Our findings suggested that the concentrations of curcumin less than 50 μM does not significantly induce cytotoxicity in primary TM cells, which is consistent with previous reports.
15 Sameermahmood et al.
16 demonstrated that human retinal endothelial cell (HREC) treated with 30 μM of curcumin inhibited SDF-1α-induced cell migration through blockage of Ca
2+ influx and reduction of PI3K/AKT signaling. Premanand et al.
17 also indicated 10 μM of curcumin induced the HRECs apoptosis partly by the control of iROS production. Bian et al.
18 reported that 20 μM of curcumin effectively reduced glycated human serum albumin-induced IL-8 and MCP-1 secretion in human RPE cells. In addition, several other publications further suggested curcumin doses between 10 and 20 μM are effective in RPE cells.
19–21
Oxidative stress has been known to contribute to the TM failure in glaucoma. H
2O
2 exposure of cultured TM cells caused excessive iROS release through mitochondria. The production of iROS in turn results in sustained stress response, which contributes to the malfunction of TM in glaucoma.
7 Meanwhile, iROS also significantly promotes NF-κB activity, which has been reported to activate the expressions of inflammatory markers in glaucoma.
22 The inflammatory markers upregulated by NF-κB activation including IL-6, ELAM-1, IL-1α, and IL-8
7 were analyzed in this study. In addition, the accumulation of senescent cells and reduction of the cell numbers are well-known outcomes of oxidative stress in glaucoma.
23 Activity of SA-β-gal was applied as a senescence marker in our study, while iROS-promoted protein carbonylation, a type of protein oxidation, was monitored as well. Our results showed that curcumin treatment in H
2O
2-exposed TM cells was able to effectively downregulate the aforementioned markers induced by oxidative stress.
Curcumin is an antioxidative substance that possesses the ability to reduce inflammation. It has been reported recently that curcumin-treated L02 cells exhibited less cytotoxicity and genotoxicity induced by quinocetone, confirming the ability of curcumin to scavenge excessive iROS and resist oxidative stress.
24 Trabecular meshwork cells under chronic oxidative stress produce sustained amount of iROS through the mitochondria. Jat et al.
25 found that curcumin could elevate endogenous glutathione levels, thus protecting the mitochondrial integrity and enhancing the mitochondrial defense system to attenuate oxidative stress. Our results also confirmed that the levels of iROS were significantly lowered in the curcumin-treated group in comparison with the H
2O
2-treated group. Therefore, the effects of curcumin on the reduction of iROS may be considered as the major contribution to the relief of oxidative stress. The activation of iROS induced by chronic oxidative stress in TM cells frequently upregulates inflammatory markers, including IL-6, ELAM-1, IL-1α, and IL-8. Long et al.
26 revealed that curcumin ameliorated inflammation by targeting NF-κB pathway, leading to the downregulation of IL-6. Kumar et al.
27 reported that pretreatment of human umbilical vein endothelial cells with curcumin for 1 hour completely blocked the expression of ELAM-1. Das and Vinayak
28 demonstrated that curcumin was able to inhibit carcinogenesis by suppressing proinflammatory cytokine IL-1α through AP-1 in mice. McFarlin et al.
29 found that oral supplementation with curcumin effectively reduced inflammatory biomarkers, such as IL-8. Consistent with these reports, our data indicated curcumin was a potent anti-inflammatory substance, with the capabity to inhibit oxidative stress induced inflammatory markers in TM cells, including IL-6, ELAM-1, IL-1α, and IL-8.
In addition to the protective roles of curcumin against oxidative stress-induced oxidation and inflammation, curcumin also demonstrated a potent antiapoptotic effect after oxidative insults. Our results showed that curcumin treatment inhibited SA-β-gal activity, and decreased the apoptotic cell numbers, indicating curcumin protected TM from oxidative stress by preserving its celluar intergrity and viability. Lin et al.
30 found that curcumin exerted its antiapoptotic and anti-inflammatory functions through maintaining CISD2 levels in neural cells, indicating a possible molecular mechanism.