Mitogen-activated protein (MAP) kinases are involved in interleukin-1 (IL-1)-induced IL-6 synthesis in osteoblasts: modulation not of p38 MAP kinase, but of p42/p44 MAP kinase by IL-1-activated protein kinase C.

We previously reported that interleukin-1 alpha (IL-1 alpha)- induced activation of protein kinase C (PKC) via phosphatidylcholine-specific phospholipase C (PC-PLC) limits IL-6 synthesis induced by IL-1 alpha itself in osteoblast-like MC3T3-E1 cells. In the present study, we further investigated the mechanism behind IL-1 alpha-induced IL-6 synthesis in MC3T3-E1 cells. IL-1 alpha time-dependently stimulated the phosphorylation of both p42/p44 mitogen-activated protein (MAP) kinase and p38 MAP kinase. PD98059, a specific inhibitor of the upstream kinase that activates p42/p44 MAP kinase, inhibited the IL-1 alpha-induced IL-6 synthesis as well as the phosphorylation of p42/p44 MAP kinase induced by IL-1 alpha. SB203580, a specific inhibitor of p38 MAP kinase, also reduced both the phosphorylation of p38 MAP kinase and the IL-6 synthesis, 1-Oleoyl-2-acetylglycerol, an activator of PKC, suppressed the IL-1 alpha-induced IL-6 synthesis. Calphostin C, a specific inhibitor of PKC, or D-609, a specific inhibitor of PC-PLC, significantly enhanced the IL-1 alpha-induced phosphorylation of p42/p44 MAP kinase without affecting the phosphorylation of p38 MAP kinase. The phosphorylation of p42/p44 MAP kinase by IL-1 alpha was markedly increased in PKC-down-regulated MC3T3-E1 cells. Neither 12-O-tetradecanoylphorbol-13-acetate, known to be an activator of PKC, nor 1-oleoyl-2-acetylglycerol affected the phosphorylation of p38 MAP kinase induced by IL-1 alpha. These results strongly suggest that IL-1 alpha-induced IL-6 synthesis is mediated via activations of both p42/p44 MAP kinase and p38 MAP kinase in osteoblasts, and that PKC activated by IL-1 alpha itself negatively regulates IL-6 synthesis at a point upstream from p42/p44 MAP kinase.