Biosynthesis and stability of globin mRNA in cultured erythroleukemic Friend cells

Stability of polyadenylated RNA in differentiating myogenic cells

Three independent methods of measurement showed that cytoplasmic polyadenylated RNA from the differentiating myogenic cell line L8 consists of two main populations with regard to stability, one with a half-life of less than 4 h and the other with a half-life of 17--54 h. Similar results were obtained in the presence and absence of actinomycin D. During the fusion of mononucleated myoblasts into multinucleated fibers, there was an increase in both the steady-state pool of the more stable polyadenylated RNA and the proportion of stable polyadenylated RNA synthesized in pulse labelling.

The capacity of polyadenylated RNA from myogenic cells treated with actinomycin D to direct protein synthesis in a cell-free system

Cytoplasmic polyadenylated RNA of myogenic cells was shown to decay with biphasic kinetics, suggesting the existence of two main populations of mRNA with respect to stability. In the present study, the stability of mRNA extracted from actinomycin-D-treated cultures of a myogenic cell line was tested by its capacity to direct protein synthesis in the wheat germ cell-free system. The products were analyzed by dodecylsulphate/polyacrylamide gel electrophoresis. All major radioactive bands found in gels used for analyzing the products of the cell-free system directed by polyadenylated RNA extracted from untreated cultures were also found in similar gels containing products of RNA extracted after many hours of application of actinomycin D. The capacity to code for specific protein bands decays with a half-life ranging between 11 and 40 h. No fast-decaying translatable mRNA could be detected by this method. Instead, it was found that during the first 4--6 h following application of actinomycin D, the capacity of RNA to stimulate incorporation of amino acids into total acid-insoluble material increased by 20--30%. The synthesis of specific products increased by up to 100%. The possibility that the fast-decaying polyadenylated RNA or part of it is nontranslatable RNA is discussed.

A change in the stability of globin mRNA during the induction of murine erythroleukemia cells

The stability of globin mRNA in murine erythroleukemia cells (Friend cells) before and during DMSO-induced differentiation was investigated. Cells were exposed to 3H-uridine for 2 hr and then transferred to medium without the radioactive precursor. The loss of radioactivity in total RNA, poly(A)-containing RNA and globin mRNA was followed. The globin mRNA was isolated using a highly specific globin cDNA column. In uninduced cells and cells early in differentiation, the globin mRNA decays with a half-life of less than 50 hr. After 4 days of induction, the globin mRNA decays with a half-life of 17 hr, demonstrating a change in stability during the induction process. Although the stability of globin mRNA changes during induction, this is not true for total poly(A)-containing RNA. At all times of induction, the poly(A)-containing RNA decays as two populations, one with a half-life of 6 hr and the other with a half-life of 36 hr. The half-life of the rRNA also remains unchanged during differentiation.

Alterations in the protein synthetic apparatus of Friend erythroleukemia cells during their erythroid differentiation

The changes in rate of protein synthesis and cell division and the distribution of polyribosomes and globin mRNA on the polyribosomes of Friend erythroleukemia (FL) cells exposed to 2% DMSO and maintained at low cell density, were examined at different times after exposure to DMSO. The rate of protein synthesis and the capacity of cells to divide declined in concert to 50% of the level found in untreated cell cultures at 24 hours after exposure. Thereafter these rates recovered to 70% of the rate found in untreated control cultures until 96 hours post-exposure and then irreversibly declined as the cells lost the capacity to divide. The proportion of ribosomes present as polyribosomes in cells exposed to DMSO paralleled the capacity of these cells to synthesize protein. The distribution of polyribosomes analyzed by sedimentation in sucrose gradients demonstrated that a discrete, abundant class of polyribosomes composed of pentamers to heptamers appeared as early as 48 hours after exposure to DMSO. The appearance of an abundant class of polyribosomes was correlated with globin synthesis by demonstrating that a discrete class of polyribosomes arises in cells treated with the inducers hexamethylene bisacetamide and hemin.

Differentiation of murine erythroleukemic (Friend) cells: an in vitro model of erythropoiesis

Normal erythropoiesis involves differentiation of uncommitted stem cells through committed erythroid precursors into cells specialized for hemoglobin synthesis. Several aspects of this developmental sequence may be studied in murine erythroleukemic cells infected with Friend virus complex. These cells are arrested at the proerythroblast stage, yet capable of continuous growth in vitro. Maturation along an erythroid pathway is induced after treatment with a variety of agents (e.g. dimethylsulfoxide, butyric acid, hemin, ouabain). Following induction, the cells morphologically resemble normoblasts, accumulate globin mRNAs and strain-specific globins, increase heme synthesis and acquire erythrocyte membrane antigens. Cloned populations of erythroleukemic cells mature in a nonhomogeneous fashion upon induction, indicative of a stochastic response in the inductive process. This "probability of differentiation" phenotype is formally analogous to stem cell development in which hematopoietic precursor cells form a constant, dividing population from which cells are continuously maturing. Although the sequence of events involved in triggering differentiation is uncertain, cloning and cell hybridization experiments demonstrate that this phenotype is under rather stable genetic (or epigenetic) control. Recent molecular analysis shows that induced differentiation is accompanied by transcriptional activation of the globin genes rather than posttranscriptional stabilization of the globin RNAs. Further application of cellular, molecular and genetic approaches in this system may help to define specific control mechanisms in erythroid development.

Initiation of RNA synthesis in isolated nuclei

Ribonucleotide triphosphates, labeled at the beta position, were synthesized and used directly to quantify RNA chain initiation in nuclei isolated from Friend cells grown in tissue culture. At the optimal salt concentration, low-molecular-weight RNAs (4-5 S) synthesized by RNA polymerase III were the predominant species initiated. Less than 5% of the molecules were initiated by polymerase II. We calculate that 50-80% of the small RNA molecules synthesized in vitro were also initiated in vitro. Assuming that a substantial fraction of the nuclei were active in vitro, the number of 4-5 S RNA molecules initiated per nucleus was about 100 molecules/min.

Globin RNA precursor molecules: biosynthesis and process in erythroid cells

Hybridization of labeled RNA with excess amounts of DNA complementary to globin mRNA, in conjunction with a pulse-chase technique, were used to investigate the biosynthetic pathway of globin mRNA in erythroid cells. Three species of molecules sharing common sequences with globin mRNA were detected in the nuclei of these cells, two of which are larger than the cytoplasmic globin mRNA. One species was approximately 7 times larger than globin mRNA ("27S"), and the other ("15S") was only about twice the size of cytoplasmic globin mRNA. The largest species lacked poly(A) sequences, while the others contained poly(A), After chase, the large RNA species gradually disappeared ( 1/2 = 5 min), while the cytoplasmic 10S species accumulated. From these results a model is proposed describing the biosynthetic pathway of globin RNA transcription: an early transcription product is the large molecule "27S" (approximately 5000 nucleotides long) which is then cleaved into a smaller species "15S" (approximately 1500 nucleotides). This intermediate precursor is then clipped, presumably at the 5' end, and finally converted to the exported "10S" molecule (approximately 750 nucleotides) which accumulates in the cytoplasm.