A high throughput screen for neutral, magnesium-dependent sphingomyelinase (SMase) was performed. One inhibitor discovered in the screen, GW4869, functioned as a noncompetitive inhibitor of the enzyme in vitro with an IC₅₀ of 1 μm. It did not inhibit acid SMase at up to at least 150 μm. The compound was then evaluated for its ability to inhibit tumor necrosis factor (TNF)-induced activation of neutral SMase (N-SMase) in MCF7 cells. GW4869 (10 μm) partially inhibited TNF-induced sphingomyelin (SM) hydrolysis, and 20 μm of the compound was protected completely from the loss of SM. The addition of 10–20 μmGW4869 completely inhibited the initial accumulation of ceramide, whereas this effect was partially lost at later time points (24 h). These data therefore support the inhibitory action of GW4869 on N-SMase not only in vitro but also in a cellular model. The addition of GW4869 at both 10 and 20 μm did not modify cellular glutathione levels in response to TNF, suggesting that the action of GW4869 occurred downstream of the drop in glutathione, which was shown previously to occur upstream of the activation of N-SMase. Further, whereas TNF treatment also caused a 75% increase ofde novo synthesized ceramide after 20 h of incubation, GW4869, at either 10 or 20 μm, had no effect on this pathway of ceramide generation. In addition, GW4869 did not significantly impair TNF-induced NF-κB translocation to nuclei. Therefore, GW4869 does not interfere with other key TNF-mediated signaling effects. GW4869 was able, in a dose-dependent manner, to significantly protect from cell death as measured by nuclear condensation, caspase activation, PARP degradation, and trypan blue uptake. These protective effects were accompanied by significant inhibition of cytochrome c release from mitochondria and caspase 9 activation, therefore localizing N-SMase activation upstream of mitochondrial dysfunction. In conclusion, our results indicate that N-SMase activation is a necessary step for the full development of the cytotoxic program induced by TNF.

In an effort to define the actual function of the promiscuous putatively silent chemokine receptor D6, transfectants were generated in different cell types. Engagement of D6 by inflammatory CC chemokines elicited no calcium response nor chemotaxis, but resulted in efficient agonist internalization and degradation. Also in lymphatic endothelium, where this receptor is expressed in vivo, D6 did not elicit cellular responses other than ligand internalization and degradation. In particular, no evidence was obtained for D6-mediated transcytosis of chemokines in the apical-to-basal or basal-to-apical directions. These results indicate that D6 acts as an inflammatory chemokine scavenging nonactivatory decoy receptors and suggest that in lymphatic vessels D6 may function as a gatekeeper for inflammatory CC chemokines, by clearing them and preventing excessive diffusion via afferent lymphatics to lymph nodes.

Seven transmembrane receptors mediate diverse physiological responses including hormone action, olfaction, neurotransmission, and chemotaxis. Human D6 is a non-signaling seven-transmembrane receptor expressed on lymphatic endothelium interacting with most inflammatory CC-chemokines resulting in their rapid internalization. Here, we demonstrate that this scavenging activity is mediated by continuous internalization and constant surface expression of the receptor, a process involving the clathrin-coated pit-dependent pathway. D6 constitutively associates with the cytoplasmic adaptor β-arrestin, and this interaction is essential for D6 internalization. An acidic region, but not the putative phosphorylation sites in the cytoplasmic tail of D6, is critical for receptor interaction with β-arrestin and subsequent internalization. Neither the native D6 nor mutants uncoupled from β-arrestin activate any G-protein-mediated signaling pathways. Therefore, D6 may be considered a decoy receptor structurally adapted to perform chemokine scavenging.