Overexpression of C-MYC induces cell proliferation and increases expression of dihydrofolate reductase and thymidine kinase genes in HL-60 promyelocytic leukemia cells

To investigate the mechanism of c-myc induced cellular proliferation, we studied the effects of c-myc overexpression on two important DNA metabolism enzymes: thymidine kinase (TK) and Dihydrfolate reductase (DHFR). A 1320 bp cDNA fragment containing the entire coding region of human oncogene c-myc was cloned into the pLXSN retrovirus vector. A high titer virus containing human c-myc oncogene was generated from the recombinant retroviral vector plasmid and PA317 amphotrophic packaging cell line. By using HL-60 cells as targets, highly purified virus infected cells were obtained after G418 selection. Cell proliferation was determined using the cell growth curve assay. The effects of c-myc on upregulation of TK and DHFR mRNA expression was analysed by RT-PCR. The cell growth curve assays showed that on day 3 and day 5 after infection, the cell numbers in the c-myc transduced groups were 50% and 70% greater than those of the respective mock transduced controls. Similarly, significant upregulation of c-myc mRNA and protein was also seen at these time points. In the c-myc overexpressing cells , the levels of TK mRNA increased 2-3 fold and DHFR mRNA increased nearly 4-5 fold as compared to corresponding controls. These results seem to indicate that TK and DHFR are involved in the process of cellular proliferation mediated by c-myc overexpression. These genes might be the cellular targets of c-myc in cell growth and metabolism.