Human lipoproteins as a vehicle for the delivery of beta-carotene and alpha-tocopherol to HepG2 cells.

Highly differentiated human cell lines represent a useful in vitro model for the study of carotenoid uptake, metabolism, and function. Carotenoids are usually introduced into tissue culture media either in organic solvents or as micelles, whereas carotenoids are localized in lipoproteins in vivo. Initially, the stability of beta-carotene and alpha-tocopherol in micelles and human lipoproteins under standard tissue culture conditions was compared. Recovery of beta-carotene and alpha-tocopherol was 27% +/- 2% and 73% +/- 2%, respectively, after overnight incubation of micellar beta-carotene and alpha-tocopherol in serum-free medium without cells. This marked loss of beta-carotene was attenuated by inclusion of alpha-tocopherol in micelles. In contrast, recovery of beta-carotene and alpha-tocopherol was 88%-95% when medium containing the total lipoprotein fraction isolated from beta-carotene supplemented individuals was incubated overnight without cells. Cellular accumulation of beta-carotene and alpha-tocopherol from medium containing total lipoproteins (1 mg/ml) was proportional to their concentrations in the lipoprotein fraction (r = 0.94 for beta-carotene and 0.74 for alpha-tocopherol). Cells exhibited similar capability of acquiring beta-carotene and alpha-tocopherol from medium containing either low- or high-density lipoproteins. These data show that lipoproteins represent a stable vehicle for delivery of beta-carotene and alpha-tocopherol to HepG2 human liver cells.