Control by oestrogen of reversible gene expression: the vitellogenin model

Use of the protein A-gold immunocytochemical and enzyme-gold cytochemical techniques in studies of vitellogenesis

Vitellogenesis in the frog hepatocyte was investigated by applying the protein A-gold immunocytochemical and RNase-gold cytochemical techniques in conjunction with morphometric and biochemical analyses. The morphometric studies demonstrated that the surface density of rough endoplasmic reticulum (RER) and nucleolar size increased more than fourfold and 1.25-fold, respectively, while the nuclear size and the mitochondrial compartment size remained constant following estrogen treatment. Concurrently, liver RNA concentration increased 2.5-fold while protein and DNA concentrations did not change. In addition, total plasma protein more than doubled, with vitellogenin accounting for 40% of the final volume. The secretory proteins vitellogenin and protein-RcX (a nonvitellogenin, estrogen-induced plasma protein of unknown function, found in the plasma of Rana catesbeiana) were detected immunocytochemically in the RER, Golgi apparatus, and secretory granules in hepatocytes only of estrogen-treated frogs. Lysosomes also were labeled. These observations established that protein-RcX was synthesized and secreted by the hepatocyte in parallel with vitellogenin and that both of these export proteins were confined to the secretory pathway and lysosomes. Quantitation of labeling density indicated that the concentration of vitellogenin increased as it progressed along the secretory vector. Albumin was detected immunocytochemically also within these same hepatocyte entities from both untreated and treated animals. In the untreated animals, albumin concentration also increased progressively along the secretory vector. A marked alteration of albumin processing was observed following estrogen treatment. While albumin concentration in the RER was unchanged, its concentrations within the Golgi apparatus and secretory granules were lower than those observed in the RER or in counterpart compartments under control conditions. RNase-gold cytochemistry for total RNA demonstrated a 1.5-fold increase in labeling density over the nucleolus but no change in RER labeling following estrogen treatment. These labeling data, in combination with the morphometric data, suggest an increase of approximately 80% in the total amount of RNA in the nucleolus and 430% in the RER in response to estrogen. This review thus illustrates the significant contributions which can be made by gold-probe techniques, alone or in combination with morphometric and biochemical techniques, to investigations of the intracellular processing of secretory proteins.

A correlated morphometric and cytochemical study on hepatocyte nucleolar size and RNA distribution during vitellogenesis

The enzyme-gold cytochemical technique was used to label RNA in the nucleolus and rough endoplasmic reticulum (RER) of the hepatocytes of normal, male American bullfrogs (Rana catesbeiana) and in bullfrogs eight days following treatment with estradiol-17 beta. Concurrently, stereology was applied to quantitate: (1) the density of RNA labelling, and (2) changes in the size of the nucleus and nucleolus in response to estrogen treatment. In the hepatocytes from untreated frogs, specific labelling for RNA was present over the fibrillar and, to a greater extent, the granular portions of the nucleolus, and, to the greatest extent, over the RER. Following estrogen treatment, the density of RNA labelling increased over both parts of the nucleolus but was unchanged over the RER. The size of the nucleolus enlarged in response to estrogen: in combination with the increase in its RNA labelling, this suggested an increase of about 80% in the total amount of RNA in the nucleolus. Previous data on enlargement of the RER compartment, along with the present data on RNA labelling of RER, suggested that the total amount of this nucleic acid increased about 430% in this entity, in response to estrogen. However, the density of RNA labelling over the RER appears to be constant in spite of changes in the amount of RER.

Effect of estrogen on Xenopus laevis albumin mRNA levels

Accurate quantitation of concentrations of albumin mRNA by hybridization to albumin cDNA allows analysis of the estrogen effect on the Xenopus laevis albumin mRNA levels in the liver cytoplasm during the primary stimulation. Administration of estradiol-17 beta to male Xenopus laevis increases the amount of both total cellular RNA and poly(A)-RNA but has no effect on the albumin mRNA level and the amount of albumin mRNA per cell is constant.

Characterization of cloned complementary DNA covering more than 6000 nucleotides (97%) of avian vitellogenin mRNA

The messenger RNA coding for chicken vitellogenin, a precursor of the egg-yolk proteins lipovitellin and phosvitin, is synthesized in the liver following estrogen injection. This mRNA is 6600 nucleotides long. We have previously reported the cloning and preliminary characterization of some cDNA fragments representing portions of the vitellogenin mRNA [Biochem. Biophys. Acta, 606, 34--46 (1980)]. In this paper we report the full characterization of a larger series of such clones, representing almost the entire length of the mRNA, by restriction endonuclease mapping, R-loop mapping, RNA-DNA hybridization and by translation in vitro of the RNA which hybridizes to the cloned DNA. From the results we conclude that the chicken vitellogenin mRNA, unlike that of Xenopus laevis, does not vary in sequence over most of its length, although some variations in the cDNA sequences were detected particularly in clones derived from the 3' terminus of the RNA. All sequence variants appear to be present in RNA prepared from single animals. The possible origins of these minor species are discussed. Furthermore, we describe a cDNA clone complementary to an mRNA which is about the same size as vitellogenin mRNA and which codes for an egg yolk protein antigenically related to lipovitellin. This mRNA is synthesized constitutively.