Processing of LDL and HDL differed notably after initial binding via EL to the cell surface. More than 90% of the surface-bound125 I-LDL was destined for internalization and degradation, whereas about 70% of the surfacebound 125 I-HDL 3 was released back into the medium. These differences were significantly attenuated after HDL clustering was promoted using antibody against apolipoprotein A-I. At equal protein concentration of added lipoproteins the ratio of HDL 3 to VLDL bridging via EL was 0.092 compared with 0.174 via HL and 0.002 via LpL. In summary, EL mediates binding and uptake of plasma lipoproteins via a process that is independent of its enzymatic activity, requires cellular heparan sulfate proteoglycans, and is regulated by ligand clustering.

No abstract

Activated thrombin receptors on human umbilical vein endothelial cells rapidly undergo homologous desensitization, leaving the cells unable to respond to thrombin. The present studies examine the fate of activated thrombin receptors on endothelial cells and the mechanisms that restore intact receptors to the cell surface. The results show that: 1) at biologically relevant concentrations, thrombin rapidly cleaves all of its receptors on the cell surface. 2) The cleaved receptors are cleared from the cell surface in a two-phase process, with 60% being internalized within 10 min, the remainder requiring several hours. 3) The restoration of intact, thrombin-responsive receptors on the cell surface initially occurs from an intracellular pool of receptors in a process that is independent of protein synthesis. 4) Recycling of cleaved receptors either does not occur on endothelial cells or is masked by receptor clearance. 5) Subconfluent endothelial cells re-express intact receptors on the cell surface at a slower rate than confluent cells. 6) The agonist peptide, SFLLRN, also causes receptor internalization, although at concentrations greater than those required for receptor activation and desensitization. These results are distinctly different from those observed with megakaryoblastic cell lines, where > 90% of the activated thrombin receptors are internalized rapidly, up to 40% of the cleaved receptors are recycled, and no intracellular pool of intact receptors has been detected. Since the primary structure of the thrombin receptor is the same in all the cell types studied, these results demonstrate that there can be substantial differences between cell types in the mechanisms involved in the clearance of activated receptors and the re-expression on the cell surface of intact receptors capable of responding to thrombin.

Cdx1 is a homeodomain transcription factor that regulates intestine-specific gene expression. Experimental evidence suggests that Cdx1 may be involved in cell cycle regulation, but its role is ill defined and the mechanisms have not been explored. We used stable transfec-