Chylomicron remnant-vitamin A metabolism by the human hepatoma cell line HepG2.

The binding and metabolism of [3H]vitamin A-containing chylomicron (CM) remnants by the human hepatoma cell line HepG2 were studied. Mesenteric lymph chylomicrons were collected from [3H]retinol-fed rats and incubated with lipoprotein lipase to obtain CM remnants. At 4 degrees C, specific CM remnant binding was inhibited by an excess of unlabeled CM remnants. Specific binding predominated at low concentrations and approached saturation while total binding continued to increase over an extensive concentration range (0.45-32 microgram triglyceride/ml). CM remnant uptake at 37 degrees C was greater than that of CM and at least 70 times more efficient than the pinocytosis of sucrose. CM remnant binding increased with the extent of lipolysis. Addition of human apolipoprotein E enhanced both CM remnant and CM binding. After internalization, HepG2 cells hydrolyzed CM remnant-[3H]retinyl esters, and radiolabeled metabolites accumulated. As a function of the concentration of [3H]retinoid initially bound to cells, retinol and retinyl esters accumulated as the major cell-associated metabolites. In contrast, retinol was the major metabolite in the medium only at low retinoid concentrations; other more polar metabolites accumulated at higher concentrations (greater than 110 pmol retinoid/mg cell protein). The accumulation in the medium of labeled metabolites derived from CM remnant-retinoid was reduced when cells were preincubated in unlabeled retinol-supplemented media. The specific activity of retinol in the medium indicated that CM remnant-vitamin A had mixed with the cellular store prior to its secretion as retinol. These results indicate that HepG2 cells internalize CM remnants in part by specific binding sites, and that the metabolism of CM remnant-retinoids by the HepG2 cell involves retinyl ester hydrolysis and the secretion of retinol and other more polar metabolites. These processes were regulated in part by the concentration of retinoid delivered by the CM remnant and by the initial retinoid content of the cell.