Post-transcriptional regulation of the abundance of mRNAs encoding alpha-tubulin and a 94,000-dalton protein in teratocarcinoma-derived stem cells versus differentiated cells

A urokinase receptor mRNA binding protein from rabbit lung fibroblasts and mesothelial cells

The urokinase receptor (uPAR) influences several biological functions relevant to lung injury and repair, including proteolysis, cell migration, and adhesion. In malignant mesothelioma cells, we recently found that a posttranscriptional mechanism involving a cis-trans interaction between a uPAR mRNA sequence and a cytoplasmic uPAR mRNA binding protein (mRNABP) regulates uPAR gene expression (S. Shetty, A. Kumar, and S. Idell. Mol. Cell Biol. 17: 1075-1083, 1997). In this study, we sought to determine if uPAR expression in lung and pleural cells involves a similar posttranscriptional pathway. We first identified and characterized the uPAR mRNABP in rabbit tissues using gel mobility shift, ultraviolet (UV) cross-linking, and RNase protection assays and detected it in liver, heart, brain, spleen, colon, and lung. Phorbol 12-myristate 13-acetate, lipopolysaccharide, transforming growth factor-beta, tumor necrosis factor-alpha, or cycloheximide induced uPAR and uPAR mRNA expression in cultured rabbit pleural mesothelial cells and lung fibroblasts and concurrently reduced the uPAR mRNA-uPAR mRNABP interaction. Using conventional and affinity chromatography, we purified a 50-kDa uPAR mRNABP that selectively binds to a 51-nucleotide fragment of the uPAR coding region. This protein migrates as a monomer when analyzed by SDS-PAGE and UV cross-linking and does not possess intrinsic RNase activity in vitro. A uPAR mRNABP physicochemically and functionally similar to that of human malignant mesothelioma is constitutively expressed in the rabbit lung and other nonneoplastic tissues. In rabbit lung fibroblasts and mesothelial cells, expression of uPAR involves posttranscriptional regulation whereby the uPAR mRNABP appears to interact with a specific coding region cis-element to decrease the stability of uPAR mRNA.

The mouse elongation factor-2 gene: isolation and characterization of the promoter

Elongation factor 2 (EF-2) is a protein involved in peptide chain elongation in eukaryotes. We isolated the mouse EF-2 gene and characterized its promoter. We showed that the majority of enhancer elements were located within 500 bp of the flanking sequence and identified a factor binding site sequence (CGTCACGTGACGC) located between nucleotides -58 and -47 containing two CGTCA motifs separated by two nucleotides. The motif represents a half-site for binding of the cAMP response element (CRE) binding protein (CREB). Mutation analysis indicated that the presence of one CGTCA site alone conferred cAMP inducibility, but the presence of one or two CGTCA sites and spacing nucleotides elicited cAMP-independent, constitutive expression. UV cross-linking and DNA affinity chromatography revealed that three 40-, 43-, and 65-kD proteins bound to the CRE-like element. Of these, the 65-kD protein was unique to the CRE-like element. The 40-kD protein was ATF1 and the 43-kD protein with the molecular size of CREB was not CREB, on the basis of reactivity to their respective antibodies. Because ATF1 responds poorly to cAMP induction, it is likely the contributor to the constitutive expression rather than inductive expression of the CRE-like element, and, thus, the EF-2 gene.

Platelet-derived growth factor B-chain gene expression in mesangial cells: effect of phorbol ester on gene transcription and mRNA stability

We have investigated the effect of phorbol 12-myristate 13-acetate (PMA) on platelet-derived growth factor (PDGF) B-chain gene transcription as well as on mRNA stability in cultured human mesangial cells. Addition of actinomycin to cells stimulated with PMA decreases steady state levels of PDGF-B chain mRNA analysed by solution hybridization assay. PDGF-B chain gene transcription was also assayed directly by measuring elongation of transcripts in isolated nuclei followed by hybridization of labeled RNA transcripts to a cDNA encoding for PDGF-B chain. Our data show that PMA induces PDGF-B chain gene transcription by approximately 2-fold. alpha-Amanitin, an RNA polymerase II inhibitor, blocked transcription by more than 70%. In addition, we determined the effect of PMA on the halflife of PDGF-B chain mRNA directly by pulse chase method. In human mesangial cells, the PDGF-B chain mRNA exhibited halflife of approximately 105 min. In the presence of PMA, the halflife of PDGF-B chain mRNA was reduced to approximately 72 min. These studies indicate that regulation of PDGF-B chain gene by PMA in human mesangial cells involves a coordinate effort at the level of transcription and mRNA stability.

