BackgroundTransport of fatty acids within the cytosol of adipocytes and their subsequent assimilation into lipid droplets has been thoroughly investigated; however, the mechanism by which fatty acids are transported across the plasma membrane from the extracellular environment remains unclear. Since triacylglycerol-rich lipoproteins represent an abundant source of fatty acids for adipocyte utilization, we have investigated the expression levels of cell surface lipoprotein receptors and their functional contributions toward intracellular lipid accumulation; these include very low density lipoprotein receptor (VLDL-R), low density lipoprotein receptor-related protein (LRP), and heparan sulfate proteoglycans (HSPG).ResultsWe found that expression of these three lipoprotein receptors increased 5-fold, 2-fold, and 2.5-fold, respectively, during adipocyte differentiation. The major proteoglycans expressed by mature adipocytes are of high molecular weight (>500 kD) and contain both heparan and chondroitin sulfate moieties. Using ligand binding antagonists, we observed that HSPG, rather than VLDL-R or LRP, play a primary role in the uptake of DiI-lableled apoE-VLDL by mature adipocytes. In addition, inhibitors of HSPG maturation resulted in a significant reduction (>85%) in intracellular lipid accumulation.ConclusionsThese results suggest that cell surface HSPG is required for fatty acid transport across the plasma membrane of adipocytes.

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PurposeLipid accumulation within cells has been implemented in the pathogenesis of obesity and atherosclerosis, two factors that contribute to the development of cardiovascular disease. Central to obesity and atherosclerosis is the mechanism by which lipoproteins transport and carry lipids to tissues. The delivery of cholesterol and triacylglycerides to hepatic and extra-hepatic tissues is facilitated by remnant lipoproteins such as very low density lipoprotein (VLDL) and chylomicrons. The clearance of VLDL is a critical component in the development of atherosclerosis, yet the exact mechanism by which this occurs remains unclear. Prior research implements simple diffusion and receptor-mediated transport as models for lipoprotein remnant clearance. It has also been shown that heparan sulfate proteoglycan (HSPG) plays a role in the hepatic clearance of plasma remnant lipoproteins. We have investigated the expression levels of known cell surface lipoprotein receptors and their functional contributions toward intracellular lipid accumulation in adipocytes; these include very low density lipoprotein receptor (VLDL-R), low density lipoprotein receptor-related protein (LRP), and HSPG.MethodsFibroblast (3T3-L1) induced adipocytes were cultured and protein expression levels of VLDL-R, LRP and HSPG were measured. Mature adipocytes were incubated with DiI-labeled apoE-VLDL in the presence of heparin (to inhibit HSPG activity) or RAP (to inhibit LRP and VLDLR activity) and the cells were visualized by fluorescent microscopy. Adipocytes were grown with or without ligand binding antagonists for HSPG and the cells were stained with Oil Red O and lipid accumulation was quantified with spectrophotometry.ResultsProtein expression levels of all three lipoprotein receptors increased during adipocyte differentiation. Using ligand binding antagonists, we identified that HSPG, rather than VLDL-R or LRP, play a primary role in the uptake of DiI-labeled apoE-VLDL by mature adipocytes. In addition, incubation of adipocytes with xyloside inhibitors of HSPG maturation resulted in a significant reduction of intracellular lipid accumulation.ConclusionsThese results suggest that cell surface HSPG is an essential component of fatty acid transport across the plasma membrane of adipocytes. Our observations will aid in our understanding of the complex mechanism that leads to obesity. Future research in this area may elucidate a target for the reduction of intracellular lipid accumulation.

Purpose: Lipid accumulation within cells has been implemented in the pathogenesis of obesity and atherosclerosis, two factors that contribute to the development of cardiovascular disease. Central to obesity and atherosclerosis is the mechanism by which lipoproteins transport and carry lipids to tissues. The delivery of cholesterol and triacylglycerides to hepatic and extra-hepatic tissues is facilitated by remnant lipoproteins such as very low density lipoprotein (VLDL) and chylomicrons. The clearance of VLDL is a critical component in the development of atherosclerosis, yet the exact mechanism by which this occurs remains unclear. Prior research implements simple diffusion and receptor-mediated transport as models for lipoprotein remnant clearance. It has also been shown that heparan sulfate proteoglycan (HSPG) plays a role in the hepatic clearance of plasma remnant lipoproteins. We have investigated the expression levels of known cell surface lipoprotein receptors and their functional contributions toward intracellular lipid accumulation in adipocytes; these include very low density lipoprotein receptor (VLDL-R), low density lipoprotein receptor-related protein (LRP), and HSPG. Methods: Fibroblast (3T3-L1) induced adipocytes were cultured and protein expression levels of VLDL-R, LRP and HSPG were measured. Mature adipocytes were incubated with DiI-labeled apoE-VLDL in the presence of heparin (to inhibit HSPG activity) or RAP (to inhibit LRP and VLDLR activity) and the cells were visualized by fluorescent microscopy. Adipocytes were grown with or without ligand binding antagonists for HSPG and the cells were stained with Oil Red O and lipid accumulation was quantified with spectrophotometry. Results: Protein expression levels of all three lipoprotein receptors increased during adipocyte differentiation. Using ligand binding antagonists, we identified that HSPG, rather than VLDL-R or LRP, play a primary role in the uptake of DiI-labeled apoE-VLDL by mature adipocytes. In addition, incubation of adipocytes with xyloside inhibitors of HSPG maturation resulted in a significant reduction of intracellular lipid accumulation. Conclusions: These results suggest that cell surface HSPG is an essential component of fatty acid transport across the plasma membrane of adipocytes. Our observations will aid in our understanding of the complex mechanism that leads to obesity. Future research in this area may elucidate a target for the reduction of intracellular lipid accumulation.