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 divisi on due to the inability of DNA polymerase to completely replicate the chrom osomal 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 en ds of their chromosomes. Telomerase is a ribonucleoprotein complex that syn thesizes telomeric DNA onto chromosomes using its RNA component as a templa te. 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 spe cific cell cycle phases caused marked reduction in telomerase activity in G (0) and S, but not in G(1) or M. Reduction in telomerase activity correlate d with induction of Rb protein in these phases. Overexpression of full leng th Rb resulted in significant downregulation of telomerase activity. Howeve r, expression of an Rb N-terminal oligomerization domain deletion construct , a C-terminal DNA binding domain deletion construct, or a pocket domain mu tant failed to downregulate telomerase activity. We concluded that function ally intact Rb was required for cell cycle-dependent downregulation of telo merase activity in SCC lines. (C) 1999 Elsevier Science B.V. All rights res erved.