Background
It has been found that fluoride may cause cell damage by inducing intracellular calcium overload. Store-operated calcium entry (SOCE) plays an important role in maintaining intracellular calcium homeostasis, but the effect of fluoride on renal SOCE is unknown.
Objective
To explore the renal toxicity and the expression levels of the key proteins of SOCE, stromal interaction molecule 1 (STIM1) and calcium release-activated calcium modulator 1 (ORAI1) in the kidney tissues of mice exposed to fluoride subchronically.
Methods
Twenty male ICR mice were randomly divided into four groups with five mice in each group, including 0 (control group), 0.3, 3, and 30 mg·L−1 fluoride groups. The mice were given drinking water containing designed fluoride for 12 weeks. Body weight and liver and kidney organ coefficients of the mice were measured after the exposure; histopathological changes of the mouse kidney were observed; 24 h urine was collected at the end of 12 weeks of exposure to determine the levels of urine creatinine (UCr), urine calcium (UCa), albumin (ALB), and β2-microglobulin (β2-MG); the protein expression levels of STIM1 and ORAI1 in the kidney were detected by Western blotting; the fluorescence co-localization of STIM1 and ORAI1 was used to further verify the expression levels of STIM1 and ORAI1.
Results
After the exposure, there were no differences in body weight as well as liver and kidney organ coefficients among the groups (P > 0.05). Under optical microscope, the renal tubular cell showed degeneration, apical protrusion, shedding, and dilation in the 3 and 30 mg·L −1 fluoride groups. There was no statistical difference in UCr among the mice in each group (P > 0.05). Compared with the control group, the levels of UCa adjusted by UCr in the 3 and 30 mg·L −1 fluoride groups were (0.075±0.014) and (0.081±0.012) mol·mol−1 (represent by UCr per mol), which had a rising trend but showed no statistical difference. No difference was identified in the level of ALB among the groups (P > 0.05). The levels of β2-MG showed difference in different exposure groups, and the level of urine β2-MG in the 30 mg·L −1 fluoride group was (0.077±0.014) g·mol−1, higher than that in the control group (P<0.05). Based on the results of Western blotting, the protein expression levels of STIM1 and ORAI1 showed significant differences among the groups (F=18.411, 6.853; P=0.001, 0.013); compared with the control group, the expression levels of STIM1 protein increased in the 3 and 30 mg·L−1 fluoride groups (P < 0.05), and the protein expression level of ORAI1 in the 30 mg·L −1 fluoride group was increased (P < 0.05). The fluorescence co-localization results of STIM1 and ORAI1 showed that the expressions of STIM1 and ORAI1 were up-regulated in the 3 and 30 mg·L −1 fluoride groups.
Conclusion
Subchronic exposure to fluoride through drinking water can up-regulate the expression levels of STIM1 and ORAI1 in renal tissues and induce renal injury.
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