Roles of the membrane protein CD36 in fatty acid transport and metabolism

CD36 is a membrane protein found in many types of cells. It is highly expressed in cells with high levels of metabolic activities, such as adipocytes, muscle and liver cells. Accumulating evidence has suggested that CD36 plays an important role in diabetes and obesity where abnormal fatty acid uptake occurs. However, the mechanism by which CD36 regulates fatty acid uptake is poorly understood. Fatty acid uptake includes two associated events, transport across the plasma membrane and the subsequent metabolism. Deciphering the mechanism requires developing methods that allow separate determination of the role of CD36 in each step. Our laboratory has previously developed a fluorescence-based assay to monitor fatty acid transmembrane movement in real-time, and the results showed that fatty acids diffuse through the protein-free phospholipid bilayer rapidly (<2s). In this study, we used this technique to investigate fatty acid transport in a HEK293 cell line that stably expresses CD36. Our results showed that fatty acids diffuse through the plasma membrane rapidly (<10 s) and the rate is not affected by the overexpression of CD36. The same kinetics were also observed in wild type and CD36 null mouse adipocytes, suggesting that the transmembrane movement of fatty acid is independent of CD36. Our quantitative analysis of fatty acid metabolism using [14 C]- oleate revealed that CD36 significantly enhances triglyceride synthesis. Fatty acid esterification occurs in a time scale of mm, a much slower process compared to the transmembrane movement, indicating that intracellular metabolism, rather than the membrane transport, is the rate-limiting step in controlling fatty acid uptake. Single cell imaging of the cellular uptake of BODIPY-fatty acid confirmed that CD36 enhances lipid droplet accumulation. We further performed real-time PCR measurement using a PCR array that incorporated 84 genes involved in fatty acid metabolism. Several genes were identified with significant up- and down-regulation by overexpression of CD36. These genes encode enzymes involved mainly in fatty acid beta-oxidation and esterification, suggesting that beta-oxidation may also be increased, but with a predominant effect on esterification. While our findings suggest that CD36 controls fatty acid uptake by regulating intracellular metabolism, the detailed mechanism needs further investigation.