Control of c-myc regulation in normal and neoplastic cells

Transcription factors, translocations and haematological malignancies

Haematological malignancies arise as a consequence of clonal evolution driven by an accumulation of somatic mutations. Many haematological malignancies are associated with chromosome translocations that have provided powerful tools for the identification of proto-oncogenes implicated in the pathogenesis of leukaemia/lymphoma. The recent characterisation of several translocation breakpoints associated with human haematological tumours has demonstrated that genes encoding transcription factors are frequently involved. Direct alteration of transcription factor activity by somatic mutation may represent a particularly powerful leukaemogenic event.

Expression of myc-family genes in established human multiple myeloma cell lines: L-myc but not c-myc gene expression in the U-266 myeloma cell line

Deregulated c-myc expression, as a consequence of translocation of the c-myc gene to one of the immunoglobulin loci, appears to play an important role in the pathogenesis of several B-cell tumors, including Burkitt's lymphoma, mouse plasmacytoma and rat immunocytoma. This study investigated the expression of c-myc and 2 other members of the myc gene family, L- and N-myc, at the mRNA and protein level, and analyzed for possible rearrangements of these genes in the human counterpart to the mouse plasmacytoma--multiple myeloma (MM). Nine well-characterized MM cell lines were examined by using Northern- and Southern-blot analysis and immunoprecipitation. The c-myc gene was found to be highly expressed in most MM cell lines. The level of expression was comparable to that observed in the COLO 320 and HL-60 cell lines, carrying amplified c-myc genes, and to that of B-cell lines with a higher proliferative activity than the MM cell lines. In the U-266 MM cell line, L-myc, but no c-myc mRNA or protein, was found. The L-myc gene was expressed in both early- and late-passage U-266 cells, suggesting that the L-myc expression was not the result of the in vitro cultivation. N-myc was not expressed in any of the MM cell lines. No rearrangements of c-myc or L-myc genes were found. We thus conclude that (a) in contrast to the corresponding mouse and rat B-cell tumors, c-myc is not frequently rearranged in MM; (b) c-myc is highly expressed in most MM lines; and (c) L-myc but not c-myc is expressed in the U-266 MM cell line.

Intestinal epithelial cells contain a high molecular weight protease subject to inhibition by anticarcinogenic protease inhibitors

Protease inhibitors have been shown to be highly effective suppressors of carcinogenesis in both in vivo and in vitro model systems. For example, the soybean derived Bowman Birk inhibitor (BBI) has been shown to inhibit colon carcinogenesis. Although the precise mechanisms by which protease inhibitors suppress carcinogenesis are not known, it is believed that these compounds exert their anticarcinogenic effects by inhibiting specific cellular protease activities involved in the induction and/or expression of the transformed phenotype. In the current report, we describe a BBI-inhibitable proteolytic activity present in intestinal epithelial cells. The protease has a mass of approximately 125 kDa, cleaves gelatin and will bind to a BBI-affinity resin. Subcellular fractionation experiments indicate that this protease is located in the 10,000 x g pellet (lysosomal/golgi fraction) of IEC17 cell homogenates. Further studies have revealed that this proteolytic activity is inhibited by BBI and DFP, but unaffected by EDTA, indicating that this enzyme is a serine protease. Our results suggest that the 125-kDa protease is a 'target enzyme' of the BBI in these cells.

Induction of apoptosis in fibroblasts by c-myc protein

Although Rat-1 fibroblasts expressing c-myc constitutively are unable to arrest growth in low serum, their numbers do not increase in culture because of substantial cell death. We show this cell death to be dependent upon expression of c-myc protein and to occur by apoptosis. Regions of the c-myc protein required for induction of apoptosis overlap with regions necessary for cotransformation, autoregulation, and inhibition of differentiation, suggesting that the apoptotic function of c-myc protein is related to its other functions. Moreover, cells with higher levels of c-myc protein are more prone to cell death upon serum deprivation. Finally, we demonstrate that deregulated c-myc expression induces apoptosis in cells growth arrested by a variety of means and at various points in the cell cycle.

