Lan Guan

Hexavalent chromium induces energy metabolism disturbance and p53-dependent cell cycle arrest via reactive oxygen species in L-02 hepatocytes

Hexavalent chromium [Cr(VI)] has become a non-negligible pollutant in the world. Cr(VI) exposure leads to severe damage to the liver, but the mechanisms involved in Cr(VI)-mediated toxicity in the liver are unclear. The present study aimed to explore whether Cr(VI) induces energy metabolism disturbance and cell cycle arrest in human L-02 hepatocytes. We showed that Cr(VI) inhibited state 3 respiration, respiratory control rate (RCR), and subsequently induced energy metabolism disturbance with decreased ATP production. Interestingly, cell cycle analysis by flow cytometry and protein expression analysis by western blotting revealed that low dose of Cr(VI) (4 uM) exposure induced S phase cell cycle arrest with decreased mediator of replication checkpoint 1 (Mrc1) and cyclin-dependent kinase 2 (CDK2), while higher doses of Cr(VI) (16, 32 uM) exposure resulted in G2/M phase arrest with decreased budding uninhibited by benzimidazoles-related 1 (BubR1) and cell division cycle 25 (CDC25). Mechanism study revealed that Cr(VI) decreased the activities of mitochondrial respiratory chain complex (MRCC) I and II, thus leading to ROS accumulation. Moreover, inhibiting ROS production by antioxidant N-acetyl-l-cysteine (NAC) rescued Cr(VI)-induced ATP depletion and cell cycle arrest. ROS-mediated p53 activation was found to involve in Cr(VI)-induced cell cycle arrest, and p53 inhibitor Pifithrin-α (PFT-α) rescued Cr(VI)-induced reduction of check point proteins Mrc1 and BubR1, thus inhibiting cell cycle arrest. In summary, the present study provides experimental evidence that Cr(VI) leads to energy metabolism disturbance and p53-dependent cell cycle arrest via ROS in L-02 hepatocytes.

Reactive Oxygen Species Play a Central Role in Hexavalent Chromium-Induced Apoptosis in Hep3B Cells without the Functional Roles of p53 and Caspase-3

Background/Aims: Hexavalent chromium [Cr(VI)] and its compounds, which have the extensive application in diverse industries including metallurgy, textile and electroplating, are known to be genotoxic and mutagenic to humans. Although it is supported by a large body of literatures that p53 and caspase-3 played key roles in Cr(VI)-induced cytotoxicity, it is clear that Cr(VI) could induce apoptosis either without activating caspase, or in a p53-independent manner. Methods: In the present study, by using Z-VAD-fmk to inhibit caspase-3 in p53-deficient Hep3B cells, we explored the effect of Cr(VI) on apoptosis induction and the related mechanisms when the functions of p53 and caspase were simultaneously blocked. Results: We found that Cr(VI) still induced DNA damage, mitochondrial injury, oxidative stress and apoptosis in Hep3B cells without the functional roles of p53 and caspase-3, and the mechanism study revealed that this was in a ROS-dependent manner since NAC co-treatment showed the protective effect against Cr(VI)-induced apoptosis. Conclusion: Our research has disclosed the mechanism involved in Cr(VI)-induced cytotoxicity following the loss of p53 and caspase-3 functions and shed light on the importance of using antioxidants for primary and secondary prevention in Cr(VI) occupational exposure populations.

Effect of hexavalent chromium on electron leakage of respiratory chain in mitochondria isolated from rat liver.

Background/Aims: In the present study, we explored reactive axygen species (ROS) production in mitochondria, the mechanism of hexavalent chromium (Cr(VI)) hepatotoxicity, and the role of protection by GSH. Methods: Intact mitochondria were isolated from rat liver tissues and mitochondrial basal respiratory rates of NADH and FADH2 respiratory chains were determined. Mitochondria were treated with Cr(VI), GSH and several complex inhibitors. Mitochondria energized by glutamate/malate were separately or jointly treated with Rotenone (Rot), diphenyleneiodonium (DPI) and antimycinA (Ant), while mitochondria energized by succinate were separately or jointly treated with Rot, DPI ‚ thenoyltrifluoroacetone (TTFA) and Ant. Results: Cr(VI) concentration-dependently induced ROS production in the NADH and FADH2 respiratory chain in liver mitochondria. Basal respiratory rate of the mitochondrial FADH2 respiratory chain was significantly higher than that of NADH respiratory chain. Hepatic mitochondrial electron leakage induced by Cr(VI) from NADH respiratory chain were mainly from ubiquinone binding sites of complex I and complex III. Conclusion: Treatment with 50µM Cr(VI) enhances forward movement of electrons through FADH2 respiratory chain and leaking through the ubiquinone binding site of complex III. Moreover, the protective effect of GSH on liver mitochondria electron leakage is through removing excess H2O2 and reducing total ROS.