Regulation of expression of the c-myc proto-oncogene

Cardiomyocyte Proliferation from Fetal- to Adult- and from Normal- to Hypertrophy and Failing Hearts

Simple Summary

Death from injury to the heart from a variety of causes remains a major cause of mortality worldwide. The cardiomyocyte, the major contracting cell of the heart, is responsible for pumping blood to the rest of the body. During fetal development, these immature cardiomyocytes are small and rapidly divide to complete development of the heart by birth when they develop structural and functional characteristics of mature cells which prevent further division. All further growth of the heart after birth is due to an increase in the size of cardiomyocytes, hypertrophy. Following the loss of functional cardiomyocytes due to coronary artery occlusion or other causes, the heart is unable to replace the lost cells. One of the significant research goals has been to induce adult cardiomyocytes to reactivate the cell cycle and repair cardiac injury. This review explores the developmental, structural, and functional changes of the growing cardiomyocyte, and particularly the sarcomere, responsible for force generation, from the early fetal period of reproductive cell growth through the neonatal period and on to adulthood, as well as during pathological response to different forms of myocardial diseases or injury. Multiple issues relative to cardiomyocyte cell-cycle regulation in normal or diseased conditions are discussed.

Abstract

The cardiomyocyte undergoes dramatic changes in structure, metabolism, and function from the early fetal stage of hyperplastic cell growth, through birth and the conversion to hypertrophic cell growth, continuing to the adult stage and responding to various forms of stress on the myocardium, often leading to myocardial failure. The fetal cell with incompletely formed sarcomeres and other cellular and extracellular components is actively undergoing mitosis, organelle dispersion, and formation of daughter cells. In the first few days of neonatal life, the heart is able to repair fully from injury, but not after conversion to hypertrophic growth. Structural and metabolic changes occur following conversion to hypertrophic growth which forms a barrier to further cardiomyocyte division, though interstitial components continue dividing to keep pace with cardiac growth. Both intra- and extracellular structural changes occur in the stressed myocardium which together with hemodynamic alterations lead to metabolic and functional alterations of myocardial failure. This review probes some of the questions regarding conditions that regulate normal and pathologic growth of the heart.

NSD3S stabilizes MYC through hindering its interaction with FBXW7

The MYC transcription factor plays a key role in cell growth control. Enhanced MYC protein stability has been found to promote tumorigenesis. Thus, understanding how MYC stability is controlled may have significant implications for revealing MYC-driven growth regulatory mechanisms in physiological and pathological processes. Our previous work identified the histone lysine methyltransferase nuclear receptor binding SET domain protein 3 (NSD3) as a MYC modulator. NSD3S, a noncatalytic isoform of NSD3 with oncogenic activity, appears to bind, stabilize, and activate the transcriptional activity of MYC. However, the mechanism by which NSD3S stabilizes MYC remains to be elucidated. To uncover the nature of the interaction and the underlying mechanism of MYC regulation by NSD3S, we characterized the binding interface between both proteins by narrowing the interface to a 15-amino acid region in NSD3S that is partially required for MYC regulation. Mechanistically, NSD3S binds to MYC and reduces the association of F-box and WD repeat domain containing 7 (FBXW7) with MYC, which results in suppression of FBXW7-mediated proteasomal degradation of MYC and an increase in MYC protein half-life. These results support a critical role for NSD3S in the regulation of MYC function and provide a novel mechanism for NSD3S oncogenic function through inhibition of FBXW7-mediated degradation of MYC.

lncRNAs and MYC: An Intricate Relationship

Long non-coding RNAs (lncRNAs) are emerging as important regulators of gene expression networks, acting either at the transcriptional level, by influencing histone modifications, or at the post-transcriptional level, by controlling mRNA stability and translation. Among the gene expression networks known to influence the process of oncogenic transformation, the one controlled by the proto-oncogene MYC is one of the most frequently deregulated in cancer. In B-cell lymphomas, the MYC gene is subject to chromosomal rearrangements that result in MYC overexpression. In many other cancers, the region surrounding MYC is subject to gene amplification. MYC expression is also controlled at the level of protein and mRNA stability. Neoplastic lesions affecting MYC expression are responsible for a drastic change in the number and the type of genes that are transcriptionally controlled by MYC, depending on differential promoter affinities. Transcriptome profiling of tumor samples has shown that several lncRNAs can be found differentially regulated by MYC in different cancer types and many of them can influence cancer cell viability and proliferation. At the same time, lncRNAs have been shown to be able to control the expression of MYC itself, both at transcriptional and post-transcriptional levels. Given that targeting the MYC-dependent transcriptional program has the potential to reach broad anticancer activity, molecular dissection of the complex regulatory mechanisms governing MYC expression will be crucial in the future for the identification of novel therapeutic strategies.

New Disubstituted Quindoline Derivatives Inhibiting Burkitt's Lymphoma Cell Proliferation by Impeding c-MYC Transcription

The c-MYC oncogene is overactivated during Burkitt's lymphoma pathogenesis. Targeting c-MYC to inhibit its transcriptional activity has emerged as an effective anticancer strategy. We synthesized four series of disubstituted quindoline derivatives by introducing the second cationic amino side chain and 5-N-methyl group based on a previous study of SYUIQ-5 (1) as c-MYC promoter G-quadruplex ligands. The in vitro evaluations showed that all new compounds exhibited higher stabilities and binding affinities, and most of them had better selectivity (over duplex DNA) for the c-MYC G-quadruplex compared to 1. Moreover, the new ligands prevented NM23-H2, a transcription factor, from effectively binding to the c-MYC G-quadruplex. Further studies showed that the selected ligand, 7a4, down-regulated c-MYC transcription by targeting promoter G-quadruplex and disrupting the NM23-H2/c-MYC interaction in RAJI cells. 7a4 could inhibit Burkitt's lymphoma cell proliferation through cell cycle arrest and apoptosis and suppress tumor growth in a human Burkitt's lymphoma xenograft.

