Regulation of the human c-myc gene: 5' noncoding sequences do not affect translation

Derivation of a structural model for the c-myc IRES

We have derived a secondary structure model for the c-myc internal ribosome entry segment (IRES) by using information from chemical probing of the c-myc IRES RNA to constrain structure prediction programs. Our data suggest that the IRES is modular in nature, and can be divided into two structural domains linked by a long unstructured region. Both domains are required for full IRES function. Domain 1 is a complex element that contains a GNNRA apical loop and an overlapping double pseudoknot motif that is topologically unique amongst published RNA structures. Domain 2, the smaller of the two, contains an apical AUUU loop. We have located the ribosome landing site and have shown that ribosomes enter in a 16 nt region downstream of the pseudoknots in a situation similar to that observed in several viral IRESs. To test the structure, several key regions of the IRES were mutated and, interestingly, it appears that some of the structural elements that we have identified function to repress c-myc IRES function. This has profound implications for de-regulation of c-myc expression by mutations occurring in the IRES.

Translational control of growth factor and proto-oncogene expression

Control of translation is now understood to be one of the major regulatory events in eukaryotic gene expression. Moreover there is evidence which suggests that aberrant expression of growth-related genes by translational mechanisms makes a significant contribution to cell transformation. However, the mechanisms which regulate translation of specific growth-related mRNAs have yet to be fully elucidated. The majority of these mRNAs have long 5' untranslated regions (UTRs) and three features which are important in translational control have been identified, namely (i) structured regions which inhibit the scanning mechanisms of translation, (ii) regulatory upstream open reading frames and (iii) internal ribosome entry segments which are capable of initiating cap-independent translation. In this review the translational regulation of specific mRNAs encoding growth factors and proto-oncogenes by these three mechanisms will be discussed, together with examples of altered translational regulation in neoplasia.

Expression of the two reovirus S1 gene products in transfected mammalian cells

Reovirus genomic segment S1, which has been implicated in the viral inhibition of cellular DNA synthesis, is transcribed into a single mRNA that encodes two proteins, the approximately 49-kDa hemagglutinin, sigma 1, and the apparently nonstructural protein, p14. These two polypeptides have been expressed in mammalian cells, together or independently, in order to assess their role in the shutdown of host DNA replication. Results obtained with transient and stable expression systems demonstrate that production of serotype 3 sigma 1 and p14 together or individually is not sufficient to change the kinetics of DNA replication in uninfected cells. However, inhibition of DNA synthesis by reovirus type 1 infection was enhanced in cells producing type 3 sigma 1 and p14 but not sigma 1 by itself. In addition, expression of p14 alone led to increased cytopathic effects following infection by either type 1 or type 3 virus. The results suggest that interactions with other viral components are required to elicit the effects of the S1-specified polypeptides on cellular DNA synthesis.

Diverse responses of human fibroblasts to a highly purified fibroblast-activating factor from the U937 line of human monocytes

The responses of human dermal fibroblasts to highly purified fibroblast-activating factor (FAF), derived from supernatants of U937 human monocytes stimulated with phorbol myristate acetate (PMA), were investigated with several in vitro assays of specific synthetic functions. The highly purified peptide was detected as a single 16,000-18,000 MW protein, by both silver staining and Western blot analysis, with an antiserum generated against a synthetic peptide representing the amino-terminal sequence of 17 amino acids. At concentrations that induced similar levels of fibroblast proliferation, transforming growth factor-beta (TGF-beta), interleukin-1 (IL-1), acidic fibroblast growth factor (aFGF) and FAF also stimulated fibroblasts to generate and release prostaglandin E2 (PGE2) and proteoglycans. TGF-beta had the least effect on proteoglycan production. In contrast, the production and secretion of collagen evoked by FAF was only minimal when compared to that observed with IL-1 and aFGF. FAF and aFGF promoted fibroblast-induced collagen gel contraction with similar potency. Thus, the profile of fibroblast effects is a specific property of each cytokine.

