Abstract
Hepatic insulin resistance (IR) is associated with liver inflammatory diseases, but molecular mechanisms for the association remained elusive. IR is known to increase activity of forkhead box-containing protein O subfamily-1 (FOXO1), a transcription factor that was recently shown to enhance proinflammatory cytokine production in macrophages and adipocytes. Here we report that overexpression of constitutively active FOXO1 markedly increased chemokine ligand 20 (CCL20) expression and secretion in HepG2 hepatoma cells treated with TNF-α. The opposite was seen when endogenous FOXO1 was silenced. FOXO1 did not bind CCL20 promoter directly; instead, it potentiated CCL20 transcription through increasing the binding of p65/p50 heterodimer to a functional nuclear factor-κB site in the human CCL20 promoter. The conditional medium from TNF-α-treated HepG2 cells stimulated migration of human peripheral blood mononuclear cells. This stimulation was significantly enhanced when FOXO1 was overexpressed, and attenuated when FOXO1 was silenced. CCL20 antibody partly blocked the synergistic effect of FOXO1 and TNF-α on peripheral blood mononuclear cells migration. Additionally, TNF-α antagonizes the insulin/Akt signal transduction, thus leading to activation of FOXO1, which is capable of mediating a transcriptional activation role in response to TNF-α on CCL20 gene expression in HepG2 cells and promotes lymphocyte chemotaxis. Furthermore, we found that FOXO1 and CCL20 were coordinately up-regulated in the insulin resistant and inflammatory cell-infiltrated liver of db/db mice, an animal model that displayed hepatic and systemic low-grade inflammation. In conclusion, our data suggest that FOXO1 links IR to lymphocyte chemotaxis in the insulin-resistant hepatocytes and livers by amplifying nuclear factor-κB-dependent hepatic CCL20 production.