Interleukin-6-induced STAT3 transactivation and Ser(727) phosphorylation involves Vav, Rac-1 and the kinase SEK-1/MKK-4 as signal transduction components

In the present study, signal transducer and activator of transcription 3 (STAT3) Ser(727) phosphorylation and transactivation was investigated in relation to activation of mitogen-activated protein (MAP) kinase family members including extracellular-signal-regulated protein kinase (ERK)-1, c-Jun N-terminal kinase (JNK)-1 and p38 ('reactivating kinase') in response to interleukin (IL)-6 stimulation. Although IL-6 can activate ERK-1 in HepG2 cells. STAT3 transactivation and Ser(727) phosphorylation were not reduced by using the MAP kinase/ERK kinase (MEK) inhibitor PD98059 or by overexpression of dominant-negative Raf, IL-6 did not activate JNK-1 in HepG2 cells and STATS was a poor substrate for JNK-1 activated by anisomycin, excluding a role for JNK1 in IL-6-induced STAT3 activation. However, SEK-1/MKK-4 [where SEK-1 stands for stress-activated protein kinase (SAPK)/ERK kinase 1, and MKK-4 stands for MAP kinase kinase 4] was activated in response to IL-6 and overexpression of dominant-negative SEK-1/MKK-4 (A-L) reduced both IL-6-induced STATS Ser(727) phosphorylation as well as STATS transactivation. Subsequently, the SEK-1/MKK-4 upstream components Vav, Rac-1 and MEKK were identified as components of a signal transduction cascade that leads to STAT3 transactivation in response to IL-S stimulation. Furthermore, inhibition of p38 kinase activity with the inhibitor SB203580 did not block STATS Ser727 phosphorylation but rather increased both basal as well as IL-6-induced STATS transactivation, indicating that p38 may act as a negative regulator of IL-6 induced STAT3 transactivation through a presently unknown mechanism. In conclusion, these data indicate that IL-6-induced STATS transactivation and Ser727 phosphorylation is independent of ERK-1 or JNK-1 activity, but involves a gp130 receptor signalling cascade that includes Vav, Rac-1, MEKK and SEK-1/MKK-4 as signal transduction components.