], as well by an inability to maintain matrix Ca2+ accumulation and normal mitochondrial volume after Ca2+ addition. The chemical nature of the nonspecific transmembrane pore has not been well established. However, two membrane molecules have been proposed to act as possible pore, i.e., the adenine nucleotide carrier [11–13] and the phosphate carrier [14,15]. Inhibition of pore opening can be achieved by a different sort of compounds, being the immunosuppressant, cyclosporine A, one of the most effective [16].
Recently, tamoxifen, a selective estrogen receptor modulator widely used in oncology and reproductive endocrinology [17,18], has been introduced as inhibitor of permeability transition [19]. The present work shows that the effect of Hg2+ on membrane leaking is abolished by tamoxifen. The results obtained indicate that, in vitro, tamoxifen protects isolated mitochondria against Hg2+-induced increased membrane permeability to Ca2+, membrane de-energization, and oxidative damage of the aconitase enzyme. Administered in vivo, tamoxifen prevents the deleterious effect of mercury on both mitochondrial Ca2+ accumulation and transmembrane potential.
DISCUSSION:
The biochemical characteristics of Hg2+-induced nephrotoxicity, in in vitro and in vivo systems, have been extensively studied [2,28–30]. A wide array of drugs and chemicals has been used to remedy mercury poisoning, among them cyclosporin A [27], dpenicillamine [31], diethyldithiocarbamate [32], captopril [3], and EGTA [2]. The present study shows that the estrogen receptor modulator, tamoxifen, effectively prevented renal injury induced by mercury toxicity, both in vitro and in vivo. The in vitro experiments show that the deleterious effect of Hg2+ on the permeability to Ca2+ and on the transmembrane energization are inhibited by tamoxifen. Similarly the in vivo experiments showed that also, under these conditions, tamoxifen protected from Hg2+-induced mitochondrial membrane leakage.
The chemicals previously used, i.e., captopril, diethyldithiocarbamate, and EDTA, plausibly operate as Hg2+ chelators through their thiol or carboxylic containing groups. Regarding the protective effect of cyclosporin A, its inhibitory action on permeability transition must be through its interaction with the enzyme cyclophilin D. In turn, such an interaction locks the adenine nucleotide carrier on the matrix side of the inner membrane closing the non-specific nanopore. In order to explain the mechanism by which tamoxifen counteracted Hg2+-induced nonspecific pore opening, we must take into account that the heavy metal induces oxidative stress. As is well known, this process underlies the increased permeability activation [33].
The mechanism by which mercury induces a massive production of reactive oxygen derived species is related, on one side, with the inhibition of the heavy metal on the respiratory chain [34,35]; in fact, Belyaeva et al. [36] reported that, in rat liver mitochondria, mercury induces mitochondrial dysfunction in a substrate specific fashion. On the other side, this massive production of reactive oxygen species is related with the depletion of reduced mitochondrial glutathione [37,38].
The results shown in this work about the failure of mitochondria to retain accumulated Ca2+ and to maintain a high level of the transmembrane electric gradient must be attributed to the Hg2+-induced oxidative stress. Direct determination of ROS, shown in Fig. 4, indicated that indeed mercury generated a high amount of them. In addition, the inactivation of the enzyme aconitase is a clear demonstration that in our experiments such a process occurred. Thus, considering the above, the protective action of tamoxifen must be ascribed to its well known scavenging action.
To this regard, Moreira et al. [39] have shown that, in brain mitochondria, tamoxifen preserves mitochondrial functions through inhibiting H2O2 formation and GSH depletion promoted by Ca2+. Further, Ek et al. [40] have demonstrated that tamoxifen reduces the incidence of ventricular tachycardia on reperfusion due to its antioxidant properties.
Taken all together the reports and the results here presented, it seems challenging to propose tamoxifen as a useful therapeutic drug addressed to counteract mercury toxicity.
For years I've read news stories about dental mercury amalgam fillings that failed to ask vital follow up questions for one to form a better understanding of the true risks involved with exposure to mercury fillings. Rarely was a toxicologist, neurologist or bio-chemist interviewed. Instead, the stories would always give dentists such prominence when promoting the safety of a substance of which they knew nothing about, all while never acknowleding the much lower levels of mercury at which our government has removed other products from the market. So now I'm doing what I can to help raise awareness of the many dangerous aspects of dental mercury fillings.
