Role of thyroid hormones in apolipoprotein A-I gene expression in rat liver.

To study the regulation of hepatic apo A-I gene expression, we measured synthesis and abundance of cellular apo A-I mRNA and its nuclear precursors in livers of hypothyroid and hyperthyroid rats. In hypothyroid animals, both synthesis and abundance of apo A-I mRNA was reduced to half of control values. After injection of a receptor-saturating dose of triiodothyronine into euthyroid rats, apo A-I gene transcription increased at 20 min, reached a maximum of 179% of control (P less than 0.01) at 3.5 h, and remained elevated for up to 48 h. The abundance of nuclear and total cellular apo A-I mRNA increased at 1 and 2 h, respectively, and exceeded the levels expected from enhanced transcription more than two fold at 24 h after hormone injection. Upon chronic administration of thyroid hormones, levels of nuclear and cytoplasmic apo A-I mRNA remained elevated but transcription of the apo A-I gene fell to 42% of control (P less than 0.01). Thus, thyroid hormones rapidly stimulate apo A-I gene transcription. Posttranscriptional events leading to increased stability of nuclear apo A-I RNA precursors become the principal mechanism for enhanced gene expression in chronic hyperthyroidism and may cause feedback inhibition of apo A-I gene transcription. Our results furthermore imply that the majority of hepatic nuclear apo A-I RNA precursors are degraded in euthyroid animals.

Apolipoprotein A-IV. A determinant for binding and uptake of high density lipoproteins by rat hepatocytes.

To identify the role of a specific apoprotein other than apoE which might be responsible for the receptor-mediated uptake of high density lipoprotein (HDL) by rat hepatocytes, 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) was combined with rat apoE, apoA-I, or apoA-IV to form apoprotein-phospholipid complexes and the complexes were tested for their binding and uptake by primary rat hepatocytes. Apoprotein-POPC complexes were labeled with the specific fluorescent probe, 1,1-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine to monitor their uptake by cultured rat hepatocytes at 37 degrees C using digital fluorescence imaging microscopy or were labeled with 125I to study their binding to hepatocytes at 4 degrees C. POPC, either alone or with apoA-I, was not internalized by rat hepatocytes while complexes containing apoE or apoA-IV were taken up by the cells. Specific binding at 4 degrees C was demonstrated for apoE-free HDL, apoA-IV X POPC, and apoE X POPC but not for apoA-I X POPC. The binding of apoE-free HDL was inhibited by apoA-IV X POPC, apoE-free HDL, and apoA-IV + apoA-I X POPC but not by apoA-I X POPC. Binding of apoA-IV X POPC was inhibited by apoE-free HDL, apoA-IV X POPC, and apoA-IV + apoA-I X POPC, but not by apoE X POPC or apoE-enriched HDL. These data indicate that apoA-IV is a ligand responsible for the rat HDL binding to primary rat hepatocytes and that apoA-IV binds to a receptor site distinct from apoE-dependent receptors such as the apoB,E or chylomicron-remnant receptor.

Effect of sucrose diet on apolipoprotein biosynthesis in rat liver. Increase in apolipoprotein E gene transcription.

A sucrose-rich diet stimulates the biosynthesis of very low density lipoproteins in rat liver. This diet also increases the triglyceride content of hepatic very low density lipoproteins and changes their apolipoprotein composition. To study the changes of hepatic apolipoprotein biogenesis in response to such a diet, we measured secretory rates of apolipoproteins A-I, B, and E in cultured rat hepatocytes. In cultures from rats fed the sucrose-rich diet the production of apolipoprotein E was increased 2-fold as compared to controls, whereas the production of apolipoproteins A-I and B was unchanged. The enhanced production of apolipoprotein E could be accounted for by a 2-fold increase in hepatic apolipoprotein E mRNA, as measured by slot blot hybridization. To characterize the mechanisms leading to the increase of liver apolipoprotein E mRNA levels we measured the transcriptional activity of the apolipoprotein E gene in a cell-free transcription system using isolated liver cell nuclei. Transcriptional activity of the apolipoprotein E gene was 7% that of albumin gene transcription in control animals. In rats fed a sucrose-rich diet the transcription rate of the apolipoprotein E gene increased to 140 +/- 11% of controls. There was no change in albumin gene transcription. Thus, a sucrose-rich diet enhances apolipoprotein E biosynthesis in rat liver, at least in part, by stimulating transcription of the apolipoprotein E gene.

Value of screening for markers of hepatitis in dialysis units.

The value of monitoring the serum activity of SGOT, as well as markers of hepatitis B virus and hepatitis A virus infections, in the patients and staff of two dialysis units has been assessed retrospectively. Sera were checked each month for SGOT and HBsAg on 406 patients and 170 staff members over a 4-year period. Anti-HBc, anti-HBs, and anti-hepatitis A antibodies were assayed on the stored sera. Only 30% of the patients had normal SGOT values (less than 65 units per ml) on all occasions. Most of the abnormal values were less than 100 units per ml and could not be explained. Viral hepatitis was a reasonable explanation for only half of those instances where the SGOT value was greater than 100 units per ml. Hepatitis A virus contributed nothing to the problem of dialysis-associated liver disease. Testing for HBsAg alone missed approximately 40% of the hepatitis B events acquired in the unit. Only two of these episodes were of epidemiologic importance, however, because the rest were recognized only after there was serologic resolution of the infection. There was a high frequency of potentially "false positive" reactions with all the antibodies tested. It is not cost-effective to monitor dialysis patients and staff regularly with anti-HBc, anti-HBs, or antibodies against hepatitis A. Initial screening with anti-HBc and anti-HBs on entry to the unit is of value but weak positive results must be interpreted with caution since approximately half of such results will prove to be nonspecific.