Intact functional domains of the retinoblastoma gene product (pRb) are required for downregulation of telomerase activity.

The ends of human chromosomes (telomeres) consist of tandem repeats of the sequence TTAGGG. Telomeres lose up to 200 base pairs of DNA per cell division due to the inability of DNA polymerase to completely replicate the chromosomal ends. Chromosomal shortening ultimately leads to senescence and cell death in normal cells. However, some immortal cells do not lose telomeric sequence during DNA replication. 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. To elucidate potential mechanisms for telomerase regulation, we tested human squamous cell carcinoma lines (SCCs) for telomerase activity. All SCC lines expressed high levels of telomerase activity. Synchronization in specific cell cycle phases caused marked reduction in telomerase activity in G0 and S, but not in G1 or M. Reduction in telomerase activity correlated with induction of Rb protein in these phases. Overexpression of full length Rb resulted in significant downregulation of telomerase activity. However, expression of an Rb N-terminal oligomerization domain deletion construct, a C-terminal DNA binding domain deletion construct, or a pocket domain mutant failed to downregulate telomerase activity. We concluded that functionally intact Rb was required for cell cycle-dependent downregulation of telomerase activity in SCC lines.