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

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 of 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 approximately 2265 nucleotides and the AFP mRNA possesed a length of approximately 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 approximately 100 adenosine residues. The AFP mRNA preparation was found to contain an average poly(A) tract of approximately 190 bases. Thus, albumin mRNA appears to contain approximately 330 untranslated nucleotides, and AFP mRNA appears to contain a similar number (approximately 285) of noncoding, nonpoly(A) bases. The purified albumin and AFP mRNA's 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. ROt 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 was found to drastically decrease during the first weeks of postnatal life, concomitant with a decline in the AFP synthetic capacity of the livers 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 10(3)-fold higher than that found in normal adult liver, whereas the content of albumin nRNA was four- to five-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.     

alpha-Fetoprotein and albumin mRNA levels in liver regeneration and carcinogenesis

Using a titration procedure, we measured the proportion of alpha-fetoprotein (AFP) and albumin mRNA in normal, regenerating, and preneoplastic rat livers. AFP mRNA constitutes approximately 0.006% of the polysomal polyadenylated RNA of normal livers and this proportion increases only slightly before the onset of DNA synthesis in liver regeneration induced by partial hepatectomy or CCl4 injury. In either model of liver regeneration, the proportion of AFP mRNA in polysomal RNA is highest approximately 24 h after the peak of DNA synthesis. The increase in the proportion of AFP mRNA in polysomal RNA is relatively small during liver regeneration (2-4-fold) but is larger (30-50-fold) in preneoplastic livers of rats fed a choline-deficient diet containing 0.1% ethionine. In contrast to those changes in AFP mRNA, albumin mRNA levels remain unchanged during liver regeneration and double in preneoplastic livers. Our results indicate that the concept of "retrodifferentiation" as it applies to liver regeneration and certain types of hepatic neoplasia needs reevaluation.     

Changes in the polyadenylated messenger RNA population during development of Dictyostelium discoideum

The program of gene expression during the life cycle of Dictyostelium discoideum has been assessed by molecular hybridization of cDNA probes with polysomal RNA extracted at the following different stages of development: vegetative growth, interphase (2.5 hr), aggregation (8 hr), postaggregation (12 hr), and preculmination (18 hr). Several different cDNA probes were used. Two probes were prepared from vegetative stage poly(A⁺) RNA, one representing all species present and the other enriched for abundant species. A third cDNA probe was prepared from preculmination stage polysomal RNA and a fourth probe consisted of the preculmination stage cDNA depleted in those species also present at the vegetative stage. Hybridization of the various probes with the different polysomal RNA preparations has revealed developmental changes in the mRNA populations. These changes were not detected in an aggregation less mutant under similar conditions of starvation. Abundant RNA species of vegetative cells were found to drop to low levels, especially during the aggregation period. Fifty percent by mass of the RNA present in polysomes at 18 hr is not present during vegetative growth. Some of the new RNA species appeared during interphase and the remaining during the postaggregation period. A gradual increase in the number of copies per cell of certain RNA species comprising both new species as well as some shared with vegetative cells was observed throughout development. Other results indicated that the composition of polysomal and cytoplasmic RNA is similar during vegetative growth but differs markedly at 18 hr of development. Also, cytoplasmic RNA at 18 hr contained, in addition to polysomal RNA, a large proportion by mass of nonpolysomal RNA similar to vegetative RNA. The number of polysomal RNA species detected by this analysis during vegetative growth and during the preculmination stage were estimated to be 3000 and 3700, respectively. The number of copies of these RNA species ranged between 30 and 2000 per cell during vegetative growth and 3 to 300 per cell in polysomes at 18 hr. Developmentally induced RNAs which were preferentially distributed among abundant and intermediate classes were estimated to number 700–900 species.     

