Differential effects of dietary cholesterol and triglycerides on the lipid homeostasis in Meibomian glands

Exocrine Meibomian glands (MG) play a central role in the ocular surface physiology by producing meibum – a lipid secretion composed of cholesteryl esters (CE), cholesterol (Chl), triacylgycerols (TAG), waxes and other types of lipids. MG were previously shown to synthesize Meibomian lipids (ML) in situ via a complex array of reactions termed meibogenesis. However, questions remain about the role of dietary lipids in meibogenesis. To establish if dietary Chl (DC) and TAG (DT) can participate in meibogenesis, we studied mice whose diet was supplemented with trace amounts of deuterated Chl (²H-Chl) and ¹³C-labeled triolein (¹³C-TO), and the products of their biosynthetic transformations were analyzed using LC/MS. We demonstrated that ²H-Chl, but not ¹³C-TO, could be directly incorporated into meibum. Furthermore, ²H-Chl was esterified into MG-specific ultra long ²H-CE, which were vastly different from plasma CE and ²H-CE. The measured ²H-Chl/Chl and ²H-CE/CE ratios in meibum increased in a time-dependent manner reaching ∼5% and ∼1.2 %, respectively. The ²H-Chl/²H-CE ratio was about 3.5x higher than that for endogenous unlabeled Chl and CE, indicating accumulation of ²H-Chl in meibum. The elongation pattern of Meibomian ²H-CE closely replicated that of unlabeled CE. On the other hand, ¹³C-TO was not detected in any of the ML samples as an intact lipid or its metabolized/hydrolyzed products. We conclude that DC can be directly esterified into MG-specific CE, while DT undergo extensive catabolic transformations before reaching MG. These findings demonstrate that DC can have a direct impact on MG and ocular surface lipid homeostasis and pathophysiology.