Changes in expression of albumin and .alpha.-fetoprotein genes during rat liver development and neoplasia

Abstract

Albumin mRNA was isolated and purified from rat liver polysomes by a combination of immunoprecipitation of specific polysomes, poly(U)-Sepharose 4B chromatography, and fractionation of the resulting poly(A)-containing RNA on a sucrose gradient. .alpha.-Fetoprotein (AFP) mRNA was isolated from Morris hepatoma 7777 by a similar procedure. The purity of the mRNA preparations was determined by analytical gel electrophoresis under denaturing conditions, analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the polypeptides synthesized in a wheat germ cell-free system, and the kinetics of hybridization to cDNA transcribed from albumin mRNA and AFP mRNA. The albumin mRNA possessed a chain length of .apprx. 2265 nucleotides and the AFP mRNA possessed a length of .apprx. 2235 nucleotides when examined under stringent denaturing conditions on agarose gels containing 10 mM methylmercury hydroxide. Analysis of poly(A) content by a hybridization assay with [3H]poly(U) revealed the presence in albumin mRNA of a poly(A) region containing .apprx. 100 adenosine residues. The AFP mRNA preparation contained an average poly(A) tract of .apprx. 190 bases. Albumin mRNA appears to contain .apprx. 330 untranslated nucleotides, and AFP mRNA appears to contain a similar number (.apprx. 285) of noncoding, nonpoly(A) bases. The purified albumin and AFP mRNA were used as templates for synthesis of full-length cDNA hybridization probes. Both of the probes selectively hybridized to their templates with kinetics expected for single RNA species the sizes of albumin and AFP mRNA. R0t analysis was used to quantitate albumin and AFP mRNA sequences during normal liver postnatal development and liver oncogenesis. The number of polysomal AFP mRNA molecules per liver drastically decreased during the first weeks of postnatal life, concomitant with a decline in the AFP synthetic capacity of the liver and in the serum concentrations of AFP. During this period, the concentration of albumin mRNA molecules per cell in the liver remained at high, approximately constant levels. In Morris hepatoma 7777, the concentration of AFP-specifying sequences was at least 103-fold higher than that found in normal adult liver, whereas the content of albumin mRNA was 4- to 5-fold lower. These changes in concentration of albumin and AFP mRNA sequences closely correlated with a parallel variation in the specific protein synthetic capacity of the tissues.