Figure 8. Schematic summary of signaling cascades which contribute to the osmotic expression of the NFAT5 gene in RPE cells. The NaCl-induced expression of the NFAT5 gene is fully dependent on intracellular calcium signaling mediated by phospholipase C (PLC) and inositol triphosphate (IP₃)-induced release of calcium from the endoplasmic reticulum (ER), as well as on activation of phospholipase A2 (PLA₂). Autocrine/paracrine purinergic receptor signaling, as well as activation of further receptors like the fibroblast growth factor (FGF) receptor, may induce release of calcium from intracellular stores. High extracellular NaCl induces a release of ATP from the cells which activates calcium-permeable P2X₇ receptors and metabotropic P2Y₂ receptors. Extracellular ATP is converted to ADP by the ectonucleotidase nucleoside triphosphate diphosphohydrolase 2 (NTPDase2); ADP activates P2Y₁ receptors. P2 receptor signaling results in activation of nucleoside transporters, which mediate a release of adenosine that activates A₁ receptors. Intracellular calcium signaling results in activation of calcium-dependent enzymes, such as protein kinase C (PKC), calpains, and PLA₂. In addition to PKA and Src tyrosine kinases, these enzymes contribute to activation of multiple intracellular signal transduction pathways, including extracellular signal–regulated kinases 1 and 2 (ERK1/2), p38 mitogen-activated protein kinase (MAPK), c-Jun NH₂-terminal kinase (JNK), and phosphatidylinositol-3 kinase (PI3K) pathways. One of the transcription factors mediating activation of the NFAT5 gene is nuclear factor κB (NF-κB).