Regulation of lipoprotein synthesis. Studies on the molecular mechanisms of lipoprotein synthesis and their regulation by estrogen in the cockerel.

We used the estrogen-treated cockerel as a model to study the regulation of very low density lipoproteins (VLDL) at the molecular level. A single injection of estrogen induced marked elevation of plasma VLDL in the cockerel. Messenger RNA (mRNA) activity for a major VLDL apoprotein (apoVLDLII) in hepatic polyribosomes was assayed in vitro, and increased at the same rate as plasma VLDL levels. Simultaneous determinations of mRNA activities for albumin and apoA-I (a major HDL apoprotein) showed that these were unchanged. Specific estrogen-binding sites were measured in the liver cell nuclei and a single class of sites with a Kd of 2 × 10−9M was observed. The number of such sites increased after estrogen treatment prior to any detectable increase in plasma VLDL. Simultaneously, RNA polymerase I and II activities were markedly stimulated by the hormone. ApoVLDLII mRNA was purified to apparent homogeneity by (1) total nucleic acid extraction, (2) zonal ultracentrifugation, (3) Sepharose 4B chromatography in EDTA, (4) Sepharose 6B chromatography, (5) Sepharose 4B chromatography in MgCU, and (6) sucrose gradient centrifugation. The in vitro translation product of apoVLDLII mRNA was about 12 amino acids larger than apoVLDLn isolated from the blood. The identification of such a product (designated pre-apoVLDLn) is compatible with Blobel's hypothesis that, like other secretory proteins, VLDLII is synthesized initially as a larger protein and the JV-terminal sequence is cleaved prior to completion of synthesis and secretion of the VLDL particle.