Interleukin-6 synthesis induced by prostaglandin E₂: Cross-talk regulation by protein kinase C

We previously showed that prostaglandin E₂ (PGE₂) stimulates multiple intracellular signaling pathways as follows: by activation of adenylate cyclase; phosphoinositide (PI)-hydrolyzing phospholipase C and phosphatidylcholine (PC)-hydrolyzing phospholipase D; and by induction of Ca²⁺ influx in osteoblast-like MC3T3-E1 cells. In this study, we investigated the effect of PGE₂ on the synthesis of interleukin-6 (IL-6) and its regulatory mechanism in MC3T3-E1 cells. PGE₂ significantly stimulated IL-6 secretion in a dose-dependent manner in the range between 1 nmol/L and 10 μmol/L. A23187, a calcium ionophore, or dibutyrgl-cAMP significantly induced IL-6 secretion. The effect of a combination of A23187 and dihutyryl-cAMP on IL-6 secretion was additive. The depletion of extracellular Ca²⁺ by EGTA reduced the PGE₂-induced IL-6 secretion. EP₁ receptor antagonist inhibited the PGE₂-induced IL-6 secretion. H-89, an inhibitor of cAMP-dependent protein kinase, decreased the PGE₂-induced IL-6 secretion. EP₂ receptor agonist alone stimulated IL-6 secretion. However, EP₄ receptor antagonist had little effect on IL-6 secretion. Calphostin C, a specific inhibitor of protein kinase C (PKC), enhanced the secretion of IL-6 induced by PGE₂. The stimulative effect of PGE₂ on IL-6 secretion was significantly enhanced in PKC downregulated MC3T3-E1 cells. Pertussis toxin enhanced PGE₂-induced IL-6 secretion. These results strongly suggest that PGE₂ stimulates IL-6 synthesis through both Ca²⁺ mobilization from extracellular space via EP₁ receptor and cAMP production via EP₂ receptor in osteoblast-like cells, and that the PKC activation by PGE₂ itself regulates oversynthesis of IL-6.