Unravelling the interplay of sphingolipids and TGF-β signaling in the human corneal stroma

PURPOSE: To delineate the role of Sphingolipids (SPLs) in the human cornea and their cross-talks with transforming growth factor beta (TGF-β) in order to develop novel, non-invasive therapies. METHODS: Human corneal fibroblasts (HCFs) were harvested from healthy donors, stimulated with Vitamin C to promote extracellular matrix assembly, treated with exogenous sphingosine-1-phosphate (S1P) or sphingosine kinase inhibitor 2 (SPHK I(2)) and isolated after 4 weeks for further analysis. RESULTS: Data showed that S1P led to a significant decrease in cellular migration where SPHK I(2) just delayed it for 24h. Significant modulation of the sphingolipid pathway was also noted. Sphingosine kinase-1 (SphK1) was significantly downregulated upon exogenous stimulation with S1P at a concentration of 5μM and Sphingosine kinase-2 (SphK2) was also significantly downregulated at concentrations of 0.01μM, 0.1μM, and 5μM; whereas no effects were observed upon stimulation with SPHK I(2.) S1PR3 was significantly downregulated by 0.1μM and 5μM S1P and upregulated by 5μM and 10μM SPHK I(2). Furthermore, both S1P and SPHK I(2) regulated corneal fibrosis markers such as alpha-smooth muscle actin, collagen I, III, and V. We also investigated the interplay between two TGF-β isoforms and S1P/SPHK I(2) treatments and found that TGF-β1 and TGF-β3 were both significantly upregulated with the 0.1μM S1P but were significantly downregulated with the 5μM S1P concentration. When TGF-β1 was compared directly to TGF-β3 expression, we observed that TGF-β3 was significantly downregulated compared to TGF-β1 in the 5μM concentration of S1P. No changes were observed upon SPHK I(2) treatment. CONCLUSION: Our study delineates the role of sphingolipids in the human cornea and highlights their different activities based on the cell/tissue type.