Partnership of PGC-1α and HNF4α in the Regulation of Lipoprotein Metabolism

Abstract

Peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha) is a transcriptional coactivator involved in several aspects of energy metabolism. It is induced or activated under different stimuli in a highly tissue-specific manner and subsequently partners with certain transcription factors in those tissues to execute various biological programs. In the fasted liver, PGC-1alpha is induced and interacts with hepatocyte nuclear factor 4alpha (HNF4alpha) and other transcription factors to activate gluconeogenesis and increase hepatic glucose output. Given the broad spectrum of liver genes responsive to HNF4alpha, we sought to determine those that were specifically targeted by the combination of PGC-1alpha and HNF4alpha. Coexpression of these two molecules in murine stem cells reveals a high induction of mRNA for apolipoproteins A-IV and C-II. Forced expression of PGC-1alpha in mouse and human hepatoma cells increases the mRNA of a subset of apolipoproteins implicated in very low density lipoprotein and triglyceride metabolism, including apolipoproteins A-IV, C-II, and C-III. Coactivation of the apoC-III/A-IV promoter region by PGC-1alpha occurs through a highly conserved HNF4alpha response element, the loss of which completely abolishes activation by PGC-1alpha and HNF4alpha. Adenoviral infusion of PGC-1alpha into live mice increases hepatic expression of apolipoproteins A-IV, C-II, and C-III and increases serum and very low density lipoprotein triglyceride levels. Conversely, knock down of PGC-1alpha in vivo causes a decrease in both apolipoprotein expression and serum triglyceride levels. These data point to a crucial role for the PGC-1alpha/HNF4alpha partnership in hepatic lipoprotein metabolism.