Intracellular pigment epithelium-derived factor contributes to triglyceride degradation

Abstract

Pigment epithelium-derived factor is well known as a secreted glycoprotein with multiple functions, such as anti-angiogenic, neuroprotective and anti-tumor activities. However, its intracellular role remains unknown. The present study was performed to demonstrate the intracellular function of pigment epithelium-derived factor on triglyceride degradation. Hepatic pigment epithelium-derived factor levels increased at the early stage and subsequently decreased after 16 weeks in high-fat-diet-fed mice compared to those in chow-fed mice. Similarly, oleic acid led to long-term downregulation of pigment epithelium-derived factor in HepG2 cells. Endogenous pigment epithelium-derived factor was an intracellular protein with cytoplasmic distribution in hepatocytes by immunostaining. Exogenous FITC-labeled pigment epithelium-derived factor could be absorbed into hepatocytes. Both signal peptide deletion and full-length pigment epithelium-derived factor transfection HeLa cells and hepatocytes promoted triglyceride degradation. Intracellular pigment epithelium-derived factor co-immunoprecipitated with adipose triglyceride lipase and promoted triglyceride degradation in an adipose triglyceride lipase-dependent manner. Additionally, pigment epithelium-derived factor bound to the C-terminal of adipose triglyceride lipase (aa268–504) and adipose triglyceride lipase-G0/G1 switch gene-2 complex simultaneously, which facilitated adipose triglyceride lipase-G0/G1 switch gene-2 translocation onto lipid droplet using bimolecular fluorescence complementation assay. Moreover, knockdown of endogenous pigment epithelium-derived factor in hepatocytes diminished triglyceride degradation. Taken together, these results indicate that hepatic pigment epithelium-derived factor was decreased in obese mice accompanied with hepatic steatosis. Intracellular pigment epithelium-derived factor binds to and facilitates adipose triglyceride lipase translocation onto lipid droplet, which promotes triglyceride degradation. These findings suggest that a decreased level of hepatic pigment epithelium-derived factor may contribute to hepatic steatosis in obesity.