Sphingosine-1-phosphate receptor expression in cardiac fibroblasts is modulated by in vitro culture conditions

The bioactive molecule sphingosine-1-phosphate (S1P) binds with high affinity to five recognized receptors (S1P1–5) to affect various tissues, including cellular responses of cardiac fibroblasts (CFbs) and myocytes. CFbs are essential components of myocardium, and detailed study of their cell signaling and physiology is required for a number of emerging disciplines. Meaningful studies on CFbs, however, necessitate methods for selective, reproducible cell isolations. Macrophages reside within normal cardiac tissues and often are isolated with CFbs. A protocol was therefore developed that significantly reduces macrophage levels and utilizes more CFb-specific markers (discoidin domain receptor-2) instead of, or in addition to, more commonly used cytoskeletal markers. Our results demonstrate that primary isolated, purified CFbs express predominantly S1P1–3; however, the relative levels of these receptor subtypes are modulated with time and by culture conditions. In coculture experiments, macrophages altered CFb S1P receptor levels relative to controls. Further investigations using known macrophage-secreted factors showed that S1P and H2O2 had minimal effects on CFb S1P1–3 expression, whereas transforming growth factor-β1, TNF-α, and PDGF-BB significantly altered all S1P receptor subtypes. Lowering FBS concentrations from 10% to 0.1% increased S1P2, whereas supplementation with either PDGF-BB or Rho-associated protein kinase inhibitor Y-27632 significantly elevated S1P3 levels. S1P2 and S1P3 receptor levels are known to regulate cell migration. Using cells isolated from either normal or S1P3-null mice, we demonstrate that S1P3 is important and necessary for CFb migration. These results highlight the importance of demonstrating CFb culture purity in functional studies of S1P and also identify conditions that modulate S1P receptor expression in CFbs.