Combined effects of chronic high glucose and fluoride exposure on kidney cells: exploratory in vitro and in vivo study

Journal of Molecular Histology (2026) 57:42 https://doi.org/10.1007/s10735-025-10670-6 ORIGINAL PAPER Combined effects of chronic high glucose and fluoride exposure on kidney cells: exploratory in vitro and in vivo study Laura Ribeiro1,2 · Heloísa Aparecida Barbosa da Silva Pereira1 · Juliana Sanches Trevizol1 Aislan Quintiliano Delgado3 · Tânia Mary Cestari1 · Marília Afonso Rabelo Buzalaf1 · Rodrigo Cardoso de Oliveira1 · Claudia Cristina Biguetti2 · Received: 23 September 2025 / Accepted: 12 December 2025 / Published online: 12 January 2026 © The Author(s) 2026 Abstract Fluoride is commonly used in dentistry to prevent dental caries, however, excessive exposure may pose risks to soft tissues, particularly the kidneys, which are responsible for approximately 60% of fluoride excretion. Diabetic nephropathy, a major complication of diabetes mellitus, may share pathogenic pathways with fluoride-induced renal toxicity. However, the combined effects of chronic hyperglycemia and fluoride exposure on kidney cells remain poorly understood. This study investigated the effects of fluoride on murine renal tubular epithelial cells (M-1) and on murine kidneys under normal and hyperglycemic conditions. M-1 cells were cultured under high-glucose conditions (22.5 mM) and/or treated with fluoride (1 µM or 5 µM) for 24 to 72 h. Diabetic C57BL/6J mice received drinking water containing fluoride (10 mgF/L or 50 mgF/L) for 21 days. Evaluations included cell viability and morphology in vitro, collagen deposition in renal tissue by birefringence analysis, and expression of the kidney injury marker KIM-1 by immunofluorescence in both models. M-1 cells exposed to fluoride alone showed increased viability at 72 h, while KIM-1 expression was elevated in high-glucose and high-glucose + 1 µM fluoride conditions, suggesting a stress or adaptive response. In diabetic mice, glomerular collagen accumulation, indicative of early fibrosis, was observed but attenuated by fluoride treatment. However, high KIM-1 levels in fluoride-treated diabetic mice, particularly at the higher dose, indicated potential kidney injury. These results highlight a complex, dual role of F in the diabetic kidney, potentially protecting against glomerular fibrosis while exacerbating tubular injury at high doses. Careful monitoring of F exposure is needed, especially in areas with endemic fluorosis and high chronic kidney disease risk. Keywords Fluoride · Diabetes · Kidney · Hyperglycemia Introduction This publication is dedicated to Tânia Mary Cestari. Tânia Mary Cestari: Deceased. Rodrigo Cardoso de Oliveira 1 Department of Biological Sciences, University of São Paulo – Bauru School of Dentistry, Alameda Octávio Pinheiro Brisolla, 9-75, Bauru, São Paulo 1712-901, Brazil 2 School of Podiatric Medicine, The University of Texas Rio Grande Valley, Harlingen, TX, USA 3 Institute of Biosciences, São Paulo State University, Botucatu, São Paulo, Brazil Fluoride (F) is a naturally occurring element and is commonly found in a variety of sources, including dental products, beverages, foods, and most notably through fluoridated water. Indeed, water fluoridation, administered by public water systems, stands as one of the most significant public health achievements of the twentieth century, and is widely recognized for its effectiveness in preventing dental caries (Dionizio et al. 2020). The World Health Organization (WHO) recommends a guideline of 1.5 mg/L for F in drinking water, while the American Dental Association (ADA) and the Department of Health and Human services suggest a lower guideline level of 0.7 mg/L (Arakawa et al. 2009; Meenakshi et al. 2024), maintaining optimal fluoride 13 42 Page 2 of 14 concentrations is critical to maximizing dental benefits while minimizing potential adverse effects. While lower chronic levels of F can be beneficial in preventing caries, exposure exceeding the recommended WHO limits raises concerns about its potential adverse effects on calcified and soft tissues (Barbier et al., 2010; Wei et al. 2014). Once absorbed in the gastrointestinal tract, F is distributed throughout the body, reaching all tissues (Lupo et al. 2011). Due to its strong affinity for calcified tissues, F predominantly accumulates in bone, dentin, and enamel, with any excess being excreted primarily through urine (Saad et al. 2022). The effects of chronic exposure of F on hard tissues are well-documented, with evidence showing that acute or chronic consumption of high concentrations can cause dental and skeletal fluorosis, characterized by hypomineralization of dental structures and brittle bones, respectively (Arakawa et al. 2009; Meenakshi et al. 2024). However, a small portion of F is absorbed by soft tissues, whereas a high F exposure can lead to non-skeletal fluorosis. Indeed, it has been shown that F can affect a wide range of soft tissues such as the kidneys, but the influence of F on these organ cells metabolism remains incompletely understood (Meenakshi et al. 2024). Previous studies have shown that non-skeletal fluorosis may disrupt crucial metabolic pathways and inhibit various enzymes, including those involved in the glycolytic pathway (Barbier et al., 2010). For example, F can affect glucose metabolism, with prolonged exposure potentially leading to insulin resistance and impaired glucose tolerance, it can interfere with the function of pancreatic beta cells and may also influence the insulin signaling pathway, contributing to insulin resistance (Meenakshi et al. 2024). In addition, some studies performed in murine models have found that F intake has been shown to increase insulin sensitivity in tissues such as the kidneys, liver, lung and spleen (Lupo et al. 2011; Trivedi et al. 1993). Other studies also performed in murine models have demonstrated that both chronic and acute ingestion of F alters protein expression in the liver, muscle, kidney and intestines (Dionizio et al. 2019; Kobayashi et al. 2009; Lima Leite et al. 2014; Lobo et al. 2015; Pereira et al. 2018). Amongst these organs, kidneys are of particular importance as they are responsible for 60% of the total daily F absorbed elimination, making them approximately 4–5 times more susceptible to F toxicity than other organs (Kobayashi et al. 2009). These findings highlight the clinical significance of excessive fluoride intake, particularly for population with metabolic disorders such as Diabetes Mellitus (DM), specifically type 2 diabetes mellitus (T2DM). DM is a persistent metabolic disorder marked by chronic hyperglycemia, resulting either from insufficient insulin production (type 1) or the unresponsiveness of insulin receptors (type 2) 13 Journal of Molecular Histology (2026) 57:42 (Hossain et al. 2024; Meenakshi et al. 2024). Type 1 diabetes mellitus (T1DM) primarily arises from autoimmune destruction of the pancreatic beta cells, while T2DM is significantly influenced by genetic factors, diet, obesity, and sedentary lifestyles ( (...truncated)