Protein kinase C δ associates with and phosphorylates Stat3 in an interleukin-6-dependent manner

Abstract

Stat3 is activated by phosphorylation on Tyr-705, which leads to dimer formation, nuclear translocation, and regulation of gene expression. Serine phosphorylation of Stat3 by mitogen-activated protein kinase has also been observed in cells responding to epidermal growth factor and shown to affect its tyrosine phosphorylation and transcriptional activity. Serine phosphorylation of Stat3 is also induced by interleukin-6 (IL-6) stimulation, which is shown to be independent of mitogen-activated protein kinase and sensitive to the Set/Thr kinase inhibitor H7. In this study, we investigated whether protein kinase C (PKC) is the kinase that is induced and responsible for Stat3 serine phosphorylation by IL-6 stimulation and which isoform of PKCs is likely to be involved. Here, we report that Stat3 was specifically associated with PKC δ in vivo in an IL-6-dependent manner in several cell types. Furthermore, Stat3 was phosphorylated by PKC δ in vivo on Ser-727, which could be inhibited either by a specific PKC δ inhibitor or by a dominant-negative mutant of PKC δ. Finally, we showed that the phosphorylation of Stat3 by PKC δ led to a negative regulation of Stat3 DNA binding and transcriptional activity. These results indicate that PKC δ is likely to be the kinase that phosphorylates Stat3 in response to IL-6 stimulation and suggest a possible regulatory role of PKC δ on Stat3 function.