Differential expression of albumin and alpha-fetoprotein genes in fetal tissues of mouse and rat

We have carried out a comparative analysis of the expression of the albumin and alpha-fetoprotein (AFP) genes in yolk sac and liver at different stages of fetal and postnatal life, in rat and mouse. Albumin and AFP mRNA levels were examined in these tissues by R0t analysis of RNA excess-cDNA hybridization data and/or by Dot blot hybridization. In addition, size analysis of the mRNA sequences were performed by electrophoretic fractionation on agarose gels containing methylmercury hydroxide and hybridization to radioactive cloned rat and mouse albumin and AFP cDNA probes. In the mouse, substantial amounts of albumin mRNA molecules were found in the yolk sac at different stages of development, while minimal levels of albumin mRNA sequences were detected in the rat yolk sac. The mouse yolk sac albumin mRNA molecules were found to be associated with the polysomes and to be functional in cell-free translation systems. In the rat, a reciprocal relationship appears to exist between the concentrations of the two mRNAs in yolk sac and embryonic liver. In contrast, in the mouse a parallel increase in both albumin and AFP mRNA levels was found in these tissues during fetal development. These results suggest that the expression of the albumin and AFP genes may be subjected to different regulatory events in these two members of the Muridae family.     

Procedure for the Preparation of Milligram Quantities of Adenovirus Messenger Ribonucleic Acid

Late after adenovirus 2 infection (18 hr), nearly all newly synthesized polysomal messenger ribonucleic acid (mRNA) is viral specified. Large amounts of adenovirus mRNA have been purified by utilizing membrane filtration at high ionic strength. With this procedure, molecules that contain polyadenylic acid [poly (A)] tracts are bound selectively, and ribosomal RNA can be separated from the viral mRNA which contains poly(A). Polysomal RNA synthesized 18 hr after infection was labeled in the presence of 0.02 mug of actinomycin D per ml and extracted at pH 9.0. This RNA annealed 40% to 3 mug of adenovirus 2 deoxyribonucleic acid; the RNA selected by membrane filtration bound 80% under the same conditions. The RNA eluted from membrane filters was 80 to 90% greater than 18S and contained species migrating as 31, 27, and 24S. Binding of polysomal RNA to individual membrane filters was linear, using as much as 300 mug of RNA per membrane. A 1.1-mg amount of viral RNA was prepared from 17.7 mg of polysomal RNA that had been purified by extraction at pH 9.0.     

Frequency distribution of pre-messenger RNA sequences in polyadenylated and non-polyadenylated nuclear RNA from Friend cells.

Hybridisation of cDNA probes for abundant and rare polysomal polyadenylated RNAs with polyadenylated and non-polyadenylated nuclear RNA from Friend cells indicated that the abundant polysomal polyadenylated RNA sequences were present at a higher concentration in the nucleus than rare polysomal sequences, but at a reduced range of concentrations. The ratio of the concentrations of abundant and rare sequences was about 3 in non-polyadenylated nuclear RNA, 9 in polyadenylated nuclear RNA and 13 in polysomal polyadenylated RNA. This suggests that polyadenylation may play a role in the quantitative selection of sequences for transport to the cytoplasm. Polyadenylation cannot be the only signal for transport, since a highly complex population of nucleus-confined polyadenylated molecules exists, each of which is present on average at less than one copy per cell.     

Sequence content of alpha-fetoprotein, albumin and fibrinogen polypeptide mRNAs in different organs, developing tissues and in liver during carcinogenesis in rats

To investigate the variable gene activities of alpha-fetoprotein, albumin and fibrinogen polypeptides as markers of 'liver specific proteins' in different developing organs or tissues, we have used specific complementary DNA probes to detect and to quantitate alpha-fetoprotein, albumin and fibrinogen polypeptide mRNA, respectively, in RNA fractions, prepared from various tissues of rats at different stages of fetal and postnatal development and from hepatomas induced by diethylnitrosamine. The results indicate that there is no consistent relationship between sequence content of alpha-fetoprotein, albumin and fibrinogen polypeptide mRNA in different developing tissues. Intestines which are like the liver also of endodermal origin do not contain alpha-fetoprotein, albumin and fibrinogen polypeptide mRNAs, while kidneys which are mesodermal in origin were found to be alpha-fetoprotein, albumin and fibrinogen polypeptide mRNA producers in neonatal life. In yolk sac, only alpha-fetoprotein and fibrinogen polypeptide mRNA could be detected. In the liver, the increased level of albumin and fibrinogen polypeptide mRNA during fetal and neonatal development is accompanied with a diminished amount of alpha-fetoprotein mRNA. The neosynthesis of alpha-fetoprotein mRNA in the liver during carcinogenesis occurred without a decreased content of albumin and fibrinogen polypeptide mRNAs. These findings suggest that complex mechanisms of gene regulation are involved in variable gene activities of alpha-fetoprotein, albumin and fibrinogen polypeptides in cells of different organs or tissues developed from a single cell.     