Platelet derived growth factor-A chain gene expression in cultured mesangial cells: regulation by phorbol ester at the level of mRNA abundance, transcription and mRNA stability

In human renal mesangial cells, platelet derived growth factor (PDGF)-A chain is subject to regulation by protein kinase C (PKC) activator, phorbol ester (phorbol 12-myristate 13-acetate, PMA). Treatment of mesangial cells with PMA increases PDGF-A chain mRNA abundance as analyzed by Northern blot hybridization. In contrast to the effect of PMA, the inactive analog phorbol had no effect on PDGF-A chain mRNA levels, while the PKC inhibitor H7 markedly reduced the PMA-induced increment in PDGF-A chain mRNA. To determine the mechanism by which PMA increases the abundance of this gene, transcription rate was measured by nuclear transcript elongation assay. Treatment of mesangial cells with PMA resulted in a 2-fold increase in PDGF-A chain gene transcription. In addition, we analyzed the effects of PMA on PDGF-A chain mRNA half-life as measured directly by pulse-chase method. PDGF-A chain mRNA has a half-life of about 106 min. The PDGF-A chain mRNA half-life was reduced by 30% (t1/2 = 74 min) when mesangial cells were incubated with PMA. Our results demonstrate that in human renal mesangial cells, the regulation of PDGF-A chain gene expression by PMA is primarily at the level of transcription.

Expression and stability of c-sis mRNA in human glioblastoma cells

The production of platelet-derived growth factor like (PDGF-like) material by glioblastomas may be involved in the conversion of normal cells to tumor cells. In an investigation of this problem, we have examined some of the properties of the platelet-derived growth factor B-chain mRNA (c-sis mRNA) by a sensitive and quantitative RNA-RNA solution hybridization method. In 5 out of 8 human glioblastoma cell lines, c-sis mRNA was present, and in the line with the highest level, there were approximately 4-10 molecules per cell. The half-lives of the c-sis mRNA in two glioblastoma cell lines were 2.6 and 3.4 h, while in human umbilical vein endothelial (HUVE) and bladder carcinoma (T24) cells they were 1.6 and 2.5 h, respectively. Inhibiting protein synthesis produced no significant alteration of the c-sis mRNA half-lives in the glioblastoma or HUVE cells. The A-U-rich sequence at the 3' end of the c-sis mRNA therefore does not appear to affect the mRNA stability in the presence of cycloheximide as it does in other transcripts. The similarity of the c-sis mRNA half-lives in normal and tumor cells suggests that regulation of stability of c-sis mRNA is not a major factor in tumorigenesis in the glioblastoma cell lines examined.

Expression of SPARC/osteonectin transcript in murine embryos and gonads

A cDNA clone, p2-4, was isolated from mouse teratocarcinoma-derived parietal endoderm-like cells and used to analyze expression of the corresponding transcript during mouse embryogenesis. Nucleotide-sequence analysis revealed extensive homology between this clone and SPARC/osteonectin cDNA cloned from mouse parietal endoderm and bovine bone cells. The SPARC/osteonectin transcript became more abundant when embryonal carcinoma (EC) cells differentiated into parietal endoderm-like cells. In embryos, the transcript began to appear in the embryo proper on day 11 and continued to be expressed throughout the gestation period. The transcript was also present in extraembryonic membranes and placenta from days 9 and 11 onward, respectively. Thus, expression of the transcript was regulated during differentiation of EC cells and during embryogenesis. In adult mice, several non-bone tissues, including testis, also expressed the transcript. Analysis of germ-cell-deficient mice indicated that non-germ-cell components of the testis expressed the transcript. Analysis of mouse testicular cell lines further suggested that the transcript was abundant in Sertoli cells and Leydig cells. Cumulus oophorus cells that envelope the ovulated egg also expressed high levels of the transcript.

