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The role of p16 in the E2F-dependent thymidine kinase regulation
M Hengstschläger1, E Hengstschläger-Ottnad, O Pusch
1University of Vienna, Department of Prenatal Diagnosis and Therapy, Austria.
Abstract:
The role of alterations of the MTS1 tumor suppressor gene on chromosome 9p21, which encodes p16, the inhibitor of cyclin-dependent-kinase-4 and 6, in tumorigenesis is not yet clear. Phosphorylation of the retinoblastoma protein by cyclin-dependent kinases 4 and 6 prevents its interaction with the transcription factor E2F, which subsequently promotes the expression of S phase regulated genes, such as thymidine kinase. Although a role of p16 in this regulation has been presumed, there is no proof so far that loss of this tumor suppressor gene really affects E2F-mediated regulations. We investigated the regulation of thymidine kinase in phytohemagglutinin-stimulated normal human lymphocytes and in the p16-negative human acute lymphoblastic leukemia cell lines, MOLT-4 and CEM. Compared to normal lymphocytes, MOLT-4 and CEM cells exhibited an altered cell cycle regulation of thymidine kinase, a much higher intracellular activity of this enzyme, and higher thymidine kinase mRNA expression. Transient expression of p16 in normal human lymphocytes caused arrest in G1, but was without effect on the cell growth of MOLT-4 and CEM cells, although all of them express functional retinoblastoma protein. Nevertheless, in the two leukemia cell lines transient overexpression of p16 reestablished the normal regulation of thymidine kinase, paralleled by an increase of the underphosphorylated form of retinoblastoma protein and decrease of free E2F bound to its motif in the thymidine kinase promoter. We demonstrate that loss of p16 causes upregulation of this DNA precursor pathway enzyme via activation of E2F by a mechanism involving retinoblastoma protein.
Insights
Loss of the p16 tumor suppressor gene disrupts normal cell cycle regulation, leading to increased thymidine kinase. Restoring p16 normalizes this regulation by affecting retinoblastoma protein and E2F activity.
Area of Science:
- Molecular Biology
- Cell Cycle Regulation
- Cancer Genetics
Background:
- The MTS1 gene encodes p16, a cyclin-dependent kinase inhibitor, crucial for cell cycle control.
- The precise role of p16 loss in tumorigenesis and its effect on E2F-mediated gene regulation remain unclear.
- Cyclin-dependent kinases 4 and 6 (CDK4/6) phosphorylate retinoblastoma protein (Rb), releasing E2F and promoting S-phase gene expression.
Purpose of the Study:
- To investigate the impact of p16 loss on thymidine kinase (TK) regulation in human lymphocytes and leukemia cell lines.
- To determine if p16 deficiency affects E2F-mediated regulation of TK.
- To elucidate the mechanism by which p16 loss influences the cell cycle and gene expression.
Main Methods:
- Analysis of thymidine kinase regulation, activity, and mRNA expression in normal lymphocytes and p16-negative leukemia cell lines (MOLT-4, CEM).
- Transient expression of p16 in normal lymphocytes and leukemia cells.
- Assessment of retinoblastoma protein phosphorylation status and E2F binding to the TK promoter.
Main Results:
- p16-negative leukemia cells (MOLT-4, CEM) showed altered cell cycle regulation of TK, higher TK activity, and increased TK mRNA compared to normal lymphocytes.
- Transient p16 expression arrested normal lymphocytes in G1 but did not affect leukemia cell growth.
- Overexpression of p16 in leukemia cells restored normal TK regulation, increased underphosphorylated Rb, and decreased free E2F bound to the TK promoter.
Conclusions:
- Loss of p16 leads to the upregulation of thymidine kinase, a DNA precursor enzyme, through E2F activation.
- The mechanism involves the retinoblastoma protein pathway, highlighting p16's critical role in maintaining normal cell cycle control and gene expression.
- These findings clarify the functional consequence of p16 loss in the context of E2F-mediated regulation and its link to cancer development.