AbstractThe regulation of low‐density lipoprotein (LDL) receptor activity in the human hepatoma cell line Hep‐G2 by serum components was examined. Incubation of dense monolayers of Hep‐G2 cells with fresh medium containing 10% fetal calf serum (FM) produced a time‐dependent increase in LDL receptor activity. Uptake and degradation of125l‐LDL was stimulated two‐ to four‐fold, as compared with that of Hep‐G2 cells cultured in the same media in which they had been grown to confluence (CM); the maximal125l‐LDL uptake plus degradation increased from 0.2 μ/mg cell protein/4 to 0.8 μg/mg cell protein/4 h. In addition, a two‐fold increase in cell surface binding of125l‐LDL to Hep‐G2 cells was observed when binding was measured at 4°C. There was no change in the “apparent” Kd. The stimulation of LDL receptor activity was suppressed in a concentration‐dependent manner by the addition of cholesterol, as LDL, to the cell medium. In contrast to the stimulation of LDL receptor activity, FM did not affect the uptake or degradation of125l‐asialoorosomucoid. Addition of FM increased the protein content per dish, and DNA synthesis was stimulated approximately five‐fold, as measured by [3H]thymidine incorporation into DNA; however, the cell number did not change. Cellular cholesterol biosynthesis was also stimulated by FM; [14C]acetate incorporation into unesterified and esterified cholesterol was increased approximately five‐fold. Incubation of Hep‐G2 cells with high‐density lipoproteins (200 μg protein/ml) or albumin (8.0 mg/ml) in the absence of the serum factor did not significantly increase the total processed125l‐LDL. Stimulation of LDL receptor activity was dependent on a heat‐stable, nondialyzable serum component that eluted in the inclusion volume of a Sephadex G‐75 column. Uptake of125l‐LDL by confluent monolayers of human skin fibroblasts was not changed by incubation with FM or by incubation with Hep‐G2 conditioned medium. Taken together, these data demonstrate that LDL receptor activity in Hep‐G2 cells is stimulated by a serum component. Furthermore, this serum factor shows some specificity for
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