Hepatic lipase treatment of chylomicron remnants increases exposure of apolipoprotein E

The consequences of hepatic lipase treatment of chylomicron remnants were studied. Rats were fed corn oil to induce production and secretion of chylomicrons and were then injected with polyclonal antiserum raised against hepatic lipase to specifically and quantitatively inhibit hepatic lipase activity in vivo. A fraction enriched in chylomicron remnants was isolated from rat plasma by a brief centrifugation step that preferentially isolates triglyceride-rich apolipoprotein (apo) B-48-containing lipoproteins. The chylomicron remnants were then treated with hepatic lipase in vitro, or incubated under identical conditions in the absence of enzyme (control incubations). Hepatic lipase-treated and control chylomicron remnants were isolated by a second brief centrifugation step using discontinuous salt gradients. Control lipoproteins were collected from one discrete band at d < 1.02 g/ml. Hepatic lipase-treated chylomicron remnants formed two discrete bands and were collected at two densities: d < 1.02 g/ml and 1.02 < d < 1.04 g/ml. The buoyant (d < 1.02 g/ml) subfraction of hepatic lipase-treated chylomicron remnants was depleted of 62% of the total phospholipid when compared to control d < 1.02 g/ml lipoproteins. The dense (1.02 < d < 1.04 g/ml) subfraction of hepatic lipase-treated chylomicron remnants was depleted of 65% of particle phospholipid content and 90% of particle triglyceride content when compared to control d < 1.02 g/ml lipoproteins. The dense (1.02 < d < 1.04 g/ml) subfraction of hepatic lipase-treated chylomicron remnants showed 5- to 7-fold greater immunoreactivity of apoE when compared to control lipoproteins in competitive displacement immunoassays. These data suggest that extensive hydrolysis of chylomicron remnant phospholipid and triglyceride leads to the formation of a dense remnant particle that contains highly exposed apoE. This increased exposure of apoE may be the key to the previously observed increased degradation of chylomicron remnants treated with hepatic lipase because more exposed apoE may bind better to cell surface lipoprotein receptors. Furthermore, the data imply that hepatic lipase cleaves chylomicron remnant phospholipid and triglyceride in a sequential fashion; hydrolytic intermediates depleted only of phospholipid precede the formation of a smaller dense remnant particle depleted of phospholipid and triglyceride.