Abstract
Purpose: :
Meibomian glands produce secretions (meibum), which form the bulk of the tear film lipid layer (TFLL). Its composition is critical for maintaining the stability of the TF whose degradation was linked to dry eye disease (DED). Oleic acid has been shown to be an intrinsic part of meibum. In DED, free unoxidized oleic acid (a surface-active compound) is increased in meibum due to the increased activity of hydrolytic enzymes. This changes the composition of the TFLL and has been previously shown in our experiments to affect its properties. Other related compounds of the free fatty acids (FFA) family routinely found in biological fluids and tissues, e.g. free linoleic acid and its oxidized derivative 13-HODE, are also known to have pronounced surface active properties. However, their effects on TFLL have not been established yet.
Methods: :
Normal human meibum samples were hard-expressed from eyelids. A series of aqueous subphases with varying concentrations of dissolved FFA (0-100µM, pH 7.4), was used to study the surface pressure (π/A) isotherms of meibum at physiological temperature of 34°C. A predetermined amount of meibum dissolved in CHCl3 was spread onto the surface of the subphase to form a Langmuir film of meibomian lipids, which served as a biophysical model of human TFLL. Then, multiple π/A isotherms were recorded in cycles.
Results: :
Studied at normal corneal temperature of 34°C without FFA, the (π/A) isotherms of meibum remained stable and noncollapsible. At concentrations exceeding the physiological level of 20µM, unoxidized FFA quickly disrupted the meibum layers. The effects of unoxidized mono- and polyunsaturated FFA were found to be similar and concentration-dependent, increasing gradually from 0 to 50µM. Minimal changes were observed in the (π/A) isotherms of meibum using a subphase containing <20 µM of oxidized FFA. Increasing the concentration up to 100 µM resulted in quick collapsing of the meibum layers. Under equal circumstances, unoxidized FFA always had a much more potent detrimental effect on the stability of meibum layers.
Conclusions: :
When present in concentrations ≥20 µM, unoxidized and oxidized FFA disrupt the meibum layers in a concentration-dependent manner, apparently by forming mixed micelles with meibomian lipids. However, oxidation of FFA diminishes their effects on meibum layers, suggesting a possible protective mechanism.
Keywords: cornea: tears/tear film/dry eye • lipids • cornea: basic science