Sphingosine kinases and sphingosine-1-phosphate are critical for transforming growth factor β-induced extracellular signal-regulated kinase 1 and 2 activation and promotion of migration and invasion of esophageal cancer cells

Transforming growth factor beta (TGF beta) plays a dual role in oncogenesis, acting as both a tumor suppressor and a tumor promoter. These disparate processes of suppression and promotion are mediated primarily by Smad and non-Smad signaling, respectively. A central issue in understanding the role of TGF beta in the progression of epithelial cancers is the elucidation of the mechanisms underlying activation of non-Smad signaling cascades. Because the potent lipid mediator sphingosine-1-phosphate (S1P) has been shown to transactivate the TGF beta receptor and activate Smad3, we examined its role in TGF beta activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) and promotion of migration and invasion of esophageal cancer cells. Both S1P and TGF beta activate ERK1/2, but only TGF beta activates Smad3. Both ligands promoted ERK1/2-dependent migration and invasion. Furthermore, TGF beta rapidly increased S1P, which was required for TGF beta-induced ERK1/2 activation, as well as migration and invasion, since downregulation of sphingosine kinases, the enzymes that produce S1P, inhibited these responses. Finally, our data demonstrate that TGF beta activation of ERK1/2, as well as induction of migration and invasion, is mediated at least in part by ligation of the S1P receptor, S1PR(2). Thus, these studies provide the first evidence that TGF beta activation of sphingosine kinases and formation of S1P contribute to non-Smad signaling and could be important for progression of esophageal cancer.