Abstract
Elevated plasma triglyceride levels represent a risk factor for premature atherosclerosis. In mice, accumulation of triglyceride-rich lipoproteins can occur if sulfation of heparan sulfate in hepatocytes is diminished, as this alters hepatic lipoprotein clearance via heparan sulfate proteoglycans (HSPGs). However, the relevant HSPG has not been determined. In this study, we found by RT-PCR analysis that mouse hepatocytes expressed the membrane proteoglycans syndecan-1, -2, and -4 and glypican-1 and -4. Analysis of available proteoglycan-deficient mice showed that only syndecan-1 mutants (Sdc1-/- mice) accumulated plasma triglycerides. Sdc1-/- mice also exhibited prolonged circulation of injected human VLDL and intestinally derived chylomicrons. We found that mice lacking both syndecan-1 and hepatocyte heparan sulfate did not display accentuated triglyceride accumulation compared with single mutants, suggesting that syndecan-1 is the primary HSPG mediating hepatic triglyceride clearance. Immunoelectron microscopy showed that syndecan-1 was expressed specifically on the microvilli of hepatocyte basal membranes, facing the space of Disse, where lipoprotein uptake occurs. Abundant syndecan-1 on wild-type murine hepatocytes exhibited saturable binding of VLDL and inhibition by heparin and facilitated degradation of VLDL. Furthermore, adenovirus-encoded syndecan-1 restored binding, uptake, and degradation of VLDL in isolated Sdc1-/- hepatocytes and the lipoprotein clearance defect in Sdc1-/- mice. These findings provide the first in vivo genetic evidence that syndecan-1 is the primary hepatocyte HSPG receptor mediating the clearance of both hepatic and intestinally derived triglyceride-rich lipoproteins.