Hexavalent chromium induces oxidative stress and mitochondria-mediated apoptosis in isolated skin fibroblasts of Indo-Pacific humpback dolphin

The increasing gas emissions and industrial wastewater discharge of anthropogenic hexavalent chromium (Cr(VI)) have been growing health concerns to the high trophic level marine mammals. Our previous studies showed that Indo-Pacific humpback dolphin (Sousa chinensis), stranded on the Pearl River Estuary region, contained exceedingly high levels of Cr in their skin-tissues. Unfortunately, the molecular toxic mechanisms on this mammal are absent, limiting our understanding of the eco-physiological impacts of Cr(VI) on dolphins. Thus, the cytotoxicity effects of Cr(VI) were analyzed on fibroblasts we isolated from the skin of S. chinensis (ScSF). This study showed that Cr(VI) markedly inhibited the viability of ScSF cells via induction of apoptosis accompanied by an increase in the production of reactive oxygen species and the population of G2/M arrest or apoptotic sub-G1 phase cells, up-regulation of p53, and activation of caspase-3. Further investigation on intracellular mechanisms indicated that Cr(VI) induced depletion of mitochondrial membrane potential in cells through regulating the expression of anti-apoptotic (Bcl-2) and pro-apoptotic (Bax) proteins, resulting in decrease of the ATP level, cytochrome c release from mitochondria into cytosol, and the activation of caspase-9. Furthermore, antioxidants N-acetylcysteine and vitamin C displayed chemoprotective activity against Cr(VI) via suppression of p53 expression, indicating that the Cr(VI)-induced cell death may be mediated by oxidative stress. Overall, these results provide insights into the potential mechanisms underlying the cytotoxicity of Cr(VI) in Indo-Pacific humpback dolphin skin cells, offer experimental support for the proposed protective role of antioxidants in Cr(VI)-induced toxicity, and suggest that Cr(VI) contamination is one of key health concern issues for the protection of Indo-Pacific humpback dolphin.