REGULATION OF LDL RECEPTOR EXPRESSION BY LUMINAL STEROL FLUX IN CACO-2 CELLS

The regulation of expression of the intestinal low density lipoprotein (LDL) receptor by luminal (apical) sterol flux was investigated in the human intestinal cell line CaCo-2. Cells were cultured on semipermeable micropore filters, which separated an upper and lower well. To the apical media were added solutions containing either taurocholate micelles alone or micelles containing sterols. Because of an efflux of cholesterol, which occurred from cells incubated with micelles alone, LDL receptor mRNA levels increased threefold. With an influx of micellar sterols, receptor mRNA levels decreased in a dose-dependent manner. Synthesis and degradation of the LDL receptor were addressed by pulse-chase experiments. In cells incubated with micelles containing 25-hydroxycholesterol, the rate of receptor synthesis was significantly decreased, whereas the rate of receptor turnover remained unchanged. As assessed by immunoblots and steady-state labeling of proteins followed by immunoprecipitation of the LDL receptor, cells incubated with micellar 25-hydroxycholesterol contained substantially less receptor protein. These cells also bound and degraded less LDL. In contrast, in cells incubated with micelles alone, the rate of receptor synthesis was increased and cells contained more LDL receptor protein, although this was not reflected in an increase in LDL binding. The results suggest that LDL receptor expression in CaCo-2 cells is regulated by luminal sterol flux and that this regulation occurs at the level of transcription.