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Cellular targets for activation by c-Myc include the DNA metabolism enzyme thymidine kinase
O Pusch1, T Soucek, E Hengstschläger-Ottnad
1Obstetrics and Gynecology, University of Vienna, Department of Prenatal Diagnosis and Therapy, Austria.
Abstract:
Although a remarkable number of genes has been identified that are either activated or repressed via c-Myc, only few of them obviously contribute to Myc's biological effect--the induction of proliferation. We found that in logarithmically growing cells overexpression of Myc specifically induces thymidine kinase (TK) mRNA expression and enzyme activity, whereas loss of one allele of Myc causes downregulation of this enzyme. We show that activation of Myc triggers high levels of this normally strictly S-phase-regulated DNA metabolism enzyme in serum arrested G0 cells and causes high and constant levels of TK expression throughout the entire ongoing cell cycle. Induction of TK by Myc requires an intact transcriptional activation domain. Myc-induced deregulation of this enzyme is paralleled by alterations of protein binding at the E2F-site of the TK promoter. We further show that cell growth arrest by the cyclin-dependent kinase inhibitor p16 is abrogated by overexpression of Myc and that co-overexpression of p16 cannot inhibit the Myc-induced up-regulation of TK expression. Our data demonstrate TK to be a cellular target of Myc independently of the status of cell proliferation and provide evidence that the transcription factor E2F might be involved in this process.
Insights
The oncogene Myc directly upregulates thymidine kinase (TK), a DNA synthesis enzyme, promoting cell proliferation. This Myc-driven TK induction occurs independently of cell cycle phase and may involve the transcription factor E2F.
Area of Science:
- Molecular Biology
- Cell Biology
- Oncology
Background:
- The c-Myc oncogene regulates numerous genes, but its precise role in inducing cell proliferation remains incompletely understood.
- Identifying direct targets of c-Myc is crucial for elucidating its biological functions.
Purpose of the Study:
- To investigate the relationship between c-Myc and thymidine kinase (TK) expression and activity.
- To determine if TK is a direct cellular target of c-Myc and its role in cell cycle regulation.
Main Methods:
- Quantitative analysis of TK mRNA and enzyme activity in cells with altered c-Myc levels.
- Assessment of TK expression in different cell cycle phases and response to growth arrest.
- Investigation of the role of Myc's transcriptional activation domain and E2F binding sites.
Main Results:
- Overexpression of c-Myc specifically induces TK mRNA and enzyme activity, while Myc allele loss downregulates TK.
- c-Myc activates TK in quiescent (G0) cells and maintains high levels throughout the cell cycle.
- Myc-induced TK upregulation requires an intact transcriptional activation domain and alters protein binding at the TK promoter's E2F site.
- c-Myc overrides p16-induced cell cycle arrest and TK expression inhibition.
Conclusions:
- Thymidine kinase (TK) is a direct cellular target of c-Myc, irrespective of the cell proliferation status.
- c-Myc-mediated TK deregulation suggests a role for the transcription factor E2F in this process.
- These findings provide insights into Myc's mechanism of inducing proliferation via metabolic enzyme regulation.