Hepatic activation of IKK/NFκB signaling induces liver fibrosis via macrophage-mediated chronic inflammation

Abstract

Liver damage in humans is induced by various insults including alcohol abuse, hepatitis B/C virus infection, autoimmune or metabolic disorders and, when persistent, leads to development of liver fibrosis. Because the nuclear factor-κB (NF-κB) system is activated in response to several of these stresses, we hypothesized that NF-κB activation in hepatocytes may contribute to fibrosis development. To activate the NF-κB signaling pathway in a time- and cell-type-specific manner in the liver, we crossed transgenic mice carrying the tetracycline-responsive transactivator under the control of the liver activator protein promotor with transgenic mice carrying a constitutively active form of the Ikbkb gene (IKK2 protein [CAIKK2]). Double-transgenic mice displayed doxycycline-regulated CAIKK2 expression in hepatocytes. Removal of doxycycline at birth led to activation of NF-κB signaling, moderate liver damage, recruitment of inflammatory cells, hepatocyte proliferation, and ultimately to spontaneous liver fibrosis development. Microarray analysis revealed prominent up-regulation of chemokines and chemokine receptors and this induction was rapidly reversed after switching off the CAIKK2 expression. Turning off the transgene expression for 3 weeks reversed stellate cell activation but did not diminish liver fibrosis. The elimination of macrophages by clodronate-liposomes attenuated NF-κB-induced liver fibrosis in a liver-injury-independent manner. Our results revealed that hepatic activation of IKK/NF-κB is sufficient to induce liver fibrosis by way of macrophage-mediated chronic inflammation. Therefore, agents controlling the hepatic NF-κB system represent attractive therapeutic tools to prevent fibrosis development in multiple chronic liver diseases.