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 Janus protein tyrosine kinases (Jaks) play critical roles in transducing growth and differentiation signals emanating from ligand-activated cytokine receptor complexes. The activation of the Jaks is hypothesized to occur as a consequence of auto-or transphosphorylation on tyrosine residues associated with ligand-induced aggregation of the receptor chains and the associated Jaks. In many kinases, regulation of catalytic activity by phosphorylation occurs on residues within the activation loop of the kinase domain. Within the Jak2 kinase domain, there is a region that has considerable sequence homology to the regulatory region of the insulin receptor and 1008, to phenylalanine essentially eliminated kinase activity, whereas mutation of Y 1008 to phenylalanine had no detectable effect on kinase activity. The mutants were also examined for the ability to reconstitute erythropoietin signaling in ␥2 cells, which lack Jak2. Consistent with the kinase activity, mutation of Y 1007 to phenylalanine eliminated the ability to restore signaling. Moreover, phosphorylation of a kinase-inactive mutant (K 882 E) was not detected, indicating that Jak2 activation during receptor aggregation is dependent on Jak2 and not another receptor-associated kinase. The results demonstrate the critical role of phosphorylation of Y 1007 in Jak2 regulation and function.The Janus tyrosine kinase (Jak) family consists of four members, Jak1, Jak2, Jak3, and Tyk2. Over the past several years, it has become evident that these kinases play a critical role in the functions of receptors of the cytokine receptor superfamily (4,7,8,15,19). In general, the Jaks associate with the membraneproximal region of one or more of the receptor chains constitutively or following ligand-induced receptor aggregation. Following ligand binding, there is a rapid induction of tyrosine phosphorylation of the receptor-associated Jaks and a striking increase in their catalytic activity in in vitro kinase assays of the immunoprecipitated Jaks. It is hypothesized that ligand-induced receptor aggregation results in the aggregation of the associated Jaks, which allows Jak auto-or transphosphorylation at sites that regulate catalytic activity. The activated Jaks subsequently tyrosine phosphorylate the receptor chains as well as a variety of substrates that are recruited to the activated receptor complex.The role of tyrosine phosphorylation of Jaks in regulation of their intrinsic kinase activity, although hypothesized, has only recently been addressed. The regulation of tyrosine kinase activity has been shown to occur in two distinct manners. The family of Src kinases are regulated through the tyrosine phosphorylation of a carboxyl-terminal site (2, 14, 16). It is speculated that this site interacts with the SH2 domain to induce an inactive conformation of the kinase domain. Activation of kinase activity is therefore associated with dephosphorylation of the carboxyl-terminal site. Alternatively, a number of receptor tyrosine kinases are catalytically activated by tyrosine ph...