Expression, regulation, and chromosomal localization of the Max gene

The Max gene encodes a protein that interacts specifically with the Myc protein to form a heterodimer with high affinity for the specific cognate DNA binding site of Myc. Here we examine the expression of Max RNA in comparison to Myc RNA during cell growth and differentiation. Two species of RNA, a major 2.0- and a minor 1.7-kilobase species, hybridized specifically to a Max cDNA probe in all human and murine cell lines that were tested. Unlike Myc, the steady-state level of Max RNA is not significantly modulated with respect to proliferation or differentiation. Max RNA is expressed in quiescent BALB/c 3T3 cells and is modestly increased 3 h after addition of serum to the quiescent cells. In contrast to Myc RNA, Max RNA does not decline immediately upon induction of differentiation of HL60 cells by dimethyl sulfoxide, and only a modest decrease of Max RNA was observed 72 h after induction of differentiation. Unlike Myc RNA, Max RNA is relatively stable with a half-life of greater than 3 h and, therefore, does not exhibit the characteristic short half-life of RNAs encoded by most immediate early genes. The human Max gene was localized to chromosome 14, band q23. With respect to the recurring abnormalities in human tumors, this region of chromosome 14 is involved in deletions in B-cell chronic lymphocytic leukemia and malignant lymphomas and in the 12;14 translocation in uterine leiomyomas.

Myc and Max function as a nucleoprotein complex

The Myc family of oncoproteins are thought to regulate proliferation and differentiation in a wide variety of cell types. Recent studies show that Myc proteins form sequence-specific DNA-binding complexes with Max, a new member of the helix-loop-helix leucine zipper protein class. The properties of the Myc-Max complex suggest a mechanism for Myc's function in both normal and neoplastic cell behavior.

Multiple rearrangements and activated expression of c-myc induced by woodchuck hepatitis virus integration in a primary liver tumour

Woodchuck hepatitis virus (WHV) is a small, partially double-stranded DNA virus. Like the related human hepatitis B virus (HBV), WHV induces acute and chronic hepatitis and hepatocellular carcinoma (HCC) in its natural host. WHV DNA integration into c-myc and N-myc, resulting in deregulated expression of these genes, has been described previously in woodchuck HCC. We have analysed a woodchuck liver tumour in which WHV DNA was integrated in the c-myc gene. The virus insertion provoked multiple alterations in one c-myc allele, probably involving secondary deletions and mutations. Integrated viral DNA, including promotor and enhancer sequences, acted as an insertional mutagen, leading to enhanced expression of heterogenous c-myc transcripts ranging from 7.2 to 14 kb in size, strikingly longer than normal 2.3-kb c-myc RNA. These results provide an additional example in which the oncogenic activation of a myc gene by cis-acting effect of WHV insertion may play a critical role in virus-induced woodchuck HCC.

Suppression of deregulated c-MYC expression in human colon carcinoma cells by chromosome 5 transfer

Two-thirds of sporadic colon carcinomas express elevated levels of the c-MYC protooncogene. In addition, most colon carcinoma cell lines show constitutive elevated expression (10- to 40-fold over normal) of MYC RNA and protein that is not modulated in response to a mitogenic stimulus. Indirect immunofluorescence has been used to detect c-MYC protein in such cell lines, in hybrid cells resulting from fusions of such lines with cells that regulate MYC normally, and in carcinoma cells to which a normal copy of chromosome 5 has been transferred by microcell fusion. The deregulated expression of c-MYC is suppressed by fusion with a cell that regulates MYC normally. In addition, transfer of chromosome 5 by microcell fusion results in suppression of deregulated expression. Suppressed cells are no longer tumorigenic in nude mice. Loss of the transferred chromosome results in reexpression of the tumorigenic phenotype and in constitutive elevated expression of MYC. These data indicate that function of a tumor-suppressor gene on chromosome 5 is necessary for the regulated expression of MYC in at least some colon cells. Loss of this suppressor results in deregulated MYC expression and is a necessary, but most likely not sufficient, event for the expression of the tumorigenic phenotype in a subset of colon carcinomas.

Transcriptional activities of the Myc and Max proteins in mammalian cells

The myc family of oncogenes exhibit deregulated expression in a host of neoplasias. Though the molecular function of the Myc protein in both normal and tumorigenic cells has remained uncertain, it has been postulated to play a role in gene transcription on the basis of amino acid homologies with known transcription factors such as MyoD (Lüscher & Eisenman, 1990). We report here the direct testing of full-length Myc and its dimerization partner, Max, on the transcriptional activity of reporter genes bearing Myc/Max binding sites. Such reporter constructs display an endogenous level of activity in transient transfections which is dependent on the presence of the CACGTG sequence. Exogenous expression of myc results in modest activation of reporter gene transcription. Similar overexpression of max results in a repression of reporter gene activity, an effect which is reversed by co-expression with c-myc. Max repression is dependent on an intact DNA binding region, while Myc activation depends on both the N-terminal activation and the C-terminal dimerization domains. These results suggest a model in which Max homodimers can act as as repressors, and Myc-Max heterodimers as activators, of potential target genes.

