Research Studies
Study Tracker
Excessive fluoride induces renal cell apoptosis and subsequent renal dysfunction.
| Toxicology Letters | Zhang Y, Xu Z, Xie P, Xiao Y, Dong S, Chen C, Chen Q. | 418:111862.
mitochondrial dysfunction Apoptosis Kidney Nephrotoxicity Animal Study

Abstract

Original abstract online at
https://www.sciencedirect.com/science/article/abs/pii/S0378427426000457

Highlights

  • Fluoride elevates urinary F– and serum creatinine in rat dose- and time-dependently.
  • Excess fluoride causes renal tubule damage; dysfunction precedes morphology changes.
  • Such high F–-induced kidney damage is driven by intrinsic (mitochondrial) apoptosis.
  • A pro-apoptotic Bax/Bcl-2 shift leads to executioner Caspase-3 activation.
  • Cellular dysregulation underpins F-induced renal dysfunction & new therapy targets.

This study investigated the mechanisms of renal injury and dysfunction induced by chronic excessive fluoride exposure. Sixty female Sprague-Dawley rats were administered sodium fluoride (NaF) at 0, 50, 100, or 150 mg/L in drinking water for 2, 4, or 6 months. NaF exposure was confirmed by elevated urinary fluoride levels. Renal function was assessed by the increase in serum creatinine, and structural damage was evaluated by histopathology. The expression of key apoptosis-related markers (Bax, Bcl-2, Caspase-3) was analyzed using qRT-PCR, immunohistochemistry, and Western blotting. Fluoride exposure caused a dose- and time-dependent increase in serum creatinine, indicating impaired glomerular filtration. Histopathological analysis confirmed significant tubular degeneration. Notably, functional impairment occurred despite no pathological change in kidney yet, suggesting that functional deficits preceded overt morphological alterations. Mechanistically, fluoride exposure induced a clear pro-apoptotic molecular shift with increased Bax and decreased Bcl-2 expression, culminating in the activation of Caspase-3. The histopathological findings were consistent with a mixed pattern of cell death. However, the integrated molecular evidence demonstrated that the activation of the intrinsic (mitochondrial) apoptotic pathway was a key mechanism contributing to the excessive fluoride-induced nephrotoxicity. In summary, a central molecular pathway involved in fluoride-induced renal injury was demonstrated, providing a foundation for assessing excessive fluoride toxicological impact.

Introduction

Fluoride, a relatively abundant element in the global crust, serves as a prophylactic agent against dental caries through appropriately controlled exposure (Iheozor-Ejiofor et al., 2024). Nevertheless, its narrow therapeutic window renders excessive intake a systemic toxicant, inducing multiorgan damage via diverse pathological cascades (Fusi et al., 2024). Among the adverse effects of fluoride, skeletal manifestations are ranging from dental fluorosis (characterized by mottled enamel discoloration) to debilitating skeletal fluorosis (marked by osteosclerosis and articular rigidity) as the most common clinical observations (Gonzalez-Gonzalez et al., 2023). Emerging evidence further implicates fluoride in mediating extraosseous pathologies across neurological, cardiovascular, renal, and reproductive systems, with distinct mechanistic underpinnings (Chen et al., 2025, Ommati et al., 2025, Talebi et al., 2025, Zhang et al., 2025).

As the principal organ governing fluoride homeostasis, the kidneys execute > 50% of systemic fluoride elimination via urinary excretion (Validandi et al., 2025). Chronic fluoride overload, however, overwhelms renal excretory capacity, culminating in both structural and functional damages of renal parenchyma (Validandi et al., 2025).

Epidemiological investigations substantiated this nephrotoxic potential. A case-control study of 8–15-year-old children demonstrated that prolonged exposure to elevated fluoride levels in drinking water correlated with the biomarkers of impaired renal function (the increase in serum creatinine, and the decrease in glomerular filtration rate [GFR]), and histopathological evidence of tubular injury (Khandare et al., 2017).

