Rearrangement at the 5'' end of amplified c-myc in human COLO 320 cells is associated with abnormal transcription.

The proto-oncogene c-myc is amplified in sublines of human COLO 320 cells carrying either homogeneously staining chromosomal regions or double minutes. COLO 320 cells carrying homogeneously staining chromosomal regions have 15 to 20 copies of an apparently normal c-myc allele and 1 to 2 copies of an abnormal c-myc allele lacking exon 1 and express high levels of a normal c-myc mRNA 2.5 kilobases in size. COLO 320 cells carrying double minutes have about 25 copies each of the normal allele and the abnormal allele but express preferentially an abnormal c-myc mRNA 2.2 kilobases in size. Nucleotide sequence analyses revealed that the break point of rearrangement resulting in the loss of exon 1 in the abnormal allele lies within a region frequently rearranged in human and murine B-cell tumors.     

Truncation of exon 1 from the c-myc gene results in prolonged c-myc mRNa stability.

The human c-myc gene consists of three exons transcribed from two distinct promoters and the function of the first, noncoding exon is unknown. In COLO 320 cells, there co-exist normal and truncated (i.e., lacking exon 1) c-myc genes, both of which are transcribed. Studies on the turnover of c-myc mRNA show that the normal mRNA has an in vivo half-life of approximately 30 min which is approximately similar to the turnover time of the mRNA in lymphoblastoid cells. However, the truncated mRNA was found to be substantially more stable. This observation was also made with a Burkitt's lymphoma cell line which has a translocated, truncated c-myc gene. Therefore truncation of the c-myc gene can cause the mRNA to be more stable than the full size product suggesting that this can be a crucial factor in the activation of the c-myc oncogene, by exon 1 loss, in chromosomal translocation. The results also suggest a role for exon 1 in the c-myc mRNA degradative mechanism.     

Aberrant c-myc RNAs of Burkitt's lymphoma cells have longer half-lives.

BL67 and BL18 are Burkitt's lymphoma cell lines with t(8;14) translocations (the breakpoint is in the first exon and first intron, respectively) in which the mu-heavy chain switch region is fused to the c-myc gene in head to head orientation. In both cell lines only aberrant c-myc RNAs are found. BL67 cells contain two c-myc RNA species of 2.4 and 3.5 kb. The 2.4-kb RNA is initiated at several cryptic promoters in the first intron. The 3.5-kb RNA is transcribed from the immunoglobulin heavy chain anti-sense strand across the breakpoint of the translocation into the first exon of the c-myc gene and is then normally spliced using the physiological splice donor and acceptor sites of the c-myc gene. BL18 contains c-myc RNA of 2.4 kb initiated at cryptic promoters in the first intron and additional RNAs of 0.90 kb and 0.74 kb transcribed from the dual c-myc promoters on the reciprocal fragment of the translocation. The cytoplasmic turnover of these RNAs differs significantly from that of the normal c-myc message. The 3.5-kb RNA of BL67 cells and the 0.90-kb and 0.74-kb RNAs of BL18 cells, which are both hybrid molecules consisting of c-myc and immunoglobulin sequences, have a half-life of several hours in contrast to the normal c-myc message with a half-life of 15 min. The aberrant 2.4-kb c-myc RNAs of BL67 and BL18 cells are also more stable than the normal c-myc message and disappear with a half-life of 50 min.(ABSTRACT TRUNCATED AT 250 WORDS)     

Recombinant interleukin 2 regulates levels of c-myc mRNA in a cloned murine T lymphocyte.

The cellular oncogene c-myc has been implicated in the regulation of growth of normal and neoplastic cells. Recently, it was suggested that c-myc gene expression may control the G0----G1-phase transition in normal lymphocytes that were stimulated to enter the cell cycle by the lectin concanavalin A (ConA). Here we describe the effects of purified recombinant interleukin 2 (rIL2) and of ConA on levels of c-myc mRNA in the noncytolytic murine T-cell clone L2. In contrast to resting (G0) primary cultures of lymphocytes, quiescent L2 cells have a higher RNA content than resting splenocytes and express receptors for interleukin 2 (IL2). Resting L2 cells are therefore best regarded as early G1-phase cells. Purified rIL2 was found to stimulate the rapid accumulation of c-myc mRNA in L2 cells. Levels of c-myc mRNA became maximal within 1 h and declined gradually thereafter. In contrast, ConA induced slower accumulation of c-myc mRNA in L2 cells, with increased levels of c-myc mRNA becoming detectable 4 to 8 h after stimulation. Experiments with the protein synthesis inhibitor cycloheximide demonstrated that the increase in levels of c-myc mRNA that were induced by ConA was a direct effect of this lectin and not secondary to IL2 production. Cyclosporin A, an immunosuppressive agent, markedly reduced the accumulation of c-myc mRNA that was induced by ConA but only slightly diminished the accumulation of c-myc mRNA that was induced by rIL2. Taken together, these data provide evidence that (i) c-myc gene expression can be regulated by at least two distinct pathways in T lymphocytes, only one of which is sensitive to cyclosporine A, and (ii) the accumulation of c-myc mRNA can be induced in T cells by IL2 during the G1 phase of the cell cycle.