The prototypic oncogene c-MYC encodes a transcription factor that can drive proliferation by promoting cell-cycle reentry. However, the mechanisms through which c-MYC achieves these effects have been unclear. Using serial analysis of gene expression, we have identified the cyclin-dependent kinase 4 (CDK4) gene as a transcriptional target of c-MYC. c-MYC induced a rapid increase in CDK4 mRNA levels through four highly conserved c-MYC binding sites within the CDK4 promoter. Cell-cycle progression is delayed in c-MYC-deficient RAT1 cells, and this delay was associated with a defect in CDK4 induction. Ectopic expression of CDK4 in these cells partially alleviated the growth defect. Thus, CDK4 provides a direct link between the oncogenic effects of c-MYC and cell-cycle regulation.

The proto-oncogene c-myc (myc) encodes a transcription factor (Myc) that promotes growth, proliferation and apoptosis. Myc has been suggested to induce these effects by induction/repression of downstream genes. Here we report the identification of potential Myc target genes in a human B cell line that grows and proliferates depending on conditional myc expression. Oligonucleotide microarrays were applied to identify downstream genes of Myc at the level of cytoplasmic mRNA. In addition, we identified potential Myc target genes in nuclear run-on experiments by changes in their transcription rate. The identified genes belong to gene classes whose products are involved in amino acid/protein synthesis, lipid metabolism, protein turnover/folding, nucleotide/DNA synthesis, transport, nucleolus function/RNA binding, transcription and splicing, oxidative stress and signal transduction. The identified targets support our current view that myc acts as a master gene for growth control and increases transcription of a large variety of genes.

The c-Myc protein (Myc) is a transcription factor, and deregulated expression of the c-myc gene (myc) is frequently found in tumours. In Burkitt's lymphoma (BL), myc is transcriptionally activated by chromosomal translocation. We have used a B-cell line called P493-6 that carries a conditional myc allele to elucidate the role of Myc in the proliferation of BL cells. Regulation of proliferation involves the coordination of cell growth (accumulation of cell mass) and cell division [1] [2] [3]. Here, we show that division of P493-6 cells was strictly dependent on the expression of the conditional myc allele and the presence of foetal calf serum (FCS). More importantly, cell growth was regulated by Myc without FCS: Myc alone induced an increase in cell size and positively regulated protein synthesis. An increase in protein synthesis is thought to be one of the causes of cell mass increase. Furthermore, Myc stimulated metabolic activities, as indicated by the acidification of culture medium and the activation of mitochondrial enzymes. Our results confirm the model that Myc is involved in the regulation of cell growth [4] and provide, for the first time, direct evidence that Myc induces cell growth, that is, an increase in cell size, uncoupled from cell division.

The product of the proto‐oncogene c‐myc (myc) is a potent activator of cell proliferation. In Burkitt lymphoma (BL), a human B‐cell tumor, myc is consistently found to be transcriptionally activated by chromosomal translocation. The mechanisms by which myc promotes cell cycle progression in B‐cells is not known. As a model for myc activation in BL cells, we have established a human EBV‐EBNA1 positive B‐cell line, P493‐6, in which myc is expressed under the control of a tetracycline regulated promoter. If the expression of myc is switched off, P493‐6 cells arrest in G0/G1 in the presence of serum. Re‐expression of myc activates the cell cycle without inducing apoptosis. myc triggers the expression of cyclin D2, cyclin E and Cdk4, followed by the activation of cyclin E‐associated kinase and hyper‐phosphorylation of Rb. The transcription factor E2F‐1 is expressed in proliferating and arrested cells at constant levels. The Cdk inhibitors p16, p21, p27 and p57 are expressed at low or not detectable levels in proliferating cells and are not induced after repression of myc. Ectopic expression of p16 inhibits cell cycle progression. These data suggest that myc triggers proliferation of P493‐6 cells by promoting the expression of a set of cell cycle activators but not by inactivating cell cycle inhibitors. Int. J. Cancer 87:787–793, 2000. © 2000 Wiley‐Liss, Inc.