The myc family of nuclear proto-oncogenes

Several members of the myc family of proto-oncogenes have been described, and some (c-, N-, and L-myc) have been characterized in considerable detail. They are united by a common gene structure and nucleotide homologies that were used to identify some of them initially. Their protein products also have scattered regions of amino acid identity or homology. Although the cellular activities of the various proteins are unknown, some members may play a role in regulating cell growth and differentiation. They share the ability to cooperate with an activated ras gene and cotransform embryonic rodent cells. In naturally occurring tumors, the members of the myc family of oncogenes appear to be activated by genetic changes (proviral insertion, chromosomal translocation, and gene amplification) that augment or otherwise disrupt normally regulated expression. The members of this family of genes differ markedly in their tissue specificity and developmental regulation of expression. This may account in part for the frequent appearance of activated c-myc genes in a wide variety of neoplasms and the limited appearance of activated N- and L-myc genes in tumors of embryonic or neural origin. The c-myc gene may be activated in tumors by a variety of mechanisms, whereas N- and L-myc appear to be activated only by gene amplification. Regulation of expression of the different myc genes also appears to occur by different mechanisms. Finally, the products of the different genes differ in may regions of the protein, and this divergence probably reflects their specific and individual functions.

Activation of cryptic promoters of human c-myc genes in microinjected Xenopus laevis oocytes

The human cellular oncogene c-myc contains two promoters at the 5' end of its first non-coding exon. Cryptic promoters located within the first intron are also activated to synthesize aberrant c-myc mRNAs in some Burkitt lymphomas with a t(8: 14) chromosome translocation in which a part of the gene structure, often the 5' non-coding exon, is truncated. We have shown elsewhere that microinjected plasmid DNA carrying a normal, intact human c-myc gene directs efficient faithful transcription from its own two promoters in Xenopus laevis oocytes. Here, I have investigated the expression of different recombinants carrying various constructs of the c-myc gene in frog oocytes in order to understand the activation mechanism of those cryptic promoters. Aberrant c-myc transcripts initiating from cryptic promoters within the first intron are undetectable when the intact or truncated c-myc gene construct is used. However, the cryptic promoters can be activated in Xenopus oocytes if the truncated c-myc gene construct is fused with simian virus 40 sequences containing a 21 base-pair repeat and the replication origin. Xenopus oocytes will be useful for further investigation of enhancing elements involved in the translocated and activated c-myc genes in Burkitt lymphoma cells.

Rat insulin-like growth factor II gene. A single gene with two promoters expressing a multitranscript family

We have characterized the single-copy rat gene encoding the protein precursor of insulin-like growth factor II (pre-pro-rIGF-II) that is located downstream from and in the same transcriptional orientation as the homologous insulin II gene (5'-insulin-IGF-II-3'). This gene consists of at least three coding exons and utilizes two promoters that generate alternate 5' non-coding exons. Multiple transcripts from both promoters appear primarily in fetal or neonatal tissues (in all of the developmental stages and tissues that we have examined), but they are extremely rare or undetectable in adult tissues, with the exception of the brain and the spinal cord. These transcripts, which exhibit characteristic developmental profiles in various tissues, differ both in the presence of one of the alternate 5' non-coding exons and in the length of their fourth exon. The possible occurrence of differential splicing or differential polyadenylation (or both) in this region is discussed.

Growth stimulation of rat primary embryo fibroblasts by the human myc gene

We have studied the ability of plasmids encoding a normal human myc protein to stimulate growth of primary rat embryo fibroblasts. We measured growth stimulation by the number of G418-resistant colonies obtained after co-transfection with plasmid pSV2neo and by the percentage of these colonies that grew in long-term culture (immortalization). Using a normal human myc gene, we detected a weak growth stimulation at the colony formation stage and a low frequency of immortalization. Replacement of the myc promoter by a heterologous promoter (mouse metallothionein I promoter) and deletion of the first non-coding exon led to a more efficient growth stimulation by both criteria. Thus, disregulation of c-myc is essential for an altered pattern of growth. Using zinc, a metallothionein inducer, we observed a slight increase in the growth rate of some transfectants, which can be measured by thymidine incorporation. However, the relative inefficiency of immortalization we observed suggests that either a high level of myc expression or participation of other genes is required for establishment in culture. Under our experimental conditions, we could not detect a transforming activity for the human myc gene and none of our myc-containing cell lines was tumorigenic in nude mice.

A rapid and simple procedure for the preparation of homogeneously labeled DNA probes

This report describes a novel technique for the preparation of homogeneously labeled DNA probes. The procedure uses a heteroduplex intermediate, which is directly prepared from a double-stranded plasmid, and therefore requires neither sub-cloning of the fragment of interest into a specialized vector nor the use of a synthetic primer. The technical advantages of the procedure are demonstrated by preparation of a probe able to determine the precise location and efficiency of utilization of the polyadenylation site used during transcription of the rat preproinsulin II gene in a transfected avian cell line.