Isolation of a clone of F9 teratocarcinoma cells "naturally" resistant to G418

Resistance to the neomycin analogue G418 forms the basis of a dominant marker selection system for mammalian (and other) cells transfected with the bacterial neo gene. This system has been particularly effective because of the low incidence of spontaneous conversion to G418 resistance in mammalian cells; no case of resistance to the drug in the absence of the bacterial genes has yet been reported to our knowledge. During the course of transfection experiments, we recently isolated a clone of F9 teratocarcinoma cells which is drug resistant yet has no detectable integrated plasmid sequences, neo RNA transcripts, or aminoglycoside phosphotransferase activity. The G418-resistant clone (F9nr7) did not display enhanced resistance to other cytotoxic drugs tested: colchicine, actinomycin D, cycloheximide, and hygromycin B. Therefore, nr7 cells differ from multidrug-resistant phenotypes previously described. However, this clone is inhibited, relative to control cells, in its response to the differentiation-inducing drugs retinoic acid and dibutyryl cAMP, which suggests that some aspects of general drug metabolism may be altered in these cells.

Modulation of protein biosynthesis during early stages of differentiation in retinoic acid treated F9 teratocarcinoma cells

Modulation of protein biosynthesis by retinoic acid during induction of differentiation of F9 teratocarcinoma stem cells was investigated by using computerized analysis of double label autoradiography of two-dimensional polyacrylamide gels. As early as 6 h after induction increased synthesis of 5 and decreased synthesis of 2 proteins occur. By 12 h after induction, synthesis of 13 proteins is elevated and by 24 h that of 17. At 24 h the range of stimulation is from two- to fourfold, as demonstrated by a 3H:14C ratio divided by the mode ratio. Examination of the Gaussian distributions of frequency of ratio indicates that many subtle changes in protein synthesis accompany the development of the new phenotype.

H4 histone messenger RNA decay in cell-free extracts initiates at or near the 3' terminus and proceeds 3' to 5'

The relative decay of four human messenger RNAs, gamma globin, delta globin, c-myc and H4 histone, were compared in a cell-free system. Under appropriate conditions, they are degraded in vitro in approximately the same relative order as in vivo: histone faster than c-myc and delta globin faster than gamma globin. Degradation of polysome-associated H4 histone mRNA and of deproteinized histone mRNA begins at or near the 3' terminus. At least a portion of the mRNA then continues to be degraded in a 3' to 5' direction. Discrete 3'-terminal degradation hold-up points are observed, suggesting that 3' to 5' degradation occurs non-uniformly. Cycloheximide and puromycin inhibit protein synthesis but do not affect the rate or directionality of histone mRNA decay in vitro. We conclude that the rate-limiting step in H4 histone mRNA decay occurs at or near the 3' terminus and that at least a portion of the mRNA molecule is subsequently degraded 3' to 5', probably via a processive exonuclease.

Post-transcriptional control of myc and p53 expression during differentiation of the embryonal carcinoma cell line F9

Teratocarcinoma cells provide us with a model system for the study of differentiation and development. One of the best characterized cell lines, the embryonal carcinoma stem cell line F9, differentiates after treatment with retinoic acid (RA) and dibutyryl cyclic AMP into parietal endoderm. This differentiation process is accompanied by the induction of several genes, for example, those encoding collagen IV, plasminogen activator and intermediate filaments like laminin. In contrast, a marked reduction of stable messenger RNA has been observed for the gene encoding p53 and for c-myc. Both cellular oncogenes seem to be involved in the regulation of cellular proliferation and neoplastic transformation. For growth-arrested 3T3 fibroblasts, growth-factor-induced changes of myc RNA are controlled at the level of transcription. In contrast, F9 cells provide a differentiation system in which cells are able to change from a tumorigenic state into non-dividing, non-tumorigenic endodermal cells. The latter process enabled us to study the regulation of myc and p53 genes in the same cells at different stages of growth, tumorigenicity and differentiation. Here we report that down-regulation of stable myc and p53 RNA during irreversible differentiation of F9 cells occurs at the post-transcriptional level. Using an in vitro nuclear transcription assay, we found that the polymerase II density on both genes remains constant during differentiation. In agreement with this interpretation, we detected myc RNA as stable transcripts in differentiated F9 cells after treatment of the cells with cycloheximide. The post-transcriptional regulatory mechanisms controlling p53 and myc stability follow different kinetics. Whereas the down-regulation of myc seems to be an early event of F9 differentiation occurring within the first 24 h, the post-transcriptional regulation of p53 occurs at a later stage (two to three days), possibly as a consequence of cell cycle changes.