Interaction between ApoB and hepatic lipase mediates the uptake of ApoB-containing lipoproteins

Hepatic lipase (HL) on the surface of hepatocytes and endothelial cells lining hepatic sinusoids, the adrenal glands, and the ovary hydrolyzes triglycerides and phospholipids of circulating lipoproteins. Its expression significantly enhances low density lipoprotein (LDL) uptake via the LDL receptor pathway. A specific interaction between LPL, a homologous molecule to HL, and apoB has been described (Choi, S. Y., Sivaram, P., Walker, D. E., Curtiss, L. K., Gretch, D. G., Sturley, S. L., Attie, A. D., Deckelbaum, R. J., and Goldberg, I. J. (1995) J. Biol. Chem. 270, 8081-8086). The present studies tested the hypothesis that HL enhances the uptake of lipoproteins by a specific interaction of HL with apoB. On a ligand blot, HL bound to apoB26, 48, and 100 but not to apoE or apoAI. HL binding to LDL in a plate assay with LDL-coated plates was significantly greater than to bovine serum albumin-coated plates. Neither heat denatured HL nor bacterial fusion protein of HL bound to LDL in the plate assays. 125I-LDL bound to HL-saturated heparin-agarose gel with a Kd of 52 nM, and somewhat surprisingly, this binding was not inhibited by excess LPL. In cell culture experiments HL enhanced the uptake of 125I-LDL at both 4 and 37 degreesC. The enhanced binding and uptake of LDL was significantly inhibited by monoclonal anti-apoB antibodies. In contrast to LPL, both amino- and carboxyl-terminal antibodies blocked the apoB interaction with HL to the same extent. Thus, we conclude that there is a unique interaction between HL and apoB that facilitates the uptake of apoB-containing lipoproteins by cells where HL is present.