Different hepatocytes express the cholesterol 7 alpha-hydroxylase gene during its circadian modulation in vivo.

Cholesterol 7 alpha-hydroxylase, the rate-limiting enzyme in bile salt synthesis from cholesterol is a P450 enzyme (CYP7A). Its expression and activity are regulated by bile salts, cholesterol, hormones and a circadian modulator. Here we define the hepatocytes contributing to the expression of the rat CYP7A gene during its in vivo circadian variation. The diurnal expression of the CYP7A messenger RNA (mRNA) was studied by in situ hybridization and correlated with the diurnal rate of CYP7A gene transcription and mRNA expression. At 10 AM, the time of lowest mRNA expression and gene transcription rate, only four to five hepatocytes, located close to the hepatic venules ("perivenular"), contained the CYP7A mRNA. At 10 PM, the time of highest mRNA expression and fastest in vitro transcription rate, approximately one half of the hepatocytes (still in a "perivenular" location) contained the cholesterol 7 alpha-hydroxylase mRNA. In addition, the measured half-life of the CYP7A mRNA was shorter at 10 AM than at 10 PM suggesting that posttranscriptional mechanisms also contributed to the observed circadian differences. Therefore, the basal transcription rate of the CYP7A gene is maintained by four to five "perivenular" hepatocytes. During the circadian variation, the rate of gene transcription increases in these "perivenular" hepatocytes, but in addition, there is recruitment of other more proximal hepatocytes to transcribe the gene. It is proposed here that the response of specific hepatocytes to the various modulators of CYP7A gene expression is dependent on the relative position of these hepatocytes within the liver cell plate.

Novel control of the position-dependent expression of genes in hepatocytes. The GLUT-1 transporter.

The basal hepatocyte phenotype is conferred by the expression of liver-specific genes. In the adult liver, the basal hepatocyte phenotype is further modified by transcriptional and post-transcriptional regulation of genes which result in the appearance of specific proteins in selected hepatocytes. One of these proteins is the erythroid/brain or GLUT-1 glucose transporter. The GLUT-1 protein is detected in the plasma membrane of only one or two hepatocytes located at the end of the liver cell plate, contiguous to the hepatic venule. The objective of this study was to define the molecular mechanisms responsible for the restricted expression of the GLUT-1 protein in rat liver. Hepatocytes were isolated from either the proximal ("periportal") or the distal ("perivenular") half of the liver cell plate. The GLUT-1 mRNA as well as the GLUT-1 protein content and intracellular distribution were defined after subcellular fractionation of each hepatocyte population. In addition, the location of the GLUT-1 protein in liver tissue was determined by confocal microscopy. We propose that the GLUT-1 gene is transcribed and the mRNA is translated by both "periportal" and "perivenular" hepatocytes. However, insertion of the GLUT-1 protein into the plasma membrane occurs only in the last two hepatocytes contiguous to the hepatic venule. In other hepatocytes, the protein remains in a different cellular compartment characterized here as a "low density microsomal" fraction.