Kruppel-like factor 4 is a mediator of proinflammatory signaling in macrophages.

ABSTRACT

Activation of macrophages is important in chronic inflammatory disease states such as atherosclerosis. Proinflammatory cytokines such as interferon-gamma (IFN-gamma), lipopolysaccharide (LPS), or tumor necrosis factor-alpha can promote macrophage activation. Conversely, anti-inflammatory factors such as transforming growth factor-beta1 (TGF-beta1) can decrease proinflammatory activation. The molecular mediators regulating the balance of these opposing effectors remain incompletely understood. Herein, we identify Kruppel-like factor 4 (KLF4) as being markedly induced in response to IFN-gamma, LPS, or tumor necrosis factor-alpha and decreased by TGF-beta1 in macrophages. Overexpression of KLF4 in J774a macrophages induced the macrophage activation marker inducible nitric-oxide synthase and inhibited the TGF-beta1 and Smad3 target gene plasminogen activator inhibitor-1 (PAI-1). Conversely, KLF4 knockdown markedly attenuated the ability of IFN-gamma, LPS, or IFN-gamma plus LPS to induce the iNOS promoter, whereas it augmented macrophage responsiveness to TGF-beta1 and Smad3 signaling. The KLF4 induction of the iNOS promoter is mediated by two KLF DNA-binding sites at -95 and -212 bp, and mutation of these sites diminished induction by IFN-gamma and LPS. We further provide evidence that KLF4 interacts with the NF-kappaB family member p65 (RelA) to cooperatively induce the iNOS promoter. In contrast, KLF4 inhibited the TGF-beta1/Smad3 induction of the PAI-1 promoter independent of KLF4 DNA binding through a novel antagonistic competition with Smad3 for the C terminus of the coactivator p300/CBP. These findings support an important role for KLF4 as a regulator of key signaling pathways that control macrophage activation.