Mammalian MYC Proteins and Cancer

The MYC family of proteins is a group of basic-helix-loop-helix-leucine zipper transcription factors that feature prominently in cancer. Overexpression of MYC is observed in the vast majority of human malignancies and promotes an extraordinary set of changes that impact cell proliferation, growth, metabolism, DNA replication, cell cycle progression, cell adhesion, differentiation, and metastasis. The purpose of this review is to introduce the reader to the mammalian family of MYC proteins, highlight important functional properties that endow them with their potent oncogenic potential, describe their mechanisms of action and of deregulation in cancer cells, and discuss efforts to target the unique properties of MYC, and of MYC-driven tumors, to treat cancer.

The c-MYC NHE III(1): function and regulation

c-MYC is an important regulator of a wide array of cellular processes necessary for normal cell growth and differentiation, and its dysregulation is one of the hallmarks of many cancers. Consequently, understanding c-MYC transcriptional activation is critical for understanding developmental and cancer biology, as well as for the development of new anticancer drugs. The nuclease hypersensitive element (NHE) III(1) region of the c-MYC promoter has been shown to be particularly important in regulating c-MYC expression. Specifically, the formation of a G-quadruplex structure appears to promote repression of c-MYC transcription. This review focuses on what is known about the formation of a G-quadruplex in the NHE III(1) region of the c-MYC promoter, as well as on those factors that are known to modulate its formation. Last, we discuss the development of small molecules that stabilize or induce the formation of G-quadruplex structures and could potentially be used as anticancer agents.

The importance of negative superhelicity in inducing the formation of G-quadruplex and i-motif structures in the c-Myc promoter: implications for drug targeting and control of gene expression

The importance of DNA supercoiling in transcriptional regulation has been known for many years, and more recently, transcription itself has been shown to be a source of this superhelicity. To mimic the effect of transcriptionally induced negative superhelicity, the G-quadruplex/i-motif-forming region in the c-Myc promoter was incorporated into a supercoiled plasmid. We show, using enzymatic and chemical footprinting, that negative superhelicity facilitates the formation of secondary DNA structures under physiological conditions. Significantly, these structures are not the same as those formed in single-stranded DNA templates. Together with the recently demonstrated role of transcriptionally induced superhelicity in maintaining a mechanosensor mechanism for controlling the firing rate of the c-Myc promoter, we provide a more complete picture of how c-Myc transcription is likely controlled. Last, these physiologically relevant G-quadruplex and i-motif structures, along with the mechanosensor mechanism for control of gene expression, are proposed as novel mechanisms for small molecule targeting of transcriptional control of c-Myc.

Regulation of cardiac beta 1-adrenergic receptor transcription during the developmental transition

The beta(1)-adrenergic receptor (beta(1)AR) gene contains binding sites for myc/max proteins within a glucocorticoid response element. Transcriptional activation of the beta(1)AR is the result of cooperative binding between c-myc and the glucocorticoid receptor on the beta(1)AR promoter. The transcriptional regulation of both beta(1)AR and c-myc are developmentally regulated. We used transcription rate assays of nuclei isolated from fetal hearts to demonstrate a fivefold increase in the transcription rate of beta(1)AR vs. postnatal hearts (P < 0.01). This was associated with a fourfold increase in c-myc transcription. Transcription rate assays performed in a rat fibroblast cell line that overexpresses c-myc (myc(+/+)) showed similarly increased beta(1)AR expression compared with the wild-type cell line. Transient transfection experiments in the myc(+/+) cells demonstrated robust expression of beta(1)AR promoter constructs, which was abrogated by mutation of the myc/max binding site or by cotransfection with a c-myc antisense expression vector. These results suggest that the regulation of cardiac beta(1)AR transcription and the expression of c-myc are tightly integrated.

Molecular cloning of pituitary tumor transforming gene 1 from ovarian tumors and its expression in tumors

Pituitary tumor transforming gene 1 (PTTG1) recently cloned from human testis is a potent oncogene and is highly expressed in all the tumors analyzed to date. However, primary structure of PTTG1 and the cell types that express PTTG1 in tumors remained undescribed. We have used the reverse transcriptase-polymerase chain reaction technique to clone PTTG1 from ovarian tumors. Nucleotide sequencing of the PTTG1 cDNAs from various ovarian tumors showed identity with that of the human testis PTTG1. To determine the cell types that express PTTG1 in normal and tumor tissues, we performed in situ hybridization using digoxigenin-labeled cRNA as a probe. Our studies revealed a high level of expression of PTTG1 mRNA in both seminomatous and non-seminomatous testicular tumors; epithelial, sex-cord and stromal cell, and germ cell tumors of the ovary; and invasive ductal, ductal in situ and infiltrating ductal carcinoma of the breast. In normal tissues, expression of PTTG1 mRNA was very low or undetectable except in testis, where PTTG1 mRNA was found to be localized to spermatocytes and spermatids. Tumors that expressed high levels of PTTG1 mRNA also exhibited high levels of expression of basic fibroblast growth factor (bFGF), suggesting a correlation between PTTG1 and bFGF expression, and further suggesting that the PTTG1 protein may be involved in tumor angiogenesis and mitogenesis.

Elevated levels of RanBP7 mRNA in colorectal carcinoma are associated with increased proliferation and are similar to the transcription pattern of the proto-oncogene c-myc

We have used suppression subtractive hybridisation, "in silico" cloning and reverse Northern dot blot analysis to identify significant up-regulation of RanBP7 transcription in a human colorectal carcinoma. Quantitative RT-PCR analyses using the Taqman system demonstrated that RanBP7 mRNA levels were elevated in 47/75 colorectal tumours. There was no significant difference in 17 matched normal and tumour pairs and reduced levels in 11. Since RanBP7 specifies a key member of nuclear transport receptors responsible for the nuclear import of histone H1 and ribosomal proteins, we investigated whether this up-regulation might be proliferation-associated. RanBP7 mRNA copy numbers were significantly correlated with those of proliferating cell nuclear antigen in both normal and cancer tissue. Interestingly, the transcription pattern of the proto-oncogene c-myc showed a similar correlation with PCNA mRNA. Our results highlight the need for the careful interpretation of quantitative data that compare mRNA levels in normal and cancer tissue.

