Children with cholestatic liver diseases including extrahepatic biliary atresia (EHBA) exhibit an acquired growth hormone (GH) resistance, in which GH secretion is normal, but hepatic expression of the GH receptor (GHR) and synthesis of anabolic target genes including IGF-I is reduced. Consequences include poor linear growth, muscle wasting, and increased post-transplant morbidity and mortality. The molecular basis for impaired GH signaling in obstructive cholestasis is not known. We hypothesize that down regulation of the GHR by TNFalpha and up regulation of Suppressors of Cytokine Signaling 3 (SOCS-3) by IL-6 may combine to reduce GH signaling in this setting. We will test this hypothesis in the following aims:
Aim 1 : Characterize cytokine regulation of liver Ghr and Socs-3 gene expression. Cytokine response elements in the mouse Socs-3 promoter will be identified by examining regulation of gene promoter reporter constructs in cultured hepatocytes. We will then determine whether TNFalpha down regulates Ghr expression via Sp1/Sp3 de-phosphorylation leading to reduced DNA binding affinity, and whether IL-6 up regulates Socs-3 expression by increasing STAT3 phosphorylation and nuclear abundance. These data will identify mechanisms by which cytokines regulate target genes involved in GH signaling via alterations in transcription factor phosphorylation.
Aim 2 : Identify mechanisms of GH resistance in obstructive cholestasis. Our preliminary studies have demonstrated that bile duct ligated (BDL) mice exhibit GH resistance which reproduces that observed in children with EHBA. This is associated with hepatic up regulation of TNFalpha and IL-6, down regulation of the Ghr, and up regulation of Socs-3. Abundance of these proteins relative to GH activation of STAT5 will be examined in sham, pair fed, and BDL wild type (WT), TNF receptor 1 (TNFR1) null, and IL-6 null mice. These data will determine whether disruption of TNFalpha or IL-6 signaling will restore GH action in obstructive cholestasis. These findings will have implications for therapeutic strategies to ameliorate acquired GH resistance in diseases complicated by liver inflammation, including chronic cholestatic liver diseases
