Bacterial lipopolysaccharide regulates the phosphorylation of the 68K protein kinase C substrate in macrophages

Bacterial lipopolysaccharide (LPS) potentiates protein kinase C (PKC)-dependent responses such as the activation of arachidonic acid metabolism in macrophages (Aderem, A.A., Cohen, D.S., Wright, S.D., and Cohn, Z.A. (1986) J. Exp. Med. 164, 165-179). Concomitantly, LPS promotes the myristoylation of a 68K PKC substrate, shown to be equivalent to the 80/87K PKC substrate found in brain and fibroblasts (Aderem, A.A., Albert, K.A., Keum M.M., Wang, J.K. Greengard P., and Cohn, Z.A. (1988) Nature 332, 362-364). We have now examined the effect of LPS on the phosphorylation of this 68K PKC substrate. We report here that LPS modifies the kinetics and extent of phosphorylation of the 68K protein. While treatment with LPS alone induces low level phosphorylation of the 68K protein, it markedly increases the rate of subsequent phorbol 12-myristate 13-acetate (PMA)-dependent phosphorylation of this protein. Phosphorylation in LPS-treated macrophages was maximal 1-2 min after administration of PMA, while maximal phosphorylation in macrophages not exposed to LPS was only achieved 6 min after addition of PMA. In addition to increasing the rate of PMA-dependent phosphorylation of the 68K protein in macrophages, LPS also promoted the phosphorylation of a novel peptide on the 68K protein. Thus while PMA stimulated the phosphorylation of two thermolytic phosphopeptides (phosphopeptides 1 and 2), the low level of phosphorylation observed with LPS alone was found to occur on phosphopeptides 1 and 2 as well as on a novel phosphopeptide (phosphopeptide 3). Furthermore, LPS treatment of macrophages potentiated phosphorylation of all three phosphopeptides when the cells were subsequently stimulated with PMA. While phosphorylation stimulated by LPS and PMA was slightly more than additive for phosphopeptides 1 and 2, it was markedly synergistic (increased 14.5-fold) for phosphopeptide 3. Phosphorylation of all three phosphopeptides occurred exclusively on serine. It is possible that LPS-induced myristoylation of the 68K protein directs it to the membrane where its phosphorylation is enhanced by its close association with PKC.