c-myc oncoprotein function

Genetic alterations of the c-myc locus in various malignancies and the ability of c-myc to transform cultured cells and induce tumors in transgenic animals attest to its central role in many neoplasms. By dissecting the c-Myc protein, a number of critical functional domains of c-Myc have been identified and characterized; these findings suggest a model for c-Myc function and intracellular activity (Fig. 4). c-Myc is synthesized in the cytoplasm and undergoes oligomerization another protein such as Max. Its nuclear localization signal allows c-Myc to be targeted to and retained in the nucleus, where the protein seeks out and binds to specific DNA sites, perhaps facilitated by c-Myc's ability to bind non-specifically to DNA. Once bound to specific DNA sequences, c-Myc then activates or inhibits transcription of a number of target genes, with consequent alterations in cell growth and differentiation. Continued studies of c-Myc and its partner Max should further elucidate the mechanisms by which c-Myc can contribute both to the regulation of normal cell growth and the alteration in that regulation in neoplasia.

Evidence that a triplex-forming oligodeoxyribonucleotide binds to the c-myc promoter in HeLa cells, thereby reducing c-myc mRNA levels

A synthetic 27-base-long oligodeoxyribonucleotide, termed PU1, has been shown to bind to duplex DNA to form a triplex at a single site within the human c-myc P1 promoter. PU1 has been administered to HeLa cells in culture to examine the feasibility of influencing transcription of the c-myc gene in vivo. It is shown that uptake of PU1 into the nucleus of HeLa cells is efficient and that the compound remains intact for at least 4 hr. In nuclei extracted from PU1-treated cells, inhibition of DNase I cleavage is detected within the c-myc P1 promoter at the target site for triplex formation. The inhibition is shown to be both site and oligodeoxyribonucleotide specific. After cellular uptake of PU1, it is shown that steady-state mRNA arising from the c-myc P1 initiation site is selectively reduced relative to total mRNA, relative to mRNA from the alternative c-myc P2 initiation site, and relative to mRNA derived from the beta-actin promoter. Significant mRNA repression is not seen upon treating cells with oligodeoxyribonucleotides that fail to bind to the P1 promoter target. Taken together, these data suggest that triplex formation can occur between an exogenous oligodeoxyribonucleotide and duplex DNA in the nucleus of treated cells.

Intrachromosomal rearrangements fusing L-myc and rlf in small-cell lung cancer

Chromosomal abnormalities affecting proto-oncogenes are frequently detected in human cancer. Oncogenes of the myc family are activated in several types of tumors as a result of gene amplification or chromosomal translocation. We have recently found the L-myc gene involved in a gene fusion in small-cell lung cancer (SCLC). This results in a chimeric protein with amino-terminal sequences from a novel gene named rif joined to L-myc. Here we present a preliminary structural characterization of the rlf-L-myc fusion gene, which has been found only in cells with an amplified L-myc gene. In addition, we have used somatic cell hybrids to assign the normal rlf locus to the same chromosome (chromosome 1) on which L-myc resides. Finally, we have been able to establish a physical linkage between rif and L-myc with pulsed-field gel electrophoresis. Our results demonstrate that normal rlf and L-myc genes are separated by less than 800 kb of DNA. Thus, the rlf-L-myc gene fusions are due to similar but not identical intrachromosomal rearrangements at 1p32. The presence of independent genetic lesions that cause the formation of identical chimeric rlf-L-myc proteins suggests a role for the fusion protein in the development of these tumors.

Intrachromosomal rearrangements fusing L-myc and rlf in small-cell lung cancer

Chromosomal abnormalities affecting proto-oncogenes are frequently detected in human cancer. Oncogenes of the myc family are activated in several types of tumors as a result of gene amplification or chromosomal translocation. We have recently found the L-myc gene involved in a gene fusion in small-cell lung cancer (SCLC). This results in a chimeric protein with amino-terminal sequences from a novel gene named rif joined to L-myc. Here we present a preliminary structural characterization of the rlf-L-myc fusion gene, which has been found only in cells with an amplified L-myc gene. In addition, we have used somatic cell hybrids to assign the normal rlf locus to the same chromosome (chromosome 1) on which L-myc resides. Finally, we have been able to establish a physical linkage between rif and L-myc with pulsed-field gel electrophoresis. Our results demonstrate that normal rlf and L-myc genes are separated by less than 800 kb of DNA. Thus, the rlf-L-myc gene fusions are due to similar but not identical intrachromosomal rearrangements at 1p32. The presence of independent genetic lesions that cause the formation of identical chimeric rlf-L-myc proteins suggests a role for the fusion protein in the development of these tumors.