β₃-Adrenergic receptor regulates hepatic apolipoprotein A-I gene expression

Area of Science:

• Cardiovascular Research
• Molecular Biology
• Pharmacology

Background:

• β₃-adrenergic receptor (β₃-AR) activation is known to increase hepatic apolipoprotein A-I (apoA-I) expression and reduce atherosclerotic plaques in vivo.
• The precise in vitro effects and molecular mechanisms of β₃-AR on apoA-I expression remain largely unknown.

Purpose of the Study:

• To elucidate the molecular mechanism by which β₃-AR activation regulates hepatic apoA-I gene expression in vitro.
• To investigate the role of protein kinase A (PKA) signaling in β₃-AR-mediated apoA-I regulation.

Main Methods:

• Hepatoma cells (HepG2) were treated with selective β₃-AR agonists/antagonists and a PKA inhibitor.
• Gene expression and protein secretion of apoA-I were quantified using RT-qPCR, Western blot, and ELISA.
• ApoA-I promoter activity was assessed using dual-luciferase reporter assays.
• Chromatin immunoprecipitation (ChIP) assays evaluated the binding of transcription factors (HNF-4, HNF-3, Egr-1) to the apoA-I promoter.

Main Results:

• β₃-AR activation significantly increased apoA-I expression, secretion, and promoter activity in hepatocytes.
• Activation of HNF-4 and HNF-3 binding to the apoA-I promoter was enhanced by β₃-AR, while Egr-1 binding was unaffected.
• PKA inhibition partially blocked the activation of the apoA-I promoter and transcription factors, and significantly reduced β₃-AR-induced apoA-I upregulation.

Conclusions:

• β₃-AR activation upregulates hepatic apoA-I expression through enhanced apoA-I promoter activity, driven by increased HNF-4 and HNF-3 binding, potentially mediated by PKA signaling.
• These findings suggest that β₃-AR may exert anti-atherosclerotic effects by boosting hepatic apoA-I expression and promoting cholesterol reverse transport.