Thimerosal Induces Apoptosis in a Neuroblastoma Model
via the cJun N-Terminal Kinase Pathway

Toxicological Sciences 92 (1). 246-253

Thimerosal Induces Apoptosis in a Neuroblastoma Model via the cJun N-Terminal Kinase Pathway

ML Herdman, A Marcelo, Y Huang, RM Niles, Dhar S & Kiningham KK. (2006).

Department of Pharmacology, Joan C. Edwards School of Medicine, 1542 Spring Valley Drive, Marshall University, Huntington, WV USA

Abstract

The cJun N-terminal kinase (JNK)-signaling pathway is activated in response to a variety of stimuli, including environ-mental insults, and has been implicated in neuronal apoptosis. In this study, we investigated the role that the JNK pathway plays in neurotoxicity caused by thimerosal, an ethylmercury-containing preservative. SK-N-SH cells treated with thimerosal (0–10mM) showed an increase in the phosphorylated (active) form of JNK and cJun with 5 and 10mM thimerosal treatment at 2 and 4 h. To
examine activator protein-1 (AP-1) transcription, cells were transfected with a pGL2 vector containing four AP-1 consensus sequences and then treated with thimerosal (0–2.5mM) for 24 h. Luciferase studies showed an increase in AP-1 transcriptional activity upon thimerosal administration. To determine the components of the AP-1 complex, cells were transfected with a dominant negative to either cFos (A-Fos) or cJun (TAM67). Reporter analysis showed that TAM67, but not A-Fos, decreased AP-1 transcriptional activity, indicating a role for cJun in this pathway. To assess which components are essential to apoptosis,
cells were treated with a cell-permeable JNK inhibitor II (SP600125) or transfected with TAM67, and the downstream effectors of apoptosis were analyzed. Cells pretreated with SP600125 showed decreases in activation of caspases 9 and 3, decreases in degradation of poly(ADP-ribose) polymerase (PARP), and decreased levels of proapoptotic Bim, in comparison to cells treated with thimerosal alone. However, cells transfected with TAM67 showed no changes in those same components. Taken together, these results indicate that thimerosal-induced neurotoxicity occurs through the JNK-signaling pathway, independent of cJun activation, leading ultimately to apoptotic cell death.

Excerpt: In recent years, controversy has surrounded the use of thimerosal in vaccines as mercury is a known neurotoxin and nephrotoxin. Since the controversy began in the late 1990’s, much of the thimerosal has been removed from vaccines administered to children in the United States. However, it remains in some, such as the influenza vaccine, and is added to multidose vials used in countries around the world. Studies concentrating on thimerosal-induced neurotoxicity are limited, and exposure guidelines, such as those set by the Food and Drug Administration, are based on research with methylmercury. Interestingly, some in vitro and in vivo studies suggest that ethylmercury may react differently than methylmercury (Aschner and Aschner, 1990; Harry et al., 2004; Magos et al., 1985). Few studies with thimerosal have focused on determining specific signaling pathways involved in neurotoxicity. Establishing these pathways may be an important step in discovering methods of alleviating toxic outcomes in patients exposed to thimerosal…. Our study is the first demonstration that thimerosal can induce the activation of JNK and AP-1 in the SK-N-SH neuroblastoma cell line. We showed that activation of cJun and AP-1 transcriptional activity following thimerosal treatment does not appear to be involved in the induction of apoptosis, as demonstrated with the studies using the cJun dominant negative. Furthermore, we were able to show that JNK is an essential upstream component of this pathway through the use of the JNK inhibitor SP600125. This compound was able to attenuate activation of downstream components of mitochondrial-mediated cell death and subsequently protect the cells from apoptosis. These results are significant because identifying specific signaling pathways activated in response to thimerosal exposure presents pharmacological targets for attenuating potential toxicity in patients exposed to thimerosal-containing products.