Curcumin ameliorates streptozotocin-induced liver damage through modulation of endoplasmic reticulum stress-mediated apoptosis in diabetic rats

Abstract

We investigated the effect of curcumin on liver injury in diabetic rats induced by streptozotocin (STZ) through modulation of endoplasmic reticulum stress (ERS) and unfolded protein response (UPR). Experimental diabetes was induced by a single intraperitoneal injection of STZ (55 mg/kg), and curcumin was given at 100 mg/kg by gavage for 56 days. We observed that curcumin improved the morphological and histopathological changes, significantly decreased hepatic ERS marker protein: glucose-regulated protein 78, and improved liver function in diabetic rats. Moreover, treatment with curcumin markedly decreased the sub-arm of the UPR signaling protein such as phospho-double-stranded RNA-dependent protein kinase-like ER kinase, CCAAT/enhancer-binding protein homologous protein, tumor necrosis factor receptor-associated factor 2, and inositol-requiring enzyme1α; and inhibited tumor necrosis factor α, interleukin 1β, phospho-p38 mitogen-activated protein kinase, and apoptosis signal-regulating kinase 1 in liver tissues of diabetic rats. Apoptotic and anti-apoptotic signaling proteins, such as cleaved caspase-3 and B-cell lymphoma 2, were significantly increased and decreased, respectively in diabetic rats; curcumin treatment prevented all of these alterations. In summary, our results indicate that curcumin has the potential to protect the diabetic liver by modulating hepatic ERS-mediated apoptosis, and provides a novel therapeutic strategy for the diabetic liver damage.

Keywords: apoptosis; curcumin; diabetes mellitus; endoplasmic reticulum stress; liver.

Figure 1.

Time-course changes in blood glucose. Blood glucose increased progressively in the untreated diabetic rats following induction of diabetes. Curcumin treatment significantly reduced blood glucose in the beginning of treatment and these were maintained throughout the study period until sacrifice. Values are means ± SEM. ∗∗p < 0.01, ∗∗∗p < 0.001 versus Normal, ^###p < 0.001 versus Control.

Figure 2.

Expression of hepatic protein involved in ERS. Representative Western blots (lower panel) show specific bands for GRP78 (A) and Caspase-12 (B); representative histograms (upper panel) show the band densities with relative β-tubulin. The blots are representatives of five independent experiments. Each bar represents mean ± SE. Normal, age-matched normal rats; Control, untreated diabetic rats; Curcumin, diabetic rats treated with curcumin 100 mg/kg/day. ∗∗p < 0.01 versus Normal, ^#p < 0.05 versus Control based on one-way ANOVA followed by Tukey's test.

Figure 3.

Expression of hepatic proteins involved in UPR signaling pathway. Representative Western blots (lower panel) show specific bands for p-PERK (A), p-IRE1α (B), and ATF6α (C); representative histograms (upper panel) show the band densities with relative β-tubulin, PERK (for p-PERK), and IRE1α (p-IRE1a). The blots are representatives of five independent experiments. Each bar represents mean ± SE. Normal, age-matched normal rats; Control, untreated diabetic rats; Curcumin, diabetic rats treated with curcumin 100 mg/kg/day. ∗p < 0.05, ∗∗∗p < 0.001 versus Normal, ^#p < 0.05 versus Control based on one-way ANOVA followed by Tukey's test.

Figure 4.

Hepatic expressions of CHOP, TRAF2, ASK1, and p-p38MAPK. Representative Western blots (lower panel) show specific bands for CHOP/GADD153 (A), TRAF2 (B), ASK1 (C), and p-p38MAPK (D); representative histograms (upper panel) show the band densities with relative β-tubulin. The blots are representatives of five independent experiments. Each bar represents mean ± SE. Normal, age-matched normal rats; Control, untreated diabetic rats; Curcumin, diabetic rats treated with curcumin 100 mg/kg/day. ∗∗p < 0.01, ∗∗∗p < 0.001 versus Normal, ^##p < 0.01, ^###p < 0.001 versus Control.

Figure 5.

Hepatic expressions of cleaved caspae-3 and bcl2. Representative Western blots (lower panel) show specific bands for cleaved caspase-3 (A) and bcl2 (B); representative histograms (upper panel) show the band densities with relative β-tubulin. The blots are representatives of five independent experiments. Each bar represents mean ± SE. Normal, age-matched normal rats; Control, untreated diabetic rats; Curcumin, diabetic rats treated with curcumin 100 mg/kg/day. ∗p < 0.05, ∗∗p < 0.01 versus Normal, ^#p < 0.05, ^##p < 0.01 versus Control.

Figure 6.

Hepatic expressions of TNFα, IL-1β, and TLR4. Representative Western blots (lower panel) show specific bands for TNFα (A), IL-1β (B), and TLR4 (C); representative histograms (upper panel) show the band densities with relative β-tubulin. The blots are representatives of five independent experiments. Each bar represents mean ± SE. Normal, age-matched normal rats; Control, untreated diabetic rats; Curcumin, diabetic rats treated with curcumin 100 mg/kg/day. ∗p < 0.05, ∗∗∗p < 0.001 versus Normal, ^#p < 0.05, ^##p < 0.01 and ^###p < 0.001 versus Control.

Figure 7.

Effect of curcumin on histopathological changes. Histological staining with PAS in liver (A) shows that glycogen contents of rat liver decreased in diabetic animals when compared with those of normal control animals, but these levels increased to near normal after treatment with curcumin. In H&E, (B) light microscopic photographs of livers of experimental animal showed the liver of normal control group, lipid accumulation indicated by the unstained area in liver tissues, microvascular fattening and focal necrosis, and portal inflammation in the untreated diabetic group; in curcumin-treated diabetic group, the severity of these changes was less than those in the untreated diabetic group. (C and C1) Immunohistochemical staining for macrophage (ED1-positive cells) and its quantification graph in each group. (D) Immunohistochemical staining for fibronectin in liver section. Each bar represents mean ± SE. Normal, age-matched normal rats; Control, untreated diabetic rats; Curcumin, diabetic rats treated with curcumin 100 mg/kg/day. ∗∗p < 0.01 versus Normal, ^##p < 0.01 versus Control.