Oncostatin M-Induced IL-6 Expression in Murine Fibroblasts Requires the Activation of Protein Kinase Cδ

Oncostatin M (OSM) is an IL-6/LIF cytokine family member whose role has been identified in a range of biological activities in vitro, including up-regulation of inflammatory gene expression and regulation of connective tissue metabolism. However, the mechanisms through which OSM regulates cellular responses are not completely understood. In this study, we show that activation of the calcium-independent or novel protein kinase C (PKC) isoform PKCdelta is a critical event during OSM-mediated up-regulation of IL-6 expression in murine fibroblasts. The pan-PKC inhibitor GF109203X (bisindolylmaleimide I) reduced secretion of IL-6; however, use of Go6976, an inhibitor of calcium-dependent PKC enzymes, did not. The PKCdelta-selective inhibitory compound rottlerin abrogated expression of IL-6 transcript and protein, but only reduced PKCdelta activity when used at higher concentrations as determined by kinase activity assay, suggesting rottlerin may inhibit IL-6 expression in a PKCdelta-independent manner. However, silencing of PKCdelta protein expression, but not the related novel isoform PKCepsilon, by use of RNA interference (i.e., small interfering RNA) demonstrated that PKCdelta is required for murine OSM (mOSM) induction of IL-6 protein secretion. Furthermore, inhibition of PI3K by use of LY294002 reduces expression of IL-6 at both the mRNA and protein level in murine fibroblasts, and we suggest that PI3K is required for activation of PKCdelta. Knockdown of phosphoinositide-dependent kinases PDK-1 or Akt1 using small interfering RNA strategies did not influence mOSM-induced IL-6 expression, suggesting mOSM uses a PI3K-PKCdelta pathway of activation independent of these kinases. Our findings illustrate a novel signaling network used by mOSM that may be important for its mediation of inflammatory processes.