Male Wistar rats exposed to low (15 ppm) and high (50 ppm) fluoride concentrations in drinking water for 40 days precented a significant dose-dependent increase in urinary beta-2-microglobulin and cystatin-C excretion rates. Histopathological analysis through light microscopy also revealed a dose-dependent trend of tubular damage severity by fluoride on renal tubular structures (Cardenas-Gonzalez et al., 2013).

In our early study, the pregnancy rate of female rats treated by different concentrations of sodium fluoride (NaF) was significantly decreased, while the pathophysiological mechanism underlying such decrease was still unknown (Dong et al., 2023).

Emerging evidence indicates that fluoride-induced nephrotoxicity is closely associated with the activation of renal cell apoptosis (Li et al., 2024). Fluorosis triggers apoptosis through multiple pathogenic pathways, including endoplasmic reticulum stress (Wang et al., 2024a), oxidative stress (Ni et al., 2020) and mitochondrial dysfunction (Wang et al., 2024b). These interconnected apoptotic mechanisms collectively may cause programmed cell death or apoptosis, leading to structural degeneration and functional impairment in renal tissues.

To address this knowledge gap between high fluorosis and kidney apoptosis and dysfunction, a female Sprague-Dawley rat model with controlled fluoride exposure parameters (varying concentrations and durations) was used to systematically investigate fluoride-induced renal apoptosis. This experimental design enabled comprehensive analysis of dose- and time-response relationships and temporal progression of apoptotic mechanisms in renal cells, aiming to clarify the molecular pathogenesis of fluoride nephrotoxicity.

Section snippets

ARRIVE statement

Animal experiments were carried out in compliance of ARRIVE Guidelines (Animal Research: Reporting of In Vivo Experiments).

Chemicals and reagents

Sodium fluoride (NaF) (Catalog No. 20180108, analytically pure) and glass slide were purchased from Tianjin Tianli Chemical Reagent Co. LTD (Tianjin, China). were acquired from Jiangsu Meimian Industry Co., LTD (Nanjing, Jiangsu, China). Pentobarbital sodium was acquired from Sigma-Aldrich (St. Louis, Missouri, USA). All other chemical reagents used were of analytical grade.

Urinary fluoride content measurement

The urinary fluoride levels in rats exposed to fluoride for 2 months, 4 months, and 6 months were shown in Fig. 1. For 2 months of NaF exposure, urinary fluoride levels in the 50 mg/L, 100 mg/L, and 150 mg/L NaF groups were increased, but only in the 100 mg/L and 150 mg/L groups reaching statistically significant difference (P<0.05 and P<0.01, respectively). For 4 months of exposure, significant difference was observed too in the 100 mg/L and 150 mg/L NaF groups (P<0.01). For 6 months of

Discussion

An intriguing observation from our experiments was the significant impact of fluoride on renal function in female rats (Mohamed, 2016). Given the role of the kidney in fluoride homeostasis, it is plausible that fluoride-induced nephrotoxicity may contribute to the overall systemic toxicity, including reproductive dysfunction, by altering fluoride pharmacokinetics. Animal model studies on fluoride toxicity have demonstrated that fluoride exposure causes structural damage to kidney tissue,

Conclusion

In conclusion, chronic fluoride exposure induces renal damage and functional impairment. Mechanistically, the study provides direct molecular evidence for the dose-dependent activation of the intrinsic apoptotic pathway in renal tissue, characterized by a pro-apoptotic shift marked by upregulated Bax and downregulated Bcl-2 expression, and subsequent Caspase-3 activation. This pathway represents a key mechanism contributing to the observed toxicity. The histopathological findings indicate a

CRediT authorship contribution statement

Yi Zhang: Validation. Zhao Xu: Validation. Pengpeng Xie: Investigation. Yating Xiao: Investigation. Siyuan Dong: Investigation. Chen Chen: Writing – review & editing. chen qun: Methodology, Conceptualization.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Financial support for this work was received from National Natural Science Foundation of China (grant number 81673115 and 82073496) and Key Laboratory of Se-enriched Products Development and Quality Control, Ministry of Agriculture and Rural Affairs/National-Local Joint Engineering Laboratory of Se-enriched Food Development (Se-2023C02). CC was supported by Australian NHMRC and The University of Queensland.

References (33)