Role of Isoprenoid Metabolism in IgE Receptor-mediated Signal Transduction

In the 2H3 subline of rat basophilic leukemia cells (RBL-2H3), IgE receptor cross-linking stimulates a signal transduction pathway that leads to the secretion of histamine, serotonin, and other inflammatory mediators; the assembly of F-actin; and the transformation of the cell surface from a microvillous to a lamellar or ruffled architecture. We report here that 20 h incubation of RBL-2H3 cells with 10 microM lovastatin, an inhibitor of 3-hydroxy-3-methyl-glutaryl coenzyme A reductase (HMG CoA reductase), inhibits both the secretory and morphologic responses to IgE receptor cross-linking. Ag-induced Ca2+ mobilization, determined from the influx and efflux of 45Ca2+, and Ag-induced 1,4,5-inositol trisphosphate production are also inhibited in lovastatin-treated RBL-2H3 cells. Under the same conditions, lovastatin does not alter cell proliferation or IgE receptor expression, and it causes only a small impairment of responses initiated by drugs that bypass the earliest steps in the receptor-activated transduction pathway (ionomycin-induced secretion and PMA-induced membrane ruffling). Receptor-mediated Ca2+ mobilization, secretion, and ruffling are all restored by 0.5- to 4-h incubation of lovastatin-treated cells with mevalonic acid, the product of HMG CoA reductase and the first committed intermediate of the isoprenoid biosynthetic pathway. In contrast, dolichol and cholesterol, which are synthesized from products of the isoprenoid pathway, do not restore receptor-activated responses. These data implicate an isoprenoid pathway intermediate in an early step in the IgE receptor-activated signal-transduction sequence. We postulate that this intermediate is required for a newly described post-translational modification of proteins, their post-synthetic isoprenylation. The substrates for this modification include the ras family of GTP-binding proteins and the gamma subunits of the heterotrimeric guanine nucleotide-binding protein.