Abstract 540: Very Low Density Lipoprotein Assembly is Required for Camp Responsive Element-binding Protein H Processing and Hepatic Apolipoprotein A-IV Expression in Mouse Models of Acute Steatosis

Background: Our previous studies have demonstrated that hepatic expression of apolipoprotein A-IV (apoA-IV) is increased in mouse models of chronic steatosis and is closely correlated with hepatic triglyceride (TG) content. We have also shown that steatosis-induced hepatic apoA-IV gene expression is regulated by processing of the nuclear transcription factor cAMP responsive element-binding protein H (CREBH). Herein, we explored the mechanisms that mediate steatosis-induced CREBH processing. Methods: We measured hepatic CREBH processing, apoA-IV gene expression, and lipid content in several mouse models of attenuated or enhanced VLDL assembly that were subjected to acute steatosis induced by a 16 hour overnight fast or by feeding a ketogenic diet for 6 days. Results: Both fasting and the ketogenic diet induced acute hepatic TG accumulation associated with increased CREBH processing and apoA-IV gene expression, which were associated with hepatic TG content in C57BL/6 mice. All mouse models of attenuated VLDL secretion (shRNA-induced apoB knock down, liver specific microsomal triglyceride transfer protein (MTP) knockout, treatment with the MTP inhibitor BMS212122, and comparative gene identification-58 (CGI-58) deficiency) had increased hepatic TG accumulation, but displayed repressed CREBH processing and reduced apoA-IV gene expression compared to controls. When deficient VLDL assembly in liver-specific MTP knockout mice was reconstituted by adenoviral infection with a human MTP transgene, steatosis-induced CREBH processing and apoA-IV expression were restored. In a mouse model of enhanced VLDL assembly (transgenic overexpression of human MTP), apoA-IV gene expression correlated with bulk hepatic TG accumulation, but not with VLDL secretion rate, indicating that other steatosis-related factors participate in apoA-IV gene regulation. Conclusions: Taken together, these data provide compelling evidence that VLDL assembly and secretion is required for hepatic CREBH processing and enhanced apoA-IV gene expression in the setting of acute steatosis. These data further suggest that CREBH and apoA-IV play central roles in VLDL-mediated hepatic lipid efflux.