Diacylglycerol kinase alpha suppresses tumor necrosis factor-alpha- -induced apoptosis of human melanoma cells through NF-kappa B activation

We investigated the implication of diacylglycerol kinase (DGK) a (type I isoform) in melanoma cells because we found that this DGK isoform was expressed in several human melanoma cell lines but not in noncancerous melanocytes. Intriguingly, the overexpression of wild-type (WT) DGK alpha, but not of its kinase-dead (KD) mutant, markedly suppressed tumor necrosis factor (TNF)-alpha-induced apoptosis of AKI human melanoma cells. In the reverse experiment, siRNA-mediated knockdown of DGK alpha significantly enhanced the apoptosis. The overexpression of other type I isoforms (DGK beta and DGK-gamma) had, on the other hand, no detectable effects on the apoptosis. These results indicate that DGK alpha specifically suppresses the TNF-alpha-induced apoptosis through its catalytic action. We found that the overexpression of DGK alpha-WT, but not of DGK alpha-KD, further enhanced the TNF-alpha-stimulated transcriptional activity of an anti-apoptotic factor, NF-kappa B. Conversely, DGK alpha-knockdown considerably inhibited the NF-kappa B activity. Moreover, an NF-kappa B inhibitor blunted the anti-apoptotic effect of DGK alpha overexpression. Together, these results strongly suggest that DGK alpha is a novel positive regulator of NF-kappa B, which suppresses TNF-alpha-induced melanoma cell apoptosis. (C) 2007 Elsevier B.V. All rights reserved.