Akt Modulates STAT3-mediated Gene Expression through a FKHR (FOXO1a)-dependent Mechanism

Marcin Kortylewski,Florian Feld,Klaus-Dieter Krüger,Gregor Bahrenberg,Richard A Roth,Hans-Georg Joost,Peter C Heinrich,Iris Behrmann,Andreas Barthel

doi:10.1074/jbc.m205403200

Abstract

The phosphatidylinositol 3-kinase/Akt pathway plays an important role in the signaling of insulin and other growth factors, which reportedly attenuate the interleukin-6 (IL-6)-mediated stimulation of acute phase plasma protein genes. We investigated the effect of the protein kinase Akt on IL-6-mediated transcriptional activation. The transient expression of constitutively active Akt inhibited the IL-6-dependent activity of the alpha(2)-macroglobulin promoter in HepG2 cells, whereas expression of an inactive mutant of phosphatidylinositol-dependent kinase 1 had the opposite effect. Since Akt is known to regulate gene expression through inactivation of the transcription factor FKHR (forkhead in rhabdomyosarcoma), we examined the effect of FKHR on STAT3-mediated transcriptional regulation. Indeed, the overexpression of FKHR specifically enhanced the activity of STAT3-dependent promoters but not that of a STAT5-responsive promoter. The effect of FKHR required the presence of functional STAT3 and was abrogated by the expression of dominant negative STAT3 mutants. Furthermore, FKHR and STAT3 were shown to coimmunoprecipitate and to colocalize in the nuclear regions of IL-6-treated HepG2 cells. Our results indicate that FKHR can modulate the IL-6-induced transcriptional activity by acting as a coactivator of STAT3.

Highlights

IL-61 is the major regulator of acute phase protein (APP) synthesis by the liver during the inflammatory response [1]
Gene Activation through a FKHR-dependent Mechanism—In agreement with previous studies [4, 5], we found that insulin inhibits the IL-6-induced expression of several acute phase plasma proteins, including ␣2-M in hepatoma cells
The effects observed using wild type forms of both kinases were less pronounced. These results suggested that Akt kinase activity can negatively influence IL-6-dependent gene expression

Summary

Introduction

IL-61 is the major regulator of acute phase protein (APP) synthesis by the liver during the inflammatory response [1]. The potential inhibitory role of growth promoting signals on the IL-6-inducible Jak/ STAT3 pathway corresponds well with the suppressed acute phase response in regenerating liver [6]. One of the major effects of signaling via the insulin receptor and other growth factor receptors is the activation of PI 3-kinase [7]. Activated Akt has been described to translocate to the nucleus [8] and to directly phosphorylate members of the forkhead family of transcription factors (9 –11). We have identified FKHR as a specific transcriptional coactivator of STAT3 This functional interaction reflects the association of both proteins and their colocalization in nuclear regions of HepG2 cells

Results

Discussion

Conclusion