Zerumbone, a Phytochemical of Subtropical Ginger, Protects against Hyperglycemia-Induced Retinal Damage in Experimental Diabetic Rats

Abstract

Diabetic retinopathy (DR), the most ordinary and specific microvascular complication of diabetes, is a disease of the retina. Zerumbone (ZER) is a monocyclic sesquiterpene compound, and based on reports, it is the predominant bioactive compound from the rhizomes of Zingiber zerumbet. The aim of the current study is to evaluate the protective effect of zerumbone against DR in streptozotocin (STZ)-induced diabetic rats. STZ-diabetic rats were treated with ZER (40 mg/kg) once a day orally for 8 weeks. ZER administration significantly (p < 0.05) lowered the levels of plasma glucose (32.5% ± 5.7% lower) and glycosylated hemoglobin (29.2% ± 3.4% lower) in STZ-diabetic rats. Retinal histopathological observations indicated that disarrangement and reduction in thickness of retinal layers were reversed in ZER-treated diabetic rats. ZER downregulated both the elevated levels of advanced glycosylated end products (AGEs) and the higher levels of the receptors for AGEs (RAGE) in retinas of diabetic rats. What's more, ZER significantly (p < 0.05) ameliorated diabetes-induced upregulation of tumor necrosis factor-α, interleukin (IL)-1 and IL-6. ZER also attenuated overexpression of vascular endothelial growth factor and intercellular adhesion molecule-1, and suppressed activation of nuclear factor (NF)-κB and apoptosis in the retinas of STZ-diabetic rats. Our results suggest ZER possesses retinal protective effects, which might be associated with the blockade of the AGEs/RAGE/NF-κB pathway and its anti-inflammatory activity.

Keywords: AGEs; NF-κB; diabetic retinopathy; zerumbone.

Figure 1

Presentation of (A) retinal morphology and (B) thickness in rats. STZ-diabetic rats were administered 40 mg/kg/day ZER (STZ + ZER), or 50 mg/kg AG (STZ + AG) by oral gavage once daily for three months. Another group of STZ-diabetic rats (STZ + vehicle) and normal rats (normal + vehicle) were administered the same volume of vehicle used to prepare the test medication solutions. All the representative images of hematoxylin and eosin (H & E) stained retinas were taken at ×200 magnification. GCL; ganglion cell layer; INL, inner nuclear layer; IPL, inner plexiform layer; NFL, nerve fiber layer; OPL, outer plexiform layer; ONL, outer nuclear layer; PRL, photoreceptor layer. Values (mean ± SEM) were obtained from eight animals in each group. ^a p < 0.05 and ^b p < 0.01 compared to vehicle-treated normal rats, respectively; ^c p < 0.05 and ^d p < 0.01 compared to the values of vehicle-treated STZ-diabetic rats, respectively.

Figure 2

Effects of the treatments on the (A) protein and (B) mRNA levels of receptors for advanced glycosylated end products (RAGE) in retina of rats. STZ-diabetic rats were administered 40 mg/kg/day ZER, or 50 mg/kg AG by oral gavage once daily for three months. Another group of STZ-diabetic rats and normal rats were administered the same volume of vehicle used to prepare the test medication solutions. Data are mean ± SEM from eight rats per group, and the experiments were repeated independently at least three times with similar results. ^a p < 0.05 and ^b p < 0.01 compared to vehicle-treated normal rats, respectively; ^c p < 0.05 and ^d p < 0.01 compared to the values of vehicle-treated STZ-diabetic rats, respectively.

Figure 3

Effects of the treatments on the (A) protein and (B) mRNA levels of inflammatory cytokines and chemokines in retina of rats. STZ-diabetic rats were administered 40 mg/kg/day ZER, or 50 mg/kg AG by oral gavage once daily for three months. Another group of STZ-diabetic rats and normal rats were administered the same volume of vehicle used to prepare the test medication solutions. Data are mean ± SEM from eight rats per group, and the experiments were repeated independently at least three times with similar results. ^a p < 0.05 and ^b p < 0.01 compared to vehicle-treated normal rats, respectively; ^c p < 0.05 and ^d p < 0.01 compared to the values of vehicle-treated STZ-diabetic rats, respectively.

Figure 3

Effects of the treatments on the (A) protein and (B) mRNA levels of inflammatory cytokines and chemokines in retina of rats. STZ-diabetic rats were administered 40 mg/kg/day ZER, or 50 mg/kg AG by oral gavage once daily for three months. Another group of STZ-diabetic rats and normal rats were administered the same volume of vehicle used to prepare the test medication solutions. Data are mean ± SEM from eight rats per group, and the experiments were repeated independently at least three times with similar results. ^a p < 0.05 and ^b p < 0.01 compared to vehicle-treated normal rats, respectively; ^c p < 0.05 and ^d p < 0.01 compared to the values of vehicle-treated STZ-diabetic rats, respectively.

Figure 4

Effects of the treatments on the retinal NF-κB activity (A) and apoptosis rate (B) in rats. STZ-diabetic rats were administered 40 mg/kg/day ZER, or 50 mg/kg AG by oral gavage once daily for three months. Another group of STZ-diabetic rats and normal rats were administered the same volume of vehicle used to prepare the test medication solutions. Data are mean ± SEM from eight rats per group, and the experiments were repeated independently at least three times with similar results. ^a p < 0.05 and ^b p < 0.01 compared to vehicle-treated normal rats, respectively; ^c p < 0.05 and ^d p < 0.01 compared to the values of vehicle-treated STZ-diabetic rats, respectively.

Figure 5

The possible action mechanisms of ZER on the amelioration of streptozotocin-induced retinal damage in rats. ZER showed the retinalprotective effects via reducing hyperglycemia to inhibit AGEs and RAGE axis, and thereby resulting in downregulation the expressions of proinflammatory cytokines, adhesion molecules and angiogenic cytokine, that in turn retards apoptosis, and consequently lessens damage in diabetic retinal tissue.