ENHANCEMENT OF LYSOLECITHIN ACYLTRANSFERASE ACTIVITY BY LDL IN THROMBIN-STIMULATED PORCINE-CULTURED ENDOTHELIAL-CELLS

Changes of intracellular activity of lysolecithin acyltransferase (LAT) during an interaction between endothelial cells (EC) and low-density lipoprotein (LDL) were investigated. Following an incubation of EC with LDL, endothelial LAT activity was assayed using [H-3]lysophosphatidylcholine as the substrate. Stimulation of EC with either thrombin (0.01-1 U/ml) or Ca2+-ionophore A23187 (10(-10) - 10(-7)M) dose- and time-dependently enhanced LAT activity in the presence of LDL (1 mg protein/ml), but no enhancement was observed in quiescent cells. Ionomycin together with 1-oleoyl-2-acetyl glycerol, a synthetic analog of diacylglycerols enhanced LAT activity in a similar degree to thrombin in the presence of LDL. Either staurosporine, a protein kinase C inhibitor or neomycin, a phospholipase C inhibitor completely blocked an increase of LAT activity in stimulated EC. Stimulation of EC with various agonists including 12-o-tetradecanoylphorbol-13-acetate, an activator of protein kinase C caused a marked increase in cellular uptake of LDL, and staurosporine inhibited the uptake. These results suggest that the transport of LDL into EC is facilitated by stimulation with thrombin and other agonists, and LDL subsequently activates intracellular LAT. Protein kinase C seems to mediate LDL uptake into EC. Intracellular regulatory roles of LDL in the presence of vasoactive substances were discussed.