β‐Carotene storage, conversion to retinoic acid, and induction of the lipocyte phenotype in hepatic stellate cells

Hepatic stellate cells (HSCs) are the major site of retinol (ROH) metabolism and storage. GRX is a permanent murine myofibroblastic cell line, derived from HSCs, which can be induced to display the fat-storing phenotype by treatment with retinoids. Little is known about hepatic or serum homeostasis of β-carotene and retinoic acid (RA), although the direct biogenesis of RA from β-carotene has been described in enterocytes. The aim of this study was to identify the uptake, metabolism, storage, and release of β-carotene in HSCs. GRX cells were plated in 25 cm2 tissue culture flasks, treated during 10 days with 3 μmol/L β-carotene and subsequently transferred into the standard culture medium. β-Carotene induced a full cell conversion into the fat-storing phenotype after 10 days. The total cell extracts, cell fractions, and culture medium were analyzed by reverse phase high-performance liquid chromatography for β-carotene and retinoids. Cells accumulated 27.48 ± 6.5 pmol/L β-carotene/106 cells, but could not convert it to ROH nor produced retinyl esters (RE). β-Carotene was directly converted to RA, which was found in total cell extracts and in the nuclear fraction (10.15 ± 1.23 pmol/L/106 cells), promoting the phenotype conversion. After 24-h chase, cells contained 20.15 ± 1.12 pmol/L β-carotene/106 cells and steadily released β-carotene into the medium (6.69 ± 1.75 pmol/ml). We conclude that HSC are the site of the liver β-carotene storage and release, which can be used for RA production as well as for maintenance of the homeostasis of circulating carotenoids in periods of low dietary uptake. © 2004 Wiley-Liss, Inc.