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IL-32α is known as a proinflammatory cytokine. However, several evidences implying its action in cells have been recently reported. In this study, we present for the first time that IL-32α plays an intracellular mediatory role in IL-6 production using constitutive expression systems for IL-32α in THP-1 cells. We show that phorbol 12-myristate 13-acetate (PMA)-induced increase in IL-6 production by IL-32α-expressing cells was higher than that by empty vector-expressing cells and that this increase occurred in a time- and dose-dependent manner. Treatment with MAPK inhibitors did not diminish this effect of IL-32α, and NF-κB signaling activity was similar in the two cell lines. Because the augmenting effect of IL-32α was dependent on the PKC activator PMA, we tested various PKC inhibitors. The pan-PKC inhibitor Gö6850 and the PKCε inhibitor Ro-31-8220 abrogated the augmenting effect of IL-32α on IL-6 production, whereas the classical PKC inhibitor Gö6976 and the PKCδ inhibitor rottlerin did not. In addition, IL-32α was co-immunoprecipitated with PMA-activated PKCε, and this interaction was totally inhibited by the PKCε inhibitor Ro-31-8220. PMA-induced enhancement of STAT3 phosphorylation was observed only in IL-32α-expressing cells, and this enhancement was inhibited by Ro-31-8220, but not by Gö6976. We demonstrate that IL-32α mediated STAT3 phosphorylation by forming a trimeric complex with PKCε and enhanced STAT3 localization onto the IL-6 promoter and thereby increased IL-6 expression. Thus, our data indicate that the intracellular interaction of IL-32α with PKCε and STAT3 promotes STAT3 binding to the IL-6 promoter by enforcing STAT3 phosphorylation, which results in increased production of IL-6.
Kang et al. (Tue,) studied this question.