Osmotic and hypoxic induction of osteopontin in retinal pigment epithelial cells: Involvement of purinergic receptor signaling

PURPOSE: Osteopontin (OPN) is a neuroprotective factor in the retina that improves photoreceptor survival. The aim of the present study was to investigate whether human RPE cells express and respond to OPN. METHODS: Hypoxia and chemical hypoxia were induced by cell culture in 0.25% O(2) and the addition of CoCl(2), respectively. Hyperosmolarity was produced by the addition of 100 mM NaCl or 200 mM sucrose. Gene expression was quantified with real-time reverse transcription (RT)–PCR, and protein secretion was investigated with enzyme-linked immunosorbent assay (ELISA). Nuclear factor of activated T cell 5 (NFAT5) was depleted with siRNA. RESULTS: The acutely isolated RPE cells and the cultured RPE cells expressed OPN. OPN gene expression was induced by hypoxia and hyperosmotic media, as well as by exogenous bFGF. High extracellular NaCl and hypoxia induced secretion of OPN. Hyperosmotic expression of the OPN gene was mediated by the p38 MAPK and ERK1/2 signal transduction pathways, and the transcriptional activities of CREB and NFAT5. The hypoxic expression of the OPN gene was mediated by the PI3K signal transduction pathway and caspase-mediated, necrosis-related pathways. Phospholipases A(2) were involved in mediating hyperosmotic and hypoxic OPN gene expression. Autocrine or paracrine P2Y(2) receptor signaling induced by extracellular ATP contributed to hyperosmotic expression of the OPN gene whereas activation of A(1) receptors by extracellularly formed adenosine contributed to thypoxic OPN gene expression. Autocrine or paracrine VEGF signaling exerted an inhibitory effect on expression of the OPN gene. Exogenous OPN induced expression and secretion of bFGF, but not of VEGF. CONCLUSIONS: The data indicated that RPE cells produce and respond to OPN; OPN expression is, in part, induced by the cellular danger signal ATP. RPE-derived neuroprotective factors such as bFGF may contribute to the prosurvival effect of OPN on photoreceptor cells.