Lipoproteins
The transport of cholesterol and triglycerides through the circulation occurs by way of lipoproteins. The triglyceride rich lipoproteins consist of chylomicrons, which arise from dietary fats, and very low density lipoproteins (VLDL), which come from de novo synthesis of triglycerides in the liver. Lipoprotein lipase (LPL) acts on these particles to generate free fatty acids for energy production in the heart and skeletal muscles and for storage in adipose tissue. The resulting remnant particles are further acted on by hepatic lipase (HL) in the liver sinusoids, and are subsequently cleared by receptors in the liver, which recognize the apolipoproteins, apoB and apoE, or bound lipases. Heparan sulfate binds to several proteins important to lipoprotein metabolism, including apoB, apoE, LPL, and HL.
We altered the expression of the biosynthetic gene GlcNAc N-deacetylase/N-sulfotransferase-1 (Ndst1) in hepatocytes using the Cre-loxP system, which resulted in ~50% reduction of sulfation of liver heparan sulfate. Mice were viable and healthy, but they accumulated triglyceride-rich lipoprotein particles resembling chylomicron remnants and VLDL. Mutant mice synthesized VLDL normally, but showed reduced plasma clearance of human VLDL and a corresponding reduction in hepatic VLDL uptake. Compounding the mutation with LDL receptor deficiency caused enhanced accumulation of both cholesterol- and triglyceride-rich particles compared to mice lacking only LDL receptors, suggesting that heparan sulfate participates in the clearance of cholesterol-rich lipoproteins as well. Post-prandial studies showed that clearance of intestinally derived lipoproteins also depends on hepatocyte heparan sulfate. These findings show that under normal physiological conditions hepatic heparan sulfate proteoglycans work in parallel with LDL receptors to clear both intestinally-derived and hepatic lipoprotein particles. Current studies focus on the individual proteoglycans that modulate lipoprotein metabolism and the endocytic mechanism by which proteoglycans mediate clearance and metabolism.
In a separate project, we discovered that mutants lacking collagen XVIII, a type of heparan sulfate proteoglycan present in the basement membranes underlying endothelial cells, also accumulate plasma triglycerides and mild hyperchylomicronemia after fat feeding. Remnant lipoprotein clearance in the liver and hepatic triglyceride synthesis remained unchanged, suggesting that the defect arose in the peripheral circulation. Hypertriglyceridemia appears to result from reduced presentation of lipoprotein lipase on the lumenal surface of the endothelium and delayed lipolysis in the peripheral circulation. This is the first report showing that triglyceride-rich lipoprotein metabolism in vivo can be affected, albeit indirectly, by a basement membrane proteoglycan. We also showed that humans with Knobloch Syndrome, caused by a null mutation in the heparan sulfate proteoglycan Type XVIII collagen, also exhibit fasting hypertriglyceridemia, a previously unrecognized phenotype in these patients.
Taken together, these studies suggest that the enzymes involved in heparan sulfate biosynthesis and specific proteoglycans constitute candidate genes for screening humans with hypertriglyceridemia of unknown origin. Alterations in heparan sulfate proteoglycan formation could also explain hypertriglyceridemia arising from various drug treatments or as sequelae to other pathological disorders, such as diabetes.
Relevant Papers
MacArthur, J.M., Bishop, J.R., Wang, L., Stanford, K.I., Bensadoun, A., Witztum, J.L., and Esko, J.D. (2007) Liver heparan sulfate proteoglycans mediate clearance of triglyceride-rich lipoproteins independently of LDL receptor family members. J. Clin. Invest. 117:153-164. Commentary: Mahley, R.W. and Huang, Y. (2007) J. Clin. Invest. 117:94-98
Bishop, J.R., Stanford, K.I. and Esko, J.D. (2008) Heparan sulfate proteoglycans and triglyceride-rich lipoprotein metabolism. Curr. Opin. Lipidol. 19:307-313.
Bishop, J.R., Stanford, K.I., Yates, J.R., Bensadoun, A., Moulton, K.S. and Esko, J.D. (2008) Deletion of the basement membrane heparan sulfate proteoglycan Type 18 Collagen causes hypertriglyceridemia in mice. PLOS Biology, submitted.