Purpose: To characterize the osmoprotective properties of L-carnitine on human corneal epithelial cell volume and apoptosis during hyperosmotic stress.
Methods: Human corneal limbal epithelial (HCLE) cells were exposed to culture medium at 300 mOsm (isotonic) or 500 mOsm (hyperosmotic) with or without L-carnitine (10 mM). Induction of apoptosis was detected by quantifying the proteolytic activity of caspase-8, caspase-9, and caspase-3/7 using caspase activity assays, the expression of tumor necrosis factor (TNF)-α with enzyme-linked immunosorbent assay, and annexin V/propidium iodide staining of HCLE cells evaluated with confocal microscopy and flow cytometry. Cell volume changes in response to hyperosmotic stress were analyzed using flow cytometry.
Results: After the HCLE cells were exposed to hyperosmotic medium (500 mOsm), the percentage of shrunken cells and damaged/dead cells (stained positively for annexin V and/or propidium iodide) was six- and three-fold, respectively, higher than that under isotonic conditions (300 mOsm). This was paralleled by an increase in TNF-α concentration in media and caspase-8, -9, and -3/7 activities (six-, four-, ten-, and twelve-fold, respectively; all showing p < 0.001). Addition of L-carnitine during hyperosmotic stress partly restored cell volume and significantly reduced the concentration of TNF-α released (p = 0.005) and caspase-9 activity (p = 0.0125). Addition of L-carnitine reduced the percentage of hyperosmolarity-induced damaged/dead cells to levels observed under isotonic conditions.
Conclusions: L-carnitine can regulate human corneal epithelial cell volume under hyperosmotic stress and ameliorate hyperosmotic stress-induced apoptosis.