Frequency distribution of mRNA and pre-mRNA in growing and differentiated Friend cells.

The frequency distribution of poly(A)+-mRNA in growing and in differentiated Friend cells has been measured by mRNA-cDNA hybridization and their differences established by heterologous hybridization of mRNA of one type and cDNA of the other. It was shown that induction of Friend cells involves an increase in abundance of a small number of mRNAs, while no specific pattern of messenger disappearance could be detected. The frequency distribution of pre-mRNA was determined by hybridizing nuclear RNA with the cDNA probes complementary to mRNA. In uninduced Friend cells, it was shown that most precursor messenger sequences are present at a single frequency of about 3 molecules per nucleus, independently of their final frequency in polysomal mRNA. In induced Friend cells, the frequency distribution of pre-mRNA is more heterogeneous and correlated to some extent with the corresponding mRNA frequency distribution.     

Complexity of poly(A+) and poly(A-) polysomal RNA in mouse liver and cultured mouse fibroblasts.

RNA excess hybridization experiments were used to measure the complexity of nuclear RNA, poly(A+) mRNA, poly(A-) mRNA, and EDTA-released polysomal RNA sedimenting at less than 80 S in mouse liver and in cultured mouse cells. With both cell types, poly(A-) RNA was found to contain 30-40% of the sequence diversity of total mRNA. In the case of liver this represents 5,700 poly(A-) molecules and 8,600 poly(A+) molecules for a total of approximately 14,300 different mRNAs. Comparison of the complexity of mRNA with that of nuclear RNA revealed that in liver and in cultured cells, mRNA has only 10-20% of the sequence diversity present in nuclear RNA. This latter observation is consistent with existing data on mammalian cells from this and other laboratories.     

Isolation of rat liver albumin messenger RNA.

Rat liver albumin messenger RNA has been purified to apparent homogeneity by means of polysome immunoprecipitation and poly(U)-Sepharose affinity chromatography. Specific polysomes synthesizing albumin were separated from total liver polysomes through a double antibody technique which allowed isolation of a specific immunoprecipitate. The albumin-polysome immunoprecipitate was dissolved in detergent and the polysomal RNA was separated from protein by sucrose gradient centrifugation. Albumin mRNA was then separated from ribosomal RNA by affinity chromatography through the binding of poly(U)-Sepharose to the polyadenylate 3' terminus of the mRNA. Pure albumin mRNA migrated as an 18 S peak on 85% formamide-containing linear sucrose gradients and as a 22 S peak on 2.5% polyacrylamide gels in sodium dodecyl sulfate. It coded for the translation of authentic liver albumin when added to a heterologous protein-synthesizing cell-free system derived from either rabbit reticulocyte lysates or wheat germ extracts. Translation analysis in reticulocyte lysates indicated that albumin polysomes were purified approximately 9-fold from total liver polysomes, and that albumin mRNA was purified approximately 74-fold from albumin polysomal RNA. The total translation product in the mRNA-dependent wheat germ system, upon addition of the pure mRNA, was identified as authentic albumin by means of gel electrophoresis and tryptic peptide chromatography.     

Poly(A)+ RNA synthesis in germinating pollen ofNicotiana tabacum L

Poly(A)⁺RNA is synthesized during the first hours of pollen germination and is rapidly incorporated into polysomal structures. After a 2-h pulse with uracil-¹⁴C, 42% of the transcribed fraction of polysomal RNA is polyadenylated. Following 4 h of germination the amount of the newly-made poly(A)⁺RNA decreases steadily at the rate of about 14% per h, whereas that of rapidly-labelled poly(A)⁻RNA continues to grow. Beginning 1 h of cultivation the ratio of poly(A)⁻/poly(A)⁺RNA increases exponentially. Similarly as in non-polyadenylated mRNA the main portion of the synthesized polysomal poly(A)⁺RNA sediments at a rate of 4 to 14 S and its mean size decreases slightly with the time of labelling. RNA isolated from nuclei and cell wall containing pollen tube fraction differed from the polysomal one in higher apeoific radioactivity and the polyadenylated RNA exhibited higher size distribution. The comparison of the results with earlier observations suggests the involvement of poly(A)in mRNA translation in pollen tubes.