Abstract
Diabetic retinopathy (DR) is one of the microvascular diabetic complications that results in severe damage to the retina characterized by an irreversible deterioration process of the tiny blood vessels within the retina which subsequently leads to vision loss. The retina has a high metabolic activity and oxygen consumption. Chronic hyperglycemia, insulin deficiency and peripheral insulin resistance can cause pathological alteration in glucose metabolism such as activation of polylol, hexosamine, PKC, poly (ADP-riobose) polymerase pathways and nonenzymatic glycosylation which collectively increases the production of mitochondrial reactive oxygen species (ROS). Such increased ROS production leads to mitochondrial DNA damage and promotes oxidative stress, inflammation and cell death. However, when oxidative stress rate exceeds the capacity of the cellular anti-oxidative stress defense system, this leads to downregulation of the anti-oxidant enzymes like catalase, superoxide dismutases and glutathione peroxidases. The progression of DR is recognized as an inflammatory process mediated by an increase in the expression of many pro-inflammatory proteins such as IL-lp, TNF-a, IL-8 and IL-18. This increase in pro-inflammatory cytokines induces the expression of the angiogenic factor VEGFA. The increased VEGFA expression in advanced stages of DR leads to the formation of a new fragile and inefficient blood vessel network, which in turn results in retinal permeability and vascular leakage. Such events in the retina lead to breakdown of the blood retinal barrier (RBR) and retinal detachment which consequently result in vision loss.As the progression of DR is recognized as an irreversible inflammatory process, targeting inflammation in DR has been proposed to be an efficient strategy to protect retinal cells from further severe damage. Many drugs and natural products such as vitamins have been used to treat inflammation in diabetes. Among these products is vitamin D (VITD) which is found in many types of food. It is also endogenously produced in humans through regulated pathway. VITD is found in two forms in the biological system: the inactive form 25(OH)2D3 and the active form l,25(OH)2D3. The active form of VITD is the principal regulator of calcium and phosphate ions; however, it is also involved in regulating gene expression in targeted cells and organs. Accumulated evidence has shown that the l,25(OH)2D3 is a potent inhibitor of many inflammatory mediators as well as a potential mediator of the anti-oxidant system function in eye diseases such as age-related macular degeneration (AMD) and DR.
To study the potential protective function of l,25(OH)2D3 in DR, the H20 2 or high glucose-treated ARPE-19 cells and STZ-induced diabetic C57BL/6 mice were used as in vitro and in vivo models. In some experiments, the ARPE-19 cells were treated with H20 2, H20 2 + VITD, high glucose, high glucose + VITD or normal glucose for 6 and 24 hours. The cell survival rate, ROS level, caspase-3/7 activity, relative gene expression of pro-inflammatory cytokines and anti-oxidant enzymes in treated cells were measured and compared to control cells. In the in vivo model, the diabetic group was rendered diabetic for 8 months and divided into two groups. The first group received 12 injections of l,25(OH)2D3 at a dose of 2.5 pg/kg for 8 weeks, while the second group received 12 injections of equivalent volume of carrier buffer. The protective effect of VITD was assessed based on the changes of relative gene expression of pro-inflammatory cytokines and anti-oxidant enzymes in the retina and retinal pigment epithelium (RPE) of diabetic and diabetic + VITD treated groups compared to the control group.
Present data have demonstrated an upregulation of pro-inflammatory cytokines and a decrease in anti-oxidant enzyme expression in both in vitro and in vivo models. Also, current data have shown that VITD treatment increased cell viability, reduced reactive oxygen species (ROS) production and caspase-3/7. The l,25(OH)2D3 treatment has demonstrated a regulatory effect on pro-inflammatory cytokines and VEGFA by suppressing their expression. It also upregulated the expression of antioxidant enzymes in the retina and RPE of diabetic mouse and therefore improved retinal and RPE cellular defense systems against inflammation and apoptosis.
Current findings have shown that the l,25(OFI)2D3 can protect from oxidative stress, inflammation and apoptosis in DR via regulating the expression of many pro-inflammatory cytokines and antioxidant enzymes. Thus, the outcome of this study has shed light on the pathogenesis of DR and suggested the potential therapeutic effect of l,25(OH)2D3 of DR.
| Date of Award | 2015 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Xinhua Shu (Supervisor) & John Craft (Supervisor) |