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, suggesti ng that these cells have a mechanism for maintaining the ends of their chro mosomes. Telomerase is a ribonucleoprotein complex that synthesizes telomer ic DNA onto chromosomes using its RNA component as a template. Recent studi es have shown that inactivation of the retinoblastoma gene product pRb and the cyclin dependent kinase inhibitor p16(INK4A), required for telomerase a ctivity in epithelial cells. We have demonstrated previously that restorati on of functional retinoblastoma (Rb) expression is sufficient to downregula te telomerase activity in carcinoma cells. To determine mechanisms by which Rb regulates telomerase expression, we examined the effects of cyclin depe ndent kinase (cdk) mediated Rb inactivation and the release of E2F-1 on tel omerase activity in human carcinoma cells. Overexpression of cdk2 and cdk4 but not a dominant negative cdk2 rescued Rb mediated downregulation of telo merase activity. Overexpression of the cdk regulatory subunit cyclin D1 als o rescued telomerase downregulation and p16 expression alone was sufficient to ablate activity. E2F-1 overexpression was sufficient to rescue Rb media ted 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 regul atory role for this transcription factor. (C) 2001 Elsevier Science B.V. Al l rights reserved.