Decrease in functional albumin mRNA during estrogen-induced vitellogenin biosynthesis in avian liver

Vitellogenin precursors in the liver of the oviparous lizard, Podarcis sicula

In reptiles, as in the other oviparous vertebrates, vitellogenin (VTG) synthesis is stimulated in the liver by ovarian estrogens. In this article, the presence of VTG precursors was detected in liver subcellular fractions of the oviparous lizard, Podarcis sicula, in the reproductive period. The rough endoplasmic reticulum (RER) and the smooth microsomal fraction (SMF), which includes smooth endoplasmic reticulum and Golgi complex, were separated by means of two different sucrose gradients. The successful separation was controlled at the electron microscope. The contents of the different compartments were extracted by means of n-octyl-beta-D-glucopiranoside detergent and subjected to SDS-PAGE. Western Blotting with homologous anti/VTG antibody revealed two immunoreactive proteins of about 84 and 70 kDa in the RER, and four proteins of about 180, 150, 60, 50 kDa in the SMF; all these proteins appeared phosphorylated and glycosylated. The differences in the molecular weight of these VTG precursors are discussed.

Retinol in avian oogenesis: molecular properties of the carrier protein

Normal embryo development in oviparous (egg-laying) species requires the coordinated targeting to growing oocytes of nutrients and regulatory molecules such as retinol, the precursor of active retinoids. Serum retinol-binding protein (RBP) is the major carrier protein for retinol in the circulatory system of vertebrates. In oviparous animals, RBP is thought to function in the delivery of retinol to yolk, in analogy to other important nutrients and vitamins known to accumulate within the oocyte. Here, immunoelectron microscopy revealed that RBP indeed accumulates in yolk, in particular in the electron-lucent phase of yolk organelles known to harbor other serum-derived yolk proteins and their receptors. To gain understanding of the RBP-mediated serum-to-yolk transport of retinol, we have characterized the chicken carrier protein at the molecular level. The essential function of RBPs is emphasized by the first known avian RBP structure, which confirms that these vitamin carriers are among the most highly conserved serum proteins known. Interestingly, by analysis of RBP hepatic RNA and serum protein levels, we identified a unique property of chicken RBP relative to other known RBPs and yolk precursors, i.e., the absence of estrogen induction. One cause of the observed reduction in RBP RNA is an estrogen-dependent decrease of RBP gene transcription. Furthermore, Northern blot analysis of tissues of the hen demonstrated a lack of RBP synthesis by the oocyte or other ovarian cells, confirming the exogenous (hepatic) origin of yolk RBP. These results provide strong evidence that chicken RBP is an essential serum-to-yolk vitamin carrier with certain properties different from those of other such transporters.

Transport of serum transthyretin into chicken oocytes. A receptor-mediated mechanism

Transthyretin (TTR) is involved in the transport of thyroid hormones and, due to its interaction with serum retinol-binding protein, also of vitamin A. The importance of both ligands in vertebrate embryonic development has prompted us to investigate the molecular details of TTR transport function in a powerful germ cell system, the rapidly growing chicken oocytes. Yolk TTR is derived from the circulatory system, since biotinylated TTR was recovered by immunoaffinity chromatography of yolk obtained from a hen previously infused with in vitro biotinylated chicken serum proteins. In concordance with the intraoocytic localization in an endosomal compartment, ligand blotting and chemical cross-linking experiments revealed the presence of a approximately 115-kDa TTR-binding oocyte membrane protein. This putative TTR receptor was not detected in chicken ovarian granulosa cells or embryonic fibroblasts and was different from the previously described oocyte-specific receptor for two estrogen-induced chicken serum lipoproteins, vitellogenin and very low density lipoprotein (Barber, D. L., Sanders, E. J., Aebersold, R., and Schneider, W. J. (1991) J. Biol. Chem. 266, 18761-18770). Furthermore, in contrast to the serum levels of the yolk precursor lipoproteins, those of TTR were not significantly changed by estrogen; thus, TTR represents a newly defined, estrogen-independent class of yolk precursor proteins. These data strongly suggest that oocytic TTR is derived from the circulation, where it is a constitutive component, and deposited into yolk as a result of endocytosis mediated by a specific receptor.

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.

Effect of post-ischaemic recovery on albumin synthesis and relative amount of translatable albumin messenger RNA in rat liver

In liver cells recovering from reversible ischaemia, total protein synthesis by postmitochondrial supernatant and membrane-bound and free polyribosomes is not different from that in sham-operated controls. However, the relative proportion of specific proteins is changed, since the incorporation of [3H]leucine in vivo into liver albumin, relative to incorporation into total protein, as determined by precipitation of labelled albumin with the specific antibody, decreases by 40-50% in post-ischaemic livers. Cell-free synthesis by membrane-bound polyribosomes and poly(A)-enriched RNA isolated from unfractionated liver homogenate shows that the decrease in albumin synthesis in liver of rats recovering from ischaemia is due to the relative decrease in translatable albumin mRNA.

Albumin is encoded by 2 messenger RNAs in Xenopus laevis

A cDNA clone library was prepared from liver poly(A) RNA pf non-estrogenized Xenopus laevis. Albumin coding sequences were screened by hybridization to a cDNA prepared from poly(A) RNA enriched by sucrose density gradient centrifugation, and by a sensitive solid-phase radioimmunoassay to detect clones that contain templates for albumin antigenic determinants. Nine clones were obtained by this approach, and all but one have the cDNA inserted in phase with the beta-lactamase gene of pBR322. Mapping of these clones with restriction endonucleases yielded 2 distinct patterns, suggestive of heterogeneity in the coding sequences. This was confirmed by heteroduplex analyses of hybrids formed between clones representative of each of the 2 classes. Both classes of albumin cDNA clones were used to select mRNAs of the same size (2.3kb) that code for peptides that are indistinguishable by SDS gel electrophoresis. Examination of the organization of the albumin genes by blot hybridization of the cDNA clones to restriction fragments of Xenopus DNA failed to detect any differences at the genomic level. The considerable diversity of the albumin cDNAs is suggestive of a multiplicity of albumin genes, rather than differential processing of a common precursor RNA.

Effect of estradiol on calcium and calcium binding in serum of thick and then-shell lines of chickens

Estradiol (10 mg/bird) was injected intramuscularly into hens of a thick-shell line and a thin-shell line as well as into roosters of both lines on day 0. Serum levels of calcium (Ca), phosphorus (P), and Ca binding involving the proteins vitellogenin (a phosphoprotein) and albumin were measured on day 0 and day 6 or 7. Serum levels of all measured Ca and P parameters and vitellogenin binding of both sexes were significantly greater, while levels of albumin binding were significantly less on day 6 or 7 than on day 0. Serum levels of non-diffusible Ca in hens and vitellogenin binding in both sexes were significantly greater in the thick-shell than in the thin-shell line. There were no significant differences between lines for total serum P of either sex. A study of the hens only revealed that the change in nondiffusible Ca and vitellogenin binding from day 0 to day 6 or 7 was significantly different between lines. The correlations between vitellogenin binding and total P, expressed as either change or percent change, were greater than .8.