It is recognized that inflammation plays a prominent role in the development and magnitude of signs and symptoms of dry eye. Tear hyperosmolarity and intracellular signaling pathway activation induced by desiccating stress initiate the production of proinflammatory cytokines, such as IL-1, IL-6, IL-8, and TNF-α, and proteolytic enzymes, such as MMP-9.
18,35,36 Using the same ICES murine dry-eye model, we previously reported dry-eye–associated increased ocular surface inflammation (a hallmark of dry-eye disease) as indicated by increased expression of IL-1β, TNF-α, IL-6, and IL-17.
26 Application of trehalose eye drop restored ocular integrity in this model, reducing expression of IL-17, which in turn reduced the IL-17–mediated expression of IL-1, TNF-α, and IL-6.
26 That TNF-α, IL-17, IL-6, and IL-1β were similarly upregulated in ICES controls in the present study further supports the use of the ICES model to mimic physiological stress experienced in dry eye. Activated MAP kinases can initiate expression of transcription factors, leading to expression of inflammatory cytokines, chemokines, and MMPs.
37 Epithelial and inflammatory cell production of MMP-9, stimulated by IL-1 and TNF-α, is suggested to impede reepithelialization of the cornea after injury.
38,39 Previous studies have shown the osmoprotectants L-carnitine and erythritol, alone or in combination, can reduce the activation/phosphorylation of MAP kinases in cultured human corneal epithelial cells in response to hyperosmolar stress.
23 Reduced expression of MAP kinases would reduce production of MMP-9, suppressing the innate immune responses associated with expression of IL-1, TNF-α, and IL-17. In the present study, continual treatment of mice with betaine, L-carnitine, or erythritol suppressed expression of proinflammatory cytokines during the development of dry eye, as well as in dry-eye mice whose dry-eye conditions were developed before the treatment. This indicates that these compounds were able to at least partially ameliorate the inflammatory responses, consistent with previous reports of the anti-inflammatory properties of L-carnitine and betaine.
40,41 It should be noted that among all the compounds, L-carnitine presented the only statistically significant, and the most beneficial effect, on lowering inflammatory responses of all the mediators investigated and demonstrated in both schedules.