The role of lipoproteins in EBV early antigen induction in Raji cells.

Synthesis of the Epstein-Barr virus (EBV) associated early antigen (EA) can be induced by a variety of agents in Raji cells, a latently EBV-infected Burkitt lymphoma line. We investigated the role of lipoproteins in this EA induction system. Cell growth was not affected by lipoprotein-deficiency, but EA induction by most combinations of the inducers TPA (tetradecanoyl-phorbol-acetate), IdU (iododeoxyuridine), n-BA (n-butyric acid), anti-IgM and EA inducing factor (EIF), was greatly reduced. Only the inducer combination TPA/n-BA was completely independent of the presence of lipoproteins, indicating a different induction pathway. Removing the lipid moieties of the culturing serum did not result in reduced EA induction. Thus, the lowered EA inducibility in lipoprotein-deficiency is due to the absence of the protein moiety (apolipoprotein). Addition of HDL or VLDL partially reconstituted the original EA inducibility, whereas LDL had no effect. Lipoproteins were particularly important during the first 4 hours of induction, the phase where inducers may act on cell membrane structures (e.g., receptors). But lipoproteins were also required throughout the incubation period, even in a late and inducer independent phase.