In vivo arterial lipoprotein lipase expression augments inflammatory responses and impairs vascular dilatation.

OBJECTIVE: Although epidemiologic data suggest that hypertriglyceridemia and elevated plasma levels of fatty acids are toxic to arteries, in vitro correlates have been inconsistent. To investigate whether increased endothelial cell expression of lipoprotein lipase (LpL), the primary enzyme creating free fatty acids from circulating triglycerides (TG), affects vascular function, we created transgenic mice that express human LpL (hLpL) driven by the promoter and enhancer of the Tie2 receptor. METHODS AND RESULTS: Mice expressing this transgene, denoted EC-hLpL and L for low and H for high expression, had decreased plasma TG levels compared with wild-type mice (WT): 106+/-31 in WT, 37+/-17 (line H), and 63+/-31 mg/dL (line L) because of a reduction in VLDL TG; plasma cholesterol and HDL levels were unaltered. Crossing a high expressing EC-hLpL transgene onto the LpL knockout background allowed for survival of the pups; TG in these mice was approximately equal to that of heterozygous LpL knockout mice. Surprisingly, under control conditions the EC-hLpL transgene did not alter arterial function or endothelial cell gene expression; however, after tumor necrosis factor (TNF)-alpha treatment, arterial vascular cell adhesion molecule-1 (VCAM-1), E-selectin, and endogenous TNF-alpha mRNA levels were increased and arteries had impaired endothelium-dependent vasodilatation. This was associated with reduced eNOS dimers. CONCLUSIONS: Therefore, we hypothesize that excess vascular wall LpL augments vascular dysfunction in the setting of inflammation.

Interaction of lipoprotein lipase and receptor-associated protein.

Receptor-associated protein (RAP) is a recognized chaperone/escort protein for members of the low density lipoprotein receptor family. In this report, we show that RAP binds to lipoprotein lipase (LPL) and may play a role in the maturation of LPL. Binding of highly purified RAP to LPL was demonstrated in vitro by solid phase assays, surface plasmon resonance, and rate zonal centrifugation. The dissociation constant for this interaction measured by the first two techniques ranged between 2.4 and 13 nM, values similar to those reported for the binding of RAP to LRP or gp330. The specificity of the interaction was demonstrated by competition with a panel of LPL monoclonal antibodies. Rate zonal centrifugation demonstrated the presence of a stable complex with an apparent Mr consistent with the formation of a complex between monomeric LPL and RAP. RAP x LPL complexes were co-immunoprecipitated in adipocyte lysates or from solutions of purified LPL and RAP. The interaction was also demonstrated in whole cells by cross-linking experiments. RAP-deficient adipocytes secreted LPL with a specific activity 2.5-fold lower than the lipase secreted by control cells. Heparin addition to cultured RAP-deficient adipocytes failed to stimulate LPL secretion in the medium, suggesting defective binding of the lipase to the plasma membrane. These studies demonstrate that RAP binds to LPL with high affinity both in purified systems and cell extracts and that RAP-deficient adipocytes secrete poorly assembled LPL. A function of RAP may be to prevent premature interaction of LPL with binding partners in the secretory pathway, namely LRP and heparan sulfate proteoglycan.