Transduction of mitogenic signals in T lymphocytes: role of inositol phospholipids for the rapid activation of ornithine decarboxylase

Treatment of human T lymphocytes with mitogenic ligands, such as concanavalin A (Con A), induces a rapid activation of the enzyme ornithine decarboxylase (ODC). This activation occurs within minutes and is completely inhibited when the cells are treated with 1 mM Li+ (in an inositol-free medium) prior to stimulation with Con A. In the presence of 1 mM myo-inositol Li+ has no effect on the Con A-induced activation of ODC. To elucidate why inositol is needed for the mitogen-induced activation of ODC in T lymphocytes, we tested the ability of different inositol metabolites to reverse the inhibitory effect of Li+. Here we report that inositol phospholipids, in addition to inositol, reverse the Li+-induced inhibition of ODC activation, while all other inositol derivatives tested were ineffective. This indicates that Li+ does not block the activation of ODC by inhibiting the generation of inositol phosphates, but rather by a mechanism which is circumvented if inositol phospholipids are added. The molecular mechanisms involved in the rapid activation of ODC by mitogens in human T lymphocytes apparently require inositol phospholipids, but are not directly mediated by inositol-1,4,5-trisphosphate (IP3) alone, diacylglycerol alone, or other inositol phosphates.