Endocrine Abstracts

Fatty liver (steatosis) can progress to non-alcoholic fatty liver disease, increasing the risk of diabetes and cardiovascular disease. Growth hormone (GH) deficiency is associated with steatosis, while raising GH reduces steatosis. To date it remains to be determined how GH mediates this hepato-protective effect. To investigate the hepatocyte-specific actions of GH, a mouse model of adult-onset hepatocyte-specific GH receptor knockdown (aHepGHRkd) was generated by treating adult mice, homozygous for the GHR-floxed allele, with an adeno-associated viral vector expressing a thyroxin-binding globulin promoter driven Cre recombinase (AAV8-TBGp-Cre). Mice treated with an AAV8-TBGp-Null serve as GHR-intact controls. We reported that 7d-post aHepGHRkd, steatosis develops associated with enhanced expression of glucokinase (GCK), ketohexokinase (KHK) and de novo lipogenesis (DNL) genes, and increased rate of DNL which was measured by deuterated water labeling. New data demonstrate the aHepGHRkd-mediated alterations in liver phenotype persist under thermoneutral conditions (mice housed at 30C). Also, the increase in cytoplasmic GCK protein (active), but not KHK, occurs just 3d-post aHepGHRkd, suggesting enhanced glycolysis may be an initiating event to drive enhanced DNL, since the expression of KHK is upregulated by carbohydrate response element binding protein (ChREBP), a transcription factor activated by glycolytic metabolites. In fact, in preliminary studies we found that knockdown of hepatocyte ChREBP, in aHepGHRkd mice, prevented the rise in KHK, but did not prevent the rise in GCK or steatosis. Since GHR signals through JAK2/STAT5b to regulate many genes in hepatocytes, we sought to determine if restoration of STAT5b activity in aHepGHRkd would normalize the liver phenotype. To this end, we co-treated a subset of aHepGHRkd mice with an AAV expressing a constitutive active form of STAT5b (AAV8-TBGp-STAT5b^CA). Since regulation of hepatic lipid accumulation is dynamically mediated by multiple hormones and substrate availability, we compared the impact of aHepGHRkd, without or with STAT5b^CA under multiple nutritional conditions: natural fasting (10h after food withdrawal at 0600h), overnight fasting (16h), or overnight fasting with 6h refeeding. Data collected thus far show hepatocyte STAT5b, in the absence of GHR, can suppress GCK expression/activity, but this is not always associated with a reduction in steatosis or DNL, depending on the nutritional state. These findings imply that the GHR may signal independent of STAT5b to suppress DNL.