Activity of an intracellular lymphocyte stimulator is independent of G-protein interactions, [Ca2+]i elevation, phosphoinositide hydrolysis, and protein kinase C translocation.

We have evaluated potential molecular mechanisms by which a group of synthetic lymphokine-like molecules, the 7,8-disubstituted guanine ribonucleosides, acts on second messenger pathways to augment the responses of murine B lymphocytes. Despite its extensive structural homology with GTP, 7-methyl-8-oxoguanosine (7-Me-8-oGuo), a prototypical disubstituted nucleoside, does not inhibit the binding of guanosine 5'-3-O-(thio)triphosphate either to purified G-proteins, or to G-proteins in situ in the plasma membrane. In contrast to anti-IgM antibodies, 7-Me-8-oGuo fails to induce elevation of intracellular free calcium in B lymphocytes. It does not elicit enhanced production of inositol phosphates in either unfractionated or large, cycling B cells (the cells on which it acts preferentially). It is unable to modify the cellular distribution or activity of protein kinase C, whereas phorbol 12-myristate 13-acetate, anti-IgD antibodies, and lipopolysaccharide do activate protein kinase C. Inhibitors of protein kinase C activity diminish stimulation of mRNA transcription by anti-IgD antibodies and lipopolysaccharide but not by 7-Me-8-oGuo. These data demonstrate that 7-Me-8-oGuo either uses a pathway distinct from that mediated by G-proteins, intracellular free calcium, inositol phosphates, and protein kinase C, or else bypasses the early events of this pathway, activating the cell at a point beyond their involvement. In either event, these nucleosides represent the only class of activator to date that is capable of driving B cell proliferation and differentiation without involvement of protein kinase C.