Abstract
Lipoprotein lipase (LPL) bound to the lumenal surface of vascular endothelial cells is responsible for the hydrolysis of triglycerides in plasma lipoproteins. Studies were performed to investigate whether human plasma lipoproteins and/or free fatty acids would release LPL which was bound to endothelial cells. Purified bovine milk LPL was incubated with cultured porcine aortic endothelial cells resulting in the association of enzyme activity with the cells. When the cells were then incubated with media containing chylomicrons or very low density lipoproteins (VLDL), a concentration-dependent decrease in the cell-associated LPL enzymatic activity was observed. In contrast, incubation with media containing low density lipoproteins or high density lipoproteins produced a much smaller decrease in the cell-associated enzymatic activity. The addition of increasing molar ratios of oleic acid:bovine serum albumin to the media also reduced enzyme activity associated with the endothelial cells. To determine whether the decrease in LPL activity was due to release of the enzyme from the cells or inactivation of the enzyme, studies were performed utilizing radioiodinated bovine LPL. Radiolabeled LPL protein was released from endothelial cells by chylomicrons, VLDL, and by free fatty acids (i.e. oleic acid bound to bovine serum albumin). The release of radiolabeled LPL by VLDL correlated with the generation of free fatty acids from the hydrolysis of VLDL triglyceride by LPL bound to the cells. Inhibition of LPL enzymatic activity by use of a specific monoclonal antibody, reduced the extent of release of 125I-LPL from the endothelial cells by the added VLDL. These results demonstrated that LPL enzymatic activity and protein were removed from endothelial cells by triglyceride-rich lipoproteins (chylomicrons and VLDL) and oleic acid. We postulate that similar mechanisms may be important in the regulation of LPL activity at the vascular endothelium.
Highlights
The results reported here demonstrate the ability of the triglyceride-rich lipoproteins, chylomicrons and VLDL, to release Lipoprotein lipase (LPL) activity and LPL protein bound to endothelial cells
Evidence that chylomicrons and VLDL moduIate the amountof LPL activity on endothelial cells was obtained in the experiments in which enzymatic activity remaining on the cells followingincubation with lipoproteins was determined
The results obtained in these experiments were similar to those obtained with radioiodinated enzyme, i.e. a marked decrease in the cell-associated enzyme activity was found after incubations with chylomicrons andVLDL, but not LDL and HDL
Summary
Binding of Purified LPL to E n d o ~ h e ~C~eallls-The binding of purified LPL was studied by incubating cells with increasing concentrations of LPL. The addition of increasing concentrations of VLDL (S2.5-50 pg of protein) decreased the cell-associated LPL activity in a concentration-dependent manner, so that at thehighest amount of VLDL used (150 pg) only about 20% of the initial LPL activity was measurable. At lower concentrations of VLDL ( 6 0 pgof protein) an increase in enzyme activity in the medium was observed. At higher concentrations of VLDL (>50 pg ofprotein) therewas a decrease in theamount of enzyme activity assayed in the medium. Addition of similar large amounts of VLDL (50-150pgof protein) reduced measurable LPL activity when assayed under identical conditions, proving that larger amounts of VLDL interfere with the LPLassay (Fig. 2B). 111 1 I less effective in decreasing cell associated LPL activity (Fig. 3). Use of much higher concentrations of these lipoproteins (LDLup to 3 mg/ml and HDLup to 6 mg/ml) failed to further reduce the cell associated activity
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
