Structure of the chicken apo very low density lipoprotein II gene

Apolipoprotein (apo) B and apoII gene expression are both estrogen-responsive in chick embryo liver but only apoII is estrogen-responsive in kidney

Estrogen regulates the hepatic synthesis of a variety of proteins required for egg yolk production in oviparous vertebrates. In chickens, two of these proteins, apolipoprotein (apo) B and apoII, comprise the major protein components of specialized very low density lipoprotein particles that transport triacylglycerols and cholesterol to the developing egg yolk. In the adult, apoB is synthesized constitutively in liver, small intestine, and kidney but is estrogen-responsive only in the liver. In this work we have examined the embryonic expression of the apoB and apoII genes in yolk sac, liver, kidney, and small intestine. The 14 kb apoB mRNA was first detected at day 3 of development in vascular yolk sac, a tissue involved in the transfer of yolk lipids into the embryonic circulation. Constitutive apoB mRNA expression was detectable in liver at day 6.5 and in kidney at day 7.5, but in intestine was barely detectable before hatching. The hepatic apoB gene acquired estrogen-responsiveness at day 6.5 and its hormone-dependent expression increased throughout development in concert with the estrogen-responsive expression of the apoII gene. In contrast, the constitutively expressed apoB gene in kidney remained unresponsive to estrogen. Surprisingly, the apoII gene was found to be responsive to estrogen in both the embryonic kidney and small intestine. ApoII mRNA induction by estrogen in kidney at day 11 was at 10% of the level in the liver but estrogen-responsiveness decreased later in development and was low in the adult.(ABSTRACT TRUNCATED AT 250 WORDS)

Estradiol-dependent transcription initiation upstream from the chicken apoVLDLII gene coding for the very-low-density apolipoprotein II

We have investigated RNAs originating from the 5'-flanking region of the chicken very-low-density apolipoprotein II (apoVLDLII) gene. S1 nuclease mapping and primer extension experiments revealed two minor upstream transcription start points located 1105 and 1530 nucleotides in front of the apoVLDLII gene. Transcription starting at these points is dependent upon estradiol as is transcription from the major start points. The transcripts are polyadenylated, but are not detectable in polysomes. Run-on assays indicated that the low concentration of the upstream initiated transcripts is due both to low transcription levels and to low transcript stability. The sequence around the upstream start points does not show strong homologies with consensus sequences of promoters for eukaryotic protein encoding genes. Nevertheless, the upstream sequences are transcribed in vivo by RNA polymerase II.

A density gradient study of the lipoprotein and apolipoprotein distribution in the chicken, Gallus domesticus

Plasma lipoproteins from 5-week old male chickens were separated over the density range 1.006-1.172 g/ml into 22 subfractions by isopycnic density gradient ultracentrifugation, in order to establish the distribution of these particles and their constituent apolipoproteins as a function of density. Lipoprotein subfractions were characterized by electrophorectic, chemical and morphological analyses, and their protein moieties were defined according to net charge at alkaline pH, molecular weight and isoelectric point. These analyses have permitted us to reevaluate the density limits of the major chicken lipoprotein classes and to determine their main characteristics, which are as follows: (1) very-low-density lipoproteins (VLDL), isolated at d less than 1.016 g/ml, were present at low concentrations (less than 0.1 mg/ml) in fasted birds; their mean diameter determined by gradient gel electrophoresis and by electron microscopy was 20.5 and 31.4 nm respectively; (2) as the the density increased from VLDL to intermediate density lipoproteins (IDL), d 1.016-l.020 g/ml) and low-density lipoproteins (LDL, d 1.020-1.046 g/ml), the lipoprotein particles contained progressively less triacylglycerol and more protein, and their Stokes diameter decreased to 20.0 nm; (3) apolipoprotein B-100 was the major apolipoprotein in lipoproteins of d less than 1.046 g/ml, with an Mr of 350000; small amounts of apolipoprotein B-100 were detectable in HDL subfractions of d less than 1.076 g/ml; urea-soluble apolipoproteins were present in this density range as minor components of Mr 38000-39000, 27000-28000 (corresponding to apolipoprotein A-1) and Mr 11000-12000; (4) high density lipoprotein (HDL, d 1.052-1.130 g/ml) was isolated as a single band, whose protein content increased progressively with increase in density; the chemical composition of HDL resembled that of human HDL2, with apolipoprotein A-1 (M 27000-28000) as the major protein component, and a protein of Mr 11000-12000 as a minor component; (5) heterogeneity was observed in the particle size and apolipoprotein distribution of HDL subfractions: two lipoprotein bands which additional apolipoproteins of Mr 13000 and 15000 were detected. These studies illustrate the inadequacy in the chicken of the density limits applied to fractionate the lipoprotein spectrum, and particularly the inappropriateness of the 1.063 g/ml density limit as the cutoff for LDL and HDL particle populations in the species.

Molecular biology in arteriosclerosis research

The topics discussed in this article illustrate how molecular biology will have a dramatic impact on arteriosclerosis research. DNA clones for a small number of relevant proteins have been isolated, and studies are underway in numerous laboratories to extend these initial studies. The techniques of molecular biology will provide major advances in our understanding of numerous proteins directly or indirectly involved in the atherogenic process. Cloning technology will solve the primary structures of many proteins that can not be purified in quantities sufficient for classical methods of analysis. Studies of regulation will benefit from the availability of DNA probes, the ability to generate site-directed antibodies, and the use of reverse genetics to identify nucleic acid sequences involved in the regulation of gene expression. Studies of gene structure and genetic polymorphisms will unravel the genetic basis for defects in lipid and lipoprotein metabolism and should provide valuable reagents for clinical screening and diagnosis. The reverse genetics approach will permit the systematic analysis of structure-function relationships at the protein level in a manner not previously possible. Each of these will contribute to our understanding of the atherogenic process and should provide insight into ways of preventing and treating arteriosclerosis.