C/EBPa regulates human adiponectin gene transcription through an intronic enhancer

Adiponectin is an adipose-derived hormone that enhances insulin sensitivity and plays an important role in regulating energy homeostasis. Here, we demonstrate that the DNA encoding the first intron of the human adiponectin gene contains an intronic enhancer that regulates adiponectin gene expression in an adipose tissue-specific manner. Insertion of the DNA encoding the first intron into reporter constructs containing the proximal adiponectin promoter (Pro-Int1-Luc) resulted in a 20-fold increase in activity relative to the promoter alone in 3T3-L1 adipocytes. Coexpression of CCAAT/enhancerbinding protein (C/EBP)a increased luciferase activity of the Pro-Int1-Luc construct 75-fold but had no effect on the constructs containing the proximal adiponectin promoter alone. At least eight potential C/EBPa response elements are located between +3000 to +10000 nucleotides within the DNA encoding the first intron, including a 34-by core sequence for the intronic enhancer that contains three tandem C/EBPa response elements. However, the intronic enhancer is not conserved between human and mouse. Overexpression or siRNA-mediated knockdown of endogenous C/EBPa significantly increased or decreased, respectively, adiponectin mRNA levels in differentiated human Chub-S7 adipocytes, while neither C/EBPb nor C/EBPd significantly affected adiponectin expression in mature adipocytes. Thus, C/EBPa is a key transcription factor for full activation of human adiponectin gene transcription in mature adipocytes through interaction with response elements in the intronic enhancer.