Lipoprotein Lipase Can Function as a Monocyte Adhesion Protein

Abstract

Lipoprotein lipase (LPL) is made by several cell types, including macrophages within the atherosclerotic lesion. LPL, a dimer of identical subunits, has high affinity for heparin and cell surface heparan sulfate proteoglycans (HSPGs). Several studies have shown that cell surface HSPGs can mediate cell binding to adhesion proteins. Here, we tested whether LPL, by virtue of its HSPG binding could mediate monocyte adhesion to surfaces. Monocyte binding to LPL-coated (1-25 micrograms/mL) tissue culture plates was 1.4- to 7-fold higher than that of albumin-treated plastic. Up to 3-fold more monocytes bound to the subendothelial matrix that had been pretreated with LPL. LPL also doubled the number of monocytes that bound to endothelial cells (ECs). Heparinase and heparitinase treatment of monocytes or incubation of monocytes with heparin decreased monocyte binding to LPL. Heparinase/heparitinase treatment of the matrix also abolished the LPL-mediated increase in monocyte binding. These results suggest that LPL dimers mediate monocyte binding by forming a "bridge" between matrix and monocyte surface HSPGs. Inhibition of LPL activity with tetrahydrolipstatin, a lipase active-site inhibitor, did not affect the LPL-mediated monocyte binding. To assess whether specific oligosaccharide sequences in HSPGs mediated monocyte binding to LPL, competition experiments were performed by using known HSPG binding proteins. Neither antithrombin nor thrombin inhibited monocyte binding to LPL. Next, we tested whether integrins were involved in monocyte binding to LPL. Surprisingly, monocyte binding to LPL-coated plastic and matrix was inhibited by approximately 35% via integrin-binding arginine-glycine-aspartic acid peptides. This result suggests that monocyte binding to LPL was mediated, in part, by monocyte cell surface integrins. In summary, our data show that LPL, which is present on ECs and in the subendothelial matrix, can augment monocyte adherence. This increase in monocyte-matrix interaction could promote macrophage accumulation within arteries.