Chemically synthesized osteocalcin alleviates NAFLD via the AMPK-FOXO1/BCL6-CD36 pathway

Nonalcoholic fatty liver disease (NAFLD) is a common chronic liver disease worldwide. Osteocalcin plays an important role in energy metabolism. In this study, we investigated the mechanism of action of chemically synthesized osteocalcin (csOCN) in ameliorating NAFLD. We demonstrated for the first time that csOCN attenuates lipid accumulation in the liver and hepatocytes by modulating CD36 protein expression. In addition, we found that the expression of p-AMPK, FOXO1 and BCL6 decreased and the expression of CD36 increased after OA/PA induction compared to the control group, and these effects were reversed by the addition of csOCN. In contrast, the therapeutic effect of csOCN was inhibited by the addition of AMPK inhibitors and BCL6 inhibitors. This finding suggested that csOCN regulates CD36 expression via the AMPK-FOXO1/BCL6 axis. In NAFLD mice, oral administration of csOCN also activated the AMPK pathway and reduced CD36 expression. Molecular docking revealed that osteocalcin has a docking site with CD36. Compared to oleic acid and palmitic acid, osteocalcin bound more strongly to CD36. Laser confocal microscopy results showed that osteocalcin colocalized with CD36 at the cell membrane. In conclusion, we demonstrated the regulatory role of csOCN in fatty acid uptake pathways for the first time; it regulates CD36 expression via the AMPK-FOXO1/BCL6 axis to reduce fatty acid uptake, and it affects fatty acid transport by may directly binding to CD36. There are indications that csOCN has potential as a CD36-targeted drug for the treatment of NAFLD.