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 Ser/ 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 ␦ in vivo in an IL-6-dependent manner in several cell types. Furthermore, Stat3 was phosphorylated by PKC ␦ in vivo on Ser-727, which could be inhibited either by a specific PKC ␦ inhibitor or by a dominant-negative mutant of PKC ␦. Finally, we showed that the phosphorylation of Stat3 by PKC ␦ led to a negative regulation of Stat3 DNA binding and transcriptional activity. These results indicate that PKC ␦ is likely to be the kinase that phosphorylates Stat3 in response to IL-6 stimulation and suggest a possible regulatory role of PKC ␦ on Stat3 function.

Elevated phosphorylation and activation of PDK-1/AKT pathway in human breast cancer

“…Although Stat3 (S727) remains correlate well with kinases in this cascade including EGFR, PDK-1, AKT, mTOR, p70S6K and S6 (Po0.05 in Table 1), phosphorylated Stat3 (Y705) did not correlate with PDK-1 phosphorylation in this study (P40.05; Table 2), despite it is associated with invasive breast carcinoma (Po0.05; Table 2). It hence suggests that Stat3 phosphorylation (Y705) was likely accomplished by different kinase(s) that were excluded from PDK-1/mTOR/p70S6K/S6K pathway and further support the notion indicating phosphorylation on these two residues was mediated by independent signalling events (Jain et al, 1999;Yokogami et al, 2000;Garcia et al, 2001;Lim and Cao, 2001;Yonezawa et al, 2004).…”

“…Activation of Stat3 requires phosphorylation on two residues S727 and Y705 (Wen et al, 1995;Jain et al, 1999;Lim and Cao, 2001). mTOR is one of the Number of breast tumours in each stage that contained positive protein phosphorylation (scores 2 and 3 upon IHC staining on TMA slides) was counted and was divided to the total number (N) of breast tumours in each group, which gave rise to the frequency (%) of tumours that retained elevated protein phosphorylation.…”

Protein kinase C-δ is involved in the inflammatory effect of IL-6 in mouse adipose cells

“…An extensive search for known potential kinases inducing Ser-727 phosphorylation of STAT3 in response to IL-6 was performed by using inhibitors to PKCδ, ERK, JNK, p38 MAPK and mTOR (data not shown), as they have previously been described to induce this phosphorylation [18][19][20][21][22]. Our study revealed PKCδ as the main target molecule.…”

“…In this study, we further characterised Ser-727 phosphorylation of STAT3 by IL-6 and its potential role in mouse 3T3-L1 adipocytes by using specific inhibitors for the kinase pathways presented above [18][19][20][21][22]. We concluded that PKCδ was not only important for Ser-727 phosphorylation, but also for the Tyr-705 phosphorylation, nuclear translocation and transcriptional properties of STAT3.…”

“…Depending on cell origin, many different kinases have been shown to increase Ser-727 phosphorylation of STAT3, such as protein kinase C-δ (PKCδ), extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), p38 mitogen-activated protein kinase (MAPK) and the mammalian target of rapamycin (mTOR) [18][19][20][21][22].…”