Temporal relationship of the orphan receptor TR3 translocation and expression with zinc-induced apoptosis in prostate cancer cells

Background: This study aimed to investigate the precise mechanism of zinc-induced apoptosis in human prostate cell lines PC-3 and LNCaP and explore its relationships with the translocation and expression of the orphan receptor TR3 and cytochrome c. Methods: The effects of zinc exposure on apoptosis levels were examined in both PC-3 and LNCaP cells. These cells were treated with exogenous ZnCl2 (100 mu M) for 0, 4, 8, 12, and 24 h. Dansylaminoethylcyclen (DAEC) fluorescent probe was applied to visualize cellular zinc localization. Zinc-induced apoptosis was identified by Hoechst nuclear staining and flow cytometry analysis. TR3 messenger RNA (mRNA) expression levels were detected by real time-quantitative polymerase chain reaction (PCR). TR3 protein localization and mitochondrial release of cytochrome c were identified by immunofluorescence microscopy. Mitochondrial membrane potential collapse under fluorescence microscopy was examined with a MitoLight probe. Results: Zinc exposure led to gradually increasing apoptosis levels in both LNCaP and PC-3 cells over the 24-h treatment period. The apoptotic effects could be observed as early as after 4 h of zinc treatment in PC-3 cells, with this being seen at 8 h in LNCaP cells. The apoptosis levels of both PC-3 and LNCaP cells began to increase significantly from 8 to 24 h when necrotic cells were also found. TR3 protein translocation from the nucleus to the mitochondria was noted and was accompanied by cytochrome c release into the cytosol from the mitochondria. Interestingly, no significant changes in TR3 mRNA expression levels were observed after zinc treatment. However, the mitochondrial membrane potential of both PC-3 and LNCaP cells gradually disappeared following extended zinc exposure. Conclusions: Zinc-induced apoptosis could be regulated through TR3 protein translocation from the nucleus to the mitochondria in prostate cancer cells without notable changes in TR3 mRNA levels. The direct effect of zinc on the mitochondria was associated with the release of cytochrome c into the cytosol through the mitochondrial targeting of TR3 protein.