Abstract 120: Genetic And Physiological Role For Hepatic C/EBPα In Plasma Lipid Metabolism

Abstract

CCAAT/enhancer-binding protein alpha (C/EBPa) is a transcription factor known to mediate glucose and lipid metabolism. Hepatic protein levels of C/EBPa are controlled by the pseudokinase Tribbles-1 ( TRIB1 ), a gene which has repeatedly been linked to plasma lipids and coronary artery disease by human genome-wide association studies. Previous work has shown that genetic perturbation of hepatic Trib1 in mice alters plasma lipids. However, it is unknown if C/EBPa governs the relationship between Trib1 and plasma lipids. To investigate this, we first reasoned that if C/EBPa does govern this relationship, then human CEBPA should also be a GWAS hit for plasma lipids in existing data. Indeed, there is a GWAS locus for HDL cholesterol and triglycerides (TGs) at Chr19q13.11, with the lead SNP located 80kb downstream of CEBPA in an intron of the annotated gene PEPD . To see if this GWAS locus is identifying C/EBPa, we performed CRISPR deletion and CRISPR interference (CRISPRi) of the SNP locus in human hepatocytes and found that doing so reduced CEBPA gene expression (CRISPR: -85.5%, p<0.01; CRISPRi: -29.7%, p<0.005), confirming that the 19q13 GWAS locus regulates CEBPA gene expression. We next sought to determine how hepatic C/EBPa regulates lipids by performing hepatic knockout of Cebpa via AAV-Cre treatment in adult mice. Hepatic knockout of C/EBPa significantly reduced plasma lipids (-21%, p<0.005). RNA-seq analysis of livers from these mice identified changes in expression of known regulators of plasma lipids, including reduced Pcsk9 expression (-60.5%, p<0.005). Subsequent ELISA analysis confirmed that mice lacking hepatic C/EBPa have reduced circulating PCSK9 (-54%, p<0.005). Crossing these mice to a transgenic mouse expressing human apoB confirmed that these mice have reduced non-HDL cholesterol (-49.7%, p<0.01). Ongoing studies seek to confirm the PCSK9 phenotype in human hepatoma cells, while also investigating how C/EBPa may regulate plasma TGs and HDL cholesterol. In summary, we demonstrate here that human CEBPA is likely a causal gene at the 19q13 plasma lipid GWAS locus, and that in mice, Cebpa is a novel regulator of plasma PCSK9 and non-HDL cholesterol. These data illustrate the translational relevance of targeting hepatic C/EBPa in adult animals.