Curcumin attenuates high glucose-induced apoptosis in renal tubular epithelial cells by enhancing the Nrf2/heme oxygenase-1 antioxidant pathway

Figure 1 Effects of curcumin on general evaluations of diabetic injuries in rats. A: Changes in body weight over time. The body weight of the control group (black line) steadily increased throughout the observation period, reflecting normal growth. In contrast, the disease model group (gray line) displayed significantly reduced weight gain compared to the control group, which is indicative of disease-associated weight loss or stunting (P < 0.05). The curcumin-treated group (light gray line) showed improved weight gain relative to the model group (P < 0.05), suggesting that curcumin mitigates weight loss associated with the disease; B: Blood glucose levels over time. The control group maintained stable blood glucose levels, while the model group exhibited significantly elevated glucose levels (P < 0.01), indicating a disease-induced hyperglycemic state. Treatment with curcumin reduced glucose levels significantly compared to the model group (P < 0.05), although they remained higher than those of the control group, suggesting a partial therapeutic effect of curcumin on glucose regulation; C: Kidney injury molecule-1 (KIM-1) concentration (pg/mL). KIM-1, a biomarker of kidney injury, was significantly increased in the model group (^aP < 0.05) compared to the control group, indicating renal damage. Curcumin treatment significantly reduced KIM-1 levels compared to the model group (^aP < 0.05), implying a protective effect on kidney tissue; D: Neutrophil gelatinase-associated lipocalin (NGAL) concentration (ng/mL). NGAL, another biomarker of kidney injury, was significantly elevated in the model group (^aP < 0.05) relative to the control group. Curcumin administration reduced NGAL levels compared to the model group (^aP < 0.05), suggesting an ameliorative effect on kidney injury; E: Microalbuminuria levels (mg/day). Microalbuminuria, a marker of renal dysfunction, was substantially higher in the model group (^aP < 0.05) than in the control group. Curcumin treatment significantly decreased microalbuminuria levels compared to the model group (^aP < 0.05), indicating a protective or restorative effect on renal function; F: Renal histopathology change. Masson staining revealed tubular dilation and deformation, brush border loss, and interstitial inflammatory infiltration in both the streptozocin (STZ) and curcumin groups. However, compared to the STZ group, curcumin-treated rats showed milder tubular damage and reduced inflammatory infiltration. No significant interstitial fibrosis was observed in any group. KIM-1: Kidney injury molecule-1; NGAL: Neutrophil gelatinase-associated lipocalin; STZ: Streptozocin.

Figure 2 Curcumin reduces renal tubular epithelial cell apoptosis under hyperglycemic conditions. A and B: Representative images and quantitative analysis of TUNEL staining in rat kidney tissue. The apoptosis index was calculated as the ratio of TUNEL-positive cells to total cells in 10 non-overlapping fields per sample (200 × magnification). Compared with the control group, TUNEL-positive cells increased in the streptozocin (STZ) group (^aP < 0.05); curcumin treatment significantly reduced TUNEL-positive cells (^aP < 0.05); C and D: Flow cytometric analysis of apoptosis in renal tubular epithelial cells cultured under high-glucose conditions, showing significant differences between groups; E-G: Immunohistochemical staining of Bcl-2 and Bax in kidney tissues. Ten fields were selected per tissue section at 200 × magnification to determine relative expression levels. The results show increased expression of Bax and decreased Bcl-2 in the STZ group (^aP < 0.05); curcumin further elevated Bcl-2 and reduced Bax expression (^aP < 0.05); H and I: Quantitative polymerase chain reaction analysis of Bcl-2 and Bax mRNA expression in renal tubular epithelial cells. High glucose upregulated Bax and downregulated Bcl-2 mRNA (^aP < 0.05); curcumin treatment further elevated Bcl-2 and suppressed Bax expression (^aP < 0.05). STZ: Streptozocin.

Figure 3 Curcumin attenuates oxidative stress in renal tubular epithelial cells through activation of the Nrf2 pathway. A and B: Flow cytometry analysis of reactive oxygen species (ROS) in renal tubular epithelial cells. Compared with the control group, ROS levels increased in the high-glucose group (^aP < 0.05); curcumin treatment significantly reduced ROS (^aP < 0.05); C-F: Immunohistochemistry of renal tissues showed increased expression of Nrf2 and heme oxygenase-1 (HO-1) and decreased Keap1 in the streptozocin group (^aP < 0.05); curcumin further elevated Nrf2/HO-1 and reduced Keap1 expression (^aP < 0.05); G-I: Quantitative polymerase chain reaction analysis of Nrf2, HO-1, and Keap1 mRNA in renal tubular epithelial cells. High glucose upregulated Nrf2 and HO-1 and downregulated Keap1 mRNA (^aP < 0.05); curcumin treatment further enhanced Nrf2/HO-1 and suppressed Keap1 expression (^aP < 0.05). STZ: Streptozocin.