Diacylglycerol kinase ␣ (DAGK␣), like all type I DAGKs, has calcium regulatory motifs that act as negative regulators of enzyme activity and localization. Accordingly, DAGK␣ is activated by phospholipase C-coupled receptors in a calcium-dependent manner. One of the first functions attributed to DAGK␣ in lymphocytes was that of regulating interleukin 2-induced cell cycle entry. Interleukin-2 nonetheless exerts its action in the absence of cytosolic calcium increase. We have studied alternative receptor-derived signals to explain calciumindependent DAGK␣ activation, and show that DAGK␣ is stimulated by Src-like kinase-dependent phosphoinositide 3 kinase (PI3K) activation in lymphocytes. Our results demonstrate that, in vivo, the increase in cellular levels of PI3K products is sufficient to induce DAGK␣ activation, allowing DAGK␣ relocation to the intact lymphocyte plasma membrane. This activation is isoformspecific, because other type I DAGKs are not subject to this type of regulation. These studies are the first to describe a pathway in which, in the absence of receptorregulated calcium increase, DAGK␣ activation and membrane localization is a direct consequence of PI3K activation.

The signal transduction pathways involving phospholipid metabolism during T-cell proliferation remain partly undefined. Herein we show that interleukin-2 caused a late ( s 12 h) rise in the intracellular phosphatidic acid content of CTLL-2 cells which was a consequence of the activation of the enzyme diacylglycerol kinase. No activation of phospholipase D was observed at similar times. Incubation of the cells with a recognized diacylglycerol kinase K K isoform inhibitor, R59499, prior to interleukin-2 stimulation was able to block cell cycle entry, diacyglycerol kinase activation and phosphatidic acid accumulation. In contrast, when R59499 was added 3 h after interleukin-2, few or no observable effects on the above three parameters were noticed. These results suggest that the early signaling employed by IL-2 involving the K K isoform of diacylglycerol kinase is sufficient to control the late increase in phosphatidic acid and that phosphatidic acid is a mitogenic agent in T-cells.z 1998 Federation of European Biochemical Societies.

No abstract