YY1 and c-Myc associate in vivo in a manner that depends on c-Myc levels

The c-Myc oncoprotein has previously been shown to associate with transcription regulator YY1 and to inhibit its activity. We show herein that endogenous c-Myc and YY1 associate in vivo and that changes in c-Myc levels, which accompany mitogenic stimulation or differentiation of cultured cells, affect the ratio of free to c-Myc-associated YY1. We have also investigated the mechanism by which association with c-Myc inhibits YY1's ability to regulate transcription. c-Myc does not block binding of YY1 to DNA. However, protein association studies suggest that c-Myc interferes with the ability of YY1 to contact basal transcription proteins TATA-binding protein and TFIIB.

c-myc oncogene expression and cell proliferation in mixed oligo-astrocytoma

Mixed gliomas (oligo-astrocytomas) are brain tumours with an admixture of 2 different cell populations: astrocytes and oligodendroglia. On the basis of histological features and behaviour, these tumours are classified as low-grade mixed gliomas (MG) and malignant mixed gliomas (MMG). We have studied the relationship between c-myc protein expression and cellular proliferation in this class of tumours. Using antibody c-33 for c-myc and PC-10 for the proliferating cell nuclear antigen (PCNA), immunohistochemistry was performed on 14 MG and 9 MMG. PCNA was increased in MMG as compared to MG in both astrocytic and oligodendroglial areas. However, more c-myc-positive cells were seen only in the astrocyte areas of MMG. Analysis of the relationship of c-myc and PCNA suggests that the correlation of c-myc with cellular proliferation is dependent on tissue type and differentiation status.

Chromatin structure and function: the heretical path to an RNA transcription factor

This review represents a synthesis of the work of the author and her collaborators through 40 years of research aimed at an understanding of chromatin composition and functional arrangement. It describes the progressive experimental stages, starting with autoradiography and protein analysis and continuing on to a more functional approach testing the template properties of intact nuclei, as well as nuclei depleted of, or reconstituted with, defined fractions extracted from the chromatin of other cell lines or tissues. As new questions were raised at each phase of these studies, the investigation was shifted from chromosomal proteins to the role of a small RNA that coextracted with one protein fraction and whose properties suggested a transcription-activating function. The active RNA was identified as a class III RNA, designated as 7SK. Its properties suggested a role in the activation of two oncogenes, the SV40 T-antigen and the mammalian C-myc gene. A detailed analysis of the c-myc gene expression during transformation induction in temperature-sensitive mammalian cells finally culminated in in vivo evidence for a role of 7SK in c-myc deregulation, using cells transfected with antisense oligonucleotides to block 7SK activity. This was followed by an investigation of promoter targeting by 7SK RNP using electrophoretic mobility shift assays with whole or 7SK-depleted cell extracts. Taken together, these studies indicate that 7SK RNP participates in transformation-dependent deregulation of the c-myc gene by activation of two c-myc minor promoters. The implications of these findings are discussed.

Expression of c-Myc in glucocorticoid-treated fibroblastic cells

Glucocorticoids inhibit proliferation of L929 fibroblastic cells in culture. Inhibition of proliferation is reversible and is not associated with changes in the plating efficiency of the cells. Flow cytometric analysis indicates that glucocorticoid-treated cells exhibit a decrease in the percentage of cells with DNA content > 2 N. Thymidine kinase expression is inhibited as cells with 2 N DNA content accumulate. These observations indicate that glucocorticoids arrest proliferation of L929 cells in the G1 phase of the cell cycle. The abundance of c-Myc mRNA does not decrease in glucocorticoid-treated cells, and c-Myc protein content in dexamethasone-treated cells is approximately the same as that detected in mid-log phase cells. Nuclear run-on transcription of c-Myc is not inhibited by glucocorticoids. These observations indicate that glucocorticoid regulation of fibroblastic cell proliferation does not involve inhibition of c-Myc transcription. Although regulation of c-Myc expression is central to the mechanism whereby glucocorticoids regulate proliferation of lymphoid cells, it is clear that different mechanisms must be involved in glucocorticoid regulation of fibroblastic cell proliferation.

Changes in promoter utilization in human and mouse c-myc genes upon transformation induction in temperature-sensitive cell lines

We have previously reported accelerated transcription and rapid accumulation of c-myc mRNAs upon induction of transformation in a temperature-sensitive mouse cell line (Gallant et al., 1989, Oncogene Res., 4:39-46). Here we have used both mouse and human cell lines transformed with a temperature-sensitive mutant of the Simian virus 40 (SV40) virus to investigate whether a shift in promoter utilization within the c-myc gene locus is part of a general mechanism that deregulates c-myc expression during transformation induction. We devised a simple and sensitive method using reverse transcription followed by radioactive polymerase chain reaction (RT-PCR) to measure the relative change in c-myc mRNAs arising from each of the four known promoters. We show that a three to fivefold increase in c-myc transcripts from the P1 and P3 promoters occurs in both human and mouse cell lines within 30 min of the shift to the permissive temperature. The major P2-initiated transcripts are not significantly effected. However, exon 3-containing RNAs increase more gradually up to 24 h postinduction and P1 and P3 transcripts, while remaining elevated, still contribute relatively little to the total c-myc RNA population. These and other results, demonstrating a transient activation of P1 and P3 promoters, suggest an indirect role of the minor transcripts in the deregulated expression of the c-myc gene in transformed cells.

Localization of c-myc protooncogene expression in the rat heart in vivo and in the isolated, perfused heart following treatment with norepinephrine

We have investigated the expression of the protooncogene c-myc in rat hearts following exposure to norepinephrine, both in vivo and in isolated perfused preparations. Both chronic and acute norepinephrine treatment produced a rapid, transient elevation of c-myc mRNA in adult rat hearts, but chronic infusion produced a second, larger increase. This expression profile was characteristic for c-myc since it was not found for four other protooncogenes. In the isolated, perfused heart, addition of norepinephrine to the perfusion buffer and elevation of perfusion pressure separately increase c-myc mRNA suggesting both direct hormonal and hemodynamic factors might be important in vivo. Immunocytochemistry showed that Myc protein accumulated predominantly in the nuclei of non-myocyte cells following norepinephrine treatment indicating that expression at the mRNA level culminated in protein synthesis. These findings suggest that the c-myc expression observed in the hypertrophying adult heart following exposure to norepinephrine may be associated with proliferating cells like fibroblasts rather than cardiomyocytes.

