Effects of Nickel at Environmentally Relevant Concentrations on Human Corneal Epithelial Cells: Oxidative Damage and Cellular Apoptosis

Abstract

Nickel (Ni) is ubiquitous in the environment and evidence has suggested that Ni can cause ocular surface inflammation, especially in fine particulate matter and personal products. Continuous daily exposure to Ni-containing dust may adversely impact the human cornea, whereas the underlying mechanism of this phenomenon remains not fully understood. Here, human corneal epithelial cells (HCEC) were employed to analyze the toxicity of Ni via detections of cell morphology, cell viability, reactive oxygen species production, cell apoptosis rate, and apoptotic gene expression levels after exposure for 24 h to uncover the damage of Ni to the cornea. A concentration-dependent inhibition of HCECs' viability and growth was observed. In particular, Ni at 100 μM significantly decreased cell viability to 76%, and many cells displayed an abnormal shape and even induced oxidative damage of HCEC by increasing ROS to 1.2 times, and further led to higher apoptosis (24%), evidenced by up-regulation of apoptotic genes Caspase-8, Caspase-9, NF-κB, IL-1β, and Caspase-3, posing a risk of dry eye. Our study suggested that Ni induces apoptosis of HCEC through oxidative damage. Therefore, Ni pollution should be comprehensively considered in health risks or toxic effects on the ocular surface.

Keywords: apoptosis; human corneal epithelial cells; nickel; ocular damage; oxidative stress.

Figure 1

Cell viability after Ni exposure (A,C) and LC₅₀ calculated by nonlinear fitting after logarithmic conversion of concentration after Ni exposure (B) and morphological changes of human corneal epithelial cells by inverted microscope (TS-100, Nikon, Tokyo, Japan) at 200× magnification (D–G) after exposure to Ni for 24 h. Each bar represents the mean ± SD of three replicates. * p < 0.05, **** p < 0.0001.

Figure 2

The fluorescence intensity (A) detected by flow cytometry via fluorescent probe DCFH-DA of human corneal epithelial cells and its histogram expressed as % of control (B) after 24 h exposure. Each bar represents the mean ± SD of three replicates. ** p < 0.01, **** p < 0.0001.

Figure 3

The apoptosis rate of human corneal epithelial cells after exposure to Ni for 24 h measured by flow cytometry (A–D). And its statistical graph calculated by Q2 + Q3 quadrant (E). Each bar represents the mean ± SD of three replicates. *** p < 0.01, **** p < 0.0001.

Figure 4

Expression of apoptotic genes Caspase-8 (A), Caspase-9 (B) and Caspase-3 (C), and inflammatory genes NF-κB (D) and IL-1β (E) in human corneal epithelial cells after exposure to Ni for 24 h. Each bar represents the mean ± SD of three replicates. * p < 0.05, *** p < 0.001, **** p < 0.0001.