Probing Mechanism for the Enhancement of Uptake of Fatty Acid into Cells by the Membrane Protein CD36

Abstract

CD36 is a membrane protein found in various cell types including adipocytes and endothelial cells. Physiological studies have suggested that CD36 is associated with insulin-resistant diabetes, however, the mechanism remains poorly understood. It is well recognized as a receptor for multiple ligands such as collagen and thrombospondin 1, and possibly acts as a catalyst to enhance the rate of transmembrane movement of fatty acid. To separate the two independent events transport across the plasma membrane and the subsequent intracellular metabolism, we have applied biophysical approaches and metabolic analyses to cells in vitro. Our results using cultured cells (HEK293 cells overexpressing CD36 and 3T3-L1 adipocyte cells) as well as mice adipocyte cells isolated from a CD36 null mouse showed that fatty acids diffuse through the plasma membrane rapidly with or without CD36. In HEK 298 cells, which normally synthesize triglycerides very slowly and to a limited extent, expression of CD36 enhanced the rate and extent of synthesis. Even in the presence of CD36, incorporation into triglycerides is a much slower process (min) relative to the transmembrane movement (sec), indicating that the rate-limiting step of the regulation of fatty acid uptake by CD36 is intracellular metabolism. Lastly, by PCR array analysis of 84 proteins, we have identified several enzymes involved in human fatty acid metabolism with gene expression levels altered by overexpression of CD36. Taken together, our results showed that CD36 increases fatty acid uptake by enhancing triglycerides synthesis rather than acting as a membrane transporter, but as yet by unidentified molecular mechanisms.