Stabilization of c-myc protein in human glioma cells

The regulation of c-myc protein, product of c-myc/genes, was studied in four glioma cell lines by Northern blot, pulse-chase dot blot, immunoblot and immunoprecipitation analyses. Northern blot analysis revealed no overexpression of c-myc transcript, and pulse-chase dot blot analysis showed normal turnover rate of c-myc transcript, suggestive of no evidence of aberrant regulation of c-myc at post-transcriptional level. The synthesis levels of c-myc protein were shown by immunoprecipitation and closely associated with the c-myc transcript levels demonstrated by Northern blot, suggestive of no evidence of aberrant translational control of c-myc, whereas they were dissociated from the accumulation levels of c-myc protein shown by immunoblot, suggestive of an evidence of aberrant regulation of c-myc at post-translational level. The mean (+/- standard deviation) half-lives of c-myc protein in four glioma cell lines were calculated from the pulse-chase immunoprecipitation analysis, and being 98 +/- 8 to 143 +/- 11 min, were about four- to sixfold longer than normal. In surgical specimens, the immunostain of c-myc protein was not found in normal astrocytes but localized heterogenously in nuclei of reactive astrocytes and glioma cells, and increased in stained cell number in proportion to malignancy. Although this study was limited to four glioma cell lines, it suggests that the c-myc protein in glioma cells may be accumulated due to its prolonged half-life contributing to an uncontrolled proliferation.

Interleukin-7 induces N-myc and c-myc expression in normal precursor B lymphocytes

Expression of both N-myc and c-myc is induced rapidly and dramatically in normal pre-B cells after stimulation with interleukin-7 (IL-7), a pre-B cell-specific growth factor. These IL-7-induced increases in N-myc and c-myc expression are mediated at the transcriptional level; for N-myc, a major portion of the induction results from release of an attenuation block between exons 1 and 2. Although c-myc expression has been shown to be regulated by many factors, these studies provide the first demonstration of N-myc regulation by a growth factor. Furthermore, mitogen-induced proliferation of more mature B-lineage cells is accompanied by induction of c-myc expression in the complete absence of N-myc expression. Thus, N-myc expression in normal B-lineage cells, as in transformed cells, is limited to cells that represent the precursor stages of this differentiation pathway. Together, these findings imply a specific function for N-myc in the early stages of B-cell development.

Complex transcriptional regulation of myc family gene expression in the developing mouse brain and liver

myc family genes (c-, N-, and L-myc) have been shown to be differentially expressed with respect to tissue type and developmental stage. To define and compare the regulatory mechanisms governing their differential developmental expression, we examined the transcriptional regulation of each myc family member during murine postnatal brain and liver development. Nuclear run-on transcription assays demonstrated that both the rate of transcriptional initiation and the degree of transcriptional blocking contribute in a complex manner to the regulation of all three genes. During postnatal brain development, the relative contribution of each transcriptional control mechanism to the regulation of myc family gene expression was found to be different for each gene. For instance, while modulation of transcriptional attenuation did not appear to contribute to the down-regulation of L-myc expression, attenuation was found to be the dominant mechanism by which steady-state N-myc mRNA levels were down-regulated. Different transcriptional strategies were found to be employed in newborn versus adult developing liver for repression of N- and L-myc expression. Undetectable steady-state N- and L-myc mRNA levels in newborn liver were associated with a very low rate of transcriptional initiation, whereas the lack of N- and L-myc expression at the adult stage was accompanied by a high rate of initiation and a striking degree of transcriptional attenuation. Transcriptional attenuation in the N-myc gene was found to map to a region encoding a potential stem-loop structure followed by a thymine tract within the first exon and was not dependent on the use of a specific transcriptional start site.

Complex transcriptional regulation of myc family gene expression in the developing mouse brain and liver

myc family genes (c-, N-, and L-myc) have been shown to be differentially expressed with respect to tissue type and developmental stage. To define and compare the regulatory mechanisms governing their differential developmental expression, we examined the transcriptional regulation of each myc family member during murine postnatal brain and liver development. Nuclear run-on transcription assays demonstrated that both the rate of transcriptional initiation and the degree of transcriptional blocking contribute in a complex manner to the regulation of all three genes. During postnatal brain development, the relative contribution of each transcriptional control mechanism to the regulation of myc family gene expression was found to be different for each gene. For instance, while modulation of transcriptional attenuation did not appear to contribute to the down-regulation of L-myc expression, attenuation was found to be the dominant mechanism by which steady-state N-myc mRNA levels were down-regulated. Different transcriptional strategies were found to be employed in newborn versus adult developing liver for repression of N- and L-myc expression. Undetectable steady-state N- and L-myc mRNA levels in newborn liver were associated with a very low rate of transcriptional initiation, whereas the lack of N- and L-myc expression at the adult stage was accompanied by a high rate of initiation and a striking degree of transcriptional attenuation. Transcriptional attenuation in the N-myc gene was found to map to a region encoding a potential stem-loop structure followed by a thymine tract within the first exon and was not dependent on the use of a specific transcriptional start site.

