Curcumin protects against hyperosmoticity-induced IL-1β elevation in human corneal epithelial cell via MAPK pathways

Increased tear osmolarity is an essential feature of dry eye disease. Curcumin, a natural polyphenol extracted from herb turmeric, has recently been reported to have anti-inflammatory effects. However, its anti-inflammatory effects have not been investigated in dry eye disease. It has been reported that elevated osmolarity achieved by adding sodium chloride to the culture medium of corneal epithelial cells increased the production of IL-1β, a proinflammation cytokine. This in vitro dry eye model was used to test the anti-inflammatory effects of curcumin. In the present study, a 450 mOsM hyperosmotic medium was produced by adding sodium chloride to the culture medium to reach a final concentration of 90 mM. Human corneal epithelial cells cultured in this hyperosmotic medium for 24 h showed an increase of IL-1β, IL-6 and TNF-α levels in the conditioned medium. IL-1β was also upregulated at mRNA levels. Activation of p38 MAP kinase (p38), JNK MAP kinase (JNK) and NF-κB in cultured corneal epithelial cells were also induced by hyperosmotic conditions. Curcumin at concentrations of 1–30 μM did not affect the cell viability of cultured corneal epithelial cells. Pretreatment of curcumin (5 μM) completely abolished the increased production of IL-1β induced by the hyperosmotic medium. Increased phosphorylation of p38 caused by high osmolarity was also completely abolished by curcumin, whereas the phosphorylation of JNK was only partially inhibited. SB 203580 (p38 inhibitor), but not SP 600125 (JNK inhibitor), completely suppressed hyperosmoticity-induced IL-1β production, indicating that the inhibition of production of IL-1β by curcumin may be achieved through the p38 signal pathway. Curcumin completely abolished a hyperosmoticity-induced increase of NF-κB p65. NF-κB inhibitor suppressed hyperosmoticity-induced IL-1β production. p38 inhibitor suppressed hyperosmoticity-induced NF-κB activation, indicating that NF-κB activation was dependent on p38 activation. The present study suggests that curcumin might have therapeutic potential for treating dry eye disease.