TELOMERE ELONGATION IN IMMORTAL HUMAN-CELLS WITHOUT DETECTABLE TELOMERASE ACTIVITY

Immortalization of human cells is often associated with reactivation o f telomerase, a ribonucleoprotein enzyme that adds TTAGGG repeats onto telomeres and compensates for their shortening. We examined whether t elomerase activation is necessary for immortalization, All normal huma n fibroblasts tested were negative for telomerase activity, Thirteen o ut of 13 DNA tumor virus-transformed cell cultures were also negative in the pre-crisis (i.e. non-immortalized) stage, Of 35 immortalized ce ll lines, 20 had telomerase activity as expected, but 15 had no detect able telomerase. The 15 telomerase-negative immortalized cell lines al l had very long and heterogeneous telomeres of up to 50 kb, Hybrids be tween telomerase-negative and telomerase-positive cells senesced. Two senescent hybrids demonstrated telomerase activity, indicating that ac tivation of telomerase is not sufficient for immortalization. Some hyb rid clones subsequently recommenced proliferation and became immortali zed either with or without telomerase activity, Those without telomera se activity also had very long and heterogeneous telomeres. Taken toge ther, these data suggest that the presence of lengthened or stabilized telomeres is necessary for immortalization, and that this may be achi eved either by the reactivation of telomerase or by a novel and as yet unidentified mechanism.