DNA repair in the c-myc proto-oncogene locus: possible involvement in susceptibility or resistance to plasmacytoma induction in BALB/c mice

This report describes an unexpected difference in the efficiency of removal of UV-induced DNA damage in the c-myc locus in splenic B lymphoblasts from two inbred strains of mice. In cells from plasmacytoma-resistant DBA/2N mice, 35% of UV-induced damage in the regulatory and 5' flank of c-myc is removed by 12 h. However, in cells from plasmacytoma-susceptible BALB/cAn mice, damage is not removed from this region. In the protein-encoding region and 3' flank of c-myc as well as in two dihydrofolate reductase gene fragments, UV damage is repaired with similar efficiency in B lymphoblasts from both strains of mice. Furthermore, in the protein-encoding portion and 3' flank of c-myc, damage is selectively removed from only the transcribed strand. No repair is detected in the nontranscribed strand. In contrast, DNA repair in the 5' flank of c-myc is not strand specific; in DNA from DBA/2N cells, UV damage is rapidly removed from both the transcribed and nontranscribed strands. In BALB/cAn cells no repair was detected in either strand in the 5'flank, consistent with the results with double-stranded, nick-translated probes to this region of c-myc. In addition to the repair studies, we have detected post-UV-damage formation: in most of the genes studied, we find that additional T4 endonuclease-sensitive sites are formed in the DNA 2 h after irradiation. Our findings provide new insights into the details of gene-specific and strand-specific DNA repair and suggest that there may be close links between DNA repair and B-cell neoplastic development.

Rapid activation and subsequent down-regulation of the human immunodeficiency virus type 1 promoter in the presence of Tat: possible mechanisms contributing to latency

The mechanism of induction of gene expression of the human immunodeficiency virus type 1 long terminal repeat (LTR) by the Tat transactivator protein was studied in a cell fusion assay. Tat causes a rapid activation of both transcription from the LTR and accumulation of hybrid LTR-chloramphenicol acetyltransferase mRNAs. Approximately 4 h after induction by Tat, expression from the LTR promoter is down-regulated, resulting in a decrease in the accumulation of LTR mRNA. This down-regulation of expression occurs in the continued presence of Tat. Protein synthesis inhibitors can block this down-regulation; therefore, the postinduction repression of expression is dependent upon de novo protein synthesis. We propose that a labile cellular protein(s) is responsible for the low levels of human immunodeficiency virus type 1 expression, possibly contributing to the establishment of a latent state of viral expression.

DNA repair in the c-myc proto-oncogene locus: possible involvement in susceptibility or resistance to plasmacytoma induction in BALB/c mice

This report describes an unexpected difference in the efficiency of removal of UV-induced DNA damage in the c-myc locus in splenic B lymphoblasts from two inbred strains of mice. In cells from plasmacytoma-resistant DBA/2N mice, 35% of UV-induced damage in the regulatory and 5' flank of c-myc is removed by 12 h. However, in cells from plasmacytoma-susceptible BALB/cAn mice, damage is not removed from this region. In the protein-encoding region and 3' flank of c-myc as well as in two dihydrofolate reductase gene fragments, UV damage is repaired with similar efficiency in B lymphoblasts from both strains of mice. Furthermore, in the protein-encoding portion and 3' flank of c-myc, damage is selectively removed from only the transcribed strand. No repair is detected in the nontranscribed strand. In contrast, DNA repair in the 5' flank of c-myc is not strand specific; in DNA from DBA/2N cells, UV damage is rapidly removed from both the transcribed and nontranscribed strands. In BALB/cAn cells no repair was detected in either strand in the 5'flank, consistent with the results with double-stranded, nick-translated probes to this region of c-myc. In addition to the repair studies, we have detected post-UV-damage formation: in most of the genes studied, we find that additional T4 endonuclease-sensitive sites are formed in the DNA 2 h after irradiation. Our findings provide new insights into the details of gene-specific and strand-specific DNA repair and suggest that there may be close links between DNA repair and B-cell neoplastic development.

Transgenic animals as a tool for studying the effect of the c-myc proto-oncogene on cardiac development

Transgenic animals provide a model system to elucidate the role of specific proteins in development. This model is now being used increasingly in the cardiovascular system to study cardiac growth and differentiation. During cardiac myocyte development a transition occurs from hyperplastic to hypertrophic growth. In the heart the switch from myocyte proliferation to terminal differentiation is synchronous with a decrease in c-myc mRNA abundance. To determine whether c-myc functions to regulate myocyte proliferation and/or differentiation, we examined the in vivo effect of increasing c-myc expression during fetal development and of preventing the decrease in c-myc mRNA expression that normally occurs during myocyte development. The model system used was a strain of transgenic mice exhibiting constitutive expression of c-myc mRNA in cardiac myocytes throughout development. Increased c-myc mRNA expression is associated with both atrial and ventricular enlargement in the transgenic mice. This increase in cardiac mass is secondary to myocyte hyperplasia, with the transgenic hearts containing greater than twice as many myocytes as nontransgenic hearts. The results of this study indicate that constitutive expression of c-myc mRNA in the heart during development results in enhanced hyperplastic growth, and suggest a regulatory role for the c-myc protooncogene in cardiac myogenesis.

Protooncogenes and growth factors associated with normal and abnormal liver growth

Hepatocyte replication during liver regeneration depends on extrinsic (circulating) and intrinsic (intrahepatic) factors. Two important growth factors produced in the regenerating liver are discussed, TGF alpha, an autocrine, stimulatory growth factor, and TGF beta, a paracrine inhibitory factor. The balance between the activities of these factors is likely to play an important role in regulating hepatocyte proliferation. The expression of some protooncogenes occurs sequentially during the first few hours after partial hepatectomy and is a marker for the entry of hepatocytes into the cell cycle (proliferative competence). As hepatocytes become competent to proliferate, they respond to TGF alpha and other growth factors and enter a proliferative phase. It is possible that TGF beta 1 serves as a stop signal for liver regeneration but the mechanisms by which TGF beta inhibits hepatocyte DNA synthesis are still unknown.

C-myc as an inducer of polycystic kidney disease in transgenic mice

In this study, a genetic model of polycystic kidney disease (PKD) has been produced in transgenic mice bearing the murine c-myc gene driven by the SV40 enhancer and the adult beta-globin promoter. These animals reproducibly develop PKD and die of renal failure. The phenotype appears to result from the overexpression of c-myc in the renal tubular epithelium and consequent abnormal cell proliferation. These transgenic mice represent a genetic model of PKD which bears similarities to human autosomal dominant PKD (ADPKD) with respect to renal morphology, renal functional alterations and dominant transmission. Study of these transgenic mice may offer valuable insights into the pathogenesis of PKD.

