Exosomal RBP4 potentiated hepatic lipid accumulation and inflammation in high-fat-diet-fed mice by promoting M1 polarization of Kupffer cells

Exosomes containing various biological cargoes have potential to be novel diagnostic biomarkers for metabolic diseases. In this study, retinol-binding protein 4 (RBP4) was found to be enriched in serum exosomes, and its increased levels could be considered as an independent risk factor for the pathogenesis of nonalcoholic fatty liver disease (NAFLD). Exosomal RBP4 (exo-RBP4), primarily derived from hepatocytes, significantly enhanced the M1-like polarization of Kupffer cells (KCs) via promoting the activation of NOX2 and NF-κB and reactive oxygen species (ROS) accumulation, resulting in the over-production of inflammatory cytokines including TNF-α. Subsequently, those excess cytokines remarkably increased the levels of intracellular free fatty acid uptake and lipogenesis-related genes (FAS and SREBP-1c) but decreased fatty acid degradation-related genes (CPT-1 and PPARα) in palmitic acid-treated LO2 cells. More notably, TNF-α significantly elevated RBP4 transcription by activating STAT3 in hepatocytes, playing a positive role in NAFLD development. Intravenous injection with RBP4 (50 μg/kg) potentiated hepatic lipid accumulation, M1-type KC proportion, and serum pro-inflammatory cytokine levels in the hepatic tissues of high-fat-diet-fed mice. Collectively, these data indicated that exo-RBP4 converted KCs to M1 subtype by mediating the NOX2/ROS/NF-κB pathway, subsequently promoting de novo lipogenesis in hepatocytes by TNF-α secretion to activate the JAK2/STAT3 signaling pathway. Therefore, this study uncovered a novel intercellular communication between the inflammatory microenvironment and lipid metabolism for fostering NAFLD progression and found the potential of exo-RBP4 as a novel diagnostic biomarker and therapeutic target for NAFLD.