Rb and E2F-1 regulate telomerase activity in human cancer cells.
The ends of human chromosomes (telomeres) lose up to 200 bp of DNA per cell division. Chromosomal shortening ultimately leads to senescence and death in normal cells. Many human carcinoma lines are immortal in vitro, suggesting that these cells have a mechanism for maintaining the ends of their chromosomes. Telomerase is a ribonucleoprotein complex that synthesizes telomeric DNA onto chromosomes using its RNA component as a template. Recent studies have shown that inactivation of the retinoblastoma gene product pRb and the cyclin dependent kinase inhibitor p16(INK4A) is required for telomerase activity in epithelial cells. We have demonstrated previously that restoration of functional retinoblastoma (Rb) expression is sufficient to downregulate telomerase activity in carcinoma cells. To determine mechanisms by which Rb regulates telomerase expression, we examined the effects of cyclin dependent kinase (cdk) mediated Rb inactivation and the release of E2F-1 on telomerase activity in human carcinoma cells. Overexpression of cdk2 and cdk4 but not a dominant negative cdk2 rescued Rb mediated downregulation of telomerase activity. Overexpression of the cdk regulatory subunit cyclin D1 also rescued telomerase downregulation and p16 expression alone was sufficient to ablate activity. E2F-1 overexpression was sufficient to rescue Rb mediated reduction of telomerase activity, but an E2F-1 mutant defective in DNA and Rb binding activities failed to produce this effect. Tumor tissue from E2F-1 -/- mice was negative for telomerase activity, indicating a key regulatory role for this transcription factor.
Crowe
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Center for Craniofacial Molecular Biology, University of Southern California, 2250 Alcazar Street, 90033, Los Angeles, CA, USA.
Nguyen
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DC
D000818:Animals; D042846:CDC2-CDC28 Kinases; D002352:Carrier Proteins; D018797:Cell Cycle Proteins; D019938:Cyclin D1 / Q000096:biosynthesis / Q000235:genetics; D051357:Cyclin-Dependent Kinase 2; D051358:Cyclin-Dependent Kinase 4; D019941:Cyclin-Dependent Kinase Inhibitor p16 / Q000096:biosynthesis / Q000235:genetics; D018844:Cyclin-Dependent Kinases / Q000037:antagonists & inhibitors / Q000096:biosynthesis / Q000235:genetics; D004268:DNA-Binding Proteins; D050684:E2F Transcription Factors; D050687:E2F1 Transcription Factor; D006801:Humans; D051379:Mice; D018345:Mice, Knockout; D017346:Protein Serine-Threonine Kinases / Q000096:biosynthesis / Q000235:genetics; D011518:Proto-Oncogene Proteins; D016160:Retinoblastoma Protein / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*; D056465:Retinoblastoma-Binding Protein 1; D019098:Telomerase / Q000378:metabolism*; D050680:Transcription Factor DP1; D014157:Transcription Factors / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*; D014407:Tumor Cells, Cultured
C533431:Arid4a protein, mouse; D002352:Carrier Proteins; D018797:Cell Cycle Proteins; D019941:Cyclin-Dependent Kinase Inhibitor p16; D004268:DNA-Binding Proteins; D050684:E2F Transcription Factors; D050687:E2F1 Transcription Factor; C493753:E2F1 protein, human; C493754:E2f1 protein, mouse; D011518:Proto-Oncogene Proteins; D016160:Retinoblastoma Protein; D056465:Retinoblastoma-Binding Protein 1; D050680:Transcription Factor DP1; D014157:Transcription Factors; D019938:Cyclin D1; D017346:Protein Serine-Threonine Kinases; D042846:CDC2-CDC28 Kinases; C495894:CDK2 protein, human; C495900:CDK4 protein, human; C495893:Cdk2 protein, mouse; C495902:Cdk4 protein, mouse; D051357:Cyclin-Dependent Kinase 2; D051358:Cyclin-Dependent Kinase 4; D018844:Cyclin-Dependent Kinases; D019098:Telomerase