Abstract
Purpose: :
Dry eye (DE) affects millions of people worldwide. The tear film lipid layer (TFLL) is produced mainly by the meibomian glands which secrete meibum. In previous research, free cholesterol (FC) and cholesterol esters (CE) were found in both normal and DE meibum. In tears, CE can be hydrolyzed into free fatty acids (FFA) and FC by lipases. In 2009, we reported that FFA can disrupt the meibum layers. However, the effects of FC were not evaluated at the time. We hypothesized that FC would also disrupt the stability of the meibum layers.
Methods: :
Human meibum and FC were mixed to create FC/meibum mixtures with mass ratios of the components ranging from 0 to 1. A Langmuir trough was used to study the surface properties of meibum at 34oC. A Tris-buffered saline solution pH 7.4 was used as the aqueous subphase. Multiple pressure/area (π/A) isotherms were recorded in cycles.
Results: :
The meibum layers remained stable and non-collapsible at any tested pressure (π). The isotherms exhibited a high degree of reproducibility. Increasing concentrations of FC mixed with meibum produced significant changes in the shape of π/A isotherms. Without FC, a pre-calculated amount of added meibum lipids occupied all the available surface of the trough (~70cm2) forming a stable elastic layer. Further compression of the layer produced a gradual increase in π. When exogenously added FC was present in the layer in amounts between 0 and 30%, there was a gradual, dose-dependent shift of the beginning of the upward slope of the π/A isotherms from 70 to 55cm2. Under the identical degrees of surface layer compression, π exerted by meibum alone was systematically higher than that with FC present.
Conclusions: :
With increasing concentrations of FC, the surface properties of the meibum layers were affected in a way that implied the condensation, destabilization, and/or disruption of the layers. Two different mechanisms are hypothesized: A) a strong aggregating and/or condensing of meibum lipids and FC on the surface, which could cause the formation of expansive lipid-free zones on the surface, and B) meibum solubilization caused by formation of its mixed micelles with FC, which would prevent the formation of a stable meibum layer (FC is much more polar than most of meibomian lipids). This effect of FC is similar, but not identical, to the one previously described for FFA. This study offers additional support to the rationale for the use of lipases inhibitors to treat DE.
Keywords: cornea: tears/tear film/dry eye • lipids • cornea: basic science