Effect of brimonidine, an α2 adrenergic agonist, on human meibomian gland epithelial cells

We recently discovered that the anti-glaucoma pharmaceuticals timolol, a beta adrenergic antagonist, and pilocarpine, a cholinergic compound, negatively influence the morphology, proliferative capacity and survival of human meibomian gland epithelial cells (HMGECs) We hypothesize that another class of anti-glaucoma drugs, the alpha 2 adrenergic agonists, also acts directly on HMGECs to affect their structure and function We tested this hypothesis Immortalized (i) HMGECs were cultured with brimonidine, as well as clonidine (alpha 2 agonist), phenylephrine (alpha l agonist), RX821002 (inverse alpha 2 agonist) and MK912 (neutral alpha 2 agonist) for up to 7 days Cells were counted with a hemocytometer, and evaluated for morphology, signaling pathway activity, protein biomarker expression, and the accumulation of neutral lipids, phospholipids and lysosomes Our findings demonstrate that brimondine treatment induces a dose-dependent decrease in Akt signaling and proliferation of iHMGECs In contrast, brimonidine also promotes a dose-dependent differentiation of iHMGECs, including an increase in neutral lipid, phospholipid and lysosome levels These effects were paralleled by an inhibition of p38 signaling, and duplicated by cellular exposure to clonidine, but not phenylephrine Brimonidine also enhanced the cellular content of sterol regulatory binding protein-1, a master regulator of lipid synthesis Of particular interest, the putative alpha 2 antagonists, RX821002 and MK912, did not interfere with brimonidine action, but rather stimulated IHMGEC differentiation. Our results support our hypothesis and demonstrate that alpha 2 adrenergic agonists act directly on iHMGECs However, these compounds do not elicit an overall negative effect Rather, the alpha 2 agonists promote the differentiation of iHMGECs.