Caspase activities were tested by their ability to cleave specific substrates. In unstimulated monocytes cultured for 24 or 48 h caspase-9 as well as caspase-3 activity is significantly increased by 9- to 10-fold (caspase-9; Fig. 4A) or 14- to 22-fold (caspase-3; Fig. 4B), as compared

with caspase activity in freshly isolated monocytes. In contrast, activation of both, caspase-9 and –3, is blocked in CXCL4-treated cells. Furthermore, CXCL4-mediated protection from caspase activation is partially reversed in the presence of SKI, indicating that activation of SphK results in an inhibition of caspase activity. Since we have shown previously, that CXCL4-mediated activation of Erk is essential for monocyte survival 3, we included the MEK/Erk SCH772984 in vitro inhibitor PD098059 in this study. Comparable to SKI, inhibition of MEK/Erk resulted in partial reversion of the CXCL4-mediated inhibition of caspases (Fig. 4A and B). These results provide evidence, that caspase activity in CXCL4-activated cells is controlled by both, SphK and Erk. As mentioned

above, we have described in a recent report that CXCL4 induces delayed activation of Erk and Erk is absolutely required for monocyte survival 3. Since pretreatment of the cells with SKI also reduces monocyte survival, we were interested whether SphK might also regulate Erk phosphorylation. To this end, isolated monocytes Tyrosine Kinase Inhibitor Library order were preincubated in the presence or absence of 9 μM SKI, 10 μM PD098059, or solvent DMSO, and subsequently stimulated with CXCL4 (4 μM) for up to 48 h. Activation of Erk was tested by western blot analysis using phospho-Erk specific antibodies. As shown in Fig. 5, CXCL4 induced phosphorylation of Erk and pretreatment of the cells with MEK/Erk inhibitor PD098059 resulted in a strong reduction of Erk phosphorylation in CXCL4-treated cells. A comparable inhibition of Erk phosphorylation is observed in CXCL4-activated monocytes when these

cells were pretreated with SKI. From these data, we have to conclude that activation of Erk Glycogen branching enzyme is located downstream of SphK (or of its sphingolipid product S1P) in CXCL4-stimulated monocytes. To examine whether SphK activity can be mimicked by its product S1P, in a next set of experiments we analyzed the effect of exogenous S1P on monocyte survival, ROS production, caspase activation, as well as Erk phosphorylation. As shown in Fig. 6A, in the absence of CXCL4, about 53.9±3.9% of the monocytes developed an apoptotic and 22.2±5.7% a necrotic staining pattern, while CXCL4-treated monocytes were efficiently protected (9.6±4.5% apoptotic and 10.1±7.3% necrotic cells). Treatment of the cells with 50 μM S1P also significantly reduced apoptosis/necrosis rates (36.2±11.2% apoptotic and 11.6±4.