KETOCONAZOLE AND 25-HYDROXYCHOLESTEROL PRODUCE RECIPROCAL CHANGES IN THE RATE OF TRANSCRIPTION OF THE HUMAN LDL RECEPTOR GENE

Sterol-dependent regulation of low-density lipoprotein (LDL) receptor gene expression was studied in the human hepatoma HepG2 cellline. Incubation of HepG2 cells with 20 mu M ketoconazole increased the level of LDL receptor mRNA. After a lag of approx. 1.0 h the level rose 6.5-fold within 8.0 h and remained elevated for up to 24 h. Incubation with 10 mu g 25-hydroxycholesterol/ml for 24 h produced a 40-50% reduction in the level of LDL receptor mRNA. Ketoconazole- and 25-hydroxrycholesterol-induced changes in LDL receptor mRNA accumulation were due to alterations in the relative rate of LDL receptor gene transcription as measured by nuclear run-on transcription. Incubation with 20 mu M ketoconazole for 4 h or 10 mu g 25-hydroxycholesterol/ml for 24 h produced a 3.6-fold increase and a 40% reduction, respectively, in the transcription rate of LDL receptor gene. Removal of the Alu-like sequence elements within the LDL receptor cDNA was required to consistently measure changes in LDL receptor gene transcription. No significant changes were noted in the half-life of LDL receptor mRNA in ketoconazole or 25-hydroxycholesterol-treated cells. These data demonstrate that sterol-dependent changes in the level of LDL receptor mRNA can be completely accounted for by changes in the rate of LDL receptor gene transcription.