Transcriptional deregulation of myc in IgH/myc 6;7 translocation carrying rat immunocytomas

We have previously shown that the reciprocal translocation t(6;7) associated with the spontaneous immunocytoma of the Louvain rat (RIC) leads to the juxtaposition of myc to the IgH cluster. In 10 of 14 tumors investigated the breakpoints on the myc carrying chromosome were clustered in a 1.5 kb region 5' of the intact gene, proximal to the myc promoters. In this paper we describe the effect of the translocation on myc transcription in the RIC system. Run-on analysis showed transcriptional attenuation in the normal rat myc gene, similar to the situation in mice and humans. The attenuation was almost completely abrogated in the three immunocytomas studied. Sequence analysis of two tumors failed to reveal any structural changes within exon 1, as found by others in Burkitt's lymphoma. We also show that the transcriptional initiation of myc mRNA is changed in the RICs. In an established line of rat fibroblasts (Rat-2), the more distal myc promoter (P2) is the preferred site of initiation. In RIC, however, only 30% of transcripts were initiated from P2. We found that 40% of the transcripts were initiated from P1 and 30% from a novel promoter, designated P1a, located between P1 and P2.

Mithramycin selectively inhibits the transcriptional activity of a transfected human c-myc gene

The G-C specific DNA binding drug mithramycin selectively inhibits expression of the c-myc gene in a number of cell types. We have tested the ability of this agent to inhibit the expression of a transfected human c-myc gene in a murine fibroblast cell line. Expression of c-myc is inhibited in the first 24 hours of mithramycin exposure (in contrast to actin gene expression, which is unaffected). Nuclear runon transcription of c-myc by nuclei isolated from mithramycin treated cells is decreased, indicating inhibition of transcription initiation. However, treatment of isolated nuclei with mithramycin also results in decreased c-myc transcription. Thus, inhibition of c-myc expression by mithramycin in these cells appears to occur at the transcriptional level and is most likely mediated at both the transcription initiation and elongation level. This suggests that mithramycin selectively interacts with the G-C rich c-myc promoter, preventing formation of the c-myc transcription initiation complex.

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine-resistant, flat-cell PC12 variants having a partial loss of transformed phenotype

We have cloned and characterized two variants of PC12 cells. MPT1 cells were selected by their resistance to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), and variant 2068 was isolated nonselectively as a large, flat-cell variant commonly occurring in PC12 cultures. Variant 2068 cells also exhibit resistance to MPTP. Karyotype analysis revealed that these variants are true derivatives of wild-type PC12 cells; however, each variant is tetraploid, whereas the wild-type parent is diploid. The two variants contain an altered level and composition of lactate dehydrogenase isoenzymes, which could account for a previously described difference in lactate metabolism. Both variants exhibit a partial loss of transformed phenotype in culture in that they are nonrefractile, grow in monolayers, and fail to multiply in soft agar. We suggest that this alteration in transformed phenotype may result in altered mitochondria and lactate dehydrogenase and thus account for their resistance to MPTP. Compared with wild-type PC12 cells, MPT1 cells have a decreased level of fos mRNA and an increased level of myc mRNA; the latter results from an increased level of transcription of exon 1 of the myc gene. Studies with hybrid cells obtained by fusing MPT1 cells with wild-type-like cells show that most, but not all, of the parameters of the MPT1 phenotype predominate.

The c-myc proto-oncogene regulates cardiac development in transgenic mice

During the maturation of the cardiac myocyte, a transition occurs from hyperplastic to hypertrophic growth. The factors that control this transition in the developing heart are unknown. Proto-oncogenes such as c-myc have been implicated in the regulation of cellular proliferation and differentiation, and in the heart the switch from myocyte proliferation to terminal differentiation is synchronous with a decrease in c-myc mRNA abundance. To determine whether c-myc can influence myocyte proliferation or differentiation, we examined the in vivo effect of increasing c-myc expression during embryogenesis and of preventing the decrease in c-myc mRNA expression that normally occurs during cardiac development. The model system used was a strain of transgenic mice exhibiting constitutive expression of c-myc mRNA in cardiac myocytes throughout development. In these transgenic mice, increased c-myc mRNA expression was found to be associated with both atrial and ventricular enlargement. This increase in cardiac mass was secondary to myocyte hyperplasia, with the transgenic hearts containing more than twice as many myocytes as did nontransgenic hearts. The results suggest that in the transgenic animals there is additional hyperplastic growth during fetal development. However, this additional proliferative growth is not reflected in abnormal myocyte maturation, as assessed by the expression of the cardiac and skeletal isoforms of alpha-actin. The results of this study indicate that constitutive expression of c-myc mRNA in the heart during development results in enhanced hyperplastic growth and suggest a regulatory role for this proto-oncogene in cardiac myogenesis.

The c-myc proto-oncogene regulates cardiac development in transgenic mice

During the maturation of the cardiac myocyte, a transition occurs from hyperplastic to hypertrophic growth. The factors that control this transition in the developing heart are unknown. Proto-oncogenes such as c-myc have been implicated in the regulation of cellular proliferation and differentiation, and in the heart the switch from myocyte proliferation to terminal differentiation is synchronous with a decrease in c-myc mRNA abundance. To determine whether c-myc can influence myocyte proliferation or differentiation, we examined the in vivo effect of increasing c-myc expression during embryogenesis and of preventing the decrease in c-myc mRNA expression that normally occurs during cardiac development. The model system used was a strain of transgenic mice exhibiting constitutive expression of c-myc mRNA in cardiac myocytes throughout development. In these transgenic mice, increased c-myc mRNA expression was found to be associated with both atrial and ventricular enlargement. This increase in cardiac mass was secondary to myocyte hyperplasia, with the transgenic hearts containing more than twice as many myocytes as did nontransgenic hearts. The results suggest that in the transgenic animals there is additional hyperplastic growth during fetal development. However, this additional proliferative growth is not reflected in abnormal myocyte maturation, as assessed by the expression of the cardiac and skeletal isoforms of alpha-actin. The results of this study indicate that constitutive expression of c-myc mRNA in the heart during development results in enhanced hyperplastic growth and suggest a regulatory role for this proto-oncogene in cardiac myogenesis.

c-myc, c-fos, and c-jun regulation in the regenerating livers of normal and H-2K/c-myc transgenic mice

We investigated the mechanisms of regulation of c-myc, c-fos, and c-jun at the early stages of liver regeneration in mice. We show that the transient increase in steady-state levels of c-myc mRNA at the start of liver regeneration is most probably regulated by posttranscriptional mechanisms. Although there was a marked increase in c-myc transcriptional initiation shortly after partial hepatectomy, a block in elongation prevented the completion of most transcripts. To gain further information on the mechanism of regulation of c-myc expression during liver regeneration, we used transgenic mice harboring the human c-myc gene driven by the H-2K promoter. In these animals, the murine c-myc responded to the growth stimulus generated by partial hepatectomy, whereas the expression of the transgene was constitutive and did not change in the regenerating liver. However, the mRNA from both genes increased markedly after cycloheximide injection, suggesting that the regulation of c-myc mRNA abundance in the regenerating liver differs from that occurring after protein synthesis inhibition. Furthermore, we show that in normal mice c-fos and c-jun mRNA levels and transcriptional rates increase within 30 min after partial hepatectomy. c-fos transcriptional elongation was restricted in nongrowing liver, but the block was partially relieved in the regenerating liver. Nevertheless, for both c-fos and c-jun, changes in steady-state mRNA detected after partial hepatectomy were much greater than the transcriptional increase. In the regenerating liver of H-2K/c-myc mice, c-fos and c-jun expression was diminished, whereas mouse c-myc expression was enhanced in comparison with that in nontransgenic animals.

The role of myc oncogenes in cell growth and differentiation

The myc oncoproteins are expressed in a wide range of normal adult and embryonic tissues. They are also found to be over-expressed in a variety of tumor types. All myc proteins are short-lived nuclear phosphoproteins thought to act as regulatory components of cell proliferation. The rapid induction of c-myc mRNA and protein following the addition of growth factors to quiescent cells, together with the short half-life of these molecules, suggests that they are sensitive and continuous indicators of external stimuli, consistent with a role in signal transduction. Furthermore, in untransformed cells, c-myc protein expression is tightly regulated, at least in part, by a mechanism of autoregulation. Deregulated expression of myc genes is a frequent observation in tumors and may lead to a cell becoming independent of one or more growth factors, with the concomitant potential for uncontrolled proliferation. Although the precise functions of the myc proteins are unknown, they all bear the hallmarks of multimeric DNA-binding proteins probably involved in the regulation of expression of specific genes.

c-myc, c-fos, and c-jun regulation in the regenerating livers of normal and H-2K/c-myc transgenic mice

We investigated the mechanisms of regulation of c-myc, c-fos, and c-jun at the early stages of liver regeneration in mice. We show that the transient increase in steady-state levels of c-myc mRNA at the start of liver regeneration is most probably regulated by posttranscriptional mechanisms. Although there was a marked increase in c-myc transcriptional initiation shortly after partial hepatectomy, a block in elongation prevented the completion of most transcripts. To gain further information on the mechanism of regulation of c-myc expression during liver regeneration, we used transgenic mice harboring the human c-myc gene driven by the H-2K promoter. In these animals, the murine c-myc responded to the growth stimulus generated by partial hepatectomy, whereas the expression of the transgene was constitutive and did not change in the regenerating liver. However, the mRNA from both genes increased markedly after cycloheximide injection, suggesting that the regulation of c-myc mRNA abundance in the regenerating liver differs from that occurring after protein synthesis inhibition. Furthermore, we show that in normal mice c-fos and c-jun mRNA levels and transcriptional rates increase within 30 min after partial hepatectomy. c-fos transcriptional elongation was restricted in nongrowing liver, but the block was partially relieved in the regenerating liver. Nevertheless, for both c-fos and c-jun, changes in steady-state mRNA detected after partial hepatectomy were much greater than the transcriptional increase. In the regenerating liver of H-2K/c-myc mice, c-fos and c-jun expression was diminished, whereas mouse c-myc expression was enhanced in comparison with that in nontransgenic animals.

Differential regulation of the N-myc gene in transfected cells and transgenic mice

The N-myc gene is expressed specifically in the early developmental stages of numerous cell lineages. To assay for sequences that could potentially regulate N-myc expression, we transfected constructs that contained murine N-myc genomic sequences linked to a reporter gene and genomic clones that contained the complete human or murine N-myc genes into cell lines that either express or do not express the endogenous N-myc gene. Following either transient or stable transfection, the introduced N-myc sequences were expressed regardless of the expression status of the endogenous gene. In contrast, when the clones containing the complete human N-myc gene were introduced into the germline of transgenic mice, expression in some transgenic lines paralleled the tissue- and stage-specific expression of the endogenous murine gene. These findings demonstrate differences in the regulation of N-myc genes in recipient cells following in vitro versus in vivo introduction, suggesting that early developmental events may play a role in the regulation of N-myc expression.

Postinduction turnoff of beta-interferon gene expression

Viral induction of the human beta-interferon (IFN-beta) gene leads to a transient accumulation of high levels of IFN-beta mRNA. Previous studies have shown that the increase in IFN-beta mRNA levels after induction is due to an increase in the rate of IFN-beta gene transcription. In this paper, we show that the rapid postinduction decrease in the level of IFN-beta mRNA is due to a combination of transcriptional repression and rapid turnover of the mRNA. This transcriptional repression can be blocked with cycloheximide, suggesting that the synthesis of a virus-inducible repressor is necessary for the postinduction turnoff of the IFN-beta gene. Analysis of the sequence requirements for IFN-beta mRNA instability revealed two regions capable of destabilizing a heterologous mRNA. One destabilizer is an AU-rich sequence in the 3' untranslated region, and the other is located 5' to the translation stop codon.

Differential regulation of the N-myc gene in transfected cells and transgenic mice

The N-myc gene is expressed specifically in the early developmental stages of numerous cell lineages. To assay for sequences that could potentially regulate N-myc expression, we transfected constructs that contained murine N-myc genomic sequences linked to a reporter gene and genomic clones that contained the complete human or murine N-myc genes into cell lines that either express or do not express the endogenous N-myc gene. Following either transient or stable transfection, the introduced N-myc sequences were expressed regardless of the expression status of the endogenous gene. In contrast, when the clones containing the complete human N-myc gene were introduced into the germline of transgenic mice, expression in some transgenic lines paralleled the tissue- and stage-specific expression of the endogenous murine gene. These findings demonstrate differences in the regulation of N-myc genes in recipient cells following in vitro versus in vivo introduction, suggesting that early developmental events may play a role in the regulation of N-myc expression.

Postinduction turnoff of beta-interferon gene expression

Viral induction of the human beta-interferon (IFN-beta) gene leads to a transient accumulation of high levels of IFN-beta mRNA. Previous studies have shown that the increase in IFN-beta mRNA levels after induction is due to an increase in the rate of IFN-beta gene transcription. In this paper, we show that the rapid postinduction decrease in the level of IFN-beta mRNA is due to a combination of transcriptional repression and rapid turnover of the mRNA. This transcriptional repression can be blocked with cycloheximide, suggesting that the synthesis of a virus-inducible repressor is necessary for the postinduction turnoff of the IFN-beta gene. Analysis of the sequence requirements for IFN-beta mRNA instability revealed two regions capable of destabilizing a heterologous mRNA. One destabilizer is an AU-rich sequence in the 3' untranslated region, and the other is located 5' to the translation stop codon.

Constitutive c-myc expression enhances proliferation of differentiating F9 teratocarcinoma cells

The c-myc protooncogene is expressed in many tumor cells as well as during normal development. In order to study the role of c-myc in differentiation, proliferation and tumorigenicity of F9 mouse teratocarcinoma cells, the pSVmyc1 plasmid constitutively expressing an active c-myc oncogene was introduced into F9 stem cells by cotransfection with the selectable marker RSVneo. Enhanced expression of c-myc did not alter the properties of F9 stem cells. Prolonged proliferation during retinoic acid induced differentiation was observed in cell clones constitutively expressing c-myc. In contrast, as determined by morphology, by immunocytochemistry for markers specific for stem cells and differentiated derivatives, and by Northern hybridization for mRNAs specific for differentiated cells, differentiation was neither inhibited nor delayed by constitutive c-myc expression. Tumorigenicity of stem cells as well as retinoic acid-treated cells--as measured by soft agar cloning efficiency and tumor formation in syngenic mice--was not altered by SVmyc1. We conclude that in F9 teratocarcinoma cells down-regulation of c-myc is related to arrest of proliferation rather than differentiation.

Dynamic expression pattern of the myc protooncogene in midgestation mouse embryos

The c-myc protooncogene in mouse embryos was shown by RNA in situ hybridization to be preferentially expressed in tissues of endodermal and mesodermal origin. Most organs developing from the ectoderm, such as skin, brain, and spinal cord, displayed low levels of c-myc RNA. The thymus represented the only hematopoietic organ with high c-myc expression. In organs and structures strongly hybridizing to c-myc probes, for example the fetal part of the placenta, gut, liver, kidney, pancreas, submandibular glands, enamel organs of the molars, and skeletal cartilage, the level of expression depended on the stage of development. Expression was observed to be correlated with proliferation, particularly during expansion and folding of partially differentiated epithelial cells.

Induction of cyclic AMP synthesis by forskolin is followed by a reduction in the expression of c-myc messenger RNA and inhibition of 3H-thymidine incorporation in human fibroblasts

We have studied the effect of increased intracellular levels of cyclic AMP on the growth response to platelet-derived growth factor (PDGF) of human foreskin fibroblasts in culture. It was found that forskolin, a potent stimulator of adenylate cyclase activity, inhibits the stimulatory effect of PDGF on 3H-thymidine incorporation with a dose dependence similar to that observed with regard to cyclic AMP formation. A time-course study indicated that forskolin has no effect on ongoing DNA synthesis but affects events in the prereplicative phase. The cell-cycle block induced by forskolin was found to be reversible; after removal of the drug, DNA synthesis was initiated after a lag period, similar to that of the prereplicative phase of control cells. Forskolin had no effect on PDGF binding, receptor autophosphorylation, or c-fos mRNA expression. However, a reduction in PDGF-induced c-myc mRNA expression was observed in cultures given forskolin. Forskolin was also found to have a marked stimulatory effect on the expression of interferon-beta 2 mRNA expression. However, we were unable to demonstrate that the growth-inhibitory effect of forskolin is mediated by interferon-beta. In conclusion, an increase in cAMP levels leads to a reversible inhibition of PDGF-induced DNA synthesis in human fibroblasts, which may be related to an inhibition of c-myc mRNA expression.

Steady-state and nuclear run-on analyses of transcription in a temperature-sensitive Chinese hamster cell mutant with a defect in RNA metabolism

We have further characterized a temperature-sensitive mutant of Chinese hamster lung fibroblasts in tissue culture with a defect in RNA metabolism. The mutant phenotype is reflected in transcription in crude extracts or in isolated nuclei, when these are made from cells shifted to the nonpermissive temperature; however, differential heat inactivation between mutant and wild-type extracts cannot be demonstrated with cell-free systems. We tentatively conclude that the mutation may affect initiation of transcription which cannot be observed in our in vitro systems. Partially purified RNA polymerase I, II, and III fractions are indistinguishable from wild type. A temperature shift does not affect transcription by RNA polymerase III measured with intact cells or by nuclear run-on experiments. The nuclear run-on and other experiments suggest that RNA polymerase II-dependent transcription is inhibited before RNA polymerase I-dependent transcription. This conclusion is also supported by Northern analyses of selected mRNAs in nonsynchronized and synchronized cells after a shift